Formation of Ni(III)-containing substances under mechanochemical solid state synthesis

K. V. Kovalenko , I. V. Samsonov , Коваленко Вадим Леонидович Коваленко В. Л. , Коток Валерий Анатолиевич Коток В. А.
Химия и современные технологии
Abstract / Full Text

Hybrid supercapacitors with nickel hydroxide electrodes are widely used as modern power sources for starting and working of different types of electric motors, especially in electric vehicles, computer and equipment UPS, etc. Current research project suggests production principle of novel low water solid state mechanochemical synthesis of advanced Ni, Ni-Co and Ni-Al hydroxides with high electrochemical activity. Coffee grinder, as low-energy activator, was used for mechanochemical synthesis of hydroxide samples from hydrated or anhydrous salt precursors. Synthesized samples were analysed by PXRD, TG, DSC, SEM, EDX, voltammogramm and charge-discharge cycling.

It was established that all samples are β-Ni(OH)2 by PXRD, TG, DSC methods. To confirm the formation of nickel hydroxide directly during the synthesis and to determine the gray substance, which was formed, a twice longer anhydrous synthesis (20x30 sec) and double alkali synthesis (with double amount of NaOH) have been carried out. Part of the reaction mixture after twice longer anhydrous synthesis was not rinsed neither ethanol solution nor water. Both samples (with rinsing and without rinsing) have been investigated. It was shown the pattern of the nickel hydroxide with high crystallinity (Fig. 4 A). This is strong evidenceof the nickel hydroxide formation as a result of solid state mechanochemical reaction before rinsing. Also it was found patterns of new substances 4Ni(OH)2·NiOOH, i.e. semi-oxidized hydroxide-oxyhydroxide. Well known, that NiOOH has a dark brown color, that’s why some amount of 4Ni(OH)2·NiOOH can change sample’s color to grey. After rinsing, crystallinity of the sample decreased and structure, similar to that previously reported in literature for βbc-Ni(OH)2 (bc – badly crystalline) was obtained. Worth noting, this structural feature was claimed to yield higher electrochemical activity. The diffraction peaks showed the presence of crystallographic stacking faults or interstratified alpha-beta phase domains. It was hypothesized that during twice longer anhydrous synthesis Ni(OH)2 particles with some thickness layer of 4Ni(OH)2·NiOOH and high activity have been obtained. During rinsing by boric acid ethanol solution 4Ni(OH)2·NiOOH could react with ethanol. As a result, the crystallinity of existing particles of nickel hydroxide was decreased, and new particles with more amorphous structure βbc-Ni(OH)2 were formed. Sample, obtained by double alkali synthesis (fig. 1 C), has high crystallinity and include also sodium nickelate (III) NaNiO2.

The direct formation of Ni(OH)2 during of mechanochemical synthesis and obtaining of 4Ni(OH)2·NiOOH and NaNiO2 was proved with PXRD.

Figure 1 − PXRD patterns of nickel hydroxides prepared by solid state mechanochemical synthesis, Cu Kα radiation: A – twice longer hydrate synthesis, without rinse; B – twice longer hydrate synthesis, with rinse; C – doubly alkali synthesis

Hydrate synthesis sample is sodium-doped Ni(OH)2 with nano-thickness (14 nm) hexagonal particles, anhydrous synthesis sample is sulfate-doped Ni(OH)2 with spherical morphology. Voltammograms and charge-discharge cycling exhibited the highest electrochemical activity of the anhydrous synthesis Ni(OH)2, and Ni-Co and Ni-Al hydroxide samples obtained from sulfate precursors. The specific capacities are 802.7, 1332.8, 1072.2 F/g at 1-2 C and 196.7, 447.4, 404.8 F/g at 15-20 C respectively. From Regone diagram, maximum energy and power densities are 34.9 kW/kg-265.2 W·h/kg and 43.1 kW/kg-400 W·h/kg (for Ni-Al and Ni-Co hydroxides respectively).

Conclusion. Suggested solid state mechanochemical synthesis, especially from sulfate anhydrous precursors, by low-energy activator (like coffee grinder) is perspective method for obtaining of the advanced Ni, Ni-Co and Ni-Al hydroxides for supercapacitor’s applications. Formation of Ni(III)-containing substances has been proved by visual observation (grey color) and PXRD.