Mass transfer mechanism during bubble evolution on the surface of photoelectrode

The limitation of the transfer of gas products within the photocatalytic system is the key to the efficiency of the water splitting, which is directly influenced by the bubble evolution process on the catalyst surface. In this study, the entire process of bubble evolution on the surface of a superhydrophilic TiO₂ photoelectrode was simultaneously measured visually and electrochemically. The bubble growth and detachment characteristics were investigated under different laser powers and constant bias voltages. An expression for the bubble coverage characterized by the bubble radius and photocurrent, which is applicable to the photoelectrocatalytic water splitting, was proposed to accurately obtain the mass transfer coefficient during bubble evolution. The mass transfer mechanism of gas product transfer in the low current density range dominated by single-phase free convection was revealed. The mass transfer coefficient was found to increase in the inertia-controlled phase and decrease in the chemical reaction-controlled phase of bubble growth. Furthermore, a method to accelerate bubble detachment from the photoelectrode surface by periodic illumination was proposed, which could significantly enhance the gas product transfer process.