Tuning Oxygen Evolution via Dual‐Mode Fe Incorporation in CaO and KCaPO ₄ Catalysts

Developing cost‐effective electrocatalysts for the oxygen evolution reaction (OER) is crucial for scalable water electrolysis. Herein, calcium oxide (CaO) and phosphate frameworks are engineered with iron sourced from rust to fabricate two catalysts: CaOFe and CPFe (KCaPO 4 ‐Fe) derived from marine biowaste. While CaOFe features surface‐dispersed Fe species, CPFe integrates Fe into the phosphate matrix (KCaPO 4 ). Comprehensive structural, morphological, and surface analyses confirm distinct environments for Fe incorporation. Extensive electrochemical studies reveal a unique performance crossover: CPFe exhibits superior activity at low overpotential ( η 10 = 297 mV) with a Tafel slope of (147 mV dec −1 ), whereas CaOFe outperforms at higher current densities ( η 100 = 424 mV) with a lower Tafel slope (107 mV dec −1 ), attributed to enhanced kinetics via surface Fe. Impedance and Bode plot analyses further confirm distinct charge‐transfer dynamics between the two catalysts. Overall, this work demonstrates how the local environment of iron governs electrocatalytic behavior, by providing mechanistic insights into tuning OER performance through waste‐derived bimetallic systems.