Effects of photo-generated gas bubbles on the performance of tandem photoelectrochemical reactors for hydrogen production

Abstract

Presence of gas bubbles in the vicinity of semiconductor electrodes interferes with their active surface areas and introduces inert voids in the electrolyte hindering its ionic conductivity. Furthermore, gas bubbles obstruct the radiation path through scattering. The aim of this work is to study the characteristic hydrogen- and oxygen-gas bubble behavior and their effects on photoelectrochemical reactor performance. Findings of gas bubble formation, electrode coverage and curtain profiles based on macroscopic bubble graphical images are reported. Effects of increased convective forces are also observed. Further, the scattering of incident light implemented through simulations based on Mie scattering theory is reported. Results show that hydrogen gas bubbles are more extensive in coverage due to formation of a froth while oxygen bubbles coalesce and rise easily. The growth of the bubble cover increases ohmic resistance reducing the current magnitude. Even at a modest current density of 10 mA/cm2, the curtain thickness may rise to 2 mm or 3 mm for oxygen and hydrogen, respectively. Light scattering increases with increasing bubble size and is more pronounced for shorter wavelengths. It is also found that presence of multiple bubbles reduces light intensity by up to 2% and highest when the bubble radius is 150 μm. Increase in both photoelectrode and electrolyte resistances as well as radiation losses due to presence of bubbles hence undermine the performance of photoelectrochemical reactors.