A Comparative Study of Electrospray and Airbrushing Processes for Deposition of Catalyst Layers

Abstract

Electrospray and airbrushing are methods for catalyst layer deposition based on spraying a suspension of the ionomer and the catalyst. Airbrushing is most popular because it can be carried out with a simple and cheap set-up, and allows for fast deposition rates with high homogeneity of the resulting layers. In the electrospray deposition, the set-up is a little more complicated because the spray must be produced in the presence of an electric field which confers special characteristics to the deposition process giving rise to different layer properties [1]. This technique allows preparing reproducible and durable catalyst layers with internal superhydrophobic character. When used in the cathode, the electrosprayed layer enhances water back-diffusion from cathode to anode, which improves membrane and anode humidification [2]. Electrosprayed layers also have low mass-transport resistance thanks to their low liquid water content while working in a fuel cell [4]. As a consequence, power conversion increases by around 20% under standard testing conditions, comparing with the conventional airbrushing method. Durability of the cells is also improved by the more homogeneous layer response [2,3]. In this communication the electrospray and airbrushing deposition processes are compared in order to explain the differences observed in the resulting catalyst layers. The electrosprayed film growths by the stacking of catalyst particles and ionomer chains under electrostatic interaction and free of solvent, which is evaporated before arriving solids to the substrate. On the other hand, airbrushing takes place by deposition of suspension droplets, so the catalyst and the ionomer arrive to the substrate together with the solvent and dispose under the same interactions prevailing in the suspension. The different conditions give rise to differences in catalyst and ionomer distribution, morphology, and porosity of the layers. In addition, the electrosprayed catalyst layers are characterized by requiring lower ionomer concentration in the departing ink, typically 15wt% with a Pt/C 20wt% catalyst, which is about half of the amount for an optimal standard airbrushed catalyst layers, and they show a sharp optimization with thickness/loading. Differences in the structure and fuel cell behavior properties will be analyzed in the light of the characteristics of the deposition process.