Abstract
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•NiO porous thin films were prepared via RDS technique.•NiO electrodes were characterized in a nitrile based electrochemical cell.•NiO electrodes were studied by means of XPS.•The XP spectra excluded the formation of phases other than NiO.•The presence of ClO4− as charge balancing species was evidenced.
Nanostructured nickel oxide (NiO) was deposited in the configuration of thin film (thickness, l=2–6μm) onto fluorine-doped tin oxide (FTO) substrates via plasma-assisted rapid discharge sintering (RDS). Electrochemical cycling of RDS NiO in 3-methoxy-propionitrile (3-MPN) revealed two characteristic peaks of NiO oxidation which were associated to the surface-confined redox processes Ni(II)→Ni(III) and Ni(III)→Ni(IV). Grazing angle X-ray photoelectron spectroscopy (XPS) was conducted ex-situ on NiO electrodes in both pristine and oxidized states. Oxidized NiO samples for XPS experiments were obtained in the potentiostatic mode through the polarization of NiO at its two characteristic potentials of oxidation. The XPS analysis allowed to ascertain the electronic structure of the nanoporous NiO framework, and verify the adsorption of perchlorate and chloride anions onto NiO surface due to the compensation of the charge stored in oxidized NiO.
XPS also revealed that the spectrum within the region characteristic of Ni 2p ionization does not vary considerably with the state of charge of the nickel centres. This finding is in evident contrast to what has been observed for the same system when it undergoes electrochemical oxidation in aqueous electrolyte.