Balancing Surface Passivation and Catalysis with Integrated BiVO₄/(Fe-Ce)O_x Photoanodes in pH 9 Borate Electrolyte

The performance of oxygen-evolving photoanodes based on bismuth vanadate (BiVO₄) is critically determined by the surface coating. While these coatings passivate surface defects, transport photogenerated holes, protect against corrosion, and aid catalysis, their optimal composition changes with operating pH, thus affecting overall performance. We use high-throughput photoelectrochemistry methods to map photoanode performance to enable the discovery of optimal composition and loading of Ce-rich sputter-deposited (Fe-Ce)O_x overlayers on undoped BiVO₄ in pH 9 borate buffer electrolyte. The optimal composition is found to be 20% Fe and 80% Ce with an optimal Fe + Ce metal loading of 0.9 nmol mm^-2. Analysis of the composition and loading dependence of (i) the photocurrent transients upon illumination toggling, (ii) stabilized photocurrent densities, and (iii) photogenerated hole-transfer efficiency reveals the confluence of phenomena that gives rise to the optimal performance yielding nearly perfect transfer efficiency over a narrow composition window.