Effects of a perpendicular ultrasonic field on planar and porous electrodes for hydrogen production in alkaline conditions.

Among water electrolysis systems, alkaline water electrolysers (AWE) are promising for large-scale hydrogen production due to their cost-effective nickel (Ni) electrodes. However, AWE faces challenges, particularly molecular hydrogen and oxygen bubbles shielding on electrode surfaces in the liquid alkaline electrolyte (KOH). Therefore, innovative solutions like ultrasonication and advanced electrode structures are explored as effective methods for bubble removal. This study investigates the hydrogen evolution reaction (HER) under two ultrasonic frequencies (20 kHz and 580 kHz) using planar and porous Ni electrodes on a copper (Cu) substrate. The planar electrode was prepared using a Ni watt bath, while the porous electrode was fabricated using the dynamic hydrogen bubble template (DHBT) method. To further align with industrial conditions, the distance between the electrodes and transducer was maintained between 10 and 15 cm. The study focused on the synergistic effect of ultrasonication and porous electrodes on mass transfer and HER performance. Results show that the porous Ni electrode reduced overpotential by -97 mV, with ultrasonication enhancing this by up to -132 mV. Furthermore, ultrasonication altered the HER mechanism for DHBT Ni, as shown by changes in the Tafel slopes, b (-250 mV/dec vs. -87 mV/dec under ultrasonication and silent conditions respectively). It was also found that the intense convection from acoustic cavitation implosion might influence HER mechanisms by hindering water adsorption at the electrode surface. Finally, the electrode-sonotrode distance was found to be critical to prevent surface degradation at shorter distances (3.5 cm), requiring further optimization for industrial purposes.