Maximizing Oxygen Evolution Performance of NiFeO_<i>x</i> Semitransparent Electrocatalysts Applicable to Photoelectrochemical Water Splitting Device

<jats:title>Abstract</jats:title><jats:p>In photoelectrochemical (PEC) water splitting, semiconductor‐based photoelectrodes can improve reaction rates and durability by incorporating cocatalysts that serve as active sites for the water splitting process. However, achieving both high light transmittance and efficient catalytic activity is essential for these cocatalysts. This study aimed to optimize the surface loading of semitransparent NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub> thin‐film electrocatalysts to enhance the oxygen evolution reaction (OER) rates while maintaining high light transmittance. NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub> thin films were deposited on fluorine‐doped SnO<jats:sub>2</jats:sub> (FTO) transparent conductive substrates, and the relationship between the NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub> loading amount (<jats:italic>Γ</jats:italic>) and the OER rate was examined using electrochemical techniques. The OER rate of NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub> on FTO (NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub>/FTO) was the highest at a <jats:italic>Γ</jats:italic> value of 0.20 μmol cm<jats:sup>−2</jats:sup>. To further explore the connection between this optimized <jats:italic>Γ</jats:italic> and PEC activity, the impact of <jats:italic>Γ</jats:italic> on the PEC OER performance of visible‐light‐absorbing <jats:italic>α</jats:italic>‐Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> semitransparent photoanodes was evaluated as a model system. Applying the optimized <jats:italic>Γ</jats:italic> of NiFeO<jats:sub><jats:italic>x</jats:italic></jats:sub> to modify the <jats:italic>α</jats:italic>‐Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> surface also led to enhanced PEC OER performance. These findings highlight the critical role of surface design, specifically the optimization of cocatalyst loading and electrocatalytic activity, in improving PEC water splitting efficiency, providing valuable guidelines for future semitransparent photoelectrode development.</jats:p>