Ultrasound-assisted electrochemical hydrogen production in a 300 kHz sonoreactor in the presence of methanol.

This work investigates sonoelectrochemical hydrogen production in a 300 kHz sonoreactor using water-methanol electrolytes, with a focus on disentangling purely sonochemical contributions from ultrasound-induced electrochemical enhancement. Ultrasound-only experiments show that methanol addition increases cavitation-driven hydrogen formation by more than an order of magnitude relative to pure water, with an optimum around 1.2 M, but absolute rates remain at the μmol min-1 level and energy efficiencies are far below those of electrolysis. In contrast, electrochemical (EC) and sonoelectrochemical (US@EC) operation at 5.0 V with 0.1 M Na2SO4 produce 10-20 μmol min-1 of hydrogen with Faradaic efficiencies of approximately 50-57%, and ultrasound consistently enhances hydrogen production by 6-32% across the entire methanol concentration range. Gas-saturation experiments reveal that Ar increases the sonochemical hydrogen yield compared with N2, yet both gases provide similar US@EC enhancements, indicating that hydrodynamic effects (bubble detachment, suppression of gas blanketing and diffusion-layer renewal) dominate over direct sonochemical hydrogen formation. Microscopic analysis further shows that 300 kHz ultrasound causes far less mechanical damage to Ti/PbO2 and graphite electrodes than low-frequency ultrasound reported in the literature, highlighting high-frequency operation as a promising strategy to obtain stable, practically meaningful ultrasound-induced enhancement in electrochemical hydrogen production.