Electrochemical noise measurements of coalescence and gas-oscillator phenomena on gas-evolving electrodes

This paper is aimed at identifying the different mechanisms of hydrogen bubble evolution on Pt disk electrodes of different sizes (0.5, 1.0, and 5.0 mm in diam) in a wide current density range. This was accomplished by means of time and frequency electrochemical noise analysis combining potential and electrolyte resistance simultaneously measured to separate ohmic and nonohmic fluctuations. At weak current densities, coalescence processes were shown to be a major component of bubble growth in the case of small electrodes while diffusion-controlled bubble growth was dominant on larger surface ones. For all electrode diameters, a new mechanism was spontaneously switched on at higher current densities to enhance evacuation of dissolved gas produced at the interface. This new mechanism was a periodical gas-oscillator collective phenomenon originated in the simultaneous evolution of bubbles at the border of the electrode where the metal-insulator discontinuity acted as a continuous gas cavity, giving preferential bubble nucleation. Once present, the gas-oscillator mechanism was a strongly dominant component of the evolution process, so that most of the bubble production was shifted from the center to the edge of the electrode.