Proton exchange membrane fuel cell (PEMFC) based vehicles are on the verge of commercialization. However, significant hurdles for further market penetration of PEMFC vehicles are cost reductions by decreasing the amount of platinum catalysts while meeting long term durability targets. The durability of catalysts is usually estimated by accelerated stress tests (ASTs) using voltage cycling.^[1] Generally, the electrochemically active surface area (ECSA) and mass activity of the cathode catalyst is analyzed over the course of 30000 voltage cycles, while effects of O₂ mass transport related losses, especially for varying cathode catalyst loadings have not yet been systematically studied.

To better understand the influence of voltage cycling based ASTs on the development of mass transport resistances and on the extent to which they can be captured by currently available in situ diagnostics (ORR kinetics in H₂/O₂, cathode proton conduction resistance by impedance, oxygen transport resistance via limiting current measurements), we conducted a comprehensive voltage loss analysis on MEAs with high- and low-loaded cathodes (0.4 and 0.1 mg_Pt/cm²) over the course of ASTs conducted in H₂/N₂ (anode/cathode) at ambient pressure, 80 °C, and 100% RH, using different voltage cycling profiles: i) triangular wave (TW) and square wave (SW) cycles between 0.6 and 1.0 V_RHE; ii) TW cycles between 0.6 and a lowered upper potential limit of 0.85 V_RHE (TW-LUPL); and, iii) a combination of a triangular sweep between 0.6 and 1.0 V_RHE with potential hold periods at the upper and lower potential (TW-H).

For all ASTs, the O₂ transport resistance scaled inversely to the cathode electrode roughness factor (rf). Since the ECSA decreases in the course of potential cycling and two different loadings were used, a wide range of rf values with their corresponding pressure-independent O₂ transport resistance was analyzed, proving its dependence on the available Pt surface area (Figure 1) as shown also by other researchers.^[2] The pressure dependent transport resistance remained essentially constant. Furthermore, ex situ thickness measurements of catalyst layers showed no cathode thinning, indicating the absence of significant carbon corrosion. While 30000 TW or SW cycles dramatically decreased the H₂/air performance for low- and high-loaded MEAs, only a negligible degradation in performance was observed when a lower upper potential limit of 0.85 V_RHE was applied, providing a guideline to limit MEA degradation under automotive conditions.

Figure 1: Pressure independent O₂ transport resistance vs the electrode’s roughness factor, rf, for high-loaded (0.4 mg_Pt/cm², blue) and low-loaded (0.1 mg_Pt/cm², orange) catalyst layers over the course of different voltage cycling ASTs.