Using Ultrasound to Effectively Homogenise Catalyst Inks: Is this Approach Still Acceptable?

Since the early 2010s, less than a handful of studies have been communicated to the hydrogen and fuel cell communities that special care should be adopted, and a systematic approach should be applied, when homogenising catalyst ink slurries using ultrasound in the form of either a laboratory-grade ultrasonic cleaning bath, or an ultrasonic probe (sonifier). In these studies, it was demonstrated that the use of power ultrasound for the homogenisation of catalyst inks can be detrimental if not used appropriately. Unfortunately, and to this day, literature still indicates that ultrasound is still used for the homogenisation of fuel cell and electrolyser catalyst ink slurries and little or even no experimental conditions are given. To what extent is this approach acceptable? This short review paper discusses the importance of using ultrasound adequately to avoid catalyst dissolution and ionomer degradation induced by acoustic cavitation as well as metallic contamination originating from the ultrasonic probe. It also sheds some light on the important aspects and effects of power ultrasound in liquids and surfaces and presents some recommendations on how to use ultrasound adequately for mixing catalyst ink formulations.