Progress and perspective of transition metal layered double hydroxides with oxygen vacancies for enhancing water splitting applications: A review

As an energy-sustainable and environmentally friendly technology, electrochemical water splitting is a highly recognized approach for the production of clean energy and overcoming the energy crisis. However, this method faces a series of challenges, such as low catalyst activity, slow kinetics, poor stability, and high cost. Many reports showed that transition metal layered double hydroxides (TM-LDHs) with unique oxygen vacancy (Ov) defects are a significant new type of candidate nanomaterials to replace precious metal catalysts. This is mainly attributed to their unique physicochemical properties, as well as the tuning of the electronic and lattice structure of the catalyst and optimization of the catalyst surface desorption/adsorption reactions achieved through the introduction of oxygen vacancies, thereby accelerating the oxygen/hydrogen evolution reaction (OER/HER) processes in water splitting. Herein, the preparation methods for engineering oxygen vacancies, and advanced techniques for characterizing oxygen vacancies in catalysts are systematically reviewed, with the aim of clarifying the role of oxygen vacancies in TM-LDHs. The effects of oxygen vacancies on regulating electronic structure, optimizing adsorption energy for intermediates, activating surrounding atomic sites, and generating new active species in TM-LDHs for efficient catalytic HER/OER and decomposition of water are analyzed. More importantly, we discuss the mechanism by which oxygen vacancies promote efficient water splitting in TM-LDHs and outline opportunities and challenges for future development.