Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes

Composites made from RuO₂·xH₂O particles supported on carbon nanofibres (CNF) have been prepared for supercapacitor electrodes. CNF, produced by Grupo Antolin Ing. SA. using a floating catalyst procedure was treated either in HCl or in HNO₃. Then the composites were obtained by impregnation of CNF with an aqueous RuCl₃·0.5H₂O solution followed by filtering and alkali solution treatment. Heat treatment at 150 °C for 2 h was done. Specific capacitance of the composites has been measured and discussed on the basis of their RuO₂·xH₂O content and RuO₂·xH₂O particle size. The composites having RuO₂·xH₂O contents below 11 wt% show RuO₂·xH₂O particles, which grow from 2 to 4 nm as the RuO₂·xH₂O content increases. The specific capacitance of supported RuO₂·xH₂O, which can be very high (up to 840 F g⁻¹), decreases as the RuO₂·xH₂O content increases and RuO₂·xH₂O particles grow. The composites having RuO₂·xH₂O contents above 11 wt% show RuO₂·xH₂O particles of nearly constant size (4 nm); the effect of increasing the RuO₂·xH₂O content is to increase the amount of particles but not the size of the particles. In these composites the specific capacitance of supported RuO₂·xH₂O is nearly constant (440 F g⁻¹) and close to bare RuO₂·xH₂O (460 F g⁻¹).     

Solid state synthesis of hydrous ruthenium oxide for supercapacitors

A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO₂·xH₂O). The procedure involves directly mixing RuCl₂·xH₂O with alkali to form RuO₂·xH₂O in a mortar at room temperature. Transmission electron microscopy (TEM) and N₂ adsorption–desorption measurement indicate that the RuO₂·xH₂O particle is approximately 30–40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO₂·xH₂O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO₂·xH₂O powder was found in an amorphous phase and the maximum capacitance of 655 F g⁻¹ was obtained by annealing at 150 °C. Higher temperatures (exceeding 175 °C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 F g⁻¹ at 175 °C to 87 F g⁻¹ at 400 °C. Also, the dependence of electrochemical performance on annealing conditions of RuO₂·xH₂O was investigated by electrical impedance spectroscopy (EIS) study.     

Tailoring of RuO2 nanoparticles by microwave assisted “Instant method” for energy storage applications

RuO₂ nanoparticles are synthesized by Instant method using Li₂CO₃ as stabilizing agent, under microwave irradiation at 60 °C and investigated for the anodic oxygen evolution reaction (OER) and for their supercapacitance properties in 0.5 M H₂SO₄ medium. Structural and morphological characterizations of RuO₂ are investigated by in situ X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDS) and Raman spectroscopy. The TEM images of as prepared material show the uniform distribution of RuO₂ nanoparticles with mean diameter of ca. 1.5 nm. Analysis on as prepared material indicates the structural formula as [RuO₂·2.6H₂O] 0.7H₂O with low crystallinity. The influence of annealing temperature on RuO₂ is studied in light of electrocatalytic activity for oxygen evolution reaction (OER) and capacitance. Electrochemical performances of RuO₂ electrodes are followed by current-potential curves, galvanostatic charge-discharge cycles and evolved oxygen measurements. The amount of oxygen gas evolved during the OER by the crystalline RuO₂ is found to be consistent with the electrical energy supplied to the catalyst. The cyclic voltammogram of RuO₂ exhibits the typical capacitance behavior with highly reversible nature. The specific capacitance of hydrous RuO₂ is found to be 737 F g⁻¹ at the scan rate of 2 mV s⁻¹, by the balanced transport of proton through the structural water and electron transport along dioxo bridges, which makes a suitable material for energy storage. The specific capacitance decreases with increase in the crystallinity of RuO₂. The present study shows the potential method to synthesize rapid and uniform nano particles of RuO₂ for water electrolysis and supercapacitors.     

Microwave-assisted hydrothermal synthesis of crystalline WO3-WO3·0.5H2O mixtures for pseudocapacitors of the asymmetric type

Crystalline tungsten oxide mixtures, WO₃-WO₃·0.5H₂O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180 °C for various periods, show capacitive-like behavior at 200 mV s⁻¹ and C_S ≈ 290 F g⁻¹ at 25 mV s⁻¹ in 0.5 M H₂SO₄ between −0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO₃·0.5H₂O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO₃ and WO₃·0.5H₂O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400 °C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO₃-WO₃·0.5H₂O anode and a RuO₂·xH₂O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg⁻¹ and 5.2 kW kg⁻¹, respectively.     

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.     

Binderless fabrication of amorphous RuO2 electrode for electrochemical capacitor using spark plasma sintering technique

The spark plasma sintering (SPS) technique was successfully used to mold a hydrous amorphous RuO₂electrode without any additives and binders. At the cyclic voltammetry (CV) scan rate of 1 mV s⁻¹, the electrochemical capacitances of the RuO₂ electrodes are 600-700 F g⁻¹ for the entire electrode. An increase in the SPS current during the compaction led to the crystallization and dehydration of RuO₂, which in turn, resulted in a significant decrease in its capacitance. There is room to improve the rate properties as we observed a steep drop in the capacitance when the CV scan rate was raised.     

Influence of the mesoporous structure on capacitance of the RuO2 electrode

The difference in capacitive performance between high and low surface area RuO₂ electrodes, synthesized with and without a mesoporous silica template, respectively, was investigated in aqueous solutions of sulfuric acid and sulfates by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). RuO₂ synthesized with the template was crystalline and the formation of the mesoporous structure with a 6.5 nm diameter was confirmed using a transmission electron microscope and the nitrogen adsorption and desorption isotherm. From the CV at the scan rate of 1 mV s⁻¹, the specific capacitance of the high surface area electrode in H₂SO₄(aq) was determined to be 200 F g⁻¹. The high surface area RuO₂ has a three times higher BET specific surface area (140 m² g⁻¹) than the low surface area sample (39 m² g⁻¹). Introducing the mesoporous structure was proved effective for increasing the capacitance per mass of the RuO₂, though not all the surface functions as a capacitor. Both the CV and EIS suggest that by increasing the charging rate or frequency, the mesoporous structure of the electrode leads to a lower capacitance decrease (higher capacitance retention) than the low surface area electrode. The EIS also indicates that the response time of the capacitor is hardly influenced by the presence of the mesoporous structure.     

Anodic deposition of porous RuO2 on stainless steel for supercapacitor studies at high current densities

Ruthenium dioxide is deposited on stainless steel (SS) substrate by galvanostatic oxidation of Ru³⁺. At high current densities employed for this purpose, there is oxidation of water to oxygen, which occurs in parallel with Ru³⁺ oxidation. The oxygen evolution consumes a major portion of the charge. The oxygen evolution generates a high porosity to RuO₂ films, which is evident from scanning electron microscopy studies. RuO₂ is identified by X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostatic charge–discharge cycling studies indicate that RuO₂/SS electrodes possess good capacitance properties. Specific capacitance of 276 F g⁻¹ is obtained at current densities as high as 20 mA cm⁻² (13.33 A g⁻¹). Porous nature of RuO₂ facilitates passing of high currents during charge–discharge cycling. RuO₂/SS electrodes are thus useful for high power supercapacitor applications.     

Electrochemical performance of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2–0.8) cathode on a ScSZ electrolyte for intermediate temperature SOFCs

Intermediate temperature solid oxide fuel cell cathode materials (Ba, Sr)Co_xFe_1−xO_3−δ [x = 0.2–0.8] (BSCF), were synthesized by a glycine-nitrate process (GNP) using Ba(NO₃)₂, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O as starting materials and glycine as an oxidizer and fuel. Electrolyte-supported symmetric BSCF/GDC/ScSZ/GDC/BSCF cells consisting of porous BSCF electrodes, a GDC buffer layer, and a ScSZ electrolyte were fabricated by a screen printing technique, and the electrochemical performance of the BSCF cathode was investigated at intermediate temperatures (500–700 °C) using AC impedance spectroscopy. Crystallization behavior was found to depend on the pH value of the precursor solution. A highly acidic precursor solution increased the single phase perovskite formation temperature. In the case of using a precursor solution with pH 2, a single perovskite phase was obtained at 1000 °C. The thermal expansion coefficient of BSCF was gradually increased from 24 × 10⁻⁶ K⁻¹ for BSCF (x = 0.2) to 31 × 10⁻⁶ K⁻¹ (400–1000 °C) for BSCF (x = 0.8), which resulted in peeling-off of the cathode from the GDC/ScSZ electrolyte. Only the BSCF (x = 0.2) cathode showed good adhesion to the GDC/ScSZ electrolyte and low polarization resistance. The area specific resistance (ASR) of the BSCF (x = 0.2) cathode was 0.183 Ω cm² at 600 °C. The ASR of other BSCF (x = 0.4, 0.6, and 0.8) cathodes, however, was much higher than that of BSCF (x = 0.2).     

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 properties of graphene nanosheets/polyaniline nanofibers composites as electrode for supercapacitors

Graphene nanosheets/polyaniline nanofibers (GNS/PANI) composites are synthesized via in situ polymerization of aniline monomer in HClO₄ solution. The PANI nanofibers homogeneously coating on the surface of GNS greatly improve the charge transfer reaction. The GNS/PANI composites exhibit better electrochemical performances than the pure individual components. A remarkable specific capacitance of 1130 F g⁻¹ (based on GNS/PANI composites) is obtained at a scan rate of 5 mV s⁻¹ in 1 M H₂SO₄ solution compared to 402 F g⁻¹ for pure PANI and 270 F g⁻¹ for GNS. The excellent performance is not only due to the GNS which can provide good electrical conductivity and high specific surface area, but also associate with a good redox activity of ordered PANI nanofibers. Moreover, the GNS/PANI composites present excellent long cycle life with 87% specific capacitance retained after 1000 charge/discharge processes. The resulting composites are promising electrode materials for high-performance electrical energy storage devices.     

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.     

Supercapacitive behaviors and their temperature dependence of sol–gel synthesized nanostructured manganese dioxide in lithium hydroxide electrolyte

In the present work, a nanostructured manganese dioxide material was synthesized by a sol–gel method starting with manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, and characterized by X-ray diffraction, infrared spectroscopic and transmission electron microscope techniques. The electrochemical properties and the influence of temperature on supercapacitive behaviors of the nano-MnO₂ electrode in 1 M LiOH electrolyte were investigated using electrochemical methods. Experimental results show that the MnO₂ electrode can exhibit an excellent pseudocapacitive behavior in 1 M LiOH electrolyte, and a high specific capacitance of 317 F g⁻¹ can be obtained at a charge/discharge current rate of 100 mA g⁻¹ and at the temperature of 25 °C. We found that temperature has a crucial influence on the discharge specific capacitance of the electrode. The specific capacitance at 25 °C is higher than that at 15 or 35 °C.     

The preparation and performance of calcium carbide-derived carbon/polyaniline composite electrode material for supercapacitors

Calcium carbide (CaC₂)-derived carbon (CCDC)/polyaniline (PANI) composite materials are prepared by in situ chemical oxidation polymerization of an aniline solution containing well-dispersed CCDC. The structure and morphology of CCDC/PANI composite are characterized by Fourier infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM) and N₂ sorption isotherms. It has been found that PANI was uniformly deposited on the surface and the inner pores of CCDC. The supercapacitive behaviors of the CCDC/PANI composite materials are investigated with cyclic voltammetry (CV), galvanostatic charge/discharge and cycle life measurements. The results show that the CCDC/PANI composite electrodes have higher specific capacitances than the as grown CCDC electrodes and higher stability than the conducting polymers. The capacitance of CCDC/PANI composite electrode is as high as 713.4 F g⁻¹ measured by cyclic voltammetry at 1 mV s⁻¹. Besides, the capacitance retention of coin supercapacitor remained 80.1% after 1000 cycles.     

Electrodeposition of mesoporous manganese dioxide supercapacitor electrodes through self-assembled triblock copolymer templates

Mesoporous manganese dioxide supercapcitor electrode materials were electrochemically deposited onto silicon substrates coated with Pt using triblock copolymer species (Pluronic P123 and F127) as the structure-directing agents. Deposited electrodes of manganese dioxide film were physically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and were electrochemically characterized by cyclic voltammetry (CV) in 0.5 M Na₂SO₄ electrolyte. Maximum specific capacitance (SC) values of 449 F g⁻¹ was obtained at a scan rate of 10 mV s⁻¹ from F127 templated mesoporous MnO₂.     

Cathodic deposition and characterization of tin oxide coatings on graphite for electrochemical supercapacitors

Amorphous tin oxide (SnO_x) was cathodically deposited onto graphite electrode in a bath containing 0.1 M stannous chloride (SnCl₂), 0.5 M sodium nitrate (NaNO₃), and 0.4 M nitric acid (HNO₃) in an aqueous solution of 50% (v/v) ethanol. The SnO_x coatings grown on graphite were characterized as typical capacitive behaviors by cyclic voltammetry (CV), chronopotentiometric (CP) in 0.5 M KCl. Specific capacitance (in milli-farad per square centimeter, C_a) changes linearly with the deposition charge up to 4.5 C cm⁻², and a maximum of as high as 355 mF cm⁻² was obtained with the SnO_x coating grown at around 5 C cm⁻². For the SnO_x coating deposited at 0.2 C cm⁻², a maximum specific capacitance (in farad per gram, C_m) of 298 and 125 F g⁻¹ was achieved from CVs at a scan rate of 10, and 200 mV s⁻¹, respectively. The value of C_m significantly gets lower from 265 to around 95 F g⁻¹ when the deposition charge increases from 0.2 to around 6.0 C cm⁻². The long cycle-life and stability of the SnO_x coatings on graphite via the presented cathodic deposition were also demonstrated.     

Potentiostatically deposited nanostructured CoxNi1−x layered double hydroxides as electrode materials for redox-supercapacitors

Cobalt–nickel layered double hydroxides (Co_xNi_1−x LDHs) were deposited onto stainless steel electrodes by the potentiostatic deposition method at −1.0 V vs. Ag/AgCl using various molar ratios of Co(NO₃)₂ and Ni(NO₃)₂ in distilled water. Their structure and surface morphology were studied by using X-ray diffraction analysis, energy dispersive X-ray spectroscopy and scanning electron microscopy. A network of Co_xNi_1−x LDH nanosheets was obtained. The nature of the cyclic voltammetry and charge–discharge curves suggested that the Co_xNi_1−x LDHs exist in the form of solid solutions. The capacitive characteristics of the Co_xNi_1−x LDHs in 1 M KOH electrolyte showed that Co_0.72Ni_0.28 LDHs had the highest specific capacitance value, 2104 F g⁻¹, which is also the highest yet reported value for oxide materials in general.     

Effect of RuO2·xH2O in anode on the performance of direct methanol fuel cells

The effect of hydrous RuO₂ (RuO₂·xH₂O) in anode on the performance of direct methanol fuel cells (DMFCs) was examined by voltammetry, methanol stripping analysis, electrochemical impedance spectroscopy, polarization measurement and chronopotentiometry. The results showed that, compared with the DMFC with conventional structures, the dynamic response and quasi-steady state performance of the RuO₂·xH₂O-introduced DMFCs were significantly improved. The DMFC with RuO₂·xH₂O layer (ROL) sandwiched between anode catalyst layer and gas diffusion layer exhibited better quasi-steady state performance than those either with ROL sandwiched between anode catalyst layer and electrolyte membrane or with RuO₂·xH₂O uniformly distributed in anode catalyst layer. The maximum power density of the DMFC with this novel structure was 16% higher than the DMFC with the conventional structure. Moreover, the dynamic response of this RuO₂·xH₂O-introduced cell was more stable during 250-hour of operation when compared with that of the conventional cell.     

LiFePO4 with enhanced performance synthesized by a novel synthetic route

Pure LiFePO₄ was synthesized by heating an amorphous LiFePO₄. The amorphous LiFePO₄ obtained through lithiation of FePO₄·xH₂O by using oxalic acid as a novel reducing agent at room temperature. FePO₄·xH₂O was prepared through co-precipitation by employing FeSO₄·7H₂O and H₃PO₄ as raw materials. X-ray diffraction (XRD), scanning electron microscopy (SEM) observations showed that LiFePO₄ composites with fine particle sizes between 100 nm and 200 nm, and with homogenous sizes distribution. The electrochemical performance of LiFePO₄ powder synthesized at 500 °C were evaluated using coin cells by galvanostatic charge/discharge. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 166 mAh g⁻¹ at the 0.1C rate, and possessed a favorable capacity cycling maintenance at the 0.1C, 0.2C, 0.5C and 1C rate.     

Optimization of MnO2/vertically aligned carbon nanotube composite for supercapacitor application

The optimization strategy for producing manganese oxide supercapacitors based on vertically aligned carbon nanotubes (VACNTs) deposited on large area electrodes is presented. A single sequential process of sputtering, annealing and plasma enhanced chemical vapour deposition (PECVD) is applied to produce dense and uniform VACNTs electrodes. As dielectric layer of the supercapacitor, manganese oxide is electrodeposited lining the surface of the VACNTs electrodes. The control of the growing parameters such as catalyst thickness layer, temperature and deposition time for tuning the density, length and diameter of the VACNTs and their structure are found to be key points for the optimization of the MnO₂ electrodeposition process in view to improve the efficiency of the supercapacitor devices.The electrochemical properties of the obtained electrodes are characterized using cyclic voltammetry and galvanostatic charge-discharge techniques. A specific capacitance of 642 Fg⁻¹ is obtained for MnO₂/VACNTs nanocomposite electrode at a scan rate of 10 mV s⁻¹.