Electrochemical studies of cobalt hydroxide — an additive for nickel electrodes

The electrochemical behavior of chemically precipitated cobalt hydroxide is studied by cyclic voltammetry and galvanostatic charge/discharge cycling. When cycled in the potential range between the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), the electrode undergoes two pairs of reactions. The pair of current peaks close to the OER is attributed to quasi-reversible oxidation of Co(OH)2 to CoOOH, whereas the pair of current peaks close to the HER is due to quasi-reversible reduction of Co(OH)2 to Co. The peak current values of both reactions do not show dependence on alkali concentration. Possible mechanisms are proposed for the reactions, in which the diffusion of dissolved Co(OH)2 prior to the electron-transfer step is considered to be the rate-determining step. Considering the fact that the Co(OH)2/CoOOH and Co(OH)2/Co reactions are separated by a potential difference of about 1.2 V, a galvanic cell is constructed by using two Co(OH)2 electrodes in 6 M KOH. On charging, the positive electrode attained a stable potential of about 0.4 V versus Hg/HgO, OH− and the negative electrode attained a stable potential of about −0.8 V versus Hg/HgO, OH−, thus resulting in an open circuit cell voltage of about 1.2 V. By discharging the cell, a capacity of 15 mA h g−1 of Co(OH)2 is obtained over about 15 charge/discharge cycles. The charged electrodes are analyzed by powder XRD and IR spectroscopy and the presence of Co and CoOOH in the negative and positive electrodes, respectively, is confirmed. Although the cell is not commercially viable, the concept of a “double hydroxide” is demonstrated akin to the “double sulfate” principle of lead-acid batteries. It is expected that Ni(OH)2, which is isostructural to Co(OH)2, may also show a similar behavior.