High Power Density Platinum Group Metal-free Cathodes for Polymer Electrolyte Fuel Cells

Abstract

Low cost and high-performing platinum group metal-free (PGM-free) cathodes have the potential to transform the economics of polymer electrolyte fuel cell (PEFC) commercialization. Significant advancements have been made recently in terms of PGM-free catalyst activity and stability. However, before PGM-free catalysts become viable in PEFCs, several technical challenges must be addressed including cathode's fabrication, ionomer integration, and transport losses. Here, we present an integrated optimization of cathode performance that was achieved by simultaneously optimizing the catalyst morphology and electrode structure for high power density. The chemically doped metal−organic framework derived Fe−N−C catalyst we used allows precise tuning of the particle size over a wide range, enabling this unique study. Our results demonstrate the careful interplay between the catalyst primary particle size and the polymer electrolyte ionomer integration. The primary particles must be sufficiently large to permit uniform ionomer thin films throughout the surrounding pores, but not so large as to impact intraparticle transport to the active sites. The content of ionomer must be carefully balanced between sufficient loading for the complete catalyst coverage and adequate proton conductivity, while not being excessive and inducing large oxygen transport losses and liquid water flooding. With the optimal 100 nm size catalyst and ionomer loading, we achieved a high power density of 410 mW/cm 2 at a rated voltage and a peak power density of 610 mW/ cm 2 in an automotive-relevant operating condition.

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High‐Power PGM‐Free Cathodes in Fuel Cells

“…It contains no large-scale agglomerates, cracks, or other common large-scale heterogeneities often found with M-N-C cathodes due to the large carbon aggregates. [25,26] Instead, the structure is consistent with the diameter of the fibers. The macroscale void distribution was quantitatively evaluated using the PoreSpy toolkit.…”

“…Our prior work has shown that 3D electrode structures (e.g., pore size, pore distribution, and catalyst geometry) and ionomer distribution strongly affected the PGM‐free cathodes in MEAs. [ 25,26 ] We further evaluated these macroscopic properties for the promising Co–N–PCNFs cathode by using nano‐CT imaging (with a resolution of about 50 nm). As developed by Komini Babu et al., [ 26 ] the nano‐CT allows us to characterize the overall ionomer distribution by using the Cs + signal in absorption contrast and the pore/solid morphology through Zernike phase contrast.…”

“…Macroporosity is limited in ZIF-derived M-N-C catalysts in electrodes and only can be created between the carbon particle agglomerates (up to 20-30 µm) due to the stacking of primary carbon particles (50-100 nm). [25] The lack of large connected pores leads to the poor dispersion of the ionomer into the interior of primary catalyst particles and thus results in lower ionic conductivity within electrodes. [26][27][28] Therefore, advancements in MEA performance require innovative electrode design with tailored micro-, meso-, and macroporosity, aiming at maximizing the density of effective TPBs and improving mass and charge transport.…”

Recent Progress in Proton-Exchange Membrane Fuel Cells Based on Metal-Nitrogen-Carbon Catalysts

“…Lister's group 83 systematically investigated the effect of catalyst particle size on this interaction. Fe-N-C catalysts with various particle sizes were used to prepare MEAs.…”

“…a) Nano-CT images of ionomer distribution in CCLs with various catalyst particle sizes; (b) schematic illustration of catalyst aggregates; (c) EIS plots for CCL with various catalyst particle sizes; (d, e) polarization curves of MEAs prepared with catalysts in various particle sizes; (f) polarization curve of MEA fabricated with the catalyst in 100 nm, measured in the H2-O2 fuel cell.Reproduced with permission83 . Copyright 2020, American Chemical Society.…”