Analysis of PEMFC Electrode Structure – Bridging the Mesoscale Gap
Authors: Workman Michael J , Webber J. Beau W. , Perry Mike L. , Darling Robert M. , More Karren L. , Mukundan Rangachary , Borup Rod L. 
Published: February 2020

Proton exchange membrane fuel cell (PEMFC) electrodes are commonly composed of Pt nanoparticles supported on conductive carbon interspersed with ionomer, creating a porous network facilitating transport of electrons, protons, both reactant and product gasses, and liquid water. For the electrochemical reactions to occur at the metallic Pt active sites in the cathode, the electrons, protons, and oxygen all need to be present, which presumably requires the confluence of carbon, ionomer, and Pt all accessible at a pore. Following reaction at this 3-phase boundary, the pore network needs to remove product water from the electrode.
Though the structure of PEMFC electrode layers has been studied for many years, the details are still unclear. The distribution of pore sizes, thickness and distribution of ionomer, and structure of carbon agglomerates are not well understood. This lack of understanding hinders rational design of catalyst layers that utilize reduced Pt loading while maintaining the performance and durability necessary for large-scale commercialization.
Part of the difficulty in obtaining accurate morphological information on these complex structures is that it is unclear what techniques provide accurate analysis of the pore structures within these materials. Common pore size analysis techniques include nitrogen adsorption (BET), mercury intrusion (MIP), x-ray tomography (XCT), transmission electron microscopy (TEM), and focused ion beam tomography (FIB-SEM). When multiple techniques are applied, even to the same sample, the results for pore size distribution and connectivity can vary widely. It is unclear which, if any, of these techniques yield results representative of the true pore structure within the catalyst layer. Further, it is not known if the lack of agreement results from sample preparation methods and analysis conditions used for each technique, or fundamental limitations of the techniques when applied to PEMFC electrode materials.
In this work, we examine results of pore analysis from multiple techniques. In addition to standard measurement techniques including BET, TEM, MIP, and FIB-SEM tomography, we employ techniques for measurement of pore size by analysis of the solid/liquid phase transition temperature using cryoporometry (Figure 1). We will discuss the results of these multiple techniques, address their utility and limitations, and attempt to present a pore size distribution in better agreement with reality that can be used as input for modeling efforts.

Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos).

Figure 1. Comparison of pore size distributions from nitrogen isotherm analysis using BJH1 (top) and cryoporometry (bottom). Top figure is PSD for TEC10E50E (CB), TEC10EA30E (GCB), and acetylene black supported Pt with specific surface areas of 779 and 219 m2 g-1 (AB800 and AB250 respectively). Bottom figure is PSD for TEC10E50E.
1) Young-Chul Park, Haruki Tokiwa, Katsuyoshi Kakinuma, Masahiro Watanabe, Makoto Uchida, Effects of carbon supports on Pt distribution, ionomer coverage and cathode performance for polymer electrolyte fuel cells, Journal of Power Sources, 315 (2016) pp.179-191

Figure 1