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1 0 3 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Decoupling the Contributions of Different Instability Mechanisms to the PEMFC Performance Decay of Non-noble Metal O2 -Reduction Catalysts, S. Ünsal (a), R. Girod (b), C. Appel (a), D. Karpov (c), M. Mermoux (d), F. Maillard (d), V.A. Saveleva (c), V. Tileli (b), T.J. Schmidt (a), J. Herranz (a), J. Am. Chem. Soc. 145, 7845 (2023); https:/doi.org/10.1021/jacs.2c12751 (a) Paul Scherrer Institut (PSI), Villigen (Switzerland) (b) École Polytechnique Fédérale de Lausanne, Lausanne (Switzerland) (c) ESRF (d) Université Grenoble Alpes − CNRS, Grenoble (France)

to quantify, for the first time, the relative impact of each of the mechanisms described above to the overall performance decay undergone by an Fe-based catalyst, as illustrated in Figure 80. The results indicated that H2O2 effects on the catalyst s ORR activity appear to depend only on the ORR charge and not on the test s operative potential. Interestingly, the chemical degradation that might originate from H2O2-derived species does not seem to significantly contribute to the performance decay observed under air operation, at least within the duration of the protocols presented here. It was also found that electrochemical carbon oxidation in the protocols at a higher potential of 0.8 V contributes to the ORR- activity loss more significantly than Fe demetallation at 0.5 V, even if the effect of the latter mechanism was not negligible. Notably, this observation is likely to be highly dependent on the potentials chosen for these tests since demetallation vs. carbon electro-oxidation is expected to be exacerbated as the potential decreases or increases, respectively. The catalyst mass transport properties worsened after both of the protocols performed using air at the cathode, demonstrating ORR activity loss. On the other hand, the mass transport systematically improved after the protocols performed under N2 both at 0.5 and 0.8 V.

Complementarily, 2D X-ray fluorescence (XRF) mapping and 3D XRF computer tomography (XRF-CT) was carried out at beamline ID16A to characterise as-prepared and post mortem electrodes of the same catalyst (Figure 81a). These techniques determined that ~ 15 % of the catalyst s initial Fe-content dissolved during one of the PEMFC stability protocols (Figure 81b). Chiefly, this result contrasted with the observations of energy-dispersive X-ray spectroscopy (EDS) measurements, which did not find statistically significant Fe losses. This was made possible by the greater sensitivity and ~ 200-fold larger (and thus statistically more meaningful) sample volume probed by XRF-CT vs. EDS.

In summary, this work describes a novel protocol that decouples the contributions of different mechanisms to the instability of Fe-based O2-reduction catalysts, while providing precious information on their demetallation through XRF-CT measurements. The results contribute to knowledge that could be used to guide the design of non- noble metal catalysts with the sufficiently high durability required for their implementation in commercials PEMFCs for transport applications.

Fig. 81: a) 3D rendering of the Fe Kα line for the pristine (left) and post mortem (right) Fe-based catalyst layers based on tomographic reconstructions of the XRF-CT measurements taken at beamline ID16A. The colour scale is limited

between 0 and 4 ng·mm 2. b) Fe distributions within the pristine (purple) and post mortem (red) catalyst layers inferred from 3D XRF-CT, with the inset depicting the total loss of Fe atoms throughout the complete volume of 6000 μm3.