Understanding the Enhanced Activity of Pt-Alloys Toward Hydrogen Oxidation Reaction in Alkaline Media

Abstract

Recent advances in anion exchange membranes (AEMs) and catalysts with greatly improved ORR kinetics are tempered by the two orders of magnitude higher overvoltages for the hydrogen oxidation/evolution reaction (HOR/HER) rates as compared to acidic pH. It has been demonstrated that Pt in conjunction with additional transition metalsm (Pt-M), either alloyed or as an ad-atom exhibit enhanced HOR activity in high pH media compared to pure Pt. However, the mechanistic origin of the activity enhancement induced by the second metal has been under extensive debate and remains unclear. Some researchers propose that the enhanced HOR activity exhibited by Pt-M is attributable to the bifunctional effects whereby optimal interaction energies of adsorption/dissociation of H2 and adsorption of hydroxyl species (OHad) on metal surfaces are achieved;1 whereas others argue that the Pt-H binding energy is the dominant factor determining the HOR activity of Pt-alloys in alkaline media.2 Herein, a series of nanoscale Pt-alloys including PtRu/C, PtNb/C, and PtCo/C with various composition and particle morphology were investigated by rotating disk electrode testing and surface-sensitive in situ x-ray absorption spectroscopy (XAS) method. In an attempt to fully rationalize all the experimental results, we propose a fundamentally novel rendition of the electrochemical double-layer structure wherein the hydroxide-species formed in the compact part of the double-layer, either on an adjacent non-precious alloying metal (-OHad in the Inner Helmholtz Plane of the double-layer) or on the precious metal (-OHad in the Outer Helmholtz Plane of the double-layer), plays an important role in enhancing HOR kinetics at high pH media. A complex relationship, heretofore unknown, exists between electrocatalysis and the electrochemical double-layer structure which will unraveled here in more detail with both theoretical and experimental results. Acknowledgements: The authors gratefully acknowledge the financial support from Arpa-e, US DOE under a grant lead by Proton On-Site, Walingford, CT. Use of the Stanford Synchrotron Radiation Light-source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Use of Beamline 2-2 at SSRL was partially supported by the National Synchrotron Light Source II, Brookhaven National Laboratory, under U.S. Department of Energy Contract No. DE-SC0012704. Use of the beamline 9-BM in Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. References (1) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der, V.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Nat Chem 2013, 5, 300. (2) Durst, J.; Siebel, A.; Simon, C.; Hasche, F.; Herranz, J.; Gasteiger, H. A. Energy & Environmental Science 2014, 7, 2255. Figure 1