State-of-the-art polymer electrolyte membrane (PEM) fuel cells often use PtM (M= Co or Ni) as cathode catalysts and Ce-based additives in the membrane to enhance their performance and durability. During operation, however, the transition metals dissolve and Co²⁺ and Ce³⁺ cations transport from their initial positions, which decreases their efficacy and can cause fouling of ionomer regions, resulting in performance losses and local membrane failure.¹ Despite general knowledge of cation transport through diffusion, migration, and convection, a thorough accounting of the cation positions and concentrations has not been available owing to an inability to monitor these processes under realistic MEA operating conditions and cell geometries. Our approach has been to leverage the unique capabilities of synchrotron microprobe X-ray fluorescence (µXRF), which enable point data collection on the millisecond-scale with spot sizes of <0.25 µm.

Here, we report the development of techniques to directly observe transient, µm-scale migration of cations, in operando. An initial cell design to isolate 1-D through-thickness cation transport was tested using traditional µXRF at the Advanced Photon Source (APS). These experiments resulted in unexpected depletion of cations from the analysis surface of the cell, more preferentially from cathode side. Results agreed qualitatively with 3-D cell modeling, which suggested that steep potential gradients form between the air/cell interface and the interior of the cell, which drives cation migration deep into the cell.²

In order to map ions beyond the front cell edge, follow-on experiments utilized the confocal µXRF capabilities at the Cornell High Energy Synchrotron Source (CHESS). This setup relies on Si collimating optics to define a fluorescent voxel which can be controlled to generate 3-D elemental maps, enabling the measurement of cation transport in the interior of the cell. Steady state depth scans taken at 50 mA/cm² show the depletion of ions from the surface (Figure 1a, inset), more preferentially from the cathode side, in agreement with the APS experiments. As shown in Figure 1, these depth profile scans were used to determine an optimized depth of ~180 µm, where depth profiles converged and were located far enough away to avoid edge effects, but near enough to the surface to get sufficient signal (~90% attenuation measured at a depth of 200 µm). Transient scans at this optimized depth were performed at 80°C and 50% RH with a current density of 100 mA/cm². These measurements, shown in Figure 1b, reveal that Ce³⁺ cations initially situated in the PEM migrate from there into cathode catalyst layer (cCL), forming a gradient in the PEM after 15 minutes. Upon removal of load, Ce³⁺ rapidly diffuses and equilibrates between ionomer regions of the CL and PEM.

These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions.

These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions.

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).

This work is based upon research conducted at 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 and the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208.

References

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