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PRINCIPAL PUBLICATION AND AUTHORS

Tracking the Catalyst Layer Depth-Dependent Electrochemical Degradation of a Bimodal Pt/C Fuel Cell Catalyst: A Combined Operando Small- and Wide-Angle X-Ray Scattering Study, J. Schröder (a), R.K. Pittkowski (a,b), I. Martens (c), R. Chattot (c), J. Drnec (c), J. Quinson (b), J.J.K. Kirkensgaard (d), M. Arenz (a), ACS Catal. 12, 2077-2085 (2022); https:/doi.org/10.1021/acscatal.1c04365 (a) Department of Chemistry, Biochemistry, and Pharmaceutical Sciences, University of Bern (Switzerland) (b) Department of Chemistry, University of Copenhagen (Denmark) (c) ESRF (d) Niels Bohr Institute and Department of Food Science, University of Copenhagen (Denmark)

REFERENCES

[1] B.G. Pollet et al., Curr. Opin. Electrochem. 16, 90-95 (2019). [2] K.J.J. Mayrhofer et al., Electrochem. Commun. 10, 1144- 1147 (2008).

PEMFC-powered vehicles are a promising alternative to combustion engines [1], in particular in the heavy- duty sector. For such applications, extended lifetimes of the catalysts are required. To study the degradation behaviour of PEMFC catalysts, accelerated stress tests (ASTs) simulating load-cycle driving conditions were applied. A bimodal Pt/C catalyst was prepared by mixing two commercial nanoparticle catalysts with distinguishable particle size populations referred to as smaller and larger size populations. The bimodal catalyst was chosen as a model system to distinguish between local and macroscopic Ostwald ripening, i.e., the particle growth of larger particles at the expense of smaller ones by a redeposition of either metallic or ionic species, respectively.

Applying a combination of operando SAXS and WAXS (Figures 125a and c) at beamline ID31 allowed to probe both the particle size and the change in relative particle numbers in the two size populations during the electrochemical degradation tests. SAXS is a strong technique to determine the mean particle size of a catalyst. In the present case, the mean particle size of both size populations in the bimodal catalyst could be determined as function of the AST cycles. However, the absolute or relative number of particles in the two populations was difficult to extract from the SAXS fitting. Simulations showed that the fit was ambiguous with respect to the intensity of the scattering data. Analysing the obtained WAXS diffraction patterns, simultaneously varying both the crystallite size and phase fraction during the Rietveld refinement, was not possible as the intertwining of both variables led to an average coherent domain size. However, using the average particle sizes obtained from the SAXS refinement as input values for the crystallite domain size in the Rietveld refinement resulted in volume fractions of both populations, and a detailed analysis of the degradation process could be obtained.

The acquired results show that the particle sizes of both size populations increase during the applied AST cycles. This is the case for all depths analysed throughout the complete catalyst layers (Figure 125b). The behaviour can be explained by a local Ostwald ripening process via mobile Pt species that are formed on the support (Figure 125e). However, the phase fraction of the smaller population decreases in the depth closest to the electrolyte-catalyst interface, while it increases in the middle catalyst depth (Figure 125d). As the phase fractions are a relative quantification of both size populations, the increase of the fraction of the smaller nanoparticles in the middle catalyst depth can be explained by a detachment of the larger particles from the support [2] (Figure 125e).

For the complex bimodal Pt/C catalyst system, only the combination of operando SAXS and WAXS allowed to elucidate the proposed depth-dependent degradation. Combining operando SAXS and WAXS is presented as a strong technique for a qualitative and quantitative analysis of the degradation behaviour of fuel cell catalysts.