In Situ Monitoring of Co Cation Migration in an Operating MEA via Synchrotron Micro-X-Ray Fluorescence
Authors: Cai Yun , Ziegelbauer Joseph M. , Baker Andrew M. , Gu Wenbin , Kongkanand Anusorn , Mukundan Rangachary , Borup Rod L. 
Published: February 2020

In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells(1-5). While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to MEA performance degradation requires looking beyond well-researched catalyst nanoparticle degradation processes(2). Indeed, the performance of a PEM fuel cell is affected by many internal and external factors, such as fuel cell design and assembly, material degradation, operational conditions, and impurities or contaminants(6).
It is well accepted that state-of-the-art PtCo-alloy cathode electrocatalysts will release Co cations under operation, and the subsequent migration of these cations into the membrane can result in unacceptably large performance losses(1, 7, 8). The effect of Co cations changes with operating conditions giving their mobility in the fuel cell ionomer phase. Despite this general knowledge, a thorough accounting of the Co cation positions and concentrations has been impossible due to an inability to monitor these processes under realistic (humidified) MEA operating conditions. Our approach in this work was to leverage the unique capabilities of beamline 2-ID-D at the Advanced Photon Source to: 1) directly monitor Co2+ diffusion and mobility in an operating H2/air MEA for the first time, and 2) provide real-time data to feed into predictive degradation/loss models. For these proof-of-concept studies, we pre-exchanged standard Nafion® membranes with Co cations and used standard Pt/C for both the anode and cathode catalysts. XRF maps of the Co signal revealed that the Co cation distributions in the MEA membrane were affected by the feed-gases and current densities. These distributions, under certain circumstances, were time-dependent. Future experiments will leverage this technique to probe the effects of different MEA designs and operating conditions on overall current distributions.

Acknowledgements

We are grateful to Barry Lai and Zhonghou Cai at APS 2-ID-D to setup and assistance during the u-XRF experimental runs. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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