X-ray absorption spectroscopy studies of nickel oxide thin film electrodes for supercapacitors

Nickel oxide films were synthesized by electrochemical precipitation of Ni(OH)2 followed by heat-treatment in air at various temperatures (200-600 degreesC). Their structure and electrochemical properties were studied by cyclic voltarnmetry, X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). XRD results showed that the nickel oxide obtained at 250 degreesC or above has a crystalline NiO structure. The specific capacitance of the oxide depends on the heat-treatment temperature, showing a maximum value at 300 degreesC. XAS results revealed that the non-stoichiometric nickel oxide (Ni1-xO) approached the stoichiometric NiO structure with increasing heat-treatment temperature due to the defect healing effect. The defective nature of the nickel oxide could be utilized to improve its specific capacitance for supercapacitor application. (C) 2002 Elsevier Science Ltd. All rights reserved.