Performance and Durability of Mn-Based Platinum Group-Metal Free Catalyst in Membrane Electrode Assemblies

Abstract

Proton exchange membrane fuel cells (PEMFCs) have drawn increasing attention in automotive and residential applications. To significantly reduce the cost of the PEMFC stacks, current platinum-group metal (PGM) catalysts must be replaced by PGM-free catalysts. Although Fe-based PGM-free catalysts (Fe-N-C) have exhibited promisingly high activity for the oxygen reduction reaction (ORR)1, they suffer from insufficient stability. Additionally, a major issue associated with Fe-containing catalysts is that dissolved iron ions can facilitate Fenton’s reaction thus forming a strong radical known to degrade the ionomer and membrane2.Mn-based catalysts have recently been developed, aiming to improve the catalyst durability and mitigate the Fenton’s reaction3. However, the catalyst stability and integration into membrane electrode assemblies (MEAs) have yet to be studied. In this work, density function theory (DFT) has been applied to study the active sites and their degradation mechanism. Computational fluid dynamic (CFD) modeling was also applied to study water, oxygen and proton transport in the electrode. Electrode fabrication optimizations were conducted based on the guidance from the CFD results. The optimizations include ionomer content, electrode porosity and MEA fabrication approaches It has been found that Mn-based catalysts showed improved MEA durability compared to Fe-based catalysts due to the higher carbon graphitization degree, reduced de-metallization and mitigation of peroxide radical formation. However, there is still a significant performance degradation compared to PGM-based MEAs. X-ray absorption has been applied to examine the catalyst and corresponding MEAs before and after durability. The degradation is primarily attributed to the oxidation and agglomeration of active sites. Acknowledgement: The project is financially supported by the Department of Energy’s HFTO under the Grant DE-EE0008075. Reference:1. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science, 332, 443, 2011.2. F Coms, H Xu, T. McCallum, C. Mittelsteadt, ECS Trans. 64, 389, 2014.3. Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nature Catalysis 2018, 1 (12), 935-945.