Considerations on Oxygen Bubble Formation and Evolution on BiVO4 Porous Anodes Used in Water Splitting Photoelectrochemical Cells

The important problem of the bubble formation and evolution on a bismuth vanadate anode, and their consequence on the efficiency of a photoelectrochemical cell is analyzed. Although bubbles represent the way in which the gas products are collected to be stored, they first tend to stick on the electrode surface, decreasing the effective area, increasing the interfacial electric resistance, and, thus, increasing the losses. Starting with general thermodynamic considerations on the process of bubble generation, two ideal surfaces have been imagined: the inner surface-inside the pores-where the main electrochemical processes take place, and the outer surface, on which the bubbles stick until they are large enough to leave the surface and be collected. A percolation approach has been used in order to explain the time variation (decrease) of the photoelectric process during the bubble generation. On the basis of the percolation approach, a fitting function has been proposed in order to analyze the experimental data recorded for a bismuth vanadate porous photoanode tested into a home-designed photoelectrochemical cell at several bias voltages, under dark and light conditions.