Metallenes for electrocatalytic nitrate reduction

Metallenes, the latest emerging class of two-dimensional (2D) metals composed of atomically thin layers, have attracted considerable attention for their unique physiochemical and electrochemical properties in energy conversion reactions. Notably, noble-metal-based metallenes exhibit exceptional activity and selectivity in the electrocatalytic nitrate reduction reaction (NO3RR) to NH3, owing to the intrinsic activity of noble metals combined with the distinctive electronic and geometric characteristics of 2D ultrathin nanosheets. This review highlights the recent advancements in noble metal metallenes for NO3RR, beginning with an overview of the fundamentals of nitrate reduction, including the reaction mechanism and associated challenges. We also delve into the properties, synthesis, characterization, and reaction mechanisms of NO3RR catalyzed by metallenes. Furthermore, we emphasize the modifications and applications of various noble metallenes for NO3RR, with a particular focus on the underlying structure-function relationship. Lastly, we identify the remaining challenges and propose future perspectives for the development of more advanced metallenes in the context of NO3RR.