Catalysts Based on Earth-Abundant Elements for Catalyzing O2 Reduction in Acidic Electrolytes

Catalysis of the oxygen reduction reaction (ORR) is a key challenge for high power density fuel cells and metal-air batteries. Today, the polymer electrolyte membrane fuel cell (PEMFC) is the most advanced fuel cell type for transportation applications, where high power density is required. The advanced technology readiness of the PEMFC leans on the existence of stable and extremely thin polymer membranes and of highly active platinum-based catalysts for oxygen reduction and hydrogen oxidation. The acidic nature of PEM is however disadvantageous to the electrocatalysis of the sluggish ORR. While advances in platinum-based catalysts are still ongoing,¹ economic and resource-based constraints are incentives to start designing today the next generation of ORR catalysts. The latter should ideally be free of rare elements and, in particular, free of Platinum-Group Metals (PGMs) since those metals are all mined from the same ore. Important advances in non-PGM catalysts have been witnessed since 2009,^2-4 which is promising toward the long-haul goal of more affordable PEMFC stacks and decreased reliance on limited natural resources.

The three aspects of i) synthesis, ii) electrochemical characterisation and iii) spectroscopic characterisation of Fe- and Co-N-C catalysts synthesized using Metal-Organic Frameworks (MOFs) will be discussed. The design of novel sacrificial MOFs and approaches for preparing the catalyst precursors that are subsequently pyrolyzed will be presented. The versatility of MOF materials with respect to the ligand chemistry and topological structure is immense, resulting in thousands of different possible materials. Such materials just started being investigated as sacrificial precursors for the synthesis of Fe(Co)-N-C materials.^5-6-7 Results obtained from the electrochemical investigation of the key characteristics of those ORR catalysts, namely initial ORR activity, power performance and durability, will be discussed. In particular, the degradation mechanisms related to transient high-voltage and H₂O₂ generation during steady-state operation are important.⁸ Both these degradation types correspond to various oxidative attacks of the carbon-based catalysts. The degradation induced by minute amounts of H₂O₂ can lead to a decreased ORR activity and/or decreased mass-transport properties of the cathode, depending on the nature of the transition metal element. Ex situ chemical treatment of catalysts with various amounts of H₂O₂ is a useful approach for simulating the steady-state operando degradation in fuel cell. In this respect, cobalt will be shown to be advantageous compared to iron for long term durability of metal-N-C catalysts. Third, a better identification of the nature and structure of the metal-centred active sites with spectroscopic methods (Mössbauer spectroscopy and X-ray absorption spectroscopy) will be presented. Comparison of computed spectra to experimentally measured spectra is important for identifying the active site structure and for supporting the DFT calculation approaches on model sites with some experimental evidence for the existence of metal-N_x-C_y moieties embedded in or at the edge of graphene sheets.

References

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[3] G. Wu et al., Science 332 (6028), (2011), 443-447.

[4] F. Jaouen et al., Energy & Env. Sci. 4, (2011) 114-130.

[5] Proietti et al., Nature Commun. 2 (2011), 416.

[6] D. Zhao et al., Adv. Materials 26 (7), (2014) 1093-1097.

[7] J.Y. Cheon et al, Scientific reports, 3 (2013) 2715

[8] V. Goellner et al, Phys. Chem. Chem. Phys. 16 (2014) 18454-18462.