Non-PGM ORR Catalysts Based on Transition Metals Alternative to Iron

Non-platinum group metal (non-PGM) catalysts with high volumetric activity and low hydrogen peroxide yields are potential replacements for platinum-based catalysts for the oxygen reduction reaction (ORR). State-of-the-art non-PGM ORR catalyst synthesized from dual nitrogen precursors (cyanamide and polyaniline) have been presented before by our group [1]. These complex heterogeneous catalyst systems are formed from high-temperature synthesis of nitrogen, carbon, and transition metal precursors. Although Fe-N-C catalysts have demonstrated high ORR activity and low peroxide yields, Fenton (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) reactions generating hydroxyl radicals are a great concern for the durability of polymer electrolyte fuel cells due to membrane degradation. In an effort to mitigate these risks, development of iron-free non-PGM catalysts with similar ORR activity to the Fe-N-C system but enhanced durability, alternative transition metal precursors are being pursued. In this presentation, we will present further development of the dual nitrogen precursor synthesis approach replacing Fe with precursors of alternative transition metals: Co, Mn, and Ni. A systematic approach to compare structural and electrochemical characterization of all Me-N-C systems (Me = Fe, Co, Mn, Ni) will be developed for better understanding and further optimization of alternative-metal non-PGM ORR catalysts. Furthermore, atomic-scale structures of simplified Me-Nx active site(s), obtained from density functional theory (DFT), aiding in the synthesis of non-PGM catalysts with high concentration of active site (those with simultaneously high ORR activity and practical stability), will be presented. Relative formation energies serving as descriptors of active-site structure stability will be calculated via DFT with uniform atomic reference states. Previously, O-binding had been utilized as an activity descriptor for ORR catalysts [2], but for non-PGM catalysts, OH-binding is often the potential determining step in the ORR reaction pathway. Thus, this study will present the ORR activity of simplified Me-Nx structures on zig-zag edge of graphene nanoribbons, providing a comparison of both *OH-coordinated and uncoordinated structures and predicting the ORR activity trends from such alternative metal sites to those of iron. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. [1] Chung, H.T., Holby, E.F., Purdy, G.M., Babu, S.K., Litster, S., Cullen, D.A. More, K.L., Zelenay, P. (2015). “Combining Nitrogen Precursors in Synthesis of Non-Precious Metal ORR Catalysts with Improved Fuel Cell Performance.” ECS Meeting Abstracts, MA2015-02 (37), 1278. [2] Norskov, J.K., Rossmeisl, J., Logadottir, A., Lindqvist, L., Kitchin, J.R., Bligaard, T, Jónsson, H. (2004). “Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode.” The Journal of Physical Chemistry B, 108 (46), 17886-17892.