Bifunctional Catalysts for Alkaline Oxygen Reduction Reaction via Promotion of Ligand and Ensemble Effects at Ag/MnOx Nanodomains

Achieving synergy between inexpensive metals and metal oxides is a key challenge for the development of highly active, economical catalysts. We report the synthesis and characterization of a highly active oxygen reduction reaction(ORR) catalyst composed of Ag particles (3 nm) in intimate contact with thin (similar to 1 nm) MnOx domains on Vulcan carbon (VC) as shown via electron microscopy. A new electroless co-deposition scheme, whereby MnO4- ions are reduced by carbon, formed nanosized MnOx reduction centers for Ag nanoparticle deposition. A bifunctional mechanism for ORR is proposed, in which the HO2- intermediate is formed electrochemically and is regenerated via disproportionation into OH- and O-2. A 3x mass activity enhancement is observed for Ag-MnOx/VC (125 mA/mg(Ag+MnOx)) over the linear combination of pure component activities using rotating disk voltammetry. The Ag-MnOx/VC mass activity is comparable to commercial Pd/VC (111 inA/rng(Pd)) and Pt/VC (136 mA/mg(Pt)). Furthermore, the number of electrons transferred for ORR reaches 3.5 for Ag-MnOx, higher than for MnOx (2.8) and close to the full four-electron ORR. The synergy can be rationalized by ensemble effects, where Ag and MnOx domains facilitate the formation and disproportionation of HO2-, respectively, and ligand effects from the unique electronic interaction at the Ag-MnOx interface.