Sono-electrolysis: A look into the distinguished effects of direct and indirect sonication at high frequency ultrasounds (490 kHz)

This study investigates the impact of direct and indirect high-frequency (490 kHz) ultrasonication on alkaline water electrolysis for hydrogen production. Calorimetric analysis reveal that direct sonication transmits 54.17 W of acoustic power, corresponding to an efficiency of 77.39 %, whereas indirect configuration achieves substantially lower efficiencies. Hydrogen quantification shows that conventional electrolysis alone produces 0.288 mM H₂ in 60 min (0.08 mM/s), indirect sono-electrolysis improves yields modestly by 3-6 %, while direct sono-electrolysis reaches 0.777 mM in 60 min, representing a 170 % enhancement. This improvement stems from both sonolytic hydrogen generation (0.448 mM) and electrochemical synergy (0.034 mM). Electrochemical characterizations demonstrate enhanced current densities, reduced Tafel slopes, and improved kinetics for both HER and OER under direct ultrasound exposure, attributed to localized cavitation, bubble detachment, and intensified mass transport. Modeling of cavitation dynamics confirm that passing from indirect to direct sonication increases bubble compression ratios up to 8.64, resulting in order-of-magnitude increases in collapse temperature and hydrogen yields. These findings highlight the strong dependence of sono-electrolysis performance on acoustic coupling geometry, with direct configurations offering a promising pathway toward energy-efficient hydrogen production.