Synthesis and water oxidation electrocatalytic and electrochromic behaviours of mesoporous nickel oxide thin film electrodes

Nickel oxide (NiO) thin film with a high surface area is an important material in electrochemical applications. Spin coating of a preformed clear solution of ethanol, nickel nitrate, and surfactants (C12H25(OCH2CH2)10OH, C12E10 and C16H33N(CH3)3Br, CTAB) over a substrate produces a lyotropic liquid crystalline phase that is calcined into a transparent mesoporous NiO (m-NiO) thin film with a uniform film morphology and high porosity and surface area. m-NiO, coated over FTO glass, can be used as an electrode in an electrochromic device and electrocatalytic water oxidation processes with excellent efficiency (Tafel slope of 62 mV dec−1 and a low overpotential of 0.200 V at 1 mA cm−2 current density). Repetitive cyclic voltammetry and chronopotentiometry measurements convert the NiO pore walls into a NiO/Ni(OH)2 core–shell structure. In the water oxidation process, various Ni2+ surface species on the pore-walls are oxidized to Ni3+ and further to Ni4+ electrocatalytic active species. In an electrochromic device, the oxidation of the Ni2+ species to Ni3+ species reversibly contributes to the electrochromic behaviour, but if the electrochromic device is run at more positive potentials (such as those beyond water oxidation potentials), then electrochromic switching takes place between the Ni(OH)2 and NiOOH surface species.