In Situ X‐Ray Absorption Spectroscopic Study of Electrodeposited Nickel Oxide Films during Redox Reactions

The quick‐scanning extended X‐ray absorption fine structure technique of X‐ray absorption spectroscopy has been employed to monitor structural changes during the cyclic electrochemical oxidation and reduction of nickel oxide thin film electrodes. The films were prepared by anodic deposition onto a graphite substrate at 1.1 V vs. SCE from a solution of 0.05 M NiSO 4 + 0.5 M CH 3 COONa . The freshly “as‐deposited” film contained mainly Ni 2 + and Ni 4 + with highly disordered structures. The phase containing Ni 2 + can be charged in 1 M of KOH solution to a phase containing Ni 4 + . Electrochemical reduction of a charged nickel oxide film at constant current leads to the formation of α ‐ Ni ( OH ) 2 which transforms to β ‐ Ni ( OH ) 2 with time in the KOH solution. The discharged film can be recharged to form a phase which contains mostly Ni 4 + . The fraction of Ni 4 + in the film increased with charging time and reached a value of 0.8 (i.e., 80% of Ni is present as Ni 4 + ) after 38 min of charging at a constant current of 1.68 mA/cm 2 . The Ni 4 + phase is structurally similar to that of a NiO 2 phase produced by electrochemical oxidation of LiNiO 2 in a nonaqueous electrolyte as has been shown previously.