X-ray tomography reveals the contribution of different mechanisms to the instability of iron-based fuel cell catalysts

H₂-powered proton exchange membrane fuel cells (PEMFCs) are well suited for zero-emission automotive applications, but their broad commercialisation is hindered by their excessive cost. The latter would be significantly reduced if the Pt-based materials currently used to catalyse the reduction of O₂ in PEMFC cathodes could be substituted with inexpensive, iron (Fe-) based catalysts. However, these materials suffer from a rapid deterioration in performance that has been ascribed to the following mechanisms: (i) the dissolution (or demetallation) of the Fe in the active sites that catalyse the reduction of O₂; (ii) the corrosion of the carbon matrix hosting those sites; and (iii) the damage to the active sites and carbonaceous matrix caused by radicals derived from hydrogen peroxide produced as an O₂-reduction by-product. Most importantly, the relative contributions of these mechanisms to the overall instability endured by the catalyst during fuel cell operation are largely unknown.

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Fig. 1: From left to right: schematic representation of the three processes causing the instability of Fe-based catalyst layers (H₂O₂-related effects, electrochemical carbon oxidation and demetallation); their decoupling using a combination of gaseous cathode feeds (air vs. N₂), potentials (0.5 vs. 0.8 V) and hold durations; and the relative contributions to the overall kinetic current losses at 0.8 V (i_kin,0.8V) estimated through these protocols.

To shed light on this issue, four PEMFC-testing protocols were integrated in this work. These procedures involved different potential hold values and durations, along with the use of air vs. an inert gas (N₂) as the PEMFC cathode gas feed. This approach allowed to quantify for the first time the relative influence of the above mechanisms in the overall performance decay undergone by an Fe-based catalyst, as illustrated in Figure 1. Complementarily, 2D X-ray fluorescence (XRF) mapping and 3D XRF computer tomography (XRF-CT) was used at beamline ID16A to characterise as-prepared and post mortem electrodes of the same catalyst (Figure 2a), and determined that ~ 15 % of its initial Fe-content dissolved during one of the PEMFC stability protocols (Figure 2b). This finding differed from energy dispersive X-ray spectroscopy (EDS) investigations, which did not detect statistically significant Fe losses. This was made possible by the superior sensitivity and about 200-fold larger (and thus statistically more meaningful) sample volume probed by XRF-CT compared to EDS.

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Fig. 2: 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.

In summary, this work describes a novel protocol allowing to decouple 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. These findings could help to design non-noble metal catalysts with the sufficiently high durability required for their implementation in commercial PEMFCs.

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)