(40b) High Oxygen Reduction Activity From Electroless Co-Deposition of Ag and MnOx Nanodomains On Carbon

Currently there is renewed interest in alkaline fuel cells (AFCs) as an alternative to acid-based proton exchange membrane fuel cells (PEMFCs) due to potentially faster oxygen reduction reaction (ORR) kinetics and improved catalyst stability.  For AFCs less costly non-precious group metal (NPGM) catalysts supported on carbon can be highly effective.  Manganese oxides (MnOx) on carbon have been widely explored as cathode catalysts in base, as the ORR reaction intermediate, the HO₂^- anion, disproportionates back into oxygen to yield a pseudo 4 electron reduction similar to that observed for Pd/C and Pt/C.  However, the kinetics of the reaction remain sluggish, but may be improved by intimately combining the MnOx with metal nanoparticles (e.g. Ag) to gain a synergistic effect between the two for both ORR and HO₂^- disproportionation.  The key challenge here is to synthesize monodisperse, nano-sized metal and MnO₂ particles with uniform dispersion over the carbon support for achieving high activity.  

In this work a new electroless co-deposition scheme is developed whereby Ag and MnO₂ are co-reduced on vulcan carbon support for oxygen reduction in alkaline media.  By employing the surface specific, limited reaction between the redox pairs of potassium permanganate/carbon and Ag/MnOx, thin domains of manganese oxide and Ag nanoparticles (<5nm) were deposited uniformly over the carbon surface.  The Ag-MnO₂/C composite displays a mass activity of 0.98 A/mg), close to that of Pd/C (0.11A/mg) and 36% higher that MnOx/C (0.072 A/mg).  Furthermore, the onset potential for oxygen reduction is 15 mV positive that of Pd/VC, suggesting facile kinetics.  Mechanistically, the origin of the activity enhancement for Ag-MnOx/C is shown to result from fast kinetics for the chemical disproportionation of the superoxide anion (HO₂^-), an ORR intermediate formed by the partial 2 electron reduction of oxygen.  The chemical compositions of the catalysts were determined with X-ray photoelectron spectroscopy and energy dispersive spectroscopy.  Catalyst morphology was examined with high resolution electron microscopy to better understand the relationship between the catalyst structure and composition with respect to activity.