Recent advances in high-valent 3d-metal-incorporated layered double hydroxides for electrochemical water splitting

High-valent 3d-metal incorporated layered double hydroxides (LDHs) have emerged as a novel class of abundant and highly effective water splitting electrocatalysts. These materials demonstrate adjustable electronic architectures, a large number of active sites, and exceptional surface area, rendering them extremely efficient for facilitating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Recent advances have highlighted that high-valent 3d-metal (Mn³⁺/Mn⁴⁺, Cr³⁺/Cr⁴⁺, V³⁺/V⁴⁺) incorporation in LDHs enhances catalytic activity, accelerates reaction kinetics, improves charge transfer, and increases stability. While numerous reviews have covered LDHs as promising electrocatalysts for water splitting, most have primarily focused on general synthetic strategies, compositional tuning, and morphological optimization of LDHs containing 3d transition metals such as Ni, Co, Fe, Zn, and Cu. However, a critical and comprehensive analysis emphasizing the progress and advancement of high-valent 3d-metal-incorporated LDHs for water splitting remains absent. In this review, we comprehensively examine the design principles, synthesis strategies, and mechanistic insights underpinning the efficacy of high-valent 3d-metal-incorporated LDHs in water splitting. Particular attention is given to strategies including heteroatom doping, interfacial engineering, defect engineering, electronic tuning, and electrochemical activation. We also discuss state-of-the-art <i>in situ</i>/<i>operando</i> spectroscopic techniques that have deepened the understanding of active phases and dynamic surface reconstruction during catalysis. Finally, we discuss the existing obstacles and future possibilities for the rational design of next-generation LDH catalysts with improved stability, activity, and scalability for sustainable hydrogen production.