Bubbles and membranes: Electrolysis at the mesoscale

Water electrolysis, the focus of this thesis, is expected to play a key role in the energy transition by enabling the production of green hydrogen. This thesis broadly focuses on studying mass-transfer in the vicinity of gas-evolving electrodes. In attempting to do so, the thesis focuses on two specific aspects of electrolyzers - bubbles (Ch. 1-3) and membranes (Ch. 4).

The introductory chapter briefly explains the motivation behind the thesis as well as the research methodology adopted in this work. Electrolysis is a complex process involving many stochastic phenomena which make it difficult to study. The unifying research strategy adopted in this thesis is the adoption of well-controlled, microfabricated electrode architectures which enable us reduce the level of stochasticity at the electrode-electrolyte interface. These electrode designs are progressively scaled-up to study electrolytic phenomena at different scales in controlled systems.

In chapters 1 and 2, this strategy lead to the fabrication of a ring-pit electrode wherein a ring-shaped electrode surrounds a super-hydrophobic pit at its center. This hydrophobic pit acts as a preferential nucleation site for electrolytic bubbles thus achieving the spatial decoupling of the site of electrolysis and the site of bubble nucleation. This system enables us to study electrolysis on this micro-electrode system in the absence of electrode coverage due to bubbles. Bubbles in this system are observed to transition between three growth regimes each indicative of the prevailing concentration dynamics in the vicinity of the electrode. Experimental data in combination with numerical models showed that the bubbles lower the concentration of dissolved hydrogen near the electrode - leading to a decrease in the concentration overpotential.

Chapter 3 adopts a novel electrode architecture with multiple preferential bubble nucleation sites. This enables the study of electrolytic bubbles in a system that better resembles commercially adopted electrodes. The spacing between the bubble nucleation sites is found to be a key determinant of the bubble departure radius. This proves that the dominant mechanism of bubble departure is through coalescence with adjacent bubbles and opens the possibility for passive bubble control using optimally spaced nucleation sites. Analysis of the growth rates of bubbles driven by different electrolysis currents indicated the presence of a physical phenomenon that enhanced bubble growth rates with increasing current.

Chapter 4 describes the fabrication and characterization of a novel, porous silicon-based, zero-gap electrode for alkaline electrolysis. The porous silicon separator consists of an ordered array of cylindrical pores. As with the distance between the pits in Chapter 3, the distance between the pores on this electrode determines the separator resistance and gas crossover. The ionic resistance is found to increase with decreasing porosity, while the gas crossover decreases with decreasing porosity.