Comparative investigation of electrochemically synthesized (α+β) layered nickel hydroxide with mixture of α-Ni(OH)2 and β-Ni(OH)2

Abstract

Nickel hydroxides, owing to their high electrochemical activity are promising materials for chemical power sources (alkaline accumulators, lithium accumulators and hybrid supercapacitors), electrochromic devices, electrochemical oxidation of organic compounds. The most promising is nickel hydroxide that contains both α-Ni(OH)2 and β-Ni(OH)2. The aim of the research was to determine the properties of model mechanical mixtures with different ratios between α-Ni(OH)2 and β-Ni(OH)2 and to prove the hypothesis about the layered structure of nickel hydroxide samples synthesized in the slit diaphragm electrolyzer by means of comparative study. The XRD patterns of samples composed of α-Ni(OH)2 and β-Ni(OH)2 mixture show peaks corresponding to separate phases. By means of cyclic voltamperometry, it was discovered, that mixture samples do not show an oxidation peak during the first cycle, while two reduction peaks that correspond to reduction of α and β components are observed on the cathodic curve. A synergetic effect, which results in significant improvement in electrochemical activity of α-Ni(OH)2 and β-Ni(OH)2 mixture was discovered - specific reduction peak current values are 2.3 A/g (20 % α-phase), 3.5 A/g (30 % α-phase), 6.9 A/g (50 % α-phase), which exceed those of initial β-Ni(OH)2 (1.5 A/g) and α-Ni(OH)2 (2.0 A/g). By means of XRD analysis, it was discovered, that Ni(OH)2 samples synthesized in the SDE are composed of α and β and a series of intermediate structures. These samples demonstrate high electrochemical activity: presence of a defined oxidation peak during the first cycle, single reduction peaks with high specific currents of 6.5-11.7 A/g. The comparative analysis has revealed that electrochemically synthesized samples are composed of α-Ni(OH)2 and β-Ni(OH)2, but are not a mixture of these phases, owing to significantly different electrochemical behavior from mixture samples. Using mechanical modeling, the layered (α+β) structure of samples electrochemically synthesized in the slit diaphragm electrolyzer has been proved by contradiction.