Let’s Not Ignore the Ultrasonic Effects on the Preparation of Fuel Cell Materials

Abstract

This article is a follow-up paper recently published entitled ‘The importance of ultrasonic parameters in the preparation of fuel cell catalyst inks’ [1] describing the effect of low-frequency high-power ultrasound on the catalyst ink composition used for the fabrication of fuel cell electrodes. In this paper, it was shown that care should be taken when using low-frequency ultrasound whereby (i) the ultrasonic parameters such as frequency, power and duration may affect the final ink composition and rheology and therefore its electrochemical performance, (ii) the ultrasonic equipment (and make), frequencies, powers, durations and the distance between the vibrating source and the reaction vessel should be reported, (iii) the catalyst ink temperature should be monitored and regulated during the course of the experiment, (iv) immersing the ultrasonic probe into the solution may lead to contamination (arising from the erosion of the titanium alloy vibrating tip) and (v) high-shear mixing of the catalyst inks using rotor-stator mixers at high rotation speed in silent conditions should be performed, analysed and compared to ultrasonicated samples for consistency and comparison purposes between studies. A careful and systematic approach should be adopted due to the fact that low-frequency ultrasound is known to be an intensification technology offering remarkable advantages: (a) an increase in fluid degasification, de-agglomeration (and particle size reduction), dispersion, homogenisation, emulsification, atomisation, molecular degradation and chemical rates and yields and (b) an improvement of surfaces due to very efficient cleaning (mainly erosion). These ultrasonic effects are known to be caused by (a) an increase in mass transfer and heat transfer induced by extreme solution ‘mixing’ and (b) the production of cavitation bubbles undergoing very short and violent collapse within the fluid generating local ‘hotspots’ of high energy (temperatures of up to 5,000 K and pressures of up to 2,000 atms), leading to (i) radicals formation and (ii) jets of liquid of high velocity (up to 200 m s−1) near surfaces.