“The popcorn effect”: obtaining of the highly active ultrafine nickel hydroxide by microwave treatment of wet precipitate

Abstract

Nickel hydroxide is widely used as an active material of supercapacitors. The most active are samples of Ni(OH) 2 with (α+β) layered structure synthesized in a slit diaphragm electrolyzer. However, the processes that occur during filtering and drying , negatively impact electrochemical activity. The influence of microwave treatment of different times (from 0.5 to 5 min) on the structure, surface morphology and porous structure, and also on the electrochemical properties of nickel hydroxide samples prepared in a slit diaphragm electrolyzer , has been studied. A hypothesis was proposed on the existence of the “popcorn effect”: short-term high-power microwave irradiation of the wet sample would result in water boiling and internal explosion of the sample. Treated and untreated samples were studied by means of X-ray diffraction analysis, scanning electron microscopy and BET nitrogen adsorption-desorption. Electrochemical characteristics were studied by means of galvanostatic charge-discharge cycling in the supercapacitor regime. The existence of the “popcorn effect” has been confirmed by increased sample thickness after microwave treatment by 1.94 times, specific surface area 2.13 times, pore volume by 2.66 times, and average pore diameter by 1.46 times, It was discovered , that increasing treatment duration to 2–5 min leads to microwave drying. XRD results revealed the occurrence of ageing (crystallization) processes of nickel hydroxide during thermal drying and their absence upon realization of the “popcorn effect”. This results in the formation of X-ray amorphous samples. Comparative analysis of electrochemical characteristics of treated and untreated Ni(OH) 2 samples was performed. An increase of specific capacity at high current densities (80 and 120 mA/cm 2 ) for treated samples was observed: by 10.9 % upon microwave drying, 24–42 % upon realization of the “popcorn effect”. The maximum capacity of 231.1 F/g has been observed for the sample, in which the “popcorn effect” was realized the most. However, microwave treatment resulted in lower capacities at low cycling current density. This is related to the thermal treatment of the particle surface, caused by rapid boiling of water. A magnetron of a higher power is required for avoiding this negative effect