Nickel, cobalt, and manganese oxide composite as an electrode material for electrochemical supercapacitors

A composite material, Ni_1/3Co_1/3Mn_1/3(OH)₂, is synthesized by chemical precipitation method for supercapacitors' electrode material. Physical characterizations using x-ray diffraction, energy-dispersive x-ray, and scanning electron microscopy show that Ni_1/3Co_1/3Mn_1/3(OH)₂ possesses an amorphous structure and higher specific surface area (268.5 m²?g^?1), which lead to a high initial specific capacitance of 1,403 F?g^?1 in the potential window of 0–1.5 V. It may be a potential electrode material for future supercapacitor when its cycling stability and rate performance are addressed.     

Preparation and characterization of nano cobalt oxide

A simple and general method has been proposed for preparing strong violet emitting CdS quantum dots, in which a ligand exchange strategy was applied to surface passivation and functionalization with good reproducibility. The resulting quantum dots showed a visible violet luminescence with emission peak centered near 423 nm and photoluminescence quantum yields reached over 30%. Additionally, different mercapto-compounds used as ligands can make different functionalized surfaces, favoring quantum dots dispersion in different media and their further applications. It was observed that the band edge emission has the main contribution to the bright violet luminescence.     

Preparation and electrochemical investigation of the cobalt hydroxide carbonate/activated carbon nanocomposite for supercapacitor applications

Cobalt hydroxide carbonate/activated carbon (AC) composite was successfully synthesized by hydrothermal method. Morphological characterizations of cobalt hydroxide carbonate/AC composite were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results show that the cobalt hydroxide carbonate nanorods are well dispersed on the AC. Due to the synergistic effects arising from cobalt hydroxide carbonate nanorods and AC, the electrochemical performances of pure cobalt hydroxide carbonate material is significantly improved by the addition of AC. The composite shows a specific capacitance of 301.44 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH electrolyte and exhibits good cycling stability. Based on the above results, the cobalt hydroxide carbonate/AC composite shows a considerable promise as electrode for electrochemical applications.     

Preparation and electrochemical performance of novel ruthenium-manganese oxide electrode materials for electrochemical capacitors

A series of novel ruthenium-manganese oxide (denoted as Ru_nMn_1−nO_x) has been formed by oxidative co-precipitating. The precursor was obtained by mixing Mn(VII) (potassium permanganate), Mn(II) (manganese acetate) and Ru(III) (ruthenium chloride) in neutral aqueous solution at room temperature. The powder of Ru_nMn_1−nO_x was obtained by calcinating the precursor at appropriate temperature. The crystalline structure and electrochemical performance of the powder have been studied as a function of the calcination temperature. At appropriate calcination temperature (e.g. 170 °C), the powder is in hydrous amorphous phase with a high specific capacitance. When the calcination temperature reaches up to 350 °C, the crystal form of α-MnO₂ is formed, but the ruthenium oxide still keeps amorphous structure, which will lead to the decrease of specific capacitance of the composite electrode materials. The X-ray photoelectron spectroscopy (XPS) analysis shows that the powder of Ru_nMn_1−nO_x prepared in this study belongs to the composite of RuO₂-MnO₂. The results from cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) indicate that the ruthenium weight density of 9 wt% in Ru_nMn_1−nO_x can improve the cost-performance of ruthenium-manganese composite electrode.     

Preparation and electrochemical characterization of cobalt-manganese oxide as electrode materials for electrochemical capacitors

Cobalt-manganese oxide materials (CMOs) were prepared by chemical method and heat treated at 150, 400, 600, 800 and 1000 °C, respectively. The physical and electrochemical properties of the materials were characterized. The heat treatment process leads to the removal of water molecules adsorbed on the surface of CMO particles (below 400 °C) and the progressive reduction of Mn and Co ions from Mn⁴⁺ and Co³⁺ to Mn³⁺/Mn²⁺ and Co²⁺, respectively (440-1000 °C). CMOs obtained by treatment below 800 °C have poor crystallinity and a highly crystallized tetragonal phase by treatment at 1000 °C. The ratio of Mn and Co in CMOs is found by EDX analysis to be about 2:1. The electrochemical testing results indicate that the high crystallization of CMO is disadvantageous for the energy storage as electrode material of electrochemical capacitors. However, for CMOs with poor crystallinity, relatively high specific capacitances can be obtained. The incorporation of protons and ions into the CMO's lattice during electrochemical charge/discharge process leads to the distortion of crystal lattice and improvement of crystallinity of CMO. The XRD patterns show that negative electrode (NE) and positive electrode (PE) have tetragonal (Co, Mn)(Mn, Co)₂O₄ phase.     

Mechanochemical-synthesized Al-doped manganese dioxides for electrochemical supercapacitors

Al-doped MnO₂ as electrode materials for supercapacitor were synthesized by high-energy ball milling. The morphologies and structures of prepared MnO₂ were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Electrochemical investigation indicated that doped MnO₂ presented preferable electrochemical performances than un-doped MnO₂, but there were obvious capacitance decline in the first several dozen of cycles for all doped MnO₂ samples. The Al_0.05/Mn_0.95O₂ electrode, especially, showed the largest capacitance among all prepared MnO₂ samples. Excellent conductivity of Al in doped MnO₂ was considered to be responsible for enhanced electrochemical performances of doped MnO₂.     

Preparation and electrochemical performances of submicro-TiP2O7 cathode for lithium ion batteries

The TiP₂O₇ with a cubic 3?×?3?×?3 superstructure was synthesized by a liquid-assisted solid-state reaction, and characterized by x-ray diffraction, scanning electron microscopy, cyclic voltammogram, galvanostatic charge/discharge testing, and electrochemical impedance spectroscopy (EIS) technique. The results showed that there was only one step of intercalation into TiP₂O₇, corresponding to a pair redox (E _Li/Li ⁺?=?2.74/2.48 V). The initial discharge capacity of TiP₂O₇ was 110 mAh/g at a current density of 15 mA/g, and the capacity retention was 76.12 % of the initial discharge capacity after 100 cycles. The EIS of TiP₂O₇ electrode consisted of two semicircles in organic electrolyte, which was attributed to SEI resistance as well as the contact resistance, and charge transfer process, respectively. A suitable model was proposed to explain the impedance response of the insertion TiP₂O₇ material of lithium ion batteries.     

Structural and electrochemical effects of cobalt doping on lithium manganese oxide spinel

Pure LiMn₂O₄ and lithium manganese oxide spinels with partial replacement of manganese by cobalt up to 20 mole%, LiCo_xMn_2−xO₄, were prepared. The effect of extended cycling on the crystal structure was investigated. A capacity decrease with increasing cobalt content was observed in the potential range about 4100 mV vs. Li/Li⁺. Cycling behavior is significantly improved, compared to LiMn₂O₄. LiCo_xMn_2−xO₄ is discharged in a single phase reaction in the upper potential range around 4100 mV vs. Li/Li⁺, whereas pure LiMn₂O₄ shows a two phase behavior. LiMn₂O₄ shows a significant broadening of peaks in plots of differential capacity and change in shape of the voltage profile upon extended cycling. LiCo_xMn_2−xO₄ shows neither broadening nor change. Voltage profiles and plots of the differential capacity differ significantly compared to spinels with lithium substitution, Li_1+xMn_2−xO₄. In contrast to Li_1+xMn_2-xO₄, LiCo_xMn_2-xO₄ is discharged in a two step process in the range of 0 ≤ × ≤ 0,5. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Preparation, characterization, and electrochemical performances of carbon-coated TiO2 anatase

Several carbon-coated anatase TiO₂ samples have been prepared by impregnation with lactose as carbon precursor and annealed at different temperatures (650 and 700 °C). X-ray diffraction was carried out to study the anatase to rutile phase transition and to evaluate the crystallite size. Scanning electron microscopy was utilized to evaluate the morphology, and transmission electron microscopy was used to show the distribution and nature of the carbon coating. The galvanostatic measurements and cyclic voltammetry revealed better kinetics, cycling stability, and high rate capacity for the carbon-coated materials when compared with the noncoated ones.     

MnO2/graphite electrodeposited under supergravity field for supercapacitors and its electrochemical properties

MnO₂/graphite electrode material is successfully synthesized by electrodeposition under supergravity field from manganese acetate and graphite suspending solution. X-ray diffraction and field emission scanning electron microscopy show that the obtained composite is γ-MnO₂/graphite. The process of depositing the MnO₂/graphite was shown by the schematic illustration. Galvanostatic charge/discharge and cyclic voltammograms tests are applied to investigate electrochemical performances of the composite electrodes prepared under supergravity fields. MnO₂/graphite synthesized under supergravity field exhibits good discharge capacitance and the specific capacitance is 367.77 F g^?1 at current density of 0.5 A g^?1. It is found that supergravity field has effects on the electrochemical performances of MnO₂/graphite material.     

Preparation,characterization, and electrochemical performances of graphene/Ni(OH)2 hybrid nanomaterials

We report a class of core–shell nanomaterials that can be used as efficient surface-enhancement Raman scattering (SERS) substrates. The core consists of silver nanowires, prepared through a chemical reduction process, that are used to capture 4-mercaptobenzoic acid (4-MBA), a model analyte. The shell was prepared through a modified Stöber method and consists of patchy or full silica coats. The formation of silica coats was monitored via transmission electron microscopy, UV–visible spectroscopy, and phase-analysis light-scattering for measuring effective surface charge. Surprisingly, the patchy silica-coated silver nanowires are better SERS substrate than silver nanowires; nanomolar concentration of 4-MBA can be detected. In addition, “nano-matryoshka” configurations were used to quantitate/explore the effect of the electromagnetic field at the tips of the nanowire (“hot spots”) in the Raman scattering experiment.     

Inter-diffusion of cobalt and silicon through an ultra thin aluminum oxide layer

Optical emission spectroscopy of sputtered species during ion bombardment, Auger electron spectroscopy and high-resolution transmission electron microscopy were used to study the cobalt and silicon diffusion through the interfaces of Co/AlO/Si(0 0 1) hetero-structure. The results are discussed as a function of the annealing temperature of sample and show that the diffusion process at the interfaces starts for annealing temperatures above 200 °C without detectable modification of the oxide layer.     

Honeycomb-like manganese oxide nanospheres for redox supercapacitors

Honeycomb-like MnO₂ nanospheres were synthesized using stainless steel substrates by a facile chemical bath deposition method. The obtained nanospheres were about 200–400 nm in diameter and consisted of porous ultrathin nanosheets. Honeycomb-like MnO₂ nanospheres exhibited a high specific capacitance of 240 F g^?1 and 87.1% capacitance retention after 1000 cycles at a current density of 0.5 A g^?1. These remarkable electrochemical results imply great potential for applications of the honeycomb-like MnO₂ nanospheres in supercapacitors.     

Effect of calcination temperatures on the electrochemical performances of nickel oxide/reduction graphene oxide (NiO/RGO) composites synthesized by hydrothermal method

A series of NiO/RGO composites based on NiO nanoparticles anchored on layered RGO surfaces were proposed by the same hydrothermal method combined with different calcination temperatures (250, 300, 400 and 500 °C). The effects of calcination temperatures on the capacitive behaviors have been discussed by investigating the components, morphologies, surface conditions of the NiO/RGO composites. The specific capacitance values of NiO/RGO composites calcined at 250, 300, 400 and 500 °C are 950, 553, 375 and 205 F/g at the current density of 1 A/g and the corresponding capacitance retention are 91.3%, 83.9%, 71.9% and 67.3% after 1000 cycles at the current density of 10 A/g. The results suggest the calcination temperature plays an important role in the electrochemical performances of NiO/RGO composites and the electrochemical performances were deteriorated with the increasing calcination temperatures.     

Preparation of microporous activated carbon and its electrochemical performance for electric double layer capacitor

Microporous activated carbons were prepared by microwave heating petroleum coke with potassium hydroxide as activation agent. Microporous activated carbons were characterized by infrared spectroscopy, X-ray diffraction and nitrogen adsorption/desorption isotherms. Electrochemical properties of an electric double layer capacitor using microporous activated carbon as electrode materials were investigated by constant current charge-discharge and electrochemical impedance spectroscopic techniques. The results show that the specific surface area and the pore volume of microporous activated carbon increase with increasing activation time before the activation time reaches 37 min. The microporous volume totals 94.0% in the microporous activated carbons and the average pore diameter of microporous activated carbon is 2.00 nm. Microporous activated carbons prepared in the activation time of 31, 35 and 37 min are named as AC-31, AC-35 and AC-37, respectively. Compared with AC-27 electrode, the internal resistance for ions transferring in AC-31, AC-35 or AC-37 electrode is relatively small. The specific capacitance of AC-31 is the biggest among the microporous activated carbons, and it retains 279.6 F g^?1 maintaining 93.5% capacity after 200 recycling number.     

Nanostructured transition metal oxides for aqueous hybrid electrochemical supercapacitors

In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors. More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides and the cell voltage of the supercapacitor. It is shown that the cell voltage of an electrochemical supercapacitor can be increased significantly with the use of hybrid systems. The most relevant associations are Fe₃O₄ or activated carbon as the negative electrode and MnO₂ as the positive. The cell voltage of the Fe₃O₄/MnO₂ device is 1.8 V and this value was increased to 2.2 V by using activated carbon instead of Fe₃O₄. These two systems have shown superior behavior compared to a symmetric MnO₂/MnO₂ device which only works within a 1 V potential window in aqueous K₂SO₄. Furthermore, the activated carbon/MnO₂ hybrid device exhibits a real power density of 605 W/kg (maximum power density =19.0 kW/kg) with an energy density of 17.3 Wh/kg. These values compete well with those of standard electrochemical double layer capacitors working in organic electrolytes. PACS 82.47.Uv; 82.45.Fk; 82.45.Yz     

Electrochemical characterization on layered lithium ruthenate for electrochemical supercapacitors

Electrochemical characteristics of lithium ruthenate (Li_xRuO_2+0.5x·nH₂O) for electrochemical capacitors' electrode material were first examined in this paper by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. Results show that Li_xRuO_2+0.5x·nH₂O has electrochemical capacitive characteristic within the potential range of − 0.2–0.9 V (vs. SCE) in 1 M Li₂SO₄ solution. The capacitance mainly arises from pseudo-capacitance caused by lithium ions' insertion/extraction into/out of the Li_xRuO_2+0.5x·nH₂O electrode. The specific capacitance of 391 F g^− 1 can be delivered at 1 mA charge–discharge current for Li_xRuO_2+0.5x·nH₂O electrode with an energy density of 65.7 W h kg^− 1. This material also exhibits an excellent cycling performance and there is no attenuation of capacitance over 600 cycles.     

Nano-composite LiMnPO4 as new insertion electrode for electrochemical supercapacitors

Nano-composite olivine LiMnPO₄ (nC-LMP) was found to exhibit facile pseudo-capacitive characteristics in aqueous as well as non-aqueous electrolytes. We demonstrated employing nC-LMP as positive electrode in hybrid electrochemical capacitors namely Li-Ion hybrid capacitors (LIC). Adapting a simple CVD technique, nano-crystallites of LiMnPO₄ were coated with carbon monolayers of ∼2 nm thick to circumvent its poor intrinsic electronic conductivity. The novelty is that the single crystallites were intimately covered with carbon ring and networked to the neighboring crystallites via the continuous carbon wire-like connectivity as revealed from HRTEM analysis. Single electrode faradic capacitance of 3025 Fg⁻¹ (versus standard calomel reference electrode) was deduced for carbon coated LMP, the highest reported hitherto in Li⁺ aqueous electrolytes. Employing nC-LMP as working electrode versus an activated carbon (AC), we obtained a high specific energy of 28.8 Wh kg⁻¹ with appreciable stability in aqueous electrolytes whereas in nonaqueous electrolyte there is an obvious increase in energy density (35 Wh kg⁻¹) due to wider potential window. That is, a full cell version of LIC, AC|Li⁺|LMP, was fabricated and demonstrated its facile cycling characteristics via removal/insertion of Li⁺ within nC-LMP (positive electrode) and the electrosorption of Li⁺ into mesoporous carbon (AC) (negative electrode). Such cells ensured a typical battery-like charging and EDLC-like discharging characteristics of LIC type electrochemical capacitors (ECs) which are desired to enhance safety and energy densities.     

Electrode surface treatment and electrochemical impedance spectroscopy study on carbon/carbon supercapacitors

Power improvement in supercapacitors is mainly related to lowering the internal impedance. The real part of the impedance at a given frequency is called ESR (equivalent series resistance). Several contributions are included in the ESR: the electrolyte resistance (including the separator), the active material resistance (with both ionic and electronic parts) and the active material/current collector interface resistance. The first two contributions have been intensively described and studied by many authors. The first part of this paper is focused on the use of surface treatments as a way to decrease the active material/current collector impedance. Al current collector foils have been treated following a two-step procedure: electrochemical etching and sol-gel coating by a highly-covering, conducting carbonaceous material. It aims to increase the Al foil/active material surface contact leading to lower resistance. In a second part, carbon-carbon supercapacitor impedance is discussed in term of complex capacitance and complex power from electrochemical impedance spectroscopy data. This representation permits extraction of a relaxation time constant that provides important information on supercapacitor behaviour. The influence of carbon nanotubes addition on electrochemical performance of carbon/carbon supercapacitors has also been studied by electrochemical impedance spectroscopy. PACS 82.45.Yz; 81.16.-c     

Preparation of cobalt oxide thin films and its use in supercapacitor application

Present work explored a room temperature, simple and low cost chemical route for the cobalt oxide film onto copper substrate from cobalt chloride (CoCl₂·6H₂O) precursor and characterization for its structural and electrochemical properties for supercapacitor application. The morphology and crystal structure of the film were investigated by scanning electron microscopy and X-ray diffraction techniques, respectively. The electrochemical supercapacitive properties of cobalt oxide film were evaluated using cyclic voltammetry and galvanostatic charge-discharge methods. The film showed maximum specific capacitance of (165 F/g) in 1.0 M aqueous KOH electrolyte at scan rate 10 mV/s.