Bifunctional Catalysts for Alkaline Oxygen Reduction Reaction via Promotion of Ligand and Ensemble Effects at Ag/MnO x 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 (∼1 nm) MnO x 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 MnO x 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-MnO x /VC (125 mA/mgAg+MnO x ) over the linear combination of pure component activities using rotating disk voltammetry. The Ag-MnO x /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-MnO x , higher than for MnO x (2.8) and close to the full four-electron ORR. The synergy can be rationalized by ensemble effects, where Ag and MnO x domains facilitate the formation and disproportionation of HO2 –, respectively, and ligand effects from the unique electronic interaction at the Ag-MnO x interface.