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).     

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.     

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 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.     

Characterization of RuO2·xH2O with various water contents

Hydrous ruthenium oxides (RuO₂·xH₂O) with different contents of water (x) were prepared by annealing commercial RuO₂·2.6H₂O powders at different temperatures. The morphologies and crystalline structures of RuO₂·xH₂O were investigated using transmission electron microscope (TEM) and selected area electron diffraction (SAED) techniques. From the TEM images, it was observed that the particle size of RuO₂·xH₂O increased with increasing annealing temperature. From the SAED patterns, it was observed that RuO₂·xH₂O powders became an amorphous phase at annealing temperatures <116 °C and became a crystalline phase at annealing temperatures above 116 °C. Amorphous RuO₂·xH₂O prepared at 116 °C reached its maximum specific capacitance as a result of proton insertion into the bulk of RuO₂ but with smaller Ru–Ru distance in the local structure. The more disordered structure induced by proton insertion was obtained by SAED pattern from a sample annealed at 116 °C. The possible connection between the microstructure and specific capacitance of RuO₂·xH₂O is discussed.     

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.     

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.     

Low-temperature synthesis of Na2Mn5O10 for supercapacitor applications

The romanechite-like sodium manganese oxide Na₂Mn₅O₁₀ is synthesized through alkaline hydrolysis of [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄] followed by thermal calcination. Amorphous Na₂Mn₅O₁₀ is obtained at relatively low temperature (200 °C). Increasing the calcination temperature leads to highly crystalline nano-rods. Electrochemical studies demonstrate that Na₂Mn₅O₁₀ is a good candidate as positive electrode materials for supercapacitor: specific capacitances of 178, 173 and 175 F g⁻¹ are obtained for Na₂Mn₅O₁₀ calcined at different temperatures (200, 400 and 600 °C), respectively, by charge-discharge tests at 0.1 A g⁻¹. Moreover, capacitance losses of all the products in 1000 cycles are less than 3%.     

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.     

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.     

Biomolecule-assisted synthesis of cobalt sulfide nanowires for application in supercapacitors

A biomolecule-assisted hydrothermal process is developed to synthesize cobalt sulfide (CoS), in which l-cysteine is used as the sulfide source and directing molecule. By controlling the synthesis conditions, CoS nanospheres and nanowires can be assembled. The as-synthesized samples are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to study the effects of microstructure and morphology of the samples on their capacitance and conductivity. A specific capacitance, as high as 508 F g⁻¹, is achieved for CoS nanowires. This is very competitive with the best supercapacitor material, RuO₂ (720–760 F g⁻¹), but its cost is remarkably lower than RuO₂. Thus the nanowires are a promising material for low-cost, high-performance supercapacitors. This method could provide a universal green chemistry approach to synthesize other metal sulfides.     

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.     

Effects of the cationic structures of fluorohydrogenate ionic liquid electrolytes on the electric double layer capacitance

Electric double layer capacitance of an activated carbon electrode has been measured for fluorohydrogenate ionic liquids (FHILs) based on five different cations (1,3-dimethylimidazolium (DMIm⁺), 1-ethyl-3-methylimidazolium (EMIm⁺), 1-butyl-3-methylimidazolium (BMIm⁺), 1-ethyl-1-methylpyrrolidinium (EMPyr⁺), and 1-methoxymethyl-1-methylpyrrolidinium (MOMMPyr⁺)) at 25 °C. For all the FHILs, the capacitance increases with increase in charging voltage, and exhibits the maximum value around 2.7 V. The capacitances for FHILs are higher than those for EMImBF₄ or 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) in the measured range (1.0 < V < 3.2). For the three imidazolium-based FHILs, the maximum capacitance decreases with increase in the size of the cation in the order, DMIm(FH)_2.3F (178 F g⁻¹) > EMIm(FH)_2.3F (162 F g⁻¹) > BMIm(FH)_2.3F (135 F g⁻¹). On the other hand, the maximum capacitance observed for MOMMPyr(FH)_2.3F (152 F g⁻¹) is larger than that for EMPyr(FH)_2.3F (134 F g⁻¹) in spite of the larger size of MOMMPyr⁺ than EMPyr⁺, which is derived from introduction of the methoxy group. Some FHILs with low melting points exhibit a sufficient capacitance even at −40 °C (64 F g⁻¹ for EMIm(FH)_2.3F).     

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).     

Synthesis and capacitive property of hierarchical hollow manganese oxide nanospheres with large specific surface area

The hierarchical hollow manganese oxide nanospheres with both a large surface area and a layered structure have been successfully prepared by a templating-assisted hydrothermal process at 150 °C for 48 h. SiO₂ template spheres are dispersed in KMnO₄ solution, and then followed by hydrothermal treatment to forming silica/manganese oxide nanospheres with a core-shell structure. The core-shell nanospheres are etched in a NaOH solution (20 wt.%), so that the SiO₂ core is removed, and the hierarchical hollow manganese oxide nanospheres are obtained. The as-synthesized hierarchical hollow manganese oxide nanospheres present a birnessite-type manganese oxide phase with a chemical composition of Na_0.38MnO_2.14·13H₂O, and a specific surface area of 253 m² g⁻¹. The prepared materials exhibit an ideal capacitive behavior and good cycling stability in a neutral electrolyte system and the initial capacitance value is 299 F g⁻¹. Some preparation conditions including the hydrothermal temperature, dwell time and concentration of template have been also investigated.     

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.