Polypyrrole/reduced graphene oxide coated fabric electrodes for supercapacitor application

Flexible and wearable energy storage devices are strongly demanded to power smart textiles. Herein, reduced graphene oxide (RGO) and polypyrrole (PPy) were deposited on cotton fabric via thermal reduction of GO and chemical polymerization of pyrrole to prepare textile-based electrodes for supercapacitor application. The obtained PPy–RGO-fabric retained good flexibility of textile and was highly conductive, with the conductivity of 1.2 S cm⁻¹. The PPy–RGO-fabric supercapacitor showed a specific capacitance of 336 F g⁻¹ and an energy density of 21.1 Wh kg⁻¹ at a current density of 0.6 mA cm⁻². The RGO sheets served as conductor and framework under the PPy layer, which could facilitate electron transfer between RGO and PPy and restrict the swelling and shrinking of PPy, thus resulting in improved electrochemical properties respect to the PPy-fabric device.     

Effect of aqueous electrolyte on pseudocapacitive behavior of chemically synthesized La2S3 electrode

Lanthanum sulfide electrode (La₂S₃) is prepared by a low cost, simple and room temperature chemical route for energy storage. The surface morphology of La₂S₃ film is revealed through field emission scanning electron microscopy. For the energy storage purpose, the pseudocapacitive behavior of La₂S₃ electrode is studied in 1 M aqueous Na₂SO₄ and 1 M KOH electrolytes. La₂S₃ electrode achieved maximum specific capacitance of 358 F g⁻¹ at 5 mV s⁻¹ scan rate with 78% electrochemical cyclic stability over 1000 cycles in 1 M Na₂SO₄ electrolyte. The galvanostatic charge–discharge study demonstrated the energy density of 35 Wh kg⁻¹ at power density of 1.26 kW kg⁻¹. The electrochemical impedance study showed field assisted charge transfer process with relaxation time of 32 ms in 1 M Na₂SO₄ electrolyte ensuring fast redox reaction.     

Nanostructured CuO/PANI composite as supercapacitor electrode material

An in-situ polymerization method has been employed to prepare CuO/PANI nanocomposite. The prepared samples have been characterized by X-ray diffraction (XRD), FTIR spectroscopy, field emission scanning electron microscopy (FESEM), and BET analysis. Application of the prepared samples has been evaluated as supercapacitor material in 1 M Na₂SO₄ solution using cyclic voltammetry (CV) in different potential scan rates, ranging from 5 to 100 mV s⁻¹, and electrochemical impedance spectroscopy (EIS). The specific capacitance of CuO/PANI has been calculated to be as high as 185 F g⁻¹, much higher than that obtained for pure CuO nanoparticles (76 F g⁻¹). Moreover, the composite material has shown better rate capability (75% capacitance retention) in various scan rates in comparison with the pure oxide (30% retention). EIS results show that the composite material benefits from much lower charge transfer resistance, compared to CuO nanoparticles. Moreover, much better cyclic performance has been achieved for the composite material.     

Nanostructured nickel oxide ultrafine nanoparticles: Synthesis,characterization, and supercapacitive behavior

Well-dispersed NiO nanoparticles were prepared via cathodic electrodeposition followed by a heat-treatment method. The supercapacitive performance of the prepared nanoparticles was analyzed by means of cyclic voltammetry (CV) and galvanostatic charge–discharge tests at −0.2–0.5 V potential windows in 1 M KOH. The nanoparticles exhibited high specific capacitance (1623.1 F g⁻¹ at the scan rate of 5 mV s⁻¹) and good long-term cycling stability (9.6% capacity decay after 1000 cycling at the current density of 2 A g⁻¹).     

Synthesis of NiO nanospheres with ultrasonic method for supercapacitors

NiO nanospheres have been synthesized by using magnetic stirring and ultrasound methods. The structure and morphology of synthesized samples were characterized by X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. The results show the NiO nanospheres prepared by the ultrasonic method are far less than that prepared by the stirring method. Moreover, in the presence of surfactant, the average diameter of NiO nanospheres prepared by the ultrasonic method is only 1–3 nm. The capacitance of NiO electrodes was investigated with cyclic voltammetry. The NiO prepared by the ultrasonic method exhibited higher specific capacitance of ~260 F g⁻¹ at 1 A g⁻¹ current density.     

Porous network of samarium sulfide thin films for supercapacitive application

In the present letter, a novel aqueous chemical method is employed to prepare thin film of Sm₂S₃ material containing porous network of interconnected nanoparticles for supercapacitive application. The orthorhombic phase formation of Sm₂S₃ film is concluded from X–ray diffraction study. The chemical states of samarium and sulfur are determined using X–ray photoelectron spectroscopy study. The pseudocapacitive behavior of Sm₂S₃ showed a maximum specific capacitance of 248 F g⁻¹ in 1.5 M LiClO₄ electrolyte prepared in propylene carbonate electrolyte. The nature of charge and discharge curves confirmed pseudocapacitive behavior of film electrode. The highest power and energy densities of 15.6 kWh kg⁻¹ and 54.6 Wh kg⁻¹, respectively are obtained. An electrochemical stability of 94% is retained after 1500 cycles.     

Electrochemical investigation of graphene/cerium oxide nanoparticles as an electrode material for supercapacitors

Mechanisms of charge storage, stability, capacitance, morphology and response current of graphene/cerium oxide (CeO₂) nanoparticles as an electrode material for electrochemical capacitors have been investigated. Electrochemical properties of the assembled electrodes were studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in 3 M NaCl, NaOH and KOH electrolytes. Scanning electron microscopy (SEM) is used to characterize the microstructure and the nature of prepared electrodes. SEM images confirm the layered structure (12 nm thickness) of the used graphene. The proposed electrode shows a maximum specific capacitance as high as 11.09 F g⁻¹ in the potential range between −0.55 and 0.3 (V vs. SCE) at scan rate of 5 mV s⁻¹. The charge/discharge cycling test shows a good reversibility and confirms that capacitance will increase after 500 cycles by 37%.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

Na₂MnPO₄F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 2₁/n) with the lattice constants: a=13.7132 Å, b=5.3461 Å, c=13.7079 Å, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O²⁻: 2p → Mn²⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) indicates that the valence band derives mainly from O²⁻: 2p orbital with a small admixture of F⁻ character while the conduction band is made up of Mn²⁺: t_2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na₂MnPO₄F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h⁻¹. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O²⁻, F⁻) to the Mn²⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Synthesis of samarium telluride thin films by successive ionic layer adsorption and reaction (SILAR) method for supercapacitor application

The present paper deals with synthesis of samarium telluride (Sm₂Te₃) thin films using simple and low cost successive ionic layer adsorption and reaction (SILAR) method for supercapacitor application. The Sm₂Te₃ thin films are characterized by X-ray diffraction (XRD) for structural determination, energy dispersive analysis of X-ray (EDAX) for elemental composition, field emission scanning electron microscopy (FE-SEM) for surface morphological study and contact angle measurement for wettability study. The Sm₂Te₃ exhibits orthorhombic crystal structure with cloud like surface morphology. The film surface showed lyophilic behavior with contact angle of 5.7° for propylene carbonate (PC). Further, electrochemical measurements are carried out in LiClO₄–PC electrolyte using cyclic voltammetry (CV), galvanostatic charge discharge and electrochemical impedance spectroscopy (EIS) techniques. The Sm₂Te₃ film showed maximum specific capacitance and energy density of 144 F g⁻¹ and 10 W h kg⁻¹ respectively. The EIS study showed negligible change in resistive parameters after 1000 electrochemical cycles.     

Deposition of Fe3O4 on oxidized activated carbon by hydrazine reducing method for high performance supercapacitor

Oxidized activated carbon/Fe₃O₄ (AC/Fe₃O₄) composites for supercapacitor electrodes were synthesized by a reduction method. Poly(vinylpyrrolidone) was added as a dispersing agent for homogeneous deposition of Fe₃O₄ on AC. The obtained products were identified as AC/Fe₃O₄ by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. Morphological characterization of AC/Fe₃O₄ was carried out by field emission scanning electron microscopy (FE-SEM); the results clearly showed the formation of Fe₃O₄ nanoparticles about 30 nm in diameter on AC. Moreover, by using N₂ adsorption/desorption isotherm analysis, we confirmed that surface areas and pore volumes decreased with increasing Fe₃O₄ content. We also carried out electrochemical characterization of AC and AC/Fe₃O₄ composites. Remarkably, we found that the value of specific capacitance increased significantly from 99.4 F g⁻¹ of raw AC to 202.6 F g⁻¹ of AC/Fe₃O₄ composites at 10 mV s⁻¹ of scan rate. This result can be ascribed to a synergistic effect of the combination of electrical double-layer capacitance and pseudo-capacitance properties. This research represents a valuable contribution to the application of supercapacitor electrodes in regard to cost effectiveness and simple fabrication.     

Effect of interface traps for ultra-thin high-k gate dielectric based MIS devices on the capacitance-voltage characteristics

The impact of states at the Al₂O₃/Si interface on the capacitance-voltage C-V characteristics of a metal/insulator/semiconductor heterostructure (MIS) capacitor was studied by a numerical simulation, by solving Schrodinger-Poisson equations and taking the electron emission rate from the interface state into account. Efficient computation and accurate physics based capacitance model of MOS devices with advanced ultra-thin equivalent oxide thickness (EOT) (down to 2.5 nm clearly considered here) were introduced for the near future integrated circuit IC technology nodes. Due to the importance of the interface state density for a low dimension and very low oxide thickness, a high frequency C-V model has been developed to interpret the effect of interface state density traps which communicate with the Al₂O₃/Si and their influence on the C-V characteristics. We found that these states are manifested by jumping capacity in the inversion zone, for a density of interface, higher than 1 × 10¹¹ cm^− 2 eV^− 1 during a p-doping of 1 × 10¹⁸ cm^− 3. This behavior has been investigated with various doping, temperature, frequency and energy levels on the C-V curves, and compared with the MIS structure that contains a standard SiO₂ insulator.     

Preparation and characterization of bifunctional BiOClxIy solid solutions with excellent adsorption and photocatalytic abilities for removal of organic dyes

A series of BiOCl_xI_y solid solutions with bifunctional properties was prepared by a hydrolysis method. When used as potential adsorbents for removal of dyes in wastewater, the as prepared samples exhibited the excellent adsorption and photocatalytic abilities, which indicated that the removal ability had been restored completely without centrifugal separating or other chemical desorption methods. Especially to the sample 301 prepared with a 3:1 M ratio of Cl to I, its adsorption capacity could reach nearly 5.0 mg g⁻¹ within 5 min, which was at least 3 times larger than that of the individual BiOX (ca. 1.3 mg g⁻¹), and its degradation efficiency of high concentration (20 mg L⁻¹) methyl orange (MO) was up to 100% after 50 min of visible light irradiation. The relevant characterization results revealed that not only high specific surface area but also the electron structure of interlayer could be the key factors for the high adsorption ability. The possible mechanism was also discussed.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Preparation and electrochemical capacitance of MnO2 thin films doped by CuBi2O4

Manganese dioxide (MnO₂) and CuBi₂O₄-doped MnO₂ thin films with different nanostructures were deposited on indium tin oxide (ITO) glass and Ti foil substrates by using a chemical bath deposition (CBD) technique. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron microscopy (XPS). The effects of doping and substrates on electrochemical properties of MnO₂ and CuBi₂O₄-doped MnO₂ thin films on ITO glass and Ti foil were investigated. Capacitive properties of MnO₂ and CuBi₂O₄-doped MnO₂ thin films electrodes were studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode experimental setup using 0.1 M Na₂SO₄ aqueous solution as electrolyte. Specific capacitance, obtained from electrochemical measurement for the CuBi₂O₄-doped MnO₂, exhibited a higher value of 338 F g⁻¹ compared to the MnO₂ exhibiting value of 135 F g⁻¹. In addition, CuBi₂O₄-doped MnO₂ thin films on an ITO electrode had a better and satisfactory specific capacitance value, and exhibited more excellent electrochemical stability and reversibility than Ti foil substrates.     

Synthesis and characterization of the semiconductor Li0.93Cu0.07Nb3O8: Application to oxygen evolution under visible light

The semiconductor Li_0.93Cu_0.07Nb₃O₈ is prepared by soft chemistry in aqueous electrolyte via Cu²⁺ → Li⁺ exchange between copper nitrate and LiNb₃O₈. The substituted niobate crystallizes in an orthorhombic symmetry and the semiconducting and photo-electrochemical properties are investigated for the first time. The oxide exhibits a dark brown color and the UV–Visible spectroscopy gives an optical gap of 1.42 eV, due to the crystal field splitting of Cu²⁺ in octahedral site. The thermal variation of the conductivity shows that Nb: 4d-electrons are localized and the data are fitted by a small-polaron hopping model σ = σ_o exp {−0.053 eV/kT} with a carrier density thermally activated. The capacitance measurement done in ionic electrolyte (Na₂SO₄, 10⁻² M) indicates n type semiconductor with mixed valences Nb^5+/4+, due to the hetero-valent substitution Li⁺/Cu²⁺, with a flat band potential of 0.28 V_SCE and electrons density of 2.17×10¹⁷ cm⁻³. The Nyquist diagram shows mainly the bulk contribution with a diffusion process. The valence band (6.39 eV below vacuum) derives from O^2-: 2p orbital with a small admixture of Cu²⁺: 3d character while the conduction band is made up of Nb⁵⁺: 4d orbital. The material is successfully tested for the oxygen generation with an evolution rate of 87 µmol mn⁻¹ g⁻¹ under visible light (29 mW cm⁻²) and a quantum yield of 0.35%.     

Iron mediated cathodic electrosynthesis of hausmannite nanoparticles

A novel synthetic route has been proposed to prepare hausmannite nanoparticles. The synthetic route comprises an iron mediated constant current cathodic electrodeposition of manganite and heat treatment of the latter to obtain hausmannite. The obtained nanostructures have been characterized using X-ray Diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and Fourier transform Infrared Spectrometry (FTIR). The role of iron in the formation of manganite precursor has been studied by cyclic voltammetry (CV) and differential thermal analysis (DTA). A formation mechanism based on iron mediated formation of Mn³⁺ and subsequent cathodic reduction of the disproportionated products has been proposed accordingly. The prepared nanoparticles exhibited specific capacitance of 143 F g⁻¹ in 0.5 M Na₂SO₄ solution. The retained specific capacity was 87% after 2000 cycles.     

Interface states in polyfluorene-based metal–insulator–semiconductor devices

Hybrid metal–insulator–semiconductor structures based on ethyl-hexyl substituted polyfluorene (PF2/6) as the active polymer semiconductor were fabricated on a highly doped p-Si substrate with Al₂O₃ as the insulating oxide layer. We present detailed frequency-dependent capacitance–voltage (C–V) and conductance–voltage characteristics of the semiconductor/insulator interface. PF2/6 undergoes a transition to an ordered crystalline phase upon thermal cycling from its nematic-liquid crystalline phase, confirmed by our atomic force microscope images. Thermal cycling of the PF2/6 films significantly improves the quality of the (PF2/6)/Al₂O₃ interface, which is identified as a reduced hysteresis in the C–V curve and a decreased interface state density (D_it) from ∼3.9 × 10¹² eV⁻¹ cm⁻² to ∼3.3 × 10¹¹ eV⁻¹ cm⁻² at the flat-band voltage. Interface states give rise to energy levels that are confined to the polymer/insulator interface. A conductance loss peak, observed due to the capture and emission of carriers by the interface states, fits very well with a single time constant model from which the D_it values are inferred.     

Electrical and optical properties of Cu2ZnSnS4 grown by a thermal co-evaporation method and its diode application

Cu₂ZnSnS₄ (CZTS) is low cost and constitutes non-toxic materials abundant in the earth crust. Environment friendly solar cell absorber layers were fabricated by a thermal co-evaporation technique. Elemental composition of the film was stated by energy dispersive spectroscopy (EDS). Some optical and electrical properties such as absorption of light, absorption coefficient, optical band gap charge carrier density, sheet resistance and mobility were extracted. Optical band gap was found to be as 1.44 eV, besides, charge carrier density, resistivity and mobility were found as 2.14×10¹⁹ cm⁻³, 8.41×10⁻⁴ Ω cm and 3.45×10² cm² V⁻¹ s⁻¹, respectively. In this study Ag/CZTS/n-Si Schottky diode was fabricated and basic diode parameters including barrier height, ideality factor, and series resistance were concluded using current–voltage and capacitance–voltage measurements. Barrier height and ideality factor values were found from the measurements as 0.81 eV and 4.76, respectively, for Ag/CZTS/n-Si contact.     

Preparation and physical properties of the layered niobate Cu0.5Nb3O8: Application to photocatalytic hydrogen evolution

The new layered niobate Cu_0.5Nb₃O₈ is synthesized by soft chemistry in aqueous electrolyte via Cu²⁺→H⁺ exchange between copper nitrate and HNb₃O₈·H₂O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 V_SCE, an electronic density of 8.72×10¹⁹ m⁻³ and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu²⁺: 3d with a small admixture of O²⁻: 2p orbitals whereas the conduction band consists of empty Nb⁵⁺: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm⁻²) with a rate evolution of 0.31 mL g⁻¹ min⁻¹.     

Improved reliability of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) with high density silicon nitride passivation

We have systematically studied the effects of Si_xN_1 − x passivation density on the reliability of AlGaN/GaN high electron mobility transistors. Upon stressing, devices degrade in two stages, fast-mode degradation and followed by slow-mode degradation. Both degradations can be explained as different stages of pit formation at the gate-edge. Fast-mode degradation is caused by pre-existing oxygen at the Si_xN_1 − x/AlGaN interface. It is not significantly affected by the Si_xN_1 − x density. On the other hand, slow-mode degradation is associated with Si_xN_1 − x degradation. Si_xN_1 − x degrades through electric-field induced oxidation in discrete locations along the gate-edges. The size of these degraded locations ranged from 100 to 300 nm from the gate edge. There are about 16 degraded locations per 100 μm gate-width. In each degraded location, low density nano-globes are formed within the Si_xN_1 − x. Because of the low density of the degraded locations, oxygen can diffuse through these areas and oxidize the AlGaN/GaN to form pits. This slow-mode degradation can be minimized by using high density (ρ = 2.48 g/cm³) Si₃₆N₆₄ as the passivation layer. For slow-mode degradation, the median time to failure of devices with high density passivation is found to increase up to 2× as compared to the low density (ρ = 2.25 g/cm³) Si₄₃N₅₇ passivation. A model based on Johnson-Mehl-Avrami theory is proposed to explain the kinetics of pit formation.