Mass transfer mechanism of single bubble evolution on TiO2 electrode surface under decreased pressure

The complex gas-liquid mass transfer process dominated by bubbles adhered to the photoelectrode surface is a crucial factor affecting the energy and mass conversion efficiency of photoelectrochemical (PEC) water splitting reaction, which is of great significance for improving the efficiency of hydrogen production. The gas evolution process on a super-hydrophilic TiO2 electrode surface with various pressures and laser powers was studied in this paper by combining the means of visualization and the electrochemical measurement method. The empirical formulas of gas evolution efficiency and bubble coverage characterized by photocurrent and bubble radius under decreased pressure were established. The results shows that the transient gas evolution efficiency and the transient bubble coverage both increase first and then remain almost unchanged in the range of 0∼0.42 and 0∼0.2 respectively with bubble growth. The average gas evolution efficiency and average bubble coverage increase first and then decrease when pressure drops in the range of 40 kPa∼97 kPa, and both reach their maximum at 80 kPa. The coefficients of the gas-liquid mass transfer process between bubbles and surrounding solution under different pressures were solved mathematically. It can be discovered that the transient mass transfer coefficients decrease with bubble growth, and the average mass transfer coefficients increase first and then reduce with the decreased pressure, and reach the peak at 80 kPa. The gas mass production rate and the average total mass transfer coefficient exhibit a similar variation trend with respect to pressure under the same irradiation conditions. It is discovered that the principal mass transfer mechanism for gas products that diffuse and transfer into static electrolyte under low pressure and low photocurrent density is the single-phase free micro-convection.