Impact of Solvent Decomposition on Dispersion Structure of PEFC Catalyst Ink

Abstract

To achieve higher current density operation of polymer electrolyte fuel cells (PEFCs), it is required to realize higher performance catalyst layer with low oxygen transport resistance, high proton conductivity, and low Platinum loading. Because dispersion structure of catalyst ink strongly affects the catalyst layer structure, it is crucial to understand the dispersion mechanism of PEFC catalyst ink. Uemura et al. reported that the decomposition of ethanol within the catalyst ink strongly affect its dispersion [1, 2], but its mechanism was not fully understood yet. In this study, impact of solvent decomposition on the dispersion of the catalyst ink was investigated.The catalyst inks were fabricated by mixing platinum-supported carbon (TEC10V30E, Tanaka Kikinzoku Kogyo), ionomer (DE1021, Sigma-Aldrich), and water/ethanol solvent. The water/ethanol ratio, the ionomer/carbon ratio and the solid content of the catalyst ink were kept 46wt%, 0.75 and 10%, respectively. To investigate the impact of solvent decomposition, decomposition products of ethanol (aldehyde, acetic acid, and ethyl acetate) were artificially added to the catalyst ink, and its effect on dispersion were evaluated. Viscosity characteristics were measured at each shear rate in the range of 0.01 to 1000 1/s by a rotary rheometer (MCR302, Anton-Paar), and particle size distribution of the undiluted catalyst ink was measured by the laser diffraction type particle size distribution meter (LA-960V2, HORIBA). In addition, ionomer adsorption rate on the platinum-supported carbon was measured by centrifugal separation technique.Figure (a) showed the impact of adding ethanol decomposition on viscosity characteristics. The result showed that aldehyde and ethyl acetate affect the dispersion of the catalyst ink, and it was suggested that generation of aldehyde caused aggregation of the platinum-supported carbon within the ethanol-rich catalyst ink. Figure (b) showed the effect of aldehyde on ionomer adsorption ratio as a function of centrifuge time. By adding the aldehyde, the ionomer adsorption rate became lower. Because the ionomer promotes the dispersion of ionomer, low ionomer adsorption rate might cause the aggregation of platinum supported carbon, and further understanding on the dispersion mechanism is highly requested to realize stable catalyst ink.[1] Suguru Uemura et al 2019 J. Electrochem. Soc. 166 F89[2] Kaname Iida et al 2020 ECS Trans. 98 497 Figure 1