A Study on the Influence of Ionization Method on Single Cell Initial Performance: Electrospray Method

Abstract

Various catalysts showing high ORR activity have been developed over the past several years, and the results of evaluating actual fuel cells prepared using highly ORR-active catalysts have been reported. However, in contrast to rotating disk electrode (RDE) results showing high ORR activity, it is difficult to implement the high activity in actual fuel cell testinghen using the same catalysts. The most significant difference compared to the RDE is that there is an interface between the electrolyte and the surface of the catalyst in a membrane electrode assembly (MEA). In order to improve the coating properties of the electrolyte binder on the surface of the catalyst and the distribution of the electrolyte binder in the catalyst layer, various slurry[1] and catalyst layer preparation methods have been studied[2,3,4]. In this study, the catalyst layers were prepared using the electrospray (ES) method, and two different ionization methods (positive, negative) were used, based on surface charge analysis of the electrolyte binder (-35 mV) and electrolyte binder-coated catalysts (-43 mV). Structural differences in the catalyst layer were observed for the different ionization methods used. Mainly, it was observed that greater than 1 μm diameter pores were formed in the catalyst layer prepared by the positive ionization method. In contrast, with the negative ionization method, relatively uniform pores were formed in the catalyst layer. The initial performance of the MEAs prepared by the negative ionization method revealed improved initial performance over that of the positive ionization method: at 0.85 V, O2 supply, positive: 0.074, negative: 0.091 A/mgPt; at 0.65 V, air supply, positive: 1.26, negative: 2.10 A/mgPt; and O2 supply, positive: 3.86, negative 6.31 A/mgPt). The initial performance difference according to the ionization method can be understood through a structural difference in the catalyst layer analyzed by STEM and TEM, and through electrochemical analysis: i) different electrolyte binder morphology. ii) structural differences in the catalyst layer, and iii) proton transport resistance in the catalyst layer. Therefore, it was confirmed that, when the catalyst layer was prepared by the electrospray method using the same electrolyte binder content, the negative ionization method yielded around 1.5 times better initial performance, regardless of the single cell operating conditions. Acknowledgement This work was partially supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC) Project" from the New Energy and Industrial Technology Development Organization (NEDO) and the "Yamanashi Frontier for Innovation and Ecosystem Project" from Regional Innovation Ecosystems of the Ministry of Education, Culture, Sports, Science and Technology.