Metastable Two-Dimensional Materials for Electrocatalytic Energy Conversions

ConspectusAn urgent need for efficient energy conversion technologies is driving development of active and durable electrocatalysts. In recent years, two-dimensional (2D) materials have emerged as practically promising electrocatalysts because of unique physical and chemical properties. In general, a significant proportion of 2D materials are polymorphous with diverse crystal structures or stoichiometry. However, pristine 2D materials found in nature are thermodynamically stable phases with inert catalytic activity. Metastable phases, in contrast, are highly active for various electrocatalytic processes because of high-energy structures and high reactivity of nonequilibrium surfaces.Generally, the growth of metastable 2D materials requires higher formation energy compared with the thermodynamically stable phases, which are difficultly obtained in standard synthetic processes such as chemical vapor deposition and vapor transport processes. The destabilization of thermodynamically stable 2D materials via external forces facilitates the conversion of high-entropy crystal structure into metastable phases. To date, a number of approaches, including confined growth, topotactic transformation, electron donating, and chemical exfoliation, have been demonstrated for the preparation of high-performance metastable 2D electrocatalysts. As an atomic thin platform, metastable 2D materials represent an almost ideal prototype to achieve a comprehensive understanding of the fundamental principles and mechanisms of various electrocatalytic processes. In the design of metastable 2D electrocatalysts, a number of needs must be concomitantly considered, namely, (1) economic of synthesis methods, (2) product yield, (3) applicability of post-treatment for tuning electrocatalytic properties, (4) general synthesis protocols, and (5) the chemical and catalytic stabilities of metastable 2D materials.In this Account, we provide a critical and timely overview of metastable 2D materials for major electrocatalytic energy conversions based on recent research in our group. We review unique advances and challenges with metastable 2D materials, including specific design principles and typical strategies for synthesis of metastable 2D nanostructured materials with desirable characteristics. We compare advances in metastable 2D materials in selected electrocatalytic processes from fundamental through to functional. Significant emphasis is placed on design strategies for metastable 2D materials and resultant influence on intrinsic electrocatalytic performance, including electronic properties and adsorption energetics. We conclude with an appraisal of the likely opportunities and difficulties with metastable 2D electrocatalysts at the atomic level. This Account provides understandings and insights to the research of metastable 2D electrocatalysts. The current achievements of metastable 2D materials with the ultimate target of synthesizing high performance electrocatalysts may facilitate the development of heterogeneous catalysis for clean energy applications.