Studies of the activated carbons used in double-layer supercapacitors

The specific capacitance of activated carbons is determined by both the ratio of edge/basal orientation and the nature of functional group on the surface. The difference between the edge and the basal layers results from the semiconductive properties of basal layer. The ratio of edge/basal orientation can be estimated by X-ray diffraction (XRD). The wettability of activated carbons is determined by the nature of functional groups on the surface. Most of the surface groups are electrochemically active. The impact of the surface groups on electrochemical performance of the activated carbon electrodes was investigated by means of various surfactant treatments. The ac impedance and constant current discharge techniques were used. Two types of surface groups which had “capacitor-like” or “battery-like” behaviors, respectively, were revealed and discussed in detail. The surface groups with “battery-like” behavior should be avoided. “Non-symmetric” electrode arrangement should be considered for a double-layer supercapacitor in order to take the advantages of pseudo-capacitance of the surface groups with “capacitor-like” behaviors.     

Mathematical functions for optimisation of conducting polymer/activated carbon asymmetric supercapacitors

Equations routinely used to describe the properties of conventional symmetric electrochemical double-layer capacitors (EDLCs) are expanded to develop straightforward mathematical functions that can effectively describe the performance characteristics of asymmetric supercapacitors based on electrically conducting polymer and activated carbon (ECP–AC) electrodes. Formulae are developed to describe cell parameters (based on total active material mass) such as maximum specific capacitance (F g⁻¹), maximum specific energy (Wh kg⁻¹), and optimum electrode mass ratios that can be used for maximising the specific energy of asymmetric cells. The electrode mass ratios are found to have a significant impact on the swing voltages across the positive and negative electrodes. Illustrative EDLC and ECP–AC devices are explored and employed to verify the derived equations that serve to predict essential parameters of both symmetric and asymmetric systems, irrespective of electrolyte ion concentration, solvent or species and independent of voltage. The utility of the equations is demonstrated by predicting cell parameters for a number of theoretical asymmetric ECP–AC systems and used to correlate experimentally obtained parameters.     

Preparation of activated graphene/activated carbon dry composite electrode and its application in supercapacitors

采用干法电极制备工艺成功制备了活性石墨烯/活性炭复合电极片,分别用扣式电容器和软包电容器考察活性石墨烯/活性炭复合电极的电化学性能。综合结果表明,复合电极中活性石墨烯的含量为10%（质量分数）较为合适,相较于纯活性炭电极,比容量提高了10.8%。本工作验证了活性石墨烯材料在商用超级电容器中的适用性,证实了活性石墨烯是一种非常具有实际应用价值的电极材料。但目前,活性石墨烯并未真正产业化,其成本远高于商用活性炭。在未来,如何解决活性石墨烯工程制备技术难题和降低成本是材料产业界亟待解决的难题。     

A doped activated carbon prepared from polyaniline for high performance supercapacitors

A novel doped activated carbon has been prepared from H₂SO₄-doped polyaniline which is prepared by the oxypolymerization of aniline. The morphology, surface chemical composition and surface area of the carbon have been investigated by scanning electron microscope, X-ray photoelectron spectroscopy and Brunaner-Emmett-Teller measurement, respectively. Electrochemical properties of the doped activated carbon have been studied by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6 mol l⁻¹ KOH. The specific capacitance of the carbon is as high as 235 F g⁻¹, the specific capacitance hardly decreases at a high current density 11 A g⁻¹ after 10,000 cycles, which indicates that the carbon possesses excellent cycle durability and may be a promising candidate for supercapacitors.     

Nomex-derived activated carbon fibers as electrode materials in carbon based supercapacitors

Electrochemical characterization has been carried out for electrodes prepared of several activated carbon fiber samples derived from poly (m-phenylene isophthalamide) (Nomex) in an aqueous solution. Depending on the burn-off due to activation the BET surface area of the carbons was in the order of 1300–2800 m² g⁻¹, providing an extensive network of micropores. Their capability as active material for supercapacitors was evaluated by using cyclic voltammetry and impedance spectroscopy. Values for the capacitance of 175 F g⁻¹ in sulfuric acid were obtained. Further on, it was observed that the specific capacitance and the performance of the electrode increase significantly with increasing burn-off degree. We believe that this fact can be attributed to the increase of surface area and porosity with increasing burn-off.     

Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric

Electrochemical power sources that offer high energy and power densities and, can also withstand a harsh temperature range have become extremely desirable in applications ranging from civilian portable electronic devices to military weapons. In this report, we demonstrated a wide temperature withstanding supercapacitor which can be operated from 100 °C to −40 °C within a voltage window from −2 V to 2 V. The performance of the supercapacitor coin cells, assembled with nanostructured activated carbon fabric (ACF) as the electrode material and 1 M tetraethylammonium tetrafluoroborate (TEABF₄) in polypropylene carbonate (PC) solution as the electrolyte, was systematically studied within the set temperature window. The ACF supercapacitor yielded ideal rectangular shapes in cyclic voltammograms within 0-100 °C with an average mass capacitance of 90 F g⁻¹ and, 60 F g⁻¹ at −25 °C. The capacitance was still over 20 F g⁻¹ at the extremely low temperature of −40 °C. Another exciting feature of the ACF supercapacitors was that they resumed their room temperature capacitance when cooled from 100 °C and defrosted from −40 °C, demonstrating an excellent repeatability and stability. The charge-discharge behavior of the ACF supercapacitors showed long-cycle stability at extreme temperatures. These high electrochemical performances make this type of supercapacitors very promising in many practical applications.     

Preparation and characterization of composite electrodes of coconut-shell-based activated carbon and hydrous ruthenium oxide for supercapacitors

The relationship between the structure-specific capacitance (F g⁻¹) of a composite electrode consisting of activated coconut-shell carbon and hydrous ruthenium oxide (RuO_x(OH)_y) has been evaluated by impregnating various amounts of RuO_x(OH)_y into activated carbon that is specially prepared with optimum pore-size distribution. The composite electrode shows an enhanced specific capacitance of 250 F g⁻¹ in 1 M H₂SO₄ with 9 wt.% ruthenium incorporated. Chemical and structural characterization of the composites reveals a homogeneous distribution of amorphous RuO_x(OH)_y throughout the porous network of the activated carbon. Electrochemical characterization indicates an almost linear dependence of capacitance on the amount of ruthenium owing to its pseudocapacitive nature.     

New, low-cost, high-power poly(o-anisidine-co-metanilic acid)/activated carbon electrode for electrochemical supercapacitors

A poly(o-anisidine-co-metanilic acid)/activated carbon composite is evaluated as an active material for electrochemical supercapacitors. Poly(o-anisidine-co-metanilic acid) (PASM) is potentiodynamically deposited on an activated carbon (AC)-coated stainless-steel substrate, in a supporting electrolyte of 1.0 M H₂SO₄ containing dissolved o-anisidine and metanilic acid, at a sweep rate of 50 mV s⁻¹. Scanning electron micrographs show a uniformly deposited, thick PASM film on the activated carbon. Electrochemical techniques, such as impedance analysis, cyclic voltammetry and galvanostatic experiments, are carried out to investigate the suitability of the PASM/AC electrode for supercapacitor applications. A maximum specific capacitance of 576 F g⁻¹ is obtained for PASM/AC at a 5 mA cm⁻² current density.     

Influence of chemical modification of activated carbon surface on characteristics of supercapacitors

The influence of the change of Fermi level electrons position of activated carbon μ_C caused by chemical modification of its porous surface by Mn²⁺ ions on its capacitive characteristics in 7.6 m KOH, 4 m KI, 2 m ZnI₂ aqueous solutions is investigated in this work. The detection of adsorbed Mn²⁺ ions on the surface of activated carbon was carried out according to methods of secondary ionic mass spectrometry (SIMS). An increase in electronic density on the Fermi level of modified with Mn²⁺ activated carbon was determined with a help of X-ray photoelectron spectroscopy (XPS) data. Capacitive characteristics of the electrodes have been investigated by means of electrochemical impedance spectroscopy, computer modeling, and galvanic discharge. The correlation between electronic structure of modified activated carbon (MAC) and thermodynamic characteristics of ions of the used electrolytes is established. On the basic of the obtained experimental data, electrochemical system of the hybrid capacitor with a specific capacitance of 1740 F g⁻¹ and with a specific energy of 30 mWh g⁻¹ is developed.     

Effects of electrolytes and electrochemical pretreatments on the capacitive characteristics of activated carbon fabrics for supercapacitors

The capacitive characteristics of activated carbon fabrics (ACFs) coated on the graphite substrates were systematically investigated by means of cyclic voltammetry and the galvanostatic charge–discharge technique. Effects of the PVDF contents in the electronically conductive binder, electrochemical pretreatments, and the electrolytes on the capacitive performance of ACFs were compared in aqueous media. These ACF-pasted electrodes showed the more ideally capacitive responses in 1 M NaNO₃ with a specific capacitance of 76 F g⁻¹ when the electronically conductive binder contained 40 wt.% PVDF. The specific capacitance of ACF-pasted electrodes reached a maximum in 0.5 M H₂SO₄ (about 153 F g⁻¹ measured at 25 mV s⁻¹), due to the presence of a suitable density of oxygen-containing functional groups, when they were subjected to the potentiostatic polarization at 1.8 V (versus reversible hydrogen electrode (RHE)) or potentio-dynamic polarization between 1.3 and 1.8 V in NaNO₃ for 20 min. The oxygen-containing functional groups within the electrochemically pretreated ACFs were identified by means of X-ray photoelectron spectroscopy (XPS).     

Optimization of activated carbons for hydrogen storage

Activated carbons (ACs) having hydrogen storage performances among the highest reported so far (i.e. 6.6 wt. % at 77 K and 4 MPa) are presented. These materials were prepared by chemical activation of anthracite with KOH. The effects of two experimental parameters: KOH/anthracite weight ratio (W) and activation temperature (T), on the hydrogen storage capacity were studied by application of central composite design and response surface methodology. A quadratic model was developed for correlating W and T to the hydrogen storage capacity. The analysis of variance showed that W the only significant parameter in the range of the experimental conditions tested. Our optimized AC showed higher hydrogen capacity in terms of absolute and excess storage properties than the well-known MAXSORB-3.     

High performance hybrid supercapacitors with LiNi1/3Mn1/3Co1/3O2/activated carbon cathode and activated carbon anode

Hybrid supercapacitors have been studied as a next generation energy storage device that combines the advantages of supercapacitors and batteries. One important challenge of hybrid supercapacitors is to improve energy density (8.9–42 Wh/kg) with maintaining excellent power density (800–7989 W/kg) and cyclability (98.9% after 9000 cycles). Herein, we demonstrate an approach to design hybrid supercapacitors based on LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/activated carbon (AC) cathode and AC anode (NMC/AC//AC). The NMC/AC//AC hybrid supercapacitors shows outstanding electrochemical performances due to the enhanced energy and power densities. These findings suggest that the NMC/AC cathode is an effective method for high performance hybrid supercapacitors.     

Pinecone biomass-derived activated carbon: the potential electrode material for the development of symmetric and asymmetric supercapacitors

Activated carbon, from biomass (pinecone), was synthesized by conventional pyrolysis/chemical activation process and utilized for the fabrication of supercapacitor electrodes. The pinecone-activated carbon synthesized with 1:4 ratio of KOH (PAC4) showed an increase in surface area and pore density with a considerable amount of oxygen functionalities on the surface. Moreover, PAC4, as supercapacitor electrode, exhibited excellent electrochemical performances with specific capacitance value ∼185 Fg⁻¹ in 1 M H₂SO₄, which is higher than that of nonactivated pinecone carbon and 1:2 ratio KOH-based activated carbon (PAC2) (∼144 Fg⁻¹). The systematic studies were performed to design various forms of devices (symmetric and asymmetric) to investigate the effect of device architecture and operating voltage on the performance and stability of the supercapacitors. The symmetric supercapacitor, designed utilizing PAC4 in H₂SO₄ electrolyte, exhibited a maximum device-specific capacitance of 43 Fg⁻¹ with comparable specific energy/power and excellent stability (∼96% after 10 000 cycles). Moreover, a symmetric supercapacitor was specially designed using PAC4, as a positive electrode, and PAC2, as a negative electrode, under their electrolytic ion affinity, and which operates in aqueous Na₂SO₄ electrolyte for a wide cell voltage (1.8 V) and showed excellent supercapacitance performances. Also, a device was assembled with poly(3,4-ethylene dioxythiophene) (PEDOT) nanostructure, as positive electrode, and PAC4, as a negative electrode, to evaluate the feasibility of designing a hybrid supercapacitor, using polymeric nanostructure, as an electrode material along with biomass-activated carbon electrode.     

Electrode optimisation for carbon power supercapacitors

The purpose of this work was to economically prepare large electrodes (32 cm²) in order to assemble mono-cell supercapacitors (100 cm³) of 250 F and to reach powers in the range of 120–130 W (2 V/cell) with the structure selected. We have reached more than 20 F/cm³ of electrode, with a time constant of a few seconds (close to 2 s) and reached powers compatible with starting applications.     

Electrode compositions for carbon power supercapacitors

The purpose of this work was to prepare economically-large electrodes in order to assemble mono-element supercapacitors with a capacity of more than 300 F and to reach powers in the range of 125 W (2 V/element) with the selected structure. In this paper, results are presented from different binder compositions—PTFE and a mixture of CMC/PTFE—that were tested in order to increase the volumetric capacitance of the electrode. The electrode composition was adapted to each binder composition used. For cost reasons, the amount of PTFE was reduced and the mixture of CMC/PTFE was examined as a cheaper alternative. We have obtained more than 25 F cm⁻³ per electrode, with a time constant close to 3 s, and power outputs compatible with automotive applications.     

Graphene/single-walled carbon nanotube hybrids and applications in supercapacitors

双电层超级电容器作为新型清洁能源储能器件,具有安全、高功率密度和长寿命的优点。目前发展新电极材料与提高工作电压窗口是提高电容器能量密度的重要方向。本工作利用化学气相沉积法制备了石墨烯-碳纳米管杂化物,具有导电性优良、孔径可调、化学稳定性高、比表面积大（1200～1800 m2/g）的优点,同时避免了单独石墨烯或者碳纳米管电极制备过程中堆叠的缺点。并且系统研究了石墨烯-碳纳米管杂化物在水系、有机电解液和离子液体中的电容性质,考察了以活性炭为主体电极材料,石墨烯-碳纳米管为添加剂的软包电容器的性质,为开发高能量密度和高功率密度的超级电容器提供了基础。     

Effect of electrode mass ratio on aging of activated carbon based supercapacitors utilizing organic electrolytes

The accelerated degradation of carbon based supercapacitors utilizing 1 M Et₄NBF₄ in acetonitrile and in propylene carbonate as electrolyte is investigated for a constant cell voltage of 3.5 V as a function of the positive over total electrode mass ratio. The degradation rate of the supercapacitor using acetonitrile as a solvent can be decreased by increasing the mass of the positive electrode. With a mass ratio (positive electrode mass/total electrode mass) of 0.65 the degradation rate is minimum. For the capacitor utilizing propylene carbonate as a solvent a similar effect was observed. The degradation rate was smallest for a mass ratio above 0.5.     

MOF-5 and activated carbons as adsorbents for gas storage

This paper reports comparatively the capacities of two activated carbons (ACs) and MOF-5 for storing gases. It analyzes, using similar equipments and experimental procedures, the density used to convert gravimetric data to volumetric ones, measuring the density (tap and packing at different pressures). It presents data on porosity, surface area and gas storage (H₂, CH₄ and CO₂) obtained under different temperatures (77 K and RT) and pressures (0.1, 4 and 20 MPa). MOF-5 presents lower volume of narrow micropores than both ACs, making its storage at RT lower, independently of the gas used (H₂, CH₄ and CO₂) and the basis of reporting data (gravimetric or volumetric). For H₂ at 77 K the reliability of the results depends too much on the density used. It is shown that the outstanding volumetric performance of MOF-5, in relation to ACs, is due to the use of an unrealistic high density (crystal density) that, not including the adsorbent inter-particle space, gives an apparently high volumetric gas storage capacity. When a density measured similarly in both types of adsorbents is used (e.g. tap or packing densities) MOF-5 presents, for all gases and conditions studied, lower adsorption capacities on volumetric basis and storage capacities than ACs.     

Synthesis and characterization of tin oxide/carbon aerogel composite electrodes for electrochemical supercapacitors

Two types of carbon aerogel-based functional electrodes for supercapacitor applications are developed. To improve the electrochemical performance of the electrodes, carbon aerogels are doped with pseudocapacitive tin oxide either by impregnating tin oxide sol into resorcinol–formaldehyde (RF) wet gels (Method I), or by impregnating tin tetrachloride solution into carbon aerogel electrodes (Method II). The electrodes are heat-treated to 450 °C in air to activate the electrode surface and complete the oxidation of tin-precursors in the network structure of the aerogel. The effects of different impregnation methods on the physical/electrochemical properties of the composite electrodes are investigated. Microstructural and compositional variations of the electrodes with tin oxide doping are also examined by scanning electron microscopy and energy dispersive X-ray analysis. The tin oxide/carbon aerogel composite electrodes synthesized by both methods have similar specific capacitances (66–70 F g⁻¹). Composite electrodes synthesized via Method II showed better cyclic stability compared with electrodes synthesized via Method I.     

Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors

Graphene nanosheet/carbon nanotube/polyaniline (GNS/CNT/PANI) composite is synthesized via in situ polymerization. GNS/CNT/PANI composite exhibits the specific capacitance of 1035 F g⁻¹ (1 mV s⁻¹) in 6 M of KOH, which is a little lower than GNS/PANI composite (1046 F g⁻¹), but much higher than pure PANI (115 F g⁻¹) and CNT/PANI composite (780 F g⁻¹). Though a small amount of CNTs (1 wt.%) is added into GNS, the cycle stability of GNS/CNT/PANI composite is greatly improved due to the maintenance of highly conductive path as well as mechanical strength of the electrode during doping/dedoping processes. After 1000 cycles, the capacitance decreases only 6% of initial capacitance compared to 52% and 67% for GNS/PANI and CNT/PANI composites.