Supercapacitive properties of polyaniline/Nafion/hydrous RuO2 composite electrodes

Chemically prepared polyaniline is tested for its supercapacitive behaviour in an aqueous electrolyte of 1.0 M H₂SO₄. In order to improve the cycleability of the polyaniline electrode, it is made into a composite with Nafion. This composite electrode shows improved cycleability and higher specific capacitance compared with a pure polyaniline electrode. It is therefore used as a matrix for the electrochemical deposition of hydrous RuO₂. The resulting ternary composite electrode has a high specific capacitance of 475 F g⁻¹ at 100 mV s⁻¹ and 375 F g⁻¹ at 1000 mV s⁻¹ in the voltage range of −0.2 to 0.8 V versus Ag/AgCl. All three types of electrode are characterized by cyclic voltammetry and impedance anaylsis.     

Preparation and electrochemical capacitance performances of super-hydrophilic conducting polyaniline

Super-hydrophilic conducting polyaniline was prepared by surface modification of polyaniline using tetraethyl orthosilicate in water/ethanol solution, whereas its conductivity was 4.16 S cm⁻¹ at 25 °C. And its electrochemical capacitance performances as an electrode material were evaluated by the cyclic voltammetry and galvanostatic charge/discharge test in 0.1 M H₂SO₄ aqueous solution. Its initial specific capacitance was 500 F g⁻¹ at a constant current density of 1.5 A g⁻¹, and the capacitance still reached about 400 F g⁻¹ after 5000 consecutive cycles. Moreover, its capacitance retention ratio was circa 70% with the growth of current densities from 1.5 to 20 A g⁻¹, indicating excellent rate capability. It would be a promising electrode material for aqueous redox supercapacitors.     

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.     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

Capacitance studies of cobalt compound nanowires prepared via electrodeposition

The capacitor properties of cobalt compound nanowire (CCNW) electrodes, prepared by the one-step electroreduction of [Co(NH₃)₆]³⁺ in water, have been investigated. The CCNW electrode changes its various properties during its growth. During the initial growth stage, the CCNW electrodes consist of nanowires with smooth surfaces and have a specific capacitance (C_m) of 310 F g⁻¹. During the middle stage, prickles grow on the CCNW surface, leading to a reduction in its real surface area and its C_m value to 230 F g⁻¹. During the final stage, further growth of the prickles is accompanied by the fusion of the CCNWs, and hence, a drastic decrease in the real surface area. However, a maximum capacitance of C_m = 420 F g⁻¹ was obtained during this stage. This unexpected capacitance change was discussed in terms of the effects of rapid ion transfer and the electroactive material/electrolyte interface area. In addition, the aging effect and the cycle life of the CCNW electrode were also investigated.     

High energy density capacitor using coal tar pitch derived nanoporous carbon/MnO2 electrodes in aqueous electrolytes

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg⁻¹, power densities of 11 kW kg⁻¹ and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g⁻¹) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.     

Electrochemical supercapacitor behavior of Ni3(Fe(CN)6)2(H2O) nanoparticles

Nanosized Ni₃(Fe(CN)₆)₂(H₂O) was prepared by a simple co-precipitation method. The electrochemical properties of the sample as the electrode material for supercapacitor were studied by cyclic voltammetry (CV), constant charge/discharge tests and electrochemical impedance spectroscopy (EIS). A specific capacitance of 574.7 F g⁻¹ was obtained at the current density of 0.2 A g⁻¹ in the potential range from 0.3 V to 0.6 V in 1 M KNO₃ electrolyte. Approximately 87.46% of specific discharge capacitance was remained at the current density of 1.4 A g⁻¹ after 1000 cycles.     

Performance and stability of electrochemical capacitor based on anthraquinone modified activated carbon

A series of high surface area activated carbon powders modified with various loadings of electroactive anthraquinone groups was obtained by the spontaneous reduction of the corresponding in situ generated diazonium derivative on activated carbon. The diazotation and grafting reactions are fast and efficient and by varying the stoichiometry of these reactions the grafting amount can be controlled. With appropriate reaction conditions, the attachment of anthraquinone groups allows to double the capacitance of the modified carbonaceous material (195 F g⁻¹) compared to the unmodified carbon (100 F g⁻¹) due to the contribution of the redox reaction of grafted anthraquinone molecules. Long time galvanostatic charge-discharge cycling experiments were performed for composite electrodes prepared using modified carbons having two different AQ loadings (e.g. 6.7 and 11.1 wt.%). Following 10 000 charge/discharge cycles, only a 17% loss of the faradaic capacitance was observed for these two carbons. Thus, this hybrid bifunctional material appears to be an excellent candidate for application as active electrode in electrochemical capacitors.     

Application of activated carbon/DNA composite electrodes to aqueous electric double layer capacitors

A novel capacitor electrode auxiliary, deoxyribonucleic acid (DNA), is applied to an electric double layer capacitor (EDLC) containing an aqueous 3.5 M NaBr electrolyte. The present electrode is composed of activated carbon (95 wt.%) and DNA (2.5 wt.%) with polytetrafluoroethylene (PTFE) as a binder (2.5 wt.%). An EDLC cell with the DNA-loading electrodes exhibits improved rate capability and discharge capacitance. An EDLC cell with DNA-free electrodes cannot discharge above a current density of 3000 mA g⁻¹ (of the electrode), while a cell with the DNA-loading electrodes can work at least up to 6000 mA g⁻¹. Moreover, an open-circuit potential (OCP) of the DNA-loading electrode sifts negatively with ca. 0.2 V from an OCP of the corresponding electrode without DNA. It is noteworthy that a small amount of DNA loading (2.5 wt.%) to the activated carbon electrode not only improves the rate capability but also adjusts the working potential of the electrode to a more stable region.     

Nickel hydroxide/activated carbon composite electrodes for electrochemical capacitors

In this paper, a nickel hydroxide/activated carbon (AC) composite electrode for use in an electrochemical capacitor was prepared by a simple chemical precipitation method. The structure and morphology of nickel hydroxide/AC were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that nano-sized nickel hydroxide was loading on the surface of activated carbon. Electrochemical performance of the composite electrodes with different loading amount was studied by cyclic voltammetry and galvanostatic charge/discharge measurements. It was demonstrated that the introduction of a small amount of nickel hydroxide to activated carbon could promote the specific capacitance of a composite electrode. The composite electrodes have good electrochemical performance and high charge–discharge properties. Moreover, when the loading amount of nickel hydroxide was 6 wt.%, the composite electrode showed a high specific capacitance of 314.5 F g⁻¹, which is 23.3% higher than pure activated carbon (255.1 F g⁻¹). Also, the composite electrochemical capacitor exhibits a stable cyclic life in the potential range of 0–1.0 V.     

Polypyrrole/carbon aerogel composite materials for supercapacitor

Polypyrrole (PPy)/carbon aerogel (CA) composite materials with different PPy contents are prepared by chemical oxidation polymerization through ultrasound irradiation and are used as active electrode material for supercapacitor. The morphology of PPy/CA composite is examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that PPy is deposited onto the surface of CA. As evidenced by cyclic voltammetry, galvanostatic charge/discharge test and EIS measurements, PPy/CA composites show superior capacitive performances to CA, moreover, the results based on cyclic voltammograms show that the composite material has a high specific capacitance of 433 F g⁻¹, while the capacitance of CA electrode is only 174 F g⁻¹. Although the supercapacitor used PPy/CA as active electrode material has an initial capacitance loss due to the instability of PPy, the specific capacitance after 500 cycles stabilizes nearly at a fixed value.     

Characterization of MnFe2O4/LiMn2O4 aqueous asymmetric supercapacitor

A new type of asymmetric supercapacitor containing a MnFe₂O₄ negative electrode and a LiMn₂O₄ positive electrode in aqueous LiNO₃ electrolyte has been synthesized and characterized. The nanocrystalline MnFe₂O₄ anode material has a specific capacitance of 99 F g⁻¹ and the LiMn₂O₄ cathode a specific capacity of 130-100 mAh g⁻¹ under 10-100 C rate. The cell has a maximum operating voltage window of ca. 1.3 V, limited by irreversible reaction of MnFe₂O₄ toward reducing potential. The specific power and specific energy of the full-cell increase with increasing anode-to-cathode mass ratio (A/C) and saturate at A/C ∼4.0, which gives specific cell energies, based on total mass of the two electrodes, of 10 and 5.5 Wh kg⁻¹ at 0.3 and 1.8 kW kg⁻¹, respectively. The cell shows good cycling stability and exhibits significantly slower self-discharge rate than either the MnFe₂O₄ symmetric cell or the other asymmetric cells having the same cathode but different anode materials, including activated carbon fiber and MnO₂.     

High temperature carbon–carbon supercapacitor using ionic liquid as electrolyte

This paper presents results about the electrochemical and cycling characterizations of a supercapacitor cell using a microporous activated carbon as the active material and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) ionic liquid as the electrolyte. The microporous activated carbon exhibited a specific capacitance of 60 F g⁻¹ measured from the three-electrode cyclic voltammetry experiments at 20 mV s⁻¹ scan rate, with a maximum operating potential range of 4.5 V at 60 °C. A coin cell assembled with this microporous activated carbon and PYR₁₄TFSI as the electrolyte was cycled for 40,000 cycles without any change of cell resistance (9 Ω cm²), at a voltage up to 3.5 V at 60 °C, demonstrating a high cycling stability as well as a high stable specific capacitance in this ionic liquid electrolyte. These high performances make now this type of supercapacitor suitable for high temperature applications (≥60 °C).     

Pseudocapacitive properties of electrochemically prepared nickel oxides on 3-dimensional carbon nanotube film substrates

Nickel oxides on carbon nanotube electrodes (NiO_x/CNT electrodes) are prepared by depositing Ni(OH)₂ electrochemically onto carbon nanotube (CNT) film substrates with subsequent heating to 300 °C. Compared with the as deposited Ni(OH)₂ on CNT film substrates (Ni(OH)₂/CNT electrodes), the 300 °C heat treated electrode shows much high rate capability, which makes it suitable as an electrode in supercapacitor applications. X-ray photoelectron spectroscopy shows that the pseudocapacitance of the NiO_x/CNT electrodes in a 1 M KOH solution originates from redox reactions of NiO_x/NiO_xOH and Ni(OH)₂/NiOOH. The 8.9 wt.% NiO_x in the NiO_x/CNT electrode shows a NiO_x-normalized specific capacitance of 1701 F g⁻¹ with excellent high rate capability due to the 3-dimensional nanoporous network structure with an extremely thin NiO_x layer on the CNT film substrate. On the other hand, the 36.6 wt.% NiO_x/CNT electrode has a maximum geometric and volumetric capacitance of 127 mF cm⁻² and 254 F cc⁻¹, respectively, with a specific capacitance of 671 F g⁻¹, which is much lower than that of the 8.9% NiO_x electrode. This decrease in specific capacitance of the high wt.% NiO_x/CNT electrodes can be attributed to the dead volume of the oxides, high equivalent series resistance for a heavier deposit, and the ineffective ionic transportation caused by the destruction of the 3-dimensional network structure. Deconvolution analysis of the cyclic voltammograms reveals that the rate capability of the NiO_x/CNT electrodes is adversely affected by the redox reaction of Ni(OH)₂, while the adverse effects from the reaction of NiO_x is insignificant.     

Electrochemical capacitance of NiO/Ru0.35V0.65O2 asymmetric electrochemical capacitor

A designed asymmetric hybrid electrochemical capacitor was presented where NiO and Ru_0.35V_0.65O₂ as the positive and negative electrode, respectively, both stored charge through reversible faradic pseudocapacitive reactions of the anions (OH⁻) with electroactive materials. And the two electrodes had been individually tested in 1 M KOH aqueous electrolyte to define the adequate balance of the active materials in the hybrid system as well as the working voltage of the capacitor based on them. The electrochemical tests demonstrated that the maximum specific capacitance and energy density of the asymmetric hybrid electrochemical capacitor were 102.6 F g⁻¹ and 41.2 Wh kg⁻¹, respectively, delivered at a current density of 7.5 A cm⁻². And the specific energy density decreased to 23.0 Wh kg⁻¹ when the specific power density increased up to 1416.7 W kg⁻¹. The hybrid electrochemical capacitor also exhibited a good electrochemical stability with 83.5% of the initial capacitance over consecutive 1500 cycle numbers.     

Long-term cycling behavior of asymmetric activated carbon/MnO2 aqueous electrochemical supercapacitor

Activated carbon–MnO₂ hybrid electrochemical supercapacitor cells have been assembled and characterized in K₂SO₄ aqueous media. A laboratory cell achieved 195,000 cycles with stable performance. The maximal cell voltage was 2 V associated with 21 ± 2 F g⁻¹ of total composite electrode materials (including activated carbon and MnO₂, binder and conductive additive) and an equivalent serie resistance (ESR) below 1.3 Ω cm². Long-life cycling was achieved by removing dissolved oxygen from the electrolyte, which limits the corrosion of current collectors. Scaling up has been realized by assembling several electrodes in parallel to build a prismatic cell. A stable capacity of 380 F and a cell voltage of 2 V were maintained over 600 cycles. These encouraging results show the interest of developing such devices, including non-toxic and safer components as compared to the current organic-based devices.     

Multilayered films of cobalt oxyhydroxide nanowires/manganese oxide nanosheets for electrochemical capacitor

Multilayered films of cobalt oxyhydroxide nanowires (CoOOHNW) and exfoliated manganese oxide nanosheet (MONS) are fabricated by potentiostatic deposition and electrostatic self-assembly on indium-tin oxide coated glass substrates. The morphology and chemical composition of these films are characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) and the potential application as electrochemical supercapacitors are investigated using cyclic voltammetry and charge-discharge measurements. These ITO/CoOOHNW/MONS multilayered film electrodes exhibit excellent electrochemical capacitance properties, including high specific capacitance (507 F g⁻¹) and long cycling durability (less 2% capacity loss after 5000 charge/discharge cycles). These characteristics indicate that these newly developed films may find important application for electrochemical capacitors.     

Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

Dual functions of activated carbon in a positive electrode for MnO2-based hybrid supercapacitor

Utilizing the dual functions of activated carbon (AC) both as a conductive agent and an active substance of a positive electrode, a hybrid supercapacitor (AC-MnO₂&AC) with a composite of manganese dioxide (MnO₂) and activated carbon as the positive electrode (MnO₂&AC) and AC as the negative electrode is fabricated, which integrates approximate symmetric and asymmetric behaviors in the distinct parts of 2 V operating windows. MnO₂ in the positive electrode and AC in the negative electrode together form a pure asymmetric structure, which extends the operating voltage to 2 V due to the compensatory effect of opposite over-potentials. In the range of 0-1.1 V, both AC in the positive and negative electrode assemble as a symmetric structure via a parallel connection which offers more capacitance and less internal resistance. The optimal mass proportions of electrodes are calculated though a mathematical process. In a stable operating window of 2 V, the capacitance of AC-MnO₂&AC can reach 33.2 F g⁻¹. After 2500 cycles, maximum energy density is 18.2 Wh kg⁻¹ with a 4% loss compared to the initial cycle. The power density is 10.1 kW kg⁻¹ with an 8% loss.     

Ru oxide supercapacitors with high loadings and high power and energy densities

Supercapacitors with very high energy and power densities have been constructed with hydrous ruthenium oxide powder prepared by a sol–gel method and annealed at 110 °C. Novel features of the capacitors, which improve their performances, are the use of a carbon fibre paper support, a Nafion separator, and Nafion as a binder. 1 M sulfuric acid was employed as the electrolyte. The performances of the supercapacitors were characterized by cyclic voltammetry, impedance spectroscopy and constant current discharging. The interfacial capacitance increased linearly with increasing ruthenium oxide loading to at least 50 mg cm⁻² on each electrode. The gravimetric capacitance of the Ru oxide measure by impedance reached 742 F g⁻¹ (9.66 F cm⁻²) at a loading of 13.0 mg cm⁻², and an interfacial capacitance of 34.9 F cm⁻² (682 F g⁻¹) was obtained at 51.2 mg cm⁻². The average effective series resistance was 0.55 Ω, the electronic resistance of the electrodes was negligible, and their ionic resistances were <0.42 Ω. The average power density for full discharge at 1 A cm⁻² for supercapacitors with 10 mg cm⁻² Ru oxide increased by 39% when 5% Nafion binder was added. The maximum average power density for full discharge was 31.5 W g⁻¹ while the maximum energy density was 31.2 Wh kg⁻¹. At a 1 mA discharge rate a specific capacitance of 977 F g⁻¹ of Ru oxide was obtained.