Temperature impacts on the growth of hydrogen bubbles during ultrasonic vibration-enhanced hydrogen generation

Abstract

To improve the hydrogen precipitation performance on the surface of the catalytic layer of the proton exchange membrane (PEM) hydrogen cathode, ultrasonic vibration was employed to accelerate the detachment of hydrogen bubbles on the surface of the catalytic layer. Based on the energy and mechanical analyses of nano and microbubbles, the hydrogen bubble generation mechanism and the effect of temperature on bubble parameters during the evolution process when the ultrasonic field is coupled with the electric field are investigated. The nucleation frequency of the hydrogen bubbles, the relationship between the pressure and temperature and the operating temperature during the generation and detachment of bubbles as well as the detachment radius of bubbles under the action of the ultrasonic field are obtained. The effects of ultrasound and temperature on hydrogen production were verified by visual experiments. The results show that the operating temperature affects the nucleation, growth, and detachment processes of hydrogen bubbles. The effect of temperature on the nucleation frequency of bubbles mainly comes from the Gibbs free energy required for the electrolysis reaction. The bubble radius and growth rate are both related to the temperature to the power of one-third. Ultrasonic waves enhance the separation of hydrogen bubbles from the catalyst surface by acoustic cavitation and impact effects. An increase in the working temperature reduces the activation energy barriers to be overcome for the electrolysis reaction of water, which together with a decrease in the Gibbs free energy and the surface tension coefficient, leads to an increase in the nucleation frequency of the catalytic layer and a decrease in the radius of bubble detachment, and thus improves the hydrogen precipitation performance. Visualization experiments show that in actual PEM hydrogen production, ultrasonic intensification can promote the formation of nucleation sites. The ultrasonic induced fine bubble flow not only has a drag effect on the bubble, but also intensifies the polymerization growth of the bubble due to the impact of the fine bubble flow, thus speeding up the detachment of the bubble, shortening the covering time of the hydrogen bubble on the surface of the catalytic electrode, reducing the activation voltage loss and improve the hydrogen production efficiency of PEM. The experimental results show that when the electrolyte is 60°C, the maximum hydrogen production efficiency of ultrasound is increased by 7.34%, and the average hydrogen production efficiency is increased by 5.83%.