Ultrasound-enhanced water electrolysis for hydrogen production: Mechanisms, metrology and energy metrics

Ultrasound intensifies hydrogen production in water electrolysis cells by thinning boun­dary layers, accelerating bubble detachment, and, in tuned windows, modulating cavi­tation chemistry, yet cross-study claims remain difficult to compare. Focusing on ultra­sound-enhanced water electrolysis (sono-electrolysis), this review aligns reporting with IEC 61161 (radiation-force acoustic power) and IEC 62127-2:2025 (hydrophone calibra­tion); requires delivered acoustic intensity at the electrode, Idel / W·cm⁻², with stated trace­ability; pairs isothermal control with uncertainty budgets; and benchmarks perfor­mance using Δ-metrics: Δj (current-density gain at fixed cell voltage), Δη (cell voltage/over­po­tential reduction at fixed current density) and ΔH₂ (hydrogen production rate gain at matched electrical input), together with specific energy consumption (SEC, kWh·kg⁻¹ H₂). A window-based synthesis indicates that, under isothermal operation, 20 to 40 kHz with delivered intensity ≈0.2 to 1.0 W·cm⁻² reproducibly yields Δj ≈ 15 to 30 %, Δη ≈ 40 to 120 mV, ΔH₂ ≈ 10 to 30 %, and net SEC improvements of ~8 to 12 % when auxiliary loads are included, whereas at higher dose (Idel ≈ 1.0 to 1.6 W·cm⁻²) non-uniform fields, cloud shielding, and heating can saturate or reverse benefits. To prevent metric conflation, hybrid sono-hydrogen routes are reviewed separately. The review concludes by proposing a minimum reporting set-frequency, waveform/duty and pulse repetition frequency, Idel (traceability/uncertainty), geometry/stand-off, electrolyte and dissolved gas, bulk tempe­rature and runtime, gas metrology with temperature/pressure cor­rections, SEC boun­daries and replicates/statistics, and by outlining priorities for operando cavitation-electro­chemistry co-registration, geometry/void-fraction-aware scale-up, and durability under combined fields, to support reproducible, energy-accounted, and comparable studies across laboratories.