Structural Transformation of Fe ₃ O ₃ PO ₄ Nanoparticles During ON/OFF Operation for Oxygen Evolu

The development of effective catalysts for alkaline water electrolysis is strongly demanded for green hydrogen production. In the present study, we synthesized various crystalline metal oxide and phosphate nanoparticles and evaluated their catalytic performance for the oxygen evolution reaction by linear sweep voltammetry measurements. Among approximately 50 types of nanoparticles and benchmark catalysts (e.g., IrO ₂ and RuO ₂ ), Fe ₃ O ₃ PO ₄ exhibited the lowest potential (1.57 ± 0.02 V vs. RHE) at a current density of 0.5 A cm ⁻² . Furthermore, ON/OFF durability tests simulating system startup and shutdown revealed that this material exhibits high durability and a degradation rate of only 15 μV cycle ⁻¹ during 4000 cycles. Spectroscopic analyses, including inductively coupled plasma−atomic emission, Raman, X‐ray absorption, and X‐ray photoelectron spectroscopy likely suggested that Fe ₃ O ₃ PO ₄ is electrochemically activated to form a γ ‐FeOOH‐type species accompanied by leaching of phosphorus species from the structure during the reaction. In addition, PO ₄ ³⁻ species on the catalyst surface may affect both OER activity and the durability of the resultant γ ‐FeOOH electrocatalyst under reverse current conditions.