Determining electrochemically active surface area in PEM fuel cell electrodes with electrochemical impedance spectroscopy and its application to catalyst durability

•The capacitive EIS measurement was performed at different bias potentials.•EIS monitored both the degradation of the platinum catalyst and the carbon support.•A linear relationship between the pseudo capacitance and ECSA was established.•The potential dependent proportionality constants do not vary between catalysts.•These constants are universally applicable across Pt catalysts.

Here we have derived a simple expression to relate faradaic pseudo-capacitance, CF, determined by electrochemical impedance spectroscopy to the electrochemically active surface area (ECSA) of Pt electrocatalysts. To test this expression, two commercially available Pt/C catalysts were subjected to accelerated degradation testing protocol (ADTP) during which catalyst layer health was assessed using cyclic voltammetry (CV) and EIS to monitor the degradation process. CF was determined by acquiring the EIS response at two different DC bias potentials: the first at a bias potential where the faradaic process was present, and the second at a bias potential where only double layer capacitance was present which enables accurate tracking of changes in CF throughout the ADTP. A near-identical decay profile for both ECSA (determined by CV) and CF (determined by EIS) was observed, providing an excellent fit to the derived expression. Using the EIS model, similar potential-dependant proportionality constants were determined for hydrogen adsorption/desorption on each catalyst indicating that they are universally applicable across Pt catalysts. These constants can therefore be used to effectively determine ECSA values without performing CV measurements.

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