Electrochemical characteristics of new electric double layer capacitor with acidic polymer hydrogel electrolyte

The acidic polymer hydrogel electrolyte was prepared from 1 M H₂SO₄ aqueous solution, poly(vinyl alcohol) (PVA) and glutaraldehyde (GA). A new electric double layer capacitor (EDLC) with the polymer hydrogel electrolyte was assembled, and its electrochemical characteristics were investigated. As a result, the EDLC cell with the polymer hydrogel electrolyte exhibited almost the same discharge capacitance and high-rate dischargeability as that with a 1 M H₂SO₄ aqueous solution as an electrolyte. It was also found that the self-discharge was remarkably suppressed by using the polymer hydrogel electrolyte.     

Electropolymerization of high stable poly(3,4-ethylenedioxythiophene) in ionic liquids and its potential applications in electrochemical capacitor

Poly(3,4-ethylenedioxythiophene) (PEDOT) has been successfully electropolymerized using a purified 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) as both the growth medium and the supporting electrolyte. The electrochemical performance of the PEDOT thin film was investigated in 1 mol L⁻¹ H₂SO₄ solution. It possesses nearly ideal capacitive property, and its specific capacitance is about 130 F g⁻¹. Compared with other conducting polymers, enhanced cycling lifetime (up to 70,000 cycles), which is close to that of active carbon materials, was observed on repetitive redox cycling.     

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.     

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.     

Hybrid supercapacitor based on MnO2 and columned FeOOH using Li2SO4 electrolyte solution

The nano-sized columned β-FeOOH was prepared by the hydrolysis process and its electrochemical capacitance performance was evaluated for the first time in Li₂SO₄ solution. A hybrid supercapacitor based on MnO₂ positive electrode and FeOOH negative electrode in Li₂SO₄ electrolyte solution was designed. The electrochemical tests demonstrated that the hybrid supercapacitor has a energy density of 12 Wh kg⁻¹ and a power density of 3700 W kg⁻¹ based on the total weight of the electrode active materials with a voltage range 0–1.85 V. This hybrid supercapacitor also exhibits a good cycling performance and keeps 85% of initial capacity over 2000 cycles.     

Enhanced manganese dioxide supercapacitor electrodes produced by electrodeposition

Electrodeposited thin films of manganese dioxide, prepared using chronoamperometry on a platinum substrate in an electrolyte of MnSO₄ in H₂SO₄, possess a significantly higher capacitance compared to the literature materials (>2000 F g⁻¹ which is at least a 250% increase in performance) when cycled over a 0.8 V potential window in an aqueous electrolyte of 0.5 M Na₂SO₄. This excellent performance is discussed in terms of the manganese dioxide electrodeposition mechanism, in particular the growth mechanism under the preferred slow mass transport of electro-active species, and its effects on morphology. Furthermore, the origin of the enhanced capacitance is discussed, in which case we have proposed arises from contributions made by hydroxyl groups on the manganese dioxide nano-particulate surface, in addition to the fast redox reactions that are necessary for pseudo-capacitance.     

Feasibility of hydrogen production from coal electrolysis at intermediate temperatures

Hydrogen production via coal electrolysis was evaluated at intermediate temperatures (40–108 °C). A coal electrolytic cell (CEC) was designed and constructed to carry out galvanostatic experiments with concentrated electrolyte (4 M H₂SO₄). Operating temperatures above 100 °C were found to significantly improve the kinetics of electro-oxidation of coal, coal conversion, and CO₂/coal Faradaic efficiency. CO₂/coal Faradaic efficiencies and coal conversions of up to 57.29 and 3.21%, respectively, were observed at 108 °C.     

A development of direct hydrazine/hydrogen peroxide fuel cell

A direct hydrazine fuel cell using H₂O₂ as the oxidizer has been developed. The N₂H₄/H₂O₂ fuel cell is assembled by using Ni-Pt/C composite catalyst as the anode catalyst, Au/C as the cathode catalyst, and Nafion membrane as the electrolyte. Both anolyte and catholyte show significant influences on cell voltage and cell performance. The open-circuit voltage of the N₂H₄/H₂O₂ fuel cell reaches up to 1.75 V when using alkaline N₂H₄ solution as the anolyte and acidic H₂O₂ solution as the catholyte. A maximum power density of 1.02 W cm⁻² has been achieved at operation temperature of 80 °C. The number of electrons exchanged in the H₂O₂ reduction reaction on Au/C catalyst is 2.     

Allyl-functionalized ionic liquids as electrolytes for electric double-layer capacitors

Double-layer capacitor electrolytes employing allyl-functionalized ionic liquids as electrolytes with solvents have been evaluated. Imidazolium cations with allyl groups enabled the high capacitances and low resistances of electric double-layer capacitor (EDLC) cells at a wide range of temperature in spite of the large cation sizes and low ionic conductivities of the electrolytes compared to imidazolium with saturated alkyl groups, 1-ethyl-3-methylimidazolium (EMIm). The improvement of EDLC performance was noted particularly in the case of diallylimidazolium (DAIm) cation and TFSA anion. The substitution of the vinyl group increased the high capacitance only at 298 K and decreased the capacitance at low temperature and direct current resistance (DC-IR) at 243 and 298 K. The butenyl group deteriorated the capacitance and DC-IR at 243 and 298 K. The stability of EDLC cell of DAIm-BF₄/PC was inferior to that of EMIm-BF₄/PC. The addition of DMC to PC improved the stability.     

Electrochemical performance of nonflammable polymeric gel electrolyte containing triethylphosphate

Nonflammable polymeric gel electrolyte has been prepared by immobilizing 1 M LiBF₄/EC + DEC + TEP (55:25:20, v/v/v, EC: ethylene carbonate, DEC: diethyl carbonate and TEP: triethylphosphate) solution in poly(vinylidene fluoride-co-hexafluoro propylene) (PVdF-HFP) where TEP acts as a fire-retardant solvent in the gel electrolyte. The polymeric gel electrolyte has a high value of ionic conductivity of 1.76 mS cm⁻¹ at 28 °C. Thermal safety calorimetry (TSC) experiments show good thermal stability of the gel electrolyte. Cyclic voltammetry and charge/discharge cycling tests were performed on LiMn₂O₄/gel electrolyte and graphite/gel electrolyte half cells. The gel electrolyte works well for graphite/LiMn₂O₄ cell although some improvement in the cycleability of the graphite electrode is still needed.     

Application of Si-C-O glass-like compounds as negative electrode materials for lithium hybrid capacitors

The Si-C-O glass-like compound (a-SiCO) was applied to a negative electrode of a lithium hybrid capacitor (LHC) with activated carbon positive electrodes. The performance as a negative electrode (by a three-electrode system) and LHC (by a two-electrode system) was evaluated in LiClO₄ (EC-DEC) and LiBF₄ (PC) electrolytes. With a-SiCO reversible insertion/extraction of lithium ions at high current densities (0.5-2.0 A g⁻¹) was possible. By prior short-circuiting of the negative electrode with lithium metal in the electrolytes for appropriate periods, the charge/discharge performance of the assembled LHC compared favorably with an electric double layer capacitor (EDLC) made of the activated carbon used for LHC. The cycle performance of the LHC was better but the capacitance was smaller in the LiBF₄ (PC) electrolyte than in LiClO₄ (EC-DEC) electrolyte. Smaller capacitance is mainly due to lower electric conductivity and higher viscosity of LiBF₄ (PC) electrolyte than LiClO₄ (EC-DEC) electrolyte. The energy density of the assembled LHC reached a maximum of about three times that of EDLC, with the power density comparable to that of the EDLC.     

Manganese oxide electrochemical capacitor with potassium poly(acrylate) hydrogel electrolyte

An aqueous gel electrolyte has for the first time been successfully applied to the MnO₂·nH₂O-based pseudocapacitive electrochemical capacitors (ECs). The gel electrolyte is made of potassium poly(acrylate) (PAAK) polymer and aqueous solution of KCl. With the selected composition, PAAK:KCl:H₂O = 9.0%:6.7%:84.3% by weight, the gel shows no fluidity, possessing an ionic conductivity in the order of 10⁻¹ S cm⁻¹. The gel electrolyte has been found to give substantially higher specific capacitances than those in the liquid electrolyte with the same salt (KCl) composition (1 M) and high power capability (>10 kW/kg).     

High temperature carbon–carbon supercapacitor using ionic liquid as electrolyte

This paper presents results about the electrochemical and cycling characterizations of a supercapacitor cell using a microporous activated carbon as the active material and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) ionic liquid as the electrolyte. The microporous activated carbon exhibited a specific capacitance of 60 F g⁻¹ measured from the three-electrode cyclic voltammetry experiments at 20 mV s⁻¹ scan rate, with a maximum operating potential range of 4.5 V at 60 °C. A coin cell assembled with this microporous activated carbon and PYR₁₄TFSI as the electrolyte was cycled for 40,000 cycles without any change of cell resistance (9 Ω cm²), at a voltage up to 3.5 V at 60 °C, demonstrating a high cycling stability as well as a high stable specific capacitance in this ionic liquid electrolyte. These high performances make now this type of supercapacitor suitable for high temperature applications (≥60 °C).     

Evaluation of proton-conducting membranes for use in a sulfur dioxide depolarized electrolyzer

The chemical stability, sulfur dioxide transport, ionic conductivity, and electrolyzer performance have been measured for several commercially available and experimental proton exchange membranes (PEMs) for use in a sulfur dioxide depolarized electrolyzer (SDE). The SDEs function is to produce hydrogen by using the Hybrid Sulfur (HyS) Process, a sulfur-based electrochemical/thermochemical hybrid cycle. Membrane stability was evaluated using a screening process where each candidate PEM was heated at 80 °C in 60 wt% H₂SO₄ for 24 h. Following acid exposure, chemical stability for each membrane was evaluated by FTIR using the ATR sampling technique. Membrane SO₂ transport was evaluated using a two-chamber permeation cell. SO₂ was introduced into one chamber whereupon SO₂ transported across the membrane into the other chamber and oxidized to H₂SO₄ at an anode positioned immediately adjacent to the membrane. The resulting current was used to determine the SO₂ flux and SO₂ transport. Additionally, membrane electrode assemblies (MEAs) were prepared from candidate membranes to evaluate ionic conductivity and selectivity (ionic conductivity vs. SO₂ transport) which can serve as a tool for selecting membranes. MEAs were also performance tested in a HyS electrolyzer measuring current density vs. a constant cell voltage (1 V, 80 °C in SO₂ saturated 30 wt% H₂SO₄). Finally, candidate membranes were evaluated considering all measured parameters including SO₂ flux, SO₂ transport, ionic conductivity, HyS electrolyzer performance, and membrane stability. Candidate membranes included both PFSA and non-PFSA polymers and polymer blends of which the non-PFSA polymers, BPVE-6F and PBI, showed the best selectivity.     

Graphene-MnO2 and graphene asymmetrical electrochemical capacitor with a high energy density in aqueous electrolyte

The graphene-manganese oxide hybrid material has been prepared by solution-phase assembly of aqueous dispersions of graphene nanosheets and manganese oxide nanosheets at room temperature. The morphology and structure of the obtained material are examined by scanning electron microscopy, transition electron microscopy, X-ray diffraction and N₂ adsorption-desorption. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. An asymmetric electrochemical capacitor with high energy and power densities based on the graphene-manganese oxide hybrid material as positive electrode and graphene as negative electrode in a neutral aqueous Na₂SO₄ solution as electrolyte is assembled. The asymmetrical electrochemical capacitor could cycle reversibly in a voltage of 0-1.7 V and give an energy density of 10.03 Wh kg⁻¹ even at an average power density of 2.53 kW kg⁻¹. Moreover, the asymmetrical electrochemical capacitor exhibit excellent cycle stability, and the capacitance retention of the asymmetrical electrochemical capacitor is 69% after repeating the galvanostatic charge-discharge test at the constant current density of 2230 mA g⁻¹ for 10,000 cycles.     

Mesoporous activated carbon fiber as electrode material for high-performance electrochemical double layer capacitors with ionic liquid electrolyte

Activated carbon fibers (ACFs) with super high surface area and well-developed small mesopores have been prepared by pyrolyzing polyacrylonitrile fibers and NaOH activation. Their capacitive performances at room and elevated temperatures are evaluated in electrochemical double layer capacitors (EDLCs) using ionic liquid (IL) electrolyte composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO₂CF₃)₂) and 2-oxazolidinone (C₃H₅NO₂). The surface area of the ACF is as high as 3291 m² g⁻¹. The pore volume of the carbon reaches 2.162 cm³ g⁻¹, of which 66.7% is the contribution of the small mesopores of 2-5 nm. The unique microstructures enable the ACFs to have good compatibility with the IL electrolyte. The specific capacitance reaches 187 F g⁻¹ at room temperature with good cycling and self-discharge performances. As the temperature increases to 60 °C, the capacitance increases to 196 F g⁻¹, and the rate capability is dramatically improved. Therefore, the ACF can be a promising electrode material for high-performance EDLCs.     

Kinetics of oxygen reduction reaction on nanosized Pd electrocatalyst in acid media

Palladium electrocatalyst powder was prepared through PdCl₂ reduction with NaBH₄ in a THF solution at 0 °C. Characterization of the palladium phases, morphology and topography was performed by XRD, SEM, and AFM. Results from the X-ray diffraction (XRD) study showed evidence of nanocrystalline fcc hexagonal palladium formation with 4 nm in an average size. Electrocatalyst activity for the oxygen reduction reaction, ORR, was examined using cyclic voltammetry through rotating-disk electrode measurements in a 0.5 M H₂SO₄ electrolyte. Kinetic results suggest that the initial electron transfer is the rate-determining step. An apparent enthalpy of activation ΔH^°# = 89.5 ± 0.5 kJ mol⁻¹ was calculated from electrochemical results at different temperatures. Based on current densities and the enthalpy of activation results, the nanosized Pd catalysts are significantly less active for the ORR than Pt and Ru electrocatalysts.     

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.     

Performance of mesoporous carbons derived from poly(vinyl alcohol) in electrochemical capacitors

The present work shows that mesoporous materials obtained by the carbonization of mixtures of poly(vinyl alcohol) with magnesium citrate are very promising candidates for electrodes in supercapacitors. Their high performance arises essentially from a double-layer mechanism through the extent of the total surface area and one obtains at low current density (1 mA cm⁻²) values as high as 180 F g⁻¹ in aqueous 2 M H₂SO₄ electrolyte and around 100 F g⁻¹ in 1 M (C₂H₅)₄NBF₄ in acetonitrile. Moreover, in most cases the specific capacitance is reduced only by 15% at 100 mA cm⁻², as opposed to many other types of carbons which display much higher reductions.     

Ionic conducting behavior of solvent-free polymer electrolytes prepared from oxetane derivative with nitrile group

Polymer with trimethylene oxide (TMO) units prepared from ring-opening polymerization of an oxetane derivative is a candidate for the matrix of solid polymer electrolytes. We prepare an oxetane derivative with nitrile group, 3-(2-cyanoethoxymethyl)-3-ethyloxetane, CYAMEO. CYAMEO is polymerized by using a cationic initiator system. The structure of the resulted polymer, P(CYAMEO), is confirmed by NMR and FTIR spectroscopic techniques. Inorganic salts, such as lithium salts, can be dissolved in P(CYAMEO) matrix. FTIR and DSC results of P(CYAMEO)-based electrolyte films suggest that lithium ions in the P(CYAMEO) matrix interact with the nitrile side chains, mainly, and not with the oxygen atoms on the main chain of the P(CYAMEO). The conductivity at 30 °C for P(CYAMEO)-based electrolyte films, P(CYAME)10(LiX)1, is 19.6 μS cm⁻¹ (X = LiClO₄), 6.59 μS cm⁻¹ (BF₄), 6.54 μS cm⁻¹ (CF₃SO₃), and 25.0 μS cm⁻¹ (N(CF₃SO₂)₂). The rise in temperature from 30 °C to 70 °C increases their conductivity, about 30-40 times. The conductivity at 70 °C for P(CYAMEO)-based electrolyte films is 0.742 mS cm⁻¹ (X = LiClO₄) and 0.703 mS cm⁻¹ (N(CF₃SO₂)₂). Electrochemical deposition and dissolution of lithium on a nickel plate electrode are observed in the solvent-free three-electrode electrochemical cell with P(CYAMEO)10(LiX)1, (X = ClO₄ or N(CF₃SO₂)₂) electrolyte film at 55 °C.