Composition Effects on the Aqueous-Electrolyte Stability of Fe-Based O2-Reduction Catalysts for PEFC Cathodes

Abstract

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.