Competitive growth kinetics of coexisting hydrogen bubbles on Ni electrodes: role of bubble nucleation density.

Abstract

The coverage of hydrogen bubbles decreases the active area of electrodes, resulting in reduced electrochemical performance. However, bubble growth locally decreases hydrogen concentration, thereby mitigating concentration overpotential. This dual effect highlights the significance of investigating the effect of bubbles on hydrogen removal in electrode design. Since hydrogen removal primarily occurs <i>via</i> molecular transport across bubble interfaces (which drives bubble growth), we analyzed the multi-bubble growth kinetics (<i>R</i> = <i>αt</i> ^<i>β</i> ) on Ni electrodes with varying roughness to compare the hydrogen removal effect at the bubble interface. For a low-roughness (LR-surface) electrode, bubble growth follows conventional time coefficients (<i>β</i>) close to 0.5, indicating that the bubbles were in an H₂-saturated environment, where the entire bubble interface participated in hydrogen removal. The elevated bubble density on a medium-roughness (MR-surface) electrode provides additional bubble interfaces for hydrogen removal, reducing hydrogen concentration (<i>α</i> decrease from 93.91 to 63.11). The time coefficient of bubble growth remained at 0.5, confirming that the increased bubble interface was also in the hydrogen-saturated condition. In contrast, on a high-roughness (HR-surface) electrode, the competition of excessive coexisting bubbles for hydrogen molecules leads to the narrowing of the H₂-saturated region, and the top of the bubble is in the H₂-unsaturated region, indicating that not all of the additional bubble interface is involved in the hydrogen removal, which is manifested as the decrease in the time coefficient (<i>β</i> decrease from 0.5 to 0.42). Based on the experimental results, we conclude that the hydrogen removal effect does not increase linearly with increasing numbers of coexisting bubbles on the electrode. The transition in bubble growth kinetics reflects the varying degree of bubble interface involvement in hydrogen removal, which may serve as a consideration for designing the density of bubble nucleation sites on electrodes.