Influence of electrode surface roughness on ultrasonic-enhanced alkaline water electrolysis

This study investigates the influence of stainless-steel electrode surface roughness on the performance of alkaline water electrolysis under ultrasonic excitation. Four electrodes with distinct roughness levels were tested using a 30 wt.% KOH electrolyte and a probe-type ultrasonic processor operating at 26 kHz and 60 W. The experiments combined electrochemical characterization with high-speed imaging to evaluate hydrogen bubble dynamics and flow behavior through optical flow analysis. Results showed that ultrasound application promoted modest yet consistent improvements in current density (up to 4%) and voltage reduction, particularly at low voltages where buoyancy and surface tension forces compete. Among the tested surfaces, the electrode with intermediate roughness (S#01) exhibited the best overall performance, balancing active area and bubble detachment efficiency. In contrast, excessively smooth or rough surfaces demonstrated inferior behavior, suggesting the existence of a most favorable roughness within the tested range for maximizing ultrasonic benefits. Velocity field analyses revealed that ultrasonic vibration disrupted the directional flow, induced transient bubble clustering (“pseudo-coalescence”), and enhanced local mixing, effects that dissipated once the excitation ceased. The findings provide mechanistic insight into the coupled effects of surface topography and acoustic excitation on bubble dynamics, establishing a physical basis for the future design of optimized ultrasound-assisted electrolysis systems. The direct immersion of the sonotrode within the electrolyte, unlike most reported configurations, ensured efficient acoustic coupling and enabled the clear observation of these phenomena.