In-situ Measurement of Ni Valence in Nickel Hydroxide during Discharge

In this report, the change in Ni valence of a nickel hydroxide during the discharge reaction will be described. Experimental - XAFS (X-ray absorption fine structure) measurement can be conducted during the charge-discharge reaction without taking the electrode out of the cell which has been developed for this study. The electrode consisting of nickel hydroxides, cobalt oxides, methyl cellulose and Teflon, was spread on a graphite sheet which worked as a current collector. A 5M KOH aqueous solution, an Hg/HgO standard electrode and a foamed nickel plate were used as an electrolyte, a reference electrode and a counter electrode, respectively. The electrode was charged at a C/5 rate for 6 hrs and discharged at a C/5 rate to 0 V. XAFS measurements were conducted during the discharge process. Ni valence in the nickel hydroxide could be determined by the shift of the energy for Ni K-edge X-ray absorption. The beam line BL16B2 at synchrotron radiation facility SPring-8 was utilized for XAFS measurement. Results - The discharge curve is shown in Fig.1, and arrows indicate the measurement points where the current is off. The horizontal axis shows the utilization, which is defined as the ratio of the stored capacity to the calculated capacity for the reaction of Ni accompanied by one electron. Fig. 2 shows the energy of X-ray absorption for Ni K-edge (XANES spectra). The energy shifts to be lower with the decrease of the utilization. Ni valences have been determined by using the energies of Ni(OH)2 and LiNiO2 (Co, Al, Mg-doped) as standard. The measurements were also conducted at 91, 47 and 0 % using another cell. Fig. 3 shows the relation between Ni valence and the utilization. The change in Ni valence is almost one, i.e. from about 3.1 to 2.1 when the utilization is changed from 100 to 0%. The change in Co valence determined by the same measurement is also shown in Fig. 3 and is almost constant, nearly 3, independent of the utilization. Therefore, it is concluded that the redox species of the reaction in the electrode may be nickel. The reaction mechanism of a nickel hydroxide will be discussed by taking the results in this and previous 3)