(485d) Performance and Durability of Advaned Mn-Based Precious-Group Metal-Free Catalyst in Membrane Electrode Assemblies

Platinum-group metal (PGM) catalysts have unique advantages of high stability and catalytic activity for the oxygen reduction reaction (ORR), thus widely being used in proton exchange membrane fuel cells (PEMFCs). However, the scarcity and high cost of PGMs prohibit their extensive application in PEMFCs. Recently, PGM-free catalysts have attracted increasing attention due to their abundance, low cost and improved ORR catalytic activity. Fe based FeN₄ and Co-based CoN₄ catalysts have been widely investigated due to their high activities.¹ Unfortunately, cobalt-based materials also have limited availability and are known to be toxic, and iron-based materials have durability issues. Additionally, in a real membrane electrode assembly (MEA) environment, Fe-based catalysts are notorious for their induced Fenton’s reaction² which generates peroxide radicals, rapidly degrading perfluorosulfonic acid (PFSA) membrane/ionomer in the MEA. To address the instability issue of Fe-based catalyst, a Mn-based catalyst has been developed³.

In this work, novel electrode and ink formulation designs have been extensively studied to improve the MEA performance using Mn-N₄ catalyst coordination, which includes a freeze drying approach to increase electrode porosity and ionomer to catalyst (I/C) ratio tuning to enhance proton transport. We also integrate computational fluid dynamic (CFD) modeling results to better understand the impact of porosity and I/C ratio variation via oxygen concentration, oxygen reaction rate and electrolyte phase potential across the cathode catalyst layer.

The durability of Mn-based catalysts in the MEA level has also been studied in this work. It has been found that the Mn-based MEA is more stable than Fe-based catalysts, possibly due to higher degree of carbon graphitization and mitigated Fenton reaction. Testing protocols, oxygen concentration, relative humidity and temperature were all found to impact MEA durability. The degradation mechanism of Mn-based catalysts has been studied using synchrotron X-ray absorption spectroscopy (XAS). The oxidation and agglomeration of active sites have been found to be two major mechanisms for degradation.

Acknowledgement:

The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0008075.

Reference:

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