Effect of Catalyst Loading on the Degradation of d-PtCo/C Cathode Catalyst
Authors: Mukundan Rangachary , Macauley Natalie , Herrera Sergio A , Langlois David A. , Cullen David A. , More Karren L. , Myers Deborah J. , Ahluwalia Rajesh , Borup Rod L. 
Published: February 2020

The Consortium for Fuel Cell Performance and Durability (FC-PAD) was formed by the U.S. Department of Energy Fuel Cell Technologies office (FCTO) in 2015 to improve the understanding of performance and durability issues in polymer electrolyte membrane fuel cells (PEMFCs) and to help meet the DOE 2020 targets. This talk will summarize some of the durability work performed within the consortium on commercial dealloyed-PtCo/C catalyst-based membrane electrode assemblies (MEAs).
Commercial MEAs using Nafion®HP membranes, ElystPt300670 (30wt.%PtCo/high surface area C) cathode catalyst, and ElystPt200380 (20wt.%Pt/Graphitized carbon) anode catalyst were obtained from Umicore. The MEAs were assembled in a 5cm2differential cell reported by D. R. Baker et al.1and conditioned extensively before performing beginning of life (BOL) characterization. The conditioning protocol included a recovery step where the cell was operated at high RH and low temperatures and voltages to flush out impurities and maximize the performance.2,3The samples were then subjected to the DOE-recommended square wave catalyst accelerated stress test (AST) which is a 3 sec hold at 0.6V and 3 sec hold at 0.95V at 80oC and 100%RH.4Characterizations were performed after 15,000 and 30,000 cycles of the AST with the recovery protocol applied to the MEA before characterization. Electrochemical characterization included mass activity, electrochemical surface area, polarization curves, Electrochemical Impedance Spectroscopy (EIS), and oxygen transport measurements. The BOL and end-of-test catalysts were also characterized by electron microscopy, energy dispersive X-ray spectroscopy, X-ray absorption spectroscopy, and small-angle X-ray scattering to determine particle size and Pt to Co content.
The oxygen reduction reaction mass activity, electrochemical surface area, and performance in the kinetic region of the catalysts degrade with AST cycling. This loss can be attributed to both Co leaching from the catalyst and catalyst particle size increase. The effect of this degradation was most pronounced in the high current region for the MEA with low Pt loading. For example, in Figure 1, the voltage loss at 1A/cm2after 30,000 AST cycles is ~50mV for the 0.15mgPt/cm2MEA and ~200mV for the 0.05mgPt/cm2MEA. This is primarily due to increases in the pressure-independent oxygen transport resistance. The non-Fickian oxygen transport resistance of low-loaded (0.05 mgPt/cm2) MEAs is significantly larger than those of the high-loaded MEAs even at BOL and worsens significantly with ageing of the catalyst. EIS experiments in HelOx (21% O2, balance He) also revealed that there was an increase in the pressure-dependent gas transport during the AST cycling. However, little or no change was observed in the ionomer sheet resistance over the course of the durability testing, independent of the catalyst loading. These results will be compared with recent results reported for pure Pt catalyst-based MEAs subjected to similar tests.5In this talk, the analysis of the cathode catalyst composition and morphology before and after the testing will be presented in detail and correlated to the observed performance losses.

Acknowledgements

This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen).

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Figure 1