Electrochemically Generated Single-Atom Centers of MXenes for Energy Conversion

Abstract

Conspectus Single-atom catalysts (SACs) have attracted significant attention in electrocatalysis due to their ability to maximize atomic efficiency and provide well-defined active sites. However, their practical use is often limited by stability issues, since the anchored atom serving as the active site may dissolve or restructure under applied potential conditions. The material class of MXenes, two-dimensional transition-metal carbides and nitrides, offers a fundamentally different route toward SAC-like catalytic motifs. Under anodic polarization in aqueous environments, MXene basal planes can undergo water-mediated surface reconstruction to forms an electrochemically generated MXene–single-atom-center-like (MXene–SAC-like) motif in situ. These dynamically generated centers resemble archetypal SACs in their local coordination environment but differ fundamentally in that they are not created through synthetic anchoring strategies, but emerge directly under electrochemical operating conditions. In this Account, we summarize the progress of our group in understanding electrochemically generated MXene–SAC-like motifs for energy conversion processes using electronic structure theory. Pristine MXene basal planes are catalytically inactive under anodic polarization; reconstruction under applied bias generates isolated metal centers whose stability, activity, and selectivity we have mapped across the MXene compositional space. We discuss thermodynamic frameworks based on surface Pourbaix analysis and dissolution thermodynamics that provide insight into the conditions under which single-atom centers become accessible and catalytically active. These stability maps reveal a scenario-dependent W–Mo–Ta progression: W-based motifs dissolve spontaneously, Ta-based motifs are particularly robust under pre-OER conditions, whereas Mo-based motifs retain the wider OER-specific stability window at pH = 0. In addition to oxygen-, chlorine-, and nitrogen-containing reactions under anodic polarization, we also describe the use of electrochemically generated MXene–SAC-like motifs for cathodic processes by pulsing the electrochemical potential. While activity trends in electrocatalysis are commonly interpreted using adsorption free-energy descriptors, we further introduce a dual-descriptor framework that incorporates transition-state energetics into the activity ranking of homologous catalyst series. This approach provides a more complete description of activity and selectivity trends, particularly for proton-coupled electron-transfer processes involving competing reaction channels. For ammonia oxidation specifically, Brønsted–Evans–Polanyi scaling breaks down for dehydrogenated N–N coupling intermediates, and thermodynamic descriptors alone incorrectly predict the preferred coupling pathway for most MXene–SAC-like motifs. By connecting adsorption thermodynamics with reaction kinetics, this framework offers a different perspective on catalytic selectivity at electrified interfaces. The concept of electrochemically generated MXene–SAC-like motifs could provide an alternative to conventional SACs in energy conversion by providing a synthesis route based on fundamental principles of nature. While the pristine form of MXenes is catalytically inactive under anodic polarization, MXenes are easily activated by applying an electrode potential, and the electrochemically generated MXene–SAC-like motifs show high activity and selectivity in the case of competing reaction channels. This could pave the way for a sustainable design concept of durable SAC-based electrocatalysts for energy conversion.