Characterization of ALD Fuel Cell Catalysts Using Durability Testing and Electrochemical Impedance Spectroscopy

One of the barriers to commercialization for proton exchange membrane fuel cells is loss of activity of the cathode catalyst over time. This is caused by degradation in the form of particle migration and sintering as the fuel cell is subjected to harsh voltage cycling. Atomic layer deposition (ALD) utilizes a self-limiting reaction to produce uniform coatings on a particle substrate. Using ALD, a TiO2 layer was deposited on a commercial catalyst to investigate its effectiveness in preserving catalytic activity. Preliminary results from durability tests indicate improved stabilization of the catalyst in the form of a lower rate of loss of electrochemical surface area (ECSA). To further characterize the performance of the coated catalyst, the impact of the TiO2 layer on specific resistances—namely mass-transfer and charge-transfer resistances—at the cathode is being investigated using electrochemical impedance spectroscopy (EIS). In EIS, the catalyst is subjected to small sinusoidal perturbations around select voltages. The current response is measured at each voltage to find the impedance behavior, which is presented in a Nyquist plot. Equivalent circuit diagrams are constructed and fitted to extract quantitative information about resistances in the cathode half-reaction. From this information, the TiO2 layer can be adjusted to further improve performance at the cathode.