(Invited) Methods for Understanding and Mitigating High Current Density Performance Losses in Low Loaded Pt-Based PEMFCs

Abstract

Significant advances in the development of electrocatalysts that exceed the DOE target of 440 mA/mgPt (H2/O2, 0.90V, 80oC, 100% RH, ptotal=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 mgPt/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 gPGM/kWrated 2020 target.1 While some of the observed voltage losses for low-loaded Pt electrodes (< 0.1 mgPt/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 (RO2 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 RO2 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.