Mass Transfer Overpotentials in Dispersed Pt/C and De-Alloyed PtNi/C Polymer Electrolyte Fuel Cell Cathodes

While oxygen reduction reaction (ORR) mass activities, measured at high voltages, that exceed the DOE 2020 targets (>0.44 A/mg PGM @ 900 mV) have been demonstrated for carbon-supported Pt alloys and core-shell nanoparticle catalysts [1], the full performance of promising catalysts such as these have yet to be achieved in MEAs when operating at high current densities (> 1 A/cm²in air) [2]. To reach a high power output per gram PGM (8 kW/g, DOE 2017 Target), requires high utilization of the electrocatalytic sites as well as low oxygen transport resistance in the cathode catalyst layers. To determine the electrode properties limiting the performance of the de-alloyed PtNi/C based cathode, a standard Pt/C catalyst was annealed to grow the mean particle size to one that is comparable to that of d-PtNi/C catalysts (approximately 5.5 nm). Catalyst-coated membranes with cathodes comprised of a standard Pt/C, annealed Pt/C, and d-PtNi/C at loadings of 0.1 mg Pt/cm² with various ionomer to carbon ratios were fabricated, tested, and characterized under a variety of test conditions [3]. Modeling of these polarization curves to determine the sources of the observed voltage losses (i.e., purely resistive, kinetic, or mass transport) showed that mass transport losses are higher with d-PtNi/C and annealed Pt/C-based cathodes than standard non-annealed Pt/C, while the kinetic losses are lower for d-PtNi/C and standard non-annealed Pt/C as compared to the annealed Pt/C due to higher area-specific activity and higher enhanced surface area, respectively.

Acknowledgements

This work is part of a collaborative project with Johnson Matthey Fuel Cells, United Technologies Research Center, the University of Texas-Austin, and Indiana University-Purdue University of Indianapolis.  The authors wish to thank the U.S. Department of Energy’s Fuel Cell Technologies Office for support.  Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357.

References

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