Electrochemical Quartz Crystal Microbalance Determination of Nickel Formal Partial Charge Number as a Function of the Electrode Potential upon Nickel Underpotential Deposition on Platinum in Sulfuric Medium

Abstract

Although nickel underpotential deposition (UPD) on platinum surfaces has been demonstrated in sodium sulfate media, there is no consensus regarding the value of nickel formal partial charge number (ι Ni) upon adsorption on platinum. Moreover, in most studies, nickel formal partial charge number is determined as an average, calculated on the whole Ni-UPD potential range (i.e., for NiUPD coverage values varying between approximately 0 and 1). In the present contribution, the electrochemical quartz crystal microbalance technique is employed to go beyond this average ι Ni determination. A simple method is proposed to evaluate the electrode-potential-dependent (i) variations of the surface coverages of the specifically adsorbed species and (ii) corresponding nickel formal partial charge number upon adsorption on platinum. From the values of the surface coverage of the adsorbing species, one can recalculate the current densities during the anodic sweep of the cyclic voltammogram. These recalculated current densities agree with the experimental data, which validates the method. The value of the nickel formal partial charge number (ι Ni) decreases upon electrode potential increase, suggesting enhanced sulfate co-adsorption at high Ni-UPD potential. This further enables to determine the potential of zero total charge (PZTC) of the platinum electrode in the considered sulfate-containing electrolyte solutions: The presence of nickel shifts the PZTC of platinum positively by ∼100 mV compared to supporting electrolyte, in agreement with the presence of specifically adsorbed cations.