# Electrolytic Bubble Coalescence on Hydrophobic Cavity Arrays Determines Departure Radius and Lowers Electrolyte Supersaturation

## TL;DR

This study uses specially designed electrodes to control bubble formation and coalescence, improving efficiency in electrolysis.

## Contribution

The study introduces microfabricated electrodes with hydrophobic cavities to control bubble nucleation and coalescence.

## Key findings

- Closer hydrophobic pits reduce electrolyte supersaturation and increase electrode coverage.
- Bubble departure radius is strongly influenced by coalescence with neighboring bubbles.
- Increasing nucleation site spacing raises dissolved gas concentration in the electrolyte.

## Abstract

Bubble evolution on electrodes is a complex process that begins with the stochastic nucleation of bubbles on the electrode surface, followed by bubble growth due to diffusion and coalescence, and bubble departure. The stochasticity of bubble evolution on conventional electrodes is a significant challenge in efforts to study electrolytic bubbles. In this investigation, the growth of electrolytic hydrogen bubbles is studied on microfabricated silicon electrodes with arrays of hydrophobic cavities. These hydrophobic pits act as preferential nucleation sites for bubbles—thus lowering the degree of spatial‐randomness in bubble nucleation and enabling the study of bubbles growing in the presence of coalescence with greater control. Substrates with different spacings between the hydrophobic pits were fabricated. It is shown that coalescence with neighboring bubbles strongly determines the departure radius of bubbles. Further analysis of the bubble growth rate and electrode coverage indicates that closer pits decrease the electrolyte supersaturation while increasing electrode coverage.

Bubble evolution is a key source of inefficiency in electrolyzers. In this study, custom‐fabricated electrodes with arrays of preferential nucleation sites are used to overcome the spatial randomness of bubble nucleation. Bubble departure size is shown to strongly depend on coalescence. Furthermore, increasing nucleation site spacing is shown to increase the concentration of dissolved gas in the electrolyte.