Lattice Water Deprotonation Enables Potassium‐Ion Chemistries

Electrochemical water splitting is a key process in clean energy applications and usually occurs on the surface of catalytic materials. Here we report the anomalous partial water splitting, namely water deprotonation behavior within the lattice of hydrated materials modeled by Fe1‐xMgx(C2O4) • 2H2O (x ≈ 0.25‐0.43), which triggers the otherwise inactive framework into an excellent cathode material for potassium ion storage. Density functional theory suggests that redox‐active lattice Fe sites can split crystal water into hydroxyls and hydrogens in the initial charge, rendering thereafter reversible K‐ion chemistries, whereas lattice Mg sites are inactive but stabilize the entire framework. Our experiments validated the as‐predicted electrochemical behavior, and the isotopic tracing unambiguously confirmed the hydrogen evolution from crystal water. This intriguing “water deprotonation in lattice” phenomenon may open a new path for the design of cathode materials by electrolysis‐assisted electrochemistry.