Activity, Selectivity, and Methanol Tolerance of Se-Modified Ru/C Cathode Catalysts

The reaction characteristics of Se-modified Ru/C catalysts (RuSe y /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 H 2 O and H 2 O 2 formation, while methanol oxidation is in all cases negligible. Specifically, Se modification improves the O 2 reduction activity and reduces the tendency for H 2 O 2 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 RuSe y /C catalysts, while on Pt/C, it leads to a rapid increase of the H 2 O 2 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 RuSe y /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 °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 H 2 O 2 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.