Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH
Tao Cheng, Lu Wang, Boris V. Merinov, William A. Goddard III
Abstract
Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) are both 2 orders slower in alkaline electrolyte than in acidic electrolyte, but no explanation has been provided. The first step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent hydrogen binding to the electrode, a perplexing behavior observed experimentally. In this work, we carried out Quantum Mechanics Molecular Dynamics (QMMD) with explicit considerations of solvent and applied voltage (U) to in situ simulate water/Pt(100) interface in the condition of under-potential adsorption of hydrogen (H_(UPD)). We found that as U is made more negative, the electrode tends to repel water, which in turn increases the hydrogen binding. We predicted a 0.13 eV increase in hydrogen binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the experimental observation of 8 to 12 meV/pH. Thus, we conclude that the changes in water adsorption are the major causes of pH-dependent hydrogen binding on a noble metal. The new insight of critical role of surface water in modifying electrochemical reactions provides a guideline in designing HER/HOR catalyst targeting for the alkaline electrolyte.
Group Members
Cheng, T., Wang, L., Merinov, B. V., & III, W. A. G. (2018). Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH. *J. Am. Chem. Soc.*, *140*(25), 7787-7790. https://doi.org/10.1021/jacs.8b04006