Exploring the Shuffling Roles of Interfacial Water for the HER/HOR of Pt-Surfaces in Aqueous Solutions
Authors: Jia Qingying , Liu Ershuai 
Published: December 2020

Previous studies focus mainly on the inherent properties of the catalysts such as the metal-hydrogen binding energy (EM-H) to rationalize their catalytic activities toward the hydrogen evolution and oxidation reactions (HER/HOR) in aqueous solutions, as well as the HER/HOR kinetics. While this strategy proves effective in rationalizing and predicting the HER/HOR activity trend across a wide range of elements in dilute acidic solutions as per the Sabatier Principle, it is inapplicable to account for the sluggish HER/HOR kinetics of many transition metals in alkaline solutions. Recent works suggest that interfacial water plays critical roles in affecting the HER/HOR kinetics of Pt-based surfaces in alkaline solutions.1-4 However, the specific roles of interfacial water for the HER/HOR kinetics remain elusive, partly because of the lack of methods to explore the surface-electrolyte interface.

Herein, we probe the Pt electrode/electrolyte interface by depositing a variety of transition metals (TMs) onto Pt surfaces and monitoring their redox-controlled interactions with the HOR/HER intermediates and interfacial water via in situ x-ray absorption spectroscopy. Carbon monoxide stripping experiments with varied alkaline metal cations (AM+) concentrations are conducted to facilitate correlating the redox-controlled interactions to the TM-induced changes of the HOR/HER kinetics of Pt. We found that the TM-induced changes of the HOR/HER kinetics of Pt in alkaline are governed by the TM0/TM(OH)x redox potential or the metal-oxygen binding energy (ETM-O). The TMs with too strong ETM-O or too low redox potentials such as Mn or Co are dominated by TM(OH)x within the HOR potential region and can only improve the HER kinetics of Pt but not the HOR. The TMs with moderate ETM-O or redox potentials located within the HOR kinetic potential region of Pt (0-0.25 V) such as Ni and Ru can simultaneously improve the HOR/HER of Pt in alkaline, but the HOR improvement vanishes as the TM0 transforms to TM(OH)x with increasing potentials. The TMs with too high redox potentials or weak ETM-O(s) such as Cu improve neither. According to these results, we propose that the TM in either the form of TM0 or TM(OH)x can improve the HER of Pt in alkaline by hosting the OHad generated from water dissociation that is subsequently removed by the interfacial water coordinated to AM+ via the HSAB interaction: TM0/TM(OH)x-OHad-[AM(H2O)x]+ + e- ↔ TM0/TM(OH)x + OH--[AM(H2O)x]+;1 whereas only the form of TM0 can improve the HOR by hosting the water molecule with the oxygen facing-down orientation (H2O↓ad) that removes Had from the Pt atoms nearby via the L-H mechanism: Pt-Had + TM0-H2O↓ad ↔ Pt + TM0 + H3O+ + e-. By this new notion, the surface TM promotes the HOR/HER kinetics of Pt by facilitating interfacial water shuffling reaction intermediates against the electric field, rather than by weakening the electric field by negatively shifting the potential of zero free charge (pzfc)2, or by weakening the (apparent) EPt-H,5 or by facilitating the dissociation and formation of water molecules.6,7 The participation of interfacial water in the HOR/HER of Pt is further verified by kinetic isotope effect studies in both acidic and alkaline solutions.

Acknowledgements.

This work was supported by the Office of Naval Research (ONR) under award number N00014-18-1-2155. The authors declare no competing financial interests. This research used beamline 6-BM, 7-BM, and 8-ID (ISS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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