(Invited) Understanding the Complex Roles of Water Molecules in the HER/HOR Kinetics of Pt in Aqueous Solutions
Authors: Jia Qingying , Liu Ershuai , Mukerjee Sanjeev 
Published: May 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 corresponding 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 the interface plays an essential role affecting the HER/HOR kinetics of many catalysts including Pt.1-3 Within the interface the water molecules play important roles in transporting reaction intermediates throughout the double-layer region,2 which may hold the key for the pH-dependent HER/HOR kinetics of many transition metals. However, a clear-cut understanding of the roles of water during the HER/HOR in aqueous solutions is yet missing. Herein, we propose that the transport of hydroxyl (HER intermediates) and Had (HOR intermediates) from the surface to the bulk electrolyte through the double-layer region is the rate limiting step of the HER and HOR kinetics of Pt in alkaline solutions, respectively. The non-specifically adsorbed hydroxyl and Had is transported by the H2O molecules with the orientation of O facing toward the electrode surface (H2O↓). The specifically adsorbed hydroxyl (OHad) is transported by the water molecules coordinated by alkali cations [AM(H2O)x]+
as per the hard-soft acid-base (HSAB) theory. The pH-dependence of the HER/HOR kinetics of Pt in aqueous solutions,4 the surface structure dependence of the HER/HOR of Pt in alkaline solutions,5 the cation effects of the HER/HOR kinetics of a variety of Pt surfaces in alkaline solutions,1 and the improvements of the HER/HOR kinetics of Pt in alkaline solution by a second transition metal (Co, Ni, Ru, etc.) on surface6-8 can be unified into a coherent picture wherein all these factors affect the HER/HOR kinetics of Pt by affecting the water-assisted transport of HER/HOR intermediates.

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, &-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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