Emerging two-dimensional supported atomic and cluster catalysts for CO₂ electroreduction

Abstract

In recent years, the electrocatalytic carbon dioxide reduction reaction (CO₂RR), driven by renewable energy and operated under mild conditions with controllable reaction pathways, has emerged as a promising route for carbon-neutral energy conversion. Two-dimensional supported catalysts have attracted particular interest owing to their tunable electronic structures and well-defined active sites. However, how the number and spatial configuration of active centers govern CO₂RR activity and selectivity remains insufficiently understood, limiting the rational design of efficient catalysts. This review provides a comprehensive overview of recent experimental and theoretical advances in two-dimensional supported catalysts for the CO₂RR, including single-atom (SACs), double-atom (DACs), and three-atom (TACs) catalysts, and metal clusters, with an emphasis on insights obtained from density functional theory (DFT). The fundamental reaction pathways of the CO₂RR are first summarized, highlighting structure–activity relationships between active-site characteristics and catalytic performance. Subsequently, the advantages and limitations of different catalyst architectures are critically compared, and the mechanisms of CO₂ reduction to C₁ products such as CO, HCOOH, and CH₄ are systematically analyzed. Particular attention is given to the role of DFT in elucidating reaction pathways, charge transfer, and adsorption energetics, thereby revealing key descriptors governing activity and selectivity. By integrating experimental observations with theoretical insights, this review aims to provide a mechanistic framework and design principles for the development of advanced two-dimensional supported catalysts for efficient CO₂RRs.

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Yimeng Sun

Yimeng Sun received her BE degree in Energy Storage Science and Engineering from the University of Science and Technology Liaoning in 2025. She is currently a master's student in Chemical Engineering and Technology at the same institution, under the supervision of Associate Professor Lin Tao. Her research focuses on the electroreduction of carbon dioxide.

Lin Tao

Lin Tao is currently an Associate Professor at the University of Science and Technology Liaoning. He received his PhD in Metallurgical Engineering from the same institution in 2021. His research interests focus on electrochemical materials, computational chemistry, and metal oxide semiconductor gas sensors.

Yaqiong Su

Yaqiong Su is currently a full Professor in School of Chemistry, Xi’an Jiaotong University, China. He received his PhD degree in Catalysis at Eindhoven University of Technology in 2019. His main research interests are computational energy catalysis, electrochemistry of materials, and interfaces/surface-enhanced Raman Theory of Surface Enhanced Raman Spectroscopy.

Baigang An

Baigang An is currently a Professor at the University of Science and Technology Liaoning. He received his PhD in Applied Chemistry from Tianjin University in 2003. His research interests focus on energy materials and electrochemical energy storage technologies.

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1. Introduction

Against the backdrop of global climate change, CO₂ emissions have become a core focus.¹ Since the Industrial Revolution, humanity's extensive reliance on fossil fuels and significant changes in land use patterns have led to a sharp increase in the concentration of CO₂ in the atmosphere, jumping from approximately 285 ppm before industrialization to 417 ppm in 2022.² As shown in Fig. 1, the total CO₂ emissions from fossil fuels in China reached as high as 11.4 ± 1.5 PgCO₂ per year. This continuous and large-scale emission is triggering a series of severe environmental problems.^3–5 Therefore, accurately grasping the spatio-temporal characteristics of CO₂ emissions and analyzing their underlying causes have become key to formulating effective emission reduction strategies and mitigating the impacts of climate change.

Fig. 1 The spatial distribution of (a) the posteriori emissions, (b) the absolute discrepancies between the a posteriori and MEIC inventories (Mg km⁻² year⁻¹) for total annual CO₂ emissions across China, and (c) a provincial-level comparison (TgCO₂ per year) of the two emission datasets. Reproduced with permission from ref. 3, Copyright 2024 by American Chemical Society.

The utilization of CO₂ serves as a fundamental prerequisite for establishing a sustainable artificial carbon cycle. As a key feedstock in C₁ chemistry, CO₂ can currently be converted through three primary pathways. The first involves reforming CO₂ with CH₄ to produce syngas.⁶ The second entails transforming CO₂ into fuels via electrochemical, photochemical, or thermochemical reactions.⁷ The third pathway focuses on the production of high-value chemicals through catalytic hydrogenation.⁸ CO₂ transformation not only reduces fossil fuel consumption but also effectively alleviates the environmental impact of CO₂ generated from human production and daily life,⁹ making it one of the important approaches to tackle excessive CO₂ emissions. However, the CO₂ molecule is linearly symmetrical in shape with the carbon atom in the highest oxidation state. The C–O bond length is only 0.116 nm and exhibits certain triple bond characteristics, and has a bond energy as high as 750 kJ mol⁻¹, which indicates that breaking the C=O double bond requires great energy.¹⁰ In general, CO₂ molecules exhibit chemical stability, thermodynamic stability, and kinetic inertness, making their activation and transformation highly challenging.^11,12 Additionally, the CO₂RR involves complex processes entailing multiple electron transfer steps, hydrogenation, and C–C bond coupling, which result in CO₂ conversion with high energy consumption and low efficiency.

Light, electric, and thermal energy act as the requisite energy sources in the photocatalytic, electrocatalytic, and thermal catalytic reductions of CO₂, respectively.¹³ Among these approaches, the thermal catalytic CO₂RR is capable of generating a wide range of products, including various liquid fuels. However, it typically requires elevated temperatures and pressures and often depends on fossil fuel-derived energy sources.¹⁴ In contrast, photocatalysis and electrocatalysis can harness solar energy and electrical energy to drive the transformation of CO₂ into C₁ or C₂₊ products.^15,16 Consequently, photocatalytic and electrocatalytic CO₂RRs represent key pathways for achieving the sustainable utilization of new energy and are important strategies for attaining carbon neutrality. Notably, the CO₂RR stands out as a pivotal technology for transforming CO₂ into valuable energy products such as CO, HCOOH, and other C₁ compounds, owing to its efficiency and controllability.^17–22

The core function of an electrocatalyst lies in reducing the activation overpotential of the reaction. In addition, the active sites of catalysts can optimize the reaction process and decrease the activation energy by opening up an alternative reaction pathway. That is, catalysts can adsorb CO₂ molecules and generate new intermediates. By controlling the adsorption behavior of the above new intermediates and weakening the interaction force between them, a more favorable condition for the reaction is obtained. Nevertheless, the diverse range of CO₂RR products (including CO, CH₄, CH₃OH, C₂H₄, C₂H₅OH, etc.) and their similar reaction potentials²³ tend to result in the formation of mixed products. Furthermore, the hydrogen evolution reaction (HER) is a major competitive side reaction in the CO₂RR, leading to extra energy consumption and reduction of reaction efficiency. Therefore, exploring novel, high-performance CO₂RR electrocatalysts with low overpotential and the ability to inhibit the HER is of great significance.

The unique quantum confinement effects of two-dimensional (2D) materials have positioned them as a rapidly emerging and highly regarded platform in catalytic research, adjustable electronic structures, abundant active sites, large specific surface areas, and cost-effectiveness.^24–31 In the selection of carriers for atomic dispersion catalysts, apart from two-dimensional materials, porous crystal materials such as metal–organic frameworks (MOFs) and zeolites have also attracted extensive attention due to their regular and ordered pore structures, extremely high specific surface areas, and precisely modifiable coordination sites. For instance, MOFs provide an ideal space for anchoring and confining metal precursors, which is conducive to the preparation of SACs with high loading capacity.^32–34 The rigid framework and ion-exchange sites of zeolites can effectively stabilize metal cations and prevent their migration and agglomeration.^35,36 These carrier systems have demonstrated unique potential in the CO₂RR. Graphene, with its two-dimensional layered architecture^37–39 and remarkable properties, has sustained global focus. This material exhibits a high visible-light transmittance of 97.7%, a heat conduction capability up to 3 × 10³ W m⁻¹ K⁻¹ at 298 K, an electrical conduction of approximately 10⁴ Ω⁻¹ cm⁻¹, along with a specific surface area of 2630 m² g⁻¹ as well as a Young's modulus of 1.1 TPa.^40,41 Owing to these properties, it demonstrates significant potential for applications spanning energy (e.g., storage, solar cells), electronics (e.g., transparent electrodes), aerospace, and advanced materials (e.g., composites, biomedicine, catalysis, HER),^42–45 holding the potential to replace numerous traditional materials. However, its zero-bandgap nature restricts its application in the field of logic electronic devices.^46,47 Beyond graphene, a multitude of two-dimensional materials have achieved successful prediction and laboratory-based synthesis in recent years. Of the hundreds of existing 2D materials, there are single-atom^48–52 and double-atom^53–57 catalysts where metal atoms serve as active centers embedded in a 2D substrate, as well as three-atom^58–62 and metal cluster catalysts,^63–67 transition metal-supported two-dimensional carbon nitrides,^68–72 and so on. The combination of high charge carrier mobility, efficient heat dissipation, and mechanical stability in these standalone materials makes them promising for high-performance CO₂RR electrocatalysis.

Since the 20th century, as research into material structures has deepened, traditional research methods have struggled to meet specific investigative demands. However, the rapid advancements in computing technologies and quantum physical theories have enabled the computer-based simulation of material characteristics. Researchers have successively created a variety of computational methods and theoretical frameworks, including quantum mechanical first-principles, molecular dynamics, and machine learning.^73,74 Among these, quantum mechanical first-principles simulate material properties based on the atomic spatial arrangement and electronic structures.⁷⁵ The scale of the systems it can handle typically ranges from several to hundreds of atoms, and it exhibits remarkable strengths in calculating electronic structures and optomagnetic properties. Quantum chemistry addresses chemical problems by applying the classical theories of quantum mechanics, allowing the acquisition of data including molecular orbital energies and the total energy of chemical systems.⁷⁶ Density functional theory (DFT), a major breakthrough during the evolution of electronic structure theory, establishes a correspondence between the ground-state wave function and the ground-state electron density.^77,78 This approach significantly reduces computational complexity while maintaining accuracy, proving capable of conducting in-depth studies on material reaction mechanisms thus enabling the rational design of materials and screening.

However, the reduction reaction of CO₂ still faces several fundamental challenges at the mechanism level in theoretical calculation research. Firstly, the initial activation path of the reaction remains controversial, and the adsorption strength of the intermediate has a significant impact on the selectivity of the main product. The identification of key intermediates (such as COOH and OCHO) and their formation sequence are difficult to define clearly. The reason why different metal and non-metal catalysts can generate a variety of CO₂RR products is mainly because they have unique adsorption capacity and reactivity for intermediates. Secondly, the carbon–carbon coupling mechanism for generating C₂₊ products has not been fully elucidated. Multiple pathways such as CO dimerization and Co–*CHO coupling are relatively close on the reaction energy barrier, leading to disputes over the rate-determining steps of the reaction and the sources of product selectivity.^79,80 The essence of these problems lies in the fact that the current theoretical models have limitations in accurately describing the real electrochemical interface environment, including the synergistic effects of multiple factors such as solvation and interface electric fields, which have not been fully quantified. Furthermore, the dynamic structural evolution of the catalyst surface at the working potential leads to significant differences between the calculation results based on the ideal static model and the structure and properties of the actual active sites, thereby severely restricting the ability to precisely predict reaction pathways and product selectivity at the atomic scale. To effectively overcome the energy barrier in the electron-transfer-proton coupling process, accelerate the catalytic reaction rate and suppress the occurrence of unwanted side reaction pathways, it is crucial to develop and design ideal catalysts with high selectivity and low energy consumption.

This review summarizes the recent research progress in CO₂RR catalysts for converting CO₂ into one-carbon products. The catalysts include two-dimensional materials such as single-atom, diatomic, and triatomic catalysts, and metal cluster catalysts embedded in two-dimensional substrates with metal active sites, and two-dimensional carbon nitrides supported by transition metals. It reveals the structure–activity relationship between the material properties and performance. In addition, it not only elaborates on the advantages and disadvantages of different catalysts and details the mechanism of CO₂ reduction to one-carbon products but also reviews the relevant content of density functional theory (DFT) calculations and electrocatalysis. This review aims to provide a basis for more suitable catalysts and accurate mechanisms for the CO₂RR.

2. CO₂RR on 2D materials

2.1. Single-atom catalysts

SACs have become highly promising material systems in the field of CO₂RR due to their nearly 100% atomic utilization rate, unique electronic structure, and precisely controllable coordination environment. Their design principles and performance optimization strategies have been systematically expounded in many recent reviews.^81–85 These works systematically summarize the latest research progress of carbon-based SACs in the field of CO₂ electroreduction, elaborating in detail their synthesis methods, characterization techniques, electrocatalytic performance and reaction mechanisms, and highlighting the improvement effect on catalytic activity and selectivity through optimization strategies such as regulating the coordination environment and constructing carrier defects.^86–90 At the same time, the challenges and prospects in this field in terms of controllable synthesis, performance enhancement and practical application were prospected, providing a comprehensive and in-depth reference for the research and development of efficient CO₂ conversion electrocatalysts.^91,92 Based on this, this paper particularly focuses on the SAC systems with two-dimensional materials as carriers, exploring how they can achieve more precise and diverse regulation of metal active sites through the introduction of carrier characteristics.⁹³

Compared with traditional three-dimensional porous carbon or oxide carriers, two-dimensional carriers (such as graphene, carbon nitride, MXene, etc.) have clear surface geometric structures and tunable electronic properties, providing an ideal platform for constructing and stabilizing specific coordination structures of SACs.^94–97 A large number of studies have shown that by precisely regulating the coordination number, types of coordination atoms (such as N, O, S, and Cl), and coordination configurations of the metal center on the two-dimensional carrier, the physical and chemical state can be effectively optimized, thereby directing the improvements in CO₂RR performance.

Specifically, coordination number regulation is a key means to adjust the d-band center of the catalyst and change the adsorption strength of key intermediates (such as COOH). For example, compared with the common M–N₄ configuration, low coordination number Co–N₂ sites have been proven to effectively reduce the formation energy barrier of COOH, achieving up to 94% CO selectivity under appropriate overpotential.⁹⁸ Coordination atom engineering involves introducing different electronegative heteroatoms to adjust the charge distribution of the metal center. For example, pyrrole N species can stabilize the Fe³⁺ valence state and optimize the reaction pathway;⁹⁹ while axial coordination of Cl (such as Fe–N₄Cl) can significantly change the adsorption behavior of intermediates.¹⁰⁰ Moreover, the defects and pore engineering of the two-dimensional carrier (such as constructing nitrogen vacancy Ni–N₃–V structure) not only provide abundant anchoring sites for metal atoms but also enhance the mass transfer of CO₂ and electrolyte, raising the current density to industrial relevant levels (such as >100 mA cm⁻²).¹⁰¹

Further strategies include constructing double-atom sites (such as Ni–Fe and Co–Cu) to achieve dual-center cooperative catalysis, and using tandem catalytic design (coupling SACs with Cu-based catalysts) to expand the product range from C₁ (CO, HCOOH) to high-value C₂₊ products (ethylene, ethanol).^102–104 It is worth noting that advanced in situ characterization techniques have revealed that SACs may undergo dynamic structural evolution under reaction conditions (such as Fe³⁺/Fe²⁺ valence state changes), which is closely related to their final activity and selectivity.¹⁰⁵ More cutting-edge research even utilizes atomic-level-resolved in situ electrochemical STM to directly observe the dynamic migration and reorganization process of Cu single-atom sites at the reaction interface and correlate it with the selectivity switch of products.¹⁰⁶ In summary, SAC systems based on two-dimensional carriers achieve precise control of the CO₂RR pathway through multi-dimensional and cross-scale structural design, demonstrating great potential from fundamental understanding to practical applications. Since Zhang's research group published their pioneering work in 2011,¹⁰⁷ SACs have developed in a crucial direction for overcoming bottlenecks within the realm of heterogeneous catalysis. Their research demonstrated that the activity of the Pt₁/FeO_x SACs in the CO oxidation reaction exceeds that of nano-platinum catalysts by three times.^108,109 With an atomic architecture where metal centers are coordinated by non-metal atoms, this type of catalyst offers unique advantages by maximizing the exposure of active sites. Its distinctive coordination structure and electronic properties can significantly accelerate the kinetic process of the CO₂RR, thus exhibiting great potential in this field.^110–113 Notably, the agglomeration of single atoms on the substrate continues to be a challenging issue in the preparation of SACs, as it can diminish the catalyst's performance. Therefore, a suitable support is required to form strong bonds with single atoms, preventing metal aggregation. Typical supports include chalcogenides, oxides, and carbon materials.¹¹⁴ While metal oxide supports can enhance CO₂RR activity through strong interactions with single atoms,^115–117 the oxygen species on their surfaces may cause the oxidation of metal single atoms, thereby reducing catalytic efficiency. In contrast, two-dimensional carbon-centric materials possess considerable application prospects, owing to their outstanding stability in reaction environments.^118–122

With its unique structure, two-dimensional graphitic carbon nitride (g-C₃N₄) has proven effective for anchoring Cu, Pd, and Pt single atoms, serving as an excellent SAC support. This prominence stems from its cost-effectiveness, robust physicochemical stability, and facile synthesis protocols.^123–126 Sun and colleagues⁶⁹ elucidated a general mechanistic principle governing g-C₃N₄-catalyzed CO₂ conversion (Fig. 2a), non-spontaneous processes are characterized by positive reaction energies, and such processes arise when CO₂ is adsorbed on the surface or undergoes transformation via an odd count of H⁺/e⁻ pairs to form free radicals. Conversely, spontaneous processes with negative reaction energies are associated with the release of products involving even H⁺/e⁻ pairs, including CO, H₂CO, and CH₃OH. Transition metal dichalcogenides (TMDs) have also garnered growing interest, with their asymmetric layered structures^127–129 conferring unique physical properties that theoretically and experimentally enhance CO₂RR efficiency.^130–134

Fig. 2 (a) Optimized configurations of planar and corrugated 2 × 2 g-C₃N₄ sheets, showing their resulting electronic DOS. Reproduced with permission from ref. 69, Copyright 2016 by the Royal Society of Chemistry. (b) Schematic of the TM@J-WSSe structure with 28 transition metal (TM) atoms anchored onto the substrate. Reproduced with permission from ref. 136, Copyright 2025 by Elsevier. (c) Stable structures of Ge-N₄G and T_l-N₄G. Reproduced with permission from ref. 137, Copyright 2025 by American Chemical Society. (d) Schematic of first-row transition metal single atoms anchored on a g-C₃N₄ monolayer for the CO₂RR. Reproduced with permission from ref. 68, Copyright 2025 by American Chemical Society. (e) Optimized atomic structures of the investigated catalyst series. Reproduced with permission from ref. 70, Copyright 2024 by American Chemical Society. (f) Schematic diagram of the structure of M–N₄@Gr (M = Fe/Co/Ni). Reproduced with permission from ref. 110, Copyright 2022 by American Chemical Society. (g) Top and side views of the optimized geometric structures for the Fe–N_x–C catalyst series (models I–IV). Reproduced with permission from ref. 140, Copyright 2023 by American Chemical Society.

The Janus WSSe (J-WSSe) monolayer, a novel two-dimensional TMD, is synthesized via selective epitaxial atomic replacement (SEAR), where one Se layer in WSe₂ is substituted with S.¹³⁵ This architecture—with W bonded to S on one side and Se on the other—breaks inversion symmetry. Transition metal J-WSSe SACs with anchored (TM) show outstanding electrocatalytic activity, yet the potential of J-WSSe with TM anchoring and Se incorporation vacancies as high-performance CO₂RR catalysts warrants systematic investigation. Chen et al.¹³⁶ explored 2d 3d, 4d, and 5d transition metals anchored on J-WSSe via anchoring, designing and screening a library of SACs (Fig. 2b). Their findings identified Os@J-WSSe, Cu@J-WSSe, Mn@J-WSSe, and Cr@J-WSSe as optimal catalysts for CO, HCOOH, CH₃OH, and CH₄ production, respectively. Notably, Cr@J-WSSe displayed unique dynamic stability, maintaining consistent active sites pre- and post-reaction. Tan and coworkers¹³⁷ reported CO₂RR performance of p-block metal SACs supported on N-doped graphene (M–N₄G, where M = In, Tl, Ge, Sn, Pb, Sb, Bi. Fig. 2c). Experimental results showed most M–N₄G catalysts exhibit superior CO₂RR selectivity over the competing HER. As a key intermediate species, *HCOO's adsorption energy has stood out as a trustworthy indicator to evaluate activity. Among these, Ge/Pb–N₄G demonstrated exceptional electrocatalytic efficacy for HCOOH synthesis, offering insights for rational SAC design.

Anna and colleagues⁶⁸ studied the stability of first-row transition metal atoms anchored on g-C₃N₄ monolayers in the CO₂RR. As displayed in Fig. 2(d), Sc, Ti, V, and Zn have large proton affinity (ΔG(H*) < −0.3 eV), which will result in active site blockage and possibly inhibit CO₂ conversion. In contrast, Cr, Mn, Fe, Co, and Ni have moderately positive energies of proton binding (ΔG(H*) = 0.1–0.4 eV), enabling suppression of the competing HER. Among systems with relatively strong binding energies showing repulsion (Ni, Mn, Co) Ni₁/C₃N₄ stands out, as its moderate *CO adsorption energy (−0.52 eV), optimized d-band center position (−2.3 eV vs. Fermi level), and charge modulation of the metal by coordinated nitrogen sites (Δq = 0.53 e⁻) achieve an optimal balance between overpotential (η_CO2RR = 0.38 V) and selectivity (78% CO Faraday efficiency). Liu et al.⁷⁰ systematically investigated CO₂RR over metal-modified C₃N₄ nanotubes (Mn/CNNTs) in Fig. 2e for CO and formic acid production. Detailed analysis of geometric and electronic structures revealed that CNNTs’ unique adsorption sites efficiently activate CO₂ and preferentially direct the CO₂RR toward the CO pathway, enhancing selectivity. Single-atom-modified armchair CNNTs (M/aN-6) exhibit the highest CO selectivity in the CO₂RR.

Furthermore, extensive research confirms that the local coordination environment dictates the electrocatalytic activity and selectivity of SACs, serving as a cornerstone for efficient electrocatalysis. Typically, SACs’ active centers adopt coordination frameworks, with catalytic efficiency hinging on the central metal, coordination atom types/numbers, support properties, and functional group modifications.^71,72 Even subtle structural perturbations can markedly alter SACs’ performance.¹³⁸ In M–N–C systems, for instance, modifying the carbon support morphology and generating nearby defects serves to both expose active sites and regulate the metal's electronic states, leading to enhanced catalytic efficacy.¹³⁹ Yang et al.¹¹⁰ confirmed that the coordination environment modulates d-orbital electron occupancy in M–N₄@Gr (M = Fe/Co/Ni) SACs (Fig. 2f), inducing distinct electron spin polarization that influences catalytic selectivity and performance. Fe–N₄@Gr exhibits significant spin polarization, with decreasing polarization following the sequence of Fe > Co > Ni.

In the deformable coordination field, metal–ligand bond distortion alters d-orbital splitting, regulating intermediate adsorption strength. Ni's d_x²−y² and d_xy orbitals bond with N 2p orbitals, leaving few empty orbitals to accept electrons and resulting in weak CO₂ adsorption. In contrast, Fe's d_z²/d_zx orbitals and Co's d orbitals remain partially occupied, endowing Fe/Co single atoms with both electron-donor and acceptor capabilities. This work demonstrates optimized catalytic performance via precise regulation of the coordination environment around non-precious metals, underscoring its significance for SACs. Wang et al.¹⁴⁰ elucidated the structure–activity relationship of Fe–N_xC_y–C (x = 2–4, y = 0–2) catalysts in the CO₂RR in Fig. 2g, that spin characteristics of Fe sites can induce modulation of the nearby coordination environment and density of active sites. The Fe–N₃P–C structure can sustain high CO yields over a wide potential range. This study demonstrates the intrinsic structure–performance relationship revealed by the reaction itself, and suggests that the synergism between coordination environment and the density of active sites is the key to understanding and enhancing the performance of SACs.

Although SACs have great potential, the homogeneity of their active sites still requires in-depth research, and achieving their controllable synthesis under different coordination environments remains challenging. Importantly, current SAC-derived CO₂RR products are predominantly CO, with limited reports of CH₄, HCOOH, etc. There is a paucity of research on generating high-value C₂₊ hydrocarbon fuels by tuning single-atom-support interactions, emphasizing the need to develop new SAC materials for highly selective hydrocarbon synthesis.

2.2. Dual atom catalysts

The efficacy of metal catalysts stems from interactions at unsaturated centers, which facilitate the binding of key species and subsequently enhance catalytic turnover.^89,141 Enhancing metal atom utilization by rationally lowering metal loading has emerged as an effective strategy to control overall costs while preserving intrinsic catalytic activity.^141–143 Consequently, a growing body of research is focused on reducing the size of nanocatalysts, thereby increasing the number of low-coordinated metal centers. Atomic-scale catalyst design, which maximizes the utilization of every atom in supported catalysts, represents the most efficient approach. Additionally, to more flexibly enhance the optimized adsorption of active centers and intermediates, DACs with proximate diatomic sites have emerged as a novel class of catalysts. Compared with SACs and nanoparticle systems, they exhibit multiple advantages such as high metal atom loading, highly adjustable active sites, easily modifiable electronic structures, and the synergistic effect of adjacent metal centers.^144–151 Leveraging these features, DACs aim to surpass the inherent reactivity of single metals by synergistically regulating intermediate adsorption/desorption and activation processes, thereby exhibiting unprecedented catalytic performance.

As illustrated in Fig. 3(a), Liu et al.¹⁵² investigated CeO₂(110)-supported diatomic catalysts (M_α–M_β/CeO₂, where M_α–M_β are 3d/4d/5d transition metals). Analysis of binding and aggregation energies revealed that metals with lower electronegativity and higher d-orbital occupancy stabilize supported DACs. High-throughput descriptor calculations identified a series of potential DACs with both high activity (turnover frequency, TOF > 0.1 s⁻¹) and selectivity (>50%). Among these, heterogeneous Au-based DACs (e.g., Au–Fe) exhibited good performance. The introduction of Au further activated adsorbed oxygen and weakened CO binding. This theoretical study establishes a foundational framework for the rational design of dual-site catalysts. Current DAC research remains in its infancy. DACs are classified as homonuclear or heteronuclear based on the identity of the two atoms in the active center.

Fig. 3 (a) Atomic structure of CeO₂ (1 × 1 × 1) with Ce (green) and O (pink) atoms, and schematic search diagram of DACs/CeO₂(110) for CO PROX. Reproduced with permission from ref. 152, Copyright 2025 by American Chemical Society. (b) DFT study of bimetallic Ni–Fe–N–C catalysts, structural configurations (Ni–N₄, Fe–N₄, FeNi–NC, FeNi–NSC with C-gray, N-blue, S-yellow, Ni-green, Fe-cyan) and their catalytic reaction energetics, including free energy profiles and stepwise energy comparisons. Reproduced with permission from ref. 153, Copyright 2024 by American Chemical Society. (c) Computational characterization of bimetallic TM₁/TM₂-N@Gra systems showing geometric configurations, thermodynamic stability, and CO₂ adsorption geometry. Reproduced with permission from ref. 57, Copyright 2022 by Elsevier. (d) Structural and electronic properties of pristine g-CN and Fe₂-doped g-CN systems. Illustrated are the optimized geometries (top and side views), TDOS, and comparative binding energy analysis. Reproduced with permission from ref. 154, Copyright 2021 by the Royal Society of Chemistry. (e) Proposed reaction pathway for CO₂ to C₂H₅OH, C₂H₄ on N-graphene based dual-atom catalysts, geometric configurations and overall reaction network encompassing both C₁ (CH₃OH, CH₄) and C₂ products, and two distinct reaction pathways from the *CO intermediate to C₂-coupled species. Reproduced with permission from ref. 155, Copyright 2020 by the Royal Society of Chemistry. (f) DFT study of Ni-based SACs for the CO₂RR, showing COOH* adsorption configurations on NiMN₆ and NiN₄ sites. Reproduced with permission from ref. 56, Copyright 2022 by American Chemical Society.

As shown in Fig. 3(b), Huang's group¹⁵³ demonstrated that modulation via diatomic catalysts or heteroatomic ligands is a promising strategy to optimize SACs for more efficient CO₂ conversion to high-value chemicals. Their study focused on electrocatalytic CO₂ reduction to CO using an N/S-coordinated Fe–Ni diatomic pair embedded in a carbon matrix (FeNi–NSC). Gibbs free energy analysis of CO₂RR steps revealed that S coordination weakens Fe site activity and collaborates with adjacent Ni sites to regulate bidentate adsorption of the COO* intermediate, significantly reducing the CO desorption barrier and altering the rate-determining step. Wei et al.⁵⁷ proposed that the catalytic property of bimetallic atomic sites may be enhanced by regulating the binding mode of crucial intermediates supported by bonding interaction and electronic structure analyses. As shown in Fig. 3(c), their study screened four typical transition metals (TM = Fe, Co, Ni, Cu) and then constructed ten TM₁/TM₂-N@Gra to systematically assess homonuclear and heteronuclear DACs for CO₂ to C₁ products. Among them, Fe/Co and CO₂ displayed optimal limiting potentials (U_L) for HCOOH formation (−0.37 V and −0.34 V, correspondingly), indicating the best catalytic performance. The enhanced activity for CO₂ reduction was attributed to the optimal adsorption energy of the HCOOH intermediate on Fe/Co sites, as revealed by electronic structure analysis.

Bui's team¹⁵⁴ proposed a new strategy to boost CO₂RR performance by integrating well-defined structural clusters. As shown in Fig. 3(d), they designed g-CN molecular skeletons hosting various transition metal (TM) dimers for the CO₂RR, ultimately identifying five atomic pairs (Cr₂, CrFe, Mn₂, MnFe, Fe₂) with excellent active performance. The performance of Fe₂@g-CN in the CO₂RR was further optimized by regulating the integration of iron atomic clusters. It is worth noting that the introduction of Fe₁₃ atomic clusters enhanced the CO₂ adsorption capacity while effectively inhibiting hydrogen activation. In addition, it also breaks the scale relationship between the intermediates (CO and CHO), thereby significantly enhancing the performance of CO₂RR and reducing the limiting potential of the C₁ path to −0.45V. This work broadens understanding of DAC mechanisms in the CO₂RR and offers insights for rational design and integration of heterogeneous DACs with specific atomic clusters for other applications. Chen et al.¹⁵⁵ screened bimetallic dimers supported on nitrogen-doped graphene (including homonuclear and heteronuclear transition metals). As shown in Fig. 3(e), C–C coupling analysis revealed that six catalysts (Cr–Cu, Mn–Cu, Co–Co, Co–Ni, Co–Cu, and Ni–Cu) showed coupling energy barriers under the threshold (0.75 eV), with the top five showing notably lower barriers. All six DACs demonstrated superior C₂ pathway selectivity in competitive HER and CO₂RR.

Despite atomic vibrations, the bimetallic atoms and their six coordinated nitrogen atoms maintained strong bonding without obvious structural distortion, indicating excellent heat stability at 500 K. This suggests these DACs are able to stably retain their configuration under both mild and high-temperature conditions. As displayed in Fig. 3(f), Zhu et al.⁵⁶ studied CO₂ activation based on the Ni–Cu catalyst. As shown, the secondary Cu could tune the energy level of the Ni 3d orbital into the Fermi level and then enhance the emergence of the rate-determining *COOH intermediate. Remarkably, a CO TOF of 20 695 h⁻¹ was obtained at −0.6 V (vs RHE) with the maximum CO Faraday efficiency of 97.7%. In situ XAFS monitoring of dynamic structural evolution further clarified interactions between Ni centers and CO₂ molecules, along with the Ni–Cu synergistic mechanism enhancing CO₂RR activity.

Constructing structures where one metal atom binds exclusively to another (especially in dimer form) remains a major challenge. Inhomogeneity in support pores and defects hinders precise fabrication of diatomic sites. In contrast, nano synthetic catalysts or SACs outperform DACs in terms of controllability and performance.¹⁵⁶ Excessive metal precursor usage can form metal nanoparticles, while reducing dosage may promote single-atom dispersion and isolation. However, DACs exist in an intermediate state, making precise control of precursor dosage difficult and often leading to uneven metal distribution in catalytic centers. Due to imprecise metal loading control, coexistence of single atoms, atomic clusters, and even larger aggregates is inevitable, triggering active site aggregation. When catalysts contain ill-defined clusters or particles, clarifying the structure–performance relationship of dual-site metal catalysts becomes challenging. Most current DACs use carbon-based supports, with relatively few studies concerning other substrates. Thus, the feasibility and performance of DACs on non-carbon supports require further exploration.

2.3. Three-atom catalysts

TACs offer expanded possibilities for complex electrochemical reactions, due to their enhanced metal atom loading, increased contact area, and unique structural features. First, their higher metal atom loading capacity brings active sites in closer proximity.¹⁵⁷ In contrast, the formation of metal–metal or metal–bridge bonds offers a strategic approach to modulate the co-adsorption energetics of various reactants and intermediates.^158,159 TACs have many types of asymmetric adsorption sites, which may destruct the linear scaling relationship and present different catalytic behaviors. The asymmetric active site is more applicable for complex catalytic reaction with more than one step and intermediate. In addition, the cooperation among three atoms can tune the d-band electronic structure to enhance the catalytic activity.^160,161 Through such active center synergies, TACs transcend the performance's simple additive relationship improvement, demonstrating robust efficacy within multi-molecular reaction systems.¹⁶² The variety in atomic species and valence states further expands the research space for triatomic sites.

Jia et al.¹⁶³ reported that TACs behave as high current electrocatalysts for the CO₂RR because the large number of active sites in TACs benefit from both the high electrochemical surface area offered by the nanocubes and fast charge migration. The electronic properties and stability of TACs are also related to their carrier materials. By anchoring the three atoms on appropriate carriers and taking advantage of their interfacial interactions, the agglomeration of clusters can be effectively avoided,¹⁶⁴ and an optimized triatomic coordination environment can effectively enhance catalytic performance.¹⁶⁵ For example, when three metal atoms are atomically dispersed on supports such as nitrogen-doped graphene or g-C₃N₄, TACs with either random^166–168 or triangular^169,170 structures can form. Such catalysts actively participate in electrochemical reactions—including N–N bond/O–O bond cleavage or C–C coupling—via orbital coupling or non-bonding interactions between metal atoms.¹⁷¹

Zhou's group⁶⁰ explored the utility of graphdiyne-supported 3TM-GY (TM = Cu, Fe, Co) applied in the CO₂RR to generate C₁ products, as shown in Fig. 4(a). The singular triangular TM trimer anchored on graphdiyne, characterized by strong adsorption energy and a distinct curved configuration, creates a beneficial setting for CO₂ adsorption and initial activation. 3Cu-GY preferentially reduces CO₂ to CH₄, exhibiting higher selectivity than other C₁ products and surpassing most atomically dispersed electrocatalysts. At increased applied potentials, the CO₂RR on 3Cu-GY favors high-throughput production of HCOOH and CH₄, while 3Fe-GY/3Co-GY favor deeply reduced products. Hydrogen evolution is strongly inhibited, rendering CO₂RR dominant. This performance enhancement is attributed to synergies among multiple metal centers, which promote adsorption of critical species, notably HCOO* and CH* and facilitate pathways conducive to CO₂ reduction.¹⁷² Sun et al.¹⁷³ compared SACs and TACs (Fig. 4(b)), and showed that SACs maximize atomic utilization and provide uniform active sites, while TACs precisely regulate intermediate adsorption via interatomic synergies, overcoming traditional catalytic limitations. Among their findings, 3Mo-C₂N₁ and 3Ti-C₂N₁ exhibited exceptional catalytic activity for CO and HCOOH production, respectively. Swapnil et al.¹⁷⁴ investigated CO₂ adsorption via TM_n-doped C₂N monolayers, revealing that TM₃-C₂N achieves a maximum adsorption capacity of 6 CO₂ molecules per trimer—significantly outperforming TM₁-C₂N and TM₂-C₂N systems.

Fig. 4 (a) Optimized atomic structures and charge density difference distributions of the 3TM-GY catalysts. Reproduced with permission from ref. 60, Copyright 2021 by Elsevier. (b) A schematic diagram showing the stability and conductivity of TM-C₂N₁. Reproduced with permission from ref. 173, Copyright 2024 by the Royal Society of Chemistry. (c) Design strategy and synthesis implementation. Reproduced with permission from ref. 176, Copyright 2024 by American Chemical Society. (d) Top-view structure of the trimetallic TM₁TM₂TM₃@GY catalyst. Reproduced with permission from ref. 58, Copyright 2024 by Elsevier. (e) Optimized CO* adsorption configurations on TM-NiFe catalysts, with corresponding Mulliken charges and projected density of states (PDOS) of the transition metal (TM) atoms. Reproduced with permission from ref. 175, Copyright 2023 by Elsevier. (f) Structural models of SACs, DACs, and TACs on Mo₂CO₂ MXene, with their corresponding transition metal constituents, formation energies, and dissolution potentials. Reproduced with permission from ref. 172, Copyright 2023 by the Royal Society of Chemistry.

Copper demonstrates unique efficacy in catalyzing the CO₂ reduction reaction toward hydrocarbon products, owing to its unique ability to adsorb *CO intermediates. However, traditional Cu catalysts suffer from deficiencies in CH₄ selectivity and activity. As shown in Fig. 4(c), Pan's team⁵⁹ addressed this atomic-level engineering of Cu sites and mesoscale control of support porosity, enabling dynamic transformation of single-atom Cu–N₃ into N- and OH⁻-anchored Cu₃ clusters under reaction conditions. The N, OH-Cu₃ site stabilizes *CO at −0.76 eV. The kinetic barrier for its hydrogenation to *CHO is 0.85 eV, markedly lower than the C–C coupling barrier (2.03 eV), suggesting that formate formation is kinetically preferential, selectively promoting CH₄ formation. Its moderate adsorption of CO capacity and low barriers for CO hydrogenation are key to high selectivity. Additionally, dual continuous mesoporous channels create a water-deficient local microenvironment, inhibiting hydrogen evolution and prolonging *CO intermediate retention, thereby promoting deep CO₂ reduction.

Similarly, as presented in Fig. 4(d), Li et al.⁵⁸ developed a series of high-performance non-noble triatomic catalysts (TM₁TM₂TM₃@GY, where TM = Mn, Fe, Co, Ni, Cu, Mo) supported on the 2D material graphdiyne, investigating their CO₂RR selectivity. MnMoFe@GY and MnMoCu@GY displayed optimal CO₂RR performance, achieving the highest activity and selectivity among all tested catalysts, with effective hydrogen evolution inhibition. They demonstrated good experimental feasibility, stable catalytic performance, and excellent CO₂RR selectivity—confirming TACs’ potential in CO₂RR catalysis. Liu et al.¹⁷⁵ designed a series of trimetallic TM-NiFe catalysts as presented in Fig. 4(e), finding that Zn–NiFe performs exceptionally in the CO₂RR-to-CO pathway. Cr–NiFe prefers the COOH* pathway over HCOO* for HCOOH production, enabling efficient HCOOH synthesis while significantly inhibiting competing reactions (hydrogen evolution and CO₂RR-to-CO). These results highlight the abundance of CO₂RR active sites in TACs. Xiao et al.¹⁷² observed that homonuclear diatomic and triatomic catalysts supported on 2D Mo₂CO₂ materials exhibit superior CO₂RR performance compared to single-atom systems. As shown in Fig. 4(f), multinuclear reaction sites in multi-atom catalysts significantly enhance adsorption of crucial intermediates (e.g., HCOO* and CH*), which facilitates the selective reduction to CH₄ at ultra-low overpotentials. Additionally, multinuclear sites effectively promote C–C coupling, creating conditions for efficient C₂H₅OH production.

Benefiting from multi-metal synergy, high selectivity, and cost-effectiveness, TACs have attracted significant research interest in CO₂ electroreduction. However, rational TAC design faces fundamental challenges. Future efforts must address precise synthesis, stability enhancement, and industrial adaptation. Integrating advanced characterization and computational simulation will be critical to advancing TACs from laboratory research to large-scale applications, contributing to global carbon neutrality goals.

2.4. Cluster

Atomic clusters, ranging from a few to hundreds of atoms, typically exhibit a particle size measuring 1 nm, with a maximum value of 3 nm, placing them in the category of ultrafine particulate materials (Fig. 5a).¹⁷⁷ Relative to traditional nanoparticles (with sizes of 5–500 nm), their atypically small size confers unique properties absent in larger nanoparticles, arising from pronounced quantum size effects. For conventional nanoparticles, functional performance is governed by surface structure and geometry.^178–180 The electronic structure of atomic clusters demonstrates a hybrid nature, lying between molecular-like states and the band structure of bulk materials, endowing them with distinctive characteristics.^181–184 Additionally, clusters possess diverse structural features unobserved in other chemical species, including constituent atom count, element types, elemental proportions, atomic arrangement, and geometric symmetry. These features thus allow for the creation of numerous active sites tailored to adsorb and convert specific reactants and intermediates in the CO₂RR.^64,65 This ability to maintain high utilization efficiency with polymetallic atoms provides substantial flexibility in material design. Furthermore, their molecular-scale spatial configurations and electronic structures have great potential for achieving novel properties and functions, effectively promoting reactant activation and modulating the adsorption of key species (e.g., *COOH, *OCOH) to facilitate the CO₂RR.¹⁸⁵

Fig. 5 (a) Material classification by size scale. Reproduced with permission from ref. 176, Copyright 2024 by the Royal Society of Chemistry. (b) Schematic of the synthesis of Fe–NC–SBA-15. Reproduced with permission from ref. 185, Copyright 2020 by the Royal Society of Chemistry. (c) Structural model of the CoNiCuRh@FeN₄ high-entropy catalyst (ΔS_mix = 0.14 meV K⁻¹), and the correlation between its dissolution potential (U_diss) and formation energy (E_formation). Reproduced with permission from ref. 187, Copyright 2025 by Elsevier. (d) Molecular structures of three copper-bromide-phosphine complexes: [Cu₃(μ-Br)₂(bis-methyl-bisphenylphosphine)₃] Br-(CuBr–BisM), Cu₂(μ-Br)₂(1,2-phenyl-bisphenylphosphine)₂ (CuBr–12B) and Cu₂(μ-Br)₂(triphenylphosphine)₂(4-phenylpyridine)₂ (CuBr–4PP). Reproduced with permission from ref. 188, Copyright 2024 by Nature. (e) Most stable CO₂ adsorption configurations, charge density difference isosurfaces, and projected density of states (PDOS) for CO₂ on the FT Cu₃(Se)@MoSSe, FT Cu₃(S)@MoSSe, FT Cu₃(Te)@MoSTe, and FT Cu₃(S)@MoSTe catalysts. Reproduced with permission from ref. 196, Copyright 2024 by American Chemical Society. (f) Theoretical simulations of the CO/HCOOH formation pathways, electronic structure analysis (charge density and PDOS of d-/p-bands) of the ZnO, Ag, and ZnO–Ag systems, and the corresponding HER processes. Reproduced with permission from ref. 200, Copyright 2021 by American Chemical Society.

As illustrated in Fig. 5(b), Wu et al.¹⁸⁶ demonstrated that iron cluster sites anchored on mesoporous carbon materials exhibit enhanced electrocatalytic performance in the CO₂RR. Similarly, a study on CuAg bimetallic clusters¹⁸⁷ utilized DFT to analyze relationships between catalyst structure, composition, and activity, revealing that specific cluster configurations favor the formation of target products such as CH₃OH. These findings underscore the critical role of atomic arrangement within clusters in optimizing CO₂RR efficiency. Tamtaji's group¹⁸⁸ reported five configurations comprising five metals Fe, Co, Ni, Cu, and Rh. In Fig. 5(c), CoNiCuRh@FeN₄, FeN₄ serves as the active center, while the CoNiCuRh high-entropy cluster serves as a regulatory unit, achieving a low overpotential for CO production in the CO₂RR. In FeNiCuRh@CoN₄, CoN₄ synergizes with the FeNiCuRh high-entropy cluster as the active site, enabling selective reduction of CO₂ to methanol at an overpotential—outperforming traditional SACs. Fig. 5(d) highlights that binuclear Cu(I) complexes serve as efficient catalysts for CO₂-to-C₃ product conversion via C–C coupling, indicating that multinuclear arrangements can promote C₂₊ formation by offering a favorable microenvironment for CO₂ activation and subsequent coupling steps.¹⁸⁹

DFT-based explorations of alloy and bimetallic clusters are also prominent. Mechanistic analyses indicate that C–C coupling is influenced by the catalyst's ability to manage competing reactions (e.g., hydrogen evolution). Multinuclear catalysts can regulate these kinetics to favor CO₂ reduction pathways. For example, a study on AuCu clusters¹⁹⁰ employed first-principles calculations to simulate catalytic behavior, emphasizing how alloying modulates adsorption energies and reaction pathways. Similarly, investigations of Cu@CNT catalysts¹⁹¹ established that certain Ni-based clusters preferentially convert CO₂ to CO, demonstrating the utility of computational methods in predicting catalytic performance from cluster composition. Studies on defects and near-surface atomic structures have further elucidated their impact on catalytic activity. Zhang et al.¹⁹² used DFT to analyze defective Cu₄X_n clusters, finding that defects alter adsorption sites and lower activation barriers, thereby enhancing the CO₂RR. Jo et al.¹⁹³ explored surface FCC dependence, using DFT to clarify how different Cu surface facets influence HCOOH formation pathways. They found that electronic properties related to surface coordination and roughness significantly affect catalytic efficiency, emphasizing the critical role of surface engineering for nanoparticle catalysts. Xu et al.¹⁹⁴ synthesized CuO nanosheets with controllable nanostructures as effective CO₂RR catalysts, noting that surface oxygen and Cu⁺ species are not essential for C₂₊ product formation, suggesting catalytic activity is primarily determined by metal surface structure and morphology. Deng et al.¹⁹⁵ further supported size-dependent activity, combining theoretical and experimental methods to identify an optimal silver nanoparticle size (8–10 nm) balancing activity and selectivity—emphasizing the need for precise size control in CO₂RR nanoparticle catalysts.

Fig. 5e demonstrates the outstanding CO₂RR performance of the FT-Cu₃@MoSX (X = Se, Te) catalyst. CO₂ is stably adsorbed on the catalyst surface, and the charge transfer (0.59–0.97e) and orbital hybridization are significant, converting linear CO₂ into V-shaped CO₂^δ− active species (OCO bond angle 121.7°–136.8°, C=O bond length increasing to 1.30 Å). Moreover, the electron transfer efficiency at the S-terminated surface is better, with an adsorption energy (−0.51 to −0.55 eV) superior to that of the non-polar MoS₂ system. This effectively activates CO₂ and lowers the reaction energy barrier, demonstrating its high catalytic potential.¹⁹⁶ Structural engineering of atomic catalysts—including multinuclear arrangements—has been shown to provide multiple reaction pathways and synergistic effects, enhancing overall electrocatalytic activity.¹⁸⁸ Innovative catalyst design strategies include Zhang et al.'s¹⁹⁷ Zn–Ag–O catalytic materials in Fig. 5f, featuring nanoscale nanoparticles within nanopores to achieve high performance and durability, illustrating how nanoscale structural design boosts CO₂RR efficiency. Gang et al.¹⁹⁸ explored the scalability and practicality of nanoparticle catalysts, developing a one-step pyrolysis method to prepare M–N–C catalysts synthesized from carbon nanotubes and nitrogen precursors—an economical, effective approach enabling scalable CO₂RR catalysts and expanding practical application potential.

It is worth noting that, based on multi-metal cluster catalysts, high-entropy alloy clusters (HEA Clusters) are emerging as an emerging frontier. HEA clusters are typically composed of five or more principal metal elements in a nearly equimolar ratio, and their extremely high configurational entropy is expected to confer them unique electronic structures and outstanding structural stability. For instance, as mentioned earlier in this review, the CoNiCuRh@FeN₄ system^188,199 already possesses characteristics of being multi-component and having high entropy, demonstrating its unique control ability over different product pathways in the CO₂RR. Theoretically, the vast component space of HEA clusters contains infinite possibilities for electronic structures, providing an unprecedented platform for 'tailoring' CO₂RR catalysts with high activity, high selectivity, and ultra-high stability. However, the controllable synthesis, precise characterization, and elucidation of the relationship between structure and function remain significant challenges. In the future, by leveraging high-throughput computing and machine learning, the optimal combinations from the vast ‘high-entropy chemical space’ will be selected, and in situ characterization techniques for sub-nanometer-scale high-entropy systems will be developed, which will be the key to achieving breakthroughs in this field.

Theoretical and spectroscopic techniques, such as multinuclear magnetic resonance, have been used to study hyperpolarized nuclear spin behavior in multinuclear systems, informing the design of more effective catalysts. Additionally, integrating multinuclear complexes into molecular junctions to enhance thermoelectric performance highlights the broader applicability of multinuclear structures in energy-related catalysis. In summary, these studies collectively emphasize the significance of multinuclear metal complexes in advancing CO₂RR technology. Incorporating multinuclear structures into catalyst design provides a universal strategy to overcome current CO₂RR limitations, enabling more efficient, selective, and stable catalytic systems. Sustained mechanistic research and structural innovation are critical to fully harnessing the viability of multinuclear catalysts for sustainable energy CO₂ conversion technologies.

3. Catalytic performance of the main C₁ products

3.1. CO₂ reduction to CO

The CO₂RR has evolved into a pivotal approach to producing high-value chemicals and fuels, showing substantial promise for advancing global carbon neutrality objectives and tackling energy scarcity challenges.²⁰¹ Among the diverse electrochemical CO₂ conversion pathways, CO generation stands out due to its near-100% selectivity, rendering it industrially viable.²⁰² Notably, strategic modulation of electronic effects, steric hindrance, stabilization of metal-CO₂ intermediates, and rational ligand design to facilitate C–O bond cleavage have yielded significant progress in product selectivity control.²⁰³ However, kinetic barriers in electrocatalytic processes necessitate substantial overpotentials, which inadvertently promote the HER as a competing side reaction, thereby compromising CO selectivity.

The mechanism for CO₂-to-CO conversion involves a process of transferring 2 electrons (2e⁻) and 2 protons (2H⁺). The CO₂ reduction process commences with the efficient adsorption of CO₂ molecules on the surface (Fig. 6a). The adsorbed CO₂ is reduced to the *COOH intermediate. Subsequent attack by an additional H⁺ and e⁻ converts *COOH to H₂O and *CO, with the final step involving *CO desorption from the electrode surface to release CO.²⁰² Current research has confirmed that the two key bottlenecks restricting the production efficiency of carbon monoxide lie in the fact that too weak COO⁻ adsorption energy hinders the formation of COOH intermediates, while too strong *CO adsorption inhibits the desorption of the products.^204,205 Multiple studies corroborate that the *CO binding energy on Cu surfaces acts as a key activity descriptor to characterize CO₂RR products beyond 2e⁻ pathways, aligning with the Sabatier principle. Overly strong *CO binding results in catalyst poisoning, whereas weak binding causes premature CO desorption, preventing further reduction. Optimal *CO adsorption balances high CO uptake with HER suppression, thereby maintaining high CO₂RR Faraday efficiency.

Fig. 6 (a) Proposed reaction pathways and key intermediates for the CO₂RR to CO products. Reproduced with permission from ref. 201, Copyright 2025 by Elsevier. (b) Depicted are multiple aspects of the catalytic system, including the model structures, computed Gibbs free energy profiles for CO₂-to-CO conversion, the disparity between CO₂RR and HER limiting potentials, potential-dependent in situ ATR-IR spectra in CO₂-saturated 0.5 M KHCO₃, and the proposed CO₂RR mechanism on Ni-SAs@FNC. Reproduced with permission from ref. 99, Copyright 2021 by Elsevier. (c) Gibbs free energy diagram for the electrochemical CO₂RR process. Reproduced with permission from ref. 207, Copyright 2022 by American Chemical Society. (d) XAS characterization and theoretical computation of the single-atom Fe catalyst. Reproduced with permission from ref. 208, Copyright 2022 by American Chemical Society.

3.1.1. Performance and mechanisms on SACs. To address these challenges, innovative catalyst designs have been developed. Cheng et al.²⁰⁶ employed DFT to investigate single-atom bimetallic alloys, highlighting their potential as electrocatalysts enabling CO₂ reduction to C₁ hydrocarbons, with particular emphasis on Au or Ag-mediated initial CO formation. This work underscores single-atom alloys as cascade catalysts where CO generation constitutes a pivotal step. Zhang et al.⁹⁹ developed an FeN₅ single-atom supported on graphene catalyst that achieves 97% faradaic efficiency for CO production at low overpotentials, demonstrating the efficacy of atomically dispersed Fe active sites. Recent studies emphasize the critical role of atomic configuration and electronic structure tuning. Han et al.²⁰⁷ demonstrated through Fig. 6b that the fluorine-modified Ni–N₄ SACs, supported by ultrathin carbon nanosheets, maintained a CO Faradaic efficiency of over 90% (up to nearly 95%) and a hydrogen evolution efficiency of less than 10% in the range of −0.8 to −1.1 V vs. RHE after fluorine modification. Moreover, the linear scanning voltammetry reduction current was significantly higher than that of the unmodified sample, directly demonstrating that fluorine regulation greatly enhanced the activity and selectivity of CO₂ reduction. The Tafel slope dropped to 68 mV dec⁻¹ (82 mV dec⁻¹ for the unmodified sample), indicating that fluorine modification reduced the energy barrier formed by the *COOH intermediate to accelerate the reaction kinetics. After 12 hours of potentiostatic testing, the current remained above 92% of the initial value, confirming its high stability. The result of a 40% reduction in charge transfer resistance, combined with the ultrathin morphology feature, confirmed that the fluorine modification and structural design synergistically enhanced the interface charge transfer. Moreover, the optimal fluorine doping amount (3.2–4.5 at%) corresponds to the performance peak and the phenomenon that excessive doping will damage the Ni–N₄ structure, further indicating that fluorine regulation needs to be precisely quantified. The CO₂ electroreduction performance of the Ni–N₄ single-atom catalyst has been effectively optimized. Cao et al.²⁰⁸ conducted a systematic analysis using density functional theory to investigate the regulatory mechanism of the metal atom's electron loss behavior on CO₂ adsorption in N-doped carbon nanotubes loaded with transition metal monatomic catalysts (M@CNT, M@1N_CNT, M@2N_CNT, M@3N_CNT). The free energy of the CO₂ reaction pathway is shown in Fig. 6c. Ni@2N_CNT and Pd@2N_CNT exhibit an energy descent feature of 0.15 eV during the first step of hydrogenation to form the *COOH intermediate, while the free energy of this step for other coordination environment catalysts such as Cu@1N_CNT and Ni@1N_CNT is positive. The free energy curve of the M@2N_CNT system in Fig. 4 further clarifies that the formation of *COOH is spontaneous and the energy barrier for the second step of hydrogenation to generate *CO is only 0.23–0.32 eV. These data indicate that the coordination number of metal atoms and N atoms will affect the activation energy barrier of CO₂ through regulating the electron transfer efficiency. The double-N coordination (M@2N_CNT) can enhance the ability of metal atoms to lose electrons and strengthen CO₂ adsorption and activation, ultimately making Pd@2N_CNT and Ni@2N_CNT the preferred catalytic systems for CO₂ reduction to CO, providing a clear thermodynamic basis for the optimization of the coordination environment of SACs. Li et al.²⁰⁹ by doping with P, showed that the electronic structure of the active center and the coordination environment of the Fe–N–C monometallic catalyst can be regulated and optimized. The data in Fig. 6d of the study show that the Fe–N₃P coordination configuration formed by P doping reduces the free energy barrier for the formation of the *COOH intermediate from 0.63 eV in the Fe–N₄ configuration to 0.48 eV, and the desorption energy barrier of *CO decreases from 1.09 eV to 0.81 eV. Meanwhile, the free energy barrier of the HER remains at 0.24 eV (close to 0.27 eV in the Fe–N₄ configuration). The charge density distribution analysis in Fig. 6d further indicates that the introduction of P causes the local aggregation of the electron cloud at the Fe center and a reduction in the oxidation state. This not only confirms that P doping can enhance CO₂ activation and *CO desorption, inhibit competitive HER, and thereby improve the catalytic performance, but also enable the Fe–N/P–C catalyst to achieve a 98% CO faradaic efficiency at a low overpotential of 0.34 V. After a 24-hour stability test, there is no Fe atomic aggregation (no characteristic Fe–Fe bond peaks in the EXAFS spectrum), proving that P doping is an effective strategy for regulating the activity, selectivity, and stability of Fe–N–C monometallic catalysts. It provides key experimental and theoretical support for the synergistic optimization of single-atom catalytic systems with heteroatoms. The introduction of S into Co–N–C SACs was found to boost the CO₂-to-CO conversion by optimizing the electronic structure of the active centers.²¹⁰ Additionally, Wang et al.²¹¹ revealed that asymmetric coordination brings about electronic localization at Ca sites, facilitating efficient CO₂-to-CO reduction. These findings collectively demonstrate that precise atomic-level control over electronic structures is paramount for optimizing catalyst activity.

The coordinative environment and electronic structure are pivotal in determining the catalytic efficacy. Wang's group²¹² developed a catalyst featuring bismuth atomic sites co-coordinated by nitrogen and sulfur. Synthesized via a cation–anion diffusion strategy, this catalyst exhibits enhanced electrocatalytic activity.²¹³ Li et al.²¹⁴ explored low-valent Zn^δ+ single-atom sites, which demonstrated high current density and promising industrial application potential. Moreover, Song et al.²¹⁵ introduced a boron-doped Ni–N₄ SAC, where boron atoms collaborate with nickel to lower the energy barrier of *COOH generation, thereby realizing excellent performance in CO₂ reduction. Guo et al.²¹⁶ revealed a correlation between the tailored atomic environment of SnN₃O₁ sites and enhanced transformation of CO₂ into CO via regulating the intermediates’ binding energy, while simultaneously inhibiting competitive pathways such as HCOOH generation. In a related vein, Chaipraditgul et al.²¹⁷ investigated transition metal modifications on alumina supports to modulate surface interactions, though their research extended to CO₂ hydrogenation for alkene production.

3.1.2. Performance and mechanisms on dual-/tri-atom catalysts. The role of multi-atomic configurations and electronic interactions in DACs has also garnered significant attention. Jiao et al.²¹⁸ documented a diatomic catalyst featuring two adjacent Cu atoms. This configuration promotes the bimolecular step of CO₂ reduction by lowering the activation barrier, thereby exhibiting superior selectivity and stability. Hu et al.²¹⁹ validated that atomic sites can function both as active centers and electronic regulators. Similarly, Zhang et al.²²⁰ documented a Ni–Cu diatomic catalyst dispersed on N-doped carbon, demonstrating that the electronic Ni–Cu atomic interaction acts as a key driver for the pH-universal electroreduction of CO₂ to CO. These studies underscore the importance of multi-atomic configurations in tuning electronic properties and improving catalytic efficiency. Efforts have also been made to construct dual-site catalysts with tailored atomic interactions to optimize catalytic performance.

Sun et al.²²¹ showed that regulating the microenvironment of Ni–Fe binary SACs using ionic liquids can further enhance CO₂ electroreduction, indicating that microenvironment engineering is pivotal for the performance of diatomic catalysts. The synergistic effects observed in these diatomic systems are supported by in situ spectroscopic and theoretical studies, which confirm that such configurations can reduce activation energy and promote the selective generation of CO. Additionally, Sun's team¹⁷³ conducted a comparative study between single-atom and multi-atom catalysts. By designing transition metal catalysts (TM-C₂N₁) supported on single-layer nitrogen-doped graphene, they achieved CO₂ reduction to CO using both single-atom and multi-atom catalyst systems. Gibbs free energy and electronic structure studies on 3TM-C₂N₁ indicated their superior catalytic performance for CO during the CO₂RR, with relatively low limiting potentials, which offer a novel approach to promoting sustainable CO₂ conversion.

The Huang team²²² reported the Fe–Se bimetallic single-atom catalyst FeSe–NC, prepared by a selenic acid etching-assisted method, achieves efficient CO₂-to-CO conversion through the synergistic effect of Fe–N₄ and Se–C₂ sites. The core mechanism was characterized and verified by Fig. 7a and theoretical calculations. In situ ATR-FTIR showed that the introduction of Se atoms significantly weakened the adsorption strength of *CO at the Fe sites and accelerated the desorption of *CO. KIE analysis indicated an improvement in proton transfer kinetics. DFT calculations confirmed that the Fe-Se synergy not only lowered the formation energy barrier of *COOH but also optimized the thermodynamics of *CO desorption. Moreover, the negative shift of the d-band center of Fe (−0.61 to −0.88 eV) and the increase in the difference in the limit potential between the CO₂RR/HER enhanced the inhibition of the HER side reaction. Combined with the mass transfer advantage of the hierarchical porous structure, the catalyst was stably generated as CO at high FE_CO (up to 97.7%) and industrial-grade j_CO (228 mA cm⁻²).

Fig. 7 (a) In situ ATR-FTIR spectra, CO partial current densities in H₂O/D₂O, kinetic isotope effects, and theoretical calculations comparing the CO₂ reduction reaction performance and electronic structure of Fe–NC and FeSe–NC catalysts. Reproduced with permission from ref. 222, Copyright 2024 by American Chemical Society. (b) Performance evaluation of the three ML algorithms. Reproduced with permission from ref. 224, Copyright 2023 by American Chemical Society. (c) Gibbs free energy profiles for CO₂-to-CO and the HER on various structural models, a schematic of the in situ ATR-SEIRAS setup, and the corresponding spectra for the ZnNi-1000, Zn-SAC, and Ni-SAC catalysts. Reproduced with permission from ref. 225, Copyright 2024 by Elsevier. (d) Electrochemical CO₂ reduction performance in a GDE flow cell and a zero-gap MEA electrolyzer using a NiFe-DAC catalyst. Metrics include CO faradaic efficiency (FE_CO), partial current density, and comparative performance analysis against state-of-the-art catalysts across multiple scales. Reproduced with permission from ref. 223. Copyright 2024 by Elsevier.

Similarly, Han et al.²²³ investigated that the high CO selectivity of NiFe-DAC is attributed to the cooperative electronic regulation of Fe–N₅and Ni–N₄Ni–N₄ sites. As shown in Fig. 7d, the Fe–N₅ site preferentially adsorbs activated CO₂ to form the *COOH intermediate (with a binding energy similar to that of Fe-SAC), and the adjacent Ni–N₄ site weakens the Fe's adsorption of *CO through electron coupling (the desorption free energy decreases from 1.1 eV to 0.1 eV), avoiding site poisoning and accelerating the desorption of *CO, moreover, the Ni site has a stronger adsorption of H₂O, inhibiting its own adsorption of CO₂, thus enabling the CO₂ to selectively accumulate at the Fe site.

Xiong et al.²²⁴ utilized machine learning (support vector regression algorithm) to predict that the overpotential of this catalyst is 0.11 V. As shown in Fig. 7b, the decisive step for the reduction of CO₂ to CO is CO₂ → COOH*, and its overpotential is only 0.09 V. The results of the two are highly consistent, confirming the reliability of the catalytic activity. Electrochemical tests showed that at an electrode potential of −0.8 V vs RHE, the CO faradaic efficiency of this catalyst is as high as 82.12%, significantly higher than that of pure Cu (57.04%), Pd@Cu (79.25%), and Pt@Cu (71.24%). Meanwhile, the faradaic efficiency of H₂ is lower than those of the above control catalysts. This effectively inhibits the HER. In the stability experiment, after continuous electrolysis in the H-cell for 12 hours, the current density and CO faradaic efficiency did not show significant decline, and XRD characterization confirmed that its crystal structure remained stable with no significant changes. This verified the core value of PdPt@Cu as an efficient catalyst for CO₂ reduction to CO.

Furthermore, studies have shown that the performance of the ZnNi-TACs catalyst in converting CO₂ to CO is superior to that of SACs.²²⁵ As confirmed by Fig. 7c, the free energy curve of the CO₂RR for its core triatomic structure (Zn–Ni₁–Ni₂, Ni₁–Zn–Ni₂) is significantly lower than that of SACs, significantly reducing the activation energy barrier for the formation of COOH, which is more favorable for subsequent CO hydrogenation and desorption thermodynamics, and effectively inhibiting the HER. The in situ ATR-SEIRAS spectroscopy further captured the evolution of key intermediates. The peaks at 3365 cm⁻¹ and 1623 cm⁻¹ confirmed the adsorption and dissociation of H₂O on the Zn atoms, the peak at 3706 cm⁻¹ and the peak at 1482 cm⁻¹ indicated that *H was stabilized by the N atom; the intensity of the characteristic peaks at 1396 cm⁻¹ (*COOH) and 1933 cm⁻¹ (*CO) increased with the negative shift of the potential, and the peak positions and intensities were higher than those of Zn-SAC and Ni-SAC, confirming that the triatomic structure accelerates the formation and transformation of intermediates. This enables efficient and highly selective CO generation.

3.1.3. Performance and mechanisms on metal clusters. As illustrated in Fig. 8(a), Cai's team²²⁶ verified via in situ ATR-IR that local electric fields enhance *COOH adsorption, promoting CO formation during the CO₂RR. Wang et al.²²⁹ showed that atomically dispersed Pt on CeO₂ nanoparticles efficiently catalyze CO₂ reduction, with in situ DRIFTS unraveling the interplay mechanism between isolated atoms and nanoclusters. Galvita et al.²³⁰ prepared bifunctional 5% Ni/CeO₂–Fe₂O₃ (1 : 1) samples, which enable CO₂-to-CO conversion via the CH₄ + CO₂/CO₂ redox cycle. The incorporation of Ni enhances the material's activity in the temperature range of 873–973 K. During the reductive step CH₄ reacts with CO₂ on Ni sites to diminish the oxygen storage component, while in the oxidation step, the oxygen storage component is further oxidized by CO₂ to form CO. Similarly, Zhanaidarova et al.²³¹ investigated the anchoring of Re(tBu-bpy)(CO)₃Cl on MWCNTs, which increases the current density, lowers the overpotential, and maintains CO selectivity under aqueous conditions. This highlights that polyatomic catalysts supported on conductive substrates can boost CO₂ electroreduction efficiency.

Fig. 8 (a) Electrochemical performance of the CO₂RR on the catalyst structure. Reproduced with permission from ref. 226, Copyright 2022 by Wiley. (b) Depicted are the predicted *CO adsorption energies and DOS at different sites, the optimized adsorption geometries of key intermediates on N, OH-Cu₃, and the free energy profiles (at 0 V vs. RHE) for CO₂ reduction to CH₄ and for *COCO coupling on the same active site. Reproduced with permission from ref. 59, Copyright 2024 by American Chemical Society. (c) Energy profiles along the pathway for photochemical CO₂-to-CO conversion. Reproduced with permission from ref. 227, Copyright 2021 by Elsevier. (d) The catalyst library of Pd–Fe/Co/Ni/Cu bimetallic systems with different doping ratios, applied in the ethane/CO₂ reaction. Reproduced with permission from ref. 228, Copyright 2021 by Elsevier.

The role of metal/oxide interactions in regulating catalytic activity was further exemplified by Huo et al.,²³² who studied Cu/SnO_x heterostructure CNT-supported nanoparticles. Their systematic analysis revealed that component-dependent metal-oxide interplay significantly affects CO₂ electroreduction, suggesting that multi-atomic configurations can optimize the selectivity toward target products. Nanostructured Au catalysts are also employed in electrochemical CO₂ reduction. Pan et al.⁵⁹ obtained dynamic reconfiguration of N,OH–Cu₃ clusters originated from atomically single-metal sites. Fig. 8b shows Cu–N₃ sites by regulating the atomic-level structure of Cu active sites and designing the engineered macro-morphology of the carbon support. Benefiting from the moderate CO adsorption ability and low CO hydrogenation energy barrier, the N,OH–Cu₃ site attains an unprecedented Faraday efficiency of 74.2% for the CO₂-to-CH₄ conversion at an industrial-grade current density of 300 mA cm⁻². Nursanto et al.²³³ found that the CO selectivity exhibited a positive dependence on Au loading, plateauing at 78% as the morphology evolved from clustered to layered structures. The superior mass activity of 4 nm nanoparticles definitively confirmed that the nanoscale architecture dictates the CO production reactivity and selectivity.

Alfonso et al.²³⁴ further analyzed ligand-protected Au₂₅ clusters, noting that fully ligand-protected clusters lack efficiency for CO₂ reduction due to the high electrocatalytic potential required to form the key carboxyl intermediate, emphasizing the importance of surface and ligand engineering in cluster catalysts. Au₂₅ nanoclusters reported by Wu's team²³⁵ are equipped with thiol ligands, one hand to maintain the Au₂₅(SR)₁₈ structure, but cover all Au sites to CO adsorption and make the intact catalyst dead, while the other to detach from the Au₂₅(SR)₁₈/CeO₂ interface at ≥423 K to activate CO and induce low-temperature CO oxidation on the Au₂₅(SR)₁₈/CeO₂ rod-shaped catalyst. Xiao et al.²³⁶ highlighted the significance of metal–ligand cooperation in dinickel complexes, showing that bimetallic synergy significantly enhances catalytic reactivity—their Ni^IINi^II(bphpp)(AcO)₂ complex exhibits 5-fold higher reactivity than mononuclear analogs.

As indicated in Fig. 8(c), Xu et al.²²⁷ found epitaxially grown Co₉S₈ embedded with CoO, which strengthens CO₂ adsorption affinity and activates the molecule via bond length extension, facilitating reduction to CO. This underscores the value of engineering multi-atomic structures to promote CO₂ activation. Additionally, doping the catalyst matrix can regulate activity and selectivity. Han's team²³⁷ reported a high-performance atomic Fe–In–NC catalyst for CO₂ reduction, which achieved a 95% CO faradaic efficiency and surpassed Fe–NC across a broad potential range. This performance was attributed to the Fe–In d–p orbital hybridization, which concurrently promotes *CO desorption and reduces the *COOH formation barrier. Li et al.²²⁸ explored Pd doping in Fe/Ni/CeO₂ catalysts (Fig. 8(d)), where Ni₆/CeO₂ achieved 96.5% CO selectivity in ethane-CO₂ dry reforming, demonstrating excellent performance. However, Pd doping in Ni-based catalysts reduces syngas selectivity due to enhanced cleavage of C–C bonds and increased activation of CO₂ energy barriers, which is unfavorable for CO generation. Fe-based and Pd-doped Fe-based catalysts preferentially produce ethylene, with minimal impact on CO.

In summary, the production of *COOH is considered to be the rate-determining step in the conversion of CO₂-to-CO.²³⁸ Therefore, the final selectivity and energy yield of this reaction are greatly limited by the *COOH intermediate adsorption on the effective sites of the surface. Furthermore, it is reasonable to construct a strong atomic local electric field by heteroatom doping to improve the *COOH adsorption.

3.2. CO₂ reduction to HCOOH

The electrocatalytic transformation of CO₂ to liquid fuels utilizing renewable energy offers a compelling strategy to mitigate carbon footprints while addressing energy storage challenges.²³⁸ Among the diverse products of the CO₂RR, HCOOH stands out due to its high molar electron uptake and inherent energy density and chemical fuel.²³⁹ Achieving efficient HCOOH production requires innovative synthetic approaches to precisely engineer atomically dispersed active sites via atomic-level compositional tuning,²⁴⁰ alongside a deep understanding of structure–performance–activity relationships across metal species to enable knowledge-driven catalyst design.

To date, extensive efforts have focused on developing catalysts for HCOOH/HCOO⁻ production in the CO₂RR, primarily involving p-block metals.²⁴¹ This 2-electron transfer process has a complex reaction mechanism. After decades of research, the potential pathways have been identified. CO₂ is adsorbed at the active sites of the catalyst, undergoes activation and reduction processes to form intermediates such as *HCOO, *COOH or *OCHO, and eventually generates HCOOH/HCOO⁻.

3.2.1. Performance and mechanisms on SACs. In the research on the reduction of HCOOH by CO₂, SACs precisely directed the HCOO or OCHO reaction pathways through active site regulation, heteroatomic coordination modification and carrier structure optimization, effectively inhibiting side reactions and the HER, demonstrating excellent catalytic performance and selectivity. Zheng et al.²⁴² recently combined in situ spectroscopic analysis complemented by theoretical modeling calculations to reveal that Cu active sites in Pb₁Cu catalysts regulate the initial protonation step of the CO₂RR, steering the reaction toward *HCOO (rather than *COOH) formation. This pathway avoids by-product generation, achieving a Faradaic efficiency of 96% toward formate. Zhang et al.²⁴³ proposed transition metal-embedded 2D C₃N materials as stable, conductive platforms for the CO₂RR, shedding light on how stability and metallic conductance enhance electron transfer in the course of reduction. They evaluated CO₂RR performance of M-CC catalysts—formed by embedding transition metal single (TM) atoms in C=C double bond vacancies of C₃N monolayers—alongside TM-N-C (TM-N co-doped carbon) materials. M-CC catalysts facilitate electron transport in electrocatalysis, with all M-CCs showing superior selectivity for the CO₂RR over the HER. Notably, Cu-CC exhibits a low U_L of 0.68 V toward HCOOH production. Song et al.²⁴⁴ explored SACs embedded in antimonene monolayers for the CO₂RR, finding that 3d non-precious TMs anchored on antimonene exhibit superior CO₂RR selectivity over the HER. Among these, Zn-based SACs uniquely produce HCOOH, while others primarily generate CH₄. Co@antimonene exhibits a low overpotential (0.50 V), which rivals that of state-of-the-art electrocatalysts. The interaction between TMs and antimonene modulates SACs intrinsic activity, while TM binding to potential-determining step (PDS) intermediates dictates CO₂RR overpotential and product selectivity.

The He team²⁴⁵ investigated the single-atom Cu adsorbed Janus MoSSe monolayer catalyst (MoSSe-Cu). Fig. 9a clearly presents the complete reaction pathway of CO₂ → HCOOH. The rate-determining step is *OCHO → *HCOOH, and the corresponding free energy is only 0.49 eV, which is significantly lower than that of g-C₃N₄ and C₂N monolayer-loaded transition metal trimer catalysts (0.86 eV, 0.57 eV). The thermodynamic feasibility is better. From the free energy change in Fig. 9a, it can be seen that the step of *CO₂ combining with the proton-electron pair to form *OCHO has a free energy reduction of 0.78 eV, which is a spontaneous reaction, while the free energy for generating the competing intermediate *COOH is only reduced by 0.10 eV. Thermodynamically, the *OCHO path is dominant, ensuring the high selectivity of HCOOH. Chen et al.²⁴⁶ found that introducing sulfur atoms into the NiN₄ coordination structure specifically formed NiN₂S₂ (Fig. 9(b)). Their work highlights the critical role of heteroatom coordination in modulating the performance of Ni SACs, as the sulfur dopants regulate the electronic properties environment of Ni active centers to favor *OCHO adsorption and subsequent formic acid formation.

Fig. 9 (a) The reaction pathways and Gibbs free energy of CO₂ reduction to HCOOH on MoSSe-Cu. Reproduced with permission from ref. 245, Copyright 2023 by Elsevier. (b) Gibbs free energy profiles for the CO₂RR pathways to CO and HCOOH, along with the adsorption configurations of key intermediates on NiN₄, NiN₃S₁, and NiN₂S₂. Reproduced with permission from ref. 246, Copyright 2023 by the Royal Society of Chemistry. (c) The full catalytic cycle depicting the reduction of two CO₂ molecules by one equivalent of Fe@B₁₀H₁₄, computed at the B₃LYP-D₃ level. Reproduced with permission from ref. 250, Copyright 2017 by the Royal Society of Chemistry. (d) Potential reaction pathways from *HCOOH and comparative limiting potentials for CO₂-to-HCOOH conversion on single-atom transition metal catalysts supported on TMDs. Reproduced with permission from ref. 249, Copyright 2025 by American Chemical Society.

Similarly, Zeng's team²⁴⁷ explored MOF-derived 2D SACs based on transition metal-tetra hydroxybenzoquinone frameworks. These materials offer abundant accessible active sites and versatile functionalization capabilities, enabling precise tuning for specific reaction pathways—including selective reduction of CO₂. The MOF-based platform offers a flexible avenue for optimizing catalytic selectivity through structural design. Qian et al.²⁴⁸ demonstrated that doping iron into a boron cage (B₁₀H₁₄) yields an innovative catalyst (Fe@B₁₀H₁₄) capable of catalyzing CO₂ hydrogenation by means of a two-step reduction pathway. As presented in Fig. 9(c), their quantum mechanical studies revealed that this catalyst can sequentially reduce two CO₂ molecules to produce HCOOH, highlighting the potential of metal-doped boron-based frameworks in CO₂ reduction. Further research into nickel-based SACs has also been conducted to explore their catalytic performance in formic acid generation, expanding the portfolio of promising catalyst systems for this reaction. As shown in Fig. 9d, Li et al.²⁴⁹ reported the further protonation behavior of the HCOOH intermediate on the surface of Mn@NbS₂ indicates that the energy barriers required for its conversion to HOCHOH and *OCH intermediates are as high as 2.05 eV and 1.91 eV respectively, significantly higher than the energy barrier of 0.53 eV required for direct desorption to form HCOOH. Thermodynamically, this effectively inhibits the formation of by-products such as CH₄ and CH₃OH, ensuring the high selectivity of the HCOOH product. In terms of catalytic activity, Fig. 9d compares the limiting potential of Mn@NbS₂ with those of previously reported single-atom transition metal anchored two-dimensional transition metal chalcogenide (TMD) catalysts such as Ti@WTe₂, Fe@MoTe₂, and Ni@MoTe₂. This shows that the limiting potential of Mn@NbS₂ is as low as −0.53 V, which is within the optimal potential range for efficient catalytic reactions (absolute value <0.6 V), and only requires a lower applied driving voltage to achieve efficient reaction.

3.2.2. Performance and mechanisms on DACs/TACs. Liu's group²⁵¹ performed in situ DFT calculations at constant potential and microkinetic simulation analyses on 16 transition metal-main group metal combinations, as shown in Fig. 10(a). Operatively, dynamic reconstruction of CO₂ (from chemisorbed to physiosorbed states) drives the reaction to *OCHO (rather than *COOH) intermediates as a result of asymmetric charge buildup at bimetallic centers, and correlates this charge buildup with a proposed charge aggregation intensity (CAI) reactivity descriptor. Screening catalysts according to the CAI descriptor identifies the NiSb diatomic catalyst as very promising for HCOOH conversion. During formic acid synthesis, as shown in Fig. 10(b), the Cr-NiFe catalyst exhibits a preference for the COOH* pathway rather than the HCOO* pathway, with surface Cr atoms' negative charge and low d-band center enabling efficient formic acid production at an exceptional overpotential as low as 0.080 V while markedly inhibiting competing reactions.

Fig. 10 (a) Calculated binding energies of NiSn DACs in various configurations, pCOHP between Ni and Sn atoms, reaction pathways for *CO₂ protonation to COOH/OCHO, potential-dependent total energies of adsorption configurations, and free energy profiles for the CO₂-to-CO reduction process. Reproduced with permission from ref. 226, Copyright 2025 by American Chemical Society. (b) Gibbs free energy comparisons for the CO₂RR and HER, the correlation between ΔG_h* and ε_d, the HER overpotential on TM-NiFe catalysts, and mechanistic analysis of CO/HCOOH selectivity via the *COOH intermediate. Reproduced with permission from ref. 175, Copyright 2023 by Elsevier. (c) Reaction pathways and corresponding Gibbs free energy diagrams for the HER and the conversion of CO₂ to COOH/HCOO intermediates. Reproduced with permission from ref. 252, Copyright 2021 by Elsevier. (d) Correlation between the adsorption energy of products and their corresponding overpotential. Reproduced with permission from ref. 173, Copyright 2024 by the Royal Society of Chemistry.

Similarly, Fig. 10(c) shows Yang et al.²⁵² used DFT to screen M@2D-FeS₂ catalysts, revealing Co@2D-FeS₂ exhibits excellent activity and selectivity for HCOOH while strongly inhibiting the HER. Sun's team¹⁷³ compared SACs and multi-atom catalysts (MACs), developing TM-C₂N₁ for the CO₂RR. Gibbs free energy and electronic structure analyses on 3TM-C₂N₁ show favorable CO₂RR catalytic performance with low limiting potentials. Notably, 3Ti-C₂N₁ achieves optimal HCOOH production corresponding to a U_L of −0.42 V, positioned at the volcano plot vertex (Fig. 10(d)), offering a new route for sustainable CO₂ conversion. Liu et al.¹⁷⁵ designed trimetallic TM-NiFe catalysts, constructing 2D activity volcano curves to evaluate their performance in CO₂-to-CO and HCOOH conversion. Additionally, Sun et al.²⁵³ engineered PtS₂-based SACs with 3d TMs as candidate CO₂RR electrocatalysts, systematically investigating pathways to C₁ products. All TM-PtS₂ SACs show higher selectivity for C₁ products over the HER. For Sc, Ti, V, Cr, Mn, Fe, and Cu supported on PtS₂-SV (sulfur-vacancy PtS₂), HCOOH is the dominant product, while Co⁻, Ni⁻, and Zn–PtS₂ generate diverse C₁ products.

3.2.3. Performance and mechanisms on metal clusters. In catalyst design, targeted structural engineering and interface regulation have emerged as effective strategies to enhance formic acid production in CO₂ electroreduction. Wu et al.²⁵⁴ developed a 3D porous-structured bimetallic Bi–Sn aerogel and well-defined Bi–Sn interfaces. Compared to monometallic Bi or Sn catalysts, this aerogel exposes an increased number of active sites and demonstrates superior mass transfer capabilities, enabling a HCOOH faradaic efficiency (FE_COOH) of up to 93.9% and achieving superior selectivity for CO₂-to-HCOOH conversion. Wei et al.²⁵⁵ further highlighted the critical role of bimetallic interfaces by investigating surface-defined In–Cu nanoparticles (Fig. 11(a)). Their indium-based catalyst achieved a breakthrough in formic acid faradaic efficiency of 90%—significantly outperforming traditional indium-based materials. DFT calculations revealed the underlying mechanism. The In (101) crystal planes in In_γCu_γ nanoparticles effectively enhance the stability of the *OCHO species, thereby decreasing the reaction energy barrier associated with CO₂-to-HCOOH conversion while also accelerating the overall reaction kinetics.

Fig. 11 (a) Comparative free energy diagrams for CO₂ reduction to HCOOH and CO on the In (101) (red) versus CuO (111) (black) facets. Reproduced with permission from ref. 255, Copyright 2021 by Elsevier. (b) Displayed are the DFT-calculated ΔG for the CO₂-to-formate pathways on both facet and edge sites of Bi (003) and (012) planes. A schematic diagram illustrates the mechanism for selective formate formation on Bi nanosheets. Reproduced with permission from ref. 256, Copyright 2018 by Elsevier. (c) The adsorption configurations of CO₂, HCOO* and HCOOH in the Co–PbCO₃ heterojunction and uniform metal doping, as well as the free energy diagram of the CO₂RR on the surface. Reproduced with permission from ref. 238, Copyright 2023 by Elsevier. (d) The average current density obtained at different overpotentials, faradaic efficiencies, and the product selectivities of H₂, HCOOH and CO generated at different overpotentials. Reproduced with permission from ref. 257, Copyright 2017 by American Chemical Society. (e) The faradaic efficiency, bias current density, specific current and conversion frequency of CO + HCOOH and H₂ are presented. Reproduced with permission from ref. 258, Copyright 2021 by Elsevier.

At the atomic scale, Jiang et al.²⁵⁶ observed that in CO₂-saturated 0.1 M KHCO₃ solution, Pd-B/C catalysts achieve a format Faraday efficiency (η_HCOO⁻) of 70% following 2 hours of electrolysis. Even at potentials more negative than the −0.5 V mark—where CO formation (η_CO) increases—the η_HCOO⁻/η_CO selectivity ratio remains significantly higher for Pd–B/C than Pd/C, indicating that subsurface B doping on Pd preferentially promotes adsorption and formation of the format-pathway intermediate *HCOO over the CO⁻ pathway intermediate *COOH. Catalyst structural features, such as grain boundaries and nanostructures, also modulate CO₂ reduction performance. Kumar et al.²⁵⁰ then demonstrated that SnO₂ porous nanowires with diminished SnO₂ grain boundaries, and thus increased active-site density, exhibit enhanced CO₂-to-HCOOH conversion. This catalyst can initiate formic acid synthesis at a low overpotential (350 mV) and sustain a faradaic efficiency of 80% at −0.8 V vs. RHE, with higher energy conversion efficiency than similar systems.

Advances in catalyst design extend to ultrathin 2D nanostructures. Zhang et al.²⁵⁹ obtained ultrathin Bi nanosheets synthesized by liquid-phase exfoliation (Fig. 11(b)), where *OCHO intermediate formation preferentially occurs at edge sites (confirmed by lower Gibbs free energy) rather than basal planes. These nanosheets achieve an 86.0% formate Faraday efficiency at −1.1 V vs. RHE, where the current density reaches 16.5 mA cm⁻², outperforming bulk Bi due to superior conductivity and plentiful edge sites. Atomically precise clusters offer tunable selectivity. Liu et al.²⁶⁰ found that double tetrahedral Cu₈ clusters exhibit a formic acid Faraday efficiency of 92% at −1.0 V vs. RHE, surpassing cubic Cu₈ clusters. Theoretical studies reveal weaker competition with hydrogen evolution and lower *HCOO adsorption free energy on the double tetrahedral structure, clarifying how intermediate interactions govern activity and selectivity. Zhang's group²³⁹ reported Co-modified PbCO₃ electrocatalysts achieving a formate faradaic efficiency of 98.15% at −0.70 V vs. RHE (Fig. 11(c)), and this performance stems from reduced *HCOO formation barriers and enhanced adsorption at Co–Pb²⁺ sites, where Pb²⁺ charge enrichment boosts formate selectivity. Tang et al.²⁵⁷ highlighted the role of negatively charged hydrides in copper hydride nanoclusters (e.g., Cu₃₂H₂₀L₁₂. Fig. 11(d)), which direct selectivity toward formic acid over CO at low overpotentials. DFT calculations further predict weaker hydrogen evolution competitiveness under these conditions. Wang et al.²⁶¹ developed heterostructures of ultrasmall polymetallic sulfide clusters on MWCNTs (Fig. 11(e)), achieving >80% faradaic efficiency for CO₂-to-C₁ conversion (including formic acid) with a high turnover frequency (5974.62 h⁻¹) and 12-hour stability.

Thus, designing efficient electrocatalysts remains central to advancing CO₂RR for HCOOH production. Component engineering, defect engineering, and controlled morphology engineering are established strategies, but critical gaps persist including precise pore structure tuning for enhanced active surface area properties and mass transport capabilities. Future work still needs to address the issue of separating electrolytes from formic acid. The form of the product is closely related to the pH value of the system. Formic acid is formed under acidic conditions, while formate is formed under weakly alkaline conditions. Therefore, the separation process needs to be designed specifically based on this.²⁵⁸ Notably, suppressing the thermodynamically favored HER still stands as a key challenge, requiring synergistic optimization of catalyst and reactor design.

3.3. CO₂ Reduction to CH₄

3.3.1. Performance and mechanisms on SACs. SACs demonstrate unique advantages in regulating active centers during the CO₂ reduction to CH₄ reaction. Different research teams have precisely designed the coordination environment and active sites to reveal multiple efficient conversion pathways. Zhang et al. discovered²⁶² that the generation of CH₄ in Ru₁/CeO₂ originates from the hydride species (H⁻) induced by the formate (HCOO*) pathway. As shown in Fig. 12a, the Ru monomer is anchored in the vicinity of the oxygen vacancy on the (110) surface of CeO₂ in a tetrahedral coordination form. The difference in spin charge density indicates that Ru has +3 valence, and the formation energy of the oxygen vacancy is 0.37 eV (endothermic), which is consistent with the EPR and EXAFS characterization results. H₂ undergoes dissociative decomposition at the O–O_v site on the surface of Ru₁/CeO₂, generating H⁻ and H⁺ species (in situ DRIFTS detected the H⁻ characteristic peak at 2130 cm⁻¹). This process releases 0.47 eV of heat, which is thermodynamically favorable, as shown in Fig. 12a. The H⁻ species preferentially attacks the C^δ+ site of the adsorbed CO₂ to form an intermediate HCOO, and this step only requires overcoming an energy barrier of 1.00 eV and releases 0.44 eV of heat. The energy barrier for H⁺ attacking the O^δ− site of CO₂ to form COOH can be as high as 2.34 eV, making this step kinetically unfavorable, thereby enabling Ru₁/CeO₂ to exhibit high CH₄ selectivity.

Fig. 12 (a) Structure and spin charge density analysis of Ru₁/CeO₂ and Ru₂/CeO₂ catalysts for selective CO₂ hydrogenation, including adsorption configurations, reaction pathways, and calculated energy profiles. Reproduced with permission from ref. 262, Copyright 2025 by the Royal Society of Chemistry. (b) Free energy diagrams and corresponding stable intermediate structures for the electrocatalytic reduction of CO₂ to CH₄ on a Zn–N₄-graphene catalyst. Reproduced with permission from ref. 263, Copyright 2020 by American Chemical Society. (c) Free energy for converting CO₂ to CH₄ via an 8-electron process on o-RuB₂N₂@TiN, p-RuB₂N₂@TiN and RuB₃N₁@TiN. Reproduced with permission from ref. 264, Copyright 2024 by Elsevier.

Similarly, the Han team²⁶³ reported the SA-Zn/MNC single-atom catalyst, which achieves selective regulation of intermediates by the Zn-N₄ active site (Zn is +2 valence), successfully completing the CO₂ electroreduction to CH₄. Fig. 12b clearly shows its 8-electron transfer free energy pathway. CO₂ preferentially forms the OCHO intermediate at the Zn site (energy barrier 0.46 eV), rather than the key COOH (energy barrier 1.2 eV), blocking the competing pathways at the source. Fig. 12b also shows that OCHO forms bonds with the O atom through the O atom and carbon terminal protonation to form a stable configuration, with the rate-determining step being OCHOH → CHO (energy barrier 0.87 V). Each intermediate maintains stable coordination with Zn in Fig. 12b, and the OHCH₃ protonation releases CH₄, and the residual OH combines with H⁺ to form H₂O, completing the cycle. In situ ATRSEIRAS further confirmed the existence of OCH₂ and *O CH₃ and no CO signal. Combined with the catalyst's microporous structure (525 m² g⁻¹, 1.04 nm) and the charge transport advantage of nitrogen-doped carbon support, SA-Zn/MNC achieved 85% CH₄ faradaic efficiency at −1.8 V vs. SCE, −31.8 mA cm⁻² partial current density, and stable operation for 35 h.

Pan et al.²⁶⁴ further optimized the active center through boron coordination, developing a TiN-supported Ru SAC (RuB_xN_4−x@TiN) for CO₂ reduction to CH₄ as another efficient solution. Among them, o-RuB₂N₂@TiN, p-Ru B₂N₂@TiN and Ru B₃N₁@TiN in Fig. 12c exhibited the best performance. Their formation energies were negative and the structure was stable at 500 K (the fluctuation of Ru–B bond length was only 0.02 Å). The B atom acted as a Lewis acid site, reducing the CO₂ adsorption energy to −1.57 eV, extending the C–O bond to 1.3 Å, and the charge transfer amount was 0.52–0.64e⁻, achieving complete activation of CO₂. Moreover, the rate-determining step energy barrier for CH₄ generation was <0.7 eV, and the limiting potential was as low as 0.59–0.68 V, which was significantly superior to RuN₄@TiN (1.28 V) and pure TiN (1.72 V).

The excellent activity and selectivity were demonstrated by the synergistic effect of the coordination environment and the carrier, which showed a key reference for the structural design and path optimization of SACs in multi-electron transfer CO₂ reduction reactions.

3.3.2. Performance and mechanisms on DACs/TACs. In addition to SACs, bimetallic and metal cluster catalysts, through the cooperative effect between metal sites, also exhibit excellent performance in the multi-electron transfer reaction of CO₂ reduction to CH₄, providing a new direction for catalyst system design. Lu et al.²⁶⁵ reported a bimetallic catalyst (M₂-NC) loaded on nitrogen-doped graphene. Among them, Ru₂-NC exhibited the most outstandingly performance, as shown in Fig. 13a. It followed the eight-electron transfer pathway, with the rate-determining step (PDS) being *CO → *CHO, and the free energy barrier was only 0.28 eV, much lower than Fe₂-NC (0.82 eV) and CO₂-NC (1.00 eV). During the reaction process, the energy barrier for *CO to be stably adsorbed at the Ru₂ bimetallic site and then protonation to form *CHO was as low as 0.55 eV. Subsequent steps of *CHOH dihydroxylation to form *CH and gradual hydrogenation to *CH₄ were exothermic reactions (ΔG ranging from −0.06 to −0.58 eV), with the desorption free energy of *CH₄ reaching −0.22 eV, exhibiting excellent thermodynamic feasibility. The limit potential for CH₄ generation was as low as −0.28 V, and the difference between its limit potential and the limit potential of the HER was positive, effectively inhibiting competing reactions. At an applied potential of −0.4 V, the reaction free energy barrier was reduced to a negative value, achieving efficient CH₄ generation. The bimetallic cooperative effect of Ru₂-NC was precisely the core reason for optimizing the adsorption strength of intermediates and reducing the energy barriers of key steps.

Fig. 13 (a) Free energy profiles of the CO₂RR to CH₄ and corresponding adsorption configurations. Reproduced with permission from ref. 265, Copyright 2024 by Elsevier. (b) Possible C1 pathways of CO₂ reduction on the Fe₂Ir@NG, and the corresponding free energy diagrams. Data denote the ΔG of each elementary step. Reproduced with permission from ref. 266, Copyright 2022 by American Chemical Society. (c) Competition between the CO₂RR and HER. Reproduced with permission from ref. 267, Copyright 2025 by Elsevier.

The Fe₂Ir@NG bimetallic catalyst developed by Han et al.²⁶⁶ exhibits distinct reaction pathway characteristics. Fig. 13b comprehensively depicts the key process of CO₂ conversion to CH₄ on its surface. As can be seen from the figure, after CO₂ adsorption, a proton-electron pair transfer occurs to generate the CO + OH intermediate, which lowers the free energy by 0.74 eV. This step is a spontaneous reaction and does not require overcoming an energy barrier. Subsequently, CO gradually undergoes hydrogenation to form intermediates such as CHO (ΔG = 0.36 eV) and CHOH (ΔG = 0.49 eV), with the rate-determining step being CHO → CHOH. The corresponding limiting potential is only 0.49 V, significantly lower than that of the similar Fe₃@NG catalyst (0.79 V). Comparing the free energy data of the CH₃OH and CO generation pathways in the figure, the free energies of all intermediates in the CH₄ generation pathway are at a lower level, and there are no high-energy barrier bottlenecks throughout the process. The thermodynamic advantage is significant, which is attributed to the synergistic effect of Fe and Ir atoms, optimizing the adsorption strength of key intermediates such as CO and *CHO, enabling the CH₄ generation process to have a low limiting potential and high spontaneity characteristics.

The Ti₃@NG catalyst studied by the Wang team²⁶⁷ exhibits extremely high structural stability, providing a stable active environment for catalytic reactions. The adsorption process of CO₂ on its surface occurs spontaneously (with an adsorption free energy of negative value), and Ti₃@NG transfers 1.69e of charge to the CO₂ molecule, causing a significant reduction in the O–C–O bond angle and achieving efficient activation of the CO₂ molecule. The reaction path calculation in Fig. 13c indicates that the Ti₃@NG catalyst for CO₂ reduction follows the optimal path of CO₂* → COOH* → CO* → CHO* → HCHO* → H₃CO* → CH₄, with the rate-determining step being the process of combining CHO with proton-electron pairs to form HCHO. The corresponding limiting potential is only −0.53 V, which is superior to most reported catalysts for CH₄ generation. The volcano curve analysis further reveals that the adsorption intensity of Ti₃@NG for the key intermediate CO is moderate, precisely located at the summit active region of the CH₄ generation reaction, avoiding the retention of intermediates due to excessive adsorption or the inhibition of protonation due to insufficient adsorption. At the same time, its adsorption free energy for CO₂ is more negative than that of the H atom, and the limiting potential of the CO₂RR is lower than that of the HER, effectively inhibiting the competition of side reactions.

The above study achieved efficient CO₂ reduction to CH₄ by designing dual-atom (Ru₂-NC, Fe₂Ir@NG) and metal cluster (Ti₃@NG) catalysts, and optimizing the adsorption of intermediates and reaction pathways through the synergistic effect of metal sites. The core performance advantages were confirmed through free energy calculations, structural stability characterization, and selectivity analysis, providing important references for the design and application of multi-atom cooperative catalytic systems in multi-electron transfer CO₂ reduction reactions.

3.3.3. Performance and mechanisms on metal clusters. In the catalytic system for CO₂ reduction to CH₄, atomic-level doping control and nanostructure engineering are the core strategies for improving performance. Different research teams have achieved optimization of the reaction pathway and breakthroughs in catalytic performance by precisely designing the composition and structure of the catalysts. Shi et al.²⁶⁸ constructed single/multi-atomic alloy catalysts with transition metals (Fe, Co, Ni) doped into Cu13/Cu55 clusters. Through atomic-level local environment regulation, they achieved efficient conversion, as shown in Fig. 14a. The optimal configuration Co666 follows the reaction path of CO₂ → HCOO* → *OCH₂O → *OCH₂OH → CH₂O* → CH₃O* → CH₄. The continuous hydrogenation at the C site is the core mechanism. The rate-determining step (PDS) is *OH* → H₂O*, with a free energy barrier of only 0.33 eV, which is significantly better than the Cu(211) surface (0.74 eV). This performance is closely related to the coordination number of 6 (CN = 6). The CN = 6 sites can weaken the adsorption strength of HCOO* by filling the anti-bonding orbitals (the -ICOHP of Co666 is 1.30, lower than the value of 1.56 for Co565), while maintaining a moderate binding of H₂COO* to lower the energy barrier. Bader charge analysis indicates that after CO₂ adsorption, the C site gains more electrons, preferentially forming the HCOO* intermediate (rather than *COOH), effectively inhibiting the HER. Additionally, a small amount of doped Cu55-based catalyst (such as CO2Cu53) maintains a stable structure without collapse within 5 ps in the 300 K AIMD simulation, balancing activity and stability. The Fe666 and Ni666 configurations also exhibit excellent activity, with free energy barriers of 0.51 eV and superior to the corresponding CN = 5 configurations, providing key support for atomic-level precise design.

Fig. 14 (a) Coordination environment modulation strategies for CO₂ reduction and theoretical limiting potentials of CO₂-to-CH₄ conversion on Fe, Co, and Ni doped Cu₅₅ clusters, along with the free energy diagram and optimized intermediate structures for CH₄ formation on a Co666 cluster. Reproduced with permission from ref. 268, Copyright 2024 by Elsevier. (b) In situ ATR-SEIRAS spectra of Ce/CuO_x-NPs and CuO_x-NPs catalysts under varying potentials, proposed reaction pathways for electrochemical CO₂ reduction to CH₄, calculated Gibbs free energy diagrams, and projected density of states analyses for *CHO intermediate adsorption. Reproduced with permission from ref. 269, Copyright 2025 by Elsevier. (c) In situ ATR-SEIRAS spectra of Cu-CeO₂ catalysts with varying compositions (Cu-CeO₂-1.3, -3.0, -5.5), total peak area analysis of *CO and *CHO adsorbates, and the relative proportion of *CHO within their combined signal. Reproduced with permission from ref. 270, Copyright 2025 by American Chemical Society.

Similarly, the atomic-level cerium-doped copper oxide catalyst (Ce/CuO_x-NPs) reported by Chen's team²⁶⁹ achieved efficient conversion by regulating the electronic structure and the adsorption behavior of intermediates, as shown in Fig. 14b. It followed the reaction path of CO₂ → *CO → *CHO → *CH₂O → *CH₃O → CH₄. In situ ATR-SEIRAS characterization detected key intermediate signals of *CHO and *CH₃O at 1490 cm⁻¹ and 1397 cm⁻¹, without the appearance of C₂ product signals such as *OCCOH (1190 cm⁻¹). This demonstrated high selectivity. DFT calculations indicated that cerium doping reduced the energy barrier of the rapid step *CO → *CHO to 0.33 eV, which was much lower than that of pure Cu (0.82 eV) and CuO_x (1.10 eV), and simultaneously promoted the breakage of the C–O bond in *CH₃O. Projection density of states (PDOS) analysis showed that the overlap degree of the d orbitals of Cu in Ce/CuO₃ with the p orbitals of C in *CHO was significantly higher than that of pure Cu and CuO₃, strengthening the electron interaction. In electrochemical tests, the catalyst achieved a CH₄ faradaic efficiency (FE_CH4) of 67.4% at −1.6 V (vs. RHE), with a partial current density of 293 mA cm⁻². The FE_CH₄ remained above 62% within the voltage window of −1.4 to −1.75 V, and the 10-hour stability test showed no significant degradation in structure and performance, providing solid experimental and theoretical support.

In addition to atomic-level doping, the regulation of nanostructures has also achieved remarkable results. Xiong et al.²⁷⁰ developed mesoporous Cu-CeO₂ nanospherical catalysts, which achieved nano-confined effect regulation through pore size engineering, providing a new path for reaction optimization. As shown in Fig. 14c, in situ ATR-SEIRAS characterization revealed that the intensity of the *CO and *CHO characteristic peaks at 2190–2110 cm⁻¹ and 1480–1430 cm⁻¹ increased with the reduction of pore size, confirming that small-sized pores can enhance the activation of CO₂ and the accumulation of intermediates. Among them, Cu-mCeO₂-3.0 with a pore size of 3.0 nm performed the best, and the peak area ratio of *CHO to *CO was the highest, indicating that the rate of *CO hydrogenation to *CHO (the rapid step of CH₄ formation) was the fastest. This catalyst achieved a CH₄ selectivity of 66.1 ± 2.9% at −1.4 V (vs. RHE), with a partial current density of 237.6 ± 14.5 mA cm⁻², and maintained high selectivity within a wide potential window. Comparing Cu-mCeO₂-1.3 with a pore size of 1.3 nm (where *CO and *CHO coverage was too high, resulting in C–C coupling) and Cu-mCeO₂-5.5 with a pore size of 5.5 nm (with a weak confined effect), it was confirmed that CH₄ selectivity has a volcano-like relationship with pore size.

The above study optimized the catalytic performance through two strategies, atomic-level doping (doping of transition metals in Cu clusters and Ce in CuO_x) and nano-confined effect (control of mesopore diameters). It focused on three core dimensions: electronic structure, intermediate adsorption, and reaction pathway. All achieved efficient CO₂ reduction to CH₄. The common mechanism lies in precisely regulating the local environment and macroscopic structure of the catalyst, reducing the energy barriers of key steps, strengthening the conversion of target intermediates, and inhibiting side reactions. This provides a comprehensive reference for the multi-dimensional design of highly active and highly selective CO₂-to-CH₄ catalysts.

In summary, the core performance and characteristics of different types of 2D loadable catalysts in the CO₂RR process for the production of C₁ products can be summarized as shown in Table 1.

Table 1 CO₂RR reduction pathway

Target product	Catalyst category	Representative catalyst	Key advantages	Critical performance metrics
CO	SACs	FeN5@graphene, F–Ni–N4, Fe–N3P–C	100% atom utilization, high selectivity, tunable structure	FECO ≤ 98%, η ≤ 0.34V, h = 24 h
	DACs	FeNi–NSC, PdPt@Cu, Ni–Cu@N–C	Dual-site synergy, optimized *CO desorption, suppressed HER	FECO ≤ 97.7%, TOF 20 695 h^−1
	TACs	Zn–Ni1–Ni2@C, MnMoFe@graphyne	Multi-atom synergy, breaks scaling relationship	UL = 0.34 V, FECO >90%
	Clusters	Fe@meso-C, Fe–In–NC, AuCu alloycluster	Quantum-size effect, rich active sites, hybridized e-states	FECO ≤ 95%, enhanced *CO adsorption, durable
HCOOH	SACs	MoSSe–Cu, Zr1@C2N, NiN2S2	Pathway-specific, low η, robust structure	FEHCOOH ≤ 96%, E = 0.23 V
	DACs	NiSb@C, Cr–NiFe@N–C	Charge accumulation, suppresses competing reactions	η ≤ 0.08 V, FEHCOOH >90%
	TACs	3Ti-C2N1, MnMoCu@graphyne	Asymmetric active sites, favorable thermodynamics	UL = 0.42 V, strong HER suppression
	Clusters	Bi–Sn aerogel, In–Cu NP, Cu8 cluster	Prominent interface effect, fast mass transfer, dense sites	FEHCOOH ≤ 98.15%, j = 16.5 mA cm^−2
CH4	SACs	Ru1/CeO2, SA-Zn/MNC, RuB2N2@TiN	Single active center, precise intermediate tuning	FECH4 ≤ 85%, h = 35 h
	DACs	Ru2-NC, Fe2Ir@NG	Bimetallic synergy, lowers multi-e transfer barrier	UL = 0.28 V, FECH4 >82%
	TACs	Ti3@NG, N,OH-Cu3 cluster	Stable framework, moderate *CO, suppressed C–C coupling	FECH4 ≤ 74.2%, j = 300 mA cm^-2
	Clusters	Co666@Cu55, Ce/CuOx NPs, Cu-mCeO2	Tunable e-structure, nano-confinement, abundant sites	FECH4 ≤ 67.4%, j = 293 mA cm^−2

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4. Challenges and outlook

As described in this article, the significant progress of two-dimensional supported atomic-level and cluster-type catalysts in carbon dioxide reforming reactions is accompanied by a series of ongoing challenges that involve both experimental realization and theoretical understanding. Overcoming these bottlenecks is crucial for advancing this field from basic research to practical application.

4.1. Experimental challenges

Despite the remarkable achievements, the practical application of these advanced catalysts still faces numerous severe experimental challenges.²⁷¹ Precise and scalable synthesis, precisely synthesizing catalysts with uniform composition, specific atomic arrangement, and high-density structures on two-dimensional carriers remains a huge challenge. Current methods often only yield mixtures of individual atoms, clusters, and nanoparticles, which masks the true structure–activity relationship. Developing scalable and repeatable synthesis methods that can produce these catalysts at the atomic level with precision, and conduct industrial evaluations,²⁷² is of crucial importance. Operational stability and dynamic evolution, although in situ characterization has begun to reveal the dynamic characteristics of active sites under reaction conditions, the long-term structural and chemical stability of these catalysts under industrial-related current densities (e.g., greater than 200 milliamperes per square centimeter) and long-term operation (e.g., over 1000 hours) are still unclear.²⁷³ The main issues include metal atom loss, carrier corrosion/collapse, and irreversible aggregation of atoms/clusters. Designing catalysts with inherent stability or understanding and mitigating degradation pathways through advanced electrolyte engineering and carrier modification is crucial.²⁷⁴ Beyond model systems, the complexity of the actual electrochemical interface, most high-performance catalysts are evaluated in idealized hydrogen battery units using electrolytes saturated with pure carbon dioxide. Applying this evaluation method to more practical membrane electrode assemblies (MEA) or flow batteries introduces a series of complex and difficult-to-understand factors, such as complex ionic environments (e.g., the presence of impurities, substitution of cations/anions), mass transport limitations under high current densities, and the interaction of local pH values, electric fields, and water management.^275–277 Systematic research is urgently needed to bridge model conditions with equipment conditions.

Advanced associated probe characterization requires the development and application of associated probe spectroscopy and microscopy techniques, which can simultaneously provide chemical (e.g., XAS, Raman), structural (e.g., transmission electron microscopy at the same location, electrochemical scanning tunneling microscopy) and electronic information for the same catalyst under the same operating conditions. This will make it possible to directly correlate dynamic structural changes with catalytic activity and selectivity.

4.2. Frontiers of theory and calculation

Electronic interactions between catalysts and reactants, manifested through adsorption strength, ultimately dictate reaction pathways. Thus, the core objective of diverse strategies is to achieve optimal adsorption energies aligned with the Sabatier criterion. Yet, adsorption energy alone is unable to visually characterize a catalyst's intrinsic properties or their evolution during the CO₂RR. Therefore, it is necessary to design appropriate descriptors as bridges. Some general catalytic descriptors, such as valence state and d-band center theory, can be used as intrinsic properties of catalysts to qualitatively or semi-quantitatively regulate adsorption capacity and have been successfully applied to the CO₂RR.^173,278 Additionally, CO₂RR-specific descriptors, focusing on bonding along with orbital coupling between catalysts and key reaction intermediates, are actively under development.

Despite progress, challenges remain in elucidating how electronic structure relates to electrochemical performance, critical for enhancing CO₂RR efficiency and advancing practical applications. A primary hurdle is probing the dynamic surface states exhibited by catalysts and intermediates. For instance, while CO₂RR studies often emphasize transitions between intermediates when analyzing energy barrier heights and rate-determining steps, conformational changes of intermediates—critical to overall reaction kinetics—are frequently overlooked. A single intermediate, such as *COOH, can adopt multi-site adsorption configurations involving different atoms, profoundly influencing subsequent pathways and final products.^279–281

Moreover, intermediate transformations are not mere atomic additions, subtractions, or substitutions but complex dynamic processes involving bond formation/cleavage and interactions with both the catalyst and pre-adsorbed species. Concurrently, catalyst surfaces evolve significantly over reaction time—e.g., alloying in metal heterostructures—raising questions about the true active species and deactivation mechanisms under operational conditions. Understanding the evolution of actual active centers throughout long-term reaction processes is thus essential to ensuring catalyst stability and reliability. Current research largely confines catalyst-adsorbate interactions to single adsorbed molecules, neglecting cumulative effects of multiple intermediates or complex reaction patterns arising from interactions between numerous intermediates and the catalyst—phenomena common in practical scenarios. During prolonged CO₂RR, such clustering becomes more pronounced, with diverse intermediates coexisting on the surface. This oversight also extends to the transformation mode of intermediates. Because when the surface is densely covered, its transformation behavior may be completely different from that of isolated molecules, which highlights the inherent limitations of guiding experiments by simulating single-molecule dynamics.^282–284

Expanding research to encompass dynamic interactions between catalysts and multiple intermediates is therefore urgent to comprehensively understand the CO₂RR under practical conditions. DFT calculations have successfully guided the screening of atomic catalysts,^{278,285–287} but computational cost and time restrict the exploration of coordination environments around central metal atoms, rendering traditional DFT unsuitable for large-scale systems. With advancing computing power, first-principles high-throughput screening is poised to guide the design of superior catalysts, when combined with in situ characterization, such methods can also screen reaction intermediate structures, further advancing mechanistic studies of dynamics. However, these calculations typically assume ideal environments, whereas electrocatalytic processes involve highly complex external conditions and internal environments, leading to discrepancies between experimental results and theoretical simulations. Larger models are needed to fully simulate electrocatalytic environments, demanding the timely development of new methodologies.

Artificial intelligence (AI) offers innovative approaches to catalyst design, integrating advanced computing technologies e.g., DFT and machine learning (ML) assisted models.^{285,288–292} Operating under a variety of conditions can make predictions more directly relevant to real-world use—traditional high-throughput screening typically operates only under “standard” conditions, but these do not account for temperature, pressure, or pH. Building and supporting data sets and open databases are key to improving the effectiveness of the training sets for AI. Catalyst activity originates from active atoms, support materials, coordination environments, and defect distributions, and establishing multi-factor structure–activity relationships models and multi-scale simulations enables effective integration of structural information to refine comprehension of structure–activity relationships. AI-enabled design is capable of enhancing model precision and the efficiency of experimental verification, which in turn speeds up the conversion of theoretical research into real-world applications.

The future of the CO₂RR lies in the integration of precise experiments with complex theories. By resolving the intertwined experimental and computational challenges mentioned above, this field can progress from accidental discoveries to the rational design of next-generation catalysts. The ultimate goal is to develop reliable, highly selective, and energy-efficient catalytic systems that can be integrated into practical electrolyzers, converting carbon dioxide from waste into sustainable fuel and chemical sources, and making meaningful contributions to the circular carbon economy.

5. Summary

Electrochemical transformation of CO₂ into high-value chemicals or fuels can be achieved by storing sustainable energy within chemical bonds. SACs have advanced substantially in electrocatalysis, leveraging their unique geometric and electronic structures to emerge as promising alternatives to commercial noble metal-based catalysts. DACs, which exploit intermetallic dimer interactions, have also gained traction, demonstrating exceptional electrocatalytic activity and addressing key limitations of SACs. To better accommodate complex multi-electron reactions, TACs have further expanded the toolkit, offering enhanced flexibility in intermediate adsorption and enabling deeper investigations into structure–electrocatalytic activity relationships.

Among these catalyst systems, metal clusters stand out as a critical architecture for advancing the CO₂RR, especially for complex products requiring multi-step electron transfer and C–C coupling. Composed of several to hundreds of atoms (1–3 nm in size), clusters integrate the advantages of atomic-level precision and nano-scale functionality, exhibiting unique quantum size effects and hybrid electronic structures that bridge molecular-like states and bulk material band structures. Their structural diversity including constituent atom count, elemental proportions, atomic arrangement, and geometric symmetry enables the creation of abundant active sites tailored to adsorb and convert specific reactants and intermediates in the CO₂RR. Unlike isolated SACs/DACs/TACs, clusters provide a synergistic multi-nuclear environment that effectively modulates the adsorption energetics of key intermediates (e.g., *COOH, *OCHO, *CO) and reduces kinetic barriers for rate-determining steps, particularly C–C coupling for C₂₊ products and deep hydrogenation for CH₄.

In the reduction of CO₂ to C₁ products, clusters demonstrate remarkable performance advantages. For CO production, Fe-based clusters anchored on mesoporous carbon, Fe–In–NC clusters, and AuCu alloy clusters leverage quantum-size effects and rich active sites to enhance *CO adsorption and suppress the HER, achieving CO faradaic efficiencies (FE) up to 95% and durable stability. In HCOOH synthesis, Bi–Sn bimetallic aerogels, In–Cu nanoparticles, and double tetrahedral Cu₈ clusters exhibit prominent interface effects and fast mass transfer, with FE_HCOOH reaching as high as 98.15% and current densities of 16.5 mA cm⁻². For CH₄ generation, Co666@Cu55 clusters, Ce/CuO_x NPs, and mesoporous Cu–CeO₂ nanospheres rely on tunable electronic structures and nano-confinement effects to optimize intermediate adsorption and hydrogenation pathways, achieving FE_CH4 up to 74.2% at industrial-grade current densities (300 mA cm⁻²). Notably, high-entropy alloy clusters (e.g., CoNiCuRh@FeN₄) have emerged as a frontier, with their ultra-high configurational entropy endowing unique electronic structures and structural stability, enabling precise regulation of multiple product pathways.

This review highlights recent progress in classical two-dimensional material catalysts for the CO₂RR and strategies to regulate the selectivity of C₁₊ products. By systematically examining the performance of diverse electrocatalysts, we elucidate reaction mechanisms and design principles at a fundamental level. The pursuit of efficient CO₂RR catalysts reveals a fundamental design principle, in which the number and size of active sites determine the reaction pathway and product selectivity. From SACs to DACs and TACs, and finally to the development of atomic clusters, a clear scale-functional paradigm has been defined. SACs feature uniform and isolated metal centers, which exhibit excellent selectivity for simple two-electron transfer products such as CO while maximizing atomic utilization efficiency. However, their peculiar geometric structures inherently restrict the adsorption configuration of key intermediates, thereby hindering complex multi-step reactions, most crucially, C–C coupling.

To overcome this limitation, DACs and TACs introduced adjacent metal sites, creating a unique coordinated environment and thus achieving a synergy effect. This atomic-level partnership allows for the simultaneous stabilization of multiple intermediates or the cleavage of specific bonds, effectively circumventing the limitations imposed by linear scaling relationships in traditional heterogeneous catalysis. This development has opened up thermodynamic and kinetic pathways for more valuable multi-electron products such as HCOOH, C₂H₄ and CH₄. When the catalytic target further shifts to high-value C₂₊ products, atomic clusters typically composed of several to hundreds of atoms become the preferred architecture. Their multi-nuclear structure provides a set of active sites with hybrid electronic states, which are particularly good at reducing the kinetic barrier of C-C coupling, a step that is often regarded as the bottleneck for the formation of C₂₊. Therefore, the complexity of the required CO₂RR product combination directly determines the optimal scale of the active site, among which SACs exhibit excellent catalytic performance for the generation of CO. The synthesis of C₂₊ substance requires the synergy of multi-atom sets, although the latter faces greater challenges in terms of precise synthesis and structural stability.

Author contributions

Yimeng Sun: data curation, formal analysis, investigation, methodology, writing – original draft. Lin Tao: methodology, supervision, funding acquisition, conceptualization, writing – review & editing. Yaqiong Su: methodology, formal analysis, writing – review & editing. Davoud Dastan: investigation, formal analysis. Han Zhang: investigation, formal analysis. Hongwei Zhao: investigation, formal analysis. Lixiang Li: methodology, supervision, resources. Baigang An: supervision, formal analysis, resources, writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

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

Acknowledgements

The funding from the National Natural Science Foundation of China (Grant No. 52304330 and 52371224), the Natural Science Foundation of Liaoning Province (Grant No. 2024-BS-218), the University of Science and Technology Liaoning Talent Project Grants (Grant No. 6003000317), the Outstanding Youth Fund of University of Science and Technology Liaoning (Grant No. 2023YQ11), and the Youth Fund of the Education Department of Liaoning Province (Grant No. LJKQZ20222324) ais gratefully acknowledged.

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