Semitransparent Ternary NiFeCoO x Cocatalyst Layers with Optimized Loading Balancing Transparency and Oxygen Evolution Activity on FTO and Hematite Photoanodes

Semitransparent oxygen evolution reaction (OER) cocatalysts are essential for efficient photoelectrochemical (PEC) water oxidation with visible-light-absorbing semitransparent photoelectrodes; however, their surface loading must be optimized to minimize parasitic light absorption. In this study, we report a scalable wet-chemical route to ternary NiFeCoOx thin films that serve as highly transparent and active OER electrocatalysts for n-type semitransparent photoelectrodes. By systematically varying the surface loading and quantifying both visible-light transmittance and OER activity, we identify an optimal NiFeCoOx loading of 0.20 μmol cm–2, where high optical transmittance in the visible region is retained while the OER rate is maximized. At this optimized loading, a NiFeCoOx-modified n-type α-Fe2O3 model semitransparent photoanode exhibits a photocurrent density of 0.19 mA cm–2 at 1.23 V vs a reversible hydrogen electrode under simulated sunlight illumination, which is ∼1.5 times higher than that of α-Fe2O3 photoanodes modified with binary NiFeOx cocatalysts under otherwise identical conditions, demonstrating the beneficial effects of Co incorporation. This study quantitatively elucidates the trade-off between light transmittance and cocatalyst loading and establishes NiFeCoOx thin films prepared via a scalable wet-chemical process as promising semitransparent OER cocatalysts for visible-light-absorbing photoanodes in PEC water splitting.