Effect of laser spot diameter on mass transfer mechanisms in photoelectrochemical water splitting

Clarifying the mass transfer mechanism in the evolution of bubbles on the surface of photoelectrodes can help break the shackles that limit the efficiency of photoelectrochemical water splitting. This paper utilized an electrochemical workstation and a high-speed camera to synchronously record the evolution of oxygen bubbles on the surface of thin-film catalysts and their electrochemical signals. The nucleation and detachment characteristics of bubbles at different spot diameters were investigated, and the gas evolution efficiency and mass transfer coefficient were calculated. As the spot diameter increases, the nucleation potential, terminal rising velocity, and growth duration of the bubbles show an increasing trend, and the increase in the gas evolution efficiency reflects an increase in the proportion of gas molecules entering the bubbles. According to the calculations using the mass transfer model, the mass transfer coefficient decreases as the spot diameter increases. The mass transfer coefficient increased by a factor of 1.46 when the spot diameter decreased from 1176 to 700 μm. Our study showed that reducing the spot diameter can effectively improve the mass transfer of gaseous products on the photoelectrode surface and facilitate the nucleation and rapid detachment of bubbles.