High‐Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single‐Atom Catalysts with Tetradentate N 2 O 2 Coordination in a Three‐Phase Flow Cell

Super-Coordinated Nickel N4 Ni1 O2 Site Single-Atom Catalyst for Selective H2 O2 Electrosynthesis at High Current Densities

Electrochemical production of hydrogen peroxide (H₂ O₂ ) from O₂ on single-atom catalysts has attracted great attention, yet the quest for robust catalysts is driven by achieving >90 % Faradaic efficiency (FE) under industrial-relevant current densities (>100 mA cm^-2 ). Herein we synthesize a catalyst that contains single nickel site coordinated by four nitrogen and two oxygen atoms (i.e., N₄ Ni₁ O₂ ) via involving carboxyl functionalized multiwall carbon nanotubes as a substrate to provide extra O coordination to the regular NiN₄ site. It has a cathodic energy efficiency of approximately 82 % and a H₂ O₂ FE of around 96 % at 200 mA cm^-2 current density, outperforming the reported single-atom catalysts for H₂ O₂ electrosynthesis.

Electrocatalytic Oxygen Reduction to Produce Hydrogen Peroxide: Rational Design from Single-Atom Catalysts to Devices

Electrocatalytic production of hydrogen peroxide (H2O2) via the 2e- transfer route of the oxygen reduction reaction (ORR) offers a promising alternative to the energy-intensive anthraquinone process, which dominates current industrial-scale production of H2O2. The availability of cost-effective electrocatalysts exhibiting high activity, selectivity, and stability is imperative for the practical deployment of this process. Single-atom catalysts (SACs) featuring the characteristics of both homogeneous and heterogeneous catalysts are particularly well suited for H2O2 synthesis and thus, have been intensively investigated in the last few years. Herein, we present an in-depth review of the current trends for designing SACs for H2O2 production via the 2e- ORR route. We start from the electronic and geometric structures of SACs. Then, strategies for regulating these isolated metal sites and their coordination environments are presented in detail, since these fundamentally determine electrocatalytic performance. Subsequently, correlations between electronic structures and electrocatalytic performance of the materials are discussed. Furthermore, the factors that potentially impact the performance of SACs in H2O2 production are summarized. Finally, the challenges and opportunities for rational design of more targeted H2O2-producing SACs are highlighted. We hope this review will present the latest developments in this area and shed light on the design of advanced materials for electrochemical energy conversion.

Graphical abstract

Electrochemical Reactors for Continuous Decentralized H2 O2 Production

The global utilization of H₂ O₂ is currently around 4 million tons per year and is expected to continue to increase in the future. H₂ O₂ is mainly produced by the anthraquinone process, which involves multiple steps in terms of alkylanthraquinone hydrogenation/oxidation in organic solvents and liquid-liquid extraction of H₂ O₂ . The energy-intensive and environmentally unfriendly anthraquinone process does not meet the requirements of sustainable and low-carbon development. The electrocatalytic two-electron (2 e^- ) oxygen reduction reaction (ORR) driven by renewable energy (e.g. solar and wind power) offers a more economical, low-carbon, and greener route to produce H₂ O₂ . However, continuous and decentralized H₂ O₂ electrosynthesis still poses many challenges. This Minireview first summarizes the development of devices for H₂ O₂ electrosynthesis, and then introduces each component, the assembly process, and some optimization strategies.

Microenvironment Modulation in Carbon-Supported Single-Atom Catalysts for Efficient Electrocatalytic CO2 Reduction

The electrocatalytic CO 2 reduction reaction (ECRR) becomes an effective way to reduce excess CO 2 in the air and a promising strategy to maintain carbon balance. Carbon-supported single-atom catalysts (C-SACs) is a kind of cost savings and most promising catalysts for ECRR. For C-SACs, the key to achieving efficient ECRR performance is to adjusting the electronic structure of the central metal atoms by modulating their microenvironment of the catalysts. Not only the coordination numbers and hetero-atom coordination, but also the regulation of diatomic sites have a great influence on the performance of C-SACs. This review mainly focuses on recent studies for the microenvironment modulation in C-SACs for efficient ECRR. We hope that this review can contribute readers a comprehensive insight in the current research status of C-SACs for ECRR, as well as provide help for the rational design of C-SACs with better ECRR performance.

Engineering the Local Atomic Environments of Indium Single-Atom Catalysts for Efficient Electrochemical Production of Hydrogen Peroxide

The in-depth understanding of local atomic environment-property relationships of p-block metal single-atom catalysts toward the 2 e^- oxygen reduction reaction (ORR) has rarely been reported. Here, guided by first-principles calculations, we develop a heteroatom-modified In-based metal-organic framework-assisted approach to accurately synthesize an optimal catalyst, in which single In atoms are anchored by combined N,S-dual first coordination and B second coordination supported by the hollow carbon rods (In SAs/NSBC). The In SAs/NSBC catalyst exhibits a high H₂ O₂ selectivity of above 95 % in a wide range of pH. Furthermore, the In SAs/NSBC-modified natural air diffusion electrode exhibits an unprecedented production rate of 6.49 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M KOH electrolyte and 6.71 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M PBS electrolyte. This strategy enables the design of next-generation high-performance single-atom materials, and provides practical guidance for H₂ O₂ electrosynthesis.

Impact of Pore Structure on Two-Electron Oxygen Reduction Reaction in Nitrogen-Doped Carbon Materials: Rotating Ring-Disk Electrode vs. Flow Cell

The impact of pore structure on the two-electron oxygen reduction reaction (ORR) in nitrogen-doped carbon materials is currently under debate, and previous studies are mainly limited to the rotating ring-disk electrode (RRDE) rather than the practical flow cell (FC) system. In this study, assisted by a group of reliable pore models, the impact of two pore structure parameters, that is, Brunauer-Emmett-Teller surface area (S_BET ) and micropore surface fraction (f_micro ), on ORR activity and selectivity are investigated in both RRDE and FC. The ORR mass activity correlates positively to the S_BET in the RRDE and FC because a higher S_BET can host more active sites. The H₂ O₂ selectivity is independent of f_micro in the RRDE but correlates negatively to f_micro in the FC. The inconsistency results from different states of the electrode in the RRDE and the FC. These insights will guide the design of carbon materials for H₂ O₂ synthesis.

Photothermal-assisted triphase photocatalysis over a multifunctional bilayer paper

Photocatalysis as one of the future environment technologies has been investigated for decades. Despite great efforts in catalyst engineering, the widely used powder dispersion and photoelectrode systems are still restricted by sluggish interfacial mass transfer and chemical processes. Here we develop a scalable bilayer paper from commercialized TiO 2 and carbon nanomaterials, self-supported at gas-liquid-solid interfaces for photothermal-assisted triphase photocatalysis. The photogeneration of reactive oxygen species can be facilitated through fast oxygen diffusion over triphase interfaces, while the interfacial photothermal effect promotes the following free radical reaction for advanced oxidation of phenol. Under full spectrum irradiation, the triphase system shows 13 times higher reaction rate than diphase controlled system, achieving 88.4% mineralization of high concentration phenol within 90 min full spectrum irradiation. The bilayer paper also exhibits high stability over 40 times cycling experiments and sunlight driven feasibility, showing potentials for large scale photocatalytic applications by being further integrated into a triphase flow reactor.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

Selective Electrocatalytic Reduction of Oxygen to Hydroxyl Radicals via 3-Electron Pathway with FeCo Alloy Encapsulated Carbon Aerogel for Fast and Complete Removing Pollutants

We reported the selective electrochemical reduction of oxygen (O₂ ) to hydroxyl radicals (^. OH) via 3-electron pathway with FeCo alloy encapsulated by carbon aerogel (FeCoC). The graphite shell with exposed -COOH is conducive to the 2-electron reduction pathway for H₂ O₂ generation stepped by 1-electron reduction towards to ^. OH. The electrocatalytic activity can be regulated by tuning the local electronic environment of carbon shell with the electrons coming from the inner FeCo alloy. The new strategy of ^. OH generation from electrocatalytic reduction O₂ overcomes the rate-limiting step over electron transfer initiated by reduction-/oxidation-state cycle in Fenton process. Fast and complete removal of ciprofloxacin was achieved within 5 min in this proposed system, the apparent rate constant (k_obs ) was up to 1.44±0.04 min^-1 , which is comparable with the state-of-the-art advanced oxidation processes. The degradation rate almost remains the same after 50 successive runs, suggesting the satisfactory stability for practical applications.