Accelerated Stress Tests on Fuel Cell Cathode Catalysts: Thin Film (Ex-situ) Vs. Catalyst Layer (In-situ)

Abstract

For proton exchange membrane fuel cells (PEMFC), cathode catalyst durability is presently a key limiting factor to commercialization for automotive applications. The oxygen reduction reaction takes place on the cathode – transforming O2 + 4H+ + 4e- to 2H2O. Degradation occurs during normal operational load profiles for idle-to-peak power transients (rapid stop-go drive cycles), as the cathode voltage quickly switches from 0.9-1.2V (idle) to ~0.6V (peak). Cathode catalyst durability is dependent upon multiple operational factors, including; potential limits, temperature, and pH. As a result of the transient potentials, the electrochemical surface area (ECSA) of the carbon supported platinum catalyst (Pt/C) is reduced, due to a combination of Pt dissolution, Pt particle growth (migration and/or redeposition), and carbon corrosion (Pt detachment). Typically, accelerated stress tests (AST) are performed on thin films to evaluate catalyst durability and decouple contributions from ancillary fuel cell materials and structures. The thin films consists of catalyst deposited on a conductive rotating disk electrode and analyzed in liquid electrolyte (HClO4 or H2SO4) vs. a separate counter and reference electrode – ex-situ to the actual PEMFC environment. This removes the additional complexity of the catalyst layers 3-deminsional structure (containing both Pt/C and proton conducting ionomer), diffusion media and membrane – in-situ. The AST used in this study includes 25,000 potential cycles from idle to peak potential limits using either square or triangular waveforms (SW or TW, respectively) at 10 sec intervals, for both Pt/C thin films and catalyst layers. ECSA was measured periodically during the cycles, either by integration of the hydrogen desorption area or by CO stripping analysis. The following operational limits were used with a lower potential limit of 0.6V – several upper potential limits (UPL) from 0.9 to 1.2V, multiple temperature ranges between 25°C and 70°C, and different electrolyte pH ranges 0 to 1. Ex-situ AST results in 0.1 M HClO4 at 70°C show that the loss in ECSA was affected by both the UPL and the potential wave profile. For the 0.9V UPL dominated by Pt dissolution, the SW was more aggressive than the TW, resulting in a 45.6% and 36.8% ECSA loss, respectively. While the higher UPL of 1.2V had nearly identical ECSA losses for both potential wave profiles of 89%, due to the dominance of carbon corrosion. Figure 1A shows the effects of pH on AST induced ECSA losses at room temperature for the UPL of 1.2V using a TW. The pH values correspond to different concentrations of HClO4 (0.1 M HClO4 = pH of 1). As the pH is lowered there is an increase in ECSA loss. In Figure 1B the TW was found to cause more catalyst degradation at lower pH values. This potentially could be related to in-situ relative humidity dependent degradation – to be further studied in-situ on a catalyst layer. Figure 1