Charge storage performance of lithiated iron phosphate/activated carbon composite as symmetrical electrode for electrochemical capacitor

In this study, a symmetric electrochemical capacitor was fabricated by adopting a lithium iron phosphate (LiFePO₄)-activated carbon (AC) composite as the core electrode material in 1.0 M Na₂SO₃ and 1.0 M Li₂SO₄ aqueous electrolyte solutions. The composite electrodes were prepared via a facile mechanical mixing process. The structural properties of the nanocomposite electrodes were characterised by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis. The electrochemical performances of the prepared composite electrode were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy (EIS). The experimental results reveal that a maximum specific capacitance of 112.41 F/g was obtained a 40 wt% LiFePO₄ loading on an AC electrode compared with that of a pure AC electrode (76.24 F/g) in 1 M Na₂SO₃. The improvement in the capacitive performance of the 40 wt% LiFePO₄–AC composite electrode is believed to be attributed to the contribution of the synergistic effect of the electric double layer capacitance (EDLC) of the AC electrode and pseudocapacitance via the intercalation/extraction of H⁺, OH⁻, Na⁺ and SO₃²⁻ and Li⁺ ions in LiFePO₄ lattices. In contrast, it appears that the incorporation of LiFePO₄ into AC electrodes does not increase the charge storage capability when Li₂SO₄ is used as the electrolyte. This behaviour can be explained by the fact that the electrolyte system containing SO₄²⁻ only exhibits EDLC in the Fe-based electrodes. Additionally, Li⁺ ions that have lower conductivity and mobility may lead to poorer charge storage capability compared to Na⁺ ions. Overall, the results reveal that the AC composite electrodes with 40 wt% LiFePO₄ loading on a Na₂SO₃ neutral electrolyte exhibit high cycling stability and reversibility and thus display great potential for electrochemical capacitor applications.     

Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites

The electrochemical properties of composites prepared from an electrically conducting polymer poly(3,4-ethylenedioxythiophene), i.e. PEDOT and multiwalled carbon nanotubes (CNTs) have been investigated for supercapacitor application. The novel composite material was prepared by chemical or electrochemical polymerization of EDOT directly on the nanotubes or from a homogenous mixture of PEDOT and CNTs. Acetylene black (AB) has been also used as a composite component in order to evaluate whether nanotubes are giving improved properties or not. Electrodes prepared from such composites were used in supercapacitors operating in acidic (1 M H₂SO₄), alkaline (6M KOH) and organic (1 M TEABF₄ in AN) electrolytic solutions. The capacitance values were estimated by galvanostatic, voltammetry and impedance spectroscopy techniques with two- or three-electrode cell configuration. Due to the open mesoporous network of nanotubes, the easily accessible electrode/electrolyte interface allows quick charge propagation in the composite material and an efficient reversible storage of energy in PEDOT during subsequent charging/discharging cycles. The composites with AB supply quite good capacitance results, however, nanotubes as electrode component gave definitively a more homogenous dispersion of PEDOT that should give a better charge propagation. The values of capacitance for PEDOT/carbon composites ranged from 60 to 160 F/g and such material has a good cycling performance with a high stability in all the electrolytes. Organic medium is especially interesting because of higher energy stored. Another quite important advantage of this composite is its significant volumetric energy because of the high density of PEDOT.     

Influence of supporting electrolytes on the structure of electrodeposited SnO<Subscript>2</Subscript> thin films for energy storage applications

Different SnO₂ nanostructures (SnO₂Ns) were directly electrodeposited on the surface of anodized copper (Cu) substrates via the potentiostatic electrodeposition method with addition of supporting electrolytes. The effects of the supporting electrolytes and the electrodeposition parameters on the evolution of nanostructures and on the electrochemical properties of the SnO₂Ns were systematically investigated using field emission scanning electron microscope (FESEM) and electrochemical methods including cyclic voltammetry (CV) and chronoamperometry (CA). The results confirmed that SnO₂Ns exhibit alloying/de-alloying reactions with Li⁺ ions versus Ag/AgCl in aqueous electrolyte solution (LiOH·H₂O and Li₂CO₃). The super capacitor performance of the SnO₂Ns was investigated in 0.5-M Na₂SO₄ aqueous solution, and the highest specific capacitance of 110 Fg^?1 at a scan rate of 5 mV s^?1 was obtained for SnO₂ microspheres made up of nanocubes. Our study shows that supporting electrolytes and electrodeposition parameters play the significant role in the growth of SnO₂Ns and its electrochemical properties.     

Flexible polyethylene terephthalate (PET) electrodes based on single-walled carbon nanotubes (SWCNTs) for supercapacitor application

Flexible polyethylene terephthalate (PET) electrodes based on pristine single-walled carbon nanotubes (SWCNTs) and acid-treated single-walled carbon nanotubes (A-SWCNTs) were prepared by spray coating technique. Flexible A-SWCNTs electrodes showed enhanced electrochemical properties compared to the pristine SWCNTs electrodes. The electrochemical properties of the flexible A-SWCNTs electrodes were optimized with various types of aqueous electrolytes including sulfuric acid (H₂SO₄), sodium sulfate (Na₂SO₄), potassium chloride (KCl), sodium hydroxide (NaOH), and potassium hydroxide (KOH). The electrochemical performance of the A-SWCNTs electrodes as a function of bending to 30° were evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements in 1 M H₂SO₄. The specific capacitance value of the unbent A-SWCNTs electrode was 67 F g^?1, which decreased to 63 F g^?1 (94% retention) after 1000 GCD cycles. Interestingly, the specific capacitance of the unbent A-SWCNTs electrode with application of the 1000 GCD cycles was retained even after 500 bending to 30° with 6000 GCD cycles.     

Charge transport in activated carbon electrodes: the behaviour of three electrolytes vis-à-vis their specific conductance

In this study, the electrochemical performances of different aqueous electrolytes (6 M KOH, 2 M KCl and 0.5 M K₂SO₄) in activated carbon electrodes are evaluated with regard to their use in electrochemical double layer capacitor (EDLC). The results from cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were analysed. The lowest value of equivalent series resistance (ESR) and the highest values of specific capacitance and coulombic efficiency were observed, when KOH was the electrolyte. The impedance spectroscopy plots were fitted to an equivalent circuit of ladder type to evaluate the resistances to ion transport at different levels of hierarchies in the pore network. Also, the quality of the double layer capacitance at lower hierarchy that primarily contributes to the overall capacitance of the device was evaluated from the leakage resistance in the equivalent circuit. The fitted circuit parameters were further reviewed vis-à-vis the specific conductance of chosen electrolyte, and the number of successive charge–discharge cycles prior to the EIS measurements.     

Synthesis,structure, and electrochemistry of Ag-modified LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials for lithium-ion batteries

The composite of silver-modified lithium manganese oxide were prepared using thermal decomposition method of different mole ratio. Structural characterization was carried out by X-ray diffraction (XRD). XRD analysis revealed different patterns as the content of the dopant in the spinel increases. Phase analysis shows that Ag particles were dispersed on the LiMn₂O₄ surface instead of entering the spinel structure. On the other hand, the electrochemical behavior of cathode powder was examined by using two-electrode test cells consisting of a cathode, metallic lithium as anode, and a solid polymer electrolyte of 0.87PEO-0.13LiCF₃SO₃-0.10DBP. According to the electrochemical tests results, the influence of the Ag additive content on the electrochemical properties of Ag/LiMn₂O₄ composites is clearly shown.     

Photoelectrochemical properties of FTO/m-BiVO4 electrode in different electrolytes solutions under visible light irradiation

Photoelectrochemical properties of FTO/BiVO₄ electrode were investigated in different electrolytic solutions, potassium chloride (KCl) and sodium sulphate (Na₂SO₄), and under visible light irradiation condition. In order to accomplish that, an FTO/BiVO₄ electrode was built by combining the solution combustion synthesis technique with the dip-coating deposition process. The morphology and structure of the BiVO₄ electrode were investigated through X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photoelectrochemical properties were analyzed through chronoamperometry measurements. Results have shown that the FTO/BiVO₄ electrode presents higher electroactivity in the electrolyte Na₂SO₄, leading to better current stabilization, response time, and photoinduced current density, when compared to KCl electrolyte. Besides, this electrode shows excellent performance for methylene blue degradation under visible light irradiation condition. In Na₂SO₄, the electrode has shown higher degradation rate, 51 %, in contrast to 44 % in KCl, plus higher rate constant, 174?×?10^?4 min^?1 compared to 150?×?10^?4 min^?1 in KCl. Results presented in this communication leads to the indication of BiVO₄ thin films as alternate materials to use in heterogeneous photoelectrocatalysis, more specifically in decontamination of surface water.     

Electrochemical impedance spectroscopic studies on aging-dependent electrochemical degradation of p-toluene sulfonic acid-doped polypyrrole thin film

The conducting polymer polypyrrole thin film was galvanostatically polymerized on stainless steel substrate for the supercapacitor electrode. The electrochemical stability of the electrode was monitored each 30 days of aging up to 90 days. The FTIR analysis showed an increase in intensity of the absorption peaks, especially high growth of the carbonyl peaks after 90 days of aging. The electrochemical capacitance degradation of the electrode was studied using cyclic voltammetric and galvanostatic charge/discharge analysis in 1 M Na₂SO₄ electrolyte, which showed ~?53% of fading in the initial specific capacitance value after 90 days. Further, the electrochemical degradation of polypyrrole electrodes was analyzed in detail using electrochemical impedance spectroscopy. The analysis showed a large increase in the internal resistance and low-power deliverability of the electrode with respect to aging as the main reasons for the degradation of specific capacitance of the polypyrrole electrode.     

Pseudo-capacitance on exfoliated carbon fiber in sulfuric acid electrolyte

The specific capacitance of exfoliated carbon fibers (ExCF) which were synthesized from pitch-based carbon fibers showed a strong dependence with the concentration of sulfuric acid electrolyte and reached 1.4 F/m² in 18 M H₂SO₄ solution. Since the capacitance value is quite large compared with the case of conventional activated carbons, faradic reactions (charge transfer reactions) are the cause of pseudo-capacitance. ExCF, however, gave a featureless cyclic voltammogram in 18 M H₂SO₄ solution. In the case of exfoliated natural graphite, the intercalation of H₂SO₄ molecules is evidenced by redox peaks observed in the voltammograms in the same conditions. Therefore, a strong interaction between the H₂SO₄ molecules and the ExCF surface might be the reason for the origin of pseudo-capacitance with ExCF in H₂SO₄ electrolyte. PACS 81.05.Uw; 82.45.Fk; 82.47.Uv     

Ionic liquid enhanced magnesium-based polymer electrolytes for electrical double-layer capacitors

This paper describes the preparation and characterization of poly(vinyl alcohol) (PVA)-added ionic liquid-based ion conductors. The polymer electrolyte is incorporated with magnesium triflate [Mg(CF₃SO₃)₂ or MgTf] as salt and 1-butyl-3-methylimidazolium bromide (BmImBr) as ionic liquid. Differential scanning calorimetry (DSC) is carried out to investigate the glass transition temperature which is used to study the plasticizing effect of the ionic liquid. The highest conducting ionic liquid-based polymer electrolyte is used to fabricate electrical double-layer capacitors (EDLC). The electrochemical potential window is evaluated using linear sweep voltammetry (LSV). The electrochemical capacitance of the EDLC is evaluated through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). The electrochemical potential window of ionic liquid-added polymer electrolyte is extended from 1.35 to 2.6 V. Cyclic voltammetry (CV) proves the improvement in specific capacitance of the electrical double-layer capacitors (EDLCs) containing ionic liquid-added polymer electrolyte.     

KOH activated carbon fabrics as supercapacitor material

Carbon fabrics from viscose fibers activated with KOH have been investigated as possible electrode materials for electrochemical capacitors. The fibers were first pyrolysed at 400 or 600 °C, then saturated with KOH at C/KOH ratios from 1:3.5 to 1:5 and treated in the temperature range from 700 to 800 °C. The carbon fibers saturated with KOH were analysed by thermogravimetric and differential thermal analysis in order to get information on the temperature dependence of the KOH reaction with carbon. The electrochemical properties of the activated carbons were determined using three-electrode Swagelok^® type capacitors both in 4 M H₂SO₄ and 7 M KOH aqueous electrolytes. Specific capacities of ca. 340 and 270 F/g were determined in acidic and alkaline medium, respectively. We demonstrate that the electrical capacity for both negative and positive electrodes depends on the treatment method. The capacitance values are discussed taking into account the porous texture, the elemental composition and the surface functionality of the activated carbon fibers.     

SILAR deposited Bi2S3 thin film towards electrochemical supercapacitor

Bi₂S₃ thin film electrode has been synthesized by simple and low cost successive ionic layer adsorption and reaction (SILAR) method on stainless steel (SS) substrate at room temperature. The formation of interconnected nanoparticles with nanoporous surface morphology has been achieved and which is favourable to the supercapacitor applications. Electrochemical supercapacitive performance of Bi₂S₃ thin film electrode has been performed through cyclic voltammetry, charge-discharge and stability studies in aqueous Na₂SO₄ electrolyte. The Bi₂S₃ thin film electrode exhibits the specific capacitance of 289 Fg⁻¹ at 5 mVs⁻¹ scan rate in 1 M Na₂SO₄ electrolyte.     

Investigation of electrochemical performance of MgNiO2 prepared by sol-gel synthesis route for aqueous-based supercapacitor application

In this work, we have successfully synthesized MgNiO₂ using a sol-gel wet chemical synthesis technique named MNO - 3. Electrochemical measurements in the presence of aqueous 1 M Li₂SO₄ electrolyte indicate that MNO - 3 samples exhibit a capacitance value of about 30 F/g and an energy density of about 20 Wh/kg. Subsequently, in the experiment involving aqueous 0.5 M Na₂SO₄ electrolyte system, it has been found that the capacitance for MNO - 3 sample is about 34 F/g and the energy density is about 23 Wh/kg for MNO - 3 sample. Finally, in the presence of aqueous-based 1 M Mg(ClO₄)₂ electrolyte, MNO - 3 sample is found to exhibit a capacitance of about 26 F/g and an energy density of about 17 Wh/kg, respectively. In all three electrolyte systems, the MNO -3 sample exhibit a long cycle capacitance retention of greater than 85% for 1000 charge-discharge cycles.     

Electrochemical capacitive properties of micron-sized chemically grown cadmium oxide discrete crystals

Stable electrochemical capacitive properties of chemically grown cadmium oxide film electrode composed of micron-sized discrete crystals in 1 M Na₂SO₄ electrolyte with a specific capacitance of 1190 mF/g studied over 1000 cycles are reported. Structural and morphological characterizations of micron-sized discrete CdO crystals have been carried out using power X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. Electrochemical capacitive properties of micron-sized CdO discrete crystals on tin-doped indium oxide electrode have been investigated using cyclic voltammetry and chronopotentiometry.     

All solid-state supercapacitor with phosphotungstic acid as the proton-conducting electrolyte

For the first time, the proton-conducting composite phosphotungstic acid PTA/Al₂(SO₄)₃·18H₂O was used as the electrolyte of symmetric supercapacitor based on PANI. The optimum weight ratio of PTA/Al₂(SO₄)₃·18H₂O for using in this supercapacitor was also reported. Electrochemical tests prove that the supercapacitor using this kind of composite as electrolyte has high capacitance performance. Its capacitance is as high as 240 F/g at 6 mA. It was more important that it has long cycle life. After 1000 cycles, the attenuation of the capacitance is less than 10% and the coulombic efficiency is still greater than 96%.     

Morphology-controlled PANI nanowire electrode by using deposition scan rate in H<Subscript>2</Subscript>SO<Subscript>4</Subscript>/PVA polymer electrolyte for electrochemical capacitor

Polyaniline (PANI) nanowire electrode was successfully prepared using electrodeposition method. The morphology, thickness, and electrochemical performance of PANI electrode can be controlled by varying the deposition scan rates. Lower deposition scan rate results in compact and aggregates of PANI nanowire morphology. The uniform nanowire of PANI was obtained at the applied scan rate of 100 mV s^?1, and it was used as symmetric electrode coupled with H₂SO₄/polyvinyl alcohol (PVA) gel electrolyte. The different concentrations of H₂SO₄ acid in polymer electrolyte have influenced the electrochemical performance as well. The optimum specific capacitance and energy density of P100 PANI electrode in 3 M H₂SO₄/PVA gel polymer electrolyte was 377 F g^?1 and 95.4 Wh kg^?1 at the scan rate of 1 mV s^?1. The good stability of the electrode in this system is applicable to many wearable electronics applications.     

Sulfidation of a Cu submonolayer at the Au(1 1 1)/electrolyte interface - An in situ STM study

We describe the electrochemical preparation of an ultrathin copper sulfide film on Au(1 1 1) and its structural characterization by in situ STM. The first step, underpotential deposition of a Cu submonolayer from CuSO₄/H₂SO₄ solution, is followed by two electrolyte exchanges for (i) Cu-free (blank) H₂SO₄ solution and (ii) NaOH/Na₂S solution. The well-known (√3 × √3)R30° structure of the upd Cu layer is stable in the blank electrolyte for at least 2 h. After exposure to bisulfide, the Cu layer contracts and forms two-dimensional islands of two distinct ordered surface phases, i.e. a rectangular and, at higher potentials, a hexagonal phase, with Cu-free Au(1 1 1) regions between them, the latter exhibiting the characteristic (√3 × √3)R30°-S adlayer structure. Potential changes lead to a complex phase behaviour including HS⁻ ? S_x oxidation/reduction and, at strongly anodic potentials, dissolution of the Cu adlayer.     

The effect of different concentrations of Na2SnO3 on the electrochemical behaviors of the Mg-8Li electrode

In this research, the effect of the different concentrations of NaSnO₃ as the electrolyte additive in 0.7 mol L^?1 NaCl solution on the electrochemical performances of the magnesium-8lithium (Mg-8Li) electrode are investigated by methods of potentiodynamic polarization, potentiostatic current-time, electrochemical impedance technique, and scanning electron microscopy (SEM). The corrosion resistance of the Mg-8Li electrode is improved when Na₂SnO₃ is added into the electrolyte solution. The potentiostatic current-time curves show that the electrochemical behaviors of the Mg-8Li electrode in the electrolyte solution containing 0.20 mmol L^?1 Na₂SnO₃ is the best. The electrochemical impedance spectroscopy results indicate that the polarization resistance of the Mg-8Li electrode decreases in the following order with the concentrations of Na₂SnO₃: 0.05 mmol L^?1?>?0.00 mmol L^?1?>?0.30 mmol L^?1?>?0.10 mmol L^?1?>?0.20 mmol L^?1. The scanning electron microscopy studies indicate that the electrolyte additive prevents the formation of the dense oxide film on the alloy surface and facilitates the peeling off of the oxidation products.     

Rational synthesis of α-MnO2 and γ-Mn2O3 nanowires with the electrochemical characterization of α-MnO2 nanowires for supercapacitor

An acidification-hydrothermal method was developed to synthesize α-MnO₂ nanowires, which was subsequently treated with ethanol, resulting in γ-Mn₂O₃ nanowire bundles on a large scale. The electrochemical characterization was carried out by cyclic voltammetry, which indicated that the α-MnO₂ nanowires in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte was of an excellent electrode material for supercapacitor at the scan rate of 10 mV S⁻¹ in the range of 0.0-0.85 V.     

Pseudo-capacitive properties of LiCoO<Subscript>2</Subscript>/AC electrochemical capacitor in various aqueous electrolytes

LiCoO₂ particles were synthesized by a sol-gel process. X-ray diffraction analysis reveals that the prepared sample is a single phase with layered structure. A hybrid electrochemical capacitor was fabricated with LiCoO₂ as a positive electrode and activated carbon (AC) as a negative electrode in various aqueous electrolytes. Pseudo-capacitive properties of the LiCoO₂/AC electrochemical capacitor were determined by cyclic voltammetry, charge–discharge test, and electrochemical impedance measurement. The charge storage mechanism of the LiCoO₂-positive electrode in aqueous electrolyte was discussed, too. The results showed that the potential range, scan rate, species of aqueous electrolyte, and current density had great effect on capacitive properties of the hybrid capacitor. In the potential range of 0–1.4 V, it delivered a discharge specific capacitance of 45.9 Fg^–1 (based on the active mass of the two electrodes) at a current density of 100 mAg^–1 in 1 molL^–1 Li₂SO₄ aqueous electrolyte. The specific capacitance remained 41.7 Fg^–1 after 600 cycles.