Tunning the Activity of Silver Alloy Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

Abstract

A bottleneck to achieve efficient and stable electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFCs) is the relatively slow kinetics of the oxygen reduction reaction (ORR) taking place at the cathode. The rather mild alkaline conditions in the AEMFC enable the utilization of various PGM-free catalysts, which brings up new possibilities of finding abundant and inexpensive electrocatalysts without compromising the power density. From theory and previous experiments, it is known that Au and Ag bind oxygen weakly, whereas other transition metals (e.g., Pd, Cu, Fe, Ni) bind strongly to oxygenated species1. Therefore, a suitable geometric arrangement between Ag, an abundant metal with relatively high specific activity, and other metals may give rise to optimum binding of oxygen and thus a high activity. A good strategy to improve the activity of Ag-based catalysts could be a rational design of alloys that can enhance the intrinsic activity of Ag by both ensemble and electronic effects2–4. The former is based on the combination of active sites that facilitate the initial oxygen binding together with Ag active sites that help desorb hydroxide anions, whereas the latter is induced by the alloying metal (which binds oxygenated species strongly) on the electronic structure of the overlaying Ag that tunes its binding to oxygen5. Herein, we report the fabrication and characterization of Ag thin-films alloyed with Pd, Ni, Cu and Mo as a tool to investigate the ORR reaction in terms of ensemble and electronic effects and thus relate alloy composition and geometric arrangement with ORR activity. Ag thin-film model catalysts are fabricated by means of physical vapor deposition methods (PVD) with high control over the amount of material deposited. By varying alloy composition and evaluating electrochemical activity and stability, together with detailed physical characterization, the degree and extent of electronic effects are elucidated. In the same way, sputtering of the same amount of Ag on thin-films of different metals evidences the role of the alloying metal on tuning the activity of Ag through an adjustment of its d-band that changes the binding of oxygenated species, as well as through an ensemble effect that provides active sites for the first elementary steps of the reaction. These results provide insights for more tailored design of electrocatalysts in alkaline media by shedding new light on the mechanisms through which the ORR kinetics are improved in alkaline media. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004).Betancourt, L. E. et al. Enhancing ORR Performance of Bimetallic PdAg Electrocatalysts by Designing Interactions between Pd and Ag. ACS Appl. Energy Mater. 3, 2342–2349 (2020).Slanac, D. A., Hardin, W. G., Johnston, K. P. & Stevenson, K. J. Atomic ensemble and electronic effects in Ag-rich AgPd nanoalloy catalysts for oxygen reduction in alkaline media. J. Am. Chem. Soc. 134, 9812–9819 (2012).Zamora Zeledón, J. A. et al. Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction. Nat. Commun. 12, 1–9 (2021).Stamenkovic, V. et al. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chemie 118, 2963–2967 (2006).