Sonolytic and ultrasound-assisted techniques for hydrogen production: A review based on the role of ultrasound

Among the cleanest combustibles, hydrogen represents the energy carrier of the near future, thereby, techniques for hydrogen synthesis have been the subject of several research studies. The integration of ultrasound to several techniques as a promising combination has known in recent years an increasing interest owing to the physical and chemical roles of sonication and the eventual created synergetic effects. The present review aims to inspect the various techniques using ultrasound as a direct or auxiliary pathway for the generation of hydrogen. Experimental and numerical studies related to the use of power ultrasound as an isolated technique are reviewed in terms of approaches, configurations, and qualitative and quantitative observations. The combination of power ultrasound with other techniques such as electrolysis, catalysis, and photolysis, is then examined and discussed, particularly in regards to the role of ultrasound in the "ultrasound-assisted" hydrogen production, but also the expected magnitude of the kinetics enhancement and energy-saving when integrating ultrasound. It was reported in several works that the use of ultrasound for hydrogen generation may lead to an increase in the rate of produced hydrogen achieving 25%, and an improvement in energy efficiency in the range of 5-18%, while some other studies reported limited enhancement in the order of 1%. Overall, ultrasound irradiation has the advantages of enhancing mass transport, bubbles detachment from catalysts and electrodes surfaces, efficient degassing, and cleaning effect. The technological state of the art and the engineering designs are also