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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.

We investigate, theoretically, for what amplitude of the applied voltage to an electrolytic cell the concept of impedance is meaningful. The analysis is performed by means of a continuum model, by assuming the electrodes perfectly blocking. We show that, in the low-frequency range, the electrolytic cell behaves as a linear system only if the amplitude of the measurement voltage is small with respect to the thermal voltage V T = k B T / q, where k B T is the thermal energy, and q is the modulus of the electrical charge of the ions, assumed identical except for the sign of the charge. On the contrary, for large frequency, we prove that the amplitude of the applied signal has to be small with respect to a critical voltage that is frequency dependent. The same kind of analysis is presented for the case in which the diffusion coefficients of the positive ions is different from that for negative ions, and for the case where surface adsorption takes place.

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