Symmetric redox supercapacitor based on micro-fabrication with three-dimensional polypyrrole electrodes

To achieve higher energy density and power density, we have designed and fabricated a symmetric redox supercapacitor based on microelectromechanical system (MEMS) technologies. The supercapacitor consists of a three-dimensional (3D) microstructure on silicon substrate micromachined by high-aspect-ratio deep reactive ion etching (DRIE) method, two sputtered Ti current collectors and two electrochemical polymerized polypyrrole (PPy) films as electrodes. Electrochemical tests, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatical charge/discharge methods have been carried out on the single PPy electrodes and the symmetric supercapacitor in different electrolytes. The specific capacitance (capacitance per unit footprint area) and specific power (power per unit footprint area) of the PPy electrodes and symmetric supercapacitor can be calculated from the electrochemical test data. It is found that NaCl solution is a good electrolyte for the polymerized PPy electrodes. In NaCl electrolyte, single PPy electrodes exhibit 0.128 F cm⁻² specific capacitance and 1.28 mW cm⁻² specific power at 20 mV s⁻¹ scan rate. The symmetric supercapacitor presents 0.056 F cm⁻² specific capacitance and 0.56 mW cm⁻² specific power at 20 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.     

Study and optimisation of manganese oxide-based electrodes for electrochemical supercapacitors

A manganese oxide material was synthesised by an easy precipitation method based on reduction of potassium permanganate(VII) with a manganese(II) salt. The material was treated at different temperatures to study the effect of thermal treatment on capacitive property. The best capacitive performance was obtained with the material treated at 200 °C. This material was used to prepare electrodes with different amounts of polymer binder, carbon black and graphite fibres to individuate the optimal composition that gave the best electrochemical performances. It was found that graphite fibres improve the electrochemical performance of electrodes. The highest specific capacitance (267 F g⁻¹ MnO_x) was obtained with an electrode containing 70% of MnO_x, 15% of carbon black, 10% of graphite fibres and 5% of PVDF. This electrode, with CB/GF ratio of 1.5, showed a higher utilization of manganese oxide. The results reported in the present paper further confirmed that manganese oxide is a very interesting material for supercapacitor application.     

Carbon materials modified by plasma treatment as electrodes for supercapacitors

The carbon material was modified by RF plasma with various reactive gases: O₂, Ar and CO₂. Physicochemical properties of the final carbon products were characterized using different techniques such as gas adsorption method and XPS. Plasma modified materials enriched in oxygen functionalities were investigated as electrodes for supercapacitors in acidic medium. The electrochemical measurements have been carried out using cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy. The electrochemical measurements have confirmed that capacity characteristics are closely connected with a type of plasma exposition. Modification processes have an influence on the kind and amount of surface functional groups in the carbon matrix. The moderate increase of capacity of carbon materials modified by plasma has been observed using symmetric two-electrode systems. Whereas investigations made in three-electrode system proved that the suitable selection of plasma modification parameters allows to obtain promising negative and positive electrode materials for supercapacitor application.     

Polyaniline-based nickel electrodes for electrochemical supercapacitors—Influence of Triton X-100

The influence of Triton X-100 in enhancing the capacitance of polyaniline-based nickel electrodes is reported. Cyclic voltammetric experiments, galvanostatic charge–discharge studies and impedance analysis were carried out in order to investigate the applicability of the system as an electrochemical supercapacitor. A qualitative interpretation of the enhancement is provided. Fourier transform infrared (FTIR), X-ray diffraction and scanning electron microscopy techniques were employed for characterization of the electrode.     

Textural and electrochemical characterization of porous carbon nanofibers as electrodes for supercapacitors

Porous carbon nanofibers (CNFs) enriched with the graphitic structure were synthesized by thermal decomposition from a mixture containing polyethylene glycol and nickel chloride (catalyst). The textural and electrochemical properties of porous CNFs were systematically compared with those of commercially available multi-walled carbon nanotubes (MWCNTs). The high ratio of mesopores and large amount of open edges of porous CNFs with a higher specific surface area, very different from that of MWCNTs, are favorable for the penetration of electrolytes meanwhile the graphene layers of porous CNFs serve as a good electronic conductive medium of electrons. The electrochemical properties of porous CNFs and MWCNTs were characterized for the application of supercapacitors using cyclic voltammetry, galvanostatic charge–discharge method, and electrochemical impedance spectroscopic analyses. The porous CNFs show better capacitive performances (C_S = 98.4 F g⁻¹ at 25 mV s⁻¹ and an onset frequency of behaving as a capacitor at 1.31 kHz) than that of MWCNTs (C_S = 17.8 F g⁻¹ and an onset frequency at 1.01 kHz). This work demonstrates the promising capacitive properties of porous CNFs for the application of electrochemical supercapacitors.     

Metal-organic frameworks as highly efficient electrodes for long cycling stability supercapacitors

Among a large variety of energy storage technologies, supercapacitors possess special advantages such as rapid charge/discharge, high power density, safety, and environmental friendliness to meet the requirement of specific applications. The common electrode materials of supercapacitors, including porous carbon, conductive polymers, and metal oxides/hydroxides, have their own benefits and drawbacks in energy density and stability. Owing to the big surface area and controllable porosity, the metal-organic frameworks (MOFs) have been explored as important candidates for supercapacitor applications. This mini-review focuses on the recent advances of MOF-based materials including pristine MOFs, MOFs composite materials, and MOF-derived materials in the development of long cycling life supercapacitors. The devices discussed here mean those with capacitive retention rates of more than 90% after 10,000 cycles and high energy density. In addition, we also describe the fundamental knowledge of supercapacitors, highlight the stabilization mechanism of MOFs, and propose the strategies to enhance the stability of MOF-based supercapacitor electrodes.     

Photoelectrochemical and electrochemical behaviour of gold electrode modified with bilayers of polypyrrole and polyaniline

Modification of electrode surfaces with bilayers of organic/inorganic materials is shown to exhibit a variety of functional characteristics. The use of conducting polymers for bilayer construction has recently been explored by various groups. Gold surface has been modified with bilayers of polyaniline and polypyrrole, and the photoelectrochemical and electrochemical characteristics of such bilayer electrodes have been investigated and are reported in this communication.     

Enhanced capacitance and rate capability of graphene/polypyrrole composite as electrode material for supercapacitors

Graphene and polypyrrole composite (PPy/GNS) is synthesized via in situ polymerization of pyrrole monomer in the presence of graphene under acid conditions. The structure and morphology of the composite are characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectrometer (FTIR), X-rays photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). It is found that a uniform composite is formed with polypyrrole being homogeneously surrounded by graphene nanosheets (GNS). The composite is a promising candidate for supercapacitors to have higher specific capacitance, better rate capability and cycling stability than those of pure polypyrrole. The specific capacitance of PPy/GNS composite based on the three-electrode cell configuration is as high as 482 F g⁻¹ at a current density of 0.5 A g⁻¹. After 1000 cycles, the attenuation of the specific capacitance is less than 5%, indicating that composite has excellent cycling performance.     

Fabrication and electrochemical properties of carbon nanotube/polypyrrole composite film electrodes with controlled pore size

Carbon nanotube (CNT)/polypyrrole (PPy) composites with controlled pore size in a three-dimensional entangled structure of a CNT film are prepared as electrode materials for a pseudocapacitor. A CNT film electrode containing nanosize silica between the CNTs is first fabricated using an electrostatic spray deposition of a mixed suspension of CNTs and nanosize silica on to a platinium-coated silicon wafer. Later, nanosize silica is removed leaving a three-dimensional entangled structure of a CNT film. Before removal of the silica from the CNT/silica film electrode, PPy is electrochemically deposited on to the CNTs to anchor them in their entangled structure. Control of the pore size of the final CNT/PPy composite film can be achieved by changing the amount of silica in the mixed suspension of CNTs and nanosize silica. Nanosize silica acts as a sacrificial filler to change the pore size of the entangled CNT film. Scanning electron microscopy of the electrochemically prepared PPy on the CNT film substrate shows that the PPy nucleated heterogeneously and deposited on the surface of the CNTs. The specific capacitance and rate capability of the CNT/PPy composite electrode with a heavy loading of PPy of around 80 wt.% can be improved when it is made to have a three-dimensional network of entangled CNTs with interconnected pores through pore size control.     

Enhanced performance of carbon-based supercapacitors by constructing protonic and electric dual-channels in electrodes

Powdery carbonaceous materials have to use binder materials when they are integrated into electrodes for supercapacitors, which will results in high interfacial charge transfer resistances and reduced specific capacitance. To resolve the problem, protonic and electric dual-channels are constructed in electrodes by in situ synthesis of cesium hydrogen salt of phosphotungstic acid on the surface of carbonaceous materials. The cesium hydrogen salt particles are confirmed by a Fourier transform infrared spectroscopy, X-ray diffractometer and an energy dispersive X-ray spectroscopy. The electrochemical properties of as-fabricated electrodes are measured by cyclic voltammetry, galvanostatic charging-discharging, and impedance analysis with an electrochemical workstation. At a current density of 1 A g⁻¹, the electrode shows a specific capacitance of 152 F g⁻¹. Compared to the electrode without the cesium hydrogen salt, the value increases 25% at least. Furthermore, the specific capacitance retention of the electrode reaches 104% of its original capacitance after 5000 charge-discharge cycles, suggesting excellent cycling stability.     

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.     

Electrochemical cyclability mechanism for MnO2 electrodes utilized as electrochemical supercapacitors

The electrochemical cyclability mechanism of nanocrystalline MnO₂ electrodes with rock salt-type and hexagonal ?-type structures was investigated to determine the relationship between physicochemical feature evolution and the corresponding electrochemical behaviour of MnO₂ electrodes. Rock salt MnO₂ and hexagonal ?-MnO₂ electrodes, with fibrous and porous morphologies, evolve into the antifluorite-type MnO₂ with a petal-shaped nanosheet structure after electrochemical cycling, similar to that observed in nanocrystalline antifluorite-type MnO₂ electrodes after electrochemical cycling. However, a different impedance response was observed for the rock salt MnO₂ and hexagonal ?-MnO₂ electrodes during the charge–discharge cycles, compared with the improved impedance response observed for the cycled antifluorite-type MnO₂. A dissolution–redeposition mechanism is proposed to account for the impedance response of the MnO₂ electrodes with different morphologies and crystal structures.     

Physical and electrochemical characteristics of supercapacitors based on carbide derived carbon electrodes in aqueous electrolytes

FIB-SEM, XPS and gas adsorption methods have been used for the characterisation of physical properties of microporous carbide derived carbon electrodes prepared from Mo₂C at 600 °C (noted as CDC-Mo₂C). Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for supercapacitors consisting of the 1 M Na₂SO₄, KOH, tetraethyl ammonium iodide or 6 M KOH aqueous electrolyte and CDC-Mo₂C electrodes. The N₂ sorption values obtained have been correlated with electrochemical characteristics for supercapacitors in various aqueous electrolytes. The maximum gravimetric energy, E_max, and gravimetric power, P_max, for supercapacitors (taking into consideration the active material weight) have been obtained at cell voltage 0.9 V for 6 M KOH aqueous supercapacitor (E_max = 5.7 Wh kg⁻¹ and P_max = 43 kW kg⁻¹). For 1 M TEAI based SC somewhat higher E_max (6.2 Wh kg⁻¹) and comparatively low P_max (7.0 kW kg⁻¹) have been calculated.     

Enhanced electrochemical performance of nickel intercalated ZIF-67/rGO composite electrode for solid-state supercapacitors

Ni (Nickel) doped zeolitic-imidazolate framework (ZIF-67) has been prepared in presence of reduced graphene oxide (rGO) to realize a ZIF-67/rGO composite. The doping level of Ni and the ratio of rGO (wt%) in the composite have been optimized to attain desirable redox activity and electrical conductivity. A partial incorporation of redox active Ni ions to substitute Co (cobalt) ions in ZIF-67 has resulted in better electrochemical characteristics by inducing additional pseudocapacitance. A finalized composite with 33% Ni and 20% of rGO (i.e, Ni₃₃/ZIF-67/rGO₂₀) has been used as a supercapacitor electrode material to achieve a high specific capacitance of 304 F/g at a current density of 1 A/g in the presence of 1 M H₂SO₄ as an aqueous electrolyte. The above electrode has also been tested for an all-solid-state symmetric supercapacitor in the presence of a polymer gel electrolyte (PVA/1 M H₂SO₄). This device delivered high values of power and energy densities, i.e., 1 kW/kg and 21.5 Wh/kg, respectively. The device also exhibited an excellent cyclic stability. About 87% of capacitance could be retained even after 4500 charge-discharge cycles. The device has shown superior results for a working potential window of 0–2 V. The practical usefulness of the device has been demonstrated by preparing a symmetrical supercapacitor, which could energize a white LED for 8 min upon a charging of only 40 s.     

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.     

Nanoporous MnOx thin-film electrodes synthesized by electrochemical lithiation/delithiation for supercapacitors

Nanoporous MnO_x thin-film electrodes are synthesized using a combination of pulsed laser deposition (PLD) and electrochemical lithiation/delithiation methods. A dense Mn₃O₄ thin-film deposited by PLD can transform into a nanoporous MnO_x thin-film after electrochemical lithiation/delithiation. A nanoporous MnO_x thin-film electrode exhibits significantly improved supercapacitive performance compared with an as-deposited Mn₃O₄ thin-film electrode. A MnO_x thin-film finally transforms into a MnO₂ thin-film through an electrochemical oxidation process during continuous cyclic voltammetry scanning.     

Effect of poly(3,4-ethylenedioxythiophene) (PEDOT) in carbon-based composite electrodes for electrochemical supercapacitors

Electrochemical double layer supercapacitor cells were fabricated and tested using composite electrodes of activated carbon with carbon black and poly(3,4-ethylenedioxythiophene) (PEDOT), and an organic electrolyte 1 M TEABF₄/PC solution. The effect of PEDOT on the performance of the EDLC cells was explored and the cells were characterised by electrochemical impedance spectroscopy (EIS), cyclic voltammetry and galvanostatic charge-discharge. A generalised equivalent circuit model was developed for which numerical simulations were performed to determine the properties and parameters of its components from the EIS data. It was found that the proposed model fitted successfully the data of all tested cells. PEDOT enhanced the electrode and cell capacitance via its pseudo-capacitance effect up to a maximum value for an optimum PEDOT loading and greatly increased the energy density of the cell while the maximum power density has been still maintained at supercapacitor levels. Furthermore, PEDOT replaced PVDF as a binder and harmful solvent release was reduced during electrode processing. Activated carbon-carbon black composite electrodes with PEDOT as binder were found to have specific capacitance superior to that of activated carbon-carbon black electrodes with PVDF binder.     

Preparation and electrochemical properties of polysulfide polypyrrole

A novel cathode material, polysulfide polypyrrole was successfully designed and synthesized for high-energy lithium-sulfur secondary batteries. The product was characterized by FT-IR, element analysis and DSC. Character results show that the polymer was obtained with polypyrrole as backbone and S-S side groups attached to it. Polypyrrole backbone in this polymer was used not only as container but also as conductivity passage. Cycle performances of the polymer was examined as active cathode material in lithium batteries, charge-discharge experimental results indicate that the polymer has a specific capacity of 515 mAh g⁻¹ at the first cycle and 452 mAh g⁻¹ at the 20th cycle. The improved cycle properties compared to other polymer disulfides and polysulfide polymers supply a good foundation for practical application of this material in rechargeable lithium batteries.     

Microgravimetric study of the influence of the solvent on the redox properties of polypyrrol modified electrodes

The redox behavior of the polypyrrole films in the presence of LiClO₄ salt in different solvents like propylene carbonate (PC), N,N-dimetilformamide (DMF), methanol (MetOH), ethanol (EtOH), acetonitrile (ACN) and water was investigated using simultaneous electrochemical quartz crystal microbalance and cyclic voltammetry experiments. Both charge and mass changes during redox processes were rationalized in terms of multiple regression considering some solvent parameters and ionic transport characteristics. The electroactivity of PPY modified electrodes increase in the sequence PC<DMF<EtOH<MetOH<ACN<water. Also the mass gains in the sequence PC<water<EtOH<DMF<MetOH<ACN, showing clearly the influence of the solvent physico-chemical nature on the electroactivity and electroneutralization processes.