Contribution of electrocatalyst support to PEM oxidative degradation in an operating PEFC

The contribution of the electrocatalyst support to polymer electrolyte membrane (PEM) oxidative degradation in an operating polymer electrolyte fuel cell was investigated. A corrosion-resistant non-carbon catalyst support based on mixed ruthenium and silicon oxides (RuO₂-SiO₂; RSO) was compared against a benchmark carbon-based support (Vulcan XC 72; C). The rates of in-situ reactive oxygen species (ROS) generation (Pt/C: 9.0 ± 0.20 × 10^-5 s^-1; Pt/RSO: 5.9 ± 0.19 × 10^-5 s^-1) and macroscopic PEM degradation measured ex-situ as the fluoride emission rate (FER; Pt/C: 2.7 ± 0.32 × 10^-5 ppm cm^-2 s^-1; Pt/RSO: 2.5 ± 0.31 × 10^-5 ppm cm^-2 s^-1) were significantly lower for platinum supported on RSO than for platinum supported on carbon. There was an excellent correlation between the in-situ ROS generation rate and the FER, thereby confirming the causal relationship between ROS generation and PEM degradation. The lower rate of ROS generation over RSO and Pt/RSO was attributed to a lower net rate of electrochemical H₂O₂ generation during the oxygen reduction reaction (ORR). Rotating ring-disk electrode experiments confirmed that the net electrochemical H₂O₂ generation rate on Pt/RSO was about twice lower than that on Pt/C. Kinetic parameters estimated for the ORR supported a direct 4e^- pathway on both Pt/RSO (with i_k of 4.5 mAcm^-2, n = 3.97 and a Tafel slope of 64 mVdec^-1) and Pt/ C (with i_k of 4.1 mAcm^-2, n = 3.93 and a Tafel slope of 68 mVdec^-1). In conjunction with its high corrosion-resistance, this finding further illustrates the viability of RSO (and analogs such as ruthenium-titanium oxide) as outstanding PEFC electrocatalyst supports.