Energy Storage And Conversion Of Ni(OH)₂ Based Electrode System

Nickel hydroxide/nickel oxyhydroxide electrodes have aroused increasing attention in decades because of their extensive applications in various fields, for example, nickel-based alkaline batteries, electrochemical capacitors, electrocatalysis in organic synthesis, sensors, and electrochromic devices. Ni(OH)2/NiOOH has been identified as one of the most important redox couples due to its distinctive electrochemical properties such as high electrode potential, large electrochemical capacity and excellent electrochemical reversibility. The transition from Ni(OH)2 to NiOOH (oxidation of Ni(OH)2) implies the oxidative energy storage in nickel hydroxide. The searching on environmentally friendly and facile methods for the oxidative energy storage in nickel hydroxide undoubtedly has considerable significance. In this dissertation various techniques for oxidative energy storage in Ni(OH)2 electrodes were investigated.In the third chapter, the Co-substituted Ni(OH)2 samples were prepared by homogeneous precipitation from nickel nitrate solution in the presence of urea, and their structures and electrochemical performance were investigated. The results of XRD and IR indicated that the Ni(OH)2 samples with various Co contents are typical a-phase, and the crystallinity of the samples decreases with the increase in the cobalt content. The results of electrochemical experiments showed that as the cobalt content increases, the discharge capacity and electrochemical reversibility of the Co-substituted Ni(OH)2 samples are obviously improved. The discharge capacity of the Co-containing samples gradually increases with the electrochemical-cycling. The discharge capacity of the sample with 24.4% Co reached 415mAh per gram of pure Ni(OH)2 at the 260th cycle. It was also found that the doping of Co increases the oxygen evolution potential and decreases the oxidation potential of Ni(OH)2, which may be partly responsible for the improvement on the electrochemical performance of the Co-substituted Ni(OH)2 samples. In the fourth chapter, the effects of electrolyte composition and surface structure in Ni(OH)2 layer on storage of the oxidative energy of TiO2 have been clarified. The electrolytes with relatively high OH- content facilitate the oxidative energy storage of an UV-irradiated TiO2 photocatalyst in Ni(OH)2. The effects of cathodic deposition current density on the surface structure in Ni(OH)2 layer have been investigated. A porous nanostructured TiO2-Ni(OH)2 bilayer may be obtained by the cathodic electrodeposition at 1.0 mA cm-2. The porous structure may provide easy access of the surfaces to liquid electrolyte, leading to an increase in effective surface area for electrochemical reactions. The nanostructured particles may provide high-specific surface area, fast redox reactions, and shortened diffusion path in solid phase. The two factors are responsible for the enhanced oxidative energy storage of the obtained porous nanostructured TiO2-Ni(OH)2 bilayer.In the fifth chapter, the oxidative energy storage of an UV-irradiated TiO2 photocatalyst in Ni(OH)2 by a photochemical cell comprising a porous nanostructured TiO2-Ni(OH)2 bilayer photoanode and a Pt foil O2-ruducing cathode was investigated. The oxidative energy storage of the TiO2-Ni(OH)2 bilayer electrode is greatly enhanced when the photoanode is galvanically connected to the platinum cathode, resulting from the effective electron-hole separation. A nanostructured TiO2 film was prepared on ITO by a sol-gel method, and a porous nanostructured Ni(OH)2 layer was further obtained by a cathodic electrodeposition. The as-prepared porous nanostructured TiO2-Ni(OH)2 dilayer shows obviously improved UV-induced oxidative energy storage performance.In the sixth chapter, the oxidative energy storage in the Ni(OH)2 film electrodes, which was evaluated by means of galvanostatic discharge tests, has been successfully achieved by a designed novel system comprising a nickel hydroxide film electrode and a platinum O2-reducing cathode. It was found that the oxidative energy storage in the Ni(OH)2 film electrodes can be obviously enhanced in the coupling system containing the cathode electrolytes with higher oxygen content or lower pH value, resulting from improvement in the oxidative ability of oxygen as indicated by increase in the open-circuit potentials of the platinum electrode. The results of the oxidation-discharge cycle tests showed that the nickel hydroxide film electrode on nickel foil substrate in the galvanic couple system with air cathode using 1.0 M Na2SO4 (pH=2) as cathode electrolyte presents higher cycling stability and better rate discharge capability. It was further demonstrated that the oxidative energy storage in Ni(OH)2 can also be realized by utilizing other green oxidative agents (e.g. H2O2). From the views of both the oxidative energy storage and the oxidation of Ni(OH)2, it is reasonable to conclude that this work is of considerable importance.