Electrochemical quartz crystal microbalance determination of nickel formal partial charge number during nickel-underpotential deposition on platinum in sulphate media

We studied nickel-underpotential deposition (UPD) on polycrystalline platinum in sodium sulphate media using Electrochemical Quartz Crystal Microbalance (EQCM). EQCM enables to separate hydrogen and nickel adsorption/desorption contributions, which was impossible solely with electrochemistry. Thus, nickel coverage can be determined: nearly 0.8 NiUPD monolayer over platinum is deposited above nickel Nernst potential for pH 5.5. Coupling the gravimetric signal with nickel stripping peak coulometry yields the nickel formal partial charge number: ιNi,EQCM = 1.3 ± 0.13. Electrochemical measurements using Swathirajan and Bruckenstein’s method give: ιNi = 1.5 ± 0.17. Within the errors bars, both methods give similar results and ιNi lower than zNi2+zNi2+: nickel adatoms apparently remain partially charged upon UPD on platinum following anion and nickel co-adsorption.