Macro/Micro‐Environment Regulating Carbon‐Supported Single‐Atom Catalysts for Hydrogen/Oxygen Conversion Reactions

Abstract

Electrochemical storage and conversion systems, such as water electrolyzers, proton exchange membrane (PEM) fuel cells and metal-air batteries, have drawn extensive attention in the last decades. Hydrogen and oxygen conversion reactions, including hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), are important reactions in the above-mentioned systems. Specifically, water electrolyzer with HER and OER at cathode and anode, respectively, can transform electricity into clean energy (H 2 ); [4] the fuel cells can convert chemical energy into electricity with HOR at anode and ORR at the cathode; [5] the electrochemical ORR/OER is the pair of inverse reactions during the discharge/charge processes in rechargeable metal-air batteries. [6] Currently, precious metal-based electrocatalysts are still required to drive these electrochemical reactions for high performance. However, the high price and low distribution of precious metals block further commercial applications. [7] Therefore, the exploration of precious metals with low loading and/or nonprecious metal-based catalysts without the compromise of their catalytic activity has been the core of developing renewable energy.In recent years, nanosized catalysts, that is, nanoparticles, quantum dots, and clusters, have been developed rapidly for various electrochemical conversion reactions, such as HER, Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbonsupported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbonsupported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/ micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

Recent Advances in Engineered Ru‐Based Electrocatalysts for the Hydrogen/Oxygen Conversion Reactions

“…Singleatom catalysts (SACs) maximize the usage of all active atoms and enhance mass activity. [92][93][94] The high surface energy of Ru single atoms requires to be stabilized on a suitable supporting substrate to prevent agglomeration during the preparation and catalytic conditions. The heteroatomdoped carbon supports with large specific surface area and electrical conductivity are promising candidates to firmly anchor Ru single atoms for electrocatalysis.…”

A robust strategy to boost the proton transfer of heterogeneous catalysts for efficient and sustainable water oxidation towards practical applications

“…The ever-increasing environmental pollution and energy crisis have stimulated the development of various renewable energy systems or technologies to replace fossil fuels, such as overall water splitting, [1][2][3][4][5] metal-air batteries, 6 and the transformation of carbon dioxide to value-added compounds. 7 However, they all involve a kinetics-retarded electrochemical process, namely oxygen evolution reaction (OER) with multiple proton/electron transfer.…”

High-Entropy Metal–Organic Framework Arrays Boost Oxygen Evolution Electrocatalysis

“…Nowadays, the exploration of renewable energy technologies has drawn enormous attention to alleviate the ever-increasing energy and environmental problems. − Among those, electron-driven water splitting to generate hydrogen and oxygen gases has been regarded as a sustainable avenue due to not only its high energy conversion efficiency but also its capability of alleviating the greenhouse effect. − However, the cathodic process of water splitting, i.e., oxygen evolution reaction (OER), still has many issues like four-electron transfer and sluggish kinetics, which require significant overpotentials to drive the overall reaction efficiently. In industry, noble metal catalysts (like ruthenium oxide RuO 2 /iridium dioxide IrO 2 ) have been regarded as two benchmark electrocatalysts for promoting OER, ,− but their high price and scarcity significantly hamper widespread commercialization. It is therefore urgent to design efficient and cost-effective catalysts with superior activity toward OER.…”