Continuous Tuning of Fe‐O Covalency via Bioengineering for Facilitating Overall Water Splitting

Abstract The applications of Fe‐based electrocatalysts in oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are hindered due to poor stability and catalytic activity originating from rapid Fe leaching and the intrinsic electronic structure of FeOOH. Here, the study presents a strategy to precisely and continuously tune the morphology of FeOOH and the covalent characteristics of Fe–O bonds by controlling the coverage of extracellular polymeric substances (EPS) on the FeOOH surface. Operando spectroscopy and theoretical calculations reveal that regulating Fe–O covalency induces changes in intermediate adsorption strength and metal leaching, leading to a volcano‐shaped trend in durability and activity as a function of Fe–O covalency. Notably, in HER, negatively charged sites of EPS exhibit superior *H adsorption compared to bare FeOOH. In particular, EPS 2 @FeOOH exhibits excellent catalytic performance for both OER (η 10 = 240 mV) and HER (η 10 = 52 mV), with outstanding stability over 200 hours at 100 mA cm⁻ 2 . The current density also reaches 10 mA cm⁻ 2 at merely 1.51 V in a two‐electrode configuration, significantly surpassing other bifunctional electrocatalysts. This approach will provide a promising pathway to enhance the stability and activity of water electrolysis through precise modulation of EPS coverage.