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We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi­(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt –1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt –1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core–shell catalysts.

Over its lifetime in a fuel cell electric vehicle, a polymer electrolyte membrane fuel cell inevitably suffers from certain duration of dry operational conditions, where significant performance losses of the fuel cell take place. In this study, we investigate the activity changes of the fuel cell after a prolonged degradation protocol under dry operational condition, followed by various recovery procedures under wet conditions. The utilization of diluted air on the cathode side is found to be advantageous for the recovery due to the superior heat and water management. This more efficient recovery protocol allows the deconvolution of reversible and irreversible voltages losses after dry operations. A subsequent mechanistic study reveals an irreversible decrease of the effective ionomer coverage on the catalyst particles, while the proton conductivity of the catalyst layer drops. These observations point towards ionomer structural changes caused by the dry conditions. This is confirmed by post-mortem analysis via scanning electron microscope, showing clearly that ionomer redistributes and migrates, an additional mechanism which leads to the performance losses. Overall, the degradation mechanisms seem to be mitigated by higher ionomer content in the catalyst layer, while the investigated surface modification of carbon support shows minor sensitivities.

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This work demonstrates that functionalizing annealed-Pt/Ketjen black EC300j (a-Pt/KB) and dealloyed-PtNi/Ketjen black EC300j (d-PtNi/KB) catalysts using p-phenyl sulfonic acid can effectively enhance performance in the membrane electrode assemblies (MEAs) of proton exchange membrane fuel cells (PEMFCs). The functionalization increased the size of both Pt and PtNi catalyst particles and resulted in the further leaching of Ni from the PtNi catalyst while promoting the formation of nanoporous PtNi nanoparticles. The size of the SO3H-Pt/KB and SO3H-PtNi/KB carbon-based aggregates decreased dramatically, leading to the formation of catalyst layers with narrower pore size distributions. MEA tests highlighted the benefits of the surface functionalization, in which the cells with SO3H-Pt/KB and SO3H-PtNi/KB cathode catalysts showed superior high current density performance under reduced RH conditions, in comparison with cells containing annealed Pt/KB (a-Pt/KB) and de-alloyed PtNi/KB (d-PtNi/KB) catalysts. The performance improvement was particularly evident when using reactant gases with low relative humidity, indicating that the hydrophilic functional groups on the carbon improved the water retention in the cathode catalyst layer. These results show a new avenue for enhancing catalyst performance for the next generation of catalytic materials for PEMFCs.