Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

Carbon xerogels as electrochemical supercapacitors. Relation between impedance physicochemical parameters and electrochemical behaviour

The electrochemical behavior of carbon xerogels was studied with the aim of analyzing the performance of the materials used as electrochemical supercapacitors (SC) and to relate with physicochemical parameters. These materials have areas involving 1500–2000 m²/g measured with the BET equation and a range of pore size distributions.     

MnO2 nanotube and nanowire arrays by electrochemical deposition for supercapacitors

Highly ordered MnO₂ nanotube and nanowire arrays are successfully synthesized via a electrochemical deposition technique using porous alumina templates. The morphologies and microstructures of the MnO₂ nanotube and nanowire arrays are investigated by field emission scanning electron microscopy and transmission electron microscopy. Electrochemical characterization demonstrates that the MnO₂ nanotube array electrode has superior capacitive behaviour to that of the MnO₂ nanowire array electrode. In addition to high specific capacitance, the MnO₂ nanotube array electrode also exhibits good rate capability and good cycling stability, which makes it promising candidate for supercapacitors.     

Carbon nanotube/MnO2 composites synthesized by microwave-assisted method for supercapacitors with high power and energy densities

Carbon nanotube (CNT)/MnO₂ composites are synthesized by reduction of potassium permanganate under microwave irradiation. The morphology and microstructure of samples are examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Electrochemical properties are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). Birnessite-type MnO₂ homogeneously coats on the surfaces of CNTs. For CNT–15%MnO₂ composite, the specific capacitance based on MnO₂ is 944 (85% of the theoretical capacitance) and 522 F g⁻¹ at 1 and 500 mV s⁻¹, respectively. When the content of MnO₂ reaches 57 wt%, the composites have the maximum power density (45.4 kW kg⁻¹, the energy density is 25.2 Wh kg⁻¹). Therefore, CNT/MnO₂ composites prepared by microwave irradiation are promising electrode materials in hybrid vehicle systems.     

A hierarchical nanostructure consisting of amorphous MnO2, Mn3O4 nanocrystallites, and single-crystalline MnOOH nanowires for supercapacitors

In this communication, a porous hierarchical nanostructure consisting of amorphous MnO₂ (a-MnO₂), Mn₃O₄ nanocrystals, and single-crystalline MnOOH nanowires is designed for the supercapacitor application, which is prepared by a simple two-step electrochemical deposition process. Because of the gradual co-transformation of Mn₃O₄ nanocrystals and a-MnO₂ nanorods into an amorphous manganese oxide, the cycle stability of a-MnO₂ is obviously enhanced by adding Mn₃O₄. This unique ternary oxide nanocomposite with 100-cycle CV activation exhibits excellent capacitive performances, i.e., excellent reversibility, high specific capacitances (470 F g⁻¹ in CaCl₂), high power property, and outstanding cycle stability. The highly porous microstructures of this composite before and after the 10,000-cycle CV test are examined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

Characterizations of polybenzimidazole based electrochemical hydrogen pumps with various Pt loadings for H2/CO2 gas separation

Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO₂ concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO₂ and produce pure H₂ from anode outlet gases (H₂/CO₂ mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H₂/CO₂ mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm² to 0.2 mg/cm² without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.     

Electrochemical investigation of MnO2 electrode material for supercapacitors

MnO₂ electrode material is synthesized by low temperature solid state reaction between KMnO₄ and MnCl₂. Effects of the KMnO₄:MnCl₂ molar ratio on the structure, morphology and electrochemical properties of the as-prepared sample were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical tests. Results showed that the obtained MnO₂ is α-MnO₂, the average diameter is about 0.5-1.5 μm, which are constituted of nanoparticles of 20 nm. Under 100 mA g⁻¹, the specific capacitances of the prepared sample is 258.7, 219.6, 215.3, 198.5 and 209.5 F g⁻¹ at the KMnO₄/MnCl₂ molar ratio of 3:2, 2:1, 1:1, 1:2 and 2:3, respectively. And the MnO₂ sample with a KMnO₄/MnCl₂ molar ratio of 3:2 exhibits the best discharge capacitance and cycle performance. When the charge/discharge rate increases to 300 mA g⁻¹, the sample still remains initial discharge capacitance of 165.3 F g⁻¹, and the discharge capacitance is 145.9 F g⁻¹ after 200 cycles, the capacitance retention rate is 102.4% during the 20-200th cycles. Therefore, the MnO₂ sample is an excellent material for use in supercapacitors because of its large specific capacitance and good cycle performance.     

Synthesis and electrochemical performance of multi-walled carbon nanotube/polyaniline/MnO2 ternary coaxial nanostructures for supercapacitors

Multi-walled carbon nanotube (MWCNT)/polyaniline (PANI)/MnO₂ (MPM) ternary coaxial structures are fabricated as supercapacitor electrodes via a simple wet chemical method. The electrostatic interaction between negative poly(4-styrenesulfonic acid) (PSS) molecules and positive Mn²⁺ ions causes the generation of MnO₂ nanostructures on MWCNT surfaces while the introduction of PANI layers with appropriate thickness on MWCNT surfaces facilitates the formation of MWCNT/PANI/MnO₂ ternary coaxial structures. The thickness of PANI coatings is controlled by tuning the aniline/MWCNT ratio. The effect of PANI thickness on the subsequent MnO₂ nanoflakes attachment onto MWCNTs, and the MPM structures is investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FESEM). The results suggest that appropriate thickness of PANI layers is important for building MPM ternary coaxial structures without the agglomeration of MnO₂ nanoflakes. The MPM ternary coaxial structures provide large interaction area between the MnO₂ nanoflakes and electrolyte, and improve the electrochemical utilization of the hydrous MnO₂, and decrease the contact resistance between MnO₂ and PANI layer coated MWCNTs, leading to intriguing electrochemical properties for the applications in supercapacitors such as a specific capacitance of 330 Fg⁻¹ and good cycle stability.     

Electrochemical and capacitive properties of thin-layer carbon black electrodes

Electrochemical properties and porous-structure-dependent capacitive ability of commercial carbon blacks, Black Pearls 2000^® (BP) and Vulcan^® XC 72R (XC), were investigated in H₂SO₄ solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The capacitance in-depth profile is correlated to microscopic appearance of carbon blacks in the form of a thin layer applied over Au substrate from water suspensions of BP and XC. The capacitance calculated from voltammetric charge was found to depend on the sweep rate, due to porosity of investigated materials. Impedance (EIS) characteristics upon frequency-dependent charge/discharge process indicate transmission line electric behavior of BP and XC. Capacitance and resistance values obtained by simulations of EIS data, enabled estimation of capacitance and resistance profile throughout carbon black porous electrodes. Capacitance of BP carbon layer increases going from the outer surface towards the bulk of the layer. External capacitance originates from capacitive characteristics of the macroscopic surface consisting of relatively large agglomerates, while internal capacitance originates from “inner” surface of micro-porous agglomerates. Contrary to BP, opposite distribution of the total capacitance to external and internal part was found for XC, caused by its loose structure and considerably lower real surface area in comparison to BP. The XC morphology makes additionally the pseudocapacitive contribution of surface functionalities more pronounced, which indirectly shifts also the “internal” double-layer capacitive response to higher frequencies through the effect of increased wettability of the layer. Thus, the capacitance of XC surface directly exposed to the electrolyte is larger than that of the inner one, which makes it a “fully-utilized” capacitor, while increased capacitive performance of BP emerges only at very low frequencies of charging/discharging process.     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

Synthesis of hydrothermally reduced graphene/MnO2 composites and their electrochemical properties as supercapacitors

Hydrothermally reduced graphene/MnO₂ (HRG/MnO₂) composites were synthesized by dipping HRG into the mixed aqueous solution of 0.1 M KMnO₄ and 0.1 M K₂SO₄ for different periods of time at room temperature. The morphology and microstructure of the as-prepared composites were characterized by field-emission scanning electron microscopy, X-ray diffraction, Raman microscope, and X-ray photoelectron spectroscopy. The characterizations indicate that MnO₂ successfully deposited on HRG surfaces and the morphology of the HRG/MnO₂ shows a three-dimensional porous structure with MnO₂ homogenously distributing on the HRG surfaces. Capacitive properties of the synthesized composite electrodes were studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode experimental setup using 1 M Na₂SO₄ aqueous solution as electrolyte. The main results of electrochemical tests are drawn as follows: the specific capacitance value of HRG/MnO₂-200 (HRG dipped into the mixed solution of 0.1 M KMnO₄ and 0.1 M K₂SO₄ for 200 min) electrode reached 211.5 F g⁻¹ at a potential scan rate of 2 mV s⁻¹; moreover, this electrode shows a good cyclic stability and capacity retention. It is anticipated that the synthesized HRG/MnO₂ composites will find promising applications in supercapacitors and other devices in virtue of their outstanding characters of good cycle stability, low cost and environmentally benign nature.     

Optimization of MnO2/vertically aligned carbon nanotube composite for supercapacitor application

The optimization strategy for producing manganese oxide supercapacitors based on vertically aligned carbon nanotubes (VACNTs) deposited on large area electrodes is presented. A single sequential process of sputtering, annealing and plasma enhanced chemical vapour deposition (PECVD) is applied to produce dense and uniform VACNTs electrodes. As dielectric layer of the supercapacitor, manganese oxide is electrodeposited lining the surface of the VACNTs electrodes. The control of the growing parameters such as catalyst thickness layer, temperature and deposition time for tuning the density, length and diameter of the VACNTs and their structure are found to be key points for the optimization of the MnO₂ electrodeposition process in view to improve the efficiency of the supercapacitor devices.The electrochemical properties of the obtained electrodes are characterized using cyclic voltammetry and galvanostatic charge-discharge techniques. A specific capacitance of 642 Fg⁻¹ is obtained for MnO₂/VACNTs nanocomposite electrode at a scan rate of 10 mV s⁻¹.     

Hydrothermal synthesis of SnO2–V2O5 mixed oxide and electrochemical screening of carbon nano-tubes (CNT), V2O5, V2O5–CNT,and SnO2–V2O5–CNT electrodes for supercapacitor applications

Nano-scaled SnO₂–V₂O₅ mixed oxide is synthesized by a hydrothermal method in an autoclave. For comparative evaluation, V₂O₅ single oxide is prepared by a conventional process from ammonium vanadate. The capacitive behaviour of the following electrodes is studied by cyclic voltammetry in 0.1 M KCl solutions: carbon nano-tubes (CNT), V₂O₅, V₂O₅–CNT, and SnO₂–V₂O₅–CNT. At a scan rate of 100 mV s⁻¹, the SnO₂–V₂O₅–CNT electrode provides the best performance, viz., 121.4 F g⁻¹. The nano-scaled mixed oxide is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectra.     

Nanoporous MnOx thin-film electrodes synthesized by electrochemical lithiation/delithiation for supercapacitors

Nanoporous MnO_x thin-film electrodes are synthesized using a combination of pulsed laser deposition (PLD) and electrochemical lithiation/delithiation methods. A dense Mn₃O₄ thin-film deposited by PLD can transform into a nanoporous MnO_x thin-film after electrochemical lithiation/delithiation. A nanoporous MnO_x thin-film electrode exhibits significantly improved supercapacitive performance compared with an as-deposited Mn₃O₄ thin-film electrode. A MnO_x thin-film finally transforms into a MnO₂ thin-film through an electrochemical oxidation process during continuous cyclic voltammetry scanning.     

Electrochemical impedance spectroscopy of BaCeO3 modified by Ti and Y

Barium cerate exhibits high protonic conductivity, especially when modified by trivalent dopant such as Y, Yb, Nd, Sm or Dy. Unfortunately, the poor chemical stability in the presence of CO₂ is the main disadvantage of this material. One of the possible approach to get the stable protonic conductor is the preparation of solid solutions. For example, doping of BaCeO₃ with Zr leads to the improvement of the chemical stability, but the electrical properties are simultaneously corrupted.In the present work the influence of Ti, per analogy to Zr, and Y dopants on electrical properties of BaCeO₃ was investigated using the electrochemical impedance spectroscopy (EIS) technique. BaCe_1−xTi_xO_3−δ (0 ≤ x ≤ 0.3) and Ba(Ce_0.95Ti_0.05)_0.95Y_0.05O_3−δ solid electrolytes were prepared by solid-state reaction method. It was found that the changes of electrical properties due to the introduction of Ti into the BaCeO₃ lattice is caused predominantly by the modification of the grain boundary properties. The Ti doping leads to the substantial decrease of grain boundary electrical conductivity, comparing to undoped material. The introduction of yttrium dopant to the BaCe_0.95Ti_0.05O₃ lattice has the opposite effect. The total electrical conductivity increases, due to significant modification of grain boundary electrical properties.     

Compatibility of the Py24TFSI-LiTFSI ionic liquid solution with Li4Ti5O12 and LiFePO4 lithium ion battery electrodes

Ionic liquids are assuming a constantly growing importance as preferred media for the progress of various energy devices such as solar cells, supercapacitors and lithium batteries. However, their electrochemical properties are not yet fully recognized and in this paper we try to contribute to fill the gap by investigating the stability window of a sample IL-based solution, as well as its interfacial properties with two typical lithium ion battery electrodes, namely lithium titanate, Li₄Ti₅O₁₂ and lithium iron phosphate, LiFePO₄. The results of a detailed impedance spectroscopy analysis demonstrate that, although operating well within the stability domain of the selected IL-based electrolyte, both electrodes undergo passivation phenomena with the formation on their surface of a solid electrolyte interface, SEI, layer. The impedance spectra show that the resistance of the SEI is very low and stable, this suggesting that its occurrence is highly beneficial in terms of assuring reversible and safe electrode processes.     

Adjustment of electrodes potential window in an asymmetric carbon/MnO2 supercapacitor

The electrodes mass ratio of MnO₂/activated carbon supercapacitors has been varied in order to monitor its influence on the potential window of both electrodes and consequently to optimize the operating voltage. It appeared that the theoretical mass ratio (R = 2), calculated considering an equivalent charge passed across both electrodes, is underestimated. It was demonstrated that R values of 2.5-3 are better adapted for this system; the extreme potential reached for each electrode is close to the stability limits of the electrolyte and active material, allowing a maximum voltage to be reached. During galvanostatic cycling up to 2 V, the best performance was obtained with R = 2.5. The specific capacitance increased from 100 to 113 F g⁻¹ during the first 2000 cycles, then decayed up to 6000 cycles and finally stabilized at 100 F g⁻¹. SEM images of the manganese based electrode after various numbers of thousands cycles exhibited dramatic morphological modifications. The later are suspected to be due to Mn(IV) oxidation and dissolution at high potential values. Hence, the evolution of specific capacitance during cycling of the asymmetric capacitor is ascribed to structural changes at the positive electrode.     

Effects of tetrabutoxytitanium on photoelectrochemical properties of plastic-based TiO2 film electrodes for flexible dye-sensitized solar cells

The flexible DSSCs based on conducting plastic substrates are fabricated using electrodes made of tetrabutoxytitanium (TBOT) mixed with P25 TiO₂ nanoparticles at low temperature. To investigate the effects of TBOT on the flexible dye-sensitized solar cells, electrochemical impedance spectroscopy (EIS) is performed in the dark and under illumination conditions. Resistances for electron transport through TiO₂, charge-transfer resistance related to the TiO₂/redox electrolytes interface recombination, electron transport time and electron lifetime are quantified under different weight ratios of TBOT/P25. Additionally, the photovoltaic characteristics I-V curves and incident photon to current conversion efficiencies (IPCE) of flexible anodes made of different weight ratios of TBOT/P25 are obtained as well. It is found that the electrode under weight ratio 0.17 has the smallest inherent resistance, longest electron transport time and electron lifetime, lowest recombination rate and best performance with conversion efficiency 3.94%. These results indicate that after the weight ratios of TBOT/P25 is optimized, TBOT could enhance the interconnection between the TiO₂ particles, improve the conductivity of the electrode and decrease the charge recombination. Above results demonstrate that adding TBOT to TiO₂ is an easy and efficient method to improve the performance of the flexible DSSC fabricated at low temperature.     

Long-term cycling behavior of asymmetric activated carbon/MnO2 aqueous electrochemical supercapacitor

Activated carbon–MnO₂ hybrid electrochemical supercapacitor cells have been assembled and characterized in K₂SO₄ aqueous media. A laboratory cell achieved 195,000 cycles with stable performance. The maximal cell voltage was 2 V associated with 21 ± 2 F g⁻¹ of total composite electrode materials (including activated carbon and MnO₂, binder and conductive additive) and an equivalent serie resistance (ESR) below 1.3 Ω cm². Long-life cycling was achieved by removing dissolved oxygen from the electrolyte, which limits the corrosion of current collectors. Scaling up has been realized by assembling several electrodes in parallel to build a prismatic cell. A stable capacity of 380 F and a cell voltage of 2 V were maintained over 600 cycles. These encouraging results show the interest of developing such devices, including non-toxic and safer components as compared to the current organic-based devices.     

Electrochemical characterization of La0.6Ca0.4Fe0.8Ni0.2O3 cathode on Ce0.8Gd0.2O1.9 electrolyte for IT-SOFC

For Solid Oxide Fuel Cells (SOFCs) to become an economically attractive energy conversion technology, suitable materials and structures which enable operation at lower temperatures, while retaining high cell performance, must be developed. Recently, the perovskite-type La_0.6Ca_0.4Fe_0.8Ni_0.2O₃ oxide has shown potential as an intermediate temperature SOFC cathode. An equivalent circuit describing the cathode polarization resistances was constructed from analyzing impedance spectra recorded at different temperatures in oxygen. A competitive electrode polarization resistance is reported for this oxygen electrode using a Ce_0.8Gd_0.2O_1.9 electrolyte, determined by impedance spectroscopy studies of symmetrical cells sintered at 800 °C and 1000 °C. Scanning electron microscopy (SEM) studies of the symmetrical cells revealed the absence of any reaction layer between cathode and electrolyte, and a porous electrode microstructure even when sintered at a temperature of only 800 °C. The performance of this cathode shows favorable oxygen reduction reaction (ORR) properties potentially making it an excellent choice for IT-SOFC application.