Jitendra N. Tiwari, Gokul Bhaskaran, Krishan Kumar, Yun Suk Huh, Young‐Kyu Han
ABSTRACT Electrocatalytic water splitting (H 2 O → H 2 + 1/2O 2 ) is a promising pathway to produce hydrogen and oxygen gases, offering a solution for energy conversion and storage that addresses the energy crisis and mitigates climate change. In this process, electrocatalysts play a crucial role by lowering the kinetic barriers associated with water electrolysis. Therefore, diverse approaches and strategies have been developed to create electrocatalysts capable of achieving highly efficient oxygen and hydrogen evolution reactions (OER, HER) during electrochemical water splitting. Among these strategies, ex situ incorporation (during catalyst synthesis) and in situ formation (during the electrodeposition and electrochemical tests) of high‐, medium‐, and low‐valence metal cations at the molecular scale have proven to be one of the most effective methods for enhancing intrinsic activity, catalytic efficiency, selectivity, and long‐term durability. The presence of metal cations at various valence states not only helps stabilize metal atoms but also regulates their Fermi levels through strong interfacial interactions, leading to improved catalytic activity and stability. This review presents a thorough examination of metal cations (e.g., W, Ta, Mo, Co, V, Pt, Pd, Ag, Cr, Nb, Mn, Fe, Ni, Ru, Ir) with high‐, medium‐, and low‐oxidation states, utilized in OER/HER electrocatalytic reactions involving alkaline water, alkaline‐seawater, simulated‐seawater and natural seawater, based on both experimental and theoretical studies. We also discuss synthesis procedures, active sites, and physicochemical characterizations, and their applications in alkaline and seawater‐based electrolyzers, highlighting the future potential of exploiting mixed‐valence chemistry for water electrolysis.