Surface Structure Dependence of Electrochemical Processes at Monocrystalline Nickel Electrodes. Part 1: The Hydrogen Evolution Reaction
K. Liam Varvaris, Derek Esau, Fabian M. Schuett, Timo Jacob, Gregory Jerkiewicz
Abstract
Monocrystalline nickel electrodes (Ni( hkl )) prepared using the controlled atmosphere flame fusion (CAFF) method are used to conduct a detailed study of the hydrogen evolution reaction (HER) and to elucidate the influence of the surface arrangement of atoms on the mechanism and kinetics of the process. First, these electrodes are characterized using cyclic voltammetry (CV). The shape of the CV profiles, which depends on the surface structure, confirms the monocrystalline nature of the Ni( hkl ) electrodes. The mechanism and kinetics of the HER at Ni( hkl ) electrodes are analyzed by linear sweep voltammetry (LSV) measurements at a very low potential scan rate to ensure steady-state conditions. The LSV transients are used to prepare Tafel polarization plots and to determine the Tafel slope ( b ) and exchange current density ( j o ) values. Unlike in the case of polycrystalline Ni materials, these Tafel plots reveal two distinct linear regions. The b and j o values are found to depend on the surface geometry of the Ni( hkl ) electrodes. The values of j o are converted to the turnover number (TON) of H 2 molecules produced per surface atom per unit of time. An analysis of the j o and TON values reveals that the Ni(111) and Ni(110) electrodes show comparable electrocatalytic activities toward the HER; the electrocatalytic activity of the Ni(100) electrode is ca. twice lower. Cyclic voltammetry measurements are performed to examine the state of the Ni( hkl ) electrodes after the LSV measurements. They point to the development of a nickel hydride (NiH x ) in the near-surface region.