Polymer electrolyte fuel cell (PEFC) energy conversion systems have found application in transportation applications due to their high power density, low operating temperature, high energy conversion efficiency, and speed of refueling.¹ One of the key components of a PEFC is the membrane electrode assembly (MEA), which consists of a proton exchange membrane (PEM), such as a perfluorosulfonic acid (PFSA), sandwiched between two catalyst layers (CLs) serving as anode and cathode. Several studies of CL microstructure−performance relationships suggest that the electrode microstructure characteristics, such as the extent of the catalyst−ionomer interaction, continuity of the ionomer phase, and pore size distribution are critical for performance.^2-4 However, successful implementation of novel materials for PEFC electrodes is hindered by the lack of a fundamental understanding of the complex, multicomponent nature of the CL, the interplay of its components, and their independent and cooperative effects on the effective properties and performance.

In this work, ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, presence of platinum, and ionomer loading on catalyst agglomeration in CLs. Particle size distributions (PSDs), obtained from fitting the scattering data using the maximum entropy (MaxEnt) method, were used to determine the size of carbon aggregates and agglomerates from carbon-ionomer and platinum-carbon-ionomer CLs. Three types of carbon supports were chosen for the investigation: high surface area carbon (HSC), Vulcan XC-72, and graphitized Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer loadings (0.2-1.0 ionomer to carbon ratio, I/C) were studied in order to evaluate the effect of ionomer on the CL agglomerate structure. The CL agglomerate structure will be correlated with electrochemical performance to reveal the interplay of the components, and their independent and cooperative effects on performance.

References

1. B.D. McNicol, D. A. J. Rand, K. R. Williams, J. Power Sources 100 (2001) 47–59.
2. F. C. Cetinbas, X. Wang, R. K. Ahluwalia, N. N. Kariuki, R. P. Winarski, Z. Yang, J. Sharman, and D. J. Myers, J. Electrochem. Soc., 164, (2017) F1596–F1607.
3. Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources 315 (2016) 179-191.
4. S. Holdcroft, Chem. Mater., 26 (2014) 381−393.

This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the Fuel Cell Performance and Durability Consortium (FC-PAD). The Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science User Facility. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.