The effect of Cd, Co, and Zn as additives on nickel hydroxide opto-electrochemical behavior

Nickel hydroxide films were coprecipitated with the additives Cd(OH)2, Co(OH)2, and Zn(OH)2 by galvanostatic electrodeposition process. The effects of the Cd(OH)2, Co(OH)2, and Zn(OH)2 additions on electrode properties such as charge–discharge, reversibility of the electrode reaction, espectroelectrochemical behavior, superficial morphology by atomic force microscopy (AFM) images, were studied. An anomalous coprecipitation of Zn(OH)2 was observed. The different concentration of zinc nitrate on the solution affects the amount of zinc hydroxide found on the film; this behavior must be attributed to the differences on the solubility products (Kps) of the nickel and zinc hydroxides and also to the remarkable zinc amphoteric character. All additives studied displaced the oxygen evolution reaction (OER) to more positive values, and reduced the difference between the oxidation and reduction peaks (Ni(II)/Ni(III)), leading to an augmentation of the process reversibility. However, a significant loss on the electrode efficiency was observed for composite films with zinc. Lithium hydroxide solution as electrolyte showed an augmentation of the difference between the peak potential for the oxidation process and the oxygen evolution reaction, and also increased the reversibility of the redox nickel process. AFM images showed similar superficial morphologies for nickel hydroxide films and films with cobalt; on the other hand, films containing cadmium and zinc showed an inhomogeneous morphology. Nickel hydroxide film shows a differentiated spectroelectrochemical behavior when compared with composite films.