MXene Analogue: Metastable Two-Dimensional Transition Metal Nitrides for Electrocatalysis

Metastable two-dimensional (2D) materials offer a promising strategy to overcome the intrinsic limitations of thermodynamically stable phases in electrocatalysis, owing to their nonequilibrium surface states and tunable electronic structures. Among them, metastable-phase 2D transition metal nitrides (Meta-2D TMNs), structural analogues of MXenes, exhibit significant potential for electrochemical energy conversion due to their noble-metal-like electronic configurations and excellent electrical conductivity. However, the scalable synthesis and performance optimization of Meta-2D TMNs remain challenging. These challenges primarily stem from the instability of high-energy 2D structures during formation and competing side reactions, such as the decomposition of ammonia precursors that promote nitrogen triple-bond formation at elevated temperatures. This review systematically evaluates the emerging synthetic strategies and design principles for developing high-performance Meta-2D TMN-based electrocatalysts. Key topics include formation mechanisms, thermodynamic and kinetic barriers, and approaches to lower formation energy while suppressing undesirable side reactions. Particular emphasis is placed on the role of structural and compositional engineering in tuning electrocatalytic performance, thereby advancing the understanding of structure–property relationships in metastable systems. By highlighting advances in controlled synthesis, fundamental structure–property correlation, and the exploration of metastable compounds, this review provides insights into the rational design of Meta-2D TMNs and their potential applications in sustainable energy conversion and storage technologies.