апрель 2019

Synthesis and characterisation of the indigocarmine-intercalated Ni-Al layered double hydroxide as an electrochemically active substances and pigments

A. Gorbonos, Коваленко В. Л. Коваленко В. Л. , Коток В. А. Коток В. А.
Химия и современные технологии
Abstract / Full Text

In the third millennium mobility, in which a person is not tied to energy sources, plays an important role for humanity. Hybrid supercapacitors, especially with nickel hydroxide electrode, are widely used as a primary or backup power source for different devices. There are more and more “ecologically friendly” electro vehicles and vehicles with a hybrid engine with such supercapacitors power sources in our cities. Cost and parameters of such hybrid supercapacitors are determined by characteristics of nickel hydroxide. At the same time, colorized metal oxides and hydroxides are used as pigments for different applications. One of the ways for nickel hydroxide properties increasing is the synthesis of layered double hydroxides on the base Ni(OH)2. In the crystal lattice of LDH to compensate for the excess positive charge, the ions, which, as a rule, are ballast, inactive are intercalated. Intercalation by electrochemical active colorized anion is the perspective way for increasing of electrochemical activity and pigment properties of the nickel LDH.

The main aim of the project is to determine the possibility of the synthesis of Ni-Al LDH, intercalated by indigo carmine, with improved electrochemical and pigment properties. To achieve this aim it is necessary to solve several goals: 1) to carry out the synthesis of the indigo carmine-intercalated Ni-Al LDH under different pH; 2) to investigate the properties of the obtained samples, especially electrochemical and color properties.

Synthesis method. LDH samples obtaining have been carried out by the method of synthesis under constant pH: solutions of nickel and aluminum nitrates (with ratio Ni:Al=4:1), sodium alkali and indigo carmine have been flowed dropwise into the reaction glass with an initial solution, under t=60°С and vigorous stirring. Synthesis has been carried out under 2 different pH: 1) under equilibrium pH (quantity of adding alkali has been calculated according to reaction stoichiometry); 2) under рН=14 (quantity of adding alkali has been calculated according to reaction stoichiometry, quantity of adding alkali has been calculated according to reaction stoichiometry with access for support of pH of the reaction mixture). Carbonate-intercalated Ni-Al LDH as a reference samples have been synthesized by a similar method. After salt solution adding reaction mixture has been kept under temperature and stirring during 20 minutes. Obtained precipitate has been filtered by a vacuum pump, dried at 90-95ºС during 24 hours, ground, rinsed with distilled water, filtered and redried.

Characterization methods. Crystal structure and physical properties have been investigated by PXRD, DTG, and DSC. Color properties have been investigated by obtaining color coordinates by the color comparator. Color properties have been shown in the systems XYZ and CIELab. Spectral tone, color purity, and color saturation also have been calculated. Electrochemical properties have been investigated by the following methods: 1) cycling voltammetry; 2) galvanostatic charge-discharge cycling. In both case hydroxide sample mixed with graphite and PTFE and pasted on the nickel foam working electrode.Electrolyte – 6M KOH solution, reference electrode – saturated silver chloride, counter electrode – Ni mesh. In the cycling voltammetry potential range (0-500 mV) was used. Galvanostatic charge-discharge cycling has been carried out under following current densities: 20, 40, 60, 80, 120 mA/cm2

Results. By XRD it was shown that all samples have the crystal lattice of α-Ni(OH)2. Therefore they are Ni-Al layered double hydroxide. DTG curves have shown that the indigo carmine intercalation increases the thermal stability.

Color properties of synthesized samples have been investigated. It was detected, that indigo carmine-intercalated NI-Al LDH, synthesized under equilibrium pH and pH=14, have the same color characteristics: spectral tone is 486 and 483 nm, color purity is 5 и 3% respectively). But sample, synthesized under equilibrium pH, is darker, that sample, obtained under pH=14: coefficients of diffusion reflection are 9.13 and 15.42% respectively.

By cycling voltammetry method the electrochemical activities of the indigo carmine-intercalated sample have been shown. By galvanostatic charge-discharge cycling, it was detected new scientific data: there are two plateaus on the discharge curves of both types of Ni-Al LDH (indigo carmine–intercalated and carbonate-intercalated). The first plateau under potential 260-210 mV corresponds discharge of Ni(III). Nature of the second plateau under potential −900…−1000 mV now is not clear yet.

CONCLUSIONS. 1) Samples of Ni4Al layered double hydroxide, intercalated by indigo carmine, have been synthesized under constant pH: equilibrium pH and pH=14. As a reference sample carbonate-intercalated Ni4Al LDH have been synthesized under the same condition; 2) by XRD the formation of indigo carmine-intercalated Ni-Al layered double hydroxide has been proved; 3) Color properties of the samples of indigo carmine-intercalated Ni-Al LDH, obtained under different pH, have been investigated. 4) Electrochemical properties of the samples of indigo carmine-intercalated Ni-Al LDH, obtained under different pH, have been detected. Specific capacities have been recalculated into pure nickel hydroxide. It was shown that Ni4Al-indigo carmine-intercalated LDH, synthesized under pH=14, has maximum specific capacities: 193 mA·hour/g and 1908 F/g. 5) Firstly, it was identified for Ni4Al LDH the presence of a second discharge plateau under potential range −600…−1000 mV. Indigo carmine-intercalated LDH has the more pronounced second discharge plateau.