1. PROSPECTS OF USING GRAPHENE-LIKE NANOSTRUCTURES IN SUPERCAPACITORS
Alexander Shchegolkov, Evgeny Galunin, Alexey Shchegolkov, Nariman Memetov, Alexey Tkachev
Tambov State Technical University (TSTU), 106 Sovetskaya Str., Tambov, , Russia
Results
Introduction
The development of new technologies in industry and the widespread use of renewable energy sources have led to the creation of novel materials such as carbon nanostructures and a desire for employing them in energy storage devices like supercapacitors to improve specific energy characteristics of the latter.
At present, studies on enhancing these supercapacitor characteristics are conducted by increasing the electrical capacitance through the use of electrode materials with a large specific surface area.
Theoretical studies show that the maximum possible specific capacitance of the supercapacitor electrodes (20-90 µF/cm2) may be realized when using graphene or graphene-like materials. High conductivity typical of these nanostructures makes them promising candidates for using in power electronics components. This is achieved by reducing energy losses for electrical resistivity in the supercapacitor structure, which ensures high electric current values (100-1,000 A) .
Considering the aforementioned, the aim of the present research was to estimate prospects of using various graphene-like nanostructures as supercapacitor electrodes under different conditions.
Fig. 4. Сapacitance (Csp)-voltage (E) characteristics obtained for the carbon electrodes based on the G1-G3 and RM samples in 30 % H2SO4 at different potential scan rates (ν) : 1-5 mV/s (a), and 2-50 mV/s (b). The potentials are relative to the normal hydrogen electrode.
Table 1. Specific capacitance (Csp) values obtained for the electrodes based on the materials under study in 30 % H2SO4 at different potential scan rates (ν).
Sample
Scan rate (v), mV/s 5 10 20 50
100
Specific capacitance (Csp), F/g G1
168.3
154.3
139.1
108.4
90.4 G2
88.1
58.9
34.0
12.0
5.6 G3
106.9
90.3
44.9
13.1
5.1 RM
109.0
104.4
98.1
81.2
66.9
Research
The original graphene-like structures were synthesized through alkali activation of carbon materials obtained after carbonization of phenolformaldehyde resins containing additives of various carbohydrates such as carboxymethyl cellulose, sugar and dextrin.
Fig. 5. Changes in the specific (Csp) (a) and relative (С) (b) capacitance with increasing potential scan rate in 30 % H2SO4; Csp-log f dependencies obtained according to the electrochemical impedance method in 30 % H2SO4 (c), where f is the frequency (Hz).
Non-modified sample (G1)
Polyaniline-modified samples
at pH 3.0 (G2)
at pH 4.5 (G3)
Reference material (RM): Norit DLC Supra
Fig. 1. Initial graphene-like structures.
Fig 6. Changes in the specific (Csp) (a) and relative (С) (b) capacitance with increasing potential scan rate in 1 M TEATFB-A; Csp-log f dependencies obtained according to the electrochemical impedance method in 1 M TEATFB-A (c), where f is the frequency (Hz).
G1-G3, RM (0.5 g each)
CNTs «Taunit» g
Fluoroplastic suspension (1 mL)
Isopropanol
(10 mL) + + +
Conclusions
1. Among the studied graphene-like and reference activated carbon materials, the greatest Csp of the supercapacitor (168.3 F/g) in 30 % H2SO4 was found with the pristine graphene-like nanostructure (G1), being times higher than the values obtained with the other materials.
2. While studying the morphology effect of the pristine graphene-like materials (S1-S6) on the supercapacitor characteristics, high Csp values were found for all the electrode samples in 30 % H2SO4. When increasing the v from 5 to 100 mV/s, the sample activated at 800 OC during 3.5 h lost only 40 % of its capacitance, whereas the other samples lost % of their capacitance.
3. The cyclic current-voltage curves obtained in 1 M TEATFB-A were more strongly distorted when increasing the v, which might be related to severe transport limitations imposed on large organic ions. The sample activated at 750 OC during 2 h demonstrated the worst results for the Csp, which might be associated with the micro- and ultramicroporous structure of the material. At the same time, the samples activated at 700 and 800 OC during 2 and 3.5 h, respectively, showed rather high Csp values, which might be related to the ordered mesoporous structure.
4. The use of graphene and graphene-based materials as supercapacitor electrodes offers great opportunities to increase the Csp of such devices. Employing pristine impurity-free carbon nanostructures is of great importance, and in the near future it will allow to create stable supercapacitors with a large number of charge-discharge cycles.
(1) G1-G3, RM + 30% H2SO4
(2) Material with the highest Csp at step (1) – G1
Potentiometric testing (target parameter: specific capacitance Csp)
Electrode samples (S1-S6): activation by heating in a vacuum oven (time: 2 and 3.5 h; temperature: 700, 750 and 800 °C)
Ultrasonic dispersion (5 mL)
Fig. 2. Experimental scheme.
Acknowledgement
The research was conducted within the Federal Target Program "Research and Development in Priority Areas of the Scientific and Technological Complex of Russia for " (State Contract No of July 22, 2014; Unique Identifier for Applied Scientific Researches: RFMEFI57714X0091).
30 % H2SO4
1 M tetraethylam- monium tetrafluoro-borate in acetomitrile TEATFB-A
Fig. 3. Electrode sample testing.