Conductivity percolation in carbon–carbon supercapacitor electrodes

Composite electrodes which comprise a non-conductive activated carbon of large surface area (1420 m² g⁻¹) and a conductive carbon black (CB) of small surface area (220 m² g⁻¹) have been prepared and studied for their capacitive properties in aqueous KOH and Na₂SO₄ electrolytes. For either electrolyte, maximum capacitance exists at the composition believed to correspond to the percolation threshold for CB, the conductive phase. At a CB content less than the threshold, the capacitance is limited mainly by the electronic resistance on the electrode side. The interfacial surface area becomes the limiting factor as the threshold is exceeded. A maximum capacitance of 108 F g⁻¹ at a voltage sweep rate of 20 mV s⁻¹ is obtained in 1 M KOH aqueous electrolyte with a CB content of 25 wt.% (or ∼14 vol.%).     

The capacitive characteristics of supercapacitors consisting of activated carbon fabric–polyaniline composites in NaNO3

A sandwich-type supercapacitor consisting of two similar activated carbon fabric–polyaniline (ACF–PANI) composite electrodes was demonstrated to exhibit excellent performance (i.e., highly reversibility and good stability) in NaNO₃. Polyaniline with the charge density of polymerization less than or equal to 9 C cm⁻² synthesized by means of a potentiostatic method showed a high specific capacitance of 300 F g⁻¹. Influences of the polymerization charge density (i.e., the polymer loading) on the capacitive characteristics of ACF–PANI composites were compared systematically. The capacity of an ACF–PANI electrode reach ca. 3.4 F cm⁻² (a 100% increase in total capacity) when the charge density of polymerization is equal to 9 C cm⁻². The surface morphology of these ACF–PANI composites was examined by a scanning electron microscope (SEM).     

Effects of pore structure and electrolyte on the capacitive characteristics of steam- and KOH-activated carbons for supercapacitors

Four kinds of activated carbons (denoted as ACs) with specific surface area of ca. 1050 m² g⁻¹ were fabricated from fir wood and pistachio shell by means of steam activation or chemical activation with KOH. Pore structures of ACs were characterized by a t-plot method based on N₂ adsorption isotherms. The amount of mesopores within KOH-activated carbons ranged from 9.2 to 15.3% while 33.3–49.5% of mesopores were obtained for the steam-activated carbons. The pore structure, surface functional groups, and raw materials of ACs, as well as pH and the supporting electrolyte were also found to be significant factors determining the capacitive characteristics of ACs. The excellent capacitive characteristics in both acidic and neutral media and the weak dependence of the specific capacitance on the scan rate of cyclic voltammetry (CV) for the ACs derived from the pistachio shell with steam activation (denoted as P-H₂O-AC) revealed their promising potential in the application of supercapacitors. The ACs derived from fir wood with KOH activation (denoted as F-KOH-AC), on the other hand, showed the best capacitive performance in H₂SO₄ due to excellent reversibility and high specific capacitance (180 F g⁻¹ measured at 10 mV s⁻¹), which is obviously larger than 100 F g⁻¹ (a typical value of activated carbons with specific surface areas equal to/above 1000 m² g⁻¹).     

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.     

The capacitive characteristics of activated carbons—comparisons of the activation methods on the pore structure and effects of the pore structure and electrolyte on the capacitive performance

Fir wood-derived carbons activated with steam, KOH, and KOH + CO₂ were found to exhibit the high-power, low ESR, and highly reversible characteristics between −0.1 and 0.9 V in aqueous electrolytes, which were demonstrated to be promising electrode materials for supercapacitors. The pore structure of these activated carbons was systematically characterized by the t-plot method based on N₂ adsorption isotherms. Activated carbons prepared through the above three activation methods under different conditions (i.e., the gasification time of CO₂, KOH/char ratio, and activation time of steam) generally showed excellent capacitive performance in aqueous media, mainly attributed to the development of both micropores and mesopores (with the meso-pore volume ratio, V_meso/V_pore, ranging from 0.18 to 0.52). Scanning electron microscopic (SEM) photographs showed that the surface morphologies of honeycombed holes were found to depend on the activation methods. The average specific capacitance of the activated carbon with a combination of KOH etching and CO₂ gasification (with gasification time of 15 min) reached 197 F g⁻¹ between −0.1 and 0.9 V in H₂SO₄. The capacitive characteristics of steam- and KOH-activated carbons in NaNO₃ and H₂SO₄ could be roughly estimated from the pore structure and BET surface area although the correlation may be only applicable for the fir wood-derived activated carbons.     

Symmetric redox supercapacitor with conducting polyaniline electrodes

Polyaniline doped with HCl (Pani-HCl) and LiPF₆ (Pani-LiPF₆) are prepared and used as the active electrode material of symmetric redox supercapacitors. The system using Et₄NBF₄ as an electrolyte solution has lower internal resistance and larger specific discharge capacitance, and thus, it is suitable for use in a polyaniline redox supercapacitor. The capacitance of Pani-HCl decreases during ∼400 cycles and then becomes constant at ∼40 F g⁻¹. On the other hand, the polyaniline electrode doped with lithium salt like LiPF₆ shows a specific discharge capacitance of ∼107 F g⁻¹ initially and ∼84 F g⁻¹ at 9000 cycles.     

Cresol–formaldehyde based carbon aerogel as electrode material for electrochemical capacitor

Carbon aerogels have been prepared through a polycondensation of cresol (Cm) with formaldehyde (F) and an ambient pressure drying followed by carbonization at 900 °C. Modification of the porous structures of the carbon aerogel can be achieved by CO₂ activation at various temperatures (800, 850, 900 °C) for 1–3 h. This process could be considered as an alternative economic route to the classic RF gels synthesis. The obtained carbon aerogels have been attempted as electrode materials in electric double-layer capacitors. The relevant electrochemical behaviors were characterized by constant current charge–discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy in an electrolyte of 30% KOH aqueous solution. The results indicate that a mass specific capacitance of up to 78 F g⁻¹ for the non-activated aerogel can be obtained at current density 1 mA cm⁻². CO₂ activation can effectively improve the specific capacitance of the carbon aerogel. After CO₂ activation performed at 900 °C for 2 h, the specific capacitance increases to 146 F g⁻¹ at the same current. Only a slight decrease in capacitance, from 146 to 131 F g⁻¹, was observed when the current density increases from 1 to 20 mA cm⁻², indicating a stable electrochemical property of carbon aerogel electrodes in 30% KOH aqueous electrolyte with various currents.     

Preparation and optimization of RuO2-impregnated SnO2 xerogel supercapacitor

A Sb (6 mol%)-doped SnO₂ xerogel impregnated with RuO₂ nanocrystallites is prepared via an incipient-wetness method and is optimized for its electrochemical capacitance in aqueous 1 M KOH electrolyte by adjusting the calcination temperature and the RuO₂ loading. The electrode capacitance does not increase monotonically with increasing RuO₂ loading. A maximum electrode capacitance of 15 F g⁻¹, which represents a three-fold increase compared with the blank xerogel and a specific RuO₂ capacitance of 710 F g⁻¹ RuO₂, is obtained with a RuO₂ loading of 1.4 wt.% and by calcination at 200 °C. Higher loadings presumably result in a homogeneous nucleation upon drying, which causes severe reduction in the total surface area of the RuO₂ crystallites.     

Performance of supercapacitor with electrodeposited ruthenium oxide film electrodes—effect of film thickness

Thin-film ruthenium oxide electrodes are prepared by cathodic electrodeposition on a titanium substrate. Different deposition periods are used to obtain different film thicknesses. The electrodes are used to form a supercapacitor with a 0.5 M H₂SO₄ electrolyte. The specific capacitance and charge–discharge periods are found to be dependent on the electrode thickness. A maximum specific capacitance of 788 F g⁻¹ is achieved with an electrode thickness of 0.0014 g cm⁻². These results are explained by considering the morphological changes that take place with increasing film thickness.     

Manganese oxide film electrodes prepared by electrostatic spray deposition for electrochemical capacitors from the KMnO4 solution

Manganese oxide film electrodes for electrochemical capacitors were deposited on the polished Pt foils by electrostatic spray deposition (ESD) from KMnO₄ precursor solution. The electrochemical properties of electrodes were systematically studied using cyclic voltammetry (CV), constant current charge–discharge tests, and electrochemical impedance spectroscopy (EIS). The specific capacitance (SC) of thick deposited film was 149 F g⁻¹ at the very high scan rate of 500 mV s⁻¹, in comparison with 209 F g⁻¹ at the low scan rate of 5 mV s⁻¹. The electrode shows good cyclic performance. The initial SC value was 163 F g⁻¹ and 103% of the initial SC can be retained after 10,000 cycles at the scan rate of 50 mV s⁻¹.     

Comparative studies of nickel oxide films on different substrates for electrochemical supercapacitors

Thin nickel oxide (NiO) films were obtained by post-heating of the corresponding precursor films of nickel hydroxide (Ni(OH)₂) cathodically deposited onto different substrates, i.e., nickel foils, and graphite at 25 °C from a bath containing 1.5 mol L⁻¹ Ni(NO₃)₂ and 0.1 mol L⁻¹ NaNO₃ in a solvent of 50% (v/v) ethanol. The surface morphology of the obtained films was observed by scanning electron microscope (SEM). Electrochemical characterization was performed using cyclic voltammetrty (CV), chronopotentiometry (CP) and electrochemical impedance analysis (EIS). When heated at 300 °C for 2 h in air, the specific capacitance of the prepared NiO films on nickel foils and graphite, with a deposition charge of 250 mC cm⁻², were 135, 195 F g⁻¹, respectively. When the deposition charge is less than 280 mC cm⁻², the capacitance of both appears to keep the linear relationship with the deposition charge. The specific capacitance, cyclic stability of the NiO/graphite hybrid electrodes in 1 mol L⁻¹ KOH solution were superior to those on nickel foils mainly due to the favorable adhesion, the good interface behavior between graphite and the NiO films, and the extra pseudo-capacitance of the heated graphite substrates.     

Influence of the microstructure on the supercapacitive behavior of polyaniline/single-wall carbon nanotube composites

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites were prepared by in situ potentiostatic deposition of PANI onto SWCNTs at the potential of 0.75 V versus SCE, with the aim to investigate the influence of microstructure on the specific capacitance of PANI/SWCNT composites. It was found that the specific capacitance of the PANI/SWCNT composites is strongly influenced by their microstructure, which is correlated to the wt.% of the PANI deposited onto the SWCNTs. The optimum condition, corresponding to the highest specific capacitance, 463 F g⁻¹ (at 10 mA cm⁻²), was obtained for 73 wt.% PANI deposited onto SWCNTs. The specific capacitance of the PANI/SWCNT composite electrode was highly stable, with a capacitive decrease of 5% during the first 500 cycles and just 1% during the next 1000 cycles, indicative of the excellent cyclic stability of the composite for supercapacitor applications.     

Effect of pore size and surface area of carbide derived carbons on specific capacitance

This work presents a systematic study on how pore size and specific surface area (SSA) of carbon effect specific capacitance and frequency response behavior. Carbide derived carbons (CDC) produced by leaching metals from TiC and ZrC at temperatures from 600 to 1200 °C have highly tailorable microstructure and porosity, allowing them to serve as excellent model systems for porous carbons in general. BET SSA and average pore size increased with synthesis temperature and was 600–2000 m² g⁻¹ and 0.7–1.85 nm, respectively. Maximum specific capacitance in 1 M H₂SO₄ was found to occur at an intermediate synthesis temperature, 800 °C, for both ZrC and TiC derived carbons and was 190 and 150 F g⁻¹, respectively. Volumetric capacitance for TiC and ZrC derived carbons was maximum at 140 and 110 F cm⁻³. These results contradict an oft-reported axiom that increasing pore size and SSA, all other things being held constant, increases specific capacitance. A correlation between specific capacitance and SSA of micropores (less than 2 nm in diameter) has been shown. As expected, increasing pore size was found to improve the frequency response. However, CDCs with similar pore size distributions but obtained from different starting materials showed noticeable differences in impedance behavior. This highlights the importance of not only the pore size and specific surface area measured using gas sorption techniques, but also the pore shape or tortuousity, which is non-trivial to characterize, on energy storage.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

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.     

Studies on preparation and performances of carbon aerogel electrodes for the application of supercapacitor

Carbon aerogel was prepared by the polycondensation of resorcinol (R) with formaldehyde (F), and sodium carbonate was added as a catalyst (C). Physical properties of carbon aerogel were characterized by infrared spectrometer (IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that carbon aerogel is an amorphous material with a pearly network structure, and it consists of one or two diffuse X-ray peaks. The results of cyclic voltammetry indicated that the specific capacitance of a carbon aerogel electrode in 6 M KOH electrolyte was approximately 110.06 F g⁻¹. Through the galvanostatic charge/discharge measurement, it was found that the electrode is stable in KOH electrolyte, the maximum capacitance of the supercapacitor with carbon aerogel as the electrode active material was 28 F g⁻¹. Besides, the supercapacitor has long cycle life. Thus, it was thought that the carbon aerogel is an excellent electrode material for a supercapcitor.     

Porous structured vanadium oxide electrode material for electrochemical capacitors

A nano porous vanadium oxide (V₂O₅) was prepared by sol–gel method. The preparation involved elutriation of aqueous sodium meta vanadate over a cation exchange resin. The product was characterized using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, surface area analysis and thermogravimetric analysis. Electrochemical characterization was done using cyclic voltammetry in a three electrode system consisting of a saturated calomel electrode as reference electrode, platinum mesh as a counter electrode, and V₂O₅ mounted on Ti mesh as the working electrode. Two molars of aqueous KCl, NaCl and LiCl were used as electrolytes. A maximum capacitance of 214 F g⁻¹ was obtained at a scan rate of 5 mV s⁻¹ in 2 M KCl. The effect of different electrolytes and the effect of concentration of KCl on the specific capacitance of V₂O₅ were studied. Specific capacitance faded rapidly over 100 cycles in 2 M KCl at a 5 mV s⁻¹ scan rate.     

Analysis of polyaniline-based nickel electrodes for electrochemical supercapacitors

Polyaniline is deposited potentiodynamically on a nickel substrate in the presence of p-toluene sulfonic acid and the specific capacitance is estimated. The electrochemical characterisation of the electrode is carried out by means of cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge experiments. The specific capacitance is ∼4.05 × 10² F g⁻¹. This indicates the feasibility of the polyaniline-coated nickel electrode for use in electrochemical supercapacitors.     

Enhanced electrochemical stability of all-polymer redox supercapacitors with modified polypyrrole electrodes

Redox supercapacitors are attracting increasing attention as high power electrochemical sources and can either be coupled with batteries to provide peak power or replace batteries for memory back-up. In the present work, all-polymer solid-state supercapacitors with LiClO₄ and LiCF₃SO₃ doped polypyrrole electrodes and P(VDF-HFP)-PMMA based polymer gel electrolyte are fabricated. The polypyrrole electrodes are irradiated with 160 MeV Ni¹²⁺ ions at 5 × 10¹⁰, 5 × 10¹¹ and 5 × 10¹² ions cm⁻². A comparative study is made between unirradiated and irradiated supercapacitors with polypyrrole-based electrodes. An average capacitance of about 200 F gm⁻¹ is obtained. On successive charging and discharging, the capacitance decreases for supercapacitors with unirradiated electrodes but remains stable when irradiated electrodes are used. In addition, the capacitance is slightly decreased compared with that for unirradiated electrodes. Charge–discharge studies show a decrease in total charge–discharge time for supercapacitors with irradiated electrodes. The capacitance values calculated from cyclic voltammograms are higher than those determined from charge–discharge plots due to the added contribution of a leakage current. The coulombic efficiency of all the supercapacitors is about 90%.     

Novel architecture of composite electrode for optimization of lithium battery performance

We show that the polymeric binder of the composite electrode may have an important role on the lithium trivanadate Li_1.2V₃O₈ electrode performance. We describe a new tailored polymeric binder combination with controlled polymer–filler (carbon black) interactions that allows the preparation of new and more efficient electrode architecture. Using this polymeric binder, composite electrodes based on Li_1.2V₃O₈ display a room temperature cycling capacity of 280 mAh g⁻¹ (C/5 rate, 3.3–2 V) instead of 150 mAh g⁻¹ using a standard-type (poly(vinylidene fluoride)–hexafluoropropylene (PVdF–HFP) binder) composite electrode. We have coupled scanning electron microscopy (SEM) observations, galvanostatic cycling and electrochemical impedance spectroscopy in order to define and understand the impact of the microstructure of the composite electrode on its electrochemical performance. Derived from these studies, the main key factors that provide efficient charge carrier collection within the composite electrode complex medium are discussed.