MXenes enhance electrocatalytic water electrolysis of NiFe layered double hydroxides through bifunctional heterostructuring.

Transition metal-based layered double hydroxides (TM-LDHs) are among the most promising catalytic materials for the electrochemical reactions involved in energy conversion and storage technology. We systematically investigate NiFe-LDH-based electrocatalysts toward application in water electrolysis. We start with the highly accurate advanced density functional theory description of NiFe-LDH's fundamental properties, and demonstrate that coupling a spin-polarized p-band or d-band center model with the Gibbs free energy calculations explains NiFe-LDH's oxygen evolution reaction (OER) mechanism. By involving the related transient states, a reversible oxygen vacancy assisted reaction mechanism has been directly observed and motivated by the high spin transition metal impurity which is further confirmed by the time-consuming hybrid functional method. To further facilitate the electrocatalytic activity of NiFe-LDH, we study NiFe-LDH/MXene heterostructures where the essential semiconductor-to-metallic transition takes place by the additional Ti-3d orbitals and the interfacial non-covalent interaction between the two catalysts. On the basis of calculated results, we propose a link between microscopic properties and macroscopic electrocatalytic kinetics of heterogenous electrocatalysts. Accurately describing the electronic and magnetic structures of electrocatalysts leads us to a step-by-step process for tailoring desired electrocatalytic properties, especially for the high spin state contained TM-LDHs. A descriptor based on combination of the calculated d-band center of transition metal and p-band center of oxygen is the key to predicting electrochemical activity and stability of oxide electrocatalysts. From our results, we establish a design strategy for NiFe-LDH-based bifunctional electrocatalyst fabrication.