PGM-Free Electrode Microstructure Analysis and Transport Modeling

Abstract

The dependence of proton exchange membrane fuel cell (PEMFC) systems on platinum group metal (PGM) catalysts is a major barrier to these systems meeting the cost-performance targets required to be competitive with the internal combustion engines (ICEs) in the market. In the past decade, oxygen reduction reaction (ORR) catalysts derived from earth abundant materials have gained increasing interest as low cost alternatives to replace platinum catalysts. To date the most promising PGM-free catalyst candidate are the pyrolyzed transition metal (such as Fe, Co), nitrogen and carbon (M-N-C) complexes [1]. While there have been significant improvements in the ORR activity of these materials through innovative synthesis and treatment techniques, the volumetric activity (ORR activity per cm3of electrode, measured at high cell voltage where the only limiting factor is the electro-catalytic process) of these PGM-free catalysts is low relative to the incumbent carbon-supported platinum catalysts. Achieving comparable activity requires almost 10 times thicker electrodes with respect to Pt-based counterparts [2]. While the majority of the PGM-free electrode studies focus on the volumetric activity, thicker electrodes lead to increased mass and charge transport resistances especially for air cathode gas utilized for automotive applications. In this study, performance and transport properties of electrodes based on the cyanamide-polyaniline-iron (CM-PANI-Fe) catalyst developed by Los Alamos National Laboratory were investigated. The electrode microstructure was characterized by use of the nano-scale X-ray tomography (nano-CT) technique. After ion exchanging the protons in Nafion® with cesium (Cs+), the samples were scanned in absorption contrast mode to visualize Cs+intensity, and thus the ionomer, while pore morphology was obtained using Zernike phase contrast scans, as in [2]. Ionomer volume fraction distribution and pores larger than the nano-CT resolution (~20 nm) were obtained as shown in Fig. 1. By running transport simulations, the effective transport properties were calculated for various sub-volumes extracted from the nano-CT data, as illustrated in Fig. 1. Using the effective ion conductivities in a simple performance model, ORR kinetic parameters () were calculated and sources of voltage losses were identified depending on the experimental polarization data. Finally, the mass transport resistances were quantified by using the calculated kinetic ORR and effective transport parameters within a detailed cathode model that accounts for the charge, reactant and liquid water transport. It was concluded that the flooded pores are the major source of mass transport resistances for the thick PGM-free electrodes whereas the film resistance becomes dominant for thinner electrodes, as shown in Figs. 2a and 2b.