(Digital Presentation) Flattening of the Capacitance–Current Density Curve. the Effect of Surface Metallization of Ni(OH)2 on the Electrode for the Hybrid Supercapacitor
Authors: Kotok Valerii , Kovalenko Vadym , Sukhyy Konstantin , Mikolasek Miroslav , Ondrejka Peter 
Published: November 2022

During the last decades, the development of supercapacitors (SCs) has become one of the main trends in electrochemistry. This can be explained by the requirements of some autonomous devices in high currents for a short time. Another reason is that the coupling of an accumulator and a SC can be used for the accumulator's lifetime prolongation and increase its performance as well.
One of the best-known materials for SCs is nickel hydroxide. It is used in positive electrodes of asymmetric hybrid SCs.
In this work, we used the direct formation of Ni(OH)2 on nickel foam followed by its partial metallization by electroless nickel plating in order to define the effect of metallization on the electrochemical performance of formed electrodes.
The formation of electrodes was carried out by means of the reaction of nickel ammonia complex decomposition [1]. Cleaned nickel foam substrates were submersed in nickel ammonia complex solution. Full complex self-decomposition was made for two days and certain Ni(OH)2 mass was deposited on substrates. The next step for electrodes was rinsing with distilled water and drying in fresh air for one day. Formed in such a way electrodes were used to define electrode properties without metallization. Several formed electrodes were used for the further metallization step. For this electrodes with Ni(OH)2 were immersed in electrolyte for electroless nickel plating. The composition and work conditions were the following: 30 g/L NiSO4·7H2O, 10 g/L Na2H2PO4·H2O, 30 g /L (NH4)2SO4, pH=8.2, t=85 °C. As a result, several electrodes with different deposited nickel to nickel hydroxide were formed. Formed electrodes were rinsed with distilled water, and dried on air for one day. After that, cyclic voltammetry with different sweep rates (5, 10, 20, 50, 100, 500 mV/s) and charge-discharge cycling (1, 2, 3, 5, 10 A/g) were performed.
All reactants were of analytical grade and used without further purification. For the formation of all electrodes, nickel foam (NF) substrates with sizes 1×3.5 cm and the following parameters were used: purity - 99.99%, thickness - 1.6 mm, density - 346 g/m2, porosity ≥95%, 80-110 ppi. In order to estimate morphology, an optical microscope with a digital camera (Hayear 16MP) was used. For electrochemical measurements, Gamry 1010E potentiostat was used. The platinum plate was used as a counter and Ag|AgCl in 3M KCl was employed as a reference electrode. 1M KOH solution was used as an electrolyte in all experiments.
It was found that after metallization, the mass change of electrodes was negative. It was proposed that, despite the presence of nickel ions source in the electrolyte, the formed Ni(OH)2 partially or completely transforms into metallic nickel. The negative mass change was used for the estimation of nickel’s minimal mass after metallization.
The resulting characteristics of electrodes before and after metallization differed dramatically. For example, the electrodes’ color changed from greenish to grayish before and after metallization – fig 1a. The electrochemical characteristics parameters of metalized and non-metalized electrodes were different as well – fig. 1b. The most interesting was the dependence between specific capacity and current density – fig. 1c. As it is seen, the higher metallization quantity leads to capacity increase, especially in a region with higher current densities. The highest capacity in 188 F/g was found for the electrode with metallization Ni0 ≥ 29% at 10 A/g. This effect can be explained by the increasing local conductivity of Ni(OH)2 and its mechanical properties. The last can prevent Ni(OH)2 shedding from the electrode due to parallel oxygen evolution during the charging process.

Conclusions

Deposition of nickel hydroxide coupled with a further metallization process can increase the specific capacity of the electrode.
The positive effect of metallization can be caused by increasing Ni(OH)2 conductivity and its mechanical durability at oxygen evolution during the charging process.

Acknowledgment

This work was carried out within the framework of the National Scholarship Program of the Slovak Republic with the assistance of the Slovak Academic Information Agency (S.A.I.A, n.o.), as well as the VEGA 1/0529/20 grant, and by the Ministry of Education, Science, Research and Sport of Slovakia, the Slovak Research and Development Agency under the APVV-21-0278 and APVV 20-0220 contracts

References

Kovalenko, V., Kotok, V., & Bolotin, A. (2016). Definition of factors influencing on Ni(OH)2 electrochemical characteristics for supercapacitors. Eastern-European Journal of Enterprise Technologies, 5(6), 17-22. doi:10.15587/1729-4061.2016.79406

Figure 1