A first-principles analysis of the chemisorption of hydroxide on copper under electrochemical conditions: A probe of the electronic interactions that control chemisorption at the electrochemical interface

In this paper we analyze the electronic interactions that control both the binding and the adsorption of hydroxide at the water/copper metal interface under electrochemical conditions using first principle density functional theoretical calculations. The binding energy between the anion and the surface is defined herein as the chemical bond strength between the adsorbate and the metal surface in the absence of water or an applied potential. The adsorption energy for an anion to a metal surface, on the other hand, is typically defined in an electrochemical system as X-(aq) → X_ads + e^- where X here refers to the hydroxyl anion. The calculations carried out herein examine and test classic theories concerning the influence of electrode potential on the changes to the binding and adsorption of OH to Cu(1 1 1) surface. The hydroxide/water/Cu interface is chosen as a probe system and also due to the importance of its corrosion. The adsorption geometry of the hydroxyl intermediate is monitored over a range of applied potentials. When a positive surface charge density is applied, the hydroxyl interacts with a water molecule in the outer water layer to form an surface bound (OH)OH₂ adduct. The formation of the adduct leads to an increase in the capacitance of the system due to the delocalization of positive charge across the first 'inner-layer' of the solution environment. The electronic structure is analyzed in detail to establish how the energy levels of the hydroxyl and oxygen species change relative to the electrode's d-band structure as an electrochemical potential is applied. The energy levels of the surface hydroxide species follow the changes in the potential of the electrode. The results indicate that the changes in the binding energy with potential are less than 0.1 eV/V over a range of potentials from -2 V to +2.5 V.