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

Abstract

In an effort to viably implement polymer electrolyte 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 radicals capable of attacking the 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 catalysts have risen recent interest as a candidate to circumvent the issue of radical formation and provide a highly active and stable catalyst for the ORR in PEMFCs1. In this work we have developed a novel pyrolysis synthetic method to form the Mn single atom catalyst. It was found that the pyrolysis temperature and precursor selection are crucial factors for developing a highly active ORR catalyst. The developed pyrolysis leads to minimal aggregation and forms active sites on the exterior of the catalyst particle that become available to perform oxygen reduction. The active site of this catalysts was validated by x-ray absorption spectroscopy (XAS) and the catalyst showed good activity for the ORR in PEMFCs. The catalyst also demonstrated excellent stability upon voltage cycling under nitrogen where oxygen exposure is seen to decrease the activity. The mechanism of degradation due to oxygen exposure is investigated under storage and during operation by examining the structural changes to the Mn species via XAS. Here it was found that under storage in air the catalyst formed Mn(III) oxides and when the MEA was cycled under oxygen Mn(II) oxides formed.Acknowledgements: This work is financially supported by the Department of Energy’s Fuel Cell Technology Office under the award# DE-EE0008075(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.