Two dimensional oxides for oxygen evolution reactions and related device applications

Abstract

The oxygen evolution reaction (OER) is a key anode reaction for many renewable energy devices, such as electrocatalytic water splitting devices, Zn–air batteries and CO₂ electrolyzers. Transition metal oxides have been broadly explored for the OER due to their good electrochemical stability and low cost. However, their generic catalytic performance and poor electrical conductivity have greatly impeded their practical applications. Fabricating oxides into two dimensional (2D) structures can increase their surface areas and modulate the electronic structures of their catalytic sites, which is a new method to improve the OER performance. However, 2D oxides still come across some challenges for the OER, such as easy restacking of the 2D materials, poor intrinsic activity of the transition metal atoms and difficulty in their mass production. In this review, we firstly introduce the background and importance of the OER. Then, the OER mechanism and strategies to optimize the OER performance for 2D oxides are thoroughly reviewed. Their initial explorations in water splitting, Zn–air battery and CO₂ electrolyzer applications have also been considered. Finally, the current challenges and future opportunities in the synthesis and applications of 2D oxides are clearly highlighted.

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Ying Li

Ying Li obtained her bachelor's degree from Jilin University of Biomedical Engineering, China in 2020. Currently, she is a Master's Degree student at Shenzhen University under the supervision of Prof. Xingke Cai. Her research interest focuses on catalyst design and application of zinc air battery/electrolysis cells.

Yonggui Deng

Yonggui Deng is currently studying for a master's degree at the college of mechatronics and control engineering at Shenzhen University. He received his bachelor's degree in Shanghai University of Engineering Science in 2021. His research focuses on optimization design of separators for lithium metal batteries and catalysis design.

Dongqing Liu

Dongqing Liu is currently an assistant professor at Shenzhen University. She received her B.E. degree in materials science & engineering from Harbin Institute of Technology in China and PhD degree in materials science & engineering from National University of Singapore. Her research mainly focuses on lithium ion/metal batteries and electro-catalysis. She has published over 50 papers with over 1000 citations.

Qianqian Ji

Qianqian Ji is currently a postdoctoral fellow in Prof. Xingke Cai's group at Shenzhen university. She received her B.S degree in 2017 and PhD degree in 2022 from Anhui Normal University and University of Science and Technology of China, respectively. Her research interests are focused on synchrotron radiation technology and application in energy conversion and storage.

Xingke Cai

Xingke Cai is currently an assistant professor in Shenzhen University. His research mainly focuses on the preparation of two-dimensional materials, including two-dimensional oxides, borides, carbides, etc., through exfoliation of layered compounds, and their electrochemical applications. He has published over 60 papers, including 3 book chapters and more than 30 first/corresponding author papers in top-tier journals such as Chem. Soc. Rev., J. Am. Chem. Soc, Appl. Catal. B, and Small, with over 2000 citations. He has also applied for 8 patents related to the synthesis and energy storage applications of two-dimensional materials. He has served as an expert reviewer for the scientific and technological awards in Guangxi Province, as well as a reviewer for high-tech enterprise evaluations in Guangdong Province. He has received awards such as the 2022 China Industry-University-Research Cooperation Innovation Award (Individual). As a supervisor, he guided students to win the third prize in the first China Graduate “Dual Carbon” Innovation and Creativity Competition.

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1. Introduction

Traditional carbon–hydrocarbon-based fossil fuel resources are gradually being depleted due to their excessive consumption in industry and daily life. The ensuing negative effects, such as energy crisis, soaring atmospheric carbon dioxide levels and global warming, have become major challenges to the development of human society.^1,2 Realizing intermittent electrical energy (such as solar panels, wind farms, tidal energy, and hydropower) conversion, storage and CO₂ capture to reduce global greenhouse gas emissions is an effective strategy for mitigating the energy shortage and climate change. It is essential to accelerate the development of sustainable, affordable, green, and clean energy to enable sustained economic growth and geopolitical stability.³ Among the various energy storage and conversion techniques, water electrolysis for renewable hydrogen production, zinc–air batteries and CO₂ reduction, as three mainstreams, have been widely studied. Zinc–air batteries, as successfully commercialized metal–air batteries, possess significant advantages over traditional batteries, including large capacity, high specific energy, and low cost, along with green, environment-friendly operation.⁴ Hydrogen energy is recognized as having zero carbon emissions and a high calorific value, while being a source of high-efficiency, clean secondary energy. The electrochemical reduction of CO₂ into organic carbon is positively pursued for closing the carbon cycle. Among these, the OER plays a vital role as an anode half-reaction, which involves 4e⁻ charge transfer, mass transfer, and adsorption/desorption of intermediates on the substances. However, the efficiency of energy storage and conversion is hindered by the high energy barrier and sluggish dynamics of the OER.^5–7 Therefore, the development of efficient OER catalysts is highly desirable for reducing the energy barrier and accelerating reaction kinetics.

There are currently three main types of OER catalysts. The first type consists of commercial OER catalysts such as precious metal oxides, namely RuO₂ and IrO₂. These materials are regarded as benchmark electrocatalysts and exhibit excellent catalytic activity. However, their large-scale application in industry has been impeded by their high price, difficult recovery, low earth-abundance, and poor resistance of precious metals to deactivation, as well as their proneness to agglomeration or dissolution during the cycling process, leading to rapid performance degradation.^8–10 Consequently, the development of high-performance, metal-free/non-noble-metal catalysts is highly desirable. The second type comprises nonmetal-doped carbon nanomaterials.¹¹ These carbon materials possess features such as significant surface area, high electronic conductivity, and suitable catalytic properties. Nonetheless, they are easily destroyed by intermediates or by-products at high oxidation potentials or high current densities when acting as carriers or direct catalytic sites. Accordingly, the inherent catalytic performance of carbon materials is not ideal, and their stability in practical applications still faces critical challenges. The third type comprises transition-metal-based nanomaterials. These materials are promising candidate electrocatalysts for industrial applications because of their low price, numerous metal oxidation states, various structural and electronic modulation strategies, and ease of large-scale preparation.^12–14

To date, numerous investigations have been conducted on diverse transition-metal-based catalysts for the OER. These catalysts comprise transition-metal-based oxides, borides, phosphides, sulfides, layered double hydroxides (LDH), and metal–organic frameworks (MOFs).^15–20 Generally, transition metal oxides exhibit excellent OER durability owing to their stable structure at high oxidation potentials. In contrast, transition-metal-based phosphides and sulfides are prone to inevitable structural reconfiguration during reaction and reconstruction-induced aggregation of oxyhydroxide species with low electrical conductivity, resulting in a slow charge-transfer kinetics.²¹ The OER stability of these electrocatalysts is severely challenged under strongly oxidizing conditions, especially at high current densities. Considering that OER electrocatalysts should meet the requirements of high activity and long-time durability for practical applications, transition-metal oxides show promising application prospects in the industrial community. Significantly, the most widely studied transition metal oxide electrocatalysts are spinel-, rutile-, and perovskite-structured nanomaterials. Nevertheless, the particle size of nanomaterials is usually larger than 200 nm, leading to low atomic utilization.^22–24 These catalytic materials are generally stable during the OER, but their intrinsic catalytic performance is not ideal.

More strikingly, developing 2D-structured transition metal oxides is considered an effective method for optimizing the OER performance. In general, 2D-structured materials not only have more exposed active sites, but their performance also differs from that of conventional catalysts owing to the 2D size effect and unique crystal structure. The combination of 2D structures with electronic structure modulation strategies has resulted in a substantial enhancement in the OER performance of 2D oxide catalysts.

This review discusses the OER mechanism over a wide pH range. Various strategies, including loading 2D materials onto substrates, constructing defects, creating heterojunctions, and fabricating signal atoms, are used to enhance the interfacial charge transfer and mass transfer rates, lower the energy barrier, and enhance reaction efficiency, as described in detail. The applications of 2D oxides in water electrolysis devices, zinc–air batteries and CO₂ electrolyzers are also summarized. Finally, it offers an analysis of the current challenges faced and potential future directions for 2D oxides.

2. OER of 2D oxides in different electrolyte media

Electrocatalytic OERs have received significant interest due to their potential for application in renewable energy related devices. For example, water electrolysis powered by renewable electricity has been utilized to produce hydrogen for practical applications in large-scale projects, where the OER is a vital half-reaction for improving the overall efficiency over a wide pH range.²⁵ The performance of zinc–air batteries is critically determined by the overpotential of the OER half-reaction in alkaline media. Similarly, the CO₂ reduction reaction (CO₂RR) at the cathode always occurs along with the OER at the anode in neutral media.²⁶ Notably, 2D oxide catalysts tend to have large specific surface areas, abundant active sites, good stability over a wide pH range as well as high atomic utilization, which can be good candidates for the OER.²⁷ In the following sections, we first introduce typical OER mechanisms under different electrolyte conditions and then present various strategies for promoting the catalytic performance of 2D oxides.

2.1. OER mechanism of 2D oxides

Understanding OER pathways is important for exploring novel 2D electrocatalysts with outstanding activity. Generally, the OER can be described using the following equation: 4OH⁻ → 2H₂O + O₂ (g) + 4e⁻ (alkaline); 2H₂O → 4H⁺ + O₂ (g) + 4e⁻ (acidic/neutral).^28,29 However, the OER takes place at the interface between the catalyst and the electrolyte. The absorption and desorption processes occurring at the surface and interface play a pivotal role for the entire catalytic reaction, which leads to different reaction pathways and intermediates. Therefore, depending on the reaction pathway and reaction active site for the OER, the OER mechanism can be classified into different pathways: the conventional adsorbate evolution mechanism (AEM), the lattice oxygen mechanism (LOM) and the metal-and-lattice-oxygen-vacancy-site mechanism (MLOV). These mechanisms can be further differentiated based on the type of electrolyte solution (Fig. 1).³⁰ In the AEM, the transition metal atoms are considered as the active site, the formation of M–*OOH intermediate species from M–*O intermediates is the rate-determining step (RDS) of the four-electron reaction. The resulting product (O₂ molecules) completely comes from the absorbed OH⁻/H₂O species. In the LOM, lattice oxygen atoms are considered as the active sites and are directly involved in the formation of O₂, in which the M–*O coupling process has a high energy barrier. In the MLOV mechanism, both the metal sites and lattice-oxygen sites act as active centers for the OER. This model surpasses the conventional understanding of charge transfer steps occurring solely on metal sites and suggests the involvement of oxygen redox chemistry mediated by oxygen vacancies.^31,32 To improve the reaction kinetics, it is important to note that adjusting the morphology size and electronic structure reduces the RDS energy barrier, thereby affording efficient and stable OER catalysts.

Fig. 1 Different reaction pathways for the OER. Schematic illustrations of (a) AEM, (c) LOM and (e) MLOV under alkaline conditions. schematic illustrations of (b) AEM, (d) LOM and (f) MLOV under acidic/neutral conditions.

2.2. Modulation of OER performance

Two-dimensional transition metal oxide catalysts have emerged as efficient electrocatalysts. These include single-metal oxide nanosheets,³³ binary metal oxide nanosheets,^34,35 and multi-metal oxide nanosheets.³⁶ On the one hand, transition metal atoms can be used directly as reaction sites because of the intrinsic activity. On the other hand, 2D materials can also serve as support carriers, which can couple with 1D and 2D nanomaterials to form novel catalysts.^37–39 However, the shortcoming of easy stacking of 2D oxides reduces the number of surface active sites, and leads to a high energy barrier and large overpotential, resulting in low intrinsic catalytic activity.⁴⁰ To address this inefficiency, various strategies for modulating the performance of 2D oxides have been applied to achieve a comprehensive catalytic performance, including increasing the abundance of catalytic sites, boosting the activity of the catalytic site, and accelerating interfacial charge transport (Table 1). In the following section, we discuss strategies for elemental doping, morphology and structure regulation, and heterogenous engineering (Fig. 2).

Table 1 OER performance of 2D oxide catalysts reported in recent years

Catalysts	Media	Overpot-ential	Tafel Slope	Ref.
Porous MoO2	1.0 M KOH	266	N/A	41
Co/MoO2	1.0 M KOH	318	93.9	33
CuCo2O4	1.0 M KOH	260	64	34
r-CFO	1.0 M KOH	280	43	39
NF/H-CoMoO4	1.0 M KOH	295	N/A	35
MoN/MoO2-6	1.0 M KOH	292	98	42
NiO/RuO2/NF	1.0 M KOH	210	68.7	38
P-NiFe2O4	1.0 M KOH	231	49	43
Fe3O4/Co3S4	1.0 M KOH	270	56	44
N-BMO	1.0 M KOH	349	42	45
CoMoO4/MoO2	1.0 M KOH	230	51	46
NiMoO4	1.0 M KOH	239	71.8	47
Co1Fe1Mo1.8O/NF	1.0 M KOH	210	32	48
Co3O4/CoMoO4/NF	1.0 M KOH	244	52	49
NiO	1.0 M KOH	380	299	50
P-NiO	1.0 M KOH	294	92.6	51
Ni@C-MoO2/NF	1.0 M KOH	240	52.3	52
Co3Mo/CoMoO3	1.0 M KOH	∼265	64.1	53
IrW/WO3-NS-A	0.5 M H2SO4	229	88	54
4CeO2@SrIrO3	0.5 M H2SO4	238	71.7	55
R20-Mn	0.5 M H2SO4	210	54.2	56
Co3O4 UNA	1.0 M PBS	207	60	57
Pd/NiFeOx	1.0 M KOH	180	59.2	58
Pd/NiFeOx	0.5 M H2SO4	169	76.6	58
Pd/NiFeOx	1.0 M PBS	310	175.1	58
CoFe2O4 NSs	0.2 M PBS	275	42.1	59
S-NiFe2O4/NF	1.0 M PBS	494	118.1	60
S-NiFe2O4/NF	1.0 M KOH	267	36.7	60
S-doped M-SrIrO3	0.5 M H2SO4	228	58.4	61

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Fig. 2 The typical performance regulations of 2D oxides.

2.2.1. Growing on the substrate. In 2D oxides, the atoms are intricately linked by robust chemical bonding, while also featuring a surplus of unsaturated dangling bonds on the surface or the edge of the particles, which leads to high activity and a high-energy surface.⁶² When 2D oxides are prepared as a slurry and coated on electrodes, they tend to wrinkle or agglomerate. Unfortunately, these phenomena lead to a reduction in the electrochemically active area and impede charge injection from the metal electrodes. Based on these considerations, the inclusion of a support can facilitate the uniform dispersion of the active materials and enhance their contact with the reactants. 2D oxides have been directly synthesized on nickel foam (NF) substrates in numerous studies, as NF has the advantages of being oxidation/corrosion resistant, can be strongly bound to 2D oxides, and promote fast electron transfer during the OER process.^63–65 Compared with the super-flat layered structure, the mesoporous/macroporous structure formed by the nanosheets perpendicular to the NF surface can provide abundant pores to facilitate electrolyte penetration and dissociation of the gaseous product.

Pawar et al.³⁴ synthesized ultrathin CuCo₂O₄ nanosheets containing abundant nanopores on NF substrates through electrodeposition and air annealing treatment.⁶⁶ The scanning electron microscopy (SEM) images revealed that the CuCo₂O₄ nanosheet films had a larger pore density than the Co₃O₄ nanosheet films, indicating that the former have a large electrochemically active surface area (Fig. 3a and b). These ultrathin nanosheet films with nanopores promote charge and mass transfer, and this unique morphology can effectively facilitate the adsorption of reactants and the desorption of intermediates and products. In detail, to eliminate the impact of the oxidation peak, the true properties the Co₃O₄ nanosheets resulted in an overpotential of 330 mV at 20 mA cm⁻², accompanied by a Tafel slope of 67 mV dec⁻¹, whereas the CuCo₂O₄ nanosheets exhibited a relatively lower overpotential of 260 mV at 20 mA cm⁻² with a Tafel slope of 64 mV dec⁻¹, revealing the more favorable electrocatalytic kinetics in the case of CuCo₂O₄. Moreover, the CuCo₂O₄ electrocatalyst exhibited outstanding durability up to 25 h at 20 mA cm⁻². Notably, the linear sweep voltammetry (LSV) curves of the Co₃O₄ and CuCo₂O₄ nanosheets showed the same overpotentials before and after the stability test. The above results illustrate that structural regulation and the introduction of a support can optimize the catalytic performance and durability of transition metal oxides in alkaline solutions (Fig. 3c and d).

Fig. 3 SEM images of the (a) Co₃O₄ and (b) CuCo₂O₄ nanosheet films; (c) and (d) OER performance of Co₃O₄, CuCo₂O₄ and Ni foam.³⁴ Reproduced with permission. Copyright 2019, Elsevier. (e) The synthetic steps of porous MoO₂ nanosheets on nickel foam directly; (f) steady-state polarization curves of Ni foam, commercial Pt/C, compact MoO₂, and porous MoO₂ in 1.0 M KOH for the HER and OER;³⁷ reproduced with permission. Copyright 2016, Wiley. Characterization of Co₃O₄ UNA. (g) SEM image and (h) AFM image of Co₃O₄ ultrathin nanosheets; (i) and (j) electrocatalytic OER in 1.0 M PBS (pH = 7) electrolyte.⁵⁷ Reproduced with permission. Copyright 2017, Springer Nature.

In recent years, studies on MoO₂ nanosheets for the catalytic OER have gradually increased due to the numerous crystal morphologies of MoO₂ (for example α-, β-, γ-, λ-, δ-, and R-MnO₂), as well as the enhanced metallic character, charge transfer capability, and conductivity of MoO₂.^67–69 Significantly, porous 2D materials combine a considerable surface area and a larger concentration of active sites per unit mass. Jin et al.³⁷ constructed porous 2D MoO₂ nanosheet arrays on the surface of NF; these assemblies were used directly without post-treatment, avoiding the addition of organic binders. As shown in Fig. 3e, porous MoO₂ was grown directly on a commercial NF substrate via a wetting chemical route, followed by an annealing process. Accordingly, the pore size dispersion in the direction parallel to the NF was 5–20 nm. As shown on the right side of Fig. 3f, the porous MoO₂ exhibited excellent catalytic activity towards the OER, showing an onset potential of approximately 1.43 V. Additionally, it showcased outstanding performance compared with nickel foam, Pt/C, and compact MoO₂, as it accomplished current densities of 10 and 20 mA cm⁻² at 1.49 and 1.51 V, respectively. Pei et al.⁴⁸ developed edge-rich Mo-doped cobalt iron oxide nanosheets assembled on NF substrates. TEM was used to examine the porous structure with edge-rich interfaces. Moreover, the proportion of Mo atoms played a pivotal role in adjusting the electronic structure. Accordingly, the Co₁Fe₁Mo_1.8ONMs@NF electrocatalyst demonstrated outstanding performance, displaying the lowest overpotentials at 240 and 250 mV for current densities of 50 and 100 mA cm⁻², respectively.

Thin films were typically synthesized by chemical vapor deposition, exfoliation, and a surfactant-assisted approach, which require special equipment and reaction conditions, or they are limited by low yields and difficulty in controlling lateral film size and structure. Furthermore, achieving high loading of ultrathin nanosheets on the electrodes without compromising their catalytic activity remains challenging. Zhang and colleagues⁵⁷ presented a simple electrodeposition method for the preparation of Co₃O₄ and Co(OH)₂ ultrathin nanosheet arrays (UNA) without the use of templates or surfactants (Fig. 3g and h). The obtained arrays show a high activity for the evolution of oxygen and hydrogen, both in alkaline and neutral media. In addition to the larger specific surface area and the higher exposure catalytic active sites resulting from the UNA structure, three additional desirable features are also achieved: efficient mass and charge transfer through the electrolyte, catalyst and electrode; high loading of ultrathin nanosheets on the electrode; and efficient release of gas bubbles from the electrode surface. In neutral media, the Co₃O₄ UNA exhibit much better catalytic performance than IrO₂ and bare NF (Fig. 3i and j). Co₃O₄ UNA can deliver current densities of 10 and 50 mA cm⁻² at overpotentials of 207 and 379 mV, respectively, much lower than IrO₂ (389 and 683 mV). The outstanding catalytic activity of Co₃O₄ UNA in neutral media is attributed to the UNA structure, which has a large active surface area and allows high electrode loading of ultrathin nanosheets without affecting their catalytic activity. The performance of the electrocatalyst is frequently affected by the formation of H₂ or O₂ bubbles, which tend to adhere to the electrode surface, limiting the active surface area and mass transport.⁷⁰ The Co₃O₄ UNA have a hydrophilic surface and a vertical nanosheet structure, allowing rapid release of O₂ and H₂ as small bubbles.⁷¹

2.2.2. Structural regulation. Combining large-surface-area 2D oxides with 1D nanoparticles/nanorods that facilitate effective electron transfer is a promising strategy for achieving enhanced electrocatalytic performance. The mixed-dimensional hierarchical architecture not only integrates the characteristics of each structural unit, but also weakens the van der Waals interactions between the ultrathin 2D nanosheets, enabling rapid electron transport pathways.^72–74 The mixed-dimensional hierarchical architecture not only integrates the distinctive characteristics of each structural unit, but also simultaneously functions as carriers and active centers, particularly when 2D nanosheets are combined with other 0D/1D species.

Xia et al.³³ anchored Co nanoparticles to MoO₂ nanosheets (Co/MoO₂) through hydrothermal treatment, followed by Ar/H₂ reduction (Fig. 4a). The nanoparticles were uniformly dispersed on the nanosheets surface, as shown in the SEM image (Fig. 4b). As observed in Fig. 4c, Co/MoO₂ had the lowest OER overpotential (318 mV at 10 mA cm⁻²) compared to CoMoO₄ (355 mV), RuO₂ (345 mV), Co (343 mV), and MoO₂ (470 mV) in 1.0 M KOH electrolyte, which displayed outstanding OER performance. Electrochemical impedance spectroscopy (EIS) was employed in order to assess the interfacial kinetics. The smaller semicircle (corresponding to R_ct) in the EIS plot suggests a faster charge transfer. It was determined that the R_ct of Co/MoO₂ was smaller than those of the reference materials (Fig. 4d). Furthermore, based on the result of charge density of states (DOS) in Fig. 4e, the electronic density of states of Co/MoO₂ is greater than the Fermi energy levels of MoO₂ and Co, which could explain the optimal reactivity of Co/MoO₂. Taken together, the data show that the Co/MoO₂ composite exhibits enhanced interfacial conductivity and electron transfer ability, with improved intrinsic activity, and it promotes the OER performance in alkaline solutions.⁷⁵

Fig. 4 (a) Schematic illustration of the fabrication of Co/MoO₂; (b) SEM image of Co/MoO₂; (c) OER polarization curves of CoMoO₄, Co/MoO₂, MoO₂, Co and RuO₂; (d) Nyquist plots collected at 500 mV vs. SCE of Co/MoO₂, MoO₂ and Co; (e) density of states of Co/MoO₂, MoO₂ and Co.³³ Copyright 2022, Royal Society of Chemistry; (f) a sketch map of the formation mechanism of core/shell Au NRs@MnO₂ nanocomposites; (g) the OER performance of Au NRs@MnO₂ with different Au/Mn ratios.⁷⁶ Reproduced with permission. Copyright 2021, Elsevier. (h) Synthetic route diagram of CeO₂@SrIrO₃ nanosheets; (i) electrochemical OER performance in 0.5 M H₂SO₄ of mass activity normalized by Ir amount.⁵⁵ Reproduced with permission. Copyright 2022, Elsevier.

Core/shell Au@MnO₂ nanocomposites are one of the most attractive combinations, with Au offering distinct optical, catalytic and surface-sensitive properties, while MnO₂ has the advantage of a broad array of crystal structures and valence states. Despite the limitations of the universal template synthesis approach, Hu et al. utilized the local surface plasmon resonance (LSPR) effect method to create a core/shell metal@metal oxide structure of Au NRs@MnO₂ nanosheets.⁷⁶ In this method, Au NRs are easily oxidized by KMnO₄ and undergo serious etching during the LSPR effect treatment, while in the absence of methanol or irradiation. Notably, KMnO₄ was not only reduced to form MnO₂,^77–79 but also maintained the structural stability of the nanorods. In addition, the electrochemical activity was significantly affected by the Au/Mn ratio. In the experiment, the optimal OER performance was achieved when the Au NRs@MnO₂ nanosheet composites had an Au/Mn ratio of 1 : 0.5 (Fig. 4g).

To date, IrO₂ and RuO₂, in particular the structurally robust IrO₂, are recognized as the available electrocatalysts for acidic OER.^80,81 Nevertheless, their terrestrial scarcity and exorbitant cost are the main bottlenecks for their large-scale application. Therefore, there is an urgent need to develop cost-effective electrocatalysts with high mass or specific activity. An original research study showed that the pseudocubic perovskite SrIrO₃ with in situ surface rearrangement of IrO_x/SrIrO₃ offered high intrinsic OER activity.⁸² Simultaneously, cerium oxide (CeO₂), rich in oxygen vacancies and reversible redox between Ce⁴⁺ and Ce³⁺, is widely used as a co-catalyst to enhance activity and stability in various catalytic fields. You and collaborators⁵⁵ firstly elaborated a 0D/2D structure of CeO₂ quantum dot decorated SrIrO₃ nanosheets (CeO₂@SrIrO₃) as the promising electrocatalyst for the OER in acidic media. The synthetic route diagram for CeO₂@SrIrO₃ nanosheets is shown in Fig. 4h. As revealed via theoretical calculations, the rearranged charge density in the heterostructure alters the adsorption behavior of oxygen intermediates, ultimately lowering the activation energy of the OER. Moreover, the introduction of CeO₂ can theoretically stabilize Sr atoms in the SrIrO₃ lattice. As expected, the optimized 4CeO₂@SrIrO₃ electrocatalyst exhibits excellent OER performance in acidic media, delivering a low overpotential of 238 mV at 10 mA cm⁻² and a long-lasting life for 50 h. Fig. 4i presents the mass activity of electrocatalysts normalized by Ir amount at 1.50 V. One can see that the 4CeO₂@SrIrO₃ electrocatalyst presents the highest mass activity of 249 A g_Ir⁻¹ among these prepared electrocatalysts, which is almost three times higher than 82.3 A g_Ir⁻¹ for the individual SrIrO₃ electrocatalyst. This 0D/2D structure involving CeO₂ quantum dots decorated SrIrO₃ nanosheets to achieve a durable and active OER under a harsh acidic environment. Theoretical calculations further unveil that CeO₂ coupling with electronic rearrangement not only lowers the energy barrier of the OER but also mitigates Sr leaching. Benefiting from the activation and stabilization dual-effect of CeO₂ coupling, the 4CeO₂@SrIrO₃ exhibits excellent OER performance.

2.2.3. Defect engineering. Defect engineering has been recognized as an effective strategy for accelerating the electrocatalytic kinetics of the OER.⁸³ Oxygen-deficient sites with unpaired electrons are commonly employed to adjust the electronic structure and geometric configuration of electrocatalysts. In particular, oxygen vacancies generate coordinative unsaturated metal active sites, which optimize the energy barrier for the adsorption of OER intermediates.⁸⁴ Oxygen vacancy engineering also could enhance the conductivity of electrocatalysts and increase the charge carrier density by driving the excitation of delocalized electrons from oxygen vacancies to the conduction band.⁸⁵ Except for anion vacancies, the induction of cation vacancies in the basal plane of layered double hydroxides could influence the electrocatalytic performance. Doping with cation vacancies has been discovered to expedite the evolution of hydroxyoxide intermediates.^86,87 This acceleration of hydroxyoxide intermediates ultimately leads to an enhancement in catalytic activity.⁸⁸ Furthermore, structural defects induced by simple electron beam irradiation also can be used to create defect-rich catalysts. Therefore, the regulation of defects represents a crucial approach in optimizing electrocatalytic performance.

Guo et al.³⁹ designed a defect-engineering strategy for obtaining the target catalyst with abundant oxygen vacancies. First, CoFe₂O₄ (CFO) nanosheets were successfully generated through a straightforward hydrothermal synthesis reaction. Oxygen-poor CoFe₂O₄ (p-CFO) was produced after treatment under a nitrogen atmosphere, whereas oxygen-rich CoFe₂O₄ (r-CFO) was created via a reduction reaction at ambient temperature (Fig. 5a). The O 1s XPS spectra of CFO showed two main peaks at 530.7 and 529.6 eV, which were attributed to hydroxy–oxygen (H–O) and metal–oxygen (M–O) bonds, respectively (Fig. 5b).^89,90 Noticeably, a new peak appeared at 531.9 eV in the spectra of p-CFO and r-CFO, which can be attributed to oxygen vacancies.^91–93 Notably, the peak area corresponding to oxygen vacancies was larger for the r-CFO nanosheet compared with p-CFO, demonstrating that the r-CFO nanosheets provide more oxygen vacancies. Furthermore, density functional theory (DFT) calculations showed that the introduction of oxygen vacancies into the r-CFO nanosheets lowered the adsorption barrier energy to 0.56 eV, thus facilitating the adsorption of the reactant on the catalyst surface. Along with oxygen vacancy doping, the slight narrowing of the bandgap was induced, which enhanced the electronic conductivity of the r-CFO nanosheets. Compared to pure CFO, the oxygen-vacancy-rich r-CFO nanosheets exhibited a smaller overpotential and Tafel slope for the OER in alkaline electrolyte. The introduction of oxygen vacancies created a small adsorption energy barrier and bandgap, improving the conductivity and charge transfer capability. Taken together, the oxygen vacancies and substrate support can also improve the intrinsic properties of catalysts.

Fig. 5 (a) Schematic diagram illustrating the synthesis process of r-CFO and the significant role of oxygen vacancies in electrocatalytic overall water splitting; (b) the XPS spectra of O1s for CFO, p-CFO and r-CFO; (c) the model of CFO-1 Ovac with one oxygen vacancy on the surfaces; (d) polarization curves of CFO, p-CFO, r-CFO and RuO₂ for the OER; (e) adsorption energy (E_ads) comparison of CFO-Fe1_oct, CFO-1O_vac-Fe1_oct and CFO-1O_vac-Fe2_oct.³⁹ Reproduced with permission. Copyright 2019, Royal Society of Chemistry. (f) EPR spectra of three BMO electrodes; digital photos of water contact angle for (g) BMO, (h) N-BMO and (i) A-BMO.⁴⁵ Reproduced with permission. Copyright 2021, Elsevier. (j) Schematic of CoFe₂O₄ nanosheets.⁵⁹ Reproduced with permission. Copyright 2019, Royal Society of Chemistry. (k) Schematic illustration of cation vacancy formation in 2D NiFe-LDH.⁸⁶ Reproduced with permission. Copyright 2022, John Wiley and Sons.

Ma et al.⁴⁵ employed a one-step hydrothermal reaction to create Bi₂MoO₆ (BMO) nanosheet arrays on NF. The surface of BMO was modified via treatment with ammonia plasma, whereby N atoms were doped into BMO and oxygen vacancies were simultaneously introduced. First, to determine the density of vacancies, electron paramagnetic resonance (EPR) was utilized as primary characterization technique.⁹⁴ Compared to the pristine state of BMO, N-BMO produced a sharp and intense oxygen defect signal, indicating that the concentration of oxygen defects was higher. Hydrophilicity is a well-known criterion for evaluating the gas-bubble-releasing behavior of electrocatalysts. As shown in Fig. 5g–i, the contact angle of N-BMO (27.8°) was smaller than that of BMO (53.7°) and A-BMO (29.0°), indicating that N-BMO had a lower barrier for the adsorption of H₂O molecules and desorption of O₂ on the electrode surface. Similarly, Chi et al.³⁵ developed a novel category of bifunctional electrocatalyst for the OER, based on ordered 3D anoxic CoMoO₄ nanosheet arrays. The target electrocatalyst was fabricated through a simple hydrothermal process, which was further enhanced by annealing under a reducing atmosphere of H₂ and ammonia. These resulting materials were denoted as NF/H-CoMoO₄ and NF/N-CoMoO₄, respectively. Then, the NF surface was completely wrapped by well-distributed 2D CoMoO₄ nanosheets. The 2D nanosheet arrays could maintain close contact with the 3D NF substrate, facilitating charge and mass transfer, resulting in stable electrocatalytic activity. Self-supported and binder-free architectured nanosheets are recognized as efficient electrodes for accelerating charge and gas diffusion. More importantly, an essential factor contributing to the improved OER performance is the existence of stable oxygen vacancies within the structure of the CoMoO₄ nanosheets. Incorporating oxygen vacancies into 2D oxide materials offers a promising approach to enhance their activity, opening up new possibilities for designing and synthesizing optimal catalysts for efficient hydrogen production through water splitting.

Previous research on nanomaterials with regulated morphology and the high density of active sites generated from Prussian blue analog (PBA) precursors has been published.⁹⁵ After thermal annealing, the majority of PBA-derived naomaterials formed porous or hollow structures. However, there is limited understanding about the transformation of PBAs into 2D ultrathin nanosheets. Fang et al.⁵⁹ created 2D ultrathin CoFe₂O₄ nanosheets with numerous oxygen vacancies by reducing PBAs with NaBH₄ at ambient temperature (Fig. 5j). With the large surface area, high density of surface-active sites, high electron transfer capability and durability, CoFe₂O₄ shows a low overpotential and Tafel slope for alkaline medium.

Interestingly, researchers have shifted their attention towards regulating cation vacancies. Wu et al.⁸⁶ reported that cation vacancies in 2D NiFe-LDH arise from the leakage of metal cations due to aprotic-solvent-solvation. The resulting defective NiFe-LDH exhibits a shorter M–O length and longer M–M distance. These cation vacancies are crucial in the surface reconstruction process of NiFe-LDH, serving as a precatalyst to enhance the performance of the OER. In situ Raman spectroscopy and DFT calculations have revealed that the presence of cation defects in NiFe-LDH facilitates the localized conversion of crystalline Ni–(OH)_x species to NiOOH species (Fig. 5k).

2.2.4. Heterogenous engineering. Constructing heterojunction catalysts via hybrid engineering can induce a synergistic effect, in which the catalytic activity exceeds that of single-component catalysts and is beneficial for highly effective OER catalysts.^96,97 The coupling of suitable components can introduce different absorption sites for reaction intermediates, which are instrumental in the production of O₂. It is important to acknowledge the challenges in studying the surface interface of multi-component systems. Integrated operando characterization techniques, ex situ characterization techniques, and theoretical calculations are crucial in unveiling the underlying reaction mechanisms.^98,99 Heterojunctions are often created when two semiconductors have similar crystal structures, atomic spacing, and coefficients of thermal expansion. Techniques such as interfacial alloying, epitaxial growth and vacuum deposition can be used to create heterojunctions.

Yu and collaborators³⁸ obtained NiO/RuO₂ heterojunction nanosheets using an in situ growth strategy, as schematically illustrated in Fig. 6a. The ultrathin nanosheets grew uniformly on the NF substrate (Fig. 6b–e). Notably, NiO/RuO₂/NF has a rough surface with vertical nanosheets crossing each other to form nano/microchannels. Based on this structure, the NiO/RuO₂/NF electrocatalyst exhibits enhanced wettability, and the electrolyte is more accessible to the active sites at the interface.¹⁰⁰ This unique structure also reduces the adhesion of air bubbles and accelerates the detachment of oxygen products,¹⁰¹ leading to enhanced mass transfer and improved catalytic performance. Multiple lattice stripes were observed in the HRTEM image (Fig. 6e). The interplanar spacings of d = 0.315, 0.254, and 0.210 nm were associated with the (110) and (011) lattice planes of RuO₂ species, as well as the (200) lattice plane of NiO, respectively. The XRD pattern of NiO/RuO₂ also confirmed that the obtained composite involved coexistence of NiO and RuO₂ species (Fig. 6f). In the XPS analysis of NiO/RuO₂/NF, a noticeable shift towards the lower binding energy region was observed for the Ru 3p peak compared to the RuO₂/NF sample. Conversely, the Ni 2p peak underwent a positive shift of approximately 0.3 eV in relation to the RuO₂/NF sample (Fig. 6g and h). This change indicates that the redistribution of electrons at the heterostructure interface was due to the difference in electronegativity between the NiO and RuO₂ species.¹⁰² Consequently, the change in electron density around the nuclei of the Ru and Ni atoms showed the opposite results. Furthermore, RuO₂ has a strong affinity for O atoms, which facilitates the desorption of *OH.¹⁰³ At the same time, NiO is advantageous for water dissociation during the OER process. Therefore, the interfacial coupling effect of NiO/RuO₂ enables *OH desorption and H₂O dissociation, thus enhances the catalytic performance for the OER. Accordingly, the overpotential of NiO/RuO₂/NF was significantly reduced to only 250 mV at a current density of 50 mA cm⁻², surpassing the performance of both RuO₂/NF (330 mV) and NiO/NF (390 mV).

Fig. 6 (a) Synthesis diagram of NiO/RuO₂/NF; (b) SEM image; (c) and (d) TEM and (e) HRTEM images; (f) XRD pattern of NiO/RuO₂; (g) and (h) the XPS spectra of Ru 3p and Ni 2p for NiO/RuO₂/NF and RuO₂/NF.³⁸ Reproduced with permission. Copyright 2022, Elsevier. (i) HRTEM image of CoO_x–RuO₂; (j) OER polarization curves and (k) Tafel plots curves of samples.¹⁰⁴ Copyright 2021, American Chemical Society.

In another study conducted by Yu et al.,¹⁰⁴ a novel approach was taken to develop CoO_x–RuO₂/NF by employing multi-metal oxides and a self-supported nanosheet structure strategy. This was achieved by depositing amorphous CoO_x-decorated crystalline self-supporting RuO₂ nanosheets onto a NF substrate through a simplified hydrothermal and annealing method. The self-supporting nanosheets exhibited several advantageous properties, including a significantly increased specific surface area, numerous exposed active sites, and a larger contact area between the active substance and the electrolyte, thereby improving the OER kinetics. The HRTEM image depicted in Fig. 6i illustrates the lattice stripes of CoO_x–RuO₂, the interplanar spacing of 0.255 nm is consistent with the (101) plane of RuO₂, as confirmed by the XRD characterization. In some regions of the HRTEM image, no discernible lattice fringes can be observed, and these areas can be attributed to the amorphous phase of the CoO_x component. The CoO_x–RuO₂/NF electrocatalyst showed high OER activity, achieving an overpotential of only 260 mV at 50 mA cm⁻² in 1.0 M KOH. This performance surpasses that of other catalysts such as RuO₂/NF, CoO_x/NF, IrO₂/C/NF, and NF (Fig. 6j). Furthermore, CoO_x–RuO₂/NF demonstrated a smaller Tafel slope value (69.6 mV dec⁻¹) compared to the reference electrocatalyst (Fig. 6k). Conspicuously, the overpotential of CoO_x–RuO₂/NF was significantly minimized to 420 mV when operating under a demanding current density of 1500 mA cm⁻². These findings highlight the superior reaction kinetics of CoO_x–RuO₂/NF for the OER process. There are many reports on the construction of multicomponent heterojunctions for achieving enhanced catalytic activity. Qian et al.³⁶ prepared a NiCo@C-NiCoMoO/NF heterojunction electrocatalyst, including N-doped graphene-decorated NiCo alloys and mesoporous NiCoMoO nanosheets grown on 3D NF. At a current density of 1000 mA cm⁻², the overpotential was remarkably reduced to just 390 mV. Furthermore, the electrocatalyst showed excellent stability over 340 h with no significant decrease in the overpotential. These results indicate that this material has high catalytic activity and prolonged durability.

The combination of bimetallic oxide nanoparticles with bimetallic oxide nanosheets to form heterojunctions is a useful approach for enhancing the OER performance. Ma et al.⁵³ constructed Co₃Mo nanoparticles/pores with CoMoO₃ nanosheets to form a heterostructure (Co₃Mo/CoMoO₃ NPSs) for the OER under alkaline conditions. The Co₃Mo/CoMoO₃ heterojunction electrocatalyst exhibited excellent OER activity at a current density of 500 mA cm⁻², while maintaining a low overpotential of 410 mV. In addition to the heterojunctions composed of the alloys and oxides mentioned above, heterojunction electrocatalysts composed of metal nitrides and oxides have also been reported. Du et al.¹⁰⁵ introduced a 2D MoN/MoO₂ heterostructure nanosheet, in which a heterojunction was formed via partial nitridation; MoN was fabricated by annealing the Mo-based precursors under an ammonia atmosphere. The non-uniform interface between MoN and MoO₂ served as an active site, effectively enhancing both the surface reaction kinetics and conductivity. Remarkably, the MoN/MoO₂-6 sample displayed a significantly low overpotential of 292 mV at a current density of 10 mA cm⁻². Taken together, constructing heterojunctions offers an effective strategy for obtaining an OER catalyst with exceptional performance.

2.2.5. Elemental doping. Single-atom catalysts (SACs), as new-frontier catalysts in research, comprise isolated metal atoms anchored on a carrier with no direct metal–metal bonds around the single-metal atoms.^106,107 SACs can achieve the same or even better performance compared to bulk metal particles, thereby offering great potential in catalytic applications. This is primarily attributed to their exceptional catalytic activity, remarkable selectivity, and near-perfect atom utilization efficiency in both homogeneous and heterogeneous catalysts.¹⁰⁸ The activity and stability of SACs under high current densities can be tuned by selecting suitable carriers and the coordination environment of the metal atoms.¹⁰⁹ In addition, the bonding between the metal atoms and the supporting substrate material ensures a stable structure of the catalyst and prevents aggregation of the single metal atoms during electrocatalysis.¹¹⁰ Compared with other carriers, metal oxides have a high ability to stabilize single atoms and superb catalytic activity, which can be accounted for by the strong bond energy between oxygen atoms and single metal atoms.^111,112 Notably, among the various structural metal oxides, 2D oxide nanosheets combine the merits of low production costs, high chemical stability under environmental conditions, considerable surface area, and high conductivity. Moreover, the restricted dimensionality of the nanosheets results in a significant increase in the carrier concentration and electron mobility on the catalyst surface.^113,114 Therefore, 2D oxide nanosheets have high potential for application as support materials for SACs.

Wang et al.¹¹⁵ conducted a study on the multistep synthesis of OER electrocatalysts, which resulted in an unprecedented achievement of loading single Ir atoms onto a nickel oxide substrate at a record-breaking high level of approximately 18 wt%. High resolution high-angle annular dark-field (HADDF) and element sensitive X-ray absorption fine spectroscopy (XAFS) were conducted to explore the monodisperse state of Ir atoms. Fig. 7a displays bright spots of atomically dispersed Ir in the off-axis projection on the surface of nickel monoxide through HADDF-STEM images. The bright spots correspond to the constituent heavy, single Ir atoms, which were overlaid on the outermost layer of the nickel monoxide nanosheet. Based on this finding, a model of the single Ir atom on the ultrathin NiO sheets is shown in Fig. 7b, while Fig. 7c illustrates the off-axis projection that corresponds to the imaging conditions shown in Fig. 7b. The projection of Ir atoms in the [111] and [211] regions of Ir–NiO clearly reveals that these Ir atoms were situated within the same Ni atom column, indicating that the Ir atoms were successfully incorporated into the NiO crystal lattice (Fig. 7d and e). Accordingly, in this projection, the Ir atoms overlapped exactly with the Ni atoms, where the bright points were recognized as Ir atoms, as indicated by the atomic line profiles (Fig. 7f–h). The main peak at approximately 1.5 Å in the extended X-ray absorption fine structure (EXAFS) spectra of Ir L-edge corresponds to the nearest Ir–O bond, which is similar to the Ir local coordination structure in IrO₂ (Fig. 7i). Furthermore, EXAFS fitting of the Ni K-edge data revealed an Ir–Ni scattering pathway, confirming single-atom Ir doping of nickel monoxide (Fig. 7j). Consequently, the HAADF-STEM and XAS results both lead to the same conclusion that Ir atoms were evenly distributed within the external layer of nickel monoxide, and they were stabilized on the oxide surface by Ir–O bonds. DFT calculations (Fig. 7k and l) further confirmed that the Ir single atoms not only serve as active sites, but also enhance the activity of the intermediates surrounding the Ni atoms. The synergy of the Ni and Ir atoms significantly improved the OER performance of nickel monoxide. Unexpectedly, the electrocatalyst exhibited an overpotential of 215 mV at 10 mA cm⁻² in alkaline electrolytes (Fig. 7m). This synthesis approach offers a promising opportunity for creating highly loaded single-atom catalysts.

Fig. 7 (a) Representative HAADF-STEM micrographs of the Ir–NiO catalyst; (b) and (c) corresponding atomic models; (d)–(g) atomic STEM images along the [111] zone axis and [211] zone axis; (h) the lines represent the line profiles for HAADF intensity analysis labeled in (f) and (g); Fourier-transformed EXAFS spectra at the (i) Ir L-edge and (j) Ni K-edge of the IrNiO catalyst and the references; (k) free energy diagrams of the OER at a potential of 1.23 V vs. RHE on perfect NiO (001) and single Ir atom doped NiO (001) (IrNiO (001)); (l) DFT-calculated 2D potential landscape for an *OH species on the Ir-NiO (001) surface, the energy is relative to that of *OH adsorbed on the pristine NiO (001) surface; (m) polarization curves for the OER.¹¹⁵ Reproduced with permission. Copyright 2021, American Chemical Society.

In addition to the abovementioned regulation strategies, the electronic structure and crystal structure of 2D oxides can be modulated by directly doping metallic atoms or non-metallic atoms into the main material. Heteroatom doping can introduce additional active sites, thereby increasing the interaction between the catalyst and reactants. More importantly, heteroatom doping could adjust the electronic structure of catalysts, modify their energy band structure and energy level distribution, and thereby regulate the adsorption performance of reactants and electronic transport properties.

Chen et al.¹¹⁶ reported the introduction of non-metallic atoms into 2D nanosheets of NiFe₂O₄ to improve the conductivity and electrocatalytic performance through functionalization with phosphate ions. Phosphate-ion-modified NiFe₂O₄ (P-NiFe₂O₄) nanosheets were grown on carbon cloth using a facile hydrothermal method, followed by a phosphorylation process. Electrochemical testing showed that the obtained P-NiFe₂O₄ nanosheets exhibited a low overpotential of 231 mV at 10 mA cm⁻² in 1.0 M KOH, along with a Tafel slope of 49 mV dec⁻¹. In addition, the P-NiFe₂O₄ nanosheets displayed superior stability, after 50 h operation at various current densities, and the chronoamperometry curves showed no evident decay. Previous research has validated that incorporating high-valence foreign atoms can tune the electronic and local coordination structures of electrocatalysts. In addition to the formerly described element P, S is a regularly employed element in anion engineering methodologies. Oxygen vacancies are formed when the migration or loss of lattice oxygen from their normal positions results in the formation of soluble and over-oxidized metals, therefore, stabilizing lattice vacancies is a top priority.^117,118 Doping is an effective technique for increasing the activity and stability of lattice oxygen, and the S element has been demonstrated to be an excellent dopant for lattice stabilization. Yu and colleagues⁶¹ present a simple anion engineering technique for improving acidic OER performance in monoclinic SrIrO₃ (M-SrIrO₃) perovskite electrocatalyst. The newly made S-doped M-SrIrO₃ electrocatalyst has a considerable OER activity with an overpotential of just 228 mV to achieve 10 mA cm⁻² and a long-lasting durability of over 20 h in a high-acid environment, outperforming commercial IrO₂ and pristine M-SrIrO₃ electrocatalysts. Additionally, the DFT simulations for the S-doped M-SrIrO₃ catalyst were organized in a systematic manner to investigate the underlying OER mechanism. The DFT results show that S incorporation can increase the electrical conductivity of M-SrIrO₃ since the band gap of S-doped M-SrIrO₃ is noticeably less than that of unadulterated M-SrIrO₃. Furthermore, injecting S reduces the strong binding of reaction intermediates (O*/OH*/OOH*) on the M-SrIrO₃ surface, hence facilitating the OER process. Theoretical calculations and experimental verifications indicate that the S doping can modify the electronic structure of M-SrIrO₃ and lower the energy barrier of the OER, resulting in noticeably increased electrocatalytic activity toward the OER.

Precious metal atom modifications on non-precious metals can alter the chemical compositions and physical characteristics of materials.¹¹⁹ However, due to the large lattice discrepancy between alien metals and the metal oxide host, directly substituting noble metal atoms into the metal oxides poses a great challenge.¹²⁰ Through hydrothermal and thermal annealing treatment, self-supporting porous 2D Pd/NiFeOx nanosheets with a sizable cluster of peony-like flowers were created by Zhang et al.⁵⁸ The obtained Pd/NiFeOx nanosheets with more exposed active sites, which improve electrical conductivity, make it easier for electrolyte to diffuse, and encourage the formation and release of bubbles on the catalyst's surface. On NiFe-based materials, the Pd atoms primarily disperse along the edges of pores or nanoflakes of the porous 2D structure and provide active sites. The electrochemical results demonstrate that Pd/NiFeO_x nanosheets exhibit outstanding catalytic activity with ultralow overpotentials of 180 mV, 169, and 310 mV at 10 mA cm⁻² for the OER, and 76, 46, and 75 mV for the hydrogen evolution reaction (HER), in 1.0 M KOH, 0.5 M H₂SO₄, and 1.0 M phosphate-buffered saline (PBS), respectively. The following characteristics can be responsible for the effective catalytic activity and outstanding stability of both the HER and OER at all pH values: (i) the flower-like cluster made of 2D nanosheets is densely packed with uniformly sized pores, which not only provides more exposed active sites but also promotes gas adsorption and resolution. (ii) The 2D structure of Pd/NiFeO_x allows for close contact between the electrolyte and the catalyst, increasing the use of exposed active sites and promoting efficient electron and mass transfer. (iii) Significant coupling of Pd atoms with NiFeO_x further improves the electrocatalytic performance. (iv) Pd atom modulation of NiFeO_x active sites greatly decreases reaction kinetics and increases charge transfer, therefore strengthening HER and OER activities. The results of this study present a fresh idea for the manufacture of a novel catalyst for overall water splitting over the pH range.

3. Applications

3.1. Hydrogen production by water splitting

In recent years, hydrogen has emerged as a promising energy source and an important raw material for industrial and non-industrial purposes.^121–123 The process of electrolytic water splitting can be divided into two half-reactions, the anode undergoes the OER, while the cathode facilitates the HER.^124,125 At the anode, water molecules decompose to form oxygen, protons, and electrons. The protons generated pass through the diaphragm between the cathode and anode chambers during the reaction. Conversely, the electron/hydroxyl groups migrate from the anode to the cathode. Finally, the protons from the cathode combine with the electrons to generate H₂.¹²⁶

Defect-rich metal-based nanosheets are considered potential catalysts for electrocatalytic water splitting. The presence of nanopores on the surface/interface creates additional active sites, significantly enhancing their mass transfer capacity. Unfortunately, the simple preparation of bifunctional nanosheet electrocatalysts remains a great challenge. Song et al.⁴⁷ reported an economical and efficient method for acquiring ultrathin Ni–Mo nanosheets (NiMoO₄) for water splitting. NiMoO₄ exhibited excellent OER performance (239 mV@10 mA cm⁻²). Meanwhile, P-doped NiMoO₄ nanosheets displayed superior HER performance (144 mV@10 mA cm⁻²). In this study, researchers assembled NiMoO₄ and P-doped NiMoO₄ into a two-electrode system to assess their effectiveness in water-splitting performance. Overall, a major breakthrough in this study is the remarkable synergistic effect achieved through defect engineering and elemental doping, providing guidance for the development of new catalysts. Although the NiMoO₄ nanosheet is a bifunctional catalyst, different catalysts have been used for electrolytic water splitting at the anode and cathode, catalyzing the OER and HER, respectively. For application to the two types of catalytic reactions, many researchers have attempted to obtain novel composite catalysts by forming a heterojunction; these catalysts can be simultaneously used for both the OER and HER. For example, Zhang et al.¹²⁷ proposed the integration of components that are active for the OER and HER into a new heterostructure by developing ruthenium–doped nickel oxide ((Ru–Ni)O_x). The (Ru–Ni)O_x electrode exhibited excellent bifunctional catalytic activity (Fig. 8b and c) with minimum overpotentials of 15 mV for the HER and 237 mV for the OER at 10 mA cm⁻², respectively. Accordingly, the (Ru–Ni)O_x bifunctional electrocatalyst was assembled as an anode and cathode catalyst in KOH solution (Fig. 8a), and the total water splitting only required 1.48 V to deliver 10 mA cm⁻², which is lower than that of the commercial coupled Pt/C and RuO₂ catalyst. Furthermore, the chronoamperometry curves at 10 mA cm⁻² and 50 mA cm⁻² demonstrated good long-term stability, indicating the potential of (Ru–Ni)O_x for hydrogen production (Fig. 8d–f). Coincidentally, Yu and collaborators³⁸ discovered that CoO_x–RuO₂/NF presents good catalytic performance for the OER and HER. Notably, both the cathode and anode were CoO_x–RuO₂/NF electrocatalysts. A comparative electrolyzer consisting of Pt/C/NF as the cathode and IrO₂/C/NF as the anode was assessed. The electrolyzer employing a CoO_x–RuO₂/NF bifunctional catalyst required only 1.49 and 1.55 V to obtain current densities of 10 and 50 mA cm⁻², respectively; these values are superior to those of Pt/C/NF||IrO₂/C/NF (1.63 V and 1.76 V). In the durability tests, the bifunctional catalyst remained stable for 72 h at 1500 mA cm⁻², with slight attenuation in the chronoamperometry curve. This outstanding durability could be attributed to the following: (i) CoO_x decorated onto RuO₂ can lead to small impedances and enhance electron transport at a high current density;¹²⁸ (ii) the NF support substrate is favorable for electrolyte delivery to the active sites and gas diffusion, thus enhancing the catalyst's ability to withstand high current density increased durability;¹²⁹ (iii) the CoO_x–RuO₂/NF nanosheets are tightly integrated onto the support substrate, which could prevent detachment of the electrocatalyst.⁵² The above results suggest that CoO_x–RuO₂/NF is promising for industrial water splitting applications.

Fig. 8 (a) Schematic illustration showing the reaction route of the water electrolysis; (b, c) HER and OER curves. (d) Overall water splitting (e) comparison of potentials of (Ru–Ni)Ox (+, −), RuO₂ (+)‖Pt/C (−), NiO (+, −), and bare NF (+, −); (f) Chronoamperometry curves of (Ru–Ni)Ox (+, −).¹²⁷ Reproduced with permission. Copyright 2021, Elsevier. Overall water splitting application in 0.5 M H₂SO₄ electrolyte; overall water splitting application in 0.5 M H₂SO₄ electrolyte: (g) working principal diagram of two-electrode water splitting devices; (h) polarization plots of 4CeO₂@SrIrO₃||Pt-C based electrolyzer and benchmark IrO₂||Pt/C based electrolyzer; (i) Faraday efficiency of the OER conducted at 10 mA cm⁻² using a 4CeO₂@SrIrO₃ electrode.⁵⁵ Reproduced with permission. Copyright 2022, Elsevier.

Strontium iridate perovskites are potential OER electrocatalysts in acidic environments. You et al.⁵⁵ reported CeO₂ quantum dot decorated SrIrO₃ nanosheet synthesis using a sol–gel and wet chemistry approach. The obtained 4CeO₂@SrIrO₃ was employed as both the anode and cathode in a two-electrode system for the overall water splitting in 0.5 M H₂SO₄ electrolyte. It was found that to achieve 10 mA cm⁻² only required a cell voltage of 1.51 V, favorably rivaling 1.66 V over the commercial noble-based water electrolyzer. The durability of 4CeO₂@SrIrO₃ was evaluated using chronoamperometry, where the electrocatalyst remained continuously stable for 50 h at 10 mA cm⁻² in harsh acidic media. The results illustrate that 4CeO₂@SrIrO₃ manifests excellent performance in the overall water splitting process. The catalyst was characterized after long-term durability testing, where there was no obvious change in the morphology before and after the test, unveiling the exceptional catalytic stability of this electrocatalyst. The stable structure and catalytic performance of the 4CeO₂@SrIrO₃ ultrathin nanosheet catalysts are attributed to three factors: (i) the surface CeO₂ coating plays a protective role to stabilize SrIrO₃. (ii) This interfacial interaction, a novel mixed-dimensional electrocatalyst presents fast rapid kinetics. (iii) The interfacial charge redistribution reduces the energy barrier of the OER. All these factors result in enhanced electrochemical performance.

Aside from crystalline 2D oxides, amorphous materials also exhibit a highly active surface containing abundant dangling bonds and numerous undercoordinated active sites, making them potential electrocatalysts for overall water splitting over a wide pH range. Do et al.¹³⁰ recently reported a simple synthetic approach to confine atomically thin Pd–PdO nanodomains within amorphous RuO₂. The Pd₂RuOx catalyst exhibits an OER overpotential of 225 mV@10 mA cm⁻² and a HER overpotential of 14 mV@10 mA cm⁻² in 1.0 M KOH. It is noteworthy that the overall water splitting voltage is only 1.49 V@10 mA cm⁻², demonstrating high mass activity and work stability. This remarkable performance can be ascribed to the ultrathin 2D morphology and amorphous structure with abundant undercoordinated active sites of the materials. This finding offers a new insight and a rational design approach for highly efficient electrocatalysts utilizing 2D amorphous oxides.

3.2. Zinc–air battery

With the advantages of safety, high power density, and environmental friendliness, rechargeable zinc–air batteries have emerged as one of the most promising next-generation energy storage batteries. They hold great potential for applications in large-scale energy storage, electric vehicles, and other consumer electronics products. However, sluggish oxygen reaction kinetics of the air cathode serves as a major bottleneck.

The working principle of rechargeable zinc–air batteries is represented by eqn (3.1). When the battery is discharged, the anode undergoes oxidation, converting Zn metal into ZnO, while at the cathode, O₂ participates in the oxygen reduction reaction (ORR) to produce OH⁻. Conversely, when the battery is charged, ZnO returns to Zn metal at the anode, and the OH⁻ will produce oxygen via an oxidation reaction at the cathode. The charge/discharge reaction of rechargeable Zn–air batteries is reversible, where the reaction is represented by the following equation:

Total reaction equation:¹³¹

2Zn + O₂ (g) ↔ 2ZnO, E = 1.66 V vs. SHE (3.1)

Electrochemical reactions of electrodes during discharge.

Zinc anode:

Zn + 4OH⁻ ↔ Zn(OH)₄²⁻ + 2e⁻ (3.2)

Zn(OH)₄²⁻ → ZnO + H₂O + 2OH⁻, E = −1.26 V vs. SHE (3.3)

Air cathode:

O₂(g) + 4e⁻ + 2H₂O → 4OH⁻, E = 0.4 V vs. SHE (3.4)

Electrochemical reactions of electrodes during charge.

Zinc anode:

ZnO + H₂O + 2e⁻ → Zn + 2OH⁻ (3.5)

Air cathode:

4OH⁻ → O₂(g) + 4e⁻ + 2H₂O (3.6)

The advancement of economical and high-performance electrocatalytic materials as replacements for noble-metal-based catalysts and enhancing the electrocatalytic activity of oxygen is the goal of large-scale practical applications. Thus far, there are few reports of 2D oxide-based anode catalysts for Zn–air batteries. Li et al.¹³² developed a strategy for doping heteroatoms into bimetallic oxides by creating a 2D structure, followed by doping, mixing, and post-doping steps. The local coordination environments around the metal catalytic sites within the 2D materials were modified. Notably, the Co³⁺/Co²⁺ and Mn³⁺/Mn²⁺ ratios increased as the electronic structure was tuned, leading to further optimization of the adsorption and desorption of intermediates during the OER.¹³³ A schematic illustration of the Zn–air rechargeable battery is shown in Fig. 9a. The LSV test results show that N/PCu_0.1Co_0.3Mn_0.6O₂/CNTs possess an overpotential of 160 mV at 10 mA cm⁻² (Fig. 9b). DFT calculations showed that the adsorption energy of the *OH radicals was markedly decreased following non-metal doping. Specifically, adsorption of the second *OH species to form the *OOH species is RDS, which corresponds to the overpotential of 0.41 V. Compared to other catalyst models, Cu_0.1Co_0.3Mn_0.6O_1.8N_0.2P_0.2 composites have the lowest energy barrier for the RDS, indicating their superior catalytic activity for the OER (Fig. 9c). Moreover, the target catalyst exhibited good ORR performance (E_1/2 = 0.82 V) and the zinc–air batteries using it as a cathode catalyst outperformed commercially available noble metal-based catalysts in terms of stability, peak power density, and voltage gap (Fig. 9d–h). Kumar et al.¹³⁴ developed a low-cost and stable trimetallic oxide catalyst NiCoMoO₄@rGO via a one-step hydrothermal method, where changes in the electrical environment of the active sites were modified via site-selective Mo substitution. Operando XAS, in situ XRD and DFT analyses revealed the crucial role of Mo atoms in reducing the overpotential and optimizing the energy barriers. In Zn–air batteries, the NiCoMoO₄@rGO electrocatalyst exhibited a significantly higher specific capacity and corresponding energy density, proving an interesting concept for designing bifunctional electrocatalysts.

Fig. 9 (a) Schematic illustration of the rechargeable Zn–air battery; (b) the OER electrocatalytic performance of LSV curves in 1.0 M KOH; (c) the potential free energy profiles of the intermediate states in the OER on Co_0.4Mn_0.6O₂, Cu_0.1Co_0.3Mn_0.6O₂. Cu_0.1Co_0.3Mn_0.6O_1.8N_0.2, and Cu_0.1Co_0.3Mn_0.6O_1.8N_0.2P_0.2 nanosheets at zero potentials; (d) the power density and discharge curves using N/P-Cu_0.1Co_0.3Mn_0.6O₂/CNTs and IrO₂-Pt/C as cathodes; (e) the charge and discharge curves; (f) digital photo of the LED lamp powered by two connected zinc-air batteries. (g) The cycling performance at 10 mA cm⁻². (h) The difference of the voltage plateaus during charge and discharge for the first and last cycle.¹³² Reproduced with permission. Copyright 2023, Elsevier.

3.3. Electrochemical conversion of CO₂

Electrocatalytic CO₂RR technology can strategically focus on producing high-value chemicals, including CO, HCOOH, C₂H₄, and CH₃CH₂OH. Currently, the production of these chemicals relies heavily on fossil fuels and requires energy-intensive processes involving high-temperature and high-pressure processes. By targeting these chemicals, electrocatalytic CO₂RR can offer an alternative and environment friendly solution to reduce dependence on traditional fuels. Despite decades of research, the process at the laboratory scale has not yet resulted in significant industrial implementation. The CO₂RR at the cathode, coupled with the OER at the anode, faces the challenge of a high energy barrier and slow kinetics of the OER, particularly in commonly used electrolytes. The overall efficiency of electrical energy to chemical fuel conversion must take into account these limitations. Therefore, OERs at electrocatalysts are also key reactions for the electrocatalytic CO₂ conversion in aqueous solution.

Mi et al.¹³⁵ reported hierarchical Co₂FeO₄ nanosheet arrays on an Fe foil carrier, which underwent a topotactic transformation process from the CoFe-LDH precursor, as bifunctional catalysts (Fig. 10a). Electrolysis was performed at a specific potential range, and gas chromatography and nuclear magnetic resonance techniques were utilized to qualitatively and quantitatively analyse products. After a constant voltage response for 1 h in CO₂ saturated 0.1 M KHCO₃, the faradaic efficiency of the CO product reached up to 92% at −1.0 V and 17% at −0.8 V. At the other end of the bifunctional membrane, the overpotential of Co₂FeO₄ at 10 mA cm⁻² is 230 mV in 1.0 M KOH, which is smaller than Ir (300 mV). The good performance of Co₂FeO₄ could be due to the mass transfer ability and rich active sites originating from the nanoarray structure. DFT calculation further confirmed the Co–O–Fe sites form *COOH and *O intermediates in Co₂FeO₄ toward oxygen evolution and CO₂ activation (Fig. 10b–e). The OER catalysts for coupling CO₂RR under near-neutral conditions have traditionally relied on precious metals. Meng et al.²⁶ developed a non-noble metal based OER electrocatalyst through one-step synthesis, and the obtained NiFe-HC electrocatalyst displays a distinctive flower-shaped plate morphology (Fig. 10f). Remarkably, the OER activity of NiFe-HC requires excess commercial IrO₂ and Pt/C. Integrating a NiFe-HC anode with a CO₂RR catalyst cathode in a CO₂ electrolyzer under 2 M KHCO₃, results in the selective conversion of CO₂ to CO with a higher than 97% faradaic efficiency at 20 mA cm⁻² (Fig. 10g). This facile approach to obtaining an inexpensive and efficient anode OER electrocatalyst could greatly boost the scalability and sustainability of the CO₂RR for industrial applications.

Fig. 10 (a) Schematic diagram for the synthesis of Co₂FeO₄ nanosheet arrays on the Fe substrate. Gibbs free energies diagrams for (b) CO₂RR and (d) OER on Co₂FeO₄ (311); Pathways for (c) CO₂RR and (e) OER on Co₂FeO₄ (311).¹³⁵ Reproduced with permission. Copyright 2020, John Wiley and Sons. (f) SEM image of NiFe-HC. (g) Experiment setup of the CO₂ electrolyzer.²⁶ Reproduced with permission. Copyright 2019, PANS.

4. Summary and outlook

2D oxides materials exhibit surprising potential for the OER, due to their large specific surface area and abundant active sites. However, pristine 2D nanosheets are prone to stacking and aggregation, and their intrinsic catalytic activities and atomic utilization are not ideal. In this review, we discuss diverse physics and chemical strategies for enhancing the OER activity of 2D oxides, including in situ growth into vertical arrays of nanosheets on a substrate, combined with other nanomaterials, heterogeneous engineering, single-atom loading of noble metals or non-metal atom on 2D oxides, and the preparation of oxygen vacancies. The progress in the application of 2D oxides as OER catalysts for electrolytic water splitting, zinc–air batteries and electrochemical conversion of CO₂ is also summarized. Clearly, 2D oxides have great practical application value as OER catalysts in industry. Based on the above discussion, we propose several research opportunities for 2D oxides.

(1) Currently, the primary research of 2D oxide-based materials for the OER focus on transition metal oxides. These include oxides of Co, Ni, Mn, Fe, Cu, and Mo. The development of 2D oxide materials with high intrinsic catalytic activity, such as noble-metal oxide and bimetallic oxide nanosheets, is expected to extend the type of 2D metal oxides.¹³⁶

(2) Although there have been many recent reports on the use of bulk oxides such as MnO₂ and RuO₂ for acidic OER, reports of the 2D structure of 3d/4d transition metal oxides are limited in acidic environments. The exploration of stable 2D oxide OER electrocatalysts under acidic conditions is crucial for achieving effective electrolytic hydrogen production.

(3) The strategies for modulating 2D oxides should be further enriched, such as the simultaneous loading of various single atoms or the direct synthesis of 2D oxides with high entropy atoms, to improve the catalytic performance and stability of the oxides.^137,138

(4) The energy devices associated with the OER are not limited zinc–air batteries, and include alkali metal–air batteries such as lithium–air batteries, sodium–air batteries, potassium–air batteries, as well as ammonia oxidation, and methanol oxidation devices.

(5) Currently, the fabrication of 2D structures using 3d transition metal materials is relatively straightforward. In contrast, the synthesis of noble metal (such as Ir, Ru, Rh, and Au) based 2D structures remain extremely difficult, as noble metal atoms prefer to form 3D close-packed structures.¹³⁹ Therefore, the synthesis of 2D oxides based on precious metal has potential for OERs in acidic media.^140,141

(6) The OER process is greatly impacted by various factors, including the reaction and service conditions, as well as their interactions, such as local pH and its gradient distribution, and the types/concentrations of exotic ions.¹⁴² As reported, inactive anions (MoO₄²⁻, PO₃³⁻) have been employed as electrolyte additives in alkaline electrolyte due to their dissolvable property.¹⁴³ Consequently, exploring the effect of inactive ions in 2D oxides is imperative for the design of efficient catalysts.

Overall, we believe that achieving efficient OER is a major challenge for the future exploration of new energy devices, and 2D oxides are critical nanomaterials for this process because of their unique structures and properties. Therefore, we expect that this review will contribute to the development of the energy conversion and storage fields.

Author contributions

X. C. and D. L. conceived the review. Y. L. and Q. J. completed the references and wrote the manuscript. Y. L. and Y. D. helped organize images. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work has been supported by funding from the Natural Science Foundation of China (No. 52003163, 22105129, 52373266 and 12305364), the Guangdong Basic and Applied Basic Research Foundation (No. 2022A1515010670 and 2022A1515011048), and the Science and Technology Innovation Commission of Shenzhen (No. KQTD20170810105439418).

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