How do bubbles affect light absorption in photoelectrodes for solar water splitting?

Abstract

This paper aims to systematically investigate the effect of gas bubble formation on the performance of a horizontal photoelectrode exposed to normally incident light during photoelectrochemical water splitting. The presence of hydrogen or oxygen gas bubbles increases the back-scattering losses from the photoelectrode, thereby decreasing the photocurrent density generated. To quantify these optical losses, the normal-hemispherical reflectance of a Si photoelectrode covered with non-absorbing cap-shaped gas bubbles was predicted using the Monte Carlo ray-tracing method. For the first time, results are reported for both monodisperse and polydisperse bubbles with diameter ranging between 0.25 and 1.75 mm, projected surface area coverage varying between 0 and 78.5%, and contact angle ranging between 0° and 180°. The normal-hemispherical reflectance of the photoelectrode was found to be independent of the bubble diameter, and spatial and size distribution for any given projected surface area coverage. However, it varied significantly with the bubble contact angle due to total internal reflection at the electrolyte/bubble interface. The normal-hemispherical reflectance also increased with increasing projected surface area coverage thereby reducing the photon flux absorbed in the photoelectrode. In fact, the photons were absorbed mostly outside the bubble projection where they were preferentially scattered by the bubbles. The area-averaged absorptance in a bubble-covered Si photoelectrode reduced by up to 18% compared with a bare photoelectrode. The results presented in this study indicate that the performance of large photoelectrodes can be improved by using hydrophilic photoelectrodes or coatings.