High power density supercapacitors based on the carbon dioxide activated d-glucose derived carbon electrodes and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid
Graphical abstract
Introduction
Decrease of fossil fuel resources and environmental impact of fuels combustion have forced the chemists to develop new technologies and materials for novel energy storage and conversion devices. Application of the electrical double–layer capacitors (EDLC, so-called supercapacitors) as energy storage devices is one possible solution to reduce the use of fossil fuels and, thus, emission of greenhouse gases by transport being extremely important in cities. EDLC has attracted a great attention as a short time energy storage system due to its high power capability, short characteristic time constant value, good coulombic reversibility (98% or higher) and excellent cyclability (over 106 cycles) [1], [2], [3], filling the gap between dielectric capacitors and traditional batteries [4], [5], [6], [7]. Differently from batteries, EDLCs store energy in the electrical double layer, where the adsorption of ions is based mainly on the electrostatic interactions. The unique characteristics of EDLCs allow them to replace or combine with batteries and fuel cells in applications where the high power pulses are important, such as the different peak power sources, digital communication devices, mobile phones, laptops, hybrid electric vehicles, wind turbines, etc. [8], [9], [10].
The electrical charge accumulated in EDLC depends on the electrochemically active surface area and, thus, on the porosity of a carbon material. However, the power density of EDLC is determined by the characteristic mass transfer and adsorption relaxation time constant values [6], [8], i. e. by the rate of mass transfer (diffusion and migration) and adsorption steps, which is determined by the hierarchical porous structure and chemical composition of the electrode materials used. Therefore, the micromesoporous carbon material characteristics (especially the ratio of micropore and mesopore surface area and pore volume) have to be optimised to increase further the specific energy and power values of supercapacitors [1], [11], [12], [13], [14].
Recently, increased attention has been paid for production of specially designed functional carbonaceous materials by hydrothermal carbonization (HTC) method [15], [16], [17], [18], [19], [20], [21]. The HTC method is very attractive due to its simplicity requiring only application of the low processing temperature (normally not higher than 300 °C), being cheap and “green”, since it does not use expensive organic solvents or catalysts. However, these carbonaceous materials prepared possess low porosity. The pore size distribution, the medium pore width and the specific surface area of the carbonized raw powder can be increased by using additional activation methods based on the application of air, carbon dioxide, steam, KOH, NaOH, H3PO4, etc., as an activation agent [22], [23].
Applicability of room-temperature ionic liquids (RTIL) as electrolytes for EDLCs has been discussed in many papers [24], [25], [26], [27], [28], [29], [30], [31] due to their high thermal stability, low vapour pressure, and wide potential region of the electrochemical stability. Unfortunately, RTILs have lower conductivity, higher viscosity and, thus, narrower low-temperature operation limit compared to the aqueous and organic solvent based electrolytes. However, compared with volatile aqueous and specially with organic electrolytes, RTILs are much safer in EDLCs for high temperature applications [29], [32], [33], [34], [35], [36], [37], [38].
In present work 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) was chosen due to the high bulk conductivity (κ = 13.6 mS cm−1) and wide potential region of electrochemical stability [25], [26], [27], [28], [37]. Activated carbons used for present study were synthesized from D-(+)-glucose according to the method described in Ref. [39].
Cyclic voltammetry (CV), constant current charge/discharge (CC), electrochemical impedance spectroscopy (EIS) and constant power discharge (CP) methods have been used to establish the influence of the physical characteristics of the activated carbon material on the electrochemical performance of EDLC.
Section snippets
Chemicals and reagents
D-(+)-glucose (≥99.5% purity, Sigma) was used without further purification. Ultrapure water (Milli-Q+, 18.2 MΩ cm, Millipore) was used for preparation of the 1 M D-(+)-glucose solution and for cleaning the resulting solid product formed during HTC process [39]. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) (Fluka Analytical, for electrochemistry, ≥ 99.0%, H2O < 200 ppm) was used as received.
Synthesis and physical characterization of activated carbon material
The hydrothermal carbonization of 1 M D-(+)-glucose solution in H2O was carried out in a
Physical characterization of activated carbon materials
The SEM images of the GDAC materials exhibit the interconnected spheres with a narrow particle size distribution from about 1.09 μm (GDAC-0h) to 0.90 μm (GDAC-12h) and relatively smooth surface structure (Fig. 1 insets). After activation, the size of the spheres reduced only slightly as a result of burn-off of some carbon during the activation step.
The first-order Raman spectra for GDAC materials, given in Supplementary Material, Fig. S1, shows that the full width at half-maximum (FWHM) values
Conclusions
The electrochemical characteristics of the electrical double-layer capacitor composed of the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) ionic liquid and carbon electrodes consisting of mainly microporous carbon material synthesised from D-(+)-glucose by hydrothermal carbonization, subsequent pyrolysis and carbon dioxide activation (with variable duration) steps were tested by the electrochemical impedance spectroscopy, cyclic voltammetry, constant current charge/discharge and the
Acknowledgements
This work was supported by the Estonian Science Foundation under Project No. 9184, Estonian Ministry of Education and Research project PUT55, European Regional Development Fund Project 3.2.0501.10-0015, European Spallation Source: Estonian Partition in ESS Instrument design, development and building and application for science research (SLOKT12026T) and Estonian Centre of Excellence in Research Project 3.2.0101-0030 “High-technology Materials for Sustainable Development” and Project IUT20-13.
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