Preparation and optimization of RuO2-impregnated SnO2 xerogel supercapacitor

A Sb (6 mol%)-doped SnO₂ xerogel impregnated with RuO₂ nanocrystallites is prepared via an incipient-wetness method and is optimized for its electrochemical capacitance in aqueous 1 M KOH electrolyte by adjusting the calcination temperature and the RuO₂ loading. The electrode capacitance does not increase monotonically with increasing RuO₂ loading. A maximum electrode capacitance of 15 F g⁻¹, which represents a three-fold increase compared with the blank xerogel and a specific RuO₂ capacitance of 710 F g⁻¹ RuO₂, is obtained with a RuO₂ loading of 1.4 wt.% and by calcination at 200 °C. Higher loadings presumably result in a homogeneous nucleation upon drying, which causes severe reduction in the total surface area of the RuO₂ crystallites.     

Nanocrystalline tin oxides and nickel oxide film anodes for Li-ion batteries

Tin oxides and nickel oxide thin film anodes have been fabricated for the first time by vacuum thermal evaporation of metallic tin or nickel, and subsequent thermal oxidation in air or oxygen ambient. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements showed that the prepared films are of nanocrystalline structure with the average particle size <100 nm. The electrochemical properties of these film electrodes were examined by galvanostatic cycling measurements and cyclic voltammetry. The composition and electrochemical properties of SnO_x (1<x<2) films strongly depend on the oxidation temperature. The reversible capacities of SnO and SnO₂ films electrodes reached 825 and 760 mAh g⁻¹, respectively, at the current density of 10 μA cm⁻² between 0.10 and 1.30 V. The SnO_x film fabricated at an oxidation temperature of 600 °C exhibited better electrochemical performance than SnO or SnO₂ film electrode. Nanocrystalline NiO thin film prepared at a temperature of 600 °C can deliver a reversible capacity of 680 mAh g⁻¹ at 10 μA cm⁻² in the voltage range 0.01–3.0 V and good cyclability up to 100 cycles.     

Electrochemical behavior of Sn/SnO2 mixtures for use as anode in lithium rechargeable batteries

Anodes derived from oxides of tin have, of late, been of considerable interest because, in principle, they can store over twice as much lithium as graphite. A nanometric matrix of Li₂O generated in situ by the electrochemical reduction of SnO₂ can provide a facile environment for the reversible alloying of lithium with tin to a maximum stoichiometry of Li_4.4Sn. However, the generation of the matrix leads to a high first-cycle irreversible capacity. With a view to increasing the reversible capacity as well as to reduce the irreversible capacity and capacity fade upon cycling, tin–tin oxide mixtures were investigated. SnO₂, synthesized by a chemical precipitation method, was mixed with tin powder at two compositions, viz., 1:2 and 2:1, ball-milled and subjected to cycling studies. A mixture of composition Sn:SnO₂ = 1:2 exhibited a specific capacity of 549 mAh g⁻¹ (13% higher than that for SnO₂) with an irreversible capacity, which was 7% lower than that for SnO₂ and a capacity fade of 1.4 mAh g⁻¹ cycle⁻¹. Electrodes with this composition also exhibited a coulombic efficiency of 99% in the 40 cycles. It appears that a matrix in which tin can be distributed without aggregation is essential for realizing tin oxide anodes with high cyclability.     

Manganese oxide film electrodes prepared by electrostatic spray deposition for electrochemical capacitors from the KMnO4 solution

Manganese oxide film electrodes for electrochemical capacitors were deposited on the polished Pt foils by electrostatic spray deposition (ESD) from KMnO₄ precursor solution. The electrochemical properties of electrodes were systematically studied using cyclic voltammetry (CV), constant current charge–discharge tests, and electrochemical impedance spectroscopy (EIS). The specific capacitance (SC) of thick deposited film was 149 F g⁻¹ at the very high scan rate of 500 mV s⁻¹, in comparison with 209 F g⁻¹ at the low scan rate of 5 mV s⁻¹. The electrode shows good cyclic performance. The initial SC value was 163 F g⁻¹ and 103% of the initial SC can be retained after 10,000 cycles at the scan rate of 50 mV s⁻¹.     

In situ polyol-assisted synthesis of nano-SnO2/carbon composite materials as anodes for lithium-ion batteries

Nano-SnO₂/carbon composite materials were synthesized in situ using the polyol method by oxidizing SnCl₂·2H₂O in the presence of a carbon matrix. All the as-synthesized composites consisted of SnO₂ nanoparticles (5–10 nm) uniformly embedded into the carbon matrix as evidenced by TEM. XRD confirmed the presence of nano-sized SnO₂ particles that are crystallized in a rutile structure and XPS revealed a tin oxidation state of +4. Cyclic voltammetry of the composites showed an irreversible peak at 1.4 V in the first cycle and a typical alloying/de-alloying process at 0.1–0.5 V. The best composite (“composite I”, 15 wt% SnO₂) showed an improved lithium storage capacity of 370 mAh g^?1 at 200 mA g^?1 (～C/2) which correspond to 32% improvement and lower capacity fade compared to commercial SnO₂ (50 nm). We have also investigated the effect of the heating method and we found that the use of a microwave was beneficial in not only shortening reaction time but also in producing smaller SnO₂ particles that are also better dispersed within the carbon matrix which also resulted in higher lithium storage capacity.     

Conductivity percolation in carbon–carbon supercapacitor electrodes

Composite electrodes which comprise a non-conductive activated carbon of large surface area (1420 m² g⁻¹) and a conductive carbon black (CB) of small surface area (220 m² g⁻¹) have been prepared and studied for their capacitive properties in aqueous KOH and Na₂SO₄ electrolytes. For either electrolyte, maximum capacitance exists at the composition believed to correspond to the percolation threshold for CB, the conductive phase. At a CB content less than the threshold, the capacitance is limited mainly by the electronic resistance on the electrode side. The interfacial surface area becomes the limiting factor as the threshold is exceeded. A maximum capacitance of 108 F g⁻¹ at a voltage sweep rate of 20 mV s⁻¹ is obtained in 1 M KOH aqueous electrolyte with a CB content of 25 wt.% (or ∼14 vol.%).     

Tin and tin-based intermetallics as new anode materials for lithium-ion cells

The galvanostatic cycling behaviour of Sn/SnSb composite electrodes has been studied in 1 mol l⁻¹ LiClO₄/propylene carbonate (PC), 1 mol l⁻¹ LiPF₆/ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1), and 1 mol l⁻¹ LiClO₄/PC saturated with trans-decalin (t-Dec). Capacities between 500 and 600 mA h g⁻¹ (with respect to the mass of active material) were obtained. Reasons for the irreversible capacities are given and film formation on lithium storage metals and alloys is discussed. The observed coulombic efficiencies were slightly higher for the EC-containing electrolyte than for the PC-based one. Alternatively, improved efficiencies and stand-time behaviour were obtained when the PC electrolyte was saturated with t-Dec, which acts as a surfactant.     

Enhanced electrochemical stability of all-polymer redox supercapacitors with modified polypyrrole electrodes

Redox supercapacitors are attracting increasing attention as high power electrochemical sources and can either be coupled with batteries to provide peak power or replace batteries for memory back-up. In the present work, all-polymer solid-state supercapacitors with LiClO₄ and LiCF₃SO₃ doped polypyrrole electrodes and P(VDF-HFP)-PMMA based polymer gel electrolyte are fabricated. The polypyrrole electrodes are irradiated with 160 MeV Ni¹²⁺ ions at 5 × 10¹⁰, 5 × 10¹¹ and 5 × 10¹² ions cm⁻². A comparative study is made between unirradiated and irradiated supercapacitors with polypyrrole-based electrodes. An average capacitance of about 200 F gm⁻¹ is obtained. On successive charging and discharging, the capacitance decreases for supercapacitors with unirradiated electrodes but remains stable when irradiated electrodes are used. In addition, the capacitance is slightly decreased compared with that for unirradiated electrodes. Charge–discharge studies show a decrease in total charge–discharge time for supercapacitors with irradiated electrodes. The capacitance values calculated from cyclic voltammograms are higher than those determined from charge–discharge plots due to the added contribution of a leakage current. The coulombic efficiency of all the supercapacitors is about 90%.     

The effects of different additives in electrolyte of AGM batteries on self-discharge

Hydrogen and oxygen evolution at the negative and positive electrodes in AGM batteries are the main reasons of self-discharging. The self-discharge of five AGM batteries was investigated by measuring different potential between two electrodes during 48 days. Five different battery electrolytes were used including 35% (w/w) H₂SO₄ without additives and the remaining contain 7.1, 9.94, and 21.3 g l⁻¹ sodium sulfate, 4 g l⁻¹ boric acid, 3 g l⁻¹ citric acid, and finally 0.7 and 1 g l⁻¹ stearic acid except one containing boric acid that the concentration of H₂SO₄ was 36% (w/w). The results revealed that the rate of self-discharge for battery without additive was 0.01 V day⁻¹. The battery with boric acid showed the lowest rate of self-discharge with 0.0025 V day⁻¹. It was also found that stearic and citric acids are comparatively appropriate additives for decreasing the self-discharge. They caused a decrease of the self-discharge rate to 0.005 and 0.0075 V day⁻¹ on appropriate concentration, respectively. In compared to other additives, sodium sulfate showed to be not an appropriate additive for decreasing battery self-discharging. The rate of 0.03 V day⁻¹ of self-discharging was obtained for the battery containing all selected concentration of sodium sulfate during first 4 days of measuring.     

Improvement in the electrochemical performance of LixV2O5 induced by Tb doping

We have studied the electrochemical behavior of Tb_0.11V₂O₅ electrodes in propylene carbonate and LiClO₄. The techniques used in the study were cyclic voltammetry, discharge/charge polarization, FTIR and XRD. The Tb_0.11V₂O₅ compound was prepared using the xerogel route and the composite electrodes were membranes prepared with PVDF-binder and carbon. The results show that the Tb doping improves the electrochemical performance of V₂O₅ due to the bonding of the earth-rare ions and the residual H₂O in the V₂O₅ structure. The capacity fading was reduced, the specific reversible capacity was 330 mA g⁻¹ (C/4, cut-off 3.7–2.0 V) and the voltage presented a large plateau at 2.5 V for the Tb_0.11V₂O₅ electrodes. The charge transfer between Li and Tb_0.11V₂O₅ does not involve the oxidation–reduction of the Tb³⁺ ions.     

Electrodeposited PtRu on cryogel carbon–Nafion supports for DMFC anodes

For a high catalytic activity of the anodes in DMFC at low noble metal content a fine dispersion of PtRu on carbon supports is required and to this purpose we prepared and investigated high specific surface area cryogel carbons of controlled mesoporosity. Two carbons CC1 and CC2 with pore-size distribution centered at 6 and 20 nm were obtained by sol–gel polycondensation of resorcinol and formaldehyde, followed by a freeze-drying procedure, a versatile and low-cost method to provide after pyrolysis carbons of controlled porosity. Electrodeposited PtRu on CC2-Nafion support with ca. 100 μg cm⁻² of Pt displayed a good catalytic activity for methanol oxidation of 85 mA mg⁻¹ of Pt after 600 s at 492 mV versus NHE and 60 °C in H₂SO₄ 0.1 M–CH₃OH 0.5 M when the Pt-to-C mass ratio was ca. 10%. The catalytic activity tests and XRD and SEM analyses demonstrated the stability of the prepared electrodes upon catalysis in the time scale of our measurements. Strategies to further increase the catalytic activity of the PtRu/cryogel carbon–Nafion electrodes for methanol oxidation are discussed.     

Evaluation of nafion based double layer capacitors by electrochemical impedance spectroscopy

In this work some electrochemical characteristics of all solid double layer capacitors prepared by high surface carbon and Nafion polymer electrolyte are reported. Carbon composite electrodes with a Nafion loading of 30 wt.% were prepared and evaluated. Nafion 115 membrane, recast Nafion membrane and 1 M H₂SO₄ solution in a matrix of glass fiber have been used as electrolyte, in the double layer capacitors. The different double layer capacitors (DLCs) have been evaluated by electrochemical impedance spectroscopy. The capacitor with a recast Nafion electrolyte exhibits a proton conductivity of about 3×10⁻² S cm⁻¹ at ambient temperature, that is higher of that reported for solid electrolytes (10⁻³ to 10⁻⁴ S cm⁻¹) in the current literature on capacitors. A maximum of specific capacitance of 13 F/g of active materials (carbon+Nafion) corresponding to 52 F/g for a single electrode measured in a three-electrode arrangement has been achieved with the capacitor with recast Nafion. The capacitance of the capacitor with recast Nafion electrolyte, evaluated in low-frequency region below 10 mHz, was practically equivalent at that with sulphuric acid electrolyte. The interpretation of the characteristics of the microporous structure of carbon material of the electrodes by impedance analysis is also discussed.     

Effect of organic acids and nano-sized ceramic doping on PEO-based solid polymer electrolytes

Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO₄, organic acids (malonic, maleic, and carboxylic acids), and/or Al₂O₃ were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi_0.5Co_0.5O₂ electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)₈LiClO₄:citric acid (99.95:0.05, w/w%) was 3.25 × 10⁻⁴ S cm⁻¹ and 1.81 × 10⁻⁴ S cm⁻¹ at 30 °C. The conductivity has further improved to 3.81 × 10⁻⁴ S cm⁻¹ at 20 °C by adding 20 w/w% Al₂O₃ filler to the (PEO)₈LiClO₄ + 0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7 V versus Li/Li⁺ for CSPE at room temperature.     

Cresol–formaldehyde based carbon aerogel as electrode material for electrochemical capacitor

Carbon aerogels have been prepared through a polycondensation of cresol (Cm) with formaldehyde (F) and an ambient pressure drying followed by carbonization at 900 °C. Modification of the porous structures of the carbon aerogel can be achieved by CO₂ activation at various temperatures (800, 850, 900 °C) for 1–3 h. This process could be considered as an alternative economic route to the classic RF gels synthesis. The obtained carbon aerogels have been attempted as electrode materials in electric double-layer capacitors. The relevant electrochemical behaviors were characterized by constant current charge–discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy in an electrolyte of 30% KOH aqueous solution. The results indicate that a mass specific capacitance of up to 78 F g⁻¹ for the non-activated aerogel can be obtained at current density 1 mA cm⁻². CO₂ activation can effectively improve the specific capacitance of the carbon aerogel. After CO₂ activation performed at 900 °C for 2 h, the specific capacitance increases to 146 F g⁻¹ at the same current. Only a slight decrease in capacitance, from 146 to 131 F g⁻¹, was observed when the current density increases from 1 to 20 mA cm⁻², indicating a stable electrochemical property of carbon aerogel electrodes in 30% KOH aqueous electrolyte with various currents.     

CuFe2O4/SnO2 nanocomposites as anodes for Li-ion batteries

Ultrafine powders of nanocrystalline CuFe₂O₄ and CuFe₂O₄/10 wt.% SnO₂ nanocomposites are prepared by a urea–nitrate combustion method. Phase pure and highly crystalline CuFe₂O₄ (tetragonal structure) and CuFe₂O₄/SnO₂ (cubic structure) are obtained after sintering at 1100 °C. The average particle size is 10–20 and 20–30 nm, respectively. Both the nanoferrite anodes have an excellent specific capacity of greater than 800 mAh g⁻¹ versus Li metal. It is concluded that SnO₂ doping improves the coulombic efficiency of copper ferrite anodes from 65 to 99.5% via an enhanced structural stability.     

Performance of supercapacitor with electrodeposited ruthenium oxide film electrodes—effect of film thickness

Thin-film ruthenium oxide electrodes are prepared by cathodic electrodeposition on a titanium substrate. Different deposition periods are used to obtain different film thicknesses. The electrodes are used to form a supercapacitor with a 0.5 M H₂SO₄ electrolyte. The specific capacitance and charge–discharge periods are found to be dependent on the electrode thickness. A maximum specific capacitance of 788 F g⁻¹ is achieved with an electrode thickness of 0.0014 g cm⁻². These results are explained by considering the morphological changes that take place with increasing film thickness.     

Nano-structured spherical porous SnO2 anodes for lithium-ion batteries

Spherical porous tin oxide was fabricated via a spray pyrolysis technique. TEM revealed that the primary SnO₂ crystals had an average size of about 5 nm. The electrochemical measurements showed that the spherical porous SnO₂ samples have excellent cyclability, which can deliver a reversible capacity of 410 mAh g⁻¹ up to 50 cycles as a negative electrode for lithium batteries. The second step of the study was to thermal treat the initial tin oxide for 3 h at 600, 800, 1000 and 1200 °C, respectively, in order to identify the particle size effect on the electrochemical performance toward lithium. It was found that the morphologies of these spherical clusters could be maintained even after thermal treatment at 1200 °C. It was also proved that finer the size of the tin oxide particles the better the electrochemical performance.     

Cu5Si–Si/C composites for lithium-ion battery anodes

Cu₅Si–Si/C composites with precursor atomic ratio of Si:Cu = 1, 2 and 4.5 have been produced by high-energy ball-milling of a mixture of copper–silicon alloy and graphite powder for anode materials of lithium-ion battery. X-ray diffraction and scanning electron microscope measurements show that Cu₅Si alloy is formed after the intensive ball milling and alloy particles along with low-crystallite Si are interspersed in graphite uniformly. Cu₅Si–Si/C composite electrodes deliver a larger reversible capacity than commercialized graphite and better cyclability than silicon. The increase of copper amount in the composites decreases reversible capacity but improves cycling performance. Cu₅Si–Si/C composite with Si:Cu = 1 demonstrates an initial reversible capacity of 612 mAh g⁻¹ at 0.2 mA cm⁻² in the voltage range from 0.02 to 1.5 V. The capacity retention is respectively 74.5 and 70.0% at the 40th cycle at the current density of 0.2 and 1 mA cm⁻².     

The capacitive characteristics of supercapacitors consisting of activated carbon fabric–polyaniline composites in NaNO3

A sandwich-type supercapacitor consisting of two similar activated carbon fabric–polyaniline (ACF–PANI) composite electrodes was demonstrated to exhibit excellent performance (i.e., highly reversibility and good stability) in NaNO₃. Polyaniline with the charge density of polymerization less than or equal to 9 C cm⁻² synthesized by means of a potentiostatic method showed a high specific capacitance of 300 F g⁻¹. Influences of the polymerization charge density (i.e., the polymer loading) on the capacitive characteristics of ACF–PANI composites were compared systematically. The capacity of an ACF–PANI electrode reach ca. 3.4 F cm⁻² (a 100% increase in total capacity) when the charge density of polymerization is equal to 9 C cm⁻². The surface morphology of these ACF–PANI composites was examined by a scanning electron microscope (SEM).     

Comparative studies of nickel oxide films on different substrates for electrochemical supercapacitors

Thin nickel oxide (NiO) films were obtained by post-heating of the corresponding precursor films of nickel hydroxide (Ni(OH)₂) cathodically deposited onto different substrates, i.e., nickel foils, and graphite at 25 °C from a bath containing 1.5 mol L⁻¹ Ni(NO₃)₂ and 0.1 mol L⁻¹ NaNO₃ in a solvent of 50% (v/v) ethanol. The surface morphology of the obtained films was observed by scanning electron microscope (SEM). Electrochemical characterization was performed using cyclic voltammetrty (CV), chronopotentiometry (CP) and electrochemical impedance analysis (EIS). When heated at 300 °C for 2 h in air, the specific capacitance of the prepared NiO films on nickel foils and graphite, with a deposition charge of 250 mC cm⁻², were 135, 195 F g⁻¹, respectively. When the deposition charge is less than 280 mC cm⁻², the capacitance of both appears to keep the linear relationship with the deposition charge. The specific capacitance, cyclic stability of the NiO/graphite hybrid electrodes in 1 mol L⁻¹ KOH solution were superior to those on nickel foils mainly due to the favorable adhesion, the good interface behavior between graphite and the NiO films, and the extra pseudo-capacitance of the heated graphite substrates.