Design of Carbon Aggregation Structure in Polymer Electrolyte Fuel Cell Catalyst Ink by Solvent Hydrophilicity

Abstract

Controlling the porous structure of the cathode catalyst layer in a polymer electrolyte fuel cell is essential to reduce the reaction and mass transport resistance of the oxygen reduction reaction. As the catalyst layer is a coating of a catalyst ink, the aggregation structure of platinum-supported carbon (Pt/C) and ionomer in ink is a key property for achieving the optimal catalyst layer.1 Previous studies reported that the weight fraction of water in solvents could control aggregation structure in ink.2-4 For example, Kumano et al.2 reported that increasing the water ratio to 1-propanol enhances the ionomer adsorption on Pt/C (Γ) and dispersion of Pt/C, causing the lowering of the viscosity of the inks. Based on the results, they speculated that water repels ionomers and promotes their adsorption on Pt/C particles. They further speculated that the ionomers adsorbed on the Pt/C particles induce repulsive interactions between Pt/C particles via electrostatic repulsive interactions among sulfonate groups of adsorber ionomers. By the enhanced repulsive interactions, Pt/C particles are dispersed, and viscosity decreases. The discussion in the previous study likely indicates that the hydrophilicity of the solvent is an essential factor controlling the aggregation structure of catalyst ink. Here, we verify this hypothesis. As a universal indicator of the hydrophilicity, we adopt the hydrogen-bond term HSP-δH of the Hansen solubility parameters. We examined how HSP-δH is correlated to the ionomer adsorption ratio to Pt/C (Γ), viscoelasticity of ink, and structural properties of ink measured by ultra-small angle X-ray scattering (USAXS).The catalyst inks were prepared by mixing Pt/C (TEC10V30E, TKK), Nafion 20 wt% solution (DE2020, Chemours), ultrapure water, and three different alcohols [ethanol (EtOH), 1-propanol (PrOH) and diacetone alcohol (DAA)]. The water weight fraction in the solvents was set to 25 - 85 wt%. Γ is plotted as a function of HSP-δH in Fig. 1. Regardless of the alcohol species, Γ increases with increasing HSP-δH. The trend is consistent with the previously reported trend observed in the mixtures of water and 1-propanol. Figures 2 (a) and (b) show the equilibrium storage modulus G’0 obtained by the rheometer and the fractal dimension D 2 of agglomerates of Pt/C particles in ink as functions of Γ. Here, D 2 is determined by fitting the unified model5 to the USAXS spectrum obtained by the synchrotron radiation (Spring-8, Japan). Both decreases with the increase of Γ. The lowering of D 2 indicates that the agglomerates of Pt/C particles are dispersed and turn to less-structured particles, as schematically shown in Fig. 2 (c). By the dispersion, G’0 decreases. Accordingly, the hydrophilicity, indeed, promotes the ionomer adsorption, and the adsorption induces the dispersion. The hydrophilicity is a key factor controlling the structure and rheology of the catalyst ink, and HSP-δH is a universal descriptor of hydrophilicity. Reference K. B. Hatzell, M. B. Dixit, S. A. Berlinger, and A. Z. Weber, J. Mater. Chem. A, 5 (39), 20527-20533 (2017). N. Kumano, K. Kudo, Y. Akimoto, M. Ishii, and H. Nakamura, Carbon, 169 429-439 (2020). S. Takahashi, T. Mashio, N. Horibe, K. Akizuki, and A. Ohma, ChemElectroChem, 2 (10), 1560-1567 (2015). T. Van Cleve, S. Khandavalli, A. Chowdhury, S. Medina, S. Pylypenko, M. Wang, K. L. More, N. Kariuki, D. J. Myers, A. Z. Weber, S. A. Mauger, M. Ulsh, and K. C. Neyerlin, ACS Appl Mater Interfaces, 11 (50), 46953-46964 (2019). G. Beaucage, H. K. Kammler, and S. E. Pratsinis, J. Appl. Crystallogr., 37 (4), 523-535 (2004). Figure captions Figure 1 Dependence of the ionomer adsorption ratio Γ on the HSP-δH of the catalyst ink solvent. Figure 2 (a) Γ dependence of the equilibrium storage modulus G’0 of the catalyst inks. G’0 is plotted on a logarithmic scale. G’0 values of inks that are not measurable due to being less than the measurable lower limit are conventionally described as 0. (b) Γ dependence of the mass fractal D 2 of Pt/C aggregates in the inks. (c) Schematic illustration of the aggregation structure of Pt/C in the solvents. Figure 1