Growth characteristics and the mass transfer mechanism of single bubble on a photoelectrode at different electrolyte concentrations

In the photoelectrochemical water splitting reaction, the bubble attached to the working electrode is an essential factor affecting the reaction resistance, current density and gas-liquid mass transfer. An experimental measurement system based on an electrochemical workstation synchronously coupled with a high-speed microscopic camera was proposed and used to systematically study the growth kinetics and mass transfer mechanism of single oxygen bubbles at different electrolyte concentrations (Na2SO4, 0.1-2.0 M) on the TiO2 photoanode surface. Under constant voltage and constant current control conditions, when the electrolyte concentration increases, the bubble detachment diameter and the bubble detachment frequency gradually decrease. The bubble coverage equation expressed in terms of gas evolution efficiency is proposed and is associated with the photocurrent and bubble radius. The average bubble coverage and average gas evolution efficiency decrease when the electrolyte concentration is increased. According to the Sherwood dimensionless number, various mass transfer coefficients during bubble growth were calculated. The results show that the average total mass transfer coefficient is positively correlated with the change trend of the electrolyte concentration, and the mass transfer coefficient of single-phase natural convection is one order of magnitude larger than the mass transfer coefficient of bubble-induced convection. Finally, a conclusion on the transient mass transfer process in the bubble evolution process was obtained, that is, the mass transfer coefficient of single-phase natural convection and the total mass transfer coefficient remain high during the first growth stage, and gradually decrease during the second growth stage. Therefore, regulating the electrolyte concentration can effectively promote the gas-liquid mass transfer in the photoelectrochemical water splitting reaction.