Bifunctional Catalysts for Alkaline Oxygen Reduction Reaction via Promotion of Ligand and Ensemble Effects at Ag/MnO_x 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) 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.

Enhanced Oxygen Reduction Reaction Activity with Electrodeposited Ag on Manganese Oxide–Graphene Supported Electrocatalyst

“…On the other hand, highly dispersed Ag and MnO x nanoparticles with close proximity to each other (approximately <2-3 nm) can provide an ensemble effect acting as a bifunctional catalyst, where these different particles complement each other by catalyzing different oxygen reduction reaction steps (4-and 2-electron reductions), according to previous results for metal-metal oxide composites for ORR[67].In this way, the HO 2 ─ intermediate generated on the Mn 3 O 4 surfaces can easily diffuse to the neighboring Ag surfaces or the Ag-MnO x interface, where it can undergo prompt disproportionation into OH ─ and O 2 for further oxygen reduction with this hybrid catalyst…”

Oxygen Reduction Reaction on Silver Catalysts in Alkaline Media: a Minireview

“…AgNPs have been used to prepare various complex composites as ORR electrocatalysts, mostly with metal oxides that enhance the catalytic activity and carbon nanomaterials that give the necessary electronic conductivity. The enhancement has often been attributed to a synergistic effect, which can be due to the ligand and ensemble effects . The ligand effect has been described as modification of the electronic structure of the components, whereas the ensemble effect arises from the cooperation between the constituents, as the ORR may proceed via 2e − pathway on one component and on another the peroxide formed can be further reduced.…”

“…The enhancement has often been attributed to a synergistic effect, which can be due to the ligand and ensemble effects. [81] The ligand effect has been described as modification of the electronic structure of the components, whereas the ensemble effect arises from the cooperation between the constituents, as the ORR may proceed via 2e À pathway on one component and on another the peroxide formed can be further reduced.…”

Carbon nanotube-supported Cu3N nanocrystals as a highly active catalyst for oxygen reduction reaction

“…The other one is direct transfer of four electrons to form H 2 O. 33 Therefore, the Koutecky-Levich plots were employed to fit the rotation speed-dependent currents to extract the ORR kinetics and electron transfer numbers of catalysts, as shown in Figure 4 (c). The Koutecky-Levich expressions linearly fit very well to the data, suggesting a firstorder reaction kinetic toward the dissolved oxygen concentration.…”