Understanding the Exposed Area Dependent Photoelectrochemical Performance of BiVO₄ Photoanodes

To enable meaningful comparisons in solar fuel production, the energy conversion efficiency of photoelectrodes must be evaluated using standardized benchmarking protocols. For solar water splitting, the photocurrent density at a defined potential in a three-electrode configuration (e.g., 1.23 V vs. RHE for water oxidation) is widely used. However, this single descriptor is often insufficient for reliable cross-comparison between studies. In many reports, the highest photocurrent densities are obtained under conditions where the entire geometric electrode area (typically several cm²) is illuminated, while only a small fraction (<0.1 cm²) is in contact with the electrolyte. This discrepancy highlights the critical role of the fabricated or illuminated-to-exposed area ratio on the measured performance. In the present study, we systematically investigate the dependence of photocurrent density on the exposed-to-fabricated area ratio of bismuth vanadate (BiVO₄) photoanodes prepared using state-of-the-art methods. By combining electrochemical, spectroscopic and structural characterization, we elucidate how this geometric factor governs the apparent photoelectrochemical response. Our results demonstrate that photocurrent metrics are highly sensitive to mismatches between illuminated, fabricated, and electrolyte-exposed areas, underscoring the need for more transparent and standardized reporting protocols to ensure meaningful benchmarking of photoelectrodes across studies.