The use of ultrasound for the fabrication of fuel cell materials

This paper reviews the use and advantages of ultrasound for the preparation of fuel cell materials which is currently an emerging research area. The review also focuses on recent studies of ultrasonic, sonochemical and sonoelectrochemical production of noble metals and fuel cell electrocatalysts, carbon supported electrocatalysts, fuel cell electrodes and membranes. It is shown that ultrasound can be used as an effective method for producing nanosize mono- and bi-metallics (<10 nm) in the absence and presence of surfactants and alcohols. In most cases, the formation of nano-metallics is attributed to radical species (H and OH) generated by water sonolysis induced by cavitation whereby the nano-metallic size strongly depends upon the ultrasonic frequency and time, the type of surfactant, alcohol and atmospheric gas. It is also shown that the sonochemical production of carbon-supported mono- and bi-metallic catalysts gives excellent electrochemical activity due to surface functionalisation of the support and better dispersion induced by ultrasound. These observations are mainly due to enhanced mass-transfer caused by asymmetrical collapse of cavitation bubbles at the surface support leading to the formation of high velocity jets of liquid being directed toward its surface. This jetting, together with acoustic streaming, is thought to lead to random punctuation and disruption of the mass-transfer at the surface.