Extending the low temperature operational limit of double-layer capacitors

This work describes the design and testing of organic electrolyte systems that extend the low temperature operational limit of double-layer capacitors (also known as supercapacitors) beyond that of typical commercially available components. Electrolytes were based on a tetraethylammonium tetrafluoroborate/acetonitrile system, modified with low melting co-solvents (such as formates, esters and cyclic ethers) to enable charging and discharging of test cells to as low as −75 °C. Cell capacitance exhibited little dependence on the electrolyte salt concentration or the nature of the co-solvent used, however, both variables strongly influenced the cell equivalent series resistance (ESR). Minimizing the increase in ESR posed the greatest design challenge, which limited realistic operation of these test cells to −55 °C (still improved relative to the typical rated limit of −40 °C for commercially available non-aqueous cells).     

Fabrication of high-power electric double-layer capacitors

The electrochemical behavior of activated carbon/carbon (AC/C) composite electrodes was investigated for high-power electric doublelayer capacitors (EDLCs). It was found that high-rate charge/discharge characteristics are affected by the resistance of the electrolyte phase in the pores of the electrode. The charge/discharge characteristics were improved by optimizing the pore-size distribution of the electrodes. The size and total volume of the macro-pores in the electrodes were controlled by mixing and burning out polymer spheres. A high-power EDLC (15V, 470 F), which can discharge as much as 500 A, was fabricated by using improved AC/C composite electrodes.     

Effect of treatment of activated carbon fiber cloth electrodes with cold plasma upon performance of electric double-layer capacitors

Charge/discharge behavior of electric double-layer capacitors composed of activated carbon fiber cloth (ACFC) electrodes and an organic electrolyte was investigated. The modification of the ACFC electrodes was performed using cold plasma generated in argon-oxygen atmosphere. The effect of the cold plasma treatment of the ACPC electrodes on the capacitor performance was discussed on the basis of the physical and chemical properties of the ACFC surface such as pore radius distribution and surface atom concentration.     

Cooperation of micro- and meso-porous carbon electrode materials in electric double-layer capacitors

The capacitive characteristics of micro- and meso-porous carbon materials have been compared in cyclic voltammetric studies and galvanostatic charge-discharge tests. Meso-porous carbon can keep certain high capacitance values at high scan rates, whereas micro-porous carbon possesses very high capacitance values at low scan rates but fades quickly as the scan rate rises up. For better performance of electric double-layer capacitors (EDLCs), the cooperative application of both kinds of carbon materials has been proposed in the following two ways: mixing both kinds of carbons in the same electrode or using the asymmetric configuration of carbon electrodes in the same EDLC. The cooperative effect on the electrochemical performance has also been addressed.     

Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors

Amphiphilic carbonaceous material (ACM), with nanoscale dispersion in alkaline aqueous solutions, is synthesized from green needle coke. As a special precursor with small particle size, plenty of functional groups and widened d₀₀₂ simultaneously, ACM guarantees subsequent ACM-based activated carbons (AACs) with high specific surface area over 3000 m² g⁻¹ as well as well-developed mesoporous structure after KOH activation. Such pore properties enable AACs’ high performances as electrode materials for electric double-layer capacitors (EDLCs). In particular, surface area up to 3347 m² g⁻¹ together with notable mesopore proportion (26.9%) gives sample AAC814 outstanding EDLC behaviors during a series of electrochemical tests including galvanostatic charge/discharge, CV and electrochemical impedance spectroscopy. The electrode gets satisfactory gravimetric and volumetric specific capacitance at the current density of 50 mA g⁻¹, up to 348 F g⁻¹ and 162 F cm⁻³, respectively. Furthermore, for the mesoporosity, there is only a slight capacitance reduction for AAC814 as the current density reaches 1000 mA g⁻¹, indicating its good rate performance. It is all the ACM's unique characteristics that make AACs a sort of competitive EDLC electrode materials, both in terms of specific capacitance and rate capability.     

Influence of pore structure on electric double-layer capacitance of template mesoporous carbons

The behavior of two types of mesoporous carbons with different pore structures (i.e. unimodal and bimodal) as electrode material in an electrochemical double-layer capacitor has been analyzed. The carbon samples were prepared using mesostructured silica materials (MSM) as templating agents. The unimodal mesoporous carbon has a BET surface area of 1550 m² g⁻¹, and a pore volume of 1.03 cm³ g⁻¹; the porosity is mainly made up of structural mesopores of ca. 3 nm that exhibit a narrow pore size distribution (PSD). The bimodal carbon shows larger surface area (1730 m² g⁻¹) and larger pore volume (1.50 cm³ g⁻¹); the porosity is composed of two types of mesopores: structural (size around 3 nm) and complementary (size around 16 nm) mesopores. Both carbons show a disordered 3-D pore structure. Heat treatments at high temperatures (1000 °C) for long times (11 h) do not significantly change the pore structure with respect to the two synthesised carbons (800 °C). From the synthesized and heat-treated carbons, electrodes were processed as composites in which the carbons, polivinilidene fluoride (PVDF) and carbon black (CB) were the components. The effect of the heat treatment and relative CB content on specific capacitance, energy density and power density were studied. We found a specific capacitance of 200 F g⁻¹ for low current density (1 mA cm⁻²) and 110 F g⁻¹ for high current density (150 mA cm⁻²). Moreover, the curve of the specific capacitance versus current density shows three regimes, which are related to the three types of pore: micropores, structural mesopores and complementary mesopores. An energy density of 3 Wh kg⁻¹ at a power density of 300 W kg⁻¹ was obtained in some particular cases.     

A review on the mechanism of the energy storage about the electrochemical double-layer capacitors #br#

本文综述了双电层电容器的储能机理研究进展,详细论述了多孔碳孔结构与电解液离子之间的相互作用,介绍了多孔碳界面双电层理论,包括最早的平行板双电层模型、考虑孔隙曲率的EDCC和EWCC模型及最新发现的充电机理。经过上述讨论,认为合成具有最优微孔尺寸、合适介孔比例和结构规整的多孔碳,是今后得到高功率密度、高能量密度多孔碳基超级电容器的最佳途径。     

Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content

20 activated carbons (ACs) were prepared by activation of four coals of different ranks (bituminous, low-ash bituminous and sub-bituminous coals, and one anthracite) with potassium hydroxide, in order to evaluate their hydrogen storage capacities at −196 °C. The effect of surface area and oxygen content on hydrogen storage was examined. Oxygen content was determined by temperature-programmed desorption. The significance of oxygen content on hydrogen storage capacity was evaluated by Analysis of Variance (ANOVA). Apparent surface areas higher than 3000 m² g⁻¹ and hydrogen adsorption as high as 6.8 wt.% were obtained. The best results were obtained with ACs from bituminous coals. No significant effect of oxygen content on hydrogen adsorption was observed. We concluded that surface area controls hydrogen storage capacity at −196 °C.     

Doping oxygen triggered electrocatalytic activity of carbon interpenetrating networks in acid electrolyte

The Fe–N–C catalysts may be promising candidates for replacing platinum group metal (PGM) catalysts to solve sluggish oxygen reduction reaction (ORR) kinetics in the proton exchange membrane fuel cells. However, the activity of Fe–N–C catalysts still has a certain gap compared with commercial Pt/C. Here, we provide a way to increase the intrinsic activity of Fe–N–C catalysts by designing active sites like ketone functional groups. A self-supporting interpenetrating network catalyst, composed of carbon nanotube (CNT) and carbon nanoparticle (CNP), is synthesized via multiple carbon sources (zinc-zeolitic imidazolate frameworks, polyaniline). The interpenetrating network features abundant ketone functional groups. The density functional theory (DFT) results prove that ketone groups can promote the ORR activity of FeN₄ active sites. This offers a new idea for improving the activity of Fe–N–C catalysts co-doped by oxygen and nitrogen in acidic systems.     

Conversion of almond shell to activated carbons: Methodical study of the chemical activation based on an experimental design and relationship with their characteristics

The objective of this work is to carry out a methodical study of the preparation conditions of almond shell based chemically activated carbons and its influence on the characteristics of the activated carbons. An experimental design was used to optimize the preparation conditions of activated carbons from almond shell via chemical activation with phosphoric acid. Temperatures from 400° to 800 °C, impregnation ratios in the range 0.5-1.5 and carbonization times varying from 30 to 120 min were defined as continuous parameters to be introduced in a simplex mixture experimental design giving a total of 12 experiments to carry out.The response surface methodology was applied in order to study the influence of all the production parameters on the selected responses. The optimization of all the characteristics of the activated carbons under the same experimental conditions is not possible because the influence of activation temperature, impregnation ratio and activation time is different. However using the response surface methodology it is possible to determine the ranges of each experimental preparation condition to obtain optimal characteristics of the activated carbon.     

Electrocatalysis of oxygen reduction on carbon nanotubes with different surface functional groups in acid and alkaline solutions

Electrochemical measurements, including cyclic voltammograms (CVs) and rotating ring disk electrode (RRDE) technique, are applied to determine the impact of surface functional groups on the electrocatalytic activity and H₂O selectivity of multi-walled carbon nanotubes (MWCNTs) for the oxygen reduction reaction (ORR) both in acid and alkaline electrolytes. Here, three types of carbon nanotubes (CNTs) involving MWCNTs, MWCNTs-COOH (MWCNTs with carboxyl groups) and MWCNTs-OH (MWCNTs with hydroxyl groups) are investigated. Both CVs and RRDE results indicate that the order of ORR electrocatalytic activity and H₂O selectivity is MWCNTs-COOH > MWCNTs-OH > MWCNTs in acid solution; while in alkaline medium, it is MWCNTs > MWCNTs-COOH > MWCNTs-OH. Furthermore, the ORR on these CNTs proceeds mainly through the two-electron reduction pathway followed by a gradual four-electron transfer reaction. Possible interpretation of the different ORR electrocatalytic activity and H₂O selectivity in these two solutions is also discussed.     

Enhanced electrochemical double-layer capacitive performance with CO2 plasma treatment on activated carbon prepared from pyrolysis of pistachio shells

This study reports an original approach based on the CO₂ plasma treatment on modification of the chemical or physical properties of activated carbon(AC) from the pistachio shells as a waste for application as electrochemical double-layer capacitors(EDLC). In the AC production experiments, impregnation ratio, impregnation pre-treatment temperature, activation temperature and activation time are investigated. In the AC modification experiments with plasma treatment, the effects of plasma gases, plasma power and plasma time are performed. The results of the different conditions indicated that the structural properties of the obtained AC were significantly dependent on the plasma and pyrolysis parameters. The surface properties of the raw AC and plasma-treated AC (PTAC) with X-ray photoelectron spectroscopy (XPS), nitrogen adsorption technique, and scanning electron microscope (SEM) are characterized. Surface area values for the raw AC and PTAC are 768 and 1250 m² g⁻¹, respectively. A change in the peak positions and an increase in the percentage of oxygen of the AC treated with CO₂ plasma were obtained from XPS results. After 15 min of CO₂ plasma activation, a significant increase in the capacitance of up to about 141% was obtained as a 118.4 F g ⁻¹ compared to 49.98 F g ⁻¹ for untreated AC. The results show that the plasma treatment on the specific surface area and surface functional groups of AC has a significant impact.     

Effect of water contamination in the organic electrolyte on the performance of activated carbon/graphite capacitors

The effect of water contamination in the electrolyte on the performance of AC/graphite capacitor has been investigated by electrochemical tests and in situ XRD measurements. The deterioration mechanisms for the charge storage ability of the electrodes in the capacitors using polluted electrolytes have also been addressed.     

Hydrogen adsorption on Pd-modified carbon nanofibres: Influence of CNF surface chemistry and impregnation procedure

Different carbon nanofibre (CNF) based materials (parent, oxidized, and impregnated with a palladium loading of 1 wt.% using different procedures) have been tested for hydrogen storage at ambient pressure. Parent CNF are completely free of oxygen surface groups, whereas treatment in nitric acid increases mainly the amount of surface anhydrides groups. Add to the surface functionalization, the solvent employed in the palladium impregnation was also varied, using both aqueous and organic precursor solutions. Thermogravimetric analyses of the hydrogen adsorption–desorption cycles suggest that the presence of theses functional groups hinders the adsorption. Concerning the presence of palladium, its influence strongly depends on the previous activation of the surface and on the solvent used for the palladium addition. The use of aqueous precursors and functionalized CNFs leads to increases in the adsorption capacity close to 100% compared to the parent CNF (12.6 vs. 6.7 cm³/g).     

Influence of spacer insulator material temperature characteristics on its electric properties

Spacer insulators are important insulating component of gas insulated switchgear(GIS). In order to analyze the influence of spacer insulator materials temperature characteristics on the electric performance, we tested the temperature distribution under work condition and the temperature characteristics of the material's relative permittivity. Then established a spacer insulator simulation model in finite element numerical simulation software to calculate its temperature and electric field. Using the temperature test results as a reference to verify the temperature calculation, and in order to analyze changes of the electric field along the surface of the spacer insulator, the temperature characteristics of relative permittivity have to be considered. Through the experimental research and simulation analysis, we found that the temperature characteristics of the spacer insulator materials have great impacts on the spacer insulator performance. When the temperature increases, the material relative permittivity also increases. In the low-temperature region of spacer insulator, the electric field intensity along the surface increases, so the electrical performance deteriorates. The conclusion of this paper can provide an important reference for further research about spacer insulator's working performance analysis.     

Analysis of electric vehicle driver recharging demand profiles and subsequent impacts on the carbon content of electric vehicle trips

This paper quantifies the recharging behaviour of a sample of electric vehicle (EV) drivers and evaluates the impact of current policy in the north east of England on EV driver recharging demand profiles. An analysis of 31,765 EV trips and 7704 EV recharging events, constituting 23,805 h of recharging, were recorded from in-vehicle loggers as part of the Switch EV trials is presented. Altogether 12 private users, 21 organisation individuals and 32 organisation pool vehicles were tracked over two successive six month trial periods. It was found that recharging profiles varied between the different user types and locations. Private users peak demand was in the evening at home recharging points. Organisation individual vehicles were recharged primarily upon arrival at work. Organisation pool users recharged at work and public recharging points throughout the working day. It is recommended that pay-as-you-go recharging be implemented at all public recharging locations, and smart meters be used to delay recharging at home and work locations until after 23:00 h to reduce peak demand on local power grids and reduce carbon emissions associated with EV recharging.     

Influence of torrefaction on properties of activated carbon obtained from physical activation of pyrolysis char

In this paper, the method of ‘torrefaction–fast pyrolysis–physical activation’ was conducted to investigate the impact of torrefaction on the properties of activated carbon through CO₂ activation. It was found that torrefaction had a significantly positive influence on the quality of activated carbon and 280°C was the optimal torrefaction temperature. The activated carbon obtained under the recommended condition had a higher yield (13.0 ± 0.3% based on dried rice husk) and most developed pore structure (specific surface area of 1090.7 m²/g). The results may be helpful for the potential utilization of high ash content biomass like rice husk.     

Surface modification of carbon fuels for direct carbon fuel cells

The direct carbon fuel cell (DCFC) is a promising power-generation device that has much higher efficiency (80%) and less emissions than conventional coal-fired power plants. Two commercial carbons (activated carbon and carbon black) pre-treated with HNO₃, HCl or air plasma are tested in a DCFC. The correlation between the surface properties and electrochemical performance of the carbon fuels is explored. The HNO₃-treated carbon fuels have the highest electrochemical reactivity in the DCFC due to the largest degree of surface oxygen functional groups. The overall effect on changing the electrochemical reactivity of carbon fuels is in the order HNO₃ > air plasma ≈ HCl. Product gas analysis indicates that complete oxidation of carbon to CO₂ can be achieved at 600–700 °C.     

Effects of oxygen-containing functional groups on the supercapacitor performance of incompletely reduced graphene oxides

Incompletely reduced graphene (RG) was prepared by reduction of graphite oxides via microwave irradiation at the power of 800 W and 1200 W, respectively. Fourier transform infrared spectra and X-ray photoelectron spectroscopy analyses showed that the RG contains flourish oxygen functional groups such as carboxyl, phenol, carbonyl, and quinone groups. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to investigate the structure, morphology of the RG samples. The electrochemical performance of the RG electrodes were measured in 5 M NaOH, 1 M H₂SO₄ and 1 M Na₂SO₄ electrolytes respectively. The supercapacitor performance of oxygen-containing functional group behavior was investigated. The specific pseudocapacitance per unit atomic percentage for either carboxyl or phenol group in 1 M H₂SO₄ and either carbonyl and quinone group in 5 M NaOH were obtained by galvano-statically charge/discharge measurements     

Influence of carbons on the structure of the negative active material of lead-acid batteries and on battery performance

It has been established that addition of carbon additives to the lead negative active material (NAM) of lead-acid batteries increase battery charge acceptance in hybrid electric vehicle mode of operation. The present work studies three types of activated carbons and two types of carbon blacks with the aim to evaluate their efficiency in improving the charge acceptance of lead-acid batteries. It has been established that the size of carbon particles and their affinity to lead are essential. If carbon particles are of nanosizes, they are incorporated into the bulk of the skeleton branches of NAM and may thus increase the latter's ohmic resistance. Their content in NAM should not exceed 0.2-0.5 wt.%. At this loading level, carbon grains are adsorbed only on the surface of NAM contributing to the increase of its specific surface area and thus improving its charge acceptance. When carbon particles are of micron sizes and have high affinity to lead, they are integrated into the skeleton structure of NAM as a structural component and act as super-capacitors, i.e. electric charges are concentrated in them and then the current is distributed along the adjacent branches of the lead skeleton with the lowest ohmic resistance. This eventually improves the charge acceptance of the negative battery plates.