G. Sakai, M. Miyazaki, T. Kijima
Aug 29, 2008
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Abstract
Nickel hydroxide (Ni(OH)2) is widely used as a cathode material in Ni-based alkaline rechargeable batteries [1,2]. Recently, nanostructural control for nickel hydroxide has stimulated interest on basic scientific research as much as practical applications. Because materials having a high surface area and low particle size often reveal better charge capacity and/or related superior electrochemical activity [3,4]. There have been many studies of synthesizing nickel hydroxide nanostructures, however, there have been only a few reports of high specific surface area of nickel hydroxide [5]. Several methods has been developed for preparing nickel hydroxide nanostructures, however, some techniques need tedious procedures or time consuming processes. These situations motivated us to prepare high-surface-area nickel hydroxide nanostructures. In this study, we report simple route to synthesize nickel hydroxide having extremely high specific surface area beyond 200 mg. Furthermore, it was found that hydrothermal treatment for the obtained high surface area nickel hydroxide generates nanosheet aggregate with higher specific surface area around 190 mg at lower hydrothermal treatment temperature of 100-120 C. High-surface-area nickel hydroxide was prepared by a simple hydrolysis technique as follows. 100 ml of 0.1 M NiCl2 aqueous solution was added dropwise into 1000 ml of 1% Tetramethylammonium hydroxide aqueous solution and a pale green suspension was obtained. The obtained suspension was aged 1 h and resulting precipitate was filtrated and washed with deionized water successively. For the hydrothermal treatment, 0.2 g of the as-prepared high-surface-area nickel hydroxide was suspended in 20 ml of deionized water, and the suspensions was transferred into a Teflonlined autoclave (50 ml) and heated at a designated temperature (100 or 120 C) for a designated period (typically 12 h). The resulting hydrothermally treated hydroxides were collected by the filtration and washed with deionized water several times. Nickel hydroxide electrodes were prepared by pressing nickel mesh sandwiched by two nickel foam substrates incorporated with a designated amount of nickel hydroxide in it for evaluation of electrochemical behavior. It was confirmed that all the diffraction peaks of the obtained as-precipitated sample are indexed as a hexagonal β-Ni(OH)2 structure according to the PDF No. 14-0117. The physicochemical sorption properties and pore parameters of the as-precipitated sample were examined by nitrogen adsorption-desorption measurements. It was found that the samples prepared by the present method depict surprisingly high specific surface areas beyond 200 m/g. To the best of our knowledge, there is no report concerning such a high specific surface area [5]. Figure 1a shows the N2 adsorption-desorption isotherms of the as-precipitated sample. It is seen that the steep step in desorption isotherms for the sample demonstrates the high pore volumes and sharp pore size distributions. Actually, as shown in Fig. 1b, the pore size distribution curve obtained from the N2 desorption isotherm branch based on BJH method for the sample indicating the presence of mesopores with a narrow distribution of pore diameter centered at 3.3 nm. Figure 2 shows the FE-SEM images of the hydrothermally treated hydroxide at 120 C. The hexagonal flake-like structure consisting from nanosheets around 50 nm in thickness and several hundreds nm in width were clearly observed. It is found that the hydrothermally treated nickel hydroxide nanosheets reveal highly surface area of 190 mg. The electrochemical behavior will be described at the presentation.