Effect of the Solvent Composition in Catalyst Inks on Viscosity, Catalyst Layer Morphology, and PEMFC Performance
Authors: Schilling Markus Achim , Li Yan-Sheng , Gasteiger Hubert Andreas 
Published: August 2024

The solvent composition in a catalyst ink is a key parameter in fabricating cathode catalyst layers (CCLs) to reach high-performance proton exchange membrane fuel cells (PEMFCs). In general, an optimized solvent composition in a catalyst ink (e.g., the H2O-to-alcohol ratio) can yield reasonably uniform and thin ionomer structures covering the catalyst surface, leading to better proton and oxygen transport resistances of the CCLs.1,2 While the impact of solvent composition on the morphology of ionomer or catalyst in both solvent dispersion and CCLs was widely investigated, those results could vary significantly and remain inconclusive in between different studies.3 This is likely due to complex interactions between the solvent system, the ionomer, and the catalyst, and additional experimental parameters (e.g., the solvent composition in the ionomer stock dispersion, or the ink mixing method) that were not controlled. Minor changes in the fabrication process can alter the final morphology of the components in a CCL and thus vary its PEMFC performance.4 To the best of our knowledge, there are only a few systematic studies correlating the effect of the solvent system on ionomer dispersions and catalyst inks to the CCL micromorphology and PEMFC performance.
In this study, we systematically examined the impact of the ratio of two commonly used solvents, 1-propanol (1-PA) and H2O, on the viscosity of ionomer dispersions (without catalysts), on the viscosity of catalyst inks (using Pt supported on a Vulcan and a Ketjenblack carbon support), on the crack formation in CCLs, and on the single-cell PEMFC performance (using 5 cm2 active area single-cell testing under differential-flow conditions). The occurrence of cracks in the CCLs was rationalized based on the rheology of the catalyst inks, the surface tension of the solvent mixtures, and the change in solvent composition during the drying process. Moreover, the influence of the ionomer stock dispersion on the viscosity of catalyst inks was investigated. This analysis shows that catalyst inks with identical compositions can have significantly different viscosities and crack formation in CCLs, depending on the ionomer stock dispersion used in the catalyst ink preparation, which highlights the importance of the processing history of the ionomer. An exemplary result is given in Figure 1, comparing the photographs of two CCLs coated on a PTFE substrate prepared with catalyst inks of identical composition. Both CCLs were fabricated with 10/90 wt% 1-PA/H2O in the catalyst inks, but different ionomer stock dispersions were used: One stock dispersion was prepared with 100 wt% H2O (Fig. 1A), and the other with a 60/40 wt% 1-PA/H2O mixture (Fig. 1B). Furthermore, we show that the shear force due to the grinding media used during the mixing of catalyst inks can trigger a gelation process of the ionomer, which strongly affects the viscosity and processibility of the catalyst ink. Finally, the impact of the solvent composition of catalyst inks on the PEMFC performance (in H2/O2 and H2/air configuration), the proton conduction resistance, and the oxygen transport resistance of the CCL were compared. Based on those results, we highlight the importance of the processing history for the fabrication of CCLs.

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Acknowledgment

This work has been supported by the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No. 826097 (GAIA). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation program, Hydrogen Europe, and Hydrogen Europe Research.

Figure 1