Effects of Solvent Composition on Viscosity and Dispersion Structure of PEFC Catalyst Ink

Abstract

A polymer electrolyte fuel cell (PEFC) is an energy device that generate electricity from the electrochemical reaction that produces water from hydrogen and oxygen. It has been expected to be used as a power source for automobiles that does not emit carbon dioxide. The catalyst layer of PEFC is nano-porous layer composed of platinum-supported carbon and ionomer. The catalyst layer is fabricated by coating and drying a catalyst ink in which these materials are dispersed in a solvent made of water and ethanol. At that time, the dispersion structure in the catalyst ink, that is, the dispersion state of the platinum-supported carbon and the ionomer affects the catalyst layer structure and the electrochemical surface area. Though the viscosity and dispersion structure of the ink changes depending on the ink composition and the shear rate during the mixing and coating process, the correlation between the viscosity and the dispersion state of the catalyst ink is not fully understood, and optimal control of dispersion structure is not established.The purpose of this study is to clarify the effect of the solvent composition, which affects the catalyst layer structure and the power generation characteristics of PEFC a lot [1], on the viscosity and dispersion structure of the catalyst ink. To achieve the purpose, the solvent composition was changed as a parameter, and the viscosity characteristics measurement and the particle size distribution measurement of the catalyst inks were carried out.For the catalyst ink, TEC10V30E (Tanaka Kikinzoku) was used as the platinum-supported carbon and DE1021 (Sigma-Aldrich) was used as the Nafion solution, and the water/ethanol ratio was changed so that the ionomer/carbon ratio and the solid content was kept 0.75 and 10%, respectively. The viscosity characteristics were measured at each shear rate in the range of 0.01 to 1000 1/s by a rotary rheometer (Anton-Paar, MCR302). The particle size distributions were measured to understand the dispersion structure of the undiluted catalyst ink, and the laser diffraction type particle size distribution meter (HORIBA, LA-960V2) made it possible by using the sample holder which seals the catalyst ink as a liquid film (thickness: 10 mm) to enhance light transmission.The viscosity characteristics of the catalyst inks were shown in Figure 1 (a). Regardless of the solvent composition, the catalyst ink showed shear-thinning behavior (higher viscosity at lower shear rate), and it was more remarkable for the ink with high ethanol ratio. The particle size distribution of the catalyst ink was shown in Figure 1 (b). The ink with an ethanol ratio of 46% had a larger particle size than the catalyst ink with an ethanol ratio of 20%, and the particle size larger than 10 mm was observed for the catalyst ink with an ethanol ratio of 60%. From the results of the particle size measurement, it was suggested that the aggregate structure of platinum-supported carbon was formed as the ethanol ratio increased. As a mechanism for a dispersion to exhibit shear-thinning behavior, it has been proposed that the aggregation structure in the liquid breaks and the viscosity decreases with the application of shear [2]. Since the difference in the aggregate particle size was observed from the particle size distribution measurement, it was suggested that the viscosity of the catalyst ink with high ethanol ratio increased in low shear rate region due to the formation of the aggregate structure and decreased in high shear rate region due to the breaking-up of the aggregate structure. Since the ink without platinum did not show shear thinning behavior, it was considered that the ethanol oxide generated by the decomposition of ethanol on the platinum catalyst caused the aggregation of the catalyst ink, and further understanding about the impacts of ethanol oxide is required to control the dispersion structure. Acknowledgment:This study is based on the results obtained from a project commissionedby the New Energy and Industrial Technology Development Organization (NEDO).Reference[1] M. Kishi, S. Moriyama, T. Mori, J.Soc. Powder Technol. Japan, 55, 366–374 (2018).[2] H. Nakamura, The Micromeritics, 62, 30-38 (2019). Figure 1