Bubble formation and concentration fields in water electrolysis

The subject matter of this thesis aims to understand the combined effect of bubbles and ion transport in gas-evolving electrochemical cells by measuring ion concentration fields and imaging bubbles on electrodes. Electrochemical processes find urgent application in the fields of energy storage (batteries, fuels) and CO2 capture and reduction, among others. Many of these systems involve the interaction of bubbles, dissolved gas and ions at the electrode. For example, the electrochemical processes of chlorine production, chlorate synthesis, (sea)water electrolysis and selective hydrogenation, which require selective production of gases for their cost effective operation, involve vigorous bubble generation at the electrodes. In this work, the well-studied hydrogen evolution reaction is used as a model system to study bubbles and measure concentrations of charged/uncharged species near electrodes.

Specifically, in the first chapter, bubbles are imaged on a hydrogen evolving inverted electrode and their appearance near the electrode is correlated with an increase in the measured current density. Following this, in the second chapter, a technique to measure time resolved pH using confocal fluorescence microscopy is established. In the third chapter, bubble dissolution rates are measured. By comparing the experimental measurement with numerical results, the anomalous dissolution rates are argued to be caused by buoyancy driven convection and bubble clustering on the electrode. Finally in chapter four, patterns of a charged pH insensitive dye are measured at a H+ reducing platinum electrode. These patterns are linked to the onset of electroconvective instability, which is known to drive currents greater than the diffusion limit in electrochemical cells.