Mechanistic studies of bubble evolution
thesisposted on 15.12.2014, 10:35 by Sarah Lucy. Gilbert
This thesis describes a study of the bubble evolution mechanisms of four systems; H2/HC1O4Au, H2/HCIO4/Pt, Cl2/NaCl/RTO and Cl2/NaCl/Ti as a function of current density, surface composition (RTO) and morphology. Three experimental techniques have been used, fast frame video (FFV), electrochemical quartz crystal microbalance (EQCM) and the pressure release bubble nucleation technique (PRBN). The three complementary techniques were utilised to provide a comprehensive study of bubble behaviour. The FFV was used to study individual bubbles, the EQCM to measure the detachment rate integrated over the nucleating surface area, and the PRBN technique was used to measure the bubble nucleation rate integrated over space and time.;In the current density range 3-44 mA cm-2 nucleation of individual hydrogen bubbles was found to be rapid at vertical gold and platinum electrodes. For both electrode materials, the detachment radius decreased as a function of current density, but the average bubble size on detachment was approximately 50% smaller on the platinum electrode. The detachment rate and number of active nucleation sites both increased as a function of current density, and the dominant detachment mechanism was coalescence between closely paired neighbouring bubbles. EQCM analysis of the overall detachment rates at the electrodes confirmed the visual observations; novel Fast Fourier Transform analysis of the frequency response from the gold electrode showed that the detachment frequency increased with current density, but there was little change in the volume of gas resident on the electrode. On the platinum electrode small bubbles detached rapidly and were characterised by small fluctuations in the EQCM resonant frequency response.;Using the FFV technique, nucleation was found to be the rate-limiting step for chlorine bubble evolution at an RTO electrode in the current density range 18-56 mA cm-2.