Tuning the Activity of Silver Alloys for the Oxygen Reduction Reaction in Alkaline Media

Ag-based catalysts have recently attracted much attentionas potentialcandidates to substitute costly Pt-based electrocatalysts for theoxygen reduction reaction (ORR) in alkaline media. Although the electrocatalyticactivity of Pt-based alloys is known to exhibit a strong dependenceon their electronic structures, a relationship between electronicstructure and the ORR mechanism in Ag-based alloys still remains tobe elucidated. Herein, by means of physical vapor deposition, we prepareAg binary thin films (CoAg, CuAg, AuAg, and FeAg) with well-controlledcompositions as a tool to investigate the ORR mechanism on Ag surfaces.The bimetallic thin films are evaluated for their ORR performancein alkaline media, and their specific activity at 0.8 V-RHE is shown to correlate with the Ag electronic structure. Even thoughall thin films show different responses to potential cycling, allbimetallic samples exhibit a surface Ag enrichment after ORR. It isshown that the ORR occurs through different mechanisms on these Ag-richsurfaces, which in turn is potential-dependent. Tafel slopes revealfaster ORR kinetics at low overpotentials on all surfaces, whereasonly CuAg surpasses pure Ag at higher overpotentials. Moreover, despitetheir incomplete O-2 reduction, CuAg and AuAg exhibit anoverall superior ORR activity over pure Ag, with a more than 2-foldincrease in specific activity at 0.8 V-RHE attributed toenhancements originating from electronic effects and surface defects,respectively. Since the potential-dependent improved ORR mechanismobserved for Ag bimetallic samples makes a rational design of Ag-basedelectrocatalysts difficult, these results aim to provide insightsfor a more tailored design of electrocatalysts by shedding light onthe mechanisms through which the ORR kinetics are improved on Ag surfacesin alkaline media.