Surface Structure Dependence of Electrochemical Processes at Monocrystalline Nickel Electrodes. Part 1: The Hydrogen Evolution Reaction

Monocrystalline nickel electrodes(Ni(hkl)) preparedusing the controlled atmosphere flame fusion (CAFF) method are usedto conduct a detailed study of the hydrogen evolution reaction (HER)and to elucidate the influence of the surface arrangement of atomson the mechanism and kinetics of the process. First, these electrodesare characterized using cyclic voltammetry (CV). The shape of theCV profiles, which depends on the surface structure, confirms themonocrystalline nature of the Ni(hkl) electrodes.The mechanism and kinetics of the HER at Ni(hkl)electrodes are analyzed by linear sweep voltammetry (LSV) measurementsat a very low potential scan rate to ensure steady-state conditions.The LSV transients are used to prepare Tafel polarization plots andto determine the Tafel slope (b) and exchange currentdensity (j (o)) values. Unlike in the caseof polycrystalline Ni materials, these Tafel plots reveal two distinctlinear regions. The b and j (o) values are found to depend on the surface geometry of theNi(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. Ananalysis of the j (o) and TON values revealsthat the Ni(111) and Ni(110) electrodes show comparable electrocatalyticactivities toward the HER; the electrocatalytic activity of the Ni(100)electrode is ca. twice lower. Cyclic voltammetrymeasurements are performed to examine the state of the Ni(hkl) electrodes after the LSV measurements. They point tothe development of a nickel hydride (NiH x ) in the near-surface region.