Electrolytically Generated Nanobubbles on HOPG Surfaces

TL;DR

This study investigates the formation and growth of nanobubbles on HOPG surfaces during water electrolysis, revealing their dynamic equilibrium, growth behavior, and the influence of voltage and electrolyte composition.

Contribution

It provides detailed real-time measurements of nanobubble growth and correlates their size with electrolysis parameters, advancing understanding of nanobubble dynamics on electrode surfaces.

Findings

Nanobubbles form preferentially on HOPG surfaces during electrolysis.

Nanobubble size and current saturation occur within about 1 minute.

Electrolyte composition affects current but not nanobubble growth behavior.

Abstract

Electrolysis of water is employed to produce surface nanobubbles on highly orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles are formed when the HOPG surface acts as negative (positive) electrode. Coverage and volume of the nanobubbles enhance with increasing voltage. The yield of hydrogen nanobubbles is much larger than the yield of oxygen nanobubbles. The growth of the individual nanobubbles during the electrolysis process is recorded in time with the help of AFM measurements and correlated with the total current. Both the size of the individual nanobubbles and the total current saturate after typical 1 minute; then the nanobubbles are in a dynamic equilibrium, meaning that they do not further grow, in spite of ongoing gas production and nonzero current. The surface area of nanobubbles shows a good correlation with the nanobubble volume growth rate, suggesting that either the electrolytic gas emerges directly at the nanobubbles' surface, or it emerges at the electrode's surface and then diffuses through the nanobubbles' surface. Moreover, the experiments reveal that the time constants of the current and the aspect ratio of nanobubbles are the same under all conditions. Replacement of pure water by water containing a small amount of sodium chloride (0.01 M) allows for larger currents, but qualitatively gives the same results.