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 Fe^II-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.

References

[1] E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz, and J.-P. Dodelet, Nat. Commun. 2, 416 (2011).

[2] T. Chung, D. A. Cullen, D. Higgins, B. T. Sneed, E. F. Holby, K. L. More, P. Zelenay, Science 357, 479 (2017).

[3] Strickland, E. Miner, Q. Jia, U. Tylus, N. Ramaswamy, W. Liang, M.-T. Sougrati, F. Jaouen, S. Mukerjee, Nat. Commun. 6, 7343 (2015).

[4] Banham, S. Ye, K. Pei, J.-I. Ozaki, T. Kishimito, Y. Imashiro, J. Power Sources 285, 334 (2015).

[5] H. Choi, C. Baldizzone, J.-P. Grote, A. K. Schuppert, F. Jaouen, K. J. J. Mayrhofer, Angew. Chem. Int. Ed. 54, 12753 (2015).

[6] Ebner, J. Herranz, B.-J. Kim, S. Henning, M. Demicheli and T. J. Schmidt, submitted.

[7] Zitolo, V. Goellner, V. Armel, M. T. Sougrati, T. Mineva, L. Stievano, E. Fonda, and F. Jaouen, Nat. Mater. 14, 937 (2015).

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 μg_catayst∙cm_geom^-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.