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The morphology of carbon supports for Pt-based proton-exchange membrane fuel cell (PEMFC) catalysts strongly determines their performance at both low and high current density. Porous carbon supports with internally deposited Pt nanoparticles sustain high kinetic activity by shielding Pt from ionomer adsorption, albeit at the expense of poor oxygen mass transport. This work systematically explores an oxidative pre-treatment of commercial Pt/Ketjenblack, termed localized oxidation, which drastically improves oxygen transport and high current density performance (up to 50% at 0.6 V). The method leverages Pt-catalyzed carbon oxidation in the immediate vicinity of internal Pt particles to increase pore accessibility. We analyze the catalyst morphology via N2 physisorption and thermogravimetric analysis (TGA), and correlate these results with extensive electrochemical characterization of low-loaded cathodes (0.06 mgPt cm−2). High current density gains are shown to result predominantly from removing microporous constrictions in the primary carbon particle. We further identify a trade-off between Pt particle sintering and pore widening dependent on the oxidation temperature, which defines an optimum degree of oxidation. Finally, we investigate the susceptibility of locally oxidized catalysts towards start-up/shut-down (SUSD) degradation. Although we find modestly accelerated degradation rates at high oxidation temperatures, this does not outweigh the performance benefit imparted by the pre-treatment.

Catalyst layers in proton exchange membrane fuel cells consist of platinum-group-metal nanocatalysts supported on carbon aggregates, forming a porous structure through which an ionomer network percolates. The local structural character of these heterogeneous assemblies is directly linked to the mass-transport resistances and subsequent cell performance losses; its three-dimensional visualization is therefore of interest. Herein we implement deep-learning-aided cryogenic transmission electron tomography for image restoration, and we quantitatively investigate the full morphology of various catalyst layers at the local-reaction-site scale. The analysis enables computation of metrics such as the ionomer morphology, coverage and homogeneity, location of platinum on the carbon supports, and platinum accessibility to the ionomer network, with the results directly compared and validated with experimental measurements. We expect that our findings and methodology for evaluating catalyst layer architectures will contribute towards linking the morphology to transport properties and overall fuel cell performance.

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