Influence of the mesoporous structure on capacitance of the RuO2 electrode

The difference in capacitive performance between high and low surface area RuO₂ electrodes, synthesized with and without a mesoporous silica template, respectively, was investigated in aqueous solutions of sulfuric acid and sulfates by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). RuO₂ synthesized with the template was crystalline and the formation of the mesoporous structure with a 6.5 nm diameter was confirmed using a transmission electron microscope and the nitrogen adsorption and desorption isotherm. From the CV at the scan rate of 1 mV s⁻¹, the specific capacitance of the high surface area electrode in H₂SO₄(aq) was determined to be 200 F g⁻¹. The high surface area RuO₂ has a three times higher BET specific surface area (140 m² g⁻¹) than the low surface area sample (39 m² g⁻¹). Introducing the mesoporous structure was proved effective for increasing the capacitance per mass of the RuO₂, though not all the surface functions as a capacitor. Both the CV and EIS suggest that by increasing the charging rate or frequency, the mesoporous structure of the electrode leads to a lower capacitance decrease (higher capacitance retention) than the low surface area electrode. The EIS also indicates that the response time of the capacitor is hardly influenced by the presence of the mesoporous structure.     

MgO-templated nitrogen-containing carbons derived from different organic compounds for capacitor electrodes

Carbons containing nitrogen (C-N composites) were derived from three commercial organic compounds, poly(vinylpyrrolidone) (PVP), polyacrylamide (PAA), and trimethylolmelamine (TMM) using the MgO template method. The C-N composites formed in nitrogen at 700-1000 °C had nitrogen content, W_N, of 3-23 mass% and the specific surface area by N₂ adsorption, S_BET, of 60-2000 m² g⁻¹ without activation. Generally high nitrogen content of the starting compound led to larger W_N, but W_N was not proportional to the N/C mole ratio in the compounds. The value of S_BET strongly depended on the compound: S_BET (PVP) > S_BET (PAA) ? S_BET (TMM). There was a tendency for W_N to decrease with increasing S_BET. The capacitance measured in 1 mol dm⁻³ H₂SO₄ by cyclic voltammetry, C_M in F g⁻¹, suggested that both W_N and S_BET are influential in gaining large C_M. For the composites with W_N > 5 mass%, the capacitance normalized by S_BET, C_A = C_M/S_BET, was 0.17-0.65 F m⁻², which was larger than the electric double layer capacitance (0.05-0.15 F m⁻²), indicating that the pseudo-capacitance contributes significantly to C_M. The value of C_A increased with increasing W_N, but a correlation between C_A and particular nitrogen species on the surface measured by XPS was obscure. It was suggested that the large C_A is not simply explained by redox reactions of the surface functional groups. The composite derived from PAA at 900 °C showed 234 F g⁻¹ at 2 mV s⁻¹ and 181 F g⁻¹ at 100 mV s⁻¹ with acceptable yield of the composite.     

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.     

Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors

Amphiphilic carbonaceous material (ACM), with nanoscale dispersion in alkaline aqueous solutions, is synthesized from green needle coke. As a special precursor with small particle size, plenty of functional groups and widened d₀₀₂ simultaneously, ACM guarantees subsequent ACM-based activated carbons (AACs) with high specific surface area over 3000 m² g⁻¹ as well as well-developed mesoporous structure after KOH activation. Such pore properties enable AACs’ high performances as electrode materials for electric double-layer capacitors (EDLCs). In particular, surface area up to 3347 m² g⁻¹ together with notable mesopore proportion (26.9%) gives sample AAC814 outstanding EDLC behaviors during a series of electrochemical tests including galvanostatic charge/discharge, CV and electrochemical impedance spectroscopy. The electrode gets satisfactory gravimetric and volumetric specific capacitance at the current density of 50 mA g⁻¹, up to 348 F g⁻¹ and 162 F cm⁻³, respectively. Furthermore, for the mesoporosity, there is only a slight capacitance reduction for AAC814 as the current density reaches 1000 mA g⁻¹, indicating its good rate performance. It is all the ACM's unique characteristics that make AACs a sort of competitive EDLC electrode materials, both in terms of specific capacitance and rate capability.     

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.     

Structure and electrochemical properties of activated polyacrylonitrile based carbon fibers containing carbon nanotubes

Solution spun polyacrylonitrile (PAN), PAN/multi-wall carbon nanotube (MWCNT), and PAN/single-wall carbon nanotube (SWCNT) fibers containing 5 wt.% carbon nanotubes were stabilized in air and activated using CO₂ and KOH. The surface area as determined by nitrogen gas adsorption was an order of magnitude higher for KOH activated fibers as compared to the CO₂ activated fibers. The specific capacitance of KOH activated PAN/SWCNT samples was as high as 250 F g⁻¹ in 6 M KOH electrolyte. Under the comparable KOH activation conditions, PAN and PAN/SWCNT fibers had comparable surface areas (BET surface area about 2200 m² g⁻¹) with pore size predominantly in the range of 1–5 nm, while surface area of PAN/MWCNT samples was significantly lower (BET surface area 970 m² g⁻¹). The highest capacitance and energy density was obtained for PAN/SWCNT samples, suggesting SWCNT advantage in charge storage. The capacitance behavior of these electrodes has also been tested in ionic liquids, and the energy density in ionic liquid is about twice the value obtained using KOH electrolyte.     

Preparation and characterization of carbonaceous materials containing nitrogen as electrochemical capacitor

Carbonaceous materials containing nitrogen (C/N materials) were prepared by a pyrolysis of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (CAN). A C/N material prepared by the pyrolysis of CAN at 1070 K (CAN-1070 K) had a C/N atomic ratio of 3.0 and a non-crystalline carbonaceous structure with a BET surface area of 880 m² g⁻¹. The material CAN-1070 K showed large capacitances of 160–180 F g⁻¹ and 110–120 F cm⁻³ in case of current density of 10 mA cm⁻² (2 A g⁻¹) by using three-electrode cell in 1 M H₂SO₄ aqueous solution, in comparison with that of activated carbon (160 F g⁻¹ and 55 F cm⁻³) having BET surface area of 2300 m² g⁻¹. ESCA study indicated that pyridinic and quarternary nitrogen atoms existed in the C/N materials, which could result in producing a pseudo-capacitance in addition to the electric double layer capacitance. Also introduction of nitrogen into the carbonaceous material could enhance the wettability of material, which might also improve the capacitance.     

Cathodic deposition and characterization of tin oxide coatings on graphite for electrochemical supercapacitors

Amorphous tin oxide (SnO_x) was cathodically deposited onto graphite electrode in a bath containing 0.1 M stannous chloride (SnCl₂), 0.5 M sodium nitrate (NaNO₃), and 0.4 M nitric acid (HNO₃) in an aqueous solution of 50% (v/v) ethanol. The SnO_x coatings grown on graphite were characterized as typical capacitive behaviors by cyclic voltammetry (CV), chronopotentiometric (CP) in 0.5 M KCl. Specific capacitance (in milli-farad per square centimeter, C_a) changes linearly with the deposition charge up to 4.5 C cm⁻², and a maximum of as high as 355 mF cm⁻² was obtained with the SnO_x coating grown at around 5 C cm⁻². For the SnO_x coating deposited at 0.2 C cm⁻², a maximum specific capacitance (in farad per gram, C_m) of 298 and 125 F g⁻¹ was achieved from CVs at a scan rate of 10, and 200 mV s⁻¹, respectively. The value of C_m significantly gets lower from 265 to around 95 F g⁻¹ when the deposition charge increases from 0.2 to around 6.0 C cm⁻². The long cycle-life and stability of the SnO_x coatings on graphite via the presented cathodic deposition were also demonstrated.     

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.     

Electrical double layer capacitors with sucrose derived carbon electrodes in ionic liquid electrolytes

Activated carbons were prepared via a pyrolysis of sucrose followed by activation in the stream of CO₂ gas for 2-6 h at 900 °C to tune the pore size distribution (PSD) and increase the specific surface area (SSA). The porosity of the activated sucrose derived carbons (ASCs) has been characterized using N₂ sorption measurements. Increasing activation time led to the significant increase in SSA and pore volume of ASCs, among which sucrose derived carbon with 6 h activation time (ASC-6 h) exhibited the highest SSA of 1941 m² g⁻¹ and the highest micropore volume of 0.87 cm³ g⁻¹. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge cycle tests have been applied to investigate the capacitive performance of the ASC electrodes in ionic liquids (ILs) at room and elevated temperatures. The ASC-6 h electrodes in ethyl-dimethyl-propyl-ammonium bis (trifluoromethylsulfonyl) imide (EdMPNTf₂N) showed specific capacitance in excess of 170 F g⁻¹ at 60 °C, whereas the same electrodes in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) showed slightly lower capacitance but significantly better rate performance.     

Effects of the cationic structures of fluorohydrogenate ionic liquid electrolytes on the electric double layer capacitance

Electric double layer capacitance of an activated carbon electrode has been measured for fluorohydrogenate ionic liquids (FHILs) based on five different cations (1,3-dimethylimidazolium (DMIm⁺), 1-ethyl-3-methylimidazolium (EMIm⁺), 1-butyl-3-methylimidazolium (BMIm⁺), 1-ethyl-1-methylpyrrolidinium (EMPyr⁺), and 1-methoxymethyl-1-methylpyrrolidinium (MOMMPyr⁺)) at 25 °C. For all the FHILs, the capacitance increases with increase in charging voltage, and exhibits the maximum value around 2.7 V. The capacitances for FHILs are higher than those for EMImBF₄ or 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) in the measured range (1.0 < V < 3.2). For the three imidazolium-based FHILs, the maximum capacitance decreases with increase in the size of the cation in the order, DMIm(FH)_2.3F (178 F g⁻¹) > EMIm(FH)_2.3F (162 F g⁻¹) > BMIm(FH)_2.3F (135 F g⁻¹). On the other hand, the maximum capacitance observed for MOMMPyr(FH)_2.3F (152 F g⁻¹) is larger than that for EMPyr(FH)_2.3F (134 F g⁻¹) in spite of the larger size of MOMMPyr⁺ than EMPyr⁺, which is derived from introduction of the methoxy group. Some FHILs with low melting points exhibit a sufficient capacitance even at −40 °C (64 F g⁻¹ for EMIm(FH)_2.3F).     

Pseudocapacitive properties of electrochemically prepared nickel oxides on 3-dimensional carbon nanotube film substrates

Nickel oxides on carbon nanotube electrodes (NiO_x/CNT electrodes) are prepared by depositing Ni(OH)₂ electrochemically onto carbon nanotube (CNT) film substrates with subsequent heating to 300 °C. Compared with the as deposited Ni(OH)₂ on CNT film substrates (Ni(OH)₂/CNT electrodes), the 300 °C heat treated electrode shows much high rate capability, which makes it suitable as an electrode in supercapacitor applications. X-ray photoelectron spectroscopy shows that the pseudocapacitance of the NiO_x/CNT electrodes in a 1 M KOH solution originates from redox reactions of NiO_x/NiO_xOH and Ni(OH)₂/NiOOH. The 8.9 wt.% NiO_x in the NiO_x/CNT electrode shows a NiO_x-normalized specific capacitance of 1701 F g⁻¹ with excellent high rate capability due to the 3-dimensional nanoporous network structure with an extremely thin NiO_x layer on the CNT film substrate. On the other hand, the 36.6 wt.% NiO_x/CNT electrode has a maximum geometric and volumetric capacitance of 127 mF cm⁻² and 254 F cc⁻¹, respectively, with a specific capacitance of 671 F g⁻¹, which is much lower than that of the 8.9% NiO_x electrode. This decrease in specific capacitance of the high wt.% NiO_x/CNT electrodes can be attributed to the dead volume of the oxides, high equivalent series resistance for a heavier deposit, and the ineffective ionic transportation caused by the destruction of the 3-dimensional network structure. Deconvolution analysis of the cyclic voltammograms reveals that the rate capability of the NiO_x/CNT electrodes is adversely affected by the redox reaction of Ni(OH)₂, while the adverse effects from the reaction of NiO_x is insignificant.     

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.     

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.     

Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries

Porous nanostructured LiFePO₄ powder with a narrow particle size distribution (100–300 nm) for high rate lithium-ion battery cathode application was obtained using an ethanol based sol–gel route employing lauric acid as a surfactant. The synthesized LiFePO₄ powders comprised of agglomerates of crystallites <65 nm in diameter exhibiting a specific surface area ranging from 8 m² g⁻¹ to 36 m² g⁻¹ depending on the absence or presence of the surfactant. The LiFePO₄ obtained using lauric acid resulted in a specific capacity of 123 mAh g⁻¹ and 157 mAh g⁻¹ at discharge rates of 10C and 1C with less than 0.08% fade per cycle, respectively. Structural and microstructural characterization were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray (EDX) analysis while electronic conductivity and specific surface area were determined using four-point probe and N₂ adsorption techniques.     

High-frequency carbon supercapacitors from polyfurfuryl alcohol

Porous carbons with controllable and narrow pore-size distributions are prepared from the chemical activation of polyfurfuryl alcohol (PFA). High apparent BET surface areas, up to 2600 m² g⁻¹ (2611 m² g⁻¹ by Density Functional Theory (DFT)), and good electrical conductivities (up to ∼130 S cm⁻¹) are obtained. By varying the potassium hydroxide: carbon precursor ratio, the preparation of carbons with different proportions of micro- and fine mesoporosity (<5 nm) can be tailored to provide an ideal electronic and ionic pore structure for electrochemical energy-storage devices, such as electrical double-layer capacitors. High specific capacitance values are obtained up to 147 F g⁻¹ in a voltage window of 2.5 V using 1 M tetraethyl ammonium tetrafluoroborate in acetonitrile. Moreover, excellent high-current and high-frequency performance is demonstrated: 100 F g⁻¹ at 225 A g⁻¹ (10 Hz) and ∼30 F g⁻¹ at 100 Hz. When comparing the performance with commercial activated carbons (ACs) of similar textural properties, the PFA-derived ACs demonstrated better performance in terms of higher capacitance values and improved rate capabilities. There is a 125% increase in capacitance values at 1 kHz.     

Electrochemical performance of mesoporous TiO2 anatase

Mesoporous TiO₂ was prepared via a sol–gel method from an ethylene glycol-based titanium-precursor in the presence of a non-ionic surfactant at pH 2. Only the anatase structure was detected after annealing, while the BET specific surface area was measured as being 90 m² g⁻¹ with a rather monomodal pore diameter close to 5 nm. Electrochemical performances were investigated by cyclic voltammetry and galvanostatic techniques. Mesoporous TiO₂ exhibits excellent rate capability (184 mAh g⁻¹ at C/5, 158 mAh g⁻¹ at 2C, 127 mAh g⁻¹ at 6C, and 95 mAh g⁻¹ at 30C) and good cycling stability.     

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.     

Growth of polyaniline nanowhiskers on mesoporous carbon for supercapacitor application

Vertically aligned polyaniline nanowhiskers (PANI-NWs) doped with (1R)-(−)-10-Camphorsulfonic acid (L-CSA) have been successfully synthesized on the external surface of ordered mesoporous carbon (CMK-3) by chemical oxidative polymerization. The specific surface area of the PANI-NWs/CMK-3 nanocomposite remains as high as 497 m² g⁻¹ by removing mesoporous silica template after the polymerization of aniline. Structural and morphological characterizations of the nanocomposite were further investigated by XRD, FTIR and FE-SEM measurements. The result shows that the nanocomposite with 40 wt% PANI applying in supercapacitor devices possesses a large specific capacitance of 470 F g⁻¹ and good capacitance retention of 90.4% is achieved after 1000 cycles at a current density of 1.0 A g⁻¹. The synergistic effect of small PANI nanowhisker arrays and well-ordered mesoporous carbon endows the composite with high electrochemical capacitance and good cycling stability.     

Electrodeposition of manganese dioxide in three-dimensional poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid)–polyaniline for supercapacitor

The preparation of composites of precise metal oxides/conducting polymers is important in studies of supercapacitors. In this work, a three-dimensional matrix of poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid)–polyaniline (PEDOT–PSS–PANI) was prepared by interfacial polymerization of ANI into PEDOT–PSS. Conductivity was enhanced by incorporating of PANI into PEDOT–PSS because of the decrease in the distance for electron shuttling along the conjugated polymeric chain. Composite electrodes were prepared by the electrodeposition of manganese dioxide (MnO₂) in a PEDOT–PSS–PANI three-dimensional matrix. The electrodes were characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry techniques. The results show a significant improvement in the specific capacitance of the composite electrode. For PEDOT–PSS the specific capacitance was of 0.23 F g⁻¹, while PEDOT–PSS–PANI and PEDOT–PSS–PANI–MnO₂ displayed values of 6.7 and 61.5 F g⁻¹, respectively. When only considering the MnO₂ mass, the composite had the specific capacitance of 372 F g⁻¹. The composite also had an excellent cyclic performance.