Recent Developments of Microenvironment Engineering of Single-Atom Catalysts for Oxygen Reduction toward Desired Activity and Selectivity

Oxygen reduction reaction (ORR) is an essential process for sustainable energy supply and sufficient chemical production in modern society. Single-atom catalysts (SACs) exhibit great potential on maximum atomic efficiency, high ORR activity, and stability, making them attractive candidates for pursuing next-generation catalysts. Despite substantial efforts being made on building diversiform single-atom active sites (SAASs), the performance of the obtained catalysts is still unsatisfactory. Fortunately, microenvironment regulation of SACs provides opportunities to improve activity and selectivity for ORR. In this review, first, ORR mechanism pathways on N-coordinated SAAS, electrochemical evaluation, and characterization of SAAS are displayed. In addition, recent developments in tuning microenvironment of SACs are systematically summarized, especially, strategies for microenvironment modulation are introduced in detail for boosting the intrinsic 4e⁻/2e⁻ ORR activity and selectivity. Theoretical calculations and cutting-edge characterization techniques are united and discussed for fundamental understanding of the synthesis–construction–performance correlations. Furthermore, the techniques for building SAAS and tuning their microenvironment are comprehensively overviewed to acquire outstanding SACs. Lastly, by proposing perspectives for the remaining challenges of SACs and infant microenvironment engineering, the future directions of ORR SACs and other analogous procedures are pointed out.     

Simultaneously Tuning Band Structure and Oxygen Reduction Pathway toward High-Efficient Photocatalytic Hydrogen Peroxide Production Using Cyano-Rich Graphitic Carbon Nitride

The demands for green production of hydrogen peroxide have triggered extensive studies in the photocatalytic synthesis, but most photocatalysts suffer from rapid charge recombination and poor 2e⁻ oxygen reduction reaction (ORR) selectivity. Here, a novel composite photocatalyst of cyano-rich graphitic carbon nitride g-C₃N₄ is fabricated in a facile manner by sodium chloride-assisted calcination on dicyandiamide. The obtained photocatalysts exhibit superior activity (7.01 mm  h⁻¹ under λ  ≥  420 nm, 16.05 mm  h⁻¹ under simulated sun conditions) for H₂O₂ production and 93% selectivity for 2e⁻ ORR, much higher than that of the state-of-the-art photocatalyst. The porous g-C₃N₄ with Na dopants and cyano groups simultaneously optimize two limiting steps of the photocatalytic 2e⁻ ORR: photoactivity, and selectivity. The cyano groups can adjust the band structure of g-C₃N₄ to achieve high activity. They also serve as oxygen adsorption sites, in which local charge polarization facilitates O₂ adsorption and protonation. With the aid of Na⁺, the O₂ is reduced to produce more superoxide radicals as the intermediate products for H₂O₂ synthesis. This work provides a facile approach to simultaneously tune photocatalytic activity and 2e⁻ ORR selectivity for boosting H₂O₂ production, and then paves the way for the practical application of g-C₃N₄ in environmental remediation and energy supply.     

Catalyst Design for Electrochemical Oxygen Reduction toward Hydrogen Peroxide

Precise electrochemical synthesis under ambient conditions has provided emerging opportunities for renewable energy utilization. Among many promising systems, the production of hydrogen peroxide (H₂O₂) from the cathodic oxygen reduction reaction (ORR) has attracted considerable interest in past decades due to the increasing market demands and the vital role of ORR in the electrocatalysis field. This work describes recent advances in cathodic materials for H₂O₂ synthesis from 2e^- ORR. By using Pt as a stereotype, the tuning knobs are overviewed, including the intrinsic binding strength of oxygenated species, the intermediate diffusion path and the isolation of Pt–Pt ensembles that enable 2e^- ORR pathway from 4e^- total reduction. This knowledge is successfully applied to other transition metal systems and leads to the discovery of more efficient alloy catalysts with balanced improvement on both activity and selectivity. In addition, mesostructure engineering and heteroatoms doping strategies on carbon‐based materials, which significantly boost the H₂O₂ production efficiency as compared to intact carbon sites, are also reviewed. Finally, future directions and challenges of transferring developed catalysts from lab scale tests to pilot plant operations are briefly outlooked.     

Dual-Engineering of Porous Structure and Carbon Edge Enables Highly Selective H2O2 Electrosynthesis

Edge engineering has emerged as a powerful strategy to activate inert carbon surfaces, and thus achieve a notable enhanced electrocatalytic activity. However, the rational manipulation of carbon edges to achieve enhanced catalytic performance remains a formidable challenge, primarily hindered by immature synthesis methods and the obscured understanding of the structure-activity relationship. Herein, an organic–inorganic hybrid co-assembly strategy is used to fabricate a series of mesoporous carbon nanofibers (MCNFs) with controllable edge site densities and the impact of edge population on electrochemical oxygen reduction reaction (ORR) pathways is investigated. The optimized MCNFs catalyst exhibits a remarkable 2e⁻ ORR performance, as evidenced by a high H₂O₂ selectivity (>90%) across a wide potential window of 0.6 V and a large cathodic current density of −3.0 mA cm⁻² (at 0.2 V vs. reversible hydrogen electrode). Strikingly, the density of carbon edge sites can be changed to tune the ORR activity and selectivity. Experimental validation and density functional theory calculations confirm that the presence of edge defects can optimize the adsorption strength of *OOH intermediates and balance the selectivity and activity of the 2e⁻ ORR process. This study provides a new path to achieve high ORR activity and 2e⁻ selectivity in carbon-based electrocatalysts.     

Carbon Black-Supported Single-Atom Co?N?C as an Efficient Oxygen Reduction Electrocatalyst for H2O2 Production in Acidic Media and Microbial Fuel Cell in Neutral Media

Single metal atom isolated in nitrogen-doped carbon materials (M N C) are effective electrocatalysts for oxygen reduction reaction (ORR), which produces H₂O₂ or H₂O via 2-electron or 4-electron process. However, most of M N C catalysts can only present high selectivity for one product, and the selectivity is usually regulated by complicated structure design. Herein, a carbon black-supported Co N C catalyst (CB@Co N C) is synthesized. Tunable 2-electron/4-electron behavior is realized on CB@Co-N-C by utilizing its H₂O₂ yield dependence on electrolyte pH and catalyst loading. In acidic media with low catalyst loading, CB@Co N C presents excellent mass activity and high selectivity for H₂O₂ production. In flow cell with gas diffusion electrode, a H₂O₂ production rate of 5.04 mol h⁻¹ g⁻¹ is achieved by CB@Co N C on electrolyte circulation mode, and a long-term H₂O₂ production of 200 h is demonstrated on electrolyte non-circulation mode. Meanwhile, CB@Co N C exhibits a dominant 4-electron ORR pathway with high activity and durability in pH neutral media with high catalyst loading. The microbial fuel cell using CB@Co N C as the cathode catalyst shows a peak power density close to that of benchmark Pt/C catalyst.     

Steering Local Electronic Configuration of Fe–N–C-Based Coupling Catalysts via Ligand Engineering for Efficient Oxygen Electroreduction

Fe–N–C materials are prospective candidates to displace platinum-group-based oxygen reduction reaction (ORR) catalysts, but their application is still impeded by the conundrums of unsatisfactory activity and stability. Herein, a feasible strategy of ligand engineering of the metal-organic framework is proposed to steer the local electronic configuration of Fe–N–C-based coupling catalysts by incorporating engineered sulfur functionalities. The obtained catalysts with rich Fe-N₄ sites and FeS nanoparticles are embedded on N/S-doped carbon (denoted as FeS/FeNSC). In this unique structure, the engineered FeS nanoparticles and oxidized sulfur synergistically induce electron redistribution and modulate electronic configuration of Fe-N₄ sites, contributing to substantially accelerated kinetics and improved activity. Consequently, the optimized FeS/FeNSC catalyst displays outstanding ORR performance with a half-wave potential of 0.91 V, better four electron pathway selectivity, lower H₂O₂ yield, and superior long-term stability. As a proof-of-concept, zinc-air batteries based on FeS/FeNSC deliver high capacity of 807.54 mA h g⁻¹, a remarkable peak power density of 256.06 mW cm⁻², and outstanding cycling stability over 600 h at 20 mA cm⁻². This study delivers an efficacious approach to manipulate the electronic configuration of Fe–N–C catalysts toward elevated catalytic activity and stability for various energy conversion/storage devices.     

Low-Coordinated Single-Atom Catalysts Modulated by Metal Ionic Liquids for Efficient CO2 Electroreduction

Modulating the coordination environment of single-atom catalysts (SACs) is an attractive approach for maximizing the catalytic activity of single-atom centers. Currently, the synthesis of low-coordinated SACs is mainly confined to increasing the pyrolysis temperature (≥900 °C) to control C─N volatile fragments. Herein, a novel and universal strategy for the low-coordinated SACs modulation is presented using transition metal (e.g., Ni, Co, Zn) ionic liquid precursors under relatively mild temperature of 600 °C, which regulates the L-shell electronic structure and decreases nearly 50% electrophilic reactivity by ionization of 4-position N atom, thereby orienting synthesis of the SACs with metal-N3 centers. The Ni-N3 SACs exhibit exceptional CO₂ electroreduction performance of 99.7% CO Faraday efficiency with an ultra-high CO partial current density of 467.55 mA·cm⁻² as well as a CO production rate up to 10417.51 µmol·h⁻¹·cm⁻² in flow cell. The superior catalytic activity achieves over twofold increase compared with the Ni-N4 SACs prepared by non-metal ionic liquid precursors due to the lower free energy of the key intermediate *COOH and the stronger adsorption energy.     

Atomically Dispersed Unsaturated Cu-N3 Sites on High-Curvature Hierarchically Porous Carbon Nanotube for Synergetic Enhanced Nitrate Electroreduction to Ammonia

Cu-based single-atom catalysts (SACs) are regarded as promising candidates for electrocatalytic reduction of nitrate to ammonia (NO₃RR) owing to the appropriate intrinsic activity and the merits of SACs. However, most reported Cu SACs are based on 4N saturated coordination and supported on planer carbon substrate, and their performances are unsatisfactory. Herein, low-coordinated Cu-N₃ SACs are designed and constructed on high-curvature hierarchically porous N-doped carbon nanotube (NCNT) via a stepwise polymerization–surface modification–electrostatic adsorption–carbonization strategy. The Cu-N₃ SACs/NCNT exhibits outstanding NO₃RR performance with maximal Faradaic efficiency of 89.64% and NH₃ yield rate of up to 30.09 mg mg_cat⁻¹ h⁻¹ (70.8 mol g_Cu⁻¹ h⁻¹), superior to most reported SACs and Cu-based catalysts. The results integrated from potassium thiocyanide poisoning experiments, online differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and density functional theory calculations demonstrate: 1) unsaturated Cu is active site; 2) Cu-N₃ SACs/NCNT possesses NO*-HNO*-H₂NO*-H₂NOH* pathway; 3) low-coordinated Cu-N₃ sites and high-curvature carbon support synergetic promote reaction dynamics and reduce rate-determining step barrier. This study inspires a synergetic enhancement catalysis strategy of creating unsaturated coordination environment and regulating support structure.     

High Yield Electrosynthesis of Hydrogen Peroxide from Water Using Electrospun CaSnO3@Carbon Fiber Membrane Catalysts with Abundant Oxygen Vacancy

Hydrogen peroxide (H₂O₂) production by electrochemical two-electron water oxidation reaction (2e-WOR) is a promising approach, where high-performance electrocatalysts play critical roles. Here, the synthesis of nanostructured CaSnO₃ confined in conductive carbon fiber membrane with abundant oxygen vacancy (O_V) as a new generation of 2e-WOR electrocatalyst is reported. The CaSnO₃@carbon fiber membrane can be directly used as a self-standing electrode, exhibiting a record-high H₂O₂ production rate of 39.8 µmol cm⁻² min⁻¹ and a selectivity of ≈90% (at 2.9 V vs reversible hydrogen electrode). The CaSnO₃@carbon fiber membrane design improves not only the electrical conductivity and stability of catalysts but also the inherent activity of CaSnO₃. Density functional theory calculation further indicates the crucial role of O_V in increasing the adsorption free energy toward oxygen intermediates associated with the competitive four-electron water oxidation reaction pathway, thus enhancing the activity and selectivity of 2e-WOR. The findings pave a new avenue to the rational design of electrocatalysts for H₂O₂ production from water.     

A General Carboxylate-Assisted Approach to Boost the ORR Performance of ZIF-Derived Fe/N/C Catalysts for Proton Exchange Membrane Fuel Cells

An Fe/N/C catalyst derived from the pyrolysis of metal–organic frameworks, for example, a zeolitic-imidazolate-framework-8 (ZIF-8), has been regarded as one of the most promising non-precious metal catalysts toward oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, its ORR mass activity is still much inferior to that of Pt, partly because of the lack of general and efficient synthetic strategies. Herein, a general carboxylate-assisted strategy that dramatically enhances the ORR mass activity of ZIF-derived Fe/N/C catalysts is reported. The carboxylate is found to promote the formation of Fe/N/C catalysts with denser accessible active sites and entangled carbon nanotubes, as well as a higher mesoporosity. These structural advantages make the carboxylate-assisted Fe/N/C catalysts show a 2–10 fold higher ORR mass activity than the common carboxylate-free one in various cases. When applied in H₂–O₂ PEMFCs, the active acetate-assisted Fe/N/C catalyst generates a peak power density of 1.33 W cm⁻², a new record of peak power density for a H₂–O₂ PEMFC with non-Pt ORR catalysts.     

Phosphorus Optimized Metastable Hexagonal-Close-Packed Phase Nickel for Efficient Hydrogen Peroxide Production in Neutral Media

Direct synthesis of hydrogen peroxide (H₂O₂) through electrochemical oxygen reduction has gained close attention yet remains a great challenge due to the slow kinetics. Herein, combining with the virtues of the native high energy state and fascinating surface environment of metastable materials and doping strategy, an efficient phosphorus-optimized metastable hexagonal-close-packed phase nickel catalyst (P-hcp Ni), belonging to the space group (P63/mmc, 194), with P doping is demonstrated. Significantly, it achieves high selectivity of 97% and a high intrinsic turnover frequency of 2.34 s⁻¹, much better than those of the stable face-centered-cubic Ni catalyst. It also displays high stability with remaining in the metastable phase after the stability test. More importantly, P-hcp Ni also achieves a productivity of 4917.2 mmol g_Ni⁻¹ h⁻¹ and an accumulated concentration of (H₂O₂) of 2.38 mol L⁻¹ after 130 h stability test in pure water with a solid electrolyte. Further investigation reveals that the P doping not only greatly enhances the stability of metastable phase, but also weakens the *OOH adsorption on the active site, promoting the high production of H₂O₂ in the neutral media.     

A Universal Seeding Strategy to Synthesize Single Atom Catalysts on 2D Materials for Electrocatalytic Applications

Single‐atom catalysts (SACs) are attracting significant attention due to their exceptional catalytic performance and stability. However, the controllable, scalable, and efficient synthesis of SACs remains a significant challenge. Herein, a new and versatile seeding approach is reported to synthesize SACs supported on different 2D materials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS₂). This method is demonstrated on the synthesis of Ni, Co, Fe, Cu, Ag, Pd single atoms as well as binary atoms of Ni and Cu codoped on 2D support materials with the mass loading of single atoms in the range of 2.8–7.9 wt%. In particular, the applicability of the new seeding strategy in electrocatalysis is demonstrate on nickel SACs supported on graphene oxide (SANi‐GO), exhibiting excellent catalytic performance for electrochemical CO₂ reduction reaction with a turnover frequency of 325.9 h^?1 at a low overpotential of 0.63 V and high selectivity of 96.5% for CO production. The facile, controllable, and scalable nature of this approach in the synthesis of SACs is expected to open new research avenues for the practical applications of SACs.     

Regulating the Spin-State of Rare-Earth Ce Single Atom Catalyst for Boosted Oxygen Reduction in Neutral Medium

The development of neutral zinc–air batteries (ZABs) is long been impeded by the sluggish oxygen reduction reaction (ORR) derived from insufficient O₂ activation and OH* blocking effect. Herein, the synthesis of a series of rare-earth Ce single-atom catalysts (CeNCs) is reported with enhanced spin-state for boosting neutral ORR. Experimental analysis and theoretical calculations indicate that the unique local coordination/geometric structure reshapes the electronic configuration of Ce sites to achieve a transition from 4d¹⁰4f¹ to 4d⁸4f³. The high-spin Ce active sites accelerate the unpaired f electrons to occupy the anti-π orbitals of O₂ and generate suitable binding strength with reaction intermediates. In neutral conditions, CeNC-40 exhibits excellent ORR performance with half-wave potentials of 0.78 V and negligible decay after 10 000 cycles. Additionally, the self-breathing ZABs based on CeNC-40 demonstrates a peak power density of 81 mW cm⁻² and impressive long-cycle stability (>1 600 cycles) at 2 mA cm⁻². This work presents an effective strategy for developing high-spin catalysts to address the challenges of neutral ZABs.     

Carbon Free and Noble Metal Free Ni2Mo6S8 Electrocatalyst for Selective Electrosynthesis of H2O2

Electrocatalytic two-electron reduction of oxygen is a promising method for producing sustainable H₂O₂ but lacks low-cost and selective electrocatalysts. Here, the Chevrel phase chalcogenide Ni₂Mo₆S₈ is presented as a novel active motif for reducing oxygen to H₂O₂ in an aqueous electrolyte. Although it has a low surface area, the Ni₂Mo₆S₈ catalyst exhibits exceptional activity for H₂O₂ synthesis with >90% H₂O₂ molar selectivity across a wide potential range. Chemical titration verified successful generation of H₂O₂ and confirmed rates as high as 90 mmol H₂O₂ g_cat⁻¹ h⁻¹. The outstanding activities are attributed to the ligand and ensemble effects of Ni that promote H₂O dissociation and proton-coupled reduction of O₂ to HOO*, and the spatial effect of the Chevrel phase structure that isolates Ni active sites to inhibit O O cleavage. The synergy of these effects delivers fast and selective production of H₂O₂ with high turn-over frequencies of ≈30 s⁻¹. In addition, the Ni₂Mo₆S₈ catalyst has a stable crystal structure that is resistive for oxidation and delivers good catalyst stability for continuous H₂O₂ production. The described Ni-Mo₆S₈ active motif can unlock new opportunities for designing Earth-abundant electrocatalysts to tune oxygen reduction for practical H₂O₂ production.     

Highly Dispersed NiO Clusters Induced Electron Delocalization of Ni?N?C Catalysts for Enhanced CO2 Electroreduction

Oxygen-regulated Ni-based single-atom catalysts (SACs) show great potential in accelerating the kinetics of electrocatalytic CO₂ reduction reaction (CO₂RR). However, it remains a challenge to precisely control the coordination environment of Ni O moieties and achieve high activity at high overpotentials. Herein, a facile carbonization coupled oxidation strategy is developed to mass produce NiO clusters-decorated Ni N C SACs that exhibit a high Faradaic efficiency of CO (maximum of 96.5%) over a wide potential range (−0.9 to −1.3 V versus reversible hydrogen electrode) and a high turnover frequency for CO production of 10 120 h⁻¹ even at the high overpotential of 1.19 V. Density functional theory calculations reveal that the highly dispersed NiO clusters induce electron delocalization of active sites and reduce the energy barriers for *COOH intermediates formation from CO₂, leading to an enhanced reaction kinetics for CO production. This study opens a new universal pathway for the construction of oxygen-regulated metal-based SACs for various catalytic applications.     

Na-Sm Bimetallic Regulation and Band Structure Engineering in CaBi2Nb2O9 to Enhance Piezo-photo-catalytic Performance

The piezoelectric enhancement of photo-catalytic activity for water splitting and pollutant degradation is a novel approach to developing renewable energy and environmental protection applications. Herein, a new form of defect engineered Na-Sm bimetallic-regulated CaBi₂Nb₂O₉ platelet is synthesized via a molten salt process for water splitting and pollutant degradation applications. An intermediate band structure is introduced by Sm-doping, and empty orbitals in the conductive band are introduced by Na-doping. These factors, combined with the increased local charge density around the Sm and Na atoms, result in an increased electrical conductivity, improved electron mobility, and provide additional electrons for enhancing catalytic reactions. As a consequence, the judicious co-doping of CaBi₂Nb₂O₉ with Sm and Na leads to a unique synergetic piezo-photo-electric effect to provide a superior piezo-photo-catalytic performance for H₂ production (158.53 µmol g⁻¹ h⁻¹) and pollutant degradation (rate constant, k = 0.257 min⁻¹). This new approach provides important insights into the application of defect engineering to exploit the cooperative doping of alkaline earth metals and rare metals to create high-performance catalysts.     

High-Loading Co Single Atoms and Clusters Active Sites toward Enhanced Electrocatalysis of Oxygen Reduction Reaction for High-Performance Zn–Air Battery

The development of precious-metal alternative electrocatalysts for oxygen reduction reaction (ORR) is highly desired for a variety of fuel cells, and single atom catalysts (SACs) have been envisaged to be the promising choice. However, there remains challenges in the synthesis of high metal loading SACs (>5 wt.%), thus limiting their electrocatalytic performance. Herein, a facile self-sacrificing template strategy is developed for fabricating Co single atoms along with Co atomic clusters co-anchored on porous-rich nitrogen-doped graphene (Co SAs/AC@NG), which is implemented by the pyrolysis of dicyandiamide with the formation of layered g-C₃N₄ as sacrificed templates, providing rich anchoring sites to achieve high Co loading up to 14.0 wt.% in Co SAs/AC@NG. Experiments combined with density functional theory calculations reveal that the co-existence of Co single atoms and clusters with underlying nitrogen doped carbon in the optimized Co₄₀SAs/AC@NG synergistically contributes to the enhanced electrocatalysis for ORR, which outperforms the state-of-the-art Pt/C catalysts with presenting a high half-wave potential (E_1/2 = 0.890 V) and robust long-term stability. Moreover, the Co₄₀SAs/AC@NG presents excellent performance in Zn–air battery with a high-peak power density (221 mW cm⁻²) and strong cycling stability, demonstrating great potential for energy storage applications.     

In Situ Topochemical Transformation of ZnIn2S4 for Efficient Photocatalytic Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran

Photocatalytic selective oxidation of 5-hydroxymethylfurfural (HMF) coupled H₂ production offers a promising approach to producing valuable chemicals. Herein, an efficient in situ topological transformation tactic is developed for producing porous O-doped ZnIn₂S₄ nanosheets for HMF oxidation cooperative with H₂ evolution. Aberration-corrected high-angle annular dark-field scanning TEM images show that the hierarchical porous O-ZIS-120 possesses abundant atomic scale edge steps and lattice defects, which is beneficial for electron accumulation and molecule adsorption. The optimal catalyst (O-ZIS-120) exhibits remarkable performance with 2,5-diformylfuran (DFF) yields of 1624 µmol h⁻¹ g⁻¹ and the selectivity of >97%, simultaneously with the H₂ evolution rate of 1522 µmol h⁻¹ g⁻¹. Mechanistic investigations through theoretical calculations show that O in the O-ZIS-120 lattice can reduce the oxidation energy barrier of hydroxyl groups of HMF. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) results reveal that DFF^* (C₄H₂(CHO)₂O^*) intermediate has a weak interaction with O-ZIS-120 and desorb as the final product. This study elucidates the topotactic structural transitions of 2D materials simultaneously with electronic structure modulation for efficient photocatalytic DFF production.     

Surface Energy Mediated Sulfur Vacancy of ZnIn2S4 Atomic Layers for Photocatalytic H2O2 Production

Constructing rich defect active site structure for material design is still a great challenge. Herein, a simple surface engineering strategy is demonstrated to construct one-unit-cell ZnIn₂S₄ atomic layers with the modulated surface energy of S vacancy. Rich surface energy can regulate and control the rich S vacancy, which ensures rich active sites, higher charge density and effective carrier transport. As a result, the ZnIn₂S₄ atomic layers with rich surface energy affords an obvious enhancement in H₂O₂ productive rate of 1592.04 µmol g⁻¹ h⁻¹, roughly 14.58 times superior to that with poor surface energy. Moreover, the in situ infrared diffuse reflection spectrum indicates that S vacancy as the oxygen reduction reaction active site is responsible for the critical intermediate *O₂⁻ and *OOH, corresponding to two-electron oxygen reduction reaction. This study provides a valuable insight and guidance for constructing controllably defects to achieve highly efficient H₂O₂ production.     

Pre-Adsorbed H-Assisted N2 Activation on Single-Atom Cadmium-O5 Decorated In2O3 for Efficient NH3 Electrosynthesis

The electrocatalytic nitrogen reduction reaction (NRR) provides a promising avenue for sustainable and decentralized green ammonia (NH₃) synthesis. To promote the NRR, the design and synthesis of efficient electrocatalysts with an elucidated reaction mechanism is critically important. Here, surface hydrogenation-facilitated NRR is demonstrated to yield NH₃ at low overpotentials on oxygen-deficient In₂O₃ plates decorated with single atom CdO₅ that have a weak N₂-binding capability. Adsorbed ^*H is calculated to be first produced via the Volmer reaction (H₂O + e⁻ → ^*H + OH⁻) and then reacts with dissolved N₂ to generate ^*N₂H₂, which is likely the rate determining step (RDS) of the whole process. Cd atoms and oxygen vacancies in In₂O₃ jointly enhance the activation of N₂ and accelerate the RDS, boosting the NRR. An NH₃ production rate of as high as 57.5 µg h⁻¹ mg_cat⁻¹ is attained at a mild potential, which is retained to a large extent even after 44 h of continuous polarization.