Activity, selectivity, and methanol tolerance of Se-modified Ru/C cathode catalysts

The reaction characteristics of Se-modified Ru/C catalysts (RuSey/C; y = 0-1) for the direct methanol fuel cell cathode application were determined in model studies, combining quantitative differential electrochemical mass spectrometry, rotating ring-disk electrode, and wall-jet disk electrode measurements. The experiments were performed under fuel cell relevant, but nevertheless, well-defined reaction conditions (continuous mass transport, elevated temperatures, and negligible internal diffusion resistance). Se modification affects both the activity for the O-2 reduction reaction (ORR) and the selectivity for H2O and H2O2 formation, while methanol oxidation is in all cases negligible. Specifically, Se modification improves the O-2 reduction activity and reduces the tendency for H2O2 formation in the technically relevant potential region of 0.6-0.8 V. The presence of methanol has little effect on the ORR characteristics on the RuSey/C catalysts, while on Pt/C, it leads to a rapid increase of the H2O2 yield at potentials of 0.4 V. Temperature-dependent measurements of the ORR activity result in activation energies of 18-24 kJ mol(-1) for the RuSey/catalysts (at 0.7 V), comparable to the value obtained on Pt/C (18 kJ mol(-1)). On Ru/C, the ORR activity increases up to 50 degrees C (E-act = 24 kJ mol(-1)) and then drops again at higher temperatures, due to surface blocking via thermally activated oxide/hydroxide formation. The additional overpotential for O-2 reduction of 0.2 V as compared to the Pt/C catalyst and, in particular, the significant H2O2 yield of at least 1% at typical cathode operation potentials, which will adversely affect the long-term stability of membrane and electrode, require significant improvements before application as cathode catalyst becomes feasible.