Elettra | SAXS Spatial SAXS mapping of humidity-induced Pt3Co catalyst degradation in polymer electrolyte fuel cells Catalyst nanostructures are essential for high performance and long durability of Polymer Electrolyte Fuel Cells (PEFCs). When fuel cell catalysts age, their structure changes depending on the operation conditions. Understanding these modifications is essential to prevent early deactivation. In particular, the role of water remains insufficiently characterised in terms of its impact on local degradation mechanisms. To address this, spatially resolved Small-Angle X-ray Scattering (SAXS) is employed to investigate how humidity affects degradation in two structurally and chemically distinct PEFC cathode catalysts: (i) a carbonsupported platinum nanoparticle catalyst (Pt/C), and (ii) a carbon-supported platinum-cobalt alloy nanoparticle catalyst (Pt3Co/C). PEFCs with both electrocatalysts are subjected to two contrasting Accelerated Stress Test (AST) protocols, both designed to simulate realistic fuel cell operation under humid conditions. The first, referred to as AST1, applied symmetric humidification with 100% Relative Humidity (RH) on both the anode and cathode sides. The second, AST2, imposed a humidity gradient with 30% RH at the anode and greater than 100% RH at the cathode, mimicking severe asymmetry commonly encountered in real-world systems. The evolution of the catalyst nanostructures is tracked across the Membrane Electrode Assembly (MEA) surface area using ex situ SAXS mapping. In combination with spatially resolved in situ data, it is possible to link local structural changes and degradation phenomena. The results reveal fundamentally distinct degradation pathways (Fig. 1). In the Pt/C catalyst, degradation is dominated by homogeneous nanoparticle growth and agglomeration of Pt clusters. The average particle diameter increased from ~1.4 nm to ~2.0–2.8 nm post-AST (Fig. 1 a). SAXS analysis revealed a consistent transformation in the catalyst layer morphology, shifting from mass fractal clusters (fractal dimension Df ≈ 2) toward denser surface fractals (Df > 3). This densification reflects reorganization into more compact agglomerates and is schematically illustrated in Fig. 1 (d). Notably, these structural changes are most pronounced near the cathode outlet, where high relative humidity persisted due to water accumulation in the flow channels. Cluster sizes in these outlet regions reached up to 7 nm (Fig. 1 b), and the characteristic SAXS length scale L expanded substantially, indicating large-scale coarsening. These nano-structural transitions corresponded with measurable declines in electrochemical surface area, confirming that Pt/C degradation under humid conditions is driven by coalescence, fractal densification, and the spatially uniform loss of nanostructural integrity, particularly in zones of water accumulation. In contrast, the Pt3Co/C alloy catalyst exhibited a markedly different and more spatially heterogeneous degradation response. While average particle sizes also increased (~1.7 nm to ~2.5–2.9 nm), SAXS revealed that cluster sizes often decreased or remained constant, particularly near the gas inlets (Fig. 1 a, b). Instead of coalescence, SAXS data pointed to cluster fragmentation and disintegration into smaller, isolated domains. This behaviour is consistent with selective metal leaching, most likely cobalt, followed by the breakup of larger agglomerates. Fractal dimension changes are observed but occurred unevenly across the catalyst layer, suggesting site-specific transformations influenced by local humidity and acidity. Unlike Pt/C, the characteristic length scale L in Pt3Co/C did not show uniform growth; rather, it decreased at the anode inlet and only showed modest increases at the outlet (Fig. 1 b). This pattern reflects dispersive degradation driven by chemical instability, producing a highly inhomogeneous structural landscape. Such instability disrupts local electron and proton transport, reducing current generation efficiency and contributing to regional performance loss. Spatial SAXS mapping across the MEA revealed consistent trends linking local humidity profiles to degradation modes in both materials. In Pt/C, degradation followed a relatively predictable, isotropic pattern influenced by humidification gradients. In Pt3Co/C, however, the degradation behaviour is more complex, with clear inlet–outlet asymmetries and abrupt spatial transitions in cluster integrity. These nano-structural differences are validated by segmented current density measurements (Fig. 1 c), confirming that regions exhibiting the most severe structural degradation also showed significant electrochemical underperformance. The Pt/C catalyst was found to degrade primarily through homogeneous coarsening and agglomeration, particularly in regions with high water accumulation. In contrast, the Pt₃Co/C alloy exhibited region-specific fragmentation and cobalt leaching, leading to chemically driven, spatially heterogeneous degradation. These contrasting behaviours underscore the importance of material-specific and spatially adaptive design strategies to mitigate localized degradation effects. A detailed understanding of how different electrocatalyst materials respond to humidity gradients is essential for the development of regionally optimized catalyst layers and for improving the long-term durability of PEFCs under real-world operating conditions. Taken together, this study demonstrates the power of SAXS as a non-destructive, spatially resolved diagnostic tool for investigating electrocatalyst degradation. By enabling quantitative mapping of nanoscale structural changes across the MEA, SAXS provides critical insight into the link between local operational conditions and catalyst failure mechanisms. Elettra Highlights 2024–2025 24
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