Oxidation states in nickel oxide electrochromism

Nickel oxide, displaying anodic colouration, shows promise as a complementary counter electrode for tungsten oxide in electrochromic devices. Consideration of the thermodynamic possibilities for nickel, as expressed in the Pourbaix diagram, suggests that the commonly-accepted reaction schemes for the colouration of nickel oxide (NiOx): Ni(OH), + OH- <-> NiOOH + e(-) + H2O and Ni(OH)(2) <-> NiOOH + H+ + e(-) may not represent the whole picture. Electrochemical Impedance Spectroscopy (EIS) plots for both sputtered and electrodeposited NiOx films have been obtained. These suggest that two distinct reactions appear to occur at different overvoltages. These seem consistent with conversion of Ni3O4 to Ni2O3; and of the latter to NiO2. Differences, as yet to be elucidated, are apparent between the sputtered and the electrodeposited films. The EIS data agree well with the results obtained by others [15,16]. Potentiostatic polarization of sputtered NiOx in KOH solution lends support to the idea, derived from EIS, of two transitions. By progressively raising the electrode potential it has proved possible to isolate these steps. Both visual and spectrophotometric observations indicate that the colouration appears to occur in two steps. The transmittance data agree well with those of others [18]. The species involved appear to be Ni3O4 as the bleached state with Ni2O3 and NiO2 as dark states. The band structure implications remain to be worked out. It is suggested, however, that the present concept may assist in the consideration of NiOx electrochromism.