One-Step Synthesis of Manganese Doped Single Atom Catalysts for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

Abstract

Current commercial catalysts for polymer electrolyte membrane fuel cells (PEMFCs) require platinum group metal (PGM) catalysts that lead to high costs, thus preventing viable commercialization of PEM fuel cells. Many investigations into PGM-free catalysts have led to the identification of single metal atom sites coordinated with 4 nitrogens as being highly active. One of the best performing metal centers in the acidic environment of the PEMFC is Iron (Fe). Highly active iron-based catalysts have been synthesized easily via a one-step method of doping Iron into a ZIF-8 framework; however, there have been membrane degradation concerns due to the Fenton reaction products that are generated by Fe. Manganese (Mn) has recently been proposed as a potential metal center for the highly active metal-N4 catalyst site as theory predicts that it will have good activity and increased stability1 . The best performing Mn-based catalysts have been synthesized from a Mn doped ZIF-8 structure, but due to the high oxophilicity of Mn and difficulty in incorporating Mn into the ZIF structure, a laborious Mn catalyst synthesis route has been adopted, which requires acid leaching, secondary doping, and multiple pyrolysis steps2. This causes problems in scaling up the synthesis and limits the active site density when compared to the Fe catalyst. We aim to use highly reactive Mn metal precursors in an air-free environment to incorporate Mn into the metal sites of ZIF. High temperature pyrolysis (1100°C) leads to manganese catalyst synthesis in one step. We show that using a highly active manganese borohydride precursor allows for the formation of a metal organic framework (MOF) when combined with 2-methylimidazole, which can subsequently be pyrolyzed to form the target catalyst. Using zinc borohydride as a secondary metal source we can also form bimetallic MOF structures with increased thermal stability and varied Mn loading. With this strategy Mn can be incorporated directly into the metal MOF sites with a wide range of Mn loading. The utilization of an air-free environment prevents metal oxide formation thus giving a facile one step synthesis of a manganese-based catalyst. (1) Liu, K.; Qiao, Z.; Hwang, S.; Liu, Z.; Zhang, H.; Su, D.; Xu, H.; Wu, G.; Wang, G. Mn- and N- Doped Carbon as Promising Catalysts for Oxygen Reduction Reaction_ Theoretical Prediction and Experimental Validation. 2018. (2) Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; et al. Atomically Dispersed Manganese Catalysts for Oxygen Reduction in Proton-Exchange Membrane Fuel Cells. Nat. Catal. 2018, 1 (12), 935–945.