Mechanistic insight into the active centers of single/dual-atom Ni/Fe-based oxygen electrocatalysts

Boosting Oxygen-Evolving Activity via Atom-Stepped Interfaces Architected with Kinetic Frustration

A highly active interface is extremely critical for the catalytic efficiency of an electrocatalyst; however, facilely tailoring its atomic packing characteristics remains challenging. Herein, a simple yet effective strategy is reported to obtain copious high-energy atomic steps at the interface via controlling the solidification behavior of glass-forming metallic liquids. By adjusting the chemical composition and cooling rate, highly faceted FeNi₃ nanocrystals are in situ formed in an FeNiB metallic glass (MG) matrix, leading to the creation of order/disorder interfaces. Benefiting from the catalytically active and stable atomic steps at the jagged interfaces, the resultant free-standing FeNi₃ nanocrystal/MG composite exhibits a low oxygen-evolving overpotential of 214 mV at 10 mA cm^-2 , a small Tafel slope of 32.4 mV dec^-1 , and good stability in alkaline media, outperforming most state-of-the-art catalysts. This approach is based on the manipulation of nucleation and crystal growth of the solid-solution nanophases (e.g., FeNi₃ ) in glass-forming liquids, so that the highly stepped interface architecture can be obtained due to the kinetic frustration effect in MGs upon undercooling. It is envisaged that the atomic-level stepped interface engineering via the physical metallurgy method can be easily extended to other MG systems, providing a new and generic paradigm for designing efficient yet cost-effective electrocatalysts.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Synergetic Dual-Atom Catalysts: The Next Boom of Atomic Catalysts

Dual-atom catalysts (DACs) are an important branch of single-atom catalysts (SACs), in which the former can effectively break the dilemma faced by the traditional SACs. The synergetic effects between bimetallic atoms provide many active sites, promising to improve catalytic performance and even catalyze more complex reactions. This paper reviews the recent research progresses of two kinds of DACs, including homonuclear and heteronuclear DACs, and their applications in oxygen reduction, carbon dioxide reduction, hydrogen evolution, oxygen evolution, Zn-air batteries, tandem catalytic reactions, and so on. In addition, in order to promote the further development of DACs, the challenges and perspectives of DACs are put forward.

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

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 carbon-supported 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 carbon-supported 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.

Theory-Guided Regulation of FeN4 Spin State by Neighboring Cu Atoms for Enhanced Oxygen Reduction Electrocatalysis in Flexible Metal-Air Batteries

Iron, nitrogen-codoped carbon (Fe-N-C) nanocomposites have emerged as viable electrocatalysts for the oxygen reduction reaction (ORR) due to the formation of FeN_x C_y coordination moieties. In this study, results from first-principles calculations show a nearly linear correlation of the energy barriers of key reaction steps with the Fe magnetic moment. Experimentally, when single Cu sites are incorporated into Fe-N-C aerogels (denoted as NCAG/Fe-Cu), the Fe centers exhibit a reduced magnetic moment and markedly enhanced ORR activity within a wide pH range of 0-14. With the NCAG/Fe-Cu nanocomposites used as the cathode catalyst in a neutral/quasi-solid aluminum-air and alkaline/quasi-solid zinc-air battery, both achieve a remarkable performance with an ultrahigh open-circuit voltage of 2.00 and 1.51 V, large power density of 130 and 186 mW cm^-2 , and good mechanical flexibility, all markedly better than those with commercial Pt/C or Pt/C-RuO₂ catalysts at the cathode.

Dissolution of the Heteroatom Dopants and Formation of Ortho-Quinone Moieties in the Doped Carbon Materials during Water Electrooxidation

Heteroatom-doped carbon materials are widely used as metal-free electrocatalysts and supporting substrates for many metal-based composites. However, almost all the current researches are based on the assumption of the self-stability of the heteroatom-doped carbon materials, neglecting their possible structural evolution during electrocatalysis, especially under harsh oxygen evolution reaction (OER) conditions. Besides, previous researches always focused on the dropcast carbon-based materials with only a few participated dopants, leading to unobservable structural evolution during the electrolysis. Here, heteroatom-doped graphite flakes (GP) with a large quantity of participated dopants are chosen as the research model to multiply the transformation during the electrolysis. Through the combination of theoretical calculations and experiments, we present the nearly complete dissolution of the heteroatoms in N-, P-, and Se-doped carbon materials in the form of the high-valence oxoanions during OER. The oxygen-abundant residues are proven to be responsible for the OER activity. Among the oxygen-containing functional groups in the residues, the ortho-quinone moieties, whose structures change with the doping elements, are finally identified as the active sites. Heteroatom-doped carbon materials as the supporting substrates for the metal-based composite experience a similar transformation, leading to unexpectedly different activity origins. Our work not only reveals the real active sites of the heteroatom-doped carbon materials for OER but also provides new insight into understanding the heteroatom-doped carbon materials as the supporting substrates for other anodic reactions.

A "Pre-Constrained Metal Twins" Strategy to Prepare Efficient Dual-Metal-Atom Catalysts for Cooperative Oxygen Electrocatalysis

Dual-metal-atom-center catalysts (DACs) are a novel frontier in oxygen electrocatalysis, boasting functional and electronic synergies between contiguous metal centers and higher catalytic activities than single-atom-center catalysts. However, the definition and catalytic mechanism of DACs configurations remain unclear. Here, a "pre-constrained metal twins" strategy is proposed to prepare contiguous FeN₄ and CoN₄ DACs with homogeneous conformations embedded in a N-doped graphitic carbon (FeCo-DACs/NC). A programmable phthalocyanines dimer is used as a structural moiety to anchor the bimetallic sites (containing Co and Fe) in a metal-organic framework (MOF) to achieve delocalized dispersion before pyrolysis. The resultant FeCo-DACs/NC exhibits excellent electrochemical performance in oxygen electrocatalysis and rechargeable Zn-air batteries. Theoretical calculations demonstrate that the synergetic interaction of adjacent metals optimizes the d-band center position of metal centers and balances the free energy of the *O intermediate, thereby improving the oxygen electrocatalytic activity. This work opens up an avenue for the rational design of DACs with tailored electronic structures and uniform geometric configurations.

Multi-atom cluster catalysts for efficient electrocatalysis

This review presents recent developments in the synthesis, modulation and characterization of multi-atom cluster catalysts for electrochemical energy applications.