Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3].

Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations.

Acknowledgement
This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

References
[1] A. Kongkanand and M. F. Mathias, “The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells,” J. Phys. Chem. Lett., vol. 7, no. 7, pp. 1127–1137, Apr. 2016.
[2] V. Yarlagadda et al., “Boosting Fuel Cell Performance with Accessible Carbon Mesopores,” ACS Energy Lett., vol. 3, no. 3, pp. 618–621, 2018.
[3] E. Padgett et al., “Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography,” J. Electrochem. Soc., vol. 165, no. 3, pp. F173–F180, 2018.