Selective two-electron and four-electron oxygen reduction reactions using Co-based electrocatalysts

Abstract

The oxygen reduction reaction (ORR) can take place via both four-electron (4e⁻) and two-electron (2e⁻) pathways. The 4e⁻ ORR, which produces water (H₂O) as the only product, is the key reaction at the cathode of fuel cells and metal–air batteries. On the other hand, the 2e⁻ ORR can be used to electrocatalytically synthesize hydrogen peroxide (H₂O₂). For the practical applications of the ORR, it is very important to precisely control the selectivity. Understanding structural effects on the ORR provides the basis to control the selectivity. Co-based electrocatalysts have been extensively studied for the ORR due to their high activity, low cost, and relative ease of synthesis. More importantly, by appropriately designing their structures, Co-based electrocatalysts can become highly selective for either the 2e⁻ or the 4e⁻ ORR. Therefore, Co-based electrocatalysts are ideal models for studying fundamental structure–selectivity relationships of the ORR. This review starts by introducing the reaction mechanism and selectivity evaluation of the ORR. Next, Co-based electrocatalysts, especially Co porphyrins, used for the ORR with both 2e⁻ and 4e⁻ selectivity are summarized and discussed, which leads to the conclusion of several key structural factors for ORR selectivity regulation. On the basis of this understanding, future works on the use of Co-based electrocatalysts for the ORR are suggested. This review is valuable for the rational design of molecular catalysts and material catalysts with high selectivity for 4e⁻ and 2e⁻ ORRs. The structural regulation of Co-based electrocatalysts also provides insights into the design and development of ORR electrocatalysts based on other metal elements.

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Zuozhong Liang

Zuozhong Liang is currently an associate research fellow at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. He received his BS degree (2011) from Qufu Normal University and PhD degree (2016) from the Beijing University of Chemical Technology under the supervision of Professor Jian-Feng Chen. In July 2016, he joined the research group of Professor Rui Cao. His research interests are in the development of bioinspired electrocatalysts used in energy-related small molecule activation.

Haitao Lei

Haitao Lei is currently an associate research fellow at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. He received his BS degree (2012) from Lanzhou University and PhD degree (2017, with professor Rui Cao) from the Renmin University of China. He served as a postdoctoral fellow at Shaanxi Normal University (2017–2020), working under the supervision of professor Rui Cao. His research interests focus on the design and development of novel functional metal complexes and related materials for the activation of energy-related small molecules.

Haoquan Zheng

Haoquan Zheng is currently a professor at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. He received his BS degree (2006) and PhD degree (2011) in applied chemistry from Shanghai Jiao Tong University under the supervision of Professor Shunai Che. He worked as a postdoctoral fellow in the group of Professor Xiaodong Zou at Stockholm University. He moved to his current position in July 2016. His research interests mainly focus on the development of hierarchical porous materials with novel structures and functions.

Hong-Yan Wang

Hong-Yan Wang is currently a professor at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. She received her PhD degree in organic chemistry from the Chinese Academy of Sciences in 2011. In 2014, she joined Shaanxi Normal University as an associate professor. Her research interests concern artificial photosynthesis based on supramolecular systems, coordination compounds with organic ligands and semiconductors. Now, she is working on the design of photochemical devices for water splitting, CO2 reduction and organic synthesis.

Wei Zhang

Wei Zhang is currently a professor at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. He received his BS degree (2007) in chemistry from Peking University in Beijing, China and PhD degree (2012) from Nanyang Technological University in Singapore with Professor Rong Xu. After postdoctoral work on photocatalytic CO2 reduction with Professor Rong Xu, he joined the faculty at Shaanxi Normal University in 2014. His research focuses on the kinetics of electrocatalytic water oxidation.

Rui Cao

Rui Cao is currently a professor at the School of Chemistry and Chemical Engineering at Shaanxi Normal University. He received his BS degree (2003) in chemistry from Peking University in Beijing, China and his PhD degree (2008) from Emory University in Atlanta, Georgia. He was the Dreyfus Postdoctoral Fellow (2009–2011) at the Massachusetts Institute of Technology under the guidance of Professor Stephen J. Lippard. In 2011, he became a professor at the Renmin University of China and he transferred to Shaanxi Normal University in 2014. His current research focuses on molecular electrocatalysis.

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1. Introduction

The oxygen reduction reaction (ORR) holds crucial significance in numerous energy storage and conversion techniques, particularly in fuel cells and metal–air batteries.^1–3 In these devices, the anode oxidation reaction of various fuels provides electrons, while the cathode ORR receives these electrons produced at anodes. In principle, oxygen (O₂) can be reduced through either a four-electron (4e⁻) reduction pathway to give water (H₂O) or a two-electron (2e⁻) reduction pathway to produce hydrogen peroxide (H₂O₂).⁴ From an energy conversion and utilization point of view, the 4e⁻ ORR pathway is more desirable. For example, the occurrence of the 4e⁻ ORR at the cathode of a hydrogen (H₂) fuel cell is essential for facilitating the efficient utilization and development of renewable hydrogen energy.⁵ This process not only enhances the overall performance of fuel cells but also contributes significantly to the advancement of sustainable energy technologies. Particularly, in a typical fuel cell, O₂ will react with protons (H⁺) and electrons (e⁻) to produce H₂O through a 4e⁻ reduction process in the presence of catalysts.^6–8 Therefore, the achievement of high 4e⁻ selectivity during the ORR process is of great importance. In contrast, the 2e⁻ reduction product H₂O₂ is an important chemical, which is widely used in medicine (disinfection and sterilization), industry (oxidant and bleaching agent), military (rocket power combustion promoter), and many other fields.^9,10 Up to now, several strategies have been reported for the production of H₂O₂,^11–13 such as the anthraquinone process,¹⁴ the direct synthesis of H₂O₂ using H₂ and O₂,^15–17 and the electrochemical ORR method through a 2e⁻ process.¹⁸ In recent years, the electrochemical 2e⁻ ORR has become a promising method to synthesize H₂O₂.^19–21 Proton exchange membrane (PEM) cells have been constructed for electrochemical production of H₂O₂. Both anode and cathode catalysts are immersed in liquid water and brought into direct contact with solid electrolytes.²² The porous solid electrolyte was applied to directly synthesize high-purity H₂O₂ with a concentration of up to 20 wt% by the electrochemical method in one step.²² This process does not require subsequent separation and purification, which also has a strong competitive advantage. H₂O₂ generated by this method is in pure water and thus can be used directly. The PEM method is particularly suitable for small-scale synthesis of dilute H₂O₂ solution.²³ Compared with the traditional high energy consumption anthraquinone oxidation method and direct H₂–O₂ synthesis method with potential explosion risk, the electrochemical reduction of O₂ to H₂O₂ is considered to be a green and mild alternative method.²⁴ Furthermore, H₂O₂ can be used as a fuel. Note that the maximum output potential of a H₂O₂ fuel cell is 1.09 V, which is close to the output potential of a H₂ fuel cell (1.23 V) and a direct methanol fuel cell (1.11 V).²⁵ The H₂O₂ fuel cell stands as a promising candidate for energy conversion devices. Therefore, designing and developing electrocatalysts that exhibit both 4e⁻ and 2e⁻ selectivity for the ORR is highly desirable.

At present, Pt-based materials, being both costly and scarce, are predominantly employed as cathode catalysts for the 4e⁻ ORR.^26–30 To overcome the constraints posed by the limited availability of precious metal Pt, there is a pressing need for extensive and intensive research into non-Pt ORR catalysts.^31–33 Numerous non-Pt catalysts, including metal-coordinated and N-doped carbon materials (M–N–C, M = Co, Fe, and Mn),^34–40 metal oxides,^41–45 metal porphyrins/corroles/phthalocyanines and their frameworks/polymers,^46–55 have been reported in recent years.^56–58 Compared with electrocatalysts for the 4e⁻ ORR, the investigation of electrocatalysts used for the 2e⁻ ORR has also received rapidly increasing attention.⁵⁹ At present, single-atomic Co–N–C materials, Co porphyrins, and carbon-based materials have been reported for the ORR with a typical 2e⁻ reduction process.^60–62 Carbon-based materials,⁶³ particularly carbon black (CB), carbon nanotubes (CNTs), graphene, mesoporous carbon hollow spheres, oxygen-doped carbon quantum dots, and other specially treated carbon materials, have garnered significant attention.^64–70 Remarkably, among these materials and molecules, only a few catalysts are capable of selectively achieving both 2e⁻ and 4e⁻ ORRs under controllable conditions. Note that Co-based electrocatalysts, especially Co porphyrins, stand out as they possess the unique ability to switch between 2e⁻ and 4e⁻ selectivity in the ORR. Therefore, Co-based electrocatalysts offer an ideal platform for studying fundamental relationships between structure and selectivity, thereby advancing the understanding in this field.

In nature, cytochrome c oxidases (CcOs), featuring an Fe porphyrin unit as their active center, selectively reduce O₂ to H₂O.⁷¹ Porphyrins, which have a macrocyclic structure constructed by the linkage of four pyrrole subunits through methylene bridges (=CH–), transform into metal porphyrin complexes upon the substitution of two pyrrole protons (N–H) with metal ions. These metal porphyrins, capable of coordinating with various metal ions, such as Mn, Fe, Co, Ni, Cu, and Zn, have a well-defined molecular structure, facilitating the investigation of electrocatalytic ORR mechanisms and the establishment of structure–selectivity relationships.⁷² Inspired by nature, a large number of transition metal-based porphyrins have been designed and synthesized for ORR electrocatalysis, offering tunable selectivity.^47,73 In metal porphyrins, O₂ initially binds to the axial site of the metal center, and during the reduction process, a terminal metal–oxo species will be formed upon the O–O cleavage and is considered to be a key intermediate during the ORR.⁷⁴ This process will lead to the formation of H₂O through a 4e⁻ reduction pathway. However, for late transition metal elements, the repulsion between electrons of metal d orbitals and oxygen p orbitals causes their terminal metal–oxo intermediates to be high in energy. As a result, the breaking of the M–O bond instead of the O–O bond will happen to generate H₂O₂, leading to a 2e⁻ reduction pathway. Notably, Co–oxo exhibits a balance of stability and reactivity, enabling mononuclear Co porphyrins to be able to theoretically catalyze both 2e⁻ and 4e⁻ ORRs. The disparity in selectivity comes from the electronic configuration and local environments surrounding the Co center, emphasizing the significance of refining the molecular structure of Co porphyrins to regulate ORR selectivity. Furthermore, Co–N–C materials, which have similar Co–N₄ coordination structures to Co porphyrins, exhibit tunable 2e⁻ and 4e⁻ selectivity for the ORR. Many Co–N–C materials with varying coordination environments have been reported for the ORR.^57,75 The insights gained from studying Co-based molecular catalysts as model systems are invaluable for the design and development of both molecular catalysts and Co-based materials, underlining the significance of the structure–selectivity relationship in this context.

This review comprehensively encapsulates recent breakthroughs in ORR catalysis, with a particular focus on Co-based electrocatalysts. In the initial section, we introduce the ORR mechanism and strategic approaches employed for fine-tuning ORR selectivity. Subsequently, we spotlight the latest advancements centered on Co porphyrins and other Co-based electrocatalysts, exploring 2e⁻versus 4e⁻ selectivity achieved through diverse innovative strategies. The discussion underscores the significance of molecular structure design, emphasizing the relationship between structure and selectivity, as well as the profound insights gained from the in-depth investigation of reaction mechanisms. Furthermore, we present a comprehensive overview of both homogeneous and heterogeneous Co-based electrocatalysts, offering a comparative analysis of their characteristics and performances. Finally, we consolidate the understanding of selectivity in this field and provide promising avenues for the future development of Co-based electrocatalysts.

2. Oxygen reduction reaction

For the ORR, CcOs utilize the Fe porphyrin/Cu structure to synergistically activate and reduce O₂ to H₂O.⁷⁶ The coordination environments surrounding these two metal ions are distinct, yet they collaborate continuously within the catalytic process, enhancing the overall efficiency of the ORR.⁷⁷ Currently, some researchers believe that the Fe^II porphyrin is the oxygen binding and activation site, and the Cu ion can affect the redox properties of Fe at the reaction center.⁷⁴ The Cu^I ion can also quickly provide an electron to promote further O₂ activation and can even directly bind with the activated O₂ molecule through electrostatic interactions to stabilize catalytic reaction intermediates. The distance between Fe and Cu ions is 5.2 Å as confirmed from the single crystal structure of CcOs.⁷⁸ The space and distance between Fe^II and Cu^I are suitable for the activation and reduction of an O₂ molecule. When forming Fe^III–O–O–Cu^II peroxo intermediates, the cleavage of the O–O bond occurs immediately to form Fe^III–OH and Cu^II–OH intermediates after accepting two protons and two electrons. Then, two H₂O molecules form after further accepting two protons and two electrons. Metal active sites were reduced to the initial Fe^II porphyrin and Cu^I site. In the enzymatic cycle of CcOs, a total of four protons and four electrons are needed.⁷¹

Generally, there are three possible ORR pathways for metal complexes such as metal porphyrins.⁷⁹ The O₂ molecule can be reduced to H₂O₂ through one 2e⁻ pathway or can be reduced to H₂O through two 4e⁻ pathways (Fig. 1a). For mononuclear metal complexes, the catalytic selectivity depends on metal centers and substituent groups. First, O₂ binds at the metal center and forms an M–O₂˙⁻ intermediate. Then, M–O₂˙⁻ receives one proton and one electron to form a M–OOH species. Subsequently, there are two possible ways for further reduction of this peroxo intermediate. In general, late transition metal (e.g., Ni and Cu) complexes prefer a 2e⁻ ORR selectivity with H₂O₂ as the product after further experiencing a one electron and one proton transfer process. In contrast, early and middle transition metal (e.g., Mn and Fe) complexes tend to catalyze the ORR through a 4e⁻ pathway with H₂O as the product after further receiving three protons and three electrons. The difference is attributed to the fact that early and middle transition metal ions are more effective than late transition metal ions in mediating the O–O bond cleavage to give terminal metal–oxo species.⁷⁴ In other words, the ease of forming metal–oxo determines the selectivity of the ORR. As the number of d electrons increases, the stability of metal–oxo decreases and its activity increases, a result caused by the repulsion between d electrons of the metal ion and p electrons of the oxygen atom (Fig. 1b). If the number of d electrons is no more than 4, metal–oxo can exist. Therefore, metal–oxo species of middle transition metal elements (e.g., Mn–oxo and Fe–oxo) are relatively more easily formed compared with that of late transition metals (e.g., Ni–oxo and Cu–oxo). As a result, middle transition metal complexes tend to experience a 4e⁻ ORR process, while late transition metals complexes tend to experience a 2e⁻ ORR process.

Fig. 1 (a) Possible 2e⁻ and 4e⁻ ORR pathways mediated by metal complexes. (b) Electronic structure analysis of M–oxo to understand its stability and activity.

In addition, M–O₂˙⁻ species can combine with another M center to form the bimetallic peroxo intermediate M–O–O–M. Subsequently, this intermediate undergoes a 4e⁻ and 4H⁺ transfer process, resulting in a 4e⁻ ORR pathway. Constructing dinuclear metal complexes has emerged as an effective strategy for facilitating the 4e⁻ ORR, with the optimal distance between metal centers being confined within 4 to 7 Å.⁸⁰ To further confirm this conclusion, theoretical calculations were conducted, utilizing dinuclear transition metal complexes as models, such as Co diporphyrin anthracene and dibenzofuran.⁸¹ Notably, Co-based diporphyrin anthracene exhibits a remarkably low energy barrier for cleaving the O–O bond of a peroxo-bridged dinuclear species through a homolytic mechanism. Consequently, the coexistence of mononuclear and bimolecular reaction pathways in mononuclear Co complexes contributes to the suboptimal ORR selectivity. In contrast to the 4e⁻ ORR, studies on the 2e⁻ selective regulation of Co complexes are much less explored. It is particularly difficult to preclude the occurrence of the bimolecular reaction mechanism, emphasizing the importance of the design and development of Co-based electrocatalysts. Such endeavors are indispensable for achieving pure 4e⁻ and 2e⁻ ORR pathways, thereby enhancing the overall performance and selectivity of these catalytic systems.

3. Two-electron reduction of O₂ to H₂O₂

Electrocatalytic synthesis of H₂O₂ through the 2e⁻ ORR, with its advantages of environmental friendliness and economic feasibility, is emerging as a potential industrial alternative to the highly polluting and energy-intensive anthraquinone cyclic process.^13,82 The strategic advantage of this technology lies in its flexible production characteristics, empowering real-time, demand-driven H₂O₂ generation with precision-tunable concentration outputs to meet diverse operational demands.⁸³ This decentralized production paradigm not only effectively mitigates the risks associated with long-distance storage and transportation of hazardous chemicals, but also overcomes the limitations of traditional centralized manufacturing in terms of application adaptability, thereby expanding H₂O₂ utilization into diversified and specialized domains.^11,84 Beyond conventional industrial applications such as pulp bleaching and chemical synthesis, this technology demonstrates unique commercialization potential in emerging environmental fields including distributed water treatment systems, on-site disinfection systems, portable air purification devices and medical-grade instant sterilization products, providing innovative solutions for building a green circular economy system.^85–87 Therefore, the 2e⁻ ORR electrocatalysts are of great significance, particularly in the context of large-scale H₂O₂ production. Recently, many kinds of electrocatalysts, such as carbon materials,^88–90 Fe-based materials,^91,92 Ni-based materials,⁹³ Sb-based materials,⁹⁴ and Sn-based materials,⁹⁵ have been reported for the 2e⁻ ORR.⁹⁶ This review focuses on Co-based electrocatalysts. Several kinds of Co-based electrocatalysts have been reported to demonstrate effectiveness in the 2e⁻ ORR process, including mononuclear Co porphyrins, porphyrin boxes, frameworks, single-atom/dual-atom Co-based materials, and a large number of other Co-based materials. Each of these systems offers unique advantages and properties, highlighting the extensive research efforts directed towards optimizing Co-based electrocatalysts for the efficient and selective production of H₂O₂.

3.1. Mononuclear Co porphyrins

Metal porphyrins and their derivatives are ubiquitous in nature and biological systems, where they perform vital functions.⁹⁷ Examples include cytochrome, heme, and chlorophyll, all of which are crucial to life processes.^79,98 For example, hemoglobin is the most widely distributed respiratory pigment. The key structure of hemoglobin contains a Fe porphyrin unit. Hemoglobin is dark red when deoxidized and red when combined with O₂. Similarly, chlorocruorin, another Fe-porphyrin-containing protein, undergoes a green-to-red transformation depending on its oxygen content. Fe porphyrins play a key role in the process of oxygen transport, storage, and activation in animals.⁷⁹ Beyond Fe porphyrins, there are many other metal-porphyrin-containing proteins, including V-porphyrin-containing hemovanadin, Mn-porphyrin-containing pinnaglobin, and Mg-porphyrin-containing chlorophyll. In addition, vitamin B₁₂, a vital member of the B-complex vitamins, inherently incorporates Co-containing porphyrin-like compounds in its structure, which is crucial to the production of bone marrow erythrocytes. Due to the similar Co–N₄ structure of vitamin B₁₂, an electrochemical ORR was conducted. The metal Co in vitamin B₁₂ exhibits redox properties, enabling the reversible reduction of vitamin B_12a (Co^III) to vitamin B_12r (Co^II) and subsequently to vitamin B_12s (Co^I) continuously in aqueous media.⁹⁹ When vitamin B₁₂ is adsorbed onto a pyrolytic graphite electrode, it acts as a catalyst, facilitating the ORR process within a broad pH range of 5.0–13.0.¹⁰⁰ Comparable findings were replicated when vitamin B₁₂ was adsorbed onto a glassy carbon electrode, further corroborating its effectiveness as a catalyst for the ORR across different electrode surfaces.¹⁰¹ These examples highlight the exceptional and inspiring role that metal porphyrins play in energy transfer, sparking immense curiosity and research efforts among chemists and biologists.

Drawing inspiration from intricate designs of nature, metal porphyrins with diverse substitution groups, second sphere functional groups and axial ligands have been synthesized and extensively studied as electrocatalysts for the ORR.^102–114 As typical molecular catalysts, metal porphyrins exhibit several compelling advantages.⁷⁹ First, porphyrins encapsulate metal ions within a rigid and stable coordination environment, ensuring their durability in both acidic and basic solutions, thereby providing the way for practical applications. Second, porphyrin ligands possess redox activity, actively participating in redox processes, which not only enhances the redox chemistry of metal porphyrins but also renders them ideal for multi-electron catalytic processes. Third, metal porphyrins offer many modification sites at both meso- and β-positions, enabling the synthesis of porphyrins with diverse physical and chemical properties, thereby vastly expanding their numbers.¹¹⁵ Last but not least, the second coordination sphere of metal porphyrins can be delicately tuned through the introduction of specific substituents or coordinated metal ions, fine-tuning their catalytic activity and selectivity. Therefore, metal porphyrins have garnered widespread attention as electrocatalysts for various energy-related small molecule activation reactions, including the ORR,^{47,48,116–118} hydrogen evolution reaction (HER),^119–125 oxygen evolution reaction (OER),^126–128 and CO₂ reduction reaction (CO₂RR),^129–134 underscoring their potential as versatile catalysts for sustainable energy conversion.

Among these metal porphyrins, Co porphyrins have attracted great attention owing to their high catalytic activity and tunable selectivity.^135–139 By manipulating the surrounding environment of Co porphyrins, the realization of both 2e⁻ and 4e⁻ ORR pathways becomes feasible. To this end, a diverse range of Co porphyrins, each decorated with distinct meso-substituents and β-substituents, have been designed and thoroughly investigated.²⁵ However, only a few Co porphyrins exhibit 2e⁻ ORR selectivity with the number (n) of electrons transferred close to 2.0. These include Co 5,10,15,20-tetraphenylporphyrin (CoTPP),¹⁴⁰ Co 5,10,15,20-tetra(4-methoxyphenyl)porphyrin (CoTMOPP),¹³⁷ Co 5,10,15,20-tetra(4-cyanophenyl)porphyrin (CoTCyPP) and Co 5,10,15,20-tetra(4-cyano-2,6-dimethylphenyl)porphyrin (CoTCyDMPP),¹⁴¹ Co 5,10,15,20-tetra(N-methyl-4-pyridyl) porphyrin (CoTMPyP),¹⁴² Co 5,10,15,20-tetra(4-pyridyl) porphyrin (CoTPyP),¹⁴³ Co tetrakis-(N-methyl pyridyl)-β-octabromoporphyrin (CoTMPBr₈)¹⁴⁴ and tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin (CoTSPBr₈),¹⁴⁵ as illustrated in Fig. 2. These Co porphyrins can be used as electrocatalysts for the electrochemical 2e⁻ ORR to obtain H₂O₂. For example, CoTPP molecular catalysts exhibit a high selectivity towards H₂O₂ production (>92%) in the acidic electrolyte with a dominant 2e⁻ ORR pathway when combined with reduced graphene oxide with oxygen functionalized groups.¹⁴⁶ Leveraging a PEM electrolyzer, this composite produces an aqueous H₂O₂ solution with a concentration of 7 wt% at a high current density of 400 mA cm⁻², maintaining this performance over an extended duration of 200 h.

Fig. 2 Mononuclear Co porphyrins used for the ORR with 2e⁻ selectivity.

To enhance the catalytic activity of metal porphyrins towards the ORR, carbon materials, such as CNTs with their exceptional conductivity, are frequently introduced to form hybrid materials. Recently, CoTMOPP has emerged as a model compound to study the selectivity of the ORR.¹³⁷ Hybrid catalysts comprising CoTMOPP molecules loaded on CNTs exhibit a high H₂O₂ selectivity exceeding 94% with an n value of 2.07 measured in 0.1 M HClO₄ using a rotating ring-disk electrode (RRDE). This selectivity was confirmed by independent studies conducted in 0.5 M H₂SO₄.¹³⁸ Furthermore, even when axial ligands of CoTMOPP were modulated, the 2e⁻ selectivity of the ORR still remained. Further research delved into the substituent position effect of CoTMOPP on both the catalytic activity and selectivity of the ORR.¹⁴⁷ Two Co porphyrins, featuring three –OMe groups positioned at the 2,4,6- and 3,4,5-sites of meso-phenyl substituents, namely Co 5,10,15,20-tetra(2,4,6-methoxyphenyl)porphyrin (2,4,6-OMe-CoTPP) and Co 5,10,15,20-tetra(3,4,5-methoxyphenyl)porphyrin (3,4,5-OMe-CoTPP), were synthesized. 2,4,6-OMe-CoTPP and 3,4,5-OMe-CoTPP had n values of 2.37 and 2.33, respectively, indicating a dominant 2e⁻ ORR process measured in 0.1 M KOH. In addition, Co 5,10,15,20-tetra(pentafluorophenyl)porphyrin (CoTPFP) has been reported for the ORR.¹⁴⁸ The CoTPFP–CNT hybrid exhibits a mixed 2e⁻ and 4e⁻ selectivity for the ORR, with an n value of 3.19 measured with a RRDE. This phenomenon may be attributed to the presence of a bimolecular reaction mechanism caused by the potential formation of aggregated metal porphyrins. Consequently, inhibiting the aggregation of metal porphyrins on the support surface is crucial in regulating the ORR selectivity of Co porphyrins. More recently, the steric hindrance effect has been proposed to promote the 2e⁻ ORR selectivity of Co porphyrins. For example, a Co 5,10,15,20-tetrakis(2′,6′-dipivaloyloxyphenyl)porphyrin with eight large ester groups has been synthesized.¹⁴⁹ This Co porphyrin, when supported on carbon black, exhibits high 2e⁻ ORR selectivity with an n value of 2.20 in 0.1 M KOH, which is much smaller than an n value of 3.03 for Co 5,10,15,20-tetrakis(para-dipivaloyloxyphenyl)porphyrin without steric groups. The introduction of steric groups hinders the aggregation of Co porphyrins and prevents the occurrence of the bimolecular mechanism.

In addition to Co porphyrins featuring diverse meso-substituents, many Co porphyrins with β-substituents have been designed and synthesized to enhance 2e⁻ selectivity for the ORR. These β-substituents could effectively modulate the π-electron system of porphyrins, thereby fine-tuning electronic configurations of metal centers.¹⁵⁰ For example, Co 2,12-tetraethyl-3,8,13,18-tetramethyl-7,17-ethylacetate-porphyrin (CoTETMEAP)¹⁵¹ and Co 2,12-tetraethyl-3,8,13,18-tetramethyl-7,17-ethylpropionate-porphyrin (CoTETMEPP)¹⁵² were designed and synthesized for the ORR. These porphyrin diesters have ethyl, methyl, diacetic and dipropionic esters at their β-substituent positions. Both CoTETMEAP and CoTETMEPP exhibit a main 2e⁻ ORR selectivity with the yield of H₂O₂ close to 70% measured with a RRDE. Similarly, Co 2,3,7,8,12,13,17,18-octaethyl porphyrin (CoOEP) and Co protoporphyrin IX (CoPPIX), which have a similar molecular structure to the Fe porphyrin unit in CcOs, also exhibit a main 2e⁻ selectivity for the ORR with n values of 2.8 and 2.6, respectively, obtained using Koutecky–Levich (K–L) methods by coating Co porphyrins on a graphite disk electrode.¹⁵³ Note that numerous mononuclear Co porphyrins reported for the ORR displayed mixed 4e⁻ and 2e⁻ selectivity. These results are caused by the ORR selectivity evaluation process, which depends on RRDE and K–L measurements by coating molecular catalysts or molecule@support hybrids on electrodes. These methods will inevitably lead to the aggregation of molecular catalysts and then result in bimolecular mechanisms, which favor the 4e⁻ ORR pathway. To achieve a more precise determination of ORR selectivity, introducing homogeneous chemical reduction is of great importance. For example, building upon CoOEP, Co 2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin (CoAPP) was synthesized, demonstrating exceptional ORR catalytic activity in water/1,2-dichloroethane medium through a homogeneous chemical reduction process.¹⁵⁴ The interfacial affinity of CoAPP, in contrast to unmodified CoOEP, facilitates the 2e⁻ pathway for O₂ to H₂O₂ conversion. Furthermore, Co porphyrins incorporating both meso- and β-substituents, such as non-planar Co tetrakis-(N-methyl pyridyl)-β-octabromoporphyrin (CoTMPBr₈)¹⁴⁴ and tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin (CoTSPBr₈),¹⁴⁵ have been developed. These water-soluble Co porphyrins effectively bind O₂ and reduce it to H₂O₂via a 2e⁻ pathway. Therefore, it is crucial to verify the ORR selectivity of molecular catalysts using multiple methods.

3.2. Other Co-based molecular catalysts

In addition to Co porphyrins, various Co-centered molecular catalysts have been employed for the ORR, including Co 1,4,8,11-tetraazacyclotetradecane (Co([14]aneN₄)), Co bis-ketiminate-ligated complex Co(N₂O₂)_n, Co chlorins (Co(Ch)_n), Co corroles, and Co phthalocyanines (CoPc) (Fig. 3). Notably, cyclic tetraamine–Co^III complexes have garnered great attention due to their porphyrin-like N₄ macrocyclic structure.¹⁵⁵ The [Co([14]aneN₄)(H₂O)₂]³⁺ complex has been characterized and studied for its role in oxygen oxidation–reduction processes.^156,157 The homogeneous catalytic ORR mechanism of this Co^III complex was studied through the manipulation of the O₂-to-Co ratio.¹⁵⁸ The 2e⁻ ORR selectivity of [Co([14]aneN₄)(H₂O)₂]³⁺ was confirmed. Recently, a series of bis-ketiminate-ligated Co complexes [Co(N₂O₂)_n] were synthesized and found to exhibit a high yield of H₂O₂ (>93%) across different substituents.¹⁵⁹ The rate-limiting step is identified as the protonation process of Co^III–hydroperoxide species.¹⁶⁰ Another noteworthy class of Co-based catalysts are Co chlorin [Co^II(Ch)_n], sharing structural similarities with metal porphyrins in terms of their M–N₄ coordination geometry.¹⁶¹ Specifically, Co(Ch)₁ has a selective 2e⁻ ORR pathway in the presence of HClO₄ in benzonitrile, with the catalytic turnover number 30 000 times higher than that of CoOEP. Furthermore, fine-tuning substituents of these Co chlorin complexes allows for modulation of both Co redox potentials and, consequently, their catalytic activities.¹⁶² For instance, the introduction of a methoxycarbonyl group into the chlorin ligand results in Co(Ch)₂, which exhibits a 36-fold increase in the observed rate constant compared to Co(Ch)₁, demonstrating the effectiveness of substituent manipulation in regulating catalytic activity. In addition, a Co corrole [Co^III(tpfc)(Py)₂] (tpfc = 5,10,15-tris(pentafluorophenyl)-corrole and Py = pyridine) has been reported for the 2e⁻ ORR with a H₂O₂ production yield of 100% by ferrocene and 9,10-dihydro-10-methylacridine in the presence of HClO₄ in CH₃CN.¹⁶³

Fig. 3 Mononuclear Co-based molecular catalysts used for the ORR with a 2e⁻ selectivity.

Furthermore, metal phthalocyanines, characterized by their analogous macrocyclic architectures, have garnered significant attention for their applications in the ORR.^164–166 In 1964, Co phthalocyanine emerged as a pioneering cathode catalyst for the ORR in fuel cells, marking a crucial milestone in the field.¹⁶⁷ This discovery has since inspired the design and synthesis of numerous Co phthalocyanine derivatives featuring diverse substituents. One example is the water-soluble Co tetrasulfonate phthalocyanine (CoTSPc), which has demonstrated obvious selectivity towards the 2e⁻ ORR pathway.¹⁶⁸ More recently, innovative achievements have been made with the development of saddle-distorted Co 1,4,8,11,15,18,22,25-octaphenylphthalocyaninato (Co(Ph₈Pc)), which represents a novel addition to the phthalocyanine family.¹⁶⁹ Co(Ph₈Pc) showed 2e⁻ ORR selectivity when mediated by ferrocene derivatives under acidic conditions, underscoring its potential in catalyzing ORR processes. These findings underscore the effectiveness of manipulating coordination environments of Co ions to achieve high 2e⁻ ORR selectivity.

3.3. Co-based porphyrin boxes

In the pursuit of Co-based electrocatalysts and their electrocatalytic assessment for the ORR, a common challenge lies in eliminating the aggregation of molecular catalysts, such as metal porphyrins, which can complicate the reaction pathway by introducing a mixed 2e⁻ and 4e⁻ ORR process. To address this issue, researchers have constructed supramolecular Co porphyrin boxes, utilizing Co porphyrins as fundamental building blocks (Fig. 4a–c).¹⁷⁰ These Co porphyrin boxes have a remarkable preference for the 2e⁻ ORR pathway measured in pH 7 phosphate buffer with a RRDE. In contrast, the control sample CoTPP showed a mixed 2e⁻ and 4e⁻ selectivity for the ORR, resulting in 50% H₂O₂ yield. This result is attributed to the aggregation of CoTPP when mixed with CNTs, which facilitates the bimolecular 4e⁻ ORR pathway. This approach emphasizes the potential to tune ORR selectivity by manipulating building blocks, thereby fine-tuning the pore volume, window size, and inter-metal distances within boxes.¹⁷¹ By utilizing this design principle, researchers can expand the horizon of porphyrin boxes, offering a platform for optimizing ORR performance.

Fig. 4 Synthetic procedure (a) and computed structures (b) and (c) of Co porphyrin boxes. Reprinted with permission from ref. 170. Copyright 2018, Wiley-VCH.

3.4. Co-based frameworks

Immobilizing metal complexes within confined frameworks also emerges as an effective strategy to eliminate the aggregation of molecular catalysts. Recently, a groundbreaking hydrogen-bonded Co porphyrin framework with pore sizes of 15.7 and 18.6 Å was successfully constructed (Fig. 5a and b).¹⁷² This Co-porphyrin-based framework shows a 2e⁻ selectivity for the ORR with high H₂O₂ production (>90%). Co 5,10,15,20-tetra(4-carboxylphenyl)porphyrin (CoTCPP) served as the precursor for the assembly of this framework, using hydrogen bonds formed by carboxyl groups. Notably, the arrangement of Co porphyrins precluded direct face-to-face interactions. In addition, Co phthalocyanine-based covalent organic frameworks (COFs) were designed and synthesized for electrocatalytic production of H₂O₂.¹⁷³ A notable example involved the utilization of Co hexadecafluorophthalocyaninato (CoPcF₁₆) as the catalytic active site. By incorporating 1,2,4,5-benzenetetrathiol (BTT) and 1,2,4,5-tetrahydroxybenzene (THB) as linkers, the CoPc-S-COF and the CoPc-O-COF were obtained, respectively (Fig. 5c–e). Their structures were validated through X-ray diffraction (XRD) measurements, matching simulated stacking models. Both COFs exhibit dominant 2e⁻ ORR selectivity, with n values ranging from 2.0 to 2.2. Notably, the CoPc-S-COF has remarkable long-term stability (20 h) under high current densities (125 mA cm⁻²) in a flow cell for H₂O₂ generation. Moreover, structures of these COFs could be tuned by employing other organic linkers, offering further versatility. For example, the employment of 1,2,4,5-benzenetetramine (BTM) and 3,3′-diaminobenzidine (DAB) led to the synthesis of piperazine-linked CoPc-based COFs, designated as the CoPc-BTM-COF and the CoPc-DAB-COF, respectively.¹⁷⁴ These CoPc-based COFs exhibit excellent photocatalytic 2e⁻ ORR selectivity, operating efficiently under visible light irradiation (λ > 400 nm).

Fig. 5 Synthetic procedure (a) and crystal structures (b) of Co porphyrin frameworks. Reprinted with permission from ref. 172. Copyright 2022, Springer Nature Ltd. (c) Synthetic procedure of the CoPc-O-COF and the CoPc-S-COF. Simulated stacking models of (d) the CoPc-O-COF and (e) the CoPc-S-COF. Reprinted with permission from ref. 173. Copyright 2024, Springer Nature Ltd.

3.5. Single-atom Co-based electrocatalysts

In recent years, single-atom catalysts (SACs) have become a research hotspot in heterogeneous catalysis, primarily due to their exceptional atomic utilization, excellent activity, and unparalleled selectivity.^175–177 Inspired by Co porphyrins, great progress has been made in the design and synthesis of single metal site Co–N–C catalysts.^35,178–185 For example, a series of monoatomic catalysts loaded on N-doped graphene were reported, featuring various metals (M = Mn, Fe, Co, Ni, and Cu).¹⁸⁶ Theoretical calculations agreed that single-atom Co–N–C possesses an optimal adsorption energy for the crucial OOH* intermediate, leading to an enhanced H₂O₂ generation rate. Particularly, under specific conditions (e.g., 0.1 M HClO₄), Co–N–C has achieved high H₂O₂ production yield exceeding 90%. Furthermore, the distinct roles played by different Co–N₄ configurations in governing ORR pathways have been elucidated. Three types of Co–N₄ electrocatalysts were prepared through the high-temperature pyrolysis method by selecting different precursors.¹⁸⁷ Specifically, 4-dimethylaminopyridine, Co(NO₃)₂, and a carbon support were selected to prepare Co–N SAC_Dp with pyrrole-type Co–N₄ coordination (Fig. 6a). In contrast, 2-methylimidazole was applied to obtain Co–N SAC_Mm with pyridine-type Co–N₄ coordination (Fig. 6b). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of Co–N SAC_Dp and Co–N SAC_Mm confirm the formation of SACs. Co–N SAC_Dp facilitates the 2e⁻ ORR process with a high H₂O₂ production rate, while Co–N SAC_Mm catalyzes the 4e⁻ process (Fig. 6c–e). The moderate adsorption capability of pyrrole-type N for the OOH* intermediate contributes to the stability of the O–O bond, ultimately leading to a remarkable H₂O₂ selectivity of up to 94% (Fig. 6d). This high selectivity can be attributed to the structural similarity between pyrrole-type Co–N₄ and Co porphyrin, indicating the profound influence of molecular architecture on catalytic performance.

Fig. 6 (a) and (b) HAADF-STEM images, (c) LSV data measured with a RRDE, (d) H₂O₂ selectivity, and (e) schematic illustration of the H₂O₂ production rate of a pyrrole-type CoN₄ catalyst (Co–N SAC_Dp) and a pyridine-type CoN₄ catalyst (Co–N SAC_Mm). Reprinted with permission from ref. 187. Copyright 2022, American Chemical Society.

Moreover, atomic-level fine-tuning of the coordination environment around Co centers in Co–N–C materials offers a powerful method to modulate their selectivity towards the ORR.¹⁸⁸ For example, unlike conventional Co–N₄ catalysts, a new Co–N₅ SAC has been reported for the 2e⁻ ORR to produce H₂O₂, exhibiting a Faraday efficiency of 95.6%.¹⁸⁹ Notably, the Co–N₅ SAC exhibits remarkable resistance to chloride ions in simulated seawater without compromising ORR performance. In contrast to basal plane-anchored Co–N₄ configurations embedded in the carbon matrix, both the edge-hosted Co–N₄ active sites¹⁹⁰ and edge-hosted Co–N₂ sites¹⁹¹ demonstrate superior catalytic 2e⁻ ORR selectivity. In addition, controlled synthesis of asymmetric Co–C/N/O SACs with well-defined coordination environments enables systematic evaluation of coordination-dependent selectivity and activity for the 2e⁻ ORR pathway.¹⁹² For example, a single-atom Co-based material, named CoN₂O₂, has been reported.¹⁹³ In this material, two N atoms in the traditional Co–N₄ configuration were replaced with two O atoms, leading to a unique composition. CoN₂O₂ exhibits a dominant 2e⁻ selectivity for the ORR, achieving a H₂O₂ production yield of more than 95% under acidic electrosynthesis conditions. Theoretical calculations provide further support for the crucial role of the binding strength of the OOH* intermediate on active sites of catalysts in determining the production of H₂O₂.^194,195 The adsorption of the OOH* intermediate must be carefully balanced, neither too strong nor too weak, to optimize the selectivity and efficiency of the ORR process.⁶⁰ More recently, a series of Co–N_xO_4−x SACs were reported for the ORR.¹⁹⁶ The CoN₂O₂ SAC exhibits an optimal compromise between mass activity and H₂O₂ selectivity. In contrast, the Co–O₄ SAC shows a high 2e⁻ ORR selectivity albeit with poor activity. Therefore, the strategic manipulation of metal coordination microenvironments enables effective control over activity–selectivity tradeoffs in multi-electron electrocatalysis.

Simultaneously, engineering the second coordination sphere provides an additional dimension to optimize catalytic ORR selectivity. For example, a Co–N–C material featuring atomic Co–N_x–C sites integrated with oxygen functional groups was prepared by a unique approach involving the pyrolysis of a Co porphyrin-based polymer. This material has a 2e⁻ selectivity for the ORR, achieving a H₂O₂ production yield exceeding 80%.¹⁹⁷ Similarly, a single-atom Co–N₄ material with positioned adjacent epoxy groups has been reported.¹⁹⁸ The resulting material shows an optimal 2e⁻ selectivity for the ORR, resulting in a high H₂O₂ production yield (>97%). Theoretical calculations elucidated that the presence of epoxy groups effectively suppresses the cleavage of the O–O bond on the modified Co–N₄ structure, thereby promoting the desired 2e⁻ pathway.

3.6. Dual-atom Co-based electrocatalysts

Beyond SACs, dual-atom catalysts (DACs) have attracted widespread attention in recent years.^199,200 Typically, Co-based SACs tend to undergo a 2e⁻ ORR process.¹⁸⁶ However, Co-based DACs exhibit a preferential 4e⁻ ORR pathway,²⁰¹ attributed to the synergistic interplay between two adjacent Co sites. Nevertheless, it is noteworthy that dinuclear Co-based electrocatalysts may also engage in the 2e⁻ ORR process. For example, Co-based DACs with a Co₂N₄O₂ coordination motif (Co₂-DACs) were prepared through a macrocyclic precursor mediated pyrolysis method (Fig. 7).²⁰² The transmission electron microscopy (TEM) image shows obvious square sheet morphology with a size of about 100 nm (Fig. 7a). The HAADF-STEM image and extended X-ray absorption fine structure (EXAFS) spectra confirmed the formation of Co₂-DACs (Fig. 7b and c). Within a wide potential range (0.2–0.6 V versus reversible hydrogen electrode (vs. RHE); all potentials reported in aqueous solutions in this review are referenced to the RHE unless otherwise specified), Co₂-DACs exhibit a high 2e⁻ ORR selectivity (>95%) compared to Co-SACs in 0.1 M HClO₄ (Fig. 7d and e). Co₂-DACs also have excellent stability with negligible decay after running 12 h in an H-type cell. Then, acidic H₂O₂ electrosynthesis was conducted for Co₂-DACs at high current densities (50–400 mA cm⁻²), which exhibited excellent stability and selectivity (Fig. 7f). Theoretical calculations demonstrated that the adsorption of OOH* and HOOH* intermediates on the Co site is weakened due to the decrease in the d-band center. Recently, bimetallic Zn/Co zeolite imidazole frameworks (ZnCo ZIFs) have been prepared, which can achieve excellent 2e⁻ ORR.²⁰³ By increasing the Zn/Co ratio within ZnCo ZIFs with the same nanocube morphology and adjusting the electronic structure of Co active sites, a Zn/Co ratio of 9/1 was found to promote a 2e⁻ ORR pathway. Theoretical calculations showed that the optimized Co site has an ideal binding strength towards OOH (OOH_ad). In addition, a dual-atom Co–In–N–C catalyst was reported for the 2e⁻ ORR in acidic media.²⁰⁴ The introduction of an In atom favors the adsorption of OH* intermediates, which in turn optimizes the adsorption of OOH* intermediates on adjacent Co sites, highlighting the effectiveness of fine-tuning the coordination environment around the Co metal center to achieve targeted 2e⁻ ORR selectivity.

Fig. 7 (a) TEM image and (b) HAADF-STEM image of Co₂-DACs. (c) Fitted EXAFS spectra of Co₂-DACs and Co-SACs. (d) LSV data and (e) H₂O₂ selectivity of Co₂-DACs, Co-SACs, and N–C materials measured with a RRDE at 1600 rpm in 0.1 M HClO₄. (f) H₂O₂ Faraday efficiency and the corresponding productivity of Co₂-DACs measured at different current densities. Reprinted with permission from ref. 202. Copyright 2024, American Chemical Society.

3.7. Other Co-based electrocatalysts

In addition to the Co-based electrocatalysts discussed above, several other Co-based materials have been reported, which also exhibit excellent performance in the electrosynthesis of H₂O₂. For example, the cyclodextrin-stabilized Co(OH)₂ nanocluster electrocatalysts show exceptional 2e⁻ ORR activity and selectivity with a high H₂O₂ production rate (5.58 mol g_catalyst⁻¹ h⁻¹) and high Faraday efficiency (98.7%–94.9%).²⁰⁵ In addition, metallic CoSe₂ has also been reported for the ORR, which exhibits high 2e⁻ ORR selectivity with a Faraday efficiency of 96.7% in an acidic electrolyte.²⁰⁶ Currently, although substantial research efforts have developed several strategies to modulate the intrinsic activity of catalysts for the 2e⁻ ORR, persistent challenges remain in bridging the gap between current design paradigms and practical application requirements.²⁴ Particularly for industrial-scale electrocatalysis, achieving optimal balance between precise synthetic control of electrocatalysts and manufacturing scalability presents a critical technological hurdle.

4. Four-electron reduction of O₂ to H₂O

From an energy conversion and storage point of view, the 4e⁻ reduction of O₂ to H₂O is more advantageous. The difference in ORR products can be traced back to the electrocatalysts employed. Given the rapid developments in fuel cells and metal–air batteries, the significance of the 4e⁻ ORR pathway is increasing. Many kinds of Co-based electrocatalysts have been reported for improving the 4e⁻ ORR process, including mononuclear, dinuclear, and trinuclear Co-based molecular catalysts, as well as Co-based materials such as aggregates, metal–organic frameworks (MOFs), COFs, polymers, and dual-atom Co–M-based materials.

4.1. Mononuclear Co-based molecular electrocatalysts

4.1.1. Effect of proton transfer. In nature, Fe porphyrin within cytochrome P450 monooxygenase plays a crucial role in the activation and subsequent utilization of O₂. Recent advancements in the high-resolution crystallographic structure of cytochrome P450BM-3 demonstrated the existence of internal solvent water channels surrounding the Fe metal center through hydrogen-bond networks (Fig. 8a).²⁰⁷ These water molecules can tune the electronic structure and redox potential of heme and also participate in the proton-transfer process, ultimately facilitating the cleavage of the O–O bond.²⁰⁸ For the activation of the O–O bond, it is also very important to appropriately control the proton inventory. Inspired by the structure of heme, a hangman Fe porphyrin with a pendant carboxylic acid group was designed and synthesized.²⁰⁹ Using this strategy, hanging porphyrin xanthene (HPX) complexes were designed and synthesized to study the proton-coupled electron transfer (PCET) process.²¹⁰ The functional carboxyl group can pre-organize external water molecules to realize the activation of the O–O bond. The single crystal structure of hangman Fe porphyrin confirms the existence of water channels (Fig. 8b).²⁰⁹ In order to tune the selectivity of Co porphyrins in the ORR, a series of hangman Co porphyrins with different meso-substituents were synthesized.²¹¹ Specifically, a hanging carboxyl group was attached to the second coordination sphere of metal centers. This hanging functional group not only introduces a useful site for the proton transfer but also plays a crucial role in pre-organizing water molecules, thereby enhancing the proton transfer process. The introduction of a carboxyl group significantly enhanced the 4e⁻ selectivity of the ORR (Fig. 8c). Similar results were observed by studying Co corroles with a hanging carboxyl group (Fig. 8d).²¹² In addition, introducing a water-network in a Co corrole with an appended crown ether unit can also promote the proton transfer of the HER.²¹³

Fig. 8 (a) Schematic illustration of a hydrogen-bonded water channel in P450 with a threonine residue. (b) Crystal structure of Fe porphyrin with a water molecule within the hangman cleft and a hydroxyl group adsorbed on the Fe center, which forms a hydrogen bond network. Reprinted with permission from ref. 207. Copyright 2007, American Chemical Society. Molecular structures of (c) Co porphyrin and (d) Co corrole with a hanging carboxyl group via a xanthene spacer, (e) CoTPPy and (f) CoTPOH. (g) Schematic diagram showing the multiple proton relays of a Co corrole with N,N-di(2-picolyl)ethylenediamine in aqueous solutions with pH ≤ 7. Reprinted with permission from ref. 214. Copyright 2021, American Chemical Society.

To gain a deeper insight into the advantageous impact of proton transfer, commonly referred to as “proton relays”, 2-pyridyl and 2-hydroxyphenyl moieties at a specific meso-position of CoTPP were introduced, resulting in the synthesis of Co 5-(2-pyridyl)-10,15,20-triphenylporphyrin (CoTPPy) (Fig. 8e) and Co 5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin (CoTPOH) (Fig. 8f).²¹⁵ Notably, CoTPPy exhibits an enhanced 4e⁻ selectivity for H₂O production (70%) compared to CoTPOH (60%). It is crucial to recognize that, contrary to conventional assumptions, pyridinium cations do not function as direct proton relays due to their significant distance from Co–O–O* intermediates. Instead, the 2-pyridyl group exerts its influence by modulating other protons within the second coordination sphere, facilitated by water clusters. This mechanism is supported by similar findings reported for Fe porphyrins containing either one 2-pyridyl group²¹⁶ or four 2-pyridyl groups,¹¹⁰ which likewise exhibit remarkable 4e⁻ ORR selectivity. In addition, the incorporation of N,N-di(2-picolyl)ethylenediamine into a Co corrole system has been shown to enhance ORR performance (Fig. 8g).²¹⁴ This modified corrole has multiple proton relay sites in aqueous solutions with pH ≤ 7, enabling it to catalyze the 4e⁻ ORR process efficiently. In particular, the modified Co corrole exhibits an increase in half-wave potential (E_1/2 = 0.75 V), compared to its unmodified counterpart (E_1/2 = 0.60 V) in 0.5 M H₂SO₄. These functional groups, classified as Brønsted acids (e.g., carboxylic acids and pyridinium cations), effectively stabilize intermediates and improve ORR selectivity through their proton relaying capabilities.

4.1.2. Effect of charged substituents. The introduction of ancillary groups (e.g., pyridinium cation) into the surrounding of the metal active center in Co porphyrins is an effective strategy to enhance the 4e⁻ selectivity of the ORR. Beyond the facilitation of proton transfer, local electrostatic interactions in the immediate vicinity of the Co active site within the porphyrin structure have also been shown to play a crucial role in stabilizing O₂-adducts.²¹⁷ For example, Co 5,10,15,20-tetraphenylporphyrin with an ortho-position of one phenyl group containing –N(CH₃)₃⁺ (CoTPPNMe₃⁺) was designed and synthesized (Fig. 9a). This modified porphyrin can catalyze O₂ to H₂O conversion via a 4e⁻ pathway across a broad pH range (pH = 0–7.0) (Fig. 9b–d). Theoretical calculations demonstrated that peroxide and hydroperoxide intermediates in CoTPPNMe₃⁺ possess elongated O–O bond lengths, emphasizing the significance of local electrostatic stabilization of anionic intermediates for fine-tuning ORR selectivity. In parallel, a Fe porphyrin bearing four cationic, ortho-N,N,N-trimethylanilinium groups (o-[N(CH₃)₃]⁺) at equivalent positions was also designed and synthesized, yielding four distinct isomers.¹⁰⁷ In particular, the αβαβ isomer outperformed its counterparts, highlighting the critical role of specific charge interactions in enhancing ORR performance. Similarly, a Co corrole modified with an NMe₃⁺ group at its 10-phenyl substituent exhibits an E_1/2 of 0.75 V and an n value of 3.88 in 0.5 M H₂SO₄, further indicating the impact of charge-based modifications (Fig. 9e).²¹⁸ Recently, the incorporation of a cationic imidazolium group into a Co corrole molecule was explored for investigating its influence on ORR selectivity (Fig. 9f).²¹⁹ This modified Co corrole exhibits 4e⁻ ORR selectivity with an n value of 3.72 in 0.5 M H₂SO₄, reinforcing the beneficial effects of charged substituents on the 4e⁻ ORR. In addition, a 4-pyridyl-substituted Co porphyrin appended with redox-active units, specifically [Ru(NH₃)₅]²⁺, was developed and found to display a pronounced 4e⁻ selectivity for the ORR over unmodified CoTPyP.¹⁴³ Detailed reaction mechanism studies attributed this enhanced selectivity to the swift intramolecular electron transfer from [Ru(NH₃)₅]²⁺ centers, coordinated to pyridine groups, to O₂ molecules coordinated to the Co center. Comparable outcomes were achieved by substituting the porphyrin moiety with CoTCyPP or exchanging [Ru(NH₃)₅]²⁺ with [Os(NH₃)₅]²⁺ and [Os(NH₃)₅]³⁺, emphasizing the versatility of this charge-mediated selectivity enhancement strategy.^141,220 In conclusion, these findings collectively demonstrate the profound impact of charged substituents on modulating ORR selectivity.

Fig. 9 (a) Molecular structure, (b) LSV data at different pH values measured with a RRDE, (c) LSV data at different rotating speeds, and (d) K–L plots of CoTPPNMe₃⁺. Reprinted with permission from ref. 217. Copyright 2020, American Chemical Society. (e) Molecular structure of a Co corrole with an NMe₃⁺ group at its 10-phenyl substituent. (f) Molecular structure of a Co corrole hanged with a cationic imidazolium group.

4.1.3. Effect of steric hindrance. The ortho-position of the meso-phenyl substituent of CoTPP is usually covalently bound to a functional group, allowing for the modulation of ORR selectivity. In 1978, a pioneering Co 5,10,15,20-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin was designed to investigate the oxygen binding within this Co “picket fence” complex.²²¹ Recently, a series of Co porphyrins with an ortho-amido group at each meso-phenyl substituent were synthesized, aimed at elucidating the influence of steric hindrance on ORR selectivity (Fig. 10).²²² Four isomeric Co 5,10,15,20-tetra(2-pivalamidophenyl)porphyrins, designated as Co αααα, αααβ, ααββ, and αβαβ, were obtained (Fig. 10a–g). Experimental assessments, conducted by integrating these Co porphyrins with conductive CB and CNTs, reveal comparable ORR catalytic activity across all four isomers (Fig. 10h). However, a notable distinction emerged in terms of the reaction pathway. Co αααα@CB catalyzes a 4e⁻ ORR process with an n value of 3.75, attributed to enhanced O₂ binding in its pocket, whereas Co αβαβ@CB favors a 2e⁻ ORR pathway (n = 2.1). We believe that the significant steric hinderance at each side of the Co αβαβ porphyrin ring precludes the bimolecular reaction mechanism. Similar selectivity trends were observed for the respective CNT composites. Recently, theoretical calculations confirmed the preferentiality of Co αααα for the 4e⁻ ORR pathway and Co αβαβ for the 2e⁻ pathway.²²³ Further research has expanded the understanding of the effect of these Co porphyrin isomers on homogeneous catalytic ORR performance.²²⁴ All isomers reveal consistent ORR kinetics, indicative of a uniform catalytic mechanism. Notably, Co αααα exhibits the highest catalytic ORR activity, attributed to the space protection effect of the molecular pocket and hydrogen bonding with the amide group, which stabilizes the superoxide unit. Recently, the steric effect of Co corroles on the ORR was investigated.²²⁵ Additionally, the steric effect has been exploited to regulate the reaction mechanism of the HER, emphasizing its crucial role in determining electrocatalytic performance, selectivity, and reaction pathways.¹²⁰

Fig. 10 (a)–(d) Molecular structures of four isomeric Co porphyrins. Single crystal structures of Co αβαβ (e), Co-free αααα (f) and Co-free ααββ (g). (h) The n values of four isomers of Co porphyrins@CB. Reprinted with permission from ref. 222. Copyright 2021, Wiley-VCH.

4.1.4. Effect of electron transfer. To evaluate the ORR selectivity of metal porphyrins, high-conductivity carbon materials, such as CNTs, graphene, and CB, are commonly employed. Three primary strategies were developed for the preparation of composites with these materials. First, physical mixing of molecular catalysts and carbon materials is straightforward yet likely to cause catalyst aggregation.^226,227 Second, molecular catalysts can be immobilized on carbon surfaces through axial coordination or π-stacking interactions, enhancing stability and performance.^228,229 Third, molecular catalysts can be covalently connected to the surface of carbon materials.^230–232 For example, CoTPP was covalently linked on CNTs through the zwitterionic functionalization and substitution reaction (Fig. 11a).²³³ The resulting CoTPP-CNT exhibits 4e⁻ ORR selectivity with an n value of 3.7 within the pH range of 0–2.5, which changed the intrinsic 2e⁻ selectivity of CoTPP. In addition, Co tetra(o-aminophenyl)porphyrin (CoTAPP) hybrids with graphene, single-walled CNTs and multi-walled CNTs, formed through diazonium salt reactions, exhibit 4e⁻ selectivity for the ORR.²³⁴ Comparable outcomes are achieved with Co corroles covalently grafted onto CNTs (Fig. 11b), demonstrating an E_1/2 of 0.78 V with an n value of 3.73 in 0.5 M H₂SO₄.²³⁵ Covalently linked hybrids facilitate faster electron transfer between metal centers and electrode surfaces compared to physically mixed composites. Recent advancements involve coordinating Co corroles onto pyridine-functionalized CNTs, where the linker length and conjugation impact electron transfer rates and, consequently, ORR selectivity.²²⁹ Co corrole-CNTs utilizing short and conjugated linkers exhibit a large n value (3.81) compared to that with long and unconjugated linkers. In addition, Co 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenylcorrole supported on the CNT with a pyrene substituent has been developed to explore the impact of π–π interactions.²²⁸ The resulting Co corrole/CNT composite exhibits an n value of 3.8 in 0.5 M H₂SO₄, indicating a 4e⁻ ORR selectivity. In comparison, the Co corrole unsupported by the pyrene group/CNT exhibits a slightly lower n value of 3.6. This finding highlights the role of π–π interaction between the pyrene substituent and the CNT in enhancing electron transfer rates and, consequently, improving 4e⁻ ORR selectivity. All of these above results provide support for the beneficial effect of efficient electron transfer of carbon materials in improving the selectivity of the ORR.

Fig. 11 (a) Synthetic pathway of covalently linking Co porphyrins on CNTs. Reprinted with permission from ref. 233. Copyright 2009, American Chemical Society. (b) Synthetic pathway of covalently linking Co corroles on CNTs. Reprinted with permission from ref. 235. Copyright 2019, American Chemical Society. (c) Schematic illustration of heterogeneous catalysis with nanoparticles and homogeneous catalysis with CoTMOPP molecular catalysts. Reprinted with permission from ref. 236. Copyright 2021, Wiley-VCH.

In fuel cells, nanoparticles are traditionally employed in the construction of a heterogeneous catalyst layer (CL) for the ORR.²³⁶ The CL is positioned between the gas diffusion layer (GDL) and anion exchange membrane (AEM) (Fig. 11c). Nevertheless, this approach often results in a limited number of accessible active sites due to the encapsulation of nanoparticles, hindering their full potential. Recently, a breakthrough has been achieved by implementing homogeneous catalysis through the immobilization of CoTMOPP molecular catalysts onto a polyfluorene (PF) ionomer matrix, creating CoTMOPP-PF (Fig. 11c).²³⁶ This innovation offers several advantages. First, the introduction of PF fosters the development of ionic channels, facilitating mass transfer processes within the system. Second, these ionic channels provide a uniform environment for grafted molecular catalysts, enabling a high utilization rate of active sites. Third, the hydrophilic quaternary ammonium salt present on the PF chain promotes microphase separation, further enhancing the performance of catalysts. Remarkably, CoTMOPP-PF exhibits an E_1/2 of 0.81 V in 0.1 M KOH. Analysis using K–L plots reveals an n value of 4.1, indicating a 4e⁻ ORR selectivity. This exceptional selectivity is likely due to the rapid charge transfer dynamics and the occurrence of a bimolecular reaction mechanism, highlighting the effectiveness of homogeneous catalyst design.

4.1.5. Effect of the molecular shape. Metal porphyrins have a planar macrocyclic conjugated structure featuring a 26π aromatic configuration. However, in recent years, there has been a surge of interest in designing and synthesizing bent or twisted variants of these compounds and their derivatives, owing to their exceptional catalytic activity towards the ORR.^237–239 As reported, highly bent porphyrins can be synthesized through a straightforward condensation reaction involving ortho dipyrrole benzene and various aldehydes, resulting in clamp-shaped porphyrins or those with a rectangular cavity.^240,241 Recently, two clamp-shaped Co porphyrins with Ph or F₅Ph groups connected at their meso-positions of macrocycles have been reported for their applications in the ORR (Fig. 12a).²⁴² One of these clamp-shaped Co porphyrins was structurally characterized, revealing a methanol molecule coordinated to the Co center (Fig. 12b). The average bond length of Co–N is about 1.95 Å, slightly shorter than that observed in planar Co porphyrins. Additionally, the angle and distance between two clamps were found to be 60° and 7.523 Å, respectively, while the distance between Co and O in methanol was 2.194 Å (Fig. 12b). The overall crystal structure of this clamp-shaped Co porphyrin resembles that of a cofacial Pacman porphyrin. Both of these clamp-shaped Co porphyrins exhibit a tendency of 4e⁻ ORR selectivity with n values exceeding 3.0, which may be attributed to the unique bent structure of porphyrins.

Fig. 12 (a) Molecular structures of two clamp-shaped Co porphyrins. (b) Crystal structures of one clamp-shaped Co porphyrin from top view and side view with a methanol coordinated on the Co center. Reprinted with permission from ref. 242. Copyright 2023, Wiley-VCH. (c) Molecular structures of two π-extended nonplanar Co porphyrins. (d) Crystal structures of one nonplanar Co porphyrin from front view and side view with a methanol and Cl⁻ coordinated on the Co center. Reprinted with permission from ref. 243. Copyright 2024, The Royal Society of Chemistry.

More recently, two innovative π-extended and nonplanar Co porphyrins, CoDFP(VCN)₂ (2Co; DFP = di-fused porphyrin, VCN = vinyl cyanide) and CoTFPMB(VCN)₂ (3Co; TFP = tri-fused porphyrin, MB = monobenzo), were designed and synthesized for the ORR (Fig. 12c).²⁴³ The crystal structure of 3Co has been successfully obtained, revealing an average Co–N bond length of 1.93 Å (Fig. 12d). The 3Co has an E_1/2 of 0.80 V in 0.1 M KOH, indicating its promising electrocatalytic performance. Both 3Co and 2Co exhibit a 4e⁻ ORR selectivity with an n value of 3.7, indicating the crucial role played by the π-extended nonplanar curved structure in facilitating high 4e⁻ ORR selectivity. By enhancing the molecular conformation and electronic properties, this unique structural feature promotes efficient electron transfer and facilitates the desired 4e⁻ reduction pathway, thereby making these porphyrins promising candidates for ORR catalysis.

4.1.6. Effect of axial ligands. In nature, the proximal axial imidazole ligand (His³⁷⁶) increases the electron density on heme Fe porphyrin (through the so-called “push effect”), thus facilitating O₂ binding and subsequent inner layer electron transfer from Fe^II to O₂. Meanwhile, varying axial ligands (Cys, His, and Tyr) of heme Fe porphyrins can significantly alter the rate-determining steps within the ORR process.⁷⁹ For example, natural Fe-porphyrin-containing proteins, such as cytochrome P450, CcO, hemoglobin, and catalase, are often used as cofactors of biological enzymes.⁷⁹ The unique protein environment surrounding Fe porphyrin endows these enzymes with specialized biological functions. For example, cytochrome P450 has a trans axial cysteine thiolate ligand binding on Fe for O₂ activation.²⁴⁴ Hemoglobin has the function of transporting O₂, which has a trans axial histidine imidazole ligand. Catalase has a trans axial tyrosine phenolate ligand, which can convert toxic H₂O₂ into non-toxic H₂O and O₂ through a disproportionation reaction, thus protecting both animals and plants. Thus, trans axial ligands play a decisive role in defining the functionality of Fe-porphyrin-containing proteins, a phenomenon aptly termed the “push effect”.^245–247

When a coordination reaction occurs, the coordinating atoms of the axial ligand act as electron donors, contributing electrons to the empty d orbitals of the metal center. This electron donation alters the electronic density of the central metal ion, thereby modulating its catalytic reactivity. Research has centered on the performance modulation of Fe–N₄ macrocyclic complexes due to the pronounced “electron donating effect” of the axial ligand on the reactivity of Fe porphyrin.⁷⁹ However, there is a notable gap in understanding the correlation between catalytic performance and axial ligands of other metal complexes, particularly Co–N₄ macrocyclic complexes. To address this, researchers have harnessed controllable self-assembly techniques on Au electrode surfaces, crafting a monolayer model system featuring CoTMOPP coordinated with thiol ligands. By probing the ORR catalytic performance of CoTMOPP adorned with varying axial ligands in acidic media, three axial ligands possessing aromatic ring structures exhibit a descending order of coordination binding strength: 4-mercaptopyridine (MPy) > 4-aminophenylthiophenol (APT) > 4-mercaptobenzonitrile (MBN). Notably, the peak potential disparity for the ORR among Co porphyrins with these ligands reaches 80 mV, with catalytic activity improving alongside the increase of axial ligand binding strength. However, axial ligands remain ineffective in altering the fundamental 2e⁻ reaction mechanism of the ORR. Recently, CoTPFP with a pendant imidazole base was designed for the ORR.^248,249 The chemical reduction was applied with 1,1′-dimethylferrocene (Me₂Fc) as the reducing agent and triflic acid as the proton source. The introduction of an axial imidazole base with the electron push effect can promote O₂ binding, leading to the formation of a Co^III-superoxide intermediate. Comprehensive homogeneous measurements demonstrated that CoTPFP with a pendant imidazole base has a 2e⁻ ORR selectivity.

More recently, two distinct Co corroles, one tethered with an imidazole ligand and the other without, were designed and synthesized (Fig. 13a).²⁵⁰ The crystal structure of the imidazole-tethered Co corrole shows a Co–N coordination bond, with a precise bond distance of 1.929(4) Å (Fig. 13b). In contrast, the Co center of the imidazole-free Co corrole is coordinated by two pyridines. The imidazole-tethered Co corrole exhibits an E_1/2 of 0.82 V and a 4e⁻ ORR selectivity with an n value of 3.85 in 0.1 M KOH (Fig. 13c and d). In contrast, the imidazole-free Co corrole shows a lower E_1/2 of 0.75 V and an n of 3.39. Further insights into the reaction kinetics between Co corroles and O₂ highlight the significant advantage of the imidazole-tethered structure. The electronic push effect facilitated by the imidazole ligand enables efficient binding and activation of O₂, thereby enhancing the ORR selectivity of the Co corrole. This study demonstrates the profound influence of the push effect on enhancing the performance of Co corroles in ORR applications.

Fig. 13 (a) Molecular structures, (b) crystal structures, (c) LSV data, and (d) n values of Co corroles tethered with and without an intramolecular imidazole ligand. Reprinted with permission from ref. 250. Copyright 2023, Wiley-VCH.

4.1.7. Effect of conjugation. The conjugation effects inherent in metal complexes are crucial in determining 4e⁻ ORR selectivity. This phenomenon can be modulated by introducing conjugated functional groups, such as benzene, naphthalene, and pyrene rings, at either meso-substituent or β-substituent positions. While traditional complexes like CoTPP exhibit 2e⁻ ORR selectivity, recent advancements have yielded Co porphyrins with enhanced properties. Three Co porphyrins, each featuring four β- and β′-fused butano rings, were investigated for their ORR selectivity in 1.0 M HClO₄ upon adsorption on a graphite electrode (Fig. 14a). However, these Co butanoporphyrin derivatives still show an n value of 2.0. In contrast, another group of three Co porphyrins, featuring four β- and β′-fused benzo rings, demonstrate significant improvement with n values ranging from 2.6 to 3.1 under identical conditions. Furthermore, Co tetraphenyltetranaphthaloporphyrin (CoTPTNP) has an n value of 3.2 in 1.0 M H₂SO₄ using an edge-plane pyrolytic graphite (EPG) electrode compared to CoH₂TPTNP, featuring extended conjugation at β-pyrrole units, with an n value of 2.6 (Fig. 14b).²⁵¹ In contrast, CoTPP, lacking such extended conjugated functional groups, exhibits an n value of 2.0 under the same conditions. Therefore, the extended π-system of conjugated porphyrins affects d-orbitals of the Co metal center and then facilitates the 4e⁻ ORR process.

Fig. 14 Molecular structures of (a) Co porphyrin derivatives with four β- and β′-fused butano rings and benzo rings and (b) CoH₂TPTNP and CoTPTNP.

4.1.8. Effect of electron-withdrawing substituents. Electron-withdrawing substituents have commonly been integrated into Co-based electrocatalysts, such as porphyrins and corroles, to enhance their performance in the ORR.²⁵² For example, a Co porphyrin with a meso-substituent –NO₂ group exhibits preferential 4e⁻ selectivity with an n value of 3.2 (Fig. 15).¹⁴⁸ Introducing functional groups at β-sites of Co porphyrins can strongly affect the electronic structure of the Co active center compared to that at meso-sites. Mononuclear Co porphyrins with electron-withdrawing β-substituents (e.g., –NO₂) exhibit improved 4e⁻ selectivity for the ORR compared to those porphyrins without nitro functional groups such as Co 5,10,15,20-tetra(4-methylphenyl)porphyrin (CoTMPP) and Co 5,10,15,20-tetra(3,5-methoxyphenyl)porphyrin (3,5-OMe-CoTPP) (Fig. 15).²⁵³ In particular, NO₂-3,5-OMe-CoTPP exhibits an n value of 3.1 in 1.0 M HClO₄, which is larger than that of 3,5-OMe-CoTPP (n = 2.6). In order to better understand the effects of strong electron-withdrawing –NO₂ groups on ORR selectivity, a series of Co corroles containing different number of –NO₂ groups (0, 1, 2, and 3) were designed and synthesized (Fig. 15).²⁵⁴ These studies reveal a progressive increase in n values from 2.6 to 3.0 in 1.0 M HClO₄. However, the E_1/2 of these Co corroles remains unchanged. In addition, a series of para-substituted Co triphenylcorroles with different substituents (R = OMe, Me, H, F, and Cl) were designed and synthesized to understand the electron-withdrawing effect.²⁵⁵ The n values of these Co corroles increase from 2.2 to 2.8 in 1.0 M HClO₄, demonstrating that the introduction of electron-withdrawing groups increases 4e⁻ ORR selectivity. Furthermore, in order to enhance the electron-withdrawing effect, Co β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)corrole (Co(tpfc)Br₈) was designed.²⁵⁶ Co(tpfc)Br₈ exhibits a dominant 4e⁻ ORR selectivity at acidic pH with an n value of 3.9. These findings underline the crucial role of electron-withdrawing substituents in regulating the ORR selectivity of Co-based complexes.

Fig. 15 Molecular structures of Co porphyrins and corroles with electron-withdrawing substituents used for the 4e⁻ ORR.

4.1.9. The synergistic effect of substrates and molecules. Typically, depositing molecular catalysts via drop-casting onto carbon substrates results in the aggregation of molecules, hindering the accessibility and utilization of their active sites.²²⁷ Recently, MOF-supported molecular electrocatalysis was proposed for the ORR.²⁵⁷ A series of molecule@MOF materials were prepared by grafting molecular catalysts on MOFs through ligand exchange. For example, Zn-based zeolitic imidazolate framework-8 (ZIF-8) was selected as the MOF substrate, while Co 5,10,15,20-tetra(imidazolyl)porphyrin (CoTDP) was selected as the molecular catalyst (Fig. 16a). The resulting Co porphyrin@ZIF-8 catalyst exhibits a 2e⁻ selectivity with an n value of 2.6. Using this strategy, diverse molecule@MOF catalysts can be prepared by tuning the kinds of molecular catalysts and MOFs (Fig. 16b). Notably, by selecting MOFs with an inherent capability to facilitate the reduction of H₂O₂ to H₂O, like Co-based ZIF-67, it is possible to control the ORR selectivity of molecule@MOF catalysts, shifting from a 2e⁻ pathway to a 4e⁻ pathway. Moreover, research has demonstrated an enhanced 4e⁻ ORR selectivity by coordinating metal porphyrins, particularly CoTCPP, onto MOFs, such as MIL-88(Fe), MOF-5(NiCo) and UIO-66(Zr).²⁵⁸

Fig. 16 (a) Schematic illustration of the synthesis procedure of Co porphyrin@ZIF-8. (b) Diverse samples with molecular catalysts grafted on different shapes of MOFs through ligand exchange. The enlarged figure shows the proposed ORR mechanism. Reprinted with permission from ref. 257. Copyright 2021, Wiley-VCH.

4.2. Dinuclear Co-based catalysts

Currently, many kinds of dinuclear Co-based electrocatalysts could catalyze the ORR with high 4e⁻ selectivity.⁷⁹ These catalysts include face-to-face porphyrins/corroles, cofacial Pacman bisporphyrins, cofacial porphyrin prisms, and other dinuclear Co-based complexes.

4.2.1. Face-to-face porphyrins/corroles. Dinuclear Co porphyrins could realize the 4e⁻ ORR selectivity due to the synergistic effect between two neighboring Co atoms.²⁰⁷ To achieve this optimal spacing between Co atoms, two primary strategies are employed. The first one is designing metal complexes with strong intermolecular interaction to construct dimers. For example, the unsubstituted Co porphyrin, also named Co porphine, exhibits a remarkably high 4e⁻ ORR selectivity.²⁵⁹ The ORR performance was measured with Co porphyrins coated on the EPG electrode. The 4e⁻ selectivity is probably attributed to the strong tendency of porphyrins to form dimeric structures through van der Waals interactions. This phenomenon is further supported by the crystalline structure of Ni porphyrin, where cofacially arranged rings confirm the existence of dimers.²⁶⁰ Expanding on this concept, a 5,10,15,20-tetramethyl-substituted derivative of Co porphyrin was designed and synthesized.²⁶¹ Upon coating this modified porphyrin onto an EPG electrode and conducting electrochemical evaluations, it was found that O₂ was reduced to H₂O with 4e⁻ selectivity. Similarly, 5,10,15,20-tetra(2,4,6-trimethylphenyl)porphyrin also exhibits 4e⁻ selectivity for the ORR with an n value of 3.5 in 0.1 M H₂SO₄.²⁶² Collectively, these findings indicated that Co porphyrins featuring either hydrogen or small alkyl substituents at their meso-positions preferentially form dimers or aggregates through van der Waals interactions, resulting in exceptional catalytic performance with high 4e⁻ selectivity for the ORR.¹⁵³

An alternative approach to achieving the desired intermetallic distance involves the direct synthesis of dicobalt face-to-face (Co₂FTF) porphyrin dimers, where the distance between Co atoms can be precisely controlled. A series of such dimers, featuring 6-, 5-, and 4-atom amide bridges, were designed and synthesized (Fig. 17).^151,263 Co₂FTF 4 exhibits a high n value of >3.9, indicating 4e⁻ ORR selectivity. In contrast, Co₂FTF 6 displays a low n value of 3.0, suggesting a mixed 2e⁻ and 4e⁻ reduction process of the ORR.²⁶⁴ The key difference lies in the intermetallic distance, with Co₂FTF 4 having a distance of 3.4 Å between two Co atoms, significantly shorter than 6.5 Å found in Co₂FTF 6. This optimal distance in Co₂FTF 4 facilitates the formation of dioxygen complexes and oxo intermediates, ultimately leading to its superior 4e⁻ selectivity.²⁶⁵ Conversely, the longer and more flexible linker in Co₂FTF 6 fails to maintain a true “face-to-face” geometry between porphyrins, hindering the synergistic effect between two metal centers in activating and reducing O₂. To ensure close and genuine “face-to-face” contact between metal porphyrins, it is imperative to employ the shortest and most rigid connector, ensuring optimal positioning of Co atoms, promoting the cooperative effect necessary for highly selective 4e⁻ ORR catalysis.

Fig. 17 Molecular structures of dinuclear Co porphyrins Co₂FTF 6 (a), Co₂FTF 5 (b), and Co₂FTF 4 (c).

4.2.2. Cofacial Pacman bisporphyrins. Distinct from dicobalt face-to-face porphyrins, a series of dinuclear Pacman porphyrins were designed and synthesized (Fig. 18).²⁶⁶ These Pacman porphyrins feature a unique cofacial bisporphyrins architecture stabilized by a solitary rigid pillar, inherently promoting a facile face-to-face alignment of porphyrin molecules. The minimal lateral displacement between two Co-bound porphyrin subunits significantly enhances the efficiency of O₂ activation within the confined cofacial cleft. To explore the potential of this platform, multiple generations of Pacman assemblies were designed and synthesized. Initially, Co bisporphyrins derived from diporphyrin anthracene (DPA) and diporphyrin biphenylene (DPB) were obtained (Fig. 18a and b). Both Co₂DPA and Co₂DPB exhibit remarkable catalytic activity for the ORR, favoring the 4e⁻ pathway with n values ranging from 3.7 to 3.8 in a 0.5 M trifluoroacetic acid solution. Crystal structures of these metal bisporphyrins confirm the intermetallic distances of 4.6 Å for Co₂DPA and 3.8 Å for Co₂DPB (Fig. 18c and d).²⁶⁷ This observation highlights the narrow range of pocket size variation, approximately 1 Å, indicating a limited degree of conformational flexibility. This constraint, while simplifying the synthesis and structural characterization, also presents challenges in elucidating the intricate structure–selectivity relationships.

Fig. 18 Molecular structures of Co₂DPA (a) and Co₂DPB (b). Ball-and-stick representations of crystal structures of Co₂DPA (c) and Co₂DPB (d).

To refine the spacing between two Co atoms, two innovative Pacman bisporphyrins were crafted drawing inspiration from DPX (diporphyrin xanthene) and DPD (diporphyrin dibenzofuran) architectures (Fig. 19).^268,269 Crystallographic analysis reveals that the mean intermetallic distances in M₂DPX and M₂DPD stand at 4.1 and 7.5 Å, respectively. Conversely, Co₂DPD exhibits an intermetallic distance of 8.6 Å, demonstrating its remarkable flexibility and substantial vertical expansion (Fig. 19f).²⁷⁰ According to RRDE measurements, both Co₂DPX and Co₂DPD have 4e⁻ selectivity for the ORR with H₂O production yields of 72% and 80%, respectively. Furthermore, substituting one of the Co porphyrins with its corrole counterpart preserves the 4e⁻ selectivity for the ORR, showing the versatility of this design approach.²⁷¹

Fig. 19 Molecular structures of cofacial bisporphyrins Co₂DPX (a), Co₂DPD (b), Co₂DPXM (c), and Co₂DPDM (d). Crystal structures of Co₂DPX (e), Co₂DPD (f), Co₂DPXM (g), and Fe₂ODPXM (h). Values of ferrocenium concentration [Fc⁺] and corresponding n obtained from the chemical reduction of O₂ (i) and the H₂O production (j) using CoOEP, Co₂DPX, Co₂DPD, Co₂DPXM, and Co₂DPDM based on the electrocatalytic ORR and homogeneous catalysis in HClO₄ and other different acidic solutions. Reprinted with permission from ref. 207. Copyright 2007, American Chemical Society.

Moreover, the versatility of pocket dimensions of the Pacman motif enables fine-tuning of the intermetallic distance, spanning from 4.0 Å to over 8.0 Å, through placement of substituents along the macrocyclic perimeter.²⁷² By introducing methoxyaryl groups at the meso-positions of benzene rings opposite to the bridge, two distinct dinuclear Co Pacman bisporphyrins, Co₂ diporphyrin xanthene methoxyaryl (DPXM) and Co₂ diporphyrin dibenzofuran methoxyaryl (DPDM), were obtained (Fig. 19c and d).²⁷³ Notably, the intermetallic distance in Co₂DPXM was precisely adjusted to 5.91 Å (Fig. 19g). Notably, in the case of Fe₂ODPXM, a Fe–O–Fe bridge was formed, reducing the intermetallic distance to 3.5 Å.²⁷⁴ Electrochemical assessments reveal that both Co₂DPXM and Co₂DPXD exhibit comparable H₂O production percentages with Co₂DPXM achieving 52% and Co₂DPXD 46%, indicating a mixed 2e⁻ and 4e⁻ ORR pathway. For comparison, homogeneous chemical reduction experiments of these Pacman porphyrins were carried out in air-saturated benzonitrile with HClO₄ and ferrocene (Fc) (Fig. 19i). Co octaethylporphyrin (OEP) was selected as the control sample. The quantity of ferrocenium [Fc⁺] formed for CoOEP is two times that of O₂ consumed, demonstrating a 2e⁻ ORR process. In contrast, the quantity of [Fc⁺] formed is four times that of O₂ consumed for Co₂DPX, demonstrating a 4e⁻ ORR process. Mixed 2e⁻ and 4e⁻ processes observed in Co₂DPXM and Co₂DPXD align with their electrochemical behavior. Furthermore, both Co₂DPX and Co₂DPD have 4e⁻ selectivity for the ORR compared with Co₂DPXM and Co₂DPXD with methoxyaryl groups. This consistency between homogeneous and heterogeneous catalytic outcomes underscores the high reliability of these findings. In addition, the effect of acid strength on the selectivity of the ORR for Co₂DPX was investigated using the homogeneous chemical reduction method (Fig. 19j). Specifically, the 4e⁻ selectivity of the ORR was observed when the pK_a of the acid was less than 12.0 (e.g., HClO₄ and CH₃SO₃H) measured in benzonitrile, whereas acids with pK_a greater than 12.0 (e.g., CHCl₂CO₂H and CH₂ClCO₂H) failed to elicit this selectivity. Consequently, tuning the distance between two Co centers is paramount for achieving a highly selective 4e⁻ ORR process.

To gain deeper insights into the reaction mechanism, a series of spectroscopic and kinetic experiments and theoretical calculations were undertaken. Based on these investigations, a possible ORR pathway for Pacman dinuclear Co porphyrins was formulated (Fig. 20).^{80,207,274,275} First, Co^IICo^III was used as the starting state for the ORR. Upon interaction with O₂, Co^IICo^III species evolves into Co^IIICo^III superoxide, whose pK_a value serves as a determinant of the selectivity of the ORR. When the pK_a is low, it is reduced to Co^IIICo^III peroxo by one electron, and the peroxo species easily reacts with protons to afford hydrogen peroxide, while the Pacman porphyrin returns to Co^IIICo^III. In contrast, when the pK_a is high, it reacts with protons to form Co^III–HO₂–Co^III. The subsequent 2e⁻ reduction results in the cleavage of O–O bonds, forming Co^III–OH and Co^IV=O. The Co^IV–oxo rapidly becomes Co^III–OH through a PCET process. The Pacman porphyrin becomes two face-to-face Co^III–OH, which reacts with two molecules of protons to generate two molecules of water, leading to the formation of Co^IIICo^III. It is reduced to Co^IIICo^II by one electron and then starts the next cycle. Regarding cofacial Pacman porphyrins, the 4e⁻ selectivity of Co₂DPX in the presence of Fe(C₅H₄Me)₂ can be attributed to the strong binding of O₂ and reduced O₂ with a subtle intermetallic distance. This fine-tuned distance optimizes the electronic and steric interactions within the complex, leading to a highly efficient and selective 4e⁻ ORR pathway.

Fig. 20 Proposed ORR mechanism with cofacial Pacman bisporphyrins. Reprinted with permission from ref. 207. Copyright 2007, American Chemical Society.

Drawing inspiration from the success of cofacial Pacman porphyrins featuring diverse spacers, the fine-tuning of these spacers with alternative organic molecules emerges as an effective approach to modulate the distance between two Co centers. To explore this concept, CoTPFP was selected as the model Co porphyrin. Two Pacman porphyrin systems were designed, where two adjacent carbons of a benzene ring were covalently linked to two Co porphyrin moieties (named CoTPP–CoTPFP and CoTPFP–CoTPFP) (Fig. 21a and b).²⁷⁶ The single crystal structure of Pacman CoTPFP–CoTPFP reveals an optimal distance of 5.7 Å between two Co centers, ideally suited for facilitating a 4e⁻ ORR process (Fig. 21c). Pacman CoTPP–CoTPFP has superior ORR performance (E_1/2 = 0.7 V) compared to Pacman CoTPFP–CoTPFP (E_1/2 = 0.51 V) measured in a 0.5 M H₂SO₄ solution. Both Pacman porphyrin CoTPP–CoTPFP and CoTPFP–CoTPFP have a 4e⁻ ORR process with n values of 3.9 and 3.8, respectively. K–L plots further confirmed the 4e⁻ selectivity of Pacman CoTPP–CoTPFP towards the ORR. Within this system, CoTPFP serves as the primary site for O₂ activation and reduction, while CoTPP functions as a Lewis acid, enhancing O₂ binding and activation, thereby contributing to the overall ORR efficiency. This dual-role mechanism underscores the importance of spacer design in tuning the electronic and steric properties of Pacman porphyrin systems for optimized ORR performance.

Fig. 21 (a) and (b) Molecular structures of two Pacman Co porphyrins (CoTPP–CoTPFP and CoTPFP–CoTPFP). (c) Crystal structure of Pacman porphyrin CoTPFP–CoTPFP.

4.2.3. Cofacial porphyrin prisms. Cofacial bisporphyrins have emerged as promising catalysts for the 4e⁻ ORR, yet their synthesis, particularly through direct covalent linking, poses significant challenges. To circumvent these limitations, a novel approach involving coordination-driven self-assembly has been explored, leading to the construction of a diverse array of cofacial porphyrin-based architectures.^277,278 Inspired by the versatility of porphyrin boxes, a unique cofacial Co porphyrin dimer was crafted through the formation of coordination bonds. Utilizing CoTPyP as the fundamental building block, the innovative cofacial analogue complex, named “Co Prism”, was successfully synthesized.²⁷⁹ Furthermore, the integration of molecular clips, specifically the Benzo-clip, alongside alternative short Ru-based clips such as Ox- and Oxa-, was explored as a means to reduce the distance between porphyrin units. By adopting CoTPyP as the core porphyrin component, these modifications were found to impact ORR performance. The production percentages of H₂O for resulting Ox–Co and Oxa–Co prisms are 87% and 97%, respectively, demonstrating a preferential 4e⁻ ORR process. In contrast, the Benzo-Co prism has a lower H₂O production yield of 75%.²⁸⁰ Recently, a series of tailored ruthenium-based molecular clips [Ru(η₆-p-cymene)μ₂-R]₂Cl₂ (Ru–R, R = –CH₃, –H, –Cl, and –CF₃) were designed and synthesized to serve as precision spacers (Fig. 22).²⁸¹ These spacers effectively modulate the intermetallic distance in the resulting cofacial porphyrin prisms to 4.5 Å. Notably, all cofacial Co-based porphyrin prisms featuring these molecular clips exhibit 4e⁻ selectivity for the ORR with a H₂O production yield of >90%. The subtle manipulation of the steric R groups on molecular clips has emerged as a powerful tool for fine-tuning the metal–metal distance, which in turn exerts a profound influence on both the selectivity and kinetics of the ORR. Among the various Ru–R clips, the Co porphyrin prism constructed with the Ru–CH₃ clip (Co–Ru–CH₃) exhibits the best catalytic ORR activity and a dominant 4e⁻ selectivity for H₂O production (>98%) compared to its counterparts featuring alternative Ru–R clips. This finding demonstrates the crucial role of precise molecular engineering in optimizing the performance of ORR catalysts.

Fig. 22 Schematic illustration of the formation process of cofacial prisms based on Co porphyrins and different molecular clips. Reprinted with permission from ref. 281. Copyright 2021, American Chemical Society.

Two metal porphyrins, specifically CoTPyP, and four molecular clips can collaborate to form Co cofacial prisms. More recently, an innovative approach was proposed, involving the fine-tuning of molecular clip numbers to design and synthesize distinct Co porphyrin-based prisms.²⁸² This strategy was achieved by replacing one or two pyridine groups of CoTPyP with one or two phenyl (Ph) groups. The resulting Co prisms, named Co₂Py₃Ph and Co₂Py₂Ph₂, exhibit a reduced level of symmetry. The Co₂Py₃Ph prism exhibits a H₂O₂ production yield of 5% (n = 3.9), while the Co₂Py₂Ph₂ prism has a higher H₂O₂ production yield of 37% (n = 3.3). For comparison, the control sample Co₂Py₄ has an n value of 3.7 with a H₂O₂ production yield of 14.5%. Single crystal structures of Zn₂Py₄ and Zn₂Py₂Ph₂ demonstrate that the intermetallic distance of Zn–Zn for Zn₂Py₄ is 4.3 Å, while that value is 5.6 Å for Zn₂Py₂Ph₂. This result reveals a direct correlation between the symmetry of Co porphyrin prisms and the intermetallic distance of Zn–Zn. Therefore, the intermetallic difference caused by the symmetry of Co porphyrin prisms plays a critical role in modulating the selectivity of the ORR.

4.2.4. Other dinuclear Co-based complexes. Inspired by the Fe–Cu catalytic center in CcOs, a series of dinuclear Fe porphyrin-Cu model molecules that closely mimic the protein environment of their biological counterparts were designed and synthesized.^71,283 These artificial Fe/Cu complexes have been thoroughly investigated for their electrochemical activity, selectivity and underlying reaction mechanisms towards the ORR.²⁸⁴ By manipulating the coordination environment surrounding the Cu ion and Fe porphyrins, scientists have fine-tuned their performance. As shown in Fig. 23a, the initial Fe^II/Cu^I state in CcOs reacts with O₂, yielding an Fe^III–O–O–Cu^II intermediate. Experimental results demonstrate the crucial role of Cu ions in facilitating O₂ binding and contributing an electron to reduce O₂.²⁸⁵ Inspired by nature, a dinuclear [Cu^II(bpbp)(μ-OAc)₂Fe^III]-(ClO₄)₂ complex was synthesized for the ORR (bpbp = 2,6-bis[[bis(2-pyridinylmethyl)amino]methyl]-4-(1,1-dimethylethyl)-phenolate).²⁸⁶ The dinuclear FeCu complex exhibits a remarkable turnover frequency of 2.4 × 10³ s⁻¹ for the ORR, outperforming the corresponding CuCu and FeFe complexes. Notably, the FeCu complex exhibits a dominant 4e⁻ ORR selectivity, while the CuCu complex exhibits a 2e⁻ ORR selectivity with a H₂O₂ yield of 98.9%. This synergistic interplay between Fe and Cu significantly promotes the O–O bond cleavage, thereby boosting both catalytic activity and selectivity towards the ORR. Recent advancements have also introduced a series of dinuclear Co-corrole/M complexes for the ORR (M = Co, Cu, and Zn).²⁸⁷ Compared to Fe porphyrins, Co corroles have emerged as more efficient catalysts for the ORR. By coordinating second-sphere M ions with appended tris(2-pyridylmethyl)-amine (TPA) units, the Co-corrole/Co complex exhibits a remarkably high selectivity for the 4e⁻ ORR with an n value of 3.75 and excellent activity with an E_1/2 of 0.89 V in 0.1 M KOH, surpassing the Co-corrole/Cu complex (n = 3.56; E_1/2 = 0.78 V). Notably, upon reaction with O₂, the Co-corrole/Co complex formed a (TPA)Co^III–peroxo–Co^III corrole cation radical, mirroring a similar intermediate observed in nature (Fig. 23b). The Co^III/II redox couple of Co-TPA locates at 1.42 V, whereas the Cu^II/I couple of Cu-TPA locates at a lower potential of 0.33 V, hindering efficient electron transfer to activate O₂ under typical ORR measurement conditions (Fig. 23c). Therefore, the Cu^II/I couple was substituted with the Co^III/II couple to more closely mimic the natural CcO system. In a parallel study, a dinuclear Co porphyrin/Zn complex was found to exhibit a dominant 4e⁻ ORR pathway with an n value of 3.91 in a neutral aqueous solution.²⁸⁸ The pendant Zn^II ion enhances the electron transfer from Co^II to O₂via a “pull effect”, further underscoring the importance of metal ion pairings in optimizing ORR performance. Collectively, these findings offer a promising strategy to emulate the Fe^II/Cu^I active site in CcOs, achieving high selectivity and exceptional activity in converting O₂ to H₂O.

Fig. 23 The binding and activation of O₂ with (a) the Fe porphyrin/Cu site in nature and (b) the Co corrole/Co site. (c) Schematic illustration of the strategy by adapting Cu^II/I couple to Co^III/II couple conversion. Reprinted with permission from ref. 287. Copyright 2023, Wiley-VCH.

Apart from dinuclear Co corrole/M complexes, a series of dinuclear Co-based molecular catalysts featuring Schiff base calixpyrrole ligands have been reported (Fig. 24a).^289–291 These complexes show a low-spin oxidation state of +2 for both Co ions, with each ion coordinated to four N atoms of pyrrole, maintaining an average equatorial Co–N distance of 1.870 Å. Upon O₂ binding and activation, the complex (L) transforms into [Co₂(O₂)(Py)₂(L)] in tetrahydrofuran solvent, where two pyridine molecules coordinate with the Co center. The crystal structure of this complex reveals a structural resemblance to Pacman diporphyrins, characterized by an intermetallic distance of 4.151 Å. The O1–O2 bond length of 1.361(3) Å falls within the range of bridging peroxide (1.34–1.53 Å) and superoxide (1.26–1.36 Å) units.²⁷⁴ In addition, researchers also introduced the sterically-hindering aryl meso-groups, such as fluorenyl, into the ligand and prepared the superoxide [Co₂(O₂)(Py)₂(L^F)][OH]. In this structure, the O1–O2 bond distance extends to 1.389(4) Å, likely influenced by the ligand environment and hydrogen-bonding interaction between O1 and the hydroxide O1s (2.801 Å).²⁹¹ A similar intermetallic distance was observed for two Co atoms (4.15 Å). These dinuclear Co catalysts exhibit obvious 4e⁻ ORR selectivity with tunable catalytic activity through ligand structural modifications. Different from Pacman-shaped complexes, a dinuclear complex [Co^II₂(trpy)₂(μ-bpp)(μ-Cl)](PF₆)₂ was designed and synthesized using terpyridine (trpy) and bis(pyridyl)pyrazolate (bpp) as coordinating ligands (Fig. 24b).²⁹² Exposure to O₂ in methanol/acetonitrile solvent led to the formation of a Co–μ-1,2-peroxo complex. The resulting [Co^III₂(trpy)₂(μ-bpp)(μ-1,2-O₂)]³⁺ complex exhibits obvious 4e⁻ ORR selectivity in the mixed solution of acetonitrile and trifluoroacetic acid. According to the crystal structure of O₂ binding, the O–O bond distance of 1.397(2) Å in this peroxo complex is typical of dinuclear Co-based species. Further oxidation with cerium(IV) ammonium nitrate yielded another dinuclear Co-based complex with H₂O and OH coordinating separately to Co^III ions and sharing a H atom, resulting in an O–O distance of 2.415 Å. Both of these dinuclear Co-based complexes efficiently catalyze O₂ reduction to H₂O through a 4e⁻ pathway mediated by Me₈Fc with trifluoroacetic acid.

Fig. 24 (a) Molecular structures of dinuclear Co-based molecular catalysts with Schiff base calixpyrrole ligands and the corresponding crystal structures with O₂ binding and activation. Reprinted with permission from ref. 289. Copyright 2007, Wiley-VCH. (b) Synthetic procedures and crystal structures of dinuclear Co-based electrocatalysts. Reprinted with permission from ref. 292. Copyright 2012, American Chemical Society.

Conventionally, dinuclear complexes have been synthesized through the direct synthesis method. However, recent advancements have introduced a two-step specific-adsorption strategy for the fabrication of FeCo molecular hybrids.²⁹³ This methodology initially involves the direct immobilization of Fe phthalocyanines onto carbon nanotubes, followed by the introduction of Co phthalocyanines, which overlap with Fe phthalocyanines in a controlled manner, either completely or partially. The specific formation of these hybrids is facilitated by π–π stacking interactions and electron donor–acceptor interactions between two distinct metal phthalocyanines. The resulting FeCo hybrid exhibits superior ORR performance with an E_1/2 of 0.95 V, exceeding both standalone Fe phthalocyanines (E_1/2 = 0.92 V) and Co phthalocyanines (E_1/2 = 0.82 V) in 0.1 M KOH. Furthermore, the FeCo hybrid exhibits a favorable 4e⁻ ORR selectivity as confirmed through evaluations using RRDE measurements and K–L plots. This work presents a fresh perspective for the rational design of structurally precise dinuclear catalysts by leveraging the construction of molecular heterostructures.

4.3. Trinuclear Co-based electrocatalysts

Distinct from conventional dinuclear Co-centered catalysts, triangular trinuclear metal-based complexes ([MN₄]₃, M = Fe and Co) have been designed and synthesized (Fig. 25).²⁹⁴ Three [16]-annulene-like N₄-macrocycles are condensed within a single conjugated plane, interconnected by a central hexaazatrinaphthylene unit. The single Co–N₄ coordination structure within these trinuclear complexes resembles that found in Co porphyrins, yet they offer a distinct architectural advantage. When supported on CB and measured in 0.1 M KOH, the [CoN₄]₃ complex exhibits an n value of 3.7, demonstrating a high 4e⁻ ORR selectivity. This remarkable performance can be attributed to the extensive planar conjugation and the high density of active sites inherent in the trinuclear design, which facilitate the efficient cleavage of the O–O bond, thereby promoting the highly desired 4e⁻ pathway. This work not only shows the potential of triangular trinuclear metal-based complexes as efficient molecular catalysts but also establishes a solid foundation for the development of a novel generation of catalysts that can harness the unique properties of intricate molecular architectures to enhance electrochemical performance.

Fig. 25 Synthetic procedures of (a) the ligand 1,4,7,10,13,16-tris-[2'-(4''-octylphenyl-2H-pyrrole-2'-ylidenemethyl)-1H-pyrrole-5′-yl]- 5,6,11,12,17,18-hexaazatrinaphthylenes (10) and (b) trinuclear metal-N₄ electrocatalysts [MN₄]₃ (M = Fe and Co). Reprinted with permission from ref. 294. Copyright 2011, American Chemical Society.

4.4. Other Co-based material electrocatalysts

It is established that the distance between two Co centers is vital to determine the 4e⁻ selectivity of dinuclear Co molecular electrocatalysts for the ORR. To precisely control Co molecules at desired distances, multiple strategies have been devised for the fabrication of Co-based material electrocatalysts. First, the self-assembly method has emerged as a viable approach for constructing supramolecular structures or aggregates, where a degree of orderliness is inherently achieved.²⁹⁵ These ordered structures facilitate efficient electron transfer and catalytic activity. Second, the integration of Co molecules into MOFs, COFs or polymers represents another powerful strategy.⁵⁴ These frameworks and polymers offer well-defined coordination environments and relatively ordered structures, enabling precise control over catalytic sites and their interactions. Furthermore, the design and synthesis of dual-atom Co–M-based materials have garnered widespread attention.²⁹⁶ Notably, frameworks have emerged as a research focus in this field due to their unique combination of periodic topological structures, high specific surface areas, and tunable pore sizes.²⁹⁷ These attributes offer significant advantages to optimize the catalytic atomic and electronic structures, thereby enabling precise control over ORR selectivity.

4.4.1. Co porphyrin-based aggregates. The highly conjugated ring system of porphyrins enables the formation of supramolecular structures through π–π stacking interactions, exemplified by the formation of linear J-aggregates utilizing Co 5,10,15,20-tetra(4-sulfophenyl)porphyrin (CoTPPS) (Fig. 26a and b).²⁹⁸ These J-aggregates can serve as templates during electropolymerization of pyrrole, yielding CoTPPS/polypyrrole nanocomposites that efficiently catalyze the ORR via a 4e⁻ pathway in phosphate buffer solutions. Similarly, composite CoTPPS/polyaniline structures have also been prepared, showing an n value of 3.7 in a 1.0 M HCl solution.²⁹⁹ Furthermore, π-stacking of simple CoOEP molecules has yielded supramolecular assemblies that exhibit remarkable activity and 4e⁻ selectivity for the ORR.³⁰⁰ Porphyrin-based J-aggregates facilitate the formation of adjacent Co active sites, forming a bridge Co–O–O–Co adsorption mode that directs the ORR towards a 4e⁻ pathway. In addition, two-dimensional (2D) reduced graphene oxide (rGO) serves as an ideal template for assembling Co porphyrin molecules, as exemplified by the layer-by-layer construction of a Co porphyrin/rGO composite using rGO, Co²⁺, and 5,10,15,20-tetra(4-hydroxyphenyl)porphyrin (THPP) (Fig. 26c).³⁰¹ The resulting composite (Co²⁺-THPP)_n/rGO exhibits an n value of 3.85 measured in 0.1 M KOH, confirming its effectiveness in catalyzing the 4e⁻ ORR. Metal phthalocyanines have also aroused widespread concern for ORR catalysis, with microwave-induced face-to-face assembly facilitating the growth of nanorods of Fe phthalocyanines along the (001) direction.³⁰² The strong intermolecular π–π stacking in these nanorods ensures an optimal distance between two neighboring Fe atoms (4.92 Å). The resulting Fe phthalocyanines exhibit an E_1/2 of 0.91 V with 4e⁻ ORR selectivity in 0.1 M KOH. In conclusion, the construction of superstructures based on molecular catalysts, such as porphyrins and phthalocyanines, represents an effective strategy for achieving 4e⁻ ORR selectivity, highlighting the potential of these materials in fuel cell and energy storage applications.

Fig. 26 (a) Molecular structure of CoTPPS and schematic illustration of the corresponding linear J-aggregate. (b) Atomic force microscopy (AFM) image of CoTPPS aggregates and the corresponding height profiles. Reprinted with permission from ref. 298. Copyright 2007, American Chemical Society. (c) Schematic illustration of the synthesis procedure of (Co²⁺-THPP)_n/rGO. Reprinted with permission from ref. 301. Copyright 2013, Wiley-VCH.

4.4.2. Metal–organic frameworks. Recently, MOFs have attracted much more attention and developed quickly owing to their diverse applications in energy storage, catalysis, and adsorption processes.^303–305 Among these MOF materials, porphyrin-based MOFs, especially porous coordination networks (PCNs), have been extensively investigated.^306–308 These PCNs, constructed using metal 5,10,15,20-tetra(4-carboxyphenyl) porphyrin (MTCPP) and Zr–O clusters, enable precise manipulation of the distance between metal porphyrins through controlled crystal structures.³⁰⁹ For example, Co-based PCN-222 MOFs with different crystal sizes have been synthesized for the ORR.³¹⁰ The morphology and structure of PCN-222 still remain after running electrolysis for 1 h in 0.1 M HClO₄. PCN-222 with an average size of 200 nm shows a main 4e⁻ ORR selectivity. Recently, a series of porphyrin-based MOFs, named PCN-226–M (M = Fe, Co, Ni, Cu, and Zn), have been constructed using Zr-oxide chains as nodes.³¹¹ This special Zr–O chain leads to a close packing of metal porphyrins, creating two distinct pore sizes: 7.2 Å × 4.8 Å (Fig. 27a) and 5.4 Å × 4.2 Å (Fig. 27b). Crystal structures of PCN-226 indicate that metal porphyrins have two packing distances (4 Å and 7 Å). The resulting PCN-226–Co displays an E_1/2 = 0.75 V measured in 0.1 M KOH. Notably, PCN-226–Co has an n value of 3.3 measured with a RRDE, indicating the presence of a 4e⁻ pathway. To study the effect of porphyrin spacing on the ORR, theoretical calculations were carried out. The distance between two Co centers is 7 Å, which is beneficial for the adsorption of intermediates, including O*, OH*, and OOH*, resulting in excellent ORR performance (Fig. 27c–e). In addition, Al-MOF-Co has been prepared with CoTCPP as the linker and Al–O cluster as the node.³¹² Al-MOF-Co shows an n value of 3.0 in 0.1 M H₂SO₄. Therefore, porphyrin-based MOFs are suitable platforms to fine-tune the selectivity of Co porphyrins for heterogeneous ORR catalysis. Beyond metal porphyrin-based MOFs, bimetallic MOFs have also been reported for the ORR. For example, a Co–Cu MOF has been prepared with benzene-1,2,4,5-tetraamine and benzene-1,3,5-tricarboxylic acid as organic linkers.³¹³ The resulting bimetallic Co–Cu (1 : 1) MOF exhibits an E_1/2 of 0.95 V with an average n value of 3.90 in an alkaline solution. The excellent catalytic ORR activity and selectivity are attributed to the interatomic electron transfer processes between Co and Cu within the bimetallic MOF framework.

Fig. 27 Crystal structure of PCN-226–M viewed along the b axis (a) and the c axis (b). (c) Computational molecular model of two neighboring Co porphyrins. (d) Adsorption energies of intermediates (OOH*, O*, and OH*) with different Co-to-Co distances. (e) Comparison of free energy diagrams of intermediates (OOH*, O*, and OH*) for ideal catalysts and two Co porphyrins with a spacing of 7 Å. Reprinted with permission from ref. 311. Copyright 2020, American Chemical Society. (f) Schematic structure of CuPc–O₈–Co and (g) the corresponding experimental and calculated XRD patterns of CuPc–O₈–Co. Reprinted with permission from ref. 314. Copyright 2019, Wiley-VCH.

In addition to porphyrin-based frameworks, phthalocyanine-based frameworks have also attracted considerable attention.³¹⁴ For example, Cu phthalocyanine-based MOFs with square-planar Co bis(dihydroxy) complexes (Co–O₄) as connecting units (CuPc–O₈–Co) have been reported (Fig. 27f). XRD patterns of CuPc–O₈–Co are consistent with predicted computational results (Fig. 27g). CuPc–O₈–Co exhibits an E_1/2 of 0.83 V with an n value of 3.93, indicating a dominant 4e⁻ ORR pathway. This finding highlights the Co–O₄ unit as the key catalytic active site responsible for facilitating the efficient ORR process.

4.4.3. Covalent organic frameworks. Apart from MOFs, COFs, as organic polymers with porous crystal structures and strong covalent bonds, have also received widespread concern.^315,316 Particularly, Co porphyrin-based COFs have emerged as versatile electrocatalysts for the ORR, demonstrating their widespread application in this field.^317,318 For example, a series of large-area free-standing 2D COFs have been reported through the polymerization of meso-benzohydrazide-substituted metal porphyrins with tris-aldehyde linkers.³¹⁹ The formation of acylhydrazone bonds takes place at the liquid–air interface, yielding 2D Co-based COFs, which exhibit a mixed 2e⁻ and 4e⁻ ORR selectivity with an n value of 3.2 in 0.1 M KOH. This result may be attributed to the possible occurrence of a bimolecular reaction mechanism facilitated by the significant thickness of films (30 nm).

To improve the electron transfer rate during the ORR, the integration of carbon materials as conductive supports within these COFs has been proven beneficial.^320,321 This approach not only accelerates electron transfer within composite materials due to interactions between carbon materials and Co porphyrins but also enables the construction of supramolecular structures with distinct layers, promoting the bimolecular reaction mechanism. A notable example is the Hay-coupling-mediated synthesis of a Co meso-tetraethynylporphyrin (TEP)-based COF/CNT composite (Fig. 28a).³²² Co-TEP-COF/CNT shows an E_1/2 of 0.65 V and an n value of 3.93 measured in 0.5 M H₂SO₄ with a RRDE, demonstrating a 4e⁻ selectivity of the ORR. Moreover, this composite exhibits remarkable long-term stability, retaining over 95% of its initial performance after 24 h of continuous operation. Similar strategies have been employed using different porphyrin monomers, such as Co 5,10,15,20-tetra(4′-propynylphenyl)porphyrin, to construct a conjugated porous Co porphyrinylene–ethynylene framework (CoPEF).³²³ The integration of CB further enhances the electrocatalytic properties, yielding a CoPEF/CB complex with an n value of 3.88 in 0.5 M H₂SO₄ and an n value of 3.80 in 0.1 M KOH. Recently, a one-pot polymerization approach has been developed, utilizing metal salts, pyrrole, and benzene-1,4-dialdehyde (BDA) to construct porphyrin-based COFs.³²⁴Fig. 28b shows the chemical structure of a single-layer COF. For example, CNTs have been introduced to this system as the template (Fig. 28c). A Co porphyrin-based COF/CNT composite was obtained with a thickness of 4 nm surrounding the CNTs (Fig. 28d). Similarly, graphene has also been applied as the template to construct these kinds of composites (Fig. 28e and f).³²⁵ The resulting COF/graphene composite exhibits an E_1/2 of 0.81 V in 0.1 M KOH and an n value of 3.8, demonstrating a 4e⁻ selectivity for the ORR. Furthermore, this synthetic strategy can be extended to incorporate other transition metals (Mn, Fe, Ni, Cu, and Zn) into the COF structure, broadening the scope of potential electrocatalysts.³²⁶ In addition, the pyridine-functionalized reduced graphene oxide has also been applied as the template to construct Co 5,10,15,20-tetra(4-aminophenyl) porphyrin-based COFs through Schiff base condensation reaction with BDA as the linker.³²⁷ The resulting composite exhibits a larger n value (3.7–3.9) than that without pyridine groups (3.55–3.65). These advancements demonstrate the promise of Co porphyrin-based COFs as efficient and tunable electrocatalysts for the ORR.

Fig. 28 (a) Synthetic route of the composite material Co-TEP-COF/CNT. Reprinted with permission from ref. 322. Copyright 2014, American Chemical Society. (b) Schematic illustration of the molecular structure of porphyrin-based COFs. (c) Scanning electron microscopy (SEM) image and (d) TEM image of COFs@CNTs. Reprinted with permission from ref. 324. Copyright 2018, Royal Society of Chemistry. (e) SEM image and (f) STEM image of COFs@graphene. Reprinted with permission from ref. 325. Copyright 2019, Wiley-VCH.

4.4.4. Porous organic polymers. Apart from porphyrin-based frameworks, porous organic polymers (POPs) have also been applied as efficient electrocatalysts for the ORR.³²⁸ These POPs typically lack an ordered three-dimensional crystalline structure. Recent advancements have focused on the synthesis of metal corrole-based POPs, particularly those incorporating Mn, Fe, Co, and Cu.³²⁸ Compared with dianionic porphyrins, trianionic corroles effectively stabilize high-valent metal ions and possess lower symmetry, making them attractive candidates for POP construction. Using tetrakis(4-ethynylphenyl)methane (TEPM) as a linker, metal-corrole-based POPs were successfully synthesized via the Sonogashira coupling reaction (Fig. 29a). The Co POP exhibits uniform ball-like morphology with an average diameter of 1 μm (Fig. 29b). The Co POP also exhibits efficient catalytic ORR activity with an E_1/2 of 0.87 V (Fig. 29c). The Co POP has an n value of 3.91 in 0.1 M KOH measured using K–L plots, which is a 4e⁻ pathway of the ORR (Fig. 29d). This work further demonstrates the beneficial effect of the formation of polymers based on Co-based molecular catalysts. Furthermore, the pore size of this Co-based POP could be fine-tuned by adjusting the size of Co corrole units, leading to the development of POPs with varying pore diameters.³²⁹ The Co POP with a large pore size exhibits enhanced ORR performance with an E_1/2 of 0.89 V. This Co POP also exhibits 4e⁻ ORR selectivity with an n value of 3.89 in 0.1 M KOH. More recently, to further enhance proton transfer, a Co corrole-based POP was designed and prepared by introducing two phenolic hydroxyl groups on Co corrole catalytic units (Co-POP-2-OH).³³⁰ The resulting Co-POP-2-OH exhibits an E_1/2 of 0.91 V with an n value of 4.0 measured in 0.1 M KOH, highlighting the importance of efficient proton transfer at the local catalytic sites in boosting ORR activity. In addition, Co porphyrin-based porous coordination polymers were obtained by using 5,10,15,20-tetrakis(4′-([2,2′:6′,2′′-terpyridin]-4′-yl)-[1,1′-biphenyl]-4-yl)porphyrin and Co²⁺ (TTBPP–Co).³³¹ TTBPP–Co exhibits an n value of 3.56 in 0.1 M KOH. The electrochemical polymerization has become a powerful tool for polymer synthesis.³³² For example, Co 5,10,15,20-tetra(2-thienyl)porphyrin (CoTTP) was selected as a precursor to construct polymers (pCoTTP) on the glassy carbon electrode (Fig. 29e).³³³ pCoTTP shows n values of 3.9, 3.82, and 4.03 in different electrolytes with pH values of 2.0, 7.0, and 13.0, respectively. The 4e⁻ selectivity of the ORR in acidic, neutral, and basic solutions may be attributed to the formation of bimetallic Co active sites. Moreover, metal phthalocyanines, another class of compounds, can form polyphthalocyanines with strong π–π stacking, but this often limits active site exposure. To overcome this, edge-functionalized polyphthalocyanine networks were developed, expanding interlayer distances and facilitating mechanical exfoliation.³³⁴ Different aromatic acid anhydride substituents were applied to expand the interlayer distance of polyphthalocyanines to assist mechanical exfoliation. As a result, Co polyphthalocyanine exhibits an E_1/2 of 0.85 V with an n value of 3.84, indicating a 4e⁻ ORR pathway.

Fig. 29 (a) Synthetic procedure of M-corrole-based POPs. (b) SEM image, (c) LSV data at different rotation speeds, and (d) K–L plots of Co POP. Reprinted with permission from ref. 328. Copyright 2022, Wiley-VCH. (e) Synthetic procedure of pCoTTP through electrochemical polymerization. Reprinted with permission from ref. 333. Copyright 2010, American Chemical Society.

4.4.5. Dual-atom Co–M-based materials. Single-atom M–N–C materials have garnered extensive attention for their applications in the ORR, being hailed as some of the most promising non-precious electrocatalysts in this field.^36,37,335 The active centers of these materials, characterized by M–N₄ coordination structures, closely mimic the central motif of biological metal porphyrins. Among diverse M–N–C materials, Co–N–C stands out for its exceptional catalytic activity.⁵⁷ As an extension of SACs, dual-atom M₁–M₂–N–C electrocatalysts exhibit remarkable advantages in refining intrinsic kinetic processes and achieving bimetallic synergistic catalysis.^336–340 Recently, numerous diatomic M₁–M₂–N–C electrocatalysts have been reported for the ORR.^341–344 Herein, we focus on the recent progress on diatomic Co–M–N–C electrocatalysts. For example, dual-atom Fe–Co embedded on N-doped porous carbon (Co–Fe–N–C) materials were successfully synthesized via a host–guest approach (Fig. 30a).³⁴⁵ In this method, a Zn/Co ZIF structure served as the host framework, encapsulating FeCl₃ within porous architecture. Subsequent high-temperature pyrolysis yielded the Co–Fe–N–C material. The HAADF-STEM image and corresponding density profiles confirm the formation of dual-atom dimers (Fig. 30b). Furthermore, X-ray absorption fine structure (XAFS) spectroscopy indicates the coordination structure of Co–Fe dual-atom sites (Fig. 30c). In electrochemical evaluations, the Co–Fe–N–C electrocatalyst shows an E_1/2 of 0.86 V in 0.1 M HClO₄, surpassing that of both single-atom Fe–N–C and Co–N–C under identical conditions (Fig. 30d). Notably, the Co–Fe–N–C catalyst exhibits a H₂O₂ yield below 1.17% across the applied potential range, which is indicative of its excellent 4e⁻ ORR selectivity (Fig. 30e).

Fig. 30 (a) Schematic illustration of the synthetic procedure, (b) HAADF-STEM image and the corresponding density profiles, (c) proposed coordination structure, (d) LSV data, and (e) n value and H₂O₂ yield measured in 0.1 M HClO₄ for dual-atom Co–Fe–N–C. Reprinted with permission from ref. 345. Copyright 2017, American Chemical Society.

In recent years, the scientific community has witnessed a surge in the development of diverse dual-atom Co–M–N–C electrocatalysts, encompassing compositions like Co–Co,^199,201 Co–Fe,^346–348 Co–Ni,³⁴⁹ Co–Pt,³⁵⁰ Co–Ir,³⁵¹ and Co–Cu.³⁵² A proven approach to synthesize these catalysts involves integrating diatomic coordination compounds into precursor materials and subsequently subjecting them to high-temperature calcination to yield diatomic nitrogen–carbon materials. For example, bimetallic macrocyclic molecules [M₁–M₂–N₄O₂] were selected as multifunctional precursors to modify center sites of homonuclear or heteronuclear diatoms through in-plane coordination.³⁵³ Each metal atom within these molecules is coordinated with two N atoms and two μ-2 bridged O atoms, ensuring structural integrity during the assembly process within porous carbon matrices. Consequently, this approach offers a versatile method for crafting electrocatalysts featuring either homonuclear (e.g., Fe₂, Co₂, Ni₂, Cu₂, Mn₂, and Pd₂) or heteronuclear (e.g., Fe–Cu, Fe–Ni, Cu–Mn, and Cu–Co) diatomic sites. Fe–Fe–N₄O₂ exhibits excellent catalytic ORR activity with an E_1/2 of 0.81 V in 0.1 M HClO₄. The minimal yield of H₂O₂ for Fe–Fe–N₄O₂ is below 1%, indicating a highly selective 4e⁻ ORR pathway, likely facilitated by the side-bridge adsorption of O₂ molecules onto bimetallic active sites. Furthermore, researchers have used the Co–Fe–N₆–C catalyst as a model system to comprehensively study the mechanism of catalyst ORR activity changing with pH values.³⁵⁴ Consequently, M–N–C catalysts typically exhibit superior ORR activity in alkaline electrolytes, whereas their performance significantly decreases under acidic conditions.

5. Conclusion and perspectives

The electrocatalytic ORR can generate either H₂O₂via a 2e⁻ pathway or H₂O through a 4e⁻ process. Notably, the 4e⁻ ORR serves as a key reaction in many energy conversion technologies, including fuel cells and metal–air batteries. The 2e⁻ ORR holds significant value for the electrochemical synthesis of H₂O₂, which is a ubiquitous industrial oxidant with a broad range of applications. Furthermore, H₂O₂ emerges as a promising fuel owing to its ease of transportation and storage. Consequently, both the 2e⁻ and 4e⁻ processes of the ORR are indispensable in energy storage and conversion systems.

Herein, we provide a comprehensive review of the selectivity modulation of the ORR employing Co-based electrocatalysts, encompassing both molecular catalysts and material catalysts. For molecular catalysts, we focus on Co porphyrins with diverse structural configurations for the ORR. These Co porphyrins could catalyze the conversion of O₂ to H₂O₂via a 2e⁻ process or to H₂O via a 4e⁻ process through fine-tuning their molecular structures. The regulation of the coordination environment surrounding the metal center offers a powerful means to manipulate the selectivity between 2e⁻ and 4e⁻ pathways of the ORR. Several strategies are reported for mononuclear Co molecular catalysts to realize the 4e⁻ ORR process, including regulating meso- and β-substituents, promoting proton relay, constructing charge interactions, introducing steric hindrance substituents, and accelerating charge transfer processes. Furthermore, the construction of dinuclear Co molecular catalysts has emerged as an effective approach to realize the 4e⁻ process in the ORR, leveraging the synergistic effect between two Co centers. The precise intermetallic distance between these centers proves pivotal in dictating the selectivity of the ORR. By appropriately controlling the PCET, the efficient cleavage of the O–O bond within the Co–O–O–Co intermediate can be facilitated, thereby enhancing the 4e⁻ pathway. Consequently, a series of dinuclear Co-based electrocatalysts, including dicobalt face-to-face bisporphyrins, Pacman porphyrins, and Co prisms, have been reported, with the intermetallic distance fine-tuned through the modifications of linkers, spacers and molecular clips. Additionally, the fabrication of Co-based frameworks, such as MOFs and COFs, has garnered significant interest for realizing 4e⁻ ORR selectivity, owing to their tunable intermetallic distances. Inspired by molecular catalysts, Co-based single-atom catalysts (e.g., Co–N–C materials) and dual-atom catalysts (e.g., Co–M–N–C materials) are considered promising ORR electrocatalysts due to their high conductivity. Among Co-based electrocatalysts, molecular catalysts (e.g., Co porphyrins) and material catalysts (e.g., Co–N–C) exhibit distinct advantages and disadvantages.³⁵⁵ Molecular catalysts leverage atomically precise active sites and tunable electronic structures, making them ideal for investigating reaction mechanisms and structure–activity relationships. However, they suffer from complex synthetic steps and challenges in scalability. Material catalysts, with their high surface area, robust durability, and simplified synthesis (e.g., calcination), are better suited for industrial applications. Overall, molecular systems excel in fundamental research and precision catalysis, while material catalysts dominate in practicality and stability. Currently, many Co-based electrocatalysts achieve near-4e⁻ electron transfer pathways (n ≈ 3.9–4.0), significantly suppressing H₂O₂ byproduct formation. The catalytic activity of Co-based electrocatalysts approaches that of Pt-based materials with an E_1/2 of up to 0.85–0.90 V in 0.1 M KOH. These catalysts show strong application potential in alkaline fuel cells and metal–air batteries, particularly as cost-effective alternatives to noble metal catalysts in cost-sensitive fields. However, their limited activity in acidic media with E_1/2 reduced by ∼0.1–0.2 V hinders practical applications. Future efforts should focus on optimizing the coordination environment of Co active centers, developing corrosion-resistant supports, and exploring their adaptability in PEM fuel cells to bridge the gap from lab-scale breakthroughs to commercial applications.

Despite the significant progress made by the currently reported Co-based catalysts in the electrocatalytic ORR, further in-depth research studies are still required in the following aspects in the future.

(1) Designing new kinds of Co-based electrocatalysts with absolute 2e⁻ or 4e⁻ selectivity for the ORR is still highly desired. To achieve this goal, a profound understanding of the effects of the protein environment and surrounding coordination structures of the Fe porphyrin unit in CcOs will provide inspiring strategies for developing Co-based electrocatalysts. In particular, the intricate role of Cu^I intimately positioned around Fe porphyrin and its subsequent impact on the catalytic activity and selectivity of the ORR within CcOs remains an important yet underexplored frontier. Inspired by the Fe–Cu active center, many kinds of bimetallic Co-based molecular catalysts have been reported, which usually exhibit higher 4e⁻ selectivity for the ORR than mononuclear Co molecules. In addition, axial ligands and amino acid residues such as phenolic hydroxyl groups also play crucial roles in determining the catalytic activity and selectivity of the ORR. Therefore, developing bimetallic electrocatalysts with suitable intermetallic distances and surrounding functional groups is a promising direction.

(2) Eliminating or reducing the measurement error of n values caused by the homogeneous and heterogeneous catalyses is conducive to the development of electrocatalysts. Recently, many mononuclear Co-based molecular catalysts have been reported for the ORR, exhibiting remarkable 4e⁻ selectivity. Conventionally, the assessment of ORR selectivity relies on techniques such as RRDE and K–L analysis, often involving the integration of these molecular catalysts with conductive carbon materials. Nevertheless, the inevitable occurrence of molecular aggregation poses a challenge, potentially triggering bimolecular mechanisms that favor 4e⁻ selectivity in the ORR. To mitigate this limitation and achieve a more accurate determination of ORR selectivity, the combination of homogeneous chemical reduction and heterogeneous electrochemical reduction holds great importance. This dual-pronged approach not only provides a better understanding of the underlying mechanisms but also enables the precise evaluation of intrinsic catalytic properties of these Co-based molecular catalysts, unhindered by the complexities introduced by molecular aggregation.

(3) Constructing in situ measurement systems to investigate the electrocatalytic ORR process by capturing reaction intermediates is of great significance.³⁵⁶ These advanced tools enable researchers to gain unparalleled insights into reaction mechanisms that govern the ORR, facilitating a deeper understanding of how molecular catalysts interact with the reaction environment and transform oxygen into reduced products. Currently, more related intermediates such as Co–oxo and Co–O–O–Co species need to be obtained and characterized. By capturing these intermediates in real-time, scientists can understand reaction pathways and rate-limiting steps that dictate the efficiency and selectivity of the ORR. This knowledge is crucial for the rational design and optimization of next-generation electrocatalysts, aiming to achieve higher performance, durability, and cost-effectiveness. For example, many in situ instruments, such as UV-vis spectrophotometer, Raman spectrometer, and infrared spectrometer, had been applied to investigate electrocatalytic reaction processes. Recently, the electrochemical scanning tunneling microscope was applied to study the catalytic OER, ORR and CO₂RR processes, which provides a better understanding of the catalytic mechanism from the molecular level.^357–360 In summary, the development of an in situ measurement system represents a crucial step forward in the field of electrocatalysis.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (22478237, 22178213, 22325202, and 22171176), Innovation Capability Support Program of Shaanxi Province (2023KJXX-018), Fundamental Research Funds for the Central Universities (GK202309002 and GK202306001), and Research Funds of Shaanxi Normal University.

References

1. Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov and T. F. Jaramillo, Science, 2017, 355, eaad4998.
2. Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201.
3. C. Santoro, P. Bollella, B. Erable, P. Atanassov and D. Pant, Nat. Catal., 2022, 5, 473–484.
4. S. Li, L. Shi, Y. Guo, J. Wang, D. Liu and S. Zhao, Chem. Sci., 2024, 15, 11188–11228.
5. L. Yan, P. Li, Q. Zhu, A. Kumar, K. Sun, S. Tian and X. Sun, Chem, 2023, 9, 280–342.
6. X. X. Wang, M. T. Swihart and G. Wu, Nat. Catal., 2019, 2, 578–589.
7. G. Zhang, Z. Qu, W.-Q. Tao, X. Wang, L. Wu, S. Wu, X. Xie, C. Tongsh, W. Huo, Z. Bao, K. Jiao and Y. Wang, Chem. Rev., 2023, 123, 989–1039.
8. J. Fan, M. Chen, Z. Zhao, Z. Zhang, S. Ye, S. Xu, H. Wang and H. Li, Nat. Energy, 2021, 6, 475–486.
9. J. Zhang, H. Zhang, M.-J. Cheng and Q. Lu, Small, 2020, 16, 1902845.
10. E. Luo, T. Yang, J. Liang, Y. Chang, J. Zhang, T. Hu, J. Ge and J. Jia, Nano Res., 2024, 17, 4668–4681.
11. Y. Tian, D. Deng, L. Xu, M. Li, H. Chen, Z. Wu and S. Zhang, Nano-Micro Lett., 2023, 15, 122.
12. S. C. Perry, D. Pangotra, L. Vieira, L.-I. Csepei, V. Sieber, L. Wang, C. Ponce de León and F. C. Walsh, Nat. Rev. Chem., 2019, 3, 442–458.
13. X. Huang, M. Song, J. Zhang, T. Shen, G. Luo and D. Wang, Nano-Micro Lett., 2023, 15, 86.
14. J. M. Campos-Martin, G. Blanco-Brieva and J. L. G. Fierro, Angew. Chem., Int. Ed., 2006, 45, 6962–6984.
15. J. H. Lunsford, J. Catal., 2003, 216, 455–460.
16. J. K. Edwards, S. J. Freakley, A. F. Carley, C. J. Kiely and G. J. Hutchings, Acc. Chem. Res., 2014, 47, 845–854.
17. S. J. Freakley, Q. He, J. H. Harrhy, L. Lu, D. A. Crole, D. J. Morgan, E. N. Ntainjua, J. K. Edwards, A. F. Carley, A. Y. Borisevich, C. J. Kiely and G. J. Hutchings, Science, 2016, 351, 965–968.
18. Y. Pang, H. Xie, Y. Sun, M.-M. Titirici and G.-L. Chai, J. Mater. Chem. A, 2020, 8, 24996–25016.
19. K. Jiang, S. Back, A. J. Akey, C. Xia, Y. Hu, W. Liang, D. Schaak, E. Stavitski, J. K. Nørskov, S. Siahrostami and H. Wang, Nat. Commun., 2019, 10, 3997.
20. N. Wang, X. Zhao, R. Zhang, S. Yu, Z. H. Levell, C. Wang, S. Ma, P. Zou, L. Han, J. Qin, L. Ma, Y. Liu and H. L. Xin, ACS Catal., 2022, 12, 4156–4164.
21. Z. Deng, C. Ma, S. Yan, K. Dong, Q. Liu, Y. Luo, Y. Liu, J. Du, X. Sun and B. Zheng, J. Mater. Chem. A, 2021, 9, 20345–20349.
22. C. Xia, Y. Xia, P. Zhu, L. Fan and H. Wang, Science, 2019, 366, 226–231.
23. X. Zhang, X. Zhao, P. Zhu, Z. Adler, Z.-Y. Wu, Y. Liu and H. Wang, Nat. Commun., 2022, 13, 2880.
24. J.-Y. Zhang, C. Xia, H.-F. Wang and C. Tang, J. Energy Chem., 2022, 67, 432–450.
25. Y. Yamada, Y. Fukunishi, S. Yamazaki and S. Fukuzumi, Chem. Commun., 2010, 46, 7334–7336.
26. D. Y. Chung, J. M. Yoo and Y.-E. Sung, Adv. Mater., 2018, 30, 1704123.
27. R. Lin, X. Cai, H. Zeng and Z. Yu, Adv. Mater., 2018, 30, 1705332.
28. X. Ren, Q. Lv, L. Liu, B. Liu, Y. Wang, A. Liu and G. Wu, Sustainable Energy Fuels, 2020, 4, 15–30.
29. Y.-J. Wang, W. Long, L. Wang, R. Yuan, A. Ignaszak, B. Fang and D. P. Wilkinson, Energy Environ. Sci., 2018, 11, 258–275.
30. L. Huang, H. Niu, C. Xia, F. M. Li, Z. Shahid and B. Y. Xia, Adv. Mater., 2024, 36, 2404773.
31. X. Wang, Z. Li, Y. Qu, T. Yuan, W. Wang, Y. Wu and Y. Li, Chem, 2019, 5, 1486–1511.
32. N. Tian, B.-A. Lu, X.-D. Yang, R. Huang, Y.-X. Jiang, Z.-Y. Zhou and S.-G. Sun, Electrochem. Energy Rev., 2018, 1, 54–83.
33. Z. Li, B. Li, Y. Hu, S. Wang and C. Yu, Mater. Adv., 2022, 3, 779–809.
34. A. Han, Z. Zhang, J. Yang, D. Wang and Y. Li, Small, 2021, 17, e2004500.
35. Z. Liang, H. Zheng and R. Cao, ChemElectroChem, 2019, 6, 2600–2614.
36. Z. Miao, S. Li, C. Priest, T. Wang, G. Wu and Q. Li, Adv. Mater., 2022, 34, 2200595.
37. Y. Deng, J. Luo, B. Chi, H. Tang, J. Li, X. Qiao, Y. Shen, Y. Yang, C. Jia, P. Rao, S. Liao and X. Tian, Adv. Energy Mater., 2021, 11, 2101222.
38. M.-Y. Yu, Y.-F. Yao, K. Fang, L.-S. Chen, L.-P. Si and H.-Y. Liu, ACS Appl. Mater. Interfaces, 2024, 16, 16132–16144.
39. Y. Liu, M. Wang, Z. Liang and H. Zheng, Sci. China: Chem., 2024, 67, 3209–3222.
40. Y. Wang, T. Yang, X. Fan, Z. Bao, A. Tayal, H. Tan, M. Shi, Z. Liang, W. Zhang, H. Lin, R. Cao, Z. Huang and H. Zheng, Angew. Chem., Int. Ed., 2024, 63, e202313034.
41. Y. Wang, J. Li and Z. Wei, J. Mater. Chem. A, 2018, 6, 8194–8209.
42. Y. Wang, Y. Han, W. Suo, J. Zhang, X. Lai, Z. Li, Z. Liang and G. Cao, Sci. China Mater., 2023, 66, 4357–4366.
43. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780–786.
44. H. Li, S. Kelly, D. Guevarra, Z. Wang, Y. Wang, J. A. Haber, M. Anand, G. T. K. K. Gunasooriya, C. S. Abraham, S. Vijay, J. M. Gregoire and J. K. Nørskov, Nat. Catal., 2021, 4, 463–468.
45. J. Kim, W. Ko, J. M. Yoo, V. K. Paidi, H. Y. Jang, M. Shepit, J. Lee, H. Chang, H. S. Lee, J. Jo, B. H. Kim, S.-P. Cho, J. van Lierop, D. Kim, K.-S. Lee, S. Back, Y.-E. Sung and T. Hyeon, Adv. Mater., 2022, 34, 2107868.
46. J. Gu, Y. Peng, T. Zhou, J. Ma, H. Pang and Y. Yamauchi, Nano Res. Energy, 2022, 1, e9120009.
47. Z. Liang, H.-Y. Wang, H. Zheng, W. Zhang and R. Cao, Chem. Soc. Rev., 2021, 50, 2540–2581.
48. Y. Cao, Y. Mou, J. Zhang, R. Zhang and Z. Liang, Mater. Today Catal., 2024, 4, 100044.
49. H. Sun, H. Awada, H. Lei, A. Aljabour, L. Song, S. Offenthaler, R. Cao and W. Schöfberger, Mater. Today Catal., 2024, 4, 100038.
50. R. Z. Snitkoff-Sol, O. Rimon, A. M. Bond and L. Elbaz, Nat. Catal., 2024, 7, 139–147.
51. Y. Wei, L. Zhao, Y. Liang, M. Yuan, Q. Wu, Z. Xue, X. Qiu and J. Zhang, Chem. Eng. J., 2023, 476, 146575.
52. H. Zarenezhad, S. Mahini, A. Rezaei, S. Aber, A. Khataee and R. Teimuri-Mofrad, Chem. Eng. J., 2024, 495, 153540.
53. L. Wei, M. D. Hossain, G. Chen, G. A. Kamat, M. E. Kreider, J. Chen, K. Yan, Z. Bao, M. Bajdich, M. B. Stevens and T. F. Jaramillo, J. Am. Chem. Soc., 2024, 146, 13377–13390.
54. N. Lv, Q. Li, H. Zhu, S. Mu, X. Luo, X. Ren, X. Liu, S. Li, C. Cheng and T. Ma, Adv. Sci., 2023, 10, 2206239.
55. Z. Liang, G. Zhou, H. Tan, Y. Mou, J. Zhang, H. Guo, S. Yang, H. Lei, H. Zheng, W. Zhang, H. Lin and R. Cao, Adv. Mater., 2024, 36, 2408094.
56. Y. Shao, J.-P. Dodelet, G. Wu and P. Zelenay, Adv. Mater., 2019, 31, 1807615.
57. Z. Liang, H. Zheng and R. Cao, Sustainable Energy Fuels, 2020, 4, 3848–3870.
58. S. Zaman, L. Huang, A. I. Douka, H. Yang, B. You and B. Y. Xia, Angew. Chem., Int. Ed., 2021, 60, 17832–17852.
59. A. Byeon, W. C. Yun, J. M. Kim and J. W. Lee, ChemElectroChem, 2023, 10, e202300234.
60. Y. Jiang, P. Ni, C. Chen, Y. Lu, P. Yang, B. Kong, A. Fisher and X. Wang, Adv. Energy Mater., 2018, 8, 1801909.
61. N. Wang, S. Ma, P. Zuo, J. Duan and B. Hou, Adv. Sci., 2021, 8, e2100076.
62. Y.-H. Wang, B. Mondal and S. S. Stahl, ACS Catal., 2020, 10, 12031–12039.
63. Y. Bu, Y. Wang, G.-F. Han, Y. Zhao, X. Ge, F. Li, Z. Zhang, Q. Zhong and J.-B. Baek, Adv. Mater., 2021, 33, 2103266.
64. Z.-y Sang, F. Hou, S.-h Wang and J. Liang, New Carbon Mater., 2022, 37, 136–151.
65. C. Yang, F. Sun, Z. Qu, X. Li, W. Zhou and J. Gao, ACS Energy Lett., 2022, 7, 4398–4407.
66. Y. Pang, K. Wang, H. Xie, Y. Sun, M.-M. Titirici and G.-L. Chai, ACS Catal., 2020, 10, 7434–7442.
67. Y. Guo, R. Zhang, S. Zhang, H. Hong, Y. Zhao, Z. Huang, C. Han, H. Li and C. Zhi, Energy Environ. Sci., 2022, 15, 4167–4174.
68. P. Cui, L. Zhao, Y. Long, L. Dai and C. Hu, Angew. Chem., Int. Ed., 2023, 62, e202218269.
69. M. Li, B. Yin, C. Gao, J. Guo, C. Zhao, C. Jia and X. Guo, Exploration, 2023, 3, 20210233.
70. H. W. Kim, M. B. Ross, N. Kornienko, L. Zhang, J. Guo, P. Yang and B. D. McCloskey, Nat. Catal., 2018, 1, 282–290.
71. J. P. Collman, N. K. Devaraj, R. A. Decréau, Y. Yang, Y.-L. Yan, W. Ebina, T. A. Eberspacher and C. E. D. Chidsey, Science, 2007, 315, 1565–1568.
72. S. Fukuzumi, Y.-M. Lee and W. Nam, ChemCatChem, 2018, 10, 9–28.
73. Y. Li, N. Wang, H. Lei, X. Li, H. Zheng, H. Wang, W. Zhang and R. Cao, Coord. Chem. Rev., 2021, 442, 213996.
74. X.-P. Zhang, A. Chandra, Y.-M. Lee, R. Cao, K. Ray and W. Nam, Chem. Soc. Rev., 2021, 50, 4804–4811.
75. A. Kumar, S. Ibraheem, T. Anh Nguyen, R. K. Gupta, T. Maiyalagan and G. Yasin, Coord. Chem. Rev., 2021, 446, 214122.
76. M. Wikström, K. Krab and V. Sharma, Chem. Rev., 2018, 118, 2469–2490.
77. M. R. A. Blomberg, P. E. M. Siegbahn, G. T. Babcock and M. Wikström, J. Am. Chem. Soc., 2000, 122, 12848–12858.
78. S. Iwata, C. Ostermeier, B. Ludwig and H. Michel, Nature, 1995, 376, 660–669.
79. W. Zhang, W. Lai and R. Cao, Chem. Rev., 2017, 117, 3717–3797.
80. S. Fukuzumi, K. Okamoto, C. P. Gros and R. Guilard, J. Am. Chem. Soc., 2004, 126, 10441–10449.
81. H. Wan, A. W. Jensen, M. Escudero-Escribano and J. Rossmeisl, ACS Catal., 2020, 10, 5979–5989.
82. J. S. Lim, Y. J. Sa and S. H. Joo, Cell Rep. Phys. Sci., 2022, 3, 100987.
83. X. Sun, J. Yang, X. Zeng, L. Guo, C. Bie, Z. Wang, K. Sun, A. K. Sahu, M. Tebyetekerwa, T. E. Rufford and X. Zhang, Angew. Chem., Int. Ed., 2024, 63, e202414417.
84. M. Dan, R. Zhong, S. Hu, H. Wu, Y. Zhou and Z.-Q. Liu, Chem. Catal., 2022, 2, 1919–1960.
85. L. Cui, B. Chen, L. Zhang, C. He, C. Shu, H. Kang, J. Qiu, W. Jing, K. Ostrikov and Z. Zhang, Energy Environ. Sci., 2024, 17, 655–667.
86. L. Cui, B. Chen, D. Chen, C. He, Y. Liu, H. Zhang, J. Qiu, L. Liu, W. Jing and Z. Zhang, Nat. Commun., 2024, 15, 10632.
87. Y. Zeng and G. Wu, Chin. J. Catal., 2021, 42, 2149–2163.
88. Z. Tian, Q. Zhang, L. Thomsen, N. Gao, J. Pan, R. Daiyan, J. Yun, J. Brandt, N. López-Salas, F. Lai, Q. Li, T. Liu, R. Amal, X. Lu and M. Antonietti, Angew. Chem., Int. Ed., 2022, 61, e202206915.
89. Y. Long, J. Lin, F. Ye, W. Liu, D. Wang, Q. Cheng, R. Paul, D. Cheng, B. Mao, R. Yan, L. Zhao, D. Liu, F. Liu and C. Hu, Adv. Mater., 2023, 35, 2303905.
90. Y. Xia, X. Zhao, C. Xia, Z.-Y. Wu, P. Zhu, J. Y. Kim, X. Bai, G. Gao, Y. Hu, J. Zhong, Y. Liu and H. Wang, Nat. Commun., 2021, 12, 4225.
91. Z. Zhang, W. Chen, H. K. Chu, F. Xiong, K. Zhang, H. Yan, F. Meng, S. Gao, B. Ma, X. Hai and R. Zou, Angew. Chem., Int. Ed., 2024, 63, e202410123.
92. H. Xu, S. Zhang, X. Zhang, M. Xu, M. Han, L. R. Zheng, Y. Zhang, G. Wang, H. Zhang and H. Zhao, Angew. Chem., Int. Ed., 2023, 62, e202314414.
93. C. Xiao, L. Cheng, Y. Zhu, G. Wang, L. Chen, Y. Wang, R. Chen, Y. Li and C. Li, Angew. Chem., Int. Ed., 2022, 61, e202206544.
94. M. Yan, Z. Wei, Z. Gong, B. Johannessen, G. Ye, G. He, J. Liu, S. Zhao, C. Cui and H. Fei, Nat. Commun., 2023, 14, 368.
95. Y. Gu, Y. Tan, H. Tan, Y. Han, D. Cheng, F. Lin, Z. Qian, L. Zeng, S. Zhang, R. Zeng, Y. Liu, H. Guo, M. Luo and S. Guo, Nat. Synth., 2025 DOI:10.1038/s44160-024-00722-2.
96. X. Yang, Y. Zeng, W. Alnoush, Y. Hou, D. Higgins and G. Wu, Adv. Mater., 2022, 34, 2107954.
97. J. P. Collman, R. Boulatov, C. J. Sunderland and L. Fu, Chem. Rev., 2004, 104, 561–588.
98. J. M. Park, K.-I. Hong, H. Lee and W.-D. Jang, Acc. Chem. Res., 2021, 54, 2249–2260.
99. J. H. Zagal, M. Páez and C. Páez, J. Electroanal. Chem., 1987, 237, 145–148.
100. J. H. Zagal, M. J. Aguirre and M. A. Páez, J. Electroanal. Chem., 1997, 437, 45–52.
101. Q. Qiu and S. Dong, Electrochim. Acta, 1993, 38, 2297–2303.
102. X. Li, H. Lei, L. Xie, N. Wang, W. Zhang and R. Cao, Acc. Chem. Res., 2022, 55, 878–892.
103. C. W. Machan, ACS Catal., 2020, 10, 2640–2655.
104. M. L. Pegis, C. F. Wise, D. J. Martin and J. M. Mayer, Chem. Rev., 2018, 118, 2340–2391.
105. S. Bhunia, A. Rana, P. Roy, D. J. Martin, M. L. Pegis, B. Roy and A. Dey, J. Am. Chem. Soc., 2018, 140, 9444–9457.
106. L. L. Chng, C. J. Chang and D. G. Nocera, Org. Lett., 2003, 5, 2421–2424.
107. D. J. Martin and J. M. Mayer, J. Am. Chem. Soc., 2021, 143, 11423–11434.
108. D. J. Martin, B. Q. Mercado and J. M. Mayer, Sci. Adv., 2020, 6, eaaz3318.
109. D. J. Martin, C. F. Wise, M. L. Pegis and J. M. Mayer, Acc. Chem. Res., 2020, 53, 1056–1065.
110. B. D. Matson, C. T. Carver, A. Von Ruden, J. Y. Yang, S. Raugei and J. M. Mayer, Chem. Commun., 2012, 48, 11100–11102.
111. L. Xie, X.-P. Zhang, B. Zhao, P. Li, J. Qi, X. Guo, B. Wang, H. Lei, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2021, 60, 7576–7581.
112. I. Liberman, R. Shimoni, R. Ifraemov, I. Rozenberg, C. Singh and I. Hod, J. Am. Chem. Soc., 2020, 142, 1933–1940.
113. C. Costentin, H. Dridi and J.-M. Savéant, J. Am. Chem. Soc., 2015, 137, 13535–13544.
114. C. J. Chang, L. L. Chng and D. G. Nocera, J. Am. Chem. Soc., 2003, 125, 1866–1876.
115. S. Hiroto, Y. Miyake and H. Shinokubo, Chem. Rev., 2017, 117, 2910–3043.
116. Y. Wei, Y. Liang, Q. Wu, Z. Xue, L. Feng, J. Zhang and L. Zhao, Dalton Trans., 2023, 52, 14573–14582.
117. D. Nishiori, J. P. Menzel, N. Armada, E. A. Reyes Cruz, B. L. Nannenga, V. S. Batista and G. F. Moore, J. Am. Chem. Soc., 2024, 146, 11622–11633.
118. R. Yuan, S. L. George, J. Chen, Q. Wu, X. Qiu and L. Zhao, ChemNanoMat, 2023, 9, e202300027.
119. L. Xie, J. Tian, Y. Ouyang, X. Guo, W. Zhang, U.-P. Apfel, W. Zhang and R. Cao, Angew. Chem., Int. Ed., 2020, 59, 15844–15848.
120. X. Guo, N. Wang, X. Li, Z. Zhang, J. Zhao, W. Ren, S. Ding, G. Xu, J. Li, U.-P. Apfel, W. Zhang and R. Cao, Angew. Chem., Int. Ed., 2020, 59, 8941–8946.
121. H. Lei, Y. Wang, Q. Zhang and R. Cao, J. Porphyrins phthalocyanines, 2020, 24, 1361–1371.
122. N. Wang, X.-P. Zhang, J. Han, H. Lei, Q. Zhang, H. Zhang, W. Zhang, U.-P. Apfel and R. Cao, Chin. J. Catal., 2023, 45, 88–94.
123. X. Peng, M. Zhang, H. Qin, J. Han, Y. Xu, W. Li, X.-P. Zhang, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2024, 63, e202401074.
124. Q. Zhang, H. Lei, H. Guo, Y. Wang, Y. Gao, W. Zhang and R. Cao, ChemSusChem, 2022, 15, e202200086.
125. X. Peng, J. Han, X. Li, G. Liu, Y. Xu, Y. Peng, S. Nie, W. Li, X. Li, Z. Chen, H. Peng, R. Cao and Y. Fang, Chem. Commun., 2023, 59, 10777–10780.
126. H. Lv, X.-P. Zhang, K. Guo, J. Han, H. Guo, H. Lei, X. Li, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2023, 62, e202305938.
127. Y. Liu, Y. Han, Z. Zhang, W. Zhang, W. Lai, Y. Wang and R. Cao, Chem. Sci., 2019, 10, 2613–2622.
128. J. Kong, H. Qin, L. Yang, J. Zhang, Y. Peng, Y. Gao, Y. Wu, W. Nam and R. Cao, ChemPhysChem, 2024, 25, e202400017.
129. P. Zhang, J. Hu, B. Liu, J. Yang and H. Hou, Chemosphere, 2019, 219, 617–635.
130. J. Zheng, D. Zhou, J. Han, J. Liu, R. Cao, H. Lei, H. Bian and Y. Fang, J. Phys. Chem. Lett., 2022, 13, 11811–11817.
131. J. Han, N. Wang, X. Li, H. Lei, Y. Wang, H. Guo, X. Jin, Q. Zhang, X. Peng, X.-P. Zhang, W. Zhang, U.-P. Apfel and R. Cao, eScience, 2022, 2, 623–631.
132. H. Guo, Z. Liang, K. Guo, H. Lei, Y. Wang, W. Zhang and R. Cao, Chin. J. Catal., 2022, 43, 3089–3094.
133. K. Guo, X. Li, H. Lei, H. Guo, X. Jin, X.-P. Zhang, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2022, 61, e202209602.
134. K. Guo, H. Lei, X. Li, Z. Zhang, Y. Wang, H. Guo, W. Zhang and R. Cao, Chin. J. Catal., 2021, 42, 1439–1444.
135. H. Lei, Q. Zhang, Y. Wang, Y. Gao, Y. Wang, Z. Liang, W. Zhang and R. Cao, Dalton Trans., 2021, 50, 5120–5123.
136. H. Qin, Y. Wang, B. Wang, X. Duan, H. Lei, X. Zhang, H. Zheng, W. Zhang and R. Cao, J. Energy Chem., 2021, 53, 77–81.
137. K. Dong, J. Liang, Y. Ren, Y. Wang, Z. Xu, L. Yue, T. Li, Q. Liu, Y. Luo, Y. Liu, S. Gao, M. S. Hamdy, Q. Li, D. Ma and X. Sun, J. Mater. Chem. A, 2021, 9, 26019–26027.
138. Y. Zhou, Y.-F. Xing, J. Wen, H.-B. Ma, F.-B. Wang and X.-H. Xia, Sci. Bull., 2019, 64, 1158–1166.
139. S. Fukuzumi, S. Mochizuki and T. Tanaka, Inorg. Chem., 1989, 28, 2459–2465.
140. L. Ye, Y. Fang, Z. Ou, S. Xue and K. M. Kadish, Inorg. Chem., 2017, 56, 13613–13626.
141. B. Steiger and F. C. Anson, Inorg. Chem., 1997, 36, 4138–4140.
142. R. J. H. Chan, Y. O. Su and T. Kuwana, Inorg. Chem., 1985, 24, 3777–3784.
143. C. Shi and F. C. Anson, Inorg. Chem., 1992, 31, 5078–5083.
144. F. D'Souza, R. G. Deviprasad and Y.-Y. Hsieh, J. Electroanal. Chem., 1996, 411, 167–171.
145. F. D'Souza, Y.-Y. Hsieh and G. R. Deviprasad, J. Electroanal. Chem., 1997, 426, 17–21.
146. Y. Chen, C. Zhen, Y. Chen, H. Zhao, Y. Wang, Z. Yue, Q. Wang, J. Li, M. D. Gu, Q. Cheng and H. Yang, Angew. Chem., Int. Ed., 2024, 63, e202407163.
147. H. Lv, H. Guo, K. Guo, H. Lei, W. Zhang, H. Zheng, Z. Liang and R. Cao, Chin. Chem. Lett., 2021, 32, 2841–2845.
148. H. Guo, Y. Wang, K. Guo, H. Lei, Z. Liang, X.-P. Zhang and R. Cao, J. Electrochem., 2022, 28, 2214002.
149. Y. Mou, J. Zhang, H. Qin, X. Li, Z. Zeng, R. Zhang, Z. Liang and R. Cao, Chem. Commun., 2025, 61, 1878–1881.
150. R. Kumar and M. Sankar, Inorg. Chem., 2014, 53, 12706–12719.
151. J. P. Collman, M. Marrocco, P. Denisevich, C. Koval and F. C. Anson, J. Electroanal. Chem., 1979, 101, 117–122.
152. R. R. Durand and F. C. Anson, J. Electroanal. Chem., 1982, 134, 273–289.
153. E. Song, C. Shi and F. C. Anson, Langmuir, 1998, 14, 4315–4321.
154. B. Su, I. Hatay, A. Trojánek, Z. Samec, T. Khoury, C. P. Gros, J.-M. Barbe, A. Daina, P.-A. Carrupt and H. H. Girault, J. Am. Chem. Soc., 2010, 132, 2655–2662.
155. B. Bosnich, C. K. Poon and M. L. Tobe, Inorg. Chem., 1965, 4, 1102–1108.
156. G. McLendon and M. Mason, Inorg. Chem., 1978, 17, 362–365.
157. J. F. Endicott, K. P. Balakrishnan and C.-L. Wong, J. Am. Chem. Soc., 1980, 102, 5519–5526.
158. T. Geiger and F. C. Anson, J. Am. Chem. Soc., 1981, 103, 7489–7496.
159. Y.-H. Wang, M. L. Pegis, J. M. Mayer and S. S. Stahl, J. Am. Chem. Soc., 2017, 139, 16458–16461.
160. Y.-H. Wang, Z. K. Goldsmith, P. E. Schneider, C. W. Anson, J. B. Gerken, S. Ghosh, S. Hammes-Schiffer and S. S. Stahl, J. Am. Chem. Soc., 2018, 140, 10890–10899.
161. K. Mase, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 2800–2808.
162. K. Mase, K. Ohkubo and S. Fukuzumi, Inorg. Chem., 2015, 54, 1808–1815.
163. A. Rana, Y.-M. Lee, X. Li, R. Cao, S. Fukuzumi and W. Nam, ACS Catal., 2021, 11, 3073–3083.
164. S. Yang, Y. Yu, X. Gao, Z. Zhang and F. Wang, Chem. Soc. Rev., 2021, 50, 12985–13011.
165. Y. Ma, J. Li, X. Liao, W. Luo, W. Huang, J. Meng, Q. Chen, S. Xi, R. Yu, Y. Zhao, L. Zhou and L. Mai, Adv. Funct. Mater., 2020, 30, 2005000.
166. B.-H. Lee, H. Shin, A. S. Rasouli, H. Choubisa, P. Ou, R. Dorakhan, I. Grigioni, G. Lee, E. Shirzadi, R. K. Miao, J. Wicks, S. Park, H. S. Lee, J. Zhang, Y. Chen, Z. Chen, D. Sinton, T. Hyeon, Y.-E. Sung and E. H. Sargent, Nat. Catal., 2023, 6, 234–243.
167. R. Jasinski, Nature, 1964, 201, 1212–1213.
168. J. Zagal, R. K. Sen and E. Yeager, J. Electroanal. Chem., 1977, 83, 207–213.
169. T. Honda, T. Kojima and S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 4196–4206.
170. P. T. Smith, Y. Kim, B. P. Benke, K. Kim and C. J. Chang, Angew. Chem., Int. Ed., 2020, 59, 4902–4907.
171. R. D. Mukhopadhyay, Y. Kim, J. Koo and K. Kim, Acc. Chem. Res., 2018, 51, 2730–2738.
172. X. Zhao, Q. Yin, X. Mao, C. Cheng, L. Zhang, L. Wang, T.-F. Liu, Y. Li and Y. Li, Nat. Commun., 2022, 13, 2721.
173. Q. Zhi, R. Jiang, X. Yang, Y. Jin, D. Qi, K. Wang, Y. Liu and J. Jiang, Nat. Commun., 2024, 15, 678.
174. Q. Zhi, W. Liu, R. Jiang, X. Zhan, Y. Jin, X. Chen, X. Yang, K. Wang, W. Cao, D. Qi and J. Jiang, J. Am. Chem. Soc., 2022, 144, 21328–21336.
175. B. Qiao, A. Wang, X. Yang, L. F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li and T. Zhang, Nat. Chem., 2011, 3, 634–641.
176. J. Yang, W. Li, D. Wang and Y. Li, Adv. Mater., 2020, 32, 2003300.
177. R. Lang, X. Du, Y. Huang, X. Jiang, Q. Zhang, Y. Guo, K. Liu, B. Qiao, A. Wang and T. Zhang, Chem. Rev., 2020, 120, 11986–12043.
178. Z. Liang, X. Fan, H. Lei, J. Qi, Y. Li, J. Gao, M. Huo, H. Yuan, W. Zhang, H. Lin, H. Zheng and R. Cao, Angew. Chem., Int. Ed., 2018, 57, 13187–13191.
179. Z. Liang, C. Zhang, H. Yuan, W. Zhang, H. Zheng and R. Cao, Chem. Commun., 2018, 54, 7519–7522.
180. Y. Chen, S. Ji, C. Chen, Q. Peng, D. Wang and Y. Li, Joule, 2018, 2, 1242–1264.
181. Y. He, S. Hwang, D. A. Cullen, M. A. Uddin, L. Langhorst, B. Li, S. Karakalos, A. J. Kropf, E. C. Wegener, J. Sokolowski, M. Chen, D. Myers, D. Su, K. L. More, G. Wang, S. Litster and G. Wu, Energy Environ. Sci., 2019, 12, 250–260.
182. X. X. Wang, D. A. Cullen, Y.-T. Pan, S. Hwang, M. Wang, Z. Feng, J. Wang, M. H. Engelhard, H. Zhang, Y. He, Y. Shao, D. Su, K. L. More, J. S. Spendelow and G. Wu, Adv. Mater., 2018, 30, 1706758.
183. F. Wu, C. Pan, C.-T. He, Y. Han, W. Ma, H. Wei, W. Ji, W. Chen, J. Mao, P. Yu, D. Wang, L. Mao and Y. Li, J. Am. Chem. Soc., 2020, 142, 16861–16867.
184. Z. Liang, N. Kong, C. Yang, W. Zhang, H. Zheng, H. Lin and R. Cao, Angew. Chem., Int. Ed., 2021, 60, 12759–12764.
185. Z. Lin, Q. Zhang, J. Pan, C. Tsounis, A. A. Esmailpour, S. Xi, H. Y. Yang, Z. Han, J. Yun, R. Amal and X. Lu, Energy Environ. Sci., 2022, 15, 1172–1182.
186. J. Gao, H. b Yang, X. Huang, S.-F. Hung, W. Cai, C. Jia, S. Miao, H. M. Chen, X. Yang, Y. Huang, T. Zhang and B. Liu, Chem, 2020, 6, 658–674.
187. S. Chen, T. Luo, X. Li, K. Chen, J. Fu, K. Liu, C. Cai, Q. Wang, H. Li, Y. Chen, C. Ma, L. Zhu, Y.-R. Lu, T.-S. Chan, M. Zhu, E. Cortés and M. Liu, J. Am. Chem. Soc., 2022, 144, 14505–14516.
188. E. Jung, H. Shin, B.-H. Lee, V. Efremov, S. Lee, H. S. Lee, J. Kim, W. Hooch Antink, S. Park, K.-S. Lee, S.-P. Cho, J. S. Yoo, Y.-E. Sung and T. Hyeon, Nat. Mater., 2020, 19, 436–442.
189. Q. Zhao, Y. Wang, W.-H. Lai, F. Xiao, Y. Lyu, C. Liao and M. Shao, Energy Environ. Sci., 2021, 14, 5444–5456.
190. Y. Tian, M. Li, Z. Wu, Q. Sun, D. Yuan, B. Johannessen, L. Xu, Y. Wang, Y. Dou, H. Zhao and S. Zhang, Angew. Chem., Int. Ed., 2022, 61, e202213296.
191. J. Hu, W. Shang, C. Xin, J. Guo, X. Cheng, S. Zhang, S. Song, W. Liu, F. Ju, J. Hou and Y. Shi, Angew. Chem., Int. Ed., 2023, 62, e202304754.
192. L. Liu, L. Kang, J. Feng, D. G. Hopkinson, C. S. Allen, Y. Tan, H. Gu, I. Mikulska, V. Celorrio, D. Gianolio, T. Wang, L. Zhang, K. Li, J. Zhang, J. Zhu, G. Held, P. Ferrer, D. Grinter, J. Callison, M. Wilding, S. Chen, I. Parkin and G. He, Nat. Commun., 2024, 15, 4079.
193. C. Tang, L. Chen, H. Li, L. Li, Y. Jiao, Y. Zheng, H. Xu, K. Davey and S.-Z. Qiao, J. Am. Chem. Soc., 2021, 143, 7819–7827.
194. S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D. Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E. A. Paoli, R. Frydendal, T. W. Hansen, I. Chorkendorff, I. E. L. Stephens and J. Rossmeisl, Nat. Mater., 2013, 12, 1137–1143.
195. A. Verdaguer-Casadevall, D. Deiana, M. Karamad, S. Siahrostami, P. Malacrida, T. W. Hansen, J. Rossmeisl, I. Chorkendorff and I. E. L. Stephens, Nano Lett., 2014, 14, 1603–1608.
196. K. Sun, R. Lu, Y. Liu, J. Webb, M. Hanif, Y. Zhao, Z. Wang and G. I. N. Waterhouse, Angew. Chem., Int. Ed., 2025, 64, e202416070.
197. B.-Q. Li, C.-X. Zhao, J.-N. Liu and Q. Zhang, Adv. Mater., 2019, 31, 1808173.
198. Q. Zhang, X. Tan, N. M. Bedford, Z. Han, L. Thomsen, S. Smith, R. Amal and X. Lu, Nat. Commun., 2020, 11, 4181.
199. X. Wang, L. Xu, C. Li, C. Zhang, H. Yao, R. Xu, P. Cui, X. Zheng, M. Gu, J. Lee, H. Jiang and M. Huang, Nat. Commun., 2023, 14, 7210.
200. Y. Li, Y. Li, H. Sun, L. Gao, X. Jin, Y. Li, Z. Lv, L. Xu, W. Liu and X. Sun, Nano-Micro Lett., 2024, 16, 139.
201. M. Xiao, H. Zhang, Y. Chen, J. Zhu, L. Gao, Z. Jin, J. Ge, Z. Jiang, S. Chen, C. Liu and W. Xing, Nano Energy, 2018, 46, 396–403.
202. H. Huang, M. Sun, S. Li, S. Zhang, Y. Lee, Z. Li, J. Fang, C. Chen, Y.-X. Zhang, Y. Wu, Y. Che, S. Qian, W. Zhu, C. Tang, Z. Zhuang, L. Zhang and Z. Niu, J. Am. Chem. Soc., 2024, 146, 9434–9443.
203. C. Zhang, L. Yuan, C. Liu, Z. Li, Y. Zou, X. Zhang, Y. Zhang, Z. Zhang, G. Wei and C. Yu, J. Am. Chem. Soc., 2023, 145, 7791–7799.
204. J. Du, G. Han, W. Zhang, L. Li, Y. Yan, Y. Shi, X. Zhang, L. Geng, Z. Wang, Y. Xiong, G. Yin and C. Du, Nat. Commun., 2023, 14, 4766.
205. D. Qi, J. Xu, Y. Zhou, H. Zhang, J. Shi, K. He, Y. Yuan, J. Luo, S. Wang and Y. Wang, Angew. Chem., Int. Ed., 2023, 62, e202307355.
206. X.-L. Zhang, X. Su, Y.-R. Zheng, S.-J. Hu, L. Shi, F.-Y. Gao, P.-P. Yang, Z.-Z. Niu, Z.-Z. Wu, S. Qin, R. Wu, Y. Duan, C. Gu, X.-S. Zheng, J.-F. Zhu and M.-R. Gao, Angew. Chem., Int. Ed., 2021, 60, 26922–26931.
207. J. Rosenthal and D. G. Nocera, Acc. Chem. Res., 2007, 40, 543–553.
208. D. C. Haines, D. R. Tomchick, M. Machius and J. A. Peterson, Biochemistry, 2001, 40, 13456–13465.
209. C.-Y. Yeh, C. J. Chang and D. G. Nocera, J. Am. Chem. Soc., 2001, 123, 1513–1514.
210. C. J. Chang, C.-Y. Yeh and D. G. Nocera, J. Org. Chem., 2002, 67, 1403–1406.
211. R. McGuire Jr., D. K. Dogutan, T. S. Teets, J. Suntivich, Y. Shao-Horn and D. G. Nocera, Chem. Sci., 2010, 1, 411–414.
212. D. K. Dogutan, S. A. Stoian, R. McGuire, M. Schwalbe, T. S. Teets and D. G. Nocera, J. Am. Chem. Soc., 2011, 133, 131–140.
213. X. Li, B. Lv, X.-P. Zhang, X. Jin, K. Guo, D. Zhou, H. Bian, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2022, 61, e202114310.
214. J. Han, N. Wang, X. Li, W. Zhang and R. Cao, J. Phys. Chem. C, 2021, 125, 24805–24813.
215. S. Sinha, M. Ghosh and J. J. Warren, ACS Catal., 2019, 9, 2685–2691.
216. S. Sinha, M. S. Aaron, J. Blagojevic and J. J. Warren, Chem. – Eur. J., 2015, 21, 18072–18075.
217. R. Zhang and J. J. Warren, J. Am. Chem. Soc., 2020, 142, 13426–13434.
218. Y. Gao, H. Lei, Z. Bao, X. Liu, L. Qin, Z. Yin, H. Li, S. Huang, W. Zhang and R. Cao, Phys. Chem. Chem. Phys., 2023, 25, 4604–4610.
219. Y. Gao, H. Lei, H. Guo, J. Meng, Q. Zhang, Q. Zhao, J. Li, Z. Zhou, W. Feng, W. Zhang and R. Cao, J. Porphyrins phthalocyanines, 2023, 27, 719–727.
220. C. Shi and F. C. Anson, Inorg. Chem., 1996, 35, 7928–7931.
221. J. P. Collman, J. I. Brauman, K. M. Doxsee, T. R. Halbert, S. E. Hayes and K. S. Suslick, J. Am. Chem. Soc., 1978, 100, 2761–2766.
222. B. Lv, X. Li, K. Guo, J. Ma, Y. Wang, H. Lei, F. Wang, X. Jin, Q. Zhang, W. Zhang, R. Long, Y. Xiong, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2021, 60, 12742–12746.
223. X. Chen, Y. Dai, H. Zhang and X. Zhao, Colloids Surf., A, 2023, 663, 131091.
224. T. Liu, H. Qin, Y. Xu, X. Peng, W. Zhang and R. Cao, ACS Catal., 2024, 14, 6644–6649.
225. Y. Xu, X. Jin, J. Zhang, H. Qin, B. Mei, T. Liu, S. Lei, S. Li, Y. Bo, X. Li and R. Cao, ACS Catal., 2024, 14, 14350–14355.
226. Y. Zhao, Z.-Y. Huang, W.-Y. Xie, S.-J. Huang, B. Wan, W.-C. Chen, H.-Y. Liu and L.-P. Si, Nanotechnology, 2023, 34, 495603.
227. Q. Zhu, C. L. Rooney, H. Shema, C. Zeng, J. A. Panetier, E. Gross, H. Wang and L. R. Baker, Nat. Catal., 2024, 7, 987–999.
228. H. Lei, C. Liu, Z. Wang, Z. Zhang, M. Zhang, X. Chang, W. Zhang and R. Cao, ACS Catal., 2016, 6, 6429–6437.
229. X. Li, H. Qin, J. Han, X. Jin, Y. Xu, S. Yang, W. Zhang and R. Cao, Adv. Funct. Mater., 2024, 34, 2310820.
230. X. Li, H. Lei, J. Liu, X. Zhao, S. Ding, Z. Zhang, X. Tao, W. Zhang, W. Wang, X. Zheng and R. Cao, Angew. Chem., Int. Ed., 2018, 57, 15070–15075.
231. X. Jin, X. Li, H. Lei, K. Guo, B. Lv, H. Guo, D. Chen, W. Zhang and R. Cao, J. Energy Chem., 2021, 63, 659–666.
232. M. Tasleem, M. Yadav, V. Ganesan and M. Sankar, Langmuir, 2023, 39, 8075–8082.
233. W. Zhang, A. U. Shaikh, E. Y. Tsui and T. M. Swager, Chem. Mater., 2009, 21, 3234–3241.
234. S. K. Kim and S. Jeon, Electrochem. Commun., 2012, 22, 141–144.
235. J. Meng, H. Lei, X. Li, J. Qi, W. Zhang and R. Cao, ACS Catal., 2019, 9, 4551–4560.
236. R. Ren, X. Wang, H. Chen, H. A. Miller, I. Salam, J. R. Varcoe, L. Wu, Y. Chen, H.-G. Liao, E. Liu, F. Bartoli, F. Vizza, Q. Jia and Q. He, Angew. Chem., Int. Ed., 2021, 60, 4049–4054.
237. N. Liu, W. R. Osterloh, H. Huang, X. Tang, P. Mei, D. Kuzuhara, Y. Fang, J. Pan, H. Yamada, F. Qiu, K. M. Kadish and S. Xue, Inorg. Chem., 2022, 61, 3563–3572.
238. C. J. Kingsbury and M. O. Senge, Coord. Chem. Rev., 2021, 431, 213760.
239. T. Ishizuka, N. Grover, C. J. Kingsbury, H. Kotani, M. O. Senge and T. Kojima, Chem. Soc. Rev., 2022, 51, 7560–7630.
240. S. Xue, N. Liu, P. Mei, D. Kuzuhara, M. Zhou, J. Pan, H. Yamada and F. Qiu, Chem. Commun., 2021, 57, 12808–12811.
241. D. Kuzuhara, W. Furukawa, A. Kitashiro, N. Aratani and H. Yamada, Chem. Eur. J., 2016, 22, 10671–10678.
242. S. Xue, W. Ryan Osterloh, X. Lv, N. Liu, Y. Gao, H. Lei, Y. Fang, Z. Sun, P. Mei, D. Kuzuhara, N. Aratani, H. Yamada, R. Cao, K. M. Kadish and F. Qiu, Angew. Chem., Int. Ed., 2023, 62, e202218567.
243. A. S. Bulbul, V. Rathour, V. Ganesan and M. Sankar, Chem. Commun., 2024, 60, 3146–3149.
244. B. Meunier, S. P. de Visser and S. Shaik, Chem. Rev., 2004, 104, 3947–3980.
245. S. Samanta, P. K. Das, S. Chatterjee and A. Dey, J. Porphyrins phthalocyanines, 2015, 19, 92–108.
246. S. Chatterjee, K. Sengupta, S. Samanta, P. K. Das and A. Dey, Inorg. Chem., 2015, 54, 2383–2392.
247. S. Samanta, P. K. Das, S. Chatterjee, K. Sengupta, B. Mondal and A. Dey, Inorg. Chem., 2013, 52, 12963–12971.
248. J. Yang, P. Li, X. Li, L. Xie, N. Wang, H. Lei, C. Zhang, W. Zhang, Y.-M. Lee, W. Zhang, R. Cao, S. Fukuzumi and W. Nam, Angew. Chem., Int. Ed., 2022, 61, e202208143.
249. X. Li, P. Li, J. Yang, L. Xie, N. Wang, H. Lei, C. Zhang, W. Zhang, Y.-M. Lee, W. Zhang, S. Fukuzumi, W. Nam and R. Cao, J. Energy Chem., 2023, 76, 617–621.
250. Y. Gao, B. Mei, Y. Wu, Q. Zhao, Z. Bao, H. Qin, Y. Xu, H. Lv, X. Peng, Y. He, T. Luo, R. Yao, W. Zhang, H. Lei and R. Cao, Chin. J. Chem., 2023, 41, 2866–2872.
251. S. Swavey and A. Eder, Inorg. Chem. Commun., 2013, 29, 14–17.
252. C. Liu, Z. Yu, F. She, J. Chen, F. Liu, J. Qu, J. M. Cairney, C. Wu, K. Liu, W. Yang, H. Zheng, Y. Chen, H. Li and L. Wei, Energy Environ. Sci., 2023, 16, 446–459.
253. B. Sun, Z. Ou, S. Yang, D. Meng, G. Lu, Y. Fang and K. M. Kadish, Dalton Trans., 2014, 43, 10809–10815.
254. B. Li, Z. Ou, D. Meng, J. Tang, Y. Fang, R. Liu and K. M. Kadish, J. Inorg. Biochem., 2014, 136, 130–139.
255. Z. Ou, A. Lü, D. Meng, S. Huang, Y. Fang, G. Lu and K. M. Kadish, Inorg. Chem., 2012, 51, 8890–8896.
256. A. Schechter, M. Stanevsky, A. Mahammed and Z. Gross, Inorg. Chem., 2012, 51, 22–24.
257. Z. Liang, H. Guo, G. Zhou, K. Guo, B. Wang, H. Lei, W. Zhang, H. Zheng, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2021, 60, 8472–8476.
258. Z. Liang, H. Guo, H. Lei and R. Cao, Chin. Chem. Lett., 2022, 33, 3999–4002.
259. C. Shi, B. Steiger, M. Yuasa and F. C. Anson, Inorg. Chem., 1997, 36, 4294–4295.
260. W. Jentzen, I. Turowska-Tyrk, W. R. Scheidt and J. A. Shelnutt, Inorg. Chem., 1996, 35, 3559–3567.
261. C. Shi and F. C. Anson, Inorg. Chem., 1998, 37, 1037–1043.
262. A. Choi, H. Jeong, S. Kim, S. Jo and S. Jeon, Electrochim. Acta, 2008, 53, 2579–2584.
263. R. R. Durand, Jr., C. S. Bencosme, J. P. Collman and F. C. Anson, J. Am. Chem. Soc., 1983, 105, 2710–2718.
264. J. P. Collman, F. C. Anson, C. E. Barnes, C. S. Bencosme, T. Geiger, E. R. Evitt, R. P. Kreh, K. Meier and R. B. Pettman, J. Am. Chem. Soc., 1983, 105, 2694–2699.
265. J. P. Collman, P. Denisevich, Y. Konai, M. Marrocco, C. Koval and F. C. Anson, J. Am. Chem. Soc., 1980, 102, 6027–6036.
266. C. K. Chang, H. Y. Liu and I. Abdalmuhdi, J. Am. Chem. Soc., 1984, 106, 2725–2726.
267. J. P. Fillers, K. G. Ravichandran, I. Abdalmuhdi, A. Tulinsky and C. K. Chang, J. Am. Chem. Soc., 1986, 108, 417–424.
268. Y. Deng, C. J. Chang and D. G. Nocera, J. Am. Chem. Soc., 2000, 122, 410–411.
269. C. J. Chang, E. A. Baker, B. J. Pistorio, Y. Deng, Z.-H. Loh, S. E. Miller, S. D. Carpenter and D. G. Nocera, Inorg. Chem., 2002, 41, 3102–3109.
270. C. J. Chang, Y. Deng, C. Shi, C. K. Chang, F. C. Anson and D. G. Nocera, Chem. Commun., 2000, 1355–1356,  10.1039/B001620I.
271. K. M. Kadish, L. Frémond, Z. Ou, J. Shao, C. Shi, F. C. Anson, F. Burdet, C. P. Gros, J.-M. Barbe and R. Guilard, J. Am. Chem. Soc., 2005, 127, 5625–5631.
272. L. L. Chng, C. J. Chang and D. G. Nocera, J. Org. Chem., 2003, 68, 4075–4078.
273. C. J. Chang, Y. Deng, S.-M. Peng, G.-H. Lee, C.-Y. Yeh and D. G. Nocera, Inorg. Chem., 2002, 41, 3008–3016.
274. C. J. Chang, Z.-H. Loh, C. Shi, F. C. Anson and D. G. Nocera, J. Am. Chem. Soc., 2004, 126, 10013–10020.
275. S. Fukuzumi, K. Okamoto, Y. Tokuda, C. P. Gros and R. Guilard, J. Am. Chem. Soc., 2004, 126, 17059–17066.
276. Y. Liu, G. Zhou, Z. Zhang, H. Lei, Z. Yao, J. Li, J. Lin and R. Cao, Chem. Sci., 2020, 11, 87–96.
277. D. Zhang, M. R. Crawley, M. Fang, L. J. Kyle and T. R. Cook, Dalton Trans., 2022, 51, 18373–18377.
278. D. Zhang, L. E. Rosch, M. R. Crawley and T. R. Cook, Inorg. Chem. Front., 2024, 11, 5557–5565.
279. A. N. Oldacre, A. E. Friedman and T. R. Cook, J. Am. Chem. Soc., 2017, 139, 1424–1427.
280. A. N. Oldacre, M. R. Crawley, A. E. Friedman and T. R. Cook, Chem. – Eur. J., 2018, 24, 10984–10987.
281. M. R. Crawley, D. Zhang, A. N. Oldacre, C. M. Beavers, A. E. Friedman and T. R. Cook, J. Am. Chem. Soc., 2021, 143, 1098–1106.
282. D. Zhang, M. R. Crawley, A. N. Oldacre, L. J. Kyle, S. N. MacMillan and T. R. Cook, Inorg. Chem., 2023, 62, 1766–1775.
283. S. M. Adam, G. B. Wijeratne, P. J. Rogler, D. E. Diaz, D. A. Quist, J. J. Liu and K. D. Karlin, Chem. Rev., 2018, 118, 10840–11022.
284. J. P. Collman, L. Fu, P. C. Herrmann and X. Zhang, Science, 1997, 275, 949–951.
285. J. P. Collman, L. Fu, P. C. Herrmann, Z. Wang, M. Rapta, M. Bröring, R. Schwenninger and B. Boitrel, Angew. Chem., Int. Ed., 1998, 37, 3397–3400.
286. H.-T. Zhang, F. Xie, Y.-H. Guo, Y. Xiao and M.-T. Zhang, Angew. Chem., Int. Ed., 2023, 62, e202310775.
287. J. Meng, H. Qin, H. Lei, X. Li, J. Fan, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2023, 62, e202312255.
288. J. Han, H. Tan, K. Guo, H. Lv, X. Peng, W. Zhang, H. Lin, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2024, 63, e202409793.
289. G. Givaja, M. Volpe, M. A. Edwards, A. J. Blake, C. Wilson, M. Schröder and J. B. Love, Angew. Chem., Int. Ed., 2007, 46, 584–586.
290. M. Volpe, H. Hartnett, J. W. Leeland, K. Wills, M. Ogunshun, B. J. Duncombe, C. Wilson, A. J. Blake, J. McMaster and J. B. Love, Inorg. Chem., 2009, 48, 5195–5207.
291. E. Askarizadeh, S. B. Yaghoob, D. M. Boghaei, A. M. Z. Slawin and J. B. Love, Chem. Commun., 2010, 46, 710–712.
292. S. Fukuzumi, S. Mandal, K. Mase, K. Ohkubo, H. Park, J. Benet-Buchholz, W. Nam and A. Llobet, J. Am. Chem. Soc., 2012, 134, 9906–9909.
293. C. Chen, Y. Li, A. Huang, X. Liu, J. Li, Y. Zhang, Z. Chen, Z. Zhuang, Y. Wu, W.-C. Cheong, X. Tan, K. Sun, Z. Xu, D. Liu, Z. Wang, K. Zhou and C. Chen, J. Am. Chem. Soc., 2023, 145, 21273–21283.
294. R. Liu, C. von Malotki, L. Arnold, N. Koshino, H. Higashimura, M. Baumgarten and K. Müllen, J. Am. Chem. Soc., 2011, 133, 10372–10375.
295. O. Ohno, Y. Kaizu and H. Kobayashi, J. Chem. Phys., 1993, 99, 4128–4139.
296. X. Xu and J. Guan, Chem. Sci., 2024, 15, 14585–14607.
297. A. Radwan, H. Jin, D. He and S. Mu, Nano-Micro Lett., 2021, 13, 132.
298. Q. Zhou, C. M. Li, J. Li, X. Cui and D. Gervasio, J. Phys. Chem. C, 2007, 111, 11216–11222.
299. Q. Zhou, C. M. Li, J. Li and J. Lu, J. Phys. Chem. C, 2008, 112, 18578–18583.
300. C. Canales and G. Ramírez, Electrochim. Acta, 2015, 173, 636–641.
301. H. Tang, H. Yin, J. Wang, N. Yang, D. Wang and Z. Tang, Angew. Chem., Int. Ed., 2013, 52, 5585–5589.
302. A. Kumar, K. Sun, X. Duan, S. Tian and X. Sun, Chem. Mater., 2022, 34, 5598–5606.
303. W. Wang, D. Chen, F. Li, X. Xiao and Q. Xu, Chem, 2024, 10, 86–133.
304. H.-Y. Li, X.-J. Kong, S.-D. Han, J. Pang, T. He, G.-M. Wang and X.-H. Bu, Chem. Soc. Rev., 2024, 53, 5626–5676.
305. H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444.
306. S. Huh, S.-J. Kim and Y. Kim, CrystEngComm, 2016, 18, 345–368.
307. S. A. Younis, D.-K. Lim, K.-H. Kim and A. Deep, Adv. Colloid Interface Sci., 2020, 277, 102108.
308. Y. Bai, Y. Dou, L.-H. Xie, W. Rutledge, J.-R. Li and H.-C. Zhou, Chem. Soc. Rev., 2016, 45, 2327–2367.
309. X. Gong, Y. Shu, Z. Jiang, L. Lu, X. Xu, C. Wang and H. Deng, Angew. Chem., Int. Ed., 2020, 59, 5326–5331.
310. G. Chen, M. B. Stevens, Y. Liu, L. A. King, J. Park, T. R. Kim, R. Sinclair, T. F. Jaramillo and Z. Bao, Small Methods, 2020, 4, 2000085.
311. M. O. Cichocka, Z. Liang, D. Feng, S. Back, S. Siahrostami, X. Wang, L. Samperisi, Y. Sun, H. Xu, N. Hedin, H. Zheng, X. Zou, H.-C. Zhou and Z. Huang, J. Am. Chem. Soc., 2020, 142, 15386–15395.
312. M. Lions, J.-B. Tommasino, R. Chattot, B. Abeykoon, N. Guillou, T. Devic, A. Demessence, L. Cardenas, F. Maillard and A. Fateeva, Chem. Commun., 2017, 53, 6496–6499.
313. M. F. Sanad, A. R. Puente Santiago, S. A. Tolba, M. A. Ahsan, O. Fernandez-Delgado, M. Shawky Adly, E. M. Hashem, M. Mahrous Abodouh, M. S. El-Shall, S. T. Sreenivasan, N. K. Allam and L. Echegoyen, J. Am. Chem. Soc., 2021, 143, 4064–4073.
314. H. Zhong, K. H. Ly, M. Wang, Y. Krupskaya, X. Han, J. Zhang, J. Zhang, V. Kataev, B. Büchner, I. M. Weidinger, S. Kaskel, P. Liu, M. Chen, R. Dong and X. Feng, Angew. Chem., Int. Ed., 2019, 58, 10677–10682.
315. J. Li, X. Jing, Q. Li, S. Li, X. Gao, X. Feng and B. Wang, Chem. Soc. Rev., 2020, 49, 3565–3604.
316. J. Chen, Y. Wang, Y. Yu, J. Wang, J. Liu, H. Ihara and H. Qiu, Exploration, 2023, 3, 20220144.
317. S. Bhunia, A. Peña-Duarte, H. Li, H. Li, M. F. Sanad, P. Saha, M. A. Addicoat, K. Sasaki, T. A. Strom, M. J. Yacamán, C. R. Cabrera, R. Seshadri, S. Bhattacharya, J.-L. Brédas and L. Echegoyen, ACS Nano, 2023, 17, 3492–3505.
318. J. Li, P. Liu, J. Yan, H. Huang and W. Song, Adv. Sci., 2023, 10, 2206165.
319. J. Tang, Z. Liang, H. Qin, X. Liu, B. Zhai, Z. Su, Q. Liu, H. Lei, K. Liu, C. Zhao, R. Cao and Y. Fang, Angew. Chem., Int. Ed., 2023, 62, e202214449.
320. J. Guo, C.-Y. Lin, Z. Xia and Z. Xiang, Angew. Chem., Int. Ed., 2018, 57, 12567–12572.
321. G. Xu, H. Lei, G. Zhou, C. Zhang, L. Xie, W. Zhang and R. Cao, Chem. Commun., 2019, 55, 12647–12650.
322. I. Hijazi, T. Bourgeteau, R. Cornut, A. Morozan, A. Filoramo, J. Leroy, V. Derycke, B. Jousselme and S. Campidelli, J. Am. Chem. Soc., 2014, 136, 6348–6354.
323. G. Lu, H. Yang, Y. Zhu, T. Huggins, Z. J. Ren, Z. Liu and W. Zhang, J. Mater. Chem. A, 2015, 3, 4954–4959.
324. B.-Q. Li, S.-Y. Zhang, B. Wang, Z.-J. Xia, C. Tang and Q. Zhang, Energy Environ. Sci., 2018, 11, 1723–1729.
325. B.-Q. Li, S.-Y. Zhang, X. Chen, C.-Y. Chen, Z.-J. Xia and Q. Zhang, Adv. Funct. Mater., 2019, 29, 1901301.
326. C.-X. Zhao, B.-Q. Li, J.-N. Liu, J.-Q. Huang and Q. Zhang, Chin. Chem. Lett., 2019, 30, 911–914.
327. Q. Zuo, G. Cheng and W. Luo, Dalton Trans., 2017, 46, 9344–9348.
328. H. Lei, Q. Zhang, Z. Liang, H. Guo, Y. Wang, H. Lv, X. Li, W. Zhang, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2022, 61, e202201104.
329. Q. Zhao, Q. Zhang, Y. Wu, Z. Xiao, Y. Peng, Y. Zhou, W. Zhang, H. Lei and R. Cao, Mater. Today Catal., 2024, 5, 100050.
330. Q. Zhao, Q. Zhang, Y. Xu, A. Han, H. He, H. Zheng, W. Zhang, H. Lei, U.-P. Apfel and R. Cao, Angew. Chem., Int. Ed., 2024, 63, e202414104.
331. Y.-F. Yao, W.-Y. Xie, S.-J. Huang, J.-S. Ye, H.-Y. Liu and X.-Y. Xiao, J. Electroanal. Chem., 2024, 952, 117987.
332. Q. Zhang, Y. Wang, Y. Wang, S. Yang, X. Wu, B. Lv, N. Wang, Y. Gao, X. Xu, H. Lei and R. Cao, Chin. Chem. Lett., 2021, 32, 3807–3810.
333. W. Chen, J. Akhigbe, C. Brückner, C. M. Li and Y. Lei, J. Phys. Chem. C, 2010, 114, 8633–8638.
334. S. Yang, Y. Yu, M. Dou, Z. Zhang and F. Wang, J. Am. Chem. Soc., 2020, 142, 17524–17530.
335. C. Wan, X. Duan and Y. Huang, Adv. Energy Mater., 2020, 10, 1903815.
336. H. Zhang, X. Jin, J.-M. Lee and X. Wang, ACS Nano, 2022, 16, 17572–17592.
337. T. Pu, J. Ding, F. Zhang, K. Wang, N. Cao, E. J. M. Hensen and P. Xie, Angew. Chem., Int. Ed., 2023, 62, e202305964.
338. T. Ding, X. Liu, Z. Tao, T. Liu, T. Chen, W. Zhang, X. Shen, D. Liu, S. Wang, B. Pang, D. Wu, L. Cao, L. Wang, T. Liu, Y. Li, H. Sheng, M. Zhu and T. Yao, J. Am. Chem. Soc., 2021, 143, 11317–11324.
339. W. Zhang, Y. Chao, W. Zhang, J. Zhou, F. Lv, K. Wang, F. Lin, H. Luo, J. Li, M. Tong, E. Wang and S. Guo, Adv. Mater., 2021, 33, 2102576.
340. Z. He, K. He, A. W. Robertson, A. I. Kirkland, D. Kim, J. Ihm, E. Yoon, G.-D. Lee and J. H. Warner, Nano Lett., 2014, 14, 3766–3772.
341. P. Zhu, X. Xiong, X. Wang, C. Ye, J. Li, W. Sun, X. Sun, J. Jiang, Z. Zhuang, D. Wang and Y. Li, Nano Lett., 2022, 22, 9507–9515.
342. Y. Xie, X. Chen, K. Sun, J. Zhang, W.-H. Lai, H. Liu and G. Wang, Angew. Chem., Int. Ed., 2023, 62, e202301833.
343. G. Yang, J. Zhu, P. Yuan, Y. Hu, G. Qu, B.-A. Lu, X. Xue, H. Yin, W. Cheng, J. Cheng, W. Xu, J. Li, J. Hu, S. Mu and J.-N. Zhang, Nat. Commun., 2021, 12, 1734.
344. A. Han, X. Wang, K. Tang, Z. Zhang, C. Ye, K. Kong, H. Hu, L. Zheng, P. Jiang, C. Zhao, Q. Zhang, D. Wang and Y. Li, Angew. Chem., Int. Ed., 2021, 60, 19262–19271.
345. J. Wang, Z. Huang, W. Liu, C. Chang, H. Tang, Z. Li, W. Chen, C. Jia, T. Yao, S. Wei, Y. Wu and Y. Li, J. Am. Chem. Soc., 2017, 139, 17281–17284.
346. M. Xiao, Y. Chen, J. Zhu, H. Zhang, X. Zhao, L. Gao, X. Wang, J. Zhao, J. Ge, Z. Jiang, S. Chen, C. Liu and W. Xing, J. Am. Chem. Soc., 2019, 141, 17763–17770.
347. M. Liu, N. Li, S. Cao, X. Wang, X. Lu, L. Kong, Y. Xu and X. H. Bu, Adv. Mater., 2022, 34, 2107421.
348. X. Zhou, J. Gao, Y. Hu, Z. Jin, K. Hu, K. M. Reddy, Q. Yuan, X. Lin and H.-J. Qiu, Nano Lett., 2022, 22, 3392–3399.
349. M. Li, H. Zhu, Q. Yuan, T. Li, M. Wang, P. Zhang, Y. Zhao, D. Qin, W. Guo, B. Liu, X. Yang, Y. Liu and Y. Pan, Adv. Funct. Mater., 2023, 33, 2210867.
350. L. Zhang, J. M. T. A. Fischer, Y. Jia, X. Yan, W. Xu, X. Wang, J. Chen, D. Yang, H. Liu, L. Zhuang, M. Hankel, D. J. Searles, K. Huang, S. Feng, C. L. Brown and X. Yao, J. Am. Chem. Soc., 2018, 140, 10757–10763.
351. M. Xiao, J. Zhu, S. Li, G. Li, W. Liu, Y.-P. Deng, Z. Bai, L. Ma, M. Feng, T. Wu, D. Su, J. Lu, A. Yu and Z. Chen, ACS Catal., 2021, 11, 8837–8846.
352. Y. Chen, J. Mao, H. Zhou, L. Xing, S. Qiao, J. Yuan, B. Mei, Z. Wei, S. Zhao, Y. Tang and C. Liu, Adv. Funct. Mater., 2023, 34, 2311664.
353. Y.-X. Zhang, S. Zhang, H. Huang, X. Liu, B. Li, Y. Lee, X. Wang, Y. Bai, M. Sun, Y. Wu, S. Gong, X. Liu, Z. Zhuang, T. Tan and Z. Niu, J. Am. Chem. Soc., 2023, 145, 4819–4827.
354. P. Li, Y. Jiao, Y. Ruan, H. Fei, Y. Men, C. Guo, Y. Wu and S. Chen, Nat. Commun., 2023, 14, 6936.
355. Y. Wang, G. I. N. Waterhouse, L. Shang and T. Zhang, Adv. Energy Mater., 2021, 11, 2003323.
356. S. Yang, X. Liu, S. Li, W. Yuan, L. Yang, T. Wang, H. Zheng, R. Cao and W. Zhang, Chem. Soc. Rev., 2024, 53, 5593–5625.
357. X. Wang, Z.-F. Cai, D. Wang and L.-J. Wan, J. Am. Chem. Soc., 2019, 141, 7665–7669.
358. X. Wang, Z.-F. Cai, Y.-Q. Wang, Y.-C. Feng, H.-J. Yan, D. Wang and L.-J. Wan, Angew. Chem., Int. Ed., 2020, 59, 16098–16103.
359. X. Wang, Y.-Q. Wang, Y.-C. Feng, D. Wang and L.-J. Wan, Chem. Soc. Rev., 2021, 50, 5832–5849.
360. X. Wang, Y.-C. Feng, Y.-Q. Wang, Z.-Y. Yi, J.-J. Duan, H.-J. Yan, D. Wang and L.-J. Wan, CCS Chem., 2023, 5, 2628–2637.

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