In Situ Quantification of Transition Metal Cation Leaching from a Pt-Alloy Cathode Catalyst in a PEM Fuel Cell

The application of Pt-alloy cathode catalysts for proton exchange membrane fuel cells (PEMFCs) is hampered by the leaching of the alloyed transition metal into the ionomer phase of the membrane electrode assembly (MEA). To date, accelerated stress tests used to assess the degradation of Pt-alloy catalysts lack non-destructive, facile diagnostics to quantify transition metal leaching in an operating PEMFC. Here, we present a method based on electrochemical impedance spectroscopy that exploits the high sensitivity of the high frequency resistance (HFR) at low relative humidity (RH) to quantify transition metal cation contamination. A series of model MEAs with varying fractions of protons intentionally exchanged by Co 2+ cations was fabricated. Based on these, we identified the HFR at 30% RH and zero current (i.e., at a homogenous Co 2+ distribution in the ionomer phase) as a robust measure of Co 2+ contamination. A calibration curve that correlates the HFR at 30% RH to the fraction of protons displaced by Co 2+ in the model MEAs could be established, which then allowed quantitative tracking of the Co leached from a Pt x Co/C cathode catalyst, both at the beginning-of-test (∼6%) and after 100,000 voltage cycles (∼30%). Capabilities and limitations of this method for Pt-alloy catalyst testing are discussed.