Hydrogen Evolution Reaction (HER) Mechanism Sensitivity to Ultrasound in Alkaline Media as a Function of Metal Activation Energy

Alkaline water electrolysis (AWE) represents a critical route for low-carbon hydrogen production, but energy losses arising from several resistances at the electrode surface hinder its worldwide industrial development. Ultrasound has emerged as a promising tool to mitigate these losses by enhancing mass transfer and improving bubble management. However, although previous studies recognize its ability to improve process efficiency, its effects on fundamental kinetics and mechanism of hydrogen evolution reaction (HER) remain unclear, especially on changes in activation energy. This study investigated these kinetic variations across five electrocatalysts (Ni, Mo, Ag, Ti and Pt) selected to span a wide range of activation energies, in a 1.0 M KOH electrolyte. Two ultrasonic frequencies (20 and 580 kHz) by using ultrasonic transducers powered by ultrasonic generators whilst positioning the electrodes at optimal acoustic nodes. Ultrasound preferentially affects the least and most active electrocatalysts, respectively improving and degrading their performance, while Tafel ( b ) analysis reveals a shift from a Volmer- to a Heyrovsky-limited mechanism for some metals. Activation energies ( E a ) were derived from the exchange current densities ( j o ) measured between 10 and 40°C, applying Arrehnius’ law. These results indicate that activation energy variation stems primarily from ultrasound-induced alterations of the HER mechanism, driven by local surface changes that either hinder or promote proper molecular configuration depending on the electrocatalyst. This finding suggests that optimizing ultrasonic parameters offers a promising strategy not only to improve bubble detachment, but also to control molecular configuration in a way that promotes the HER.