Bifunctional Catalysts for Alkaline Oxygen Reduction Reaction via Promotion of Ligand and Ensemble Effects at Ag/MnO_<i>x</i> Nanodomains

Abstract

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 (∼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 O2. A 3× mass activity enhancement is observed for Ag-MnOx/VC (125 mA/mgAg+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 mA/mgPd) and Pt/VC (136 mA/mgPt). 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.