INFLUENCE OF THE CARBONATE ION ON CHARACTERISTICS OF ELECTROCHEMICALLY SYNTHESIZED LAYERED (α+β) NICKEL HYDROXIDE

Various modifications of nickel hydroxide possess high electrochemical activity [1], which results in their wide application in various electrochemical devices. Ni(OH)2 and nickel-based double and ternary hydroxides are the main component for the active material of nickel oxide electrode in alkaline Ni-Cd, Ni–Fe and Ni–MeH accumulators [2, 3]. Additionally, nickel hydroxide is also used as a cathode material in lithium batteries [4]. Nickel hydroxide is used as active material for Faradic electrode in asymmetric supercapacitors. Ni(OH)2 is used individually [5] (nanosized [6] or ultradispersed [7]), and as a composite with nanocarbon materials (graphene oxide [8], carbon nanotubes [9]). For thin-layer supercapacitors, a film of nickel hydroxide can be formed on a conductive substrate [10]. Because thin film of Ni(OH)2 is practically transparent and NiOOH has dark-brown color, nickel hydroxide is used as electrochromic material [11, 12]. Nickel hydroxide possesses high electrocatalytic activity and is used for oxidation of various organic compounds [13, 14], and in sensors [15, 16]. Each use case requires a specific set of nickel hydroxide parameters. Directed synthesis of products with desired characteristics is the most relevant trend in the modern chemical industry. Determining the influence of synthesis parameters on the properties of resulting hydroxide is a relevant problem.


Introduction
Various modifications of nickel hydroxide possess high electrochemical activity [1], which results in their wide application in various electrochemical devices.
Ni(OH) 2 and nickel-based double and ternary hydroxides are the main component for the active material of nickel oxide electrode in alkaline Ni-Cd, Ni-Fe and Ni-MeH accumulators [2,3]. Additionally, nickel hydroxide is also used as a cathode material in lithium batteries [4].
Because thin film of Ni(OH) 2 is practically transparent and NiOOH has dark-brown color, nickel hydroxide is used as electrochromic material [11,12].
Nickel hydroxide possesses high electrocatalytic activity and is used for oxidation of various organic compounds [13,14], and in sensors [15,16].
Each use case requires a specific set of nickel hydroxide parameters. Directed synthesis of products with desired characteristics is the most relevant trend in the modern chemical industry. Determining the influence of synthesis parameters on the properties of resulting hydroxide is a relevant problem.

Literature review and problem statement
Synthesis method and parameters determine characteristics of nickel hydroxide, namely irregularity of crystal lattice [17], microstructure [18], crystallinity [19,20]. In turn, these parameters affect electrochemical activity.
It should be noted, that nickel hydroxide synthesized in the SDE has a matrix structure that is similar to organic [46] and inorganic [47] composite materials. As described in the paper [39], the matrix is formed by nickel hydroxide with the mother liquor being the filler. When particles pass along the SDE slit, their aging is induced by the field of Joule heat, that has resulted from electric current passing between anode and cathode. Such a combination of growth with partial crystallization results in the formation of a unique (α+β) layered structure. Formation and stabilization of such structure are heavy influenced by anions. The influence of anions on the properties of LDH is studied rather well [30]. Namely, the paper [32] describes the positive influence of nitrate ion on the electrochemical properties of Ni-Al LDH. While in the paper [33] it is noted that carbonate ions possess greater stabilizing properties. All these studies were conducted for LDH, crystal lattice of which requires the presence of intercalated ions in order to compensate for the excess of positive charge. At the same time, the crystal lattice of promising (α+β) Ni(OH) 2 has no excess of positive charge, making the presence of anions unnecessary. It is possible, that this is why the influence of anions other than hydroxyl is almost not studied. However, intercalation of additional anions can occur due to their inclusion into the primal particle. Upon crystallization, the included anions can stabilize metastable α-phase. At the same time, different anions can impose different degrees of stabilization on α-phase. Influence of different anions on the formation and characteristics of (α+β) Ni(OH) 2 has not been studied. At the same time, the presence of data on the influence of anions will allow improving the characteristics of the layered (α+β) Ni(OH) 2 structure.
The feature of (α+β) Ni(OH) 2 synthesis in the SDE lies in that the cathodic chamber is fed with a nickel sulfate solution with the precipitant (hydroxyl-ions) formed due to electrolysis. Thus, only sulfate ions are present in the cathodic chamber. However, the anodic chamber of the SDE is fed with NaOH, which can contain up to 5 % Na 2 CO 3 if not specifically treated. Because the diaphragm is used, the pressure difference between the anodic and cathodic chamber can lead to the transfer of carbonate ions from the anolyte to the catholyte as a result of diffusion and filtration. It can be concluded that both sulfate and carbonate can play a role in the formation of (α+β) layered Ni(OH) 2 structure. The influence of carbonate ion on the formation and characteristics of (α+β) Ni(OH) 2 can be evaluated if its transfer from the anolyte to the catholyte is prevented. In this case, it is most convenient to employ a cation-exchange membrane in Na-form for the separation of SDE chambers.

The aim and objectives of the study
The aim of the work is to study the influence of carbonate cation on the formation and properties of nickel hydroxide synthesized in the slit-diaphragm electrolyzer with the diaphragm and with the cation-exchange membrane.
To achieve the set aim, the following objectives were set: -to prepare nickel hydroxide samples in the slit diaphragm electrolyzer with the diaphragm and with the cation-exchange membrane under different current densities; -to conduct the comparative analysis of structural properties with evaluation of carbonate ion influence; -to conduct the comparative analysis of electrochemical characteristics of the samples in order to evaluate the role of carbonate ion.

Materials and methods for the preparation and analysis
of nickel hydroxide samples

1. Synthesis of nickel hydroxide samples
Analytical grade reagents were used in the study, with the exception of NaOH.
Synthesis method [36,39] is based on electrolysis in the flowthrough slit-diaphragm electrolyzer (SDE). The cathodic chamber was fed with nickel sulfate solution (concentration Ni 2+ 12.7 g/L), anodic chamber -NaOH solution (concentration 50 g/L), at a feed rate of 0.2 L/h, using the peristaltic pump. Cathode -titanium, to prevent precipitation of hydroxide onto the electrode surface, anode -insoluble nickel. The synthesis was conducted at current densities of 8, 10, 12 and 15.7 A/dm 2 [36,39]. For studying the influence of carbonate ion, the anolyte was prepared from partially carbonized alkali (5 year storage period). Carbonate content was determined by titration with 0.1N HCl solution in the presence of phenolphthalein and methyl orange.
Current flow through the cathode results in hydrogen evolution and formation of hydroxyl ions, which reacted with nickel cations forming a nickel hydroxide. Formed hydroxide particles flow across the cathode in the electrolyzer slit and are subjected to Joule heat. The precipitate is removed from the electrolyzer along with the catholyte flow. Hydroxide was immediately vacuum filtered and dried at 90 °С overnight. It was then ground, sifted through 71 µm mesh, washed from soluble salts and dried again.
Two series of nickel hydroxide samples were prepared for comparative study ( Table 1). The first series of samples was prepared with low-permeability diaphragm Dioramic, separating the cathodic and anodic chambers. The second series was prepared with cation-exchange membrane Nafion in Na-form as the separator.

2. Study on characteristics of nickel hydroxide samples
Crystal structure of the samples was studied by means of X-ray diffraction analysis (XRD) using the DRON-3 diffractometer (Russia) (Co-Kα radiation, scan range 10-90° 2θ, scan rate 0.1°/s).
Electrochemical properties of nickel hydroxide were studied by means of: а) cyclic voltammetry in a special YSE-2 cell using the digital potentiostat Ellins Р-8 (Russia). The working electrode was prepared by pasting a mixture of nickel hydroxide (81 % wt.), graphite (16 % wt.) and polytetrafluoroethylene (PTFE) (3 % wt.) [50] onto the nickel foam current collector [49]. Electrolyte -6М КОН. Counter-electrode -nickel mesh, reference electrode -Ag/AgCl (KCl sat.). The electrode was cycled between 200-700 mV (vs Ag/AgCl (KCl sat.)) at a scan rate of 1 mV/s; b) galvanostatic charge-discharge cycling in the accumulator mode. The study was conducted using a custom-made accumulator model. The working electrode was made by pasting a mixture of nickel hydroxide (81 % wt.), graphite (16 % wt.) and PTFE (3 % wt.) [50] on a special current collector (Ni mesh welded onto Ni foil). Electrolyte -6М КОН. Counter-electrode -cadmium (CdO+graphite+oil, pasted onto a special current collector). Cadmium electrode was specifically charged after preparation. The capacity of the counter-electrode was 2 times higher than that of the working electrode, thus the capacity of the accumulator model is determined by the capacity of the nickel-oxide electrode. Reference electrode -Ag/AgCl (KCl sat.). Charge -18 hours with 20 % overcharge (accumulator was charged by 120 % of the theoretical capacity of nickel hydroxide mass on the electrode, accounting for oxygen evolution). The discharge was conducted at the 0.2C rate (discharge current was 0.2 of the theoretic capacity of the working electrode) to 0 V. Charge and discharge were conducted on the custom-built stand, current density and potential were measured using the digital multimeter UT-70 (China). Specific capacity was calculated based the set current, discharge time and mass of hydroxide on the electrode.

1. Influence of carbonate ions on sample structure
Results of XRD analysis have revealed that the samples synthesized with the diaphragm (Fig. 1, a, c, e, g) are (α+β) Ni(OH) 2 with medium crystallinity, which is in agreement with the literature [43]. Higher curren densities result in higher crystallinity. Similar dependency is observed for the samples synthesized with the membrane (i. e. without carbonate ions) ( Fig. 1, b, d, f, h). However, the peaks corresponding to (peak at 2Θ=13°) are lower, indicating the lower content of the α-phase.

1. Influence of carbonate ions on electrochemical characteristics of samples
Results of cyclic voltammetry for the samples synthesized with the membrane (without carbonate) are shown in Fig. 2. Cyclic curves of the samples S0.2-8М and S0.2-10M show a pronounced first charge peak, with one charge and one discharge peak on the subsequent cycles. At the same time, the samples S0.2-12М and S0. 2-15.7M show no pronounced first peak, and two charge peaks on the cycles 2-5. Fluctuation of current density indicates the occurrence of side-reactions. Fig. 3 shows the specific capacities of all synthesized samples.
It can be observed that specific capacity of the samples synthesized with the diaphragm (with carbonate) and with the membrane (no carbonate), depends on synthesis current density and shows maximum at 12 A/dm 2 . At the same time, it is worth noting, that for all synthesis current densities, the samples synthesized in the presence of carbonates (with the diaphragm) have higher capacities than those without carbonate (with membrane).

Discussion of study results on the influence of carbonate ions on characteristics on nickel hydroxide samples
Before synthesis, the concentration of carbonate ions in the anolyte was determined. The concentration of sodium carbonate was 2.8 g/l. Filtration flow was determined using a U-shaped water manometer. The pressure difference between the anodic and cathodic chambers was 10 cm of water (98.7 Pa). With pressure being lower in the cathodic chamber. Consequently, the filtering flow was directed from the anodic chamber to the cathodic chamber. Thus, it was confirmed that synthesis in the SDE with the membrane occurs in the presence of carbonate ions. The main mechanism of hydroxide formation -chemical precipitation upon the reaction of nickel cation with hydroxyls formed at the cathode. The carbonate ions leaked from the anolyte into the catholyte can take part in the formation of nickel hydroxide. Influence on the crystal structure of nickel hydroxide samples. Results of XRD analysis (Fig. 1) have revealed that the samples synthesized in the presence of carbonate ions (when the diaphragm is used) are constituted by the layered (α+β) structure that contains both forms with the corresponding peaks appearing on XRD-patterns. During synthesis in the absence of carbonate (when the membrane is used), crystal structure changes. XRD-patterns of the samples S0.2-8M and S0.2-10M show lower crystallinity in comparison to the samples S0.2-8 and S0.2-10, synthesized in the presence of carbonate. For the samples synthesized at higher current densities (samples S0.2-12M and S0.2-15.7M), rapid decrease in crystallinity was observed, along with the disappearance of peaks corresponding to the α-form. This indicates that in the absence of carbonate ions, the α-form is formed in lesser quantities. It is possible, that at higher current densities (12 and 15.7 A/dm 2 ), a bi-phase system is formed. It is possible, that carbonate ions intercalated into the primal particle become formation centers of metastable α-form and stabilize it. Sulfate anions seem to also play this role, but their structure-forming and stabilizing capability appears to be lower than that of carbonates.
Influence on electrochemical characteristics of nickel hydroxide samples. Cyclic voltammogram of the samples synthesized in the absence of carbonates (with use of membrane) (Fig. 2) showed the following. At a low current density (8 and 10 A/dm 2 ), the electrochemical behavior of the samples doesn't have any characteristic differences. However, for the samples prepared at a current density of 12 and 15.7 A/dm 2 (S0.2-12M and S0.2-15.7M), no first charge peak is observed on cyclic voltammograms (which is characteristic of the β-form). It is worth highlighting, that later cycles showed two charge peaks that correspond to the β-form and α-form. Thus, the absence of carbonate ions leads to the formation of the bi-phase system composed of β-Ni(OH) 2 and (α+β)-Ni(OH) 2 , which is in agreement with XRD results. Consequently, results of cyclic voltammetry for the samples synthesized with the diaphragm are similar to those of the monophase layered (α+β) structure of nickel hydroxide described in the literature [43]. So, cyclic voltammetry reveals the key role of carbonates in the formation of a unique monophase layered (α+β) Ni(OH) 2 structure. Charge-discharge cycling in the accumulator regime revealed that the character of specific capacity dependency on synthesis current density is the same for synthesis with the diaphragm (in the presence of carbonate) and with the membrane (in the absence of carbonate). The dependency shows the extreme character, with the maximum capacity obtained for the samples synthesized at 10 A/dm 2 (S0. It worth to note, that the study allows recommending the use of a diaphragm for industrial-scale synthesis in the SDE. It also recommended using technical-grade alkali, which is known to contain carbonate, as the anolyte to reduce costs. However, carbonate content and the presence of other anions should be controlled. The specific capacity of nickel hydroxide can also be improved by the additional introduction of sodium carbonate into the anolyte, however, this aspect requires additional studies.

Conclusions
1. Synthesis of nickel hydroxide in the slit-diaphragm electrolyzer was conducted with the use of the diaphragm and the cation-exchange membrane. The presence of the filtering anolyte flow (containing carbonate ions) into the cathodic chamber was experimentally confirmed for synthesis with the diaphragm. This confirms that the samples synthesized with the diaphragm are formed in the presence of carbonate ions, while the samples synthesized with the membrane -in the absence of carbonates.
2. By means of XRD analysis and cyclic voltammetry, the key role of carbonate anions in the formation of monophase layered (α+β) form has been revealed. The absence of carbonate results in a lower content of the α-from, and at high current densities (12 and 15.7 A/dm 2 ) a bi-phase system, composed of a mixture of the β-from and (α+β)-structure is formed.
3. Comparative analysis of electrochemical characteristics of Ni(OH) 2 , samples synthesized in the SDE in the presence (with the diaphragm) and absence (with the membrane) of carbonates has been conducted. It was discovered that the formation of the hydroxide in the SDE without carbonates leads to the reduction of specific capacity by 14.7-31.4 %. The highest specific capacity was obtained for the samples synthesized in the SDE at i=10 A/dm 2 with the diaphragm (in the presence of carbonates) and with the membrane (in the absence of carbonates), and are 216.8 and 185 mA·h/g respectively. To increase specific capacity, it is recommended to conduct a synthesis in the SDE with the use of the diaphragm and technical-grade alkali while controlling the content of carbonates and admixture of other anions. It is noted that the introduction of additional quantities of sodium carbonate into the anolyte can be promising.