Significant advances in the development of electrocatalysts that exceed the DOE target of 440 mA/mg_Pt (H₂/O₂, 0.90V, 80^oC, 100% RH, p_total=150 kPa) for ORR activity have placed proton exchange membrane fuel cells on a promising path towards achieving the DOE target of a 0.1 mg_Pt/cm²_elec cathode loading by 2020. However, unanticipated voltage losses that manifest at high current density and low Pt loading have prevented the attainment of the 0.125 g_PGM/kW_rated 2020 target.¹ While some of the observed voltage losses for low-loaded Pt electrodes (< 0.1 mg_Pt/cm²_elec) has been attributed to oxide dependent kinetics that manifest at the lower iR-free potentials (< 0.75V),² it is believed that a significant portion of this unanticipated loss stems from an oxygen transport resistance local to or associated with electrochemically accessible Pt surface area.^2-4 While the exact cause of this phenomenon remains unknown, studies have demonstrated that this loss both: 1) scales with total Pt surface area⁴ and 2) can be associated with the incorporation of ionomer into the cathode electrode.³ As such, this loss has been termed the local Pt resistance (R_O2^Pt).

In this work we have applied a variety of in-situ electrochemical diagnostics across a range of material sets (e.g. electrocatalysts, carbon supports, ionomers, and membranes) in order to understand their impact on high current density operation in low-Pt loaded electrodes. Values derived for R_O2^Pt will be compared to those determined from ex-situ measurements in an effort to elucidate the fundamental reasons for the observed performance loss.

Additionally, parallel approaches involving novel and state-of-the-art, electrocatalysts, electrodes and MEA designs aimed at mitigating performance loss at high current density and low Pt loading will be presented.

Acknowledgements

This work was funded through the DOE FC-PAD Consortium and by the U.S. Department of Energy under CRADA #CRD-14-539.

References

1. https://energy.gov/eere/fuelcells /doe-technical-targets polymer-electrolyte-membrane-fuel-cell-components

2. T. A. Greszler, D. Caulk, and P. Sinha, Journal of the Electrochemical Society, 159 (12), F831-F840 (2012).

3. H. Iden, S. Takaichi, Y. Furuya, T. Mashio, Y. Ono, and A. Ohma, Journal of Electroanalytical Chemistry, 694 37-44 (2013).

4. A. Kongkanand and M. F. Mathias, Journal of Physical Chemistry Letters, 7 (7), 1127-1137 (2016).