Unravelling the Impact of the Ionomer on the Degradation Mechanisms in Carbon-Supported Platinum Electrocatalysts: On the Path Toward Durable Proton Exchange Membrane Fuel Cells

The performance and durability of carbon-supported platinum (Pt/C) electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC) are strongly influenced by the characteristics of the carbon support, resulting in different ionomer-catalyst interactions. Our study examines how the ionomer affects the degradation of three Pt/C catalysts with distinct carbon support porosity: nonporous Vulcan, microporous Ketjenblack, and mesoporous Hollow Graphitic Spheres. The application of a voltage cycling accelerated stress test (AST) in aqueous electrolyte half-cell configurations operated at 80 °C allows us to investigate the effect on the degradation at relevant operating conditions by comparing ionomer-free catalyst films with films containing an application-relevant ionomer content. We correlate electrochemically active surface area (ECSA) losses with ex-situ diagnostic methods, including identical location and ex-situ scanning transmission electron microscopy vs secondary electron microscopy (STEM/SE-STEM) and determination of leached platinum via inductively coupled plasma mass spectrometry (ICP-MS). Our results reveal the intricate interplay between carbon-support porosity and ionomer effects on the degradation mechanisms: the nonporous carbon-supported catalyst shows enhanced ECSA loss and altered overall particle coarsening upon ionomer incorporation, which we attribute to extensive adsorption of highly acidic sulfonate groups of the ionomer on the exposed Pt nanoparticles. For the porous carbon-supported catalysts, we observe different effects depending on the location of the particles: (i) enhanced dissolution of particles outside of pores (increased SO₃^– adsorption) and (ii) protection of particles inside of pores (restricted SO₃^– adsorption) from dissolution. However, despite this significant change in the pathway and overall attenuated particle growth, the measured ECSA losses were comparable. We ultimately confirm the practical relevance of our results with complementary ASTs conducted in membrane electrode assembly (MEA) configurations. Our findings offer valuable guidance for the design of Pt/C catalysts and ionomers for optimized catalyst layers, advancing the development of more robust PEMFC technologies.