Composition Effects on the Aqueous-Electrolyte Stability of Fe-Based O2-Reduction Catalysts for PEFC Cathodes
Authors: Herranz Juan , Ebner Kathrin , Nachtegaal Maarten , Kim Baejung , Schmidt Thomas J. 
Published: February 2020

The excessive cost of polymer electrolyte fuel cells (PEFCs) could be significantly reduced if the Pt-based catalysts currently used in these devices’ cathodes could be substituted with Pt-group metal (PGM-) free catalysts based on abundant elements like iron. Such materials have been studied since the mid-sixties, and the recent refinement of their syntheses has led to catalysts displaying O2-reduction reaction (ORR) activities commensurate with those of Pt-based catalysts (if at ≈ 4-fold larger loadings).1-3 Despite this progress, the applicability of such PGM-free catalysts remains hindered by their limited stability, which has been attributed to multiple mechanisms for which the relative contribution to the overall ORR-activity decay remains poorly understood.4 Specifically, little is known regarding how the stability is affected by the catalyst’s composition,5 which preponderantly consists of ORR-active Fe-N4 like sites and Fe-agglomerates (e.g., nitrides, carbides) for which the catalytic activity remains under debate.

These relative contributions to the overall instability could be differentiated by synthesizing PGM-free catalysts with a controlled composition. With this motivation, we have developed a novel catalyst synthesis approach in which polyacrylonitrile, Na2CO3 and FeII-phenanthroline are used as the C-/N-, porosity- and iron-precursors, respectively;6 most importantly, by tuning the temperature of the first heat treatment performed on these precursors’ mixture, PGM-free catalysts with the intended, controlled composition could be prepared. Transmission electron microscopy measurements of a catalyst prepared at 650 °C suggest that this catalyst exclusively consists of Fe-N4 like sites, whereas increasing the temperature leads to the emergence of carbon-encapsulated iron-carbide agglomerates that become predominant in the 750 °C sample. These observations were confirmed by the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra displayed in Figure 1A, whereby the Fe-Fe scattering peak at ≈ 2.2 Å (uncorrected for phase shifts) can be attributed to Fe-agglomerates7 that are nevertheless present (in increasing proportions) in the 700 and 750°C samples. These three catalysts were subsequently submitted to stability tests in 0.1 M HClO4 in which their potential was held at either 0.6 or 0.9 V vs. the reversible hydrogen electrode (RHE) while periodically determining their ORR-activity by rotating disc electrode (RDE) voltammetry. As an example of these results, the catalyst prepared at 700 °C undergoes a faster deactivation when held at 0.6 vs. 0.9 V vs. RHE, which could be related to the dissolution of the iron carbide at this lower potential,5 thus indirectly confirming the ORR-imparting capacity of this Fe-agglomerate phase.3

In summary, this contribution will provide greatly-needed insight on the potential-dependent deactivation of PGM-free catalysts with a controlled composition.

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Figure 1. Fourier-transformed EXAFS spectra of a series of PGM-free catalysts with an initial iron content of 0.5 wt. % Fe and prepared at temperatures of 650, 700 or 750 °C in the first heat treatment step (A). Effect of the applied potential on the stability of the catalyst prepared at 700°C, whereby the catalyst-loaded RDE (with ≈ 500 μgcatayst∙cmgeom
-2) was held at the specified potential in N2-saturated 0.1 M HClO4 for the time specified in the X-axis, and this deairated electrolyte was temporarily saturated with O2 to determine the ORR-activity values in the Y-axis (B). Note that the initial ORR-activity of this catalyst at 0.8 V vs. RHE was 4.5 A∙g-1.

Figure 1