New Advances in Graded PEMFC Catalyst Layers

Ohmic and mass transport overpotentials remain investigation and optimization areas for Proton Exchange Membrane Fuel Cell (PEMFC) applications. One potential optimization approach involves the preferential distribution of the chemical substituents spatially in the catalyst layer. This would ultimately result in graded catalyst layers (GCLs). The spatial grading directions can thereby be in the in plane and/or through plane direction(s). Chemical constituents often graded in both directions include the binder and the catalyst[1, 2]. The ionomer, an essential constituent for PEMFC applications has also been a key focus area for many research institutes and groups. Novel optimization approaches include finding new ionomers with low equivalent weights (EWs). Typically, in a PEMFC, the highest proton conduction rate is at the membrane interface and decreases towards the GDL[3, 4]. Hence, previous studies with gradients containing high ionomer contents close to the membrane and lower ones at the GDL interface resulted in improved performances. The higher ionomer content at the membrane interface resulted in better protonic conductions, and the bigger void spaces in the GDL interface resulted in improved mass transport properties[1, 5]. Though precise electrochemical analysis is lacking, it is commonly agreed that the graded layers have lower mass transport and ohmic resistances. However, the studies encountered in the literature are almost exclusively based on high EW ionomers, such as Nafion. In contrast, low EW ionomers, which have higher protonic conductivity and water uptake capabilities have not yet been a focus of researchers. Additionally, the few reports encountered in the literature focus solely on GCLs while often disregarding other contributing effects to the overall cell voltage such as the equivalent weight. Low EW ionomers offer a wider operation range for PEMFCs in terms of relative humidities. By combining the low EW ionomers with graded catalyst layers, the operation range becomes even more flexible, and the limits are further stretched. In this study, we push the operating range limits of low EW graded catalyst layers and compare them to non-graded ones. We investigate different manufacturing techniques and use suitable scalable coating methods for the manufacturing of the electrodes. Afterwards, suitable in situ and ex situ analysis techniques are used in detail to trace down the improvements for the gradients. Among the key findings, we notice that potential improvements for low EW ionomer graded catalyst layers become apparent at considerable low relative humidities. The improvements are visible in the kinetic, ohmic, and mass transport regions. References: [1] L. Xing et al., “Membrane electrode assemblies for PEM fuel cells: A review of functional graded design and optimization,” Energy, vol. 177, pp. 445–464, 2019, doi: 10.1016/j.energy.2019.04.084. [2] S. Ebrahimi, B. Ghorbani, and K. Vijayaraghavan, “Optimization of catalyst distribution along PEMFC channel through a numerical two-phase model and genetic algorithm,” Renewable Energy, vol. 113, pp. 846–854, 2017, doi: 10.1016/j.renene.2017.06.067. [3] T. Reshetenko and A. Kulikovsky, “Impedance Spectroscopy Study of the PEM Fuel Cell Cathode with Nonuniform Nafion Loading,” J. Electrochem. Soc., vol. 164, no. 11, E3016-E3021, 2017, doi: 10.1149/2.0041711jes. [4] D. Gerteisen, “Impact of Inhomogeneous Catalyst Layer Properties on Impedance Spectra of Polymer Electrolyte Membrane Fuel Cells,” J. Electrochem. Soc., vol. 162, no. 14, F1431-F1438, 2015, doi: 10.1149/2.0511514jes. [5] Z. Xie et al., “Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells,” J. Electrochem. Soc., vol. 152, no. 6, A1171, 2005, doi: 10.1149/1.1904990.