Novel Synthesis of Manganese Single Atom Oxygen Reduction Reaction Catalyst for Enhanced Stability in Proton Exchange Membrane Fuel Cells

Abstract

In an effort to effectively implement proton exchange membrane fuel cells (PEMFCs) technology, there has been a drive for the development of platinum group metal free (PGM-free) oxygen reduction reaction (ORR) catalysts. To this end, iron based catalysts such as the single atom FeN4 catalyst have been identified as one of the most active species for the ORR in the PEMFCs . However, while initial activity of the catalyst is high, the stability of membrane and electrode assemblies (MEAs) is still poor upon extended operations. One degradation mechanism for Fe-based MEAs can be attributed to the Fenton reactions that produce highly reactive free radicals capable of attacking the FeN4 sites and membrane of the PEMFCs. Therefore, alternative catalysts that can mitigate Fenton reactions have been investigated to enhance the MEA stability in PEMFCs.Manganese single atom-based catalyst is an attractive candidate to circumvent and mitigate the issue of radical formation and to provide a highly active and stable catalyst for the ORR in PEMFCs1. In this work we investigate the nature of the MnN4 site and develop a non-contact pyrolysis synthetic method to develop the MnN4 based catalyst. In this method CVD approach is used in conjunction with appropriate Mn precursor for deposition on nitrogen doped carbon (NC) to yield ORR active sites. The non-contact pyrolysis is a preferential method as it leads to minimal aggregation and forms active sites on the exterior of the catalyst particle with a high degree of accessibility for the oxygen reduction. The formation of MnN4 active sites was investigated using in-temperature X-ray absorption spectroscopy (XAS). The XAS results exemplified the need for high temperatures for MnN4 sites to form. This can be due to the competition between the competitive formation of Mn oxides and MnN4 sites that occurs during pyrolysis and higher temperatures favor the formation of the MnN4 sites. Additionally, electron paramagnetic resonance spectroscopy is used to further understand the nature of the Mn active sites and determine the effects of site oxidation on the ORR.(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. Applied Catalysis B: Environmental, (243) p. 195-203, 2019.