Supercapacitors
Focus has been placed on new electrode materials and device fabrication with advanced performance. Supercapacitor electrode materials are classified into 3 categories: 1) High surface area carbons, 2) Transition metal oxides, 3) Conducting polymers [7].
Supercapacitors
Supercapacitors hold a very important place in energy storage systems due to their high power performance, long cycle life and low cost [1]. Metal oxides as pseudocapacitive materials form an important class of supercapacitors with high specific capacitance, high energy and power density [2].In the literature, single-walled carbon nanotube (CNT) thin films are coated on a dimethylsiloxane surface via vacuum filtration using no binder as symmetric electrodes. Specific capacitance (Csp= 22.2 F×g-1), power density (P= 41.5 kW×kg-1), capacitance retention (94%), were obtained after 500 cycles [3]. Pseudocapacitive materials, metal oxides (Ru2O, IrO2 and MnO2) and conducting polymers (polyaniline and polypyrrole etc.) are extensively used in the fabrication of supercapacitive electrodes and devices [4-6].
Introduction
The focus has been on new electrode materials and device fabrication with advanced performance. Supercapacitor electrode materials are classified into 3 categories. 1) High surface area carbons, 2) Transition metal oxides, 3) Conducting polymers [7].
Among these, manganese (IV) oxide (MnO2) is quite popular among supercapacitor electrode materials due to its low cost, natural abundance and good environmental compatibility [8, 9]. Although Yang and his group synthesized manganese (IV) oxide nanocomposites by restricting polyaniline and polypyrrole between layers, they could not achieve this in aqueous systems, but succeeded using surfactants and organic solvents (NMP) [10, 11].
The maximum specific energy in commercial supercapacitors is determined to be 5-6 Wh/kg, whereas in lead-acid batteries it is 35-40 Wh/kg. In lithium-ion batteries, the maximum specific energy is 150-200 Wh/kg. In supercapacitor cells (organic or ionic), specific energy is related to the specific capacitance of the cell and its operating voltage. It is calculated using the formula E= (C×V2)/2= (Q×V)/2. Capacitance and stored charge depend on the electrode material used and the applied voltage. The use of high-capacitance materials is crucial for increasing energy density. Generally, due to higher series resistance in supercapacitors compared to most batteries, power density may not be achieved as high as desired.
Power density is calculated using the formula; P= V2 / 4×Rs. In this formula, Rs is the equivalent series resistance (ESR) of two electrodes. Pseudocapacitors are implemented with conducting polymers. Polyacetylene, polypyrrole, polyaniline, metal oxides (RuO2, Co3O4 etc.) [13, 14] or polymer-oxide composites [15, 16] are used as electrode materials.
Electrochemical capacitors or supercapacitors have attracted considerable interest alongside batteries and fuel cells due to energy storage applications [17]. Supercapacitors serve as a power/energy bridge between commercial dielectric capacitors (high power density) and batteries (high energy density).
The development of electrode materials is very important due to cost, advanced performance, energy efficiency and environmentally friendly power sources for various daily applications. Layered graphene oxide is reduced with water and hydrazine hydrate as colloidal suspension to obtain carbon materials with high surface area.
These materials contain graphene-based carbon layers [18]. Multi-walled carbon nanotubes are chemically activated using KOH to increase specific surface area, electrical conductivity and specific capacitance. As a composite, MnO2/carbon nanotube (CNT) prepared by co-precipitation method can be applied as supercapacitor electrode material for physical and electrochemical properties.
For comparison, MnO2/CNT composite and CNT were characterized and measurements were taken in 1 M Na2SO4 solution. Using CV method at 10 mV×s-1 to 100 mV×s-1, activated specific capacitance of 250 F×g-1 and 184 F×g-1 were obtained respectively, while for MnO2/CNT composite 215 F×g-1 and 138 F×g-1 were obtained respectively. Due to CNT activation, the MnO2/CNT composite electrode gave higher results in terms of capacitance and cycle performance [19]. Polyvinylcarbazole (PVK)/CNT composite synthesis has been reported by 2 methods. The first one is direct mixing of the polymer with CNT, the other is chemical polymerization of monomer containing carbon nanoparticles [20, 21].
Results
Supercapacitors are used in many fields as energy storage devices. Supercapacitor electrode material has 3 basic requirements. 1- High capacitance, 2- Low resistance, 3- Stability.
At this point, specific surface area is not the only determining factor in capacitor performance. Especially with carbon nanotube materials, there are many factors in capacitance. 1- Specific surface area, 2- Pore size, 3- Pore size distribution, 4- Conductivity etc.
Many methods are used to increase capacitance. For example; functionalization, oxidation and doping are used to achieve high capacitance, conductivity and fast ion movement. High capacitances are achieved by preparing materials with CNT containing metal oxide, polymer or both.
Hybrid supercapacitors can provide controlled thickness through chemical bonding of oxide nanoparticles to the CNT or polymer surface. Despite all these studies, solving the stability problem of hybrid supercapacitors remains an open question.
In conclusion; supercapacitor device design and achieving high capacitance, energy and power density depends on initially optimizing many parameters such as material design, the current collector used, membrane and electrolyte type, potential window, applied method, and conducting many experiments. Our group has been conducting intensive research on this subject for many years [22-29]. Additionally, 6.5 months of collaborative work was conducted with Prof. Dr. Richard B. Kaner at the Chemistry and Biochemistry Department at University of California
Los Angeles (UCLA) and collaborations continue to the present.
Assoc. Prof. Murat Ateş - Chemistry Department - Physical Chemistry Division - Faculty of Science and Letters - Namık Kemal University
References
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