Dispersing WO3 in carbon aerogel makes an outstanding supercapacitor electrode material

Tungsten oxide, originally poor in capacitive performance, was made an excellent electrode material for supercapacitors, by dispersing it to carbon aerogels (CA), a conductive and mesoporous hosting template, that drastically improved the utilization of WO₃ for capacitance generation. The WO₃ was introduced to the CA, in a form of well-dispersed single crystalline nanoparticles of 15–40 nm in size, with a simple immersion-calcination process. A one order of magnitude improvement in specific capacitance was achieved with the present composition, from 54 F/g for WO₃ nanoparticles to 700 F/g for WO₃/CA composites (scaned at 25 mV/s in 0.5 M H₂SO₄ over a potential window of −0.3 to 0.5 V). The WO₃/CA composites exhibited an excellent high rate capability with a 60% retention in specific capacitance at 500 mV/s, almost perfect cycle efficiency of 99%, and outstanding cycling stability of only 5% decay in specific capacitance after 4000 cycles.     

Synthesis of NiMoO4 nanorods on graphene and superior electrochemical performance of the resulting ternary based composites

NiMoO₄ nanorods have been directly synthesised on graphene sheet using a facile solvothermal method with subsequent calcination treatment. As an electrode for electrochemical capacitors, the graphene-NiMoO₄ composite electrode prepared at a mass ratio of 1:8 exhibited a specific capacitance of 670 F/g at 0.3 A/g and good rate capability. When cycled at 0.5 A/g for 3000 cycles, it retained 88% of its initial capacitance with a Coulombic efficiency of ~ 80%. This study presents a pungent and environmental benign research strategy for the development of graphene-NiMoO₄ ternary based electrochemical capacitors.     

RGO/KMn8O16 composite as supercapacitor electrode with high specific capacitance

Reduced graphene oxide/cryptomelane (RGO/KMn₈O₁₆) composites are successfully synthesized from α-MnO₂ nanorods and GO using a water-bathing precipitation method. The unique structure of KMn₈O₁₆ nanorods, with a length of 2–4 μm, dispersed on the surface of RGO leads to a much enhanced electrical conductivity and ionic transport, finally achieving composites with an improved electrochemical performance. Electrochemical measurement results show a specific capacitance of 222.3 F/g at a current density of 0.2 A/g, much higher than that of the original α-MnO₂. After 500 cycles at 2.0 A/g, the RGO/KMn₈O₁₆ composite electrode still retains 92.6% of its initial specific capacitance. The excellent electrochemical performance and durability observed for this composite electrode suggest its potential application for electrochemical capacitors.     

Carbon nanotube capacitors arrays using high-k dielectrics

The electrical characteristics and fabrication process of nanocapacitor arrays using metal-high-k dielectric-carbon nanotube-metal layers (MICntM) were studied. MWCNTs arrays were fabricated using an electron beam lithography based lift-off process for catalyst definition and the high-k dielectric layer, hafnium oxide (HfO₂), was deposited using rf magnetron sputtering. The MICntM structures show high capacitance and the compatibility with high-k dielectric material and its deposition processes. MICntM capacitors arrays with sputtered HfO₂ show specific capacitance of 0.62 μF/cm². The leakage current density at 1 V is less than 5 μA/cm². The high aspect ratio of MWCNTs increases the effective electrode area and HfO₂ allows higher permittivity, hence, higher capacitance structures are realized.     

Electrophoretic deposition of MnO2-coated carbon nanotubes on a graphite sheet as a flexible electrode for supercapacitors

A flexible electrode was prepared by microwave heating deposition of manganese oxide (MnO₂) on carbon nanotubes (CNTs) followed by electrophoretic deposition of the MnO₂-coated CNTs on a flexible graphite sheet (FGS). The prepared MnO₂-coated CNTs were characterized by scanning and transmission electron microscopy, and X-ray diffraction. A uniformly thin nano-scale MnO₂ coating was formed on the surface of the CNTs. The MnO₂-coated CNTs–FGS electrode showed highly capacitive behaviour in the 0.5 M Na₂SO₄ aqueous solution, with a specific capacitance of 442.9 F/g based on MnO₂ at 2 mV/s. It exhibited an excellent cycling stability with no more than 1.1% capacitance loss after 1000 cycles at 50 mV/s.     

Metal–insulator–metal capacitors using Ba2Ti9O20 dielectric thin film

The dielectric properties of Ba₂Ti₉O₂₀ film were investigated to evaluate its potential use in metal–insulator–metal (MIM) capacitors. A homogeneous crystalline Ba₂Ti₉O₂₀ phase without any second phase developed for the film grown at 700 °C and rapid thermal annealed at 900 °C for 3 min. The 200 nm-thick Ba₂Ti₉O₂₀ film showed a capacitance density of 2.0 fF/μm² with a low dissipation factor of 0.016 at 100 kHz. The capacitance density of the film was low, but it could be increased by decreasing the thickness of the film. The leakage current density was approximately 0.094 nA/cm² at 1 V. A small linear voltage coefficient of capacitance of −690 ppm/V was obtained, together with a quadratic one of −67.41 ppm/V² and a small temperature coefficient of capacitance of −168.87 ppm/°C at 100 kHz. All these results show that the Ba₂Ti₉O₂₀ film is a good candidate material for MIM capacitors.     

Influence of the electrochemical reduction process on the performance of graphene-based capacitors

Electrochemically reduced graphene was used as the key element in the preparation of electric double-layer capacitors where the thickness of the electrode was only a few hundred nano-meters. The resultant electrodes showed different specific capacitances after pre-reduction with scanning potential windows of −1.0 to 1.6 V, −1.5 to 0 V and −1.0 to 1.0 V. Also, a specific capacitance of 246 F/g was obtained as the graphene oxide electrode was reduced with an applied potential of −1.0 to 1.0 V for 4000 s. The influence of the residual oxygen functional groups and sp² domains in electrochemically reduced graphene were investigated for capacitance performance.     

Sulfur-incorporated,porous graphene films for high performance flexible electrochemical capacitors

Graphene and its derivatives are considered potential electrode materials for flexible electrochemical capacitors (f-ECs), but their capacitive performances have to be improved for practical applications. Herein, we demonstrate fabrication of flexible sulfur (S)-incorporated reduced graphene oxide (SRGO) electrodes obtained by pyrolyzing free-standing film consisting of benzyl disulfide-functionalized graphene oxides at 900 °C. The effect of S incorporation on morphology and chemical structure of SRGO were investigated by various microscopic and spectroscopic methods. Incorporation of S and the crumpled and porous morphology of SRGO electrodes improve capacitive performance of f-ECs; SRGO f-ECs show a specific capacitance of 140.8 F/g at 1 A/g, rate capability of 91.5% retention, and cyclic performance of 93.4% after 1000 charge/discharge cycles at 4 A/g. Impressively, SRGO f-ECs exhibit excellent electrochemical and mechanical durability after 1000 charge/discharge cycles at a bending angle of 120° with values that greatly exceed those of conventional RGO-based f-ECs. This study provides a fundamental foundation of the correlation between S composition of carbon nanomaterials and their electrochemical (or surface) properties.     

A facile approach to produce holey graphene and its application in supercapacitors

We demonstrate a low-cost and simple method to prepare holey graphene nanosheets (HGNSs) by ultra-rapid heating during the process of thermal reduction/exfoliation of graphite oxide. The number density of the holes increased with the heating rate, and the size ranged between 10 and 250 nm. In addition, supercapacitors (SCs) using HGNSs as the electrode material were fabricated and their performances were evaluated. Compared to common graphene, HGNSs offered much higher capacitance values and better capability at high rates due to the much shorter cross-plane ion transport paths in the graphene stack through the large amount of holes on graphene sheets. The SC with HGNS electrodes exhibited an excellent high-rate capacitance of 170 F/g at 50 A/g in a 6 M KOH aqueous electrolyte. In the low-rate limit, the HGNS SC showed a very large specific capacitance of 350 F/g at 0.1 A/g.     

Towards high purity graphene/single-walled carbon nanotube hybrids with improved electrochemical capacitive performance

CoMgAl layered double hydroxides were prepared as catalysts for the in situ synchronous growth of graphene and single-walled carbon nanotubes (SWCNTs) from methane by chemical vapor deposition. The as-calcined CoMgAl layered double oxide (LDO) flakes served as the template for the deposition of graphene, and Co nanoparticles (NPs) embedded on the LDOs catalyzed the growth of SWCNTs. After the removal of CoMgAl LDO flakes, graphene (G)/SWCNT/Co₃O₄ hybrids with SWCNTs directly grown on the surface of graphene and 27.3 wt.% Co₃O₄ NPs encapsulated in graphene layers were available. Further removal of the Co₃O₄ NPs by a CO₂-oxidation assistant purification method induced the formation of G/SWCNT hybrids with a high carbon purity of 98.4 wt.% and a high specific surface area of 807.0 m²/g. The G/SWCNT/Co₃O₄ hybrids exhibited good electrochemical performance for pseudo-capacitors due to their high Co₃O₄ concentration and the high electrical conductivity of SWCNTs and graphene. In another aspect, the G/SWCNT hybrids can be used as excellent electrode materials for double-layer capacitors. A high capacity of 98.5 F/g_electrode was obtained at a scan rate of 10 mV/s, 78.2% of which was retained even when the scan rate increased to 500 mV/s.     

Synthesis of mesoporous carbons for supercapacitors from coal tar pitch by coupling microwave-assisted KOH activation with a MgO template

Mesoporous carbons (MCs) for supercapacitors were prepared from coal tar pitch by a microwave-assisted one-step process coupling the potassium hydroxide (KOH) activation and magnesium oxide (MgO) template. MCs were characterized by scanning electron microscope and X-ray diffraction. The results show that the specific surface area (S_BET), micropore volume and specific capacitance of MCs made by microwave heating as well as the energy density of MC capacitors pass through a maximum with increasing mass of MgO and the relative mass ratio of KOH/pitch. The S_BET of MCs varies from 1003 to 1394 m²/g. The S_BET and total pore volume of MC and microporous carbon made by microwave heating are bigger than that made by conventional heating. Under optimum conditions with the masses of coal tar pitch, MgO, KOH at 9 g, 12 g, 6 g, and the microwave power at 600 W, MC (MC_9-12-6) made at 30 min heating time shows a high specific capacitance of 224 F/g in 6 M KOH aqueous electrolyte after 1000 cycles. The results have shown that microwave-assisted rapid KOH activation coupled with the MgO template is an efficient one-step approach to the preparation of low cost yet high performance MCs for supercapacitors.     

Carbon nanotube/graphene composite for enhanced capacitive deionization performance

In this study, single-walled carbon nanotubes were combined with graphene oxide nanosheets in aqueous dispersion and then chemically reduced to form the carbon nanotube/graphene (CNT/G) composite as electrodes for capacitive deionization (CDI). The structure of the CNT/G composite was highly porous, with single-walled carbon nanotubes (SWCNTs) sandwiched between graphene sheets that functioned as spacers and provided diffusion paths for smooth and rapid ion conduction. The associated increase in the electrical double-layer capacitance enhanced capacitive deionization performance. The CNT/G composite achieved a specific capacitance of 220 F/g and an electrosorption capacity of 26.42 mg/g with 100% regeneration, showing great potential as a high performance electrode material in CDI applications.     

Porous carbon thin films for electrochemical capacitors

Activation effects on carbon films, derived from commercial aromatic polyimide films (Kapton, DuPont), in CO₂ atmosphere at 1203 K on capacitance properties were studied. Two thicknesses of polyimide films were used: 7 and 25 μm. Pore formation during the activation process progresses in two steps due to the existence of a denser surface layer and a more porous core material. In the first step micropores are opening in the dense surface region of the material with average pore diameter smaller than 1 nm. During the second step, mesopores start opening, while micropore volume remains constant with the average micropore diameter of over 1 nm, producing bimodal texture. The first step finishes after 30 min for the thinner samples while for the thicker samples it finishes after 60 min of activation. As a consequence of such textural changes during activation, the thicker sample has a maximum areal capacitance of 0.35 F/cm². The thinner sample activated for 30 min has a maximum volumetric capacitance of 220 F/cm³ and achieves a maximum gravimetric capacitance of 240 F/g when the texture becomes bimodal after 240 min of activation. These results confirm that activation of carbonized Kapton films gives promising electrode materials for supercapacitors.     

Effects of adding different morphological carbon nanomaterials on supercapacitive performance of sol–gel manganese oxide films

In this study, the supercapacitive performances of manganese oxide films were investigated by adding different carbon nanomaterials, including carbon nanocapsules (CNC), multiwalled carbon nanotubes (MWCNTs) and multi-layered graphene. The manganese oxide films were prepared with manganese acetate precursor by sol–gel method, and the post-treatment effects were also examined. With a heat-treatment above 300 °C, the as-prepared amorphous films transformed to a compound of Mn₃O₄ and Mn₂O₃ phases, and the smooth surface became rough as well. Cyclic voltammogram (CV) tests showed that the manganese oxide film, which was mixed with 0.05 wt% MWCNTs and annealed at 350 °C for 1 h, exhibited the optimized specific capacitance, 339.1 F/g. During 1000CV cycles, the specific capacitances of original manganese oxide film decreased gradually from 198.7 to 149.1 (75%) F/g. After same number of cycle tests, the modified films containing 0.025 wt% CNC, 0.05 wt% MWCNTs and 0.1 wt% graphene retained 201.8 (64.2%), 267.4 (78.9%) and 193.1 (57.4%) F/g respectively. The results indicates that the supercapacitive performance of manganese oxide films were significantly modified by carbon nanomaterials; in addition, the MWCNTs additive could also reduce the decay rate.     

Preparation and electrochemical investigation of the polyaniline/activated carbon nanocomposite for supercapacitor applications

The polyaniline (PANI)/activated carbon (AC) nanocomposite electrodes were prepared by electropolymerization of aniline monomers on the surface of AC/polyvinyl alcohol (PVA) electrodes for supercapacitor studies. Fourier transforms infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses were performed to characterize the structure and morphology of the nanocomposite electrodes. The electrochemical properties of the prepared nanocomposite electrodes and the supercapacitive behavior of the PANI, AC, and AC/PANI/PVA electrodes were investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge measurements, respectively. Morphological studies showed that a thin film of PANI has been uniformly deposited on the porous surface of AC electrode, and an ordered arrangement of nanostructures with interlinked porous network has been made. Electrochemical measurements showed that AC particles prevent the degradation of PANI chains during charge/discharge cycles. The specific capacitance of the AC/PANI/PVA nanocomposite electrode was 338.15 F/g which is higher than that of the pristine AC electrode (0.08 F/g). This is due to the contribution of PANI chains by their pseudocapacitance (redox reaction) properties. Although the specific capacitance of PANI electrode (378.57 F/g) was greater than that of the nanocomposite electrode, the cyclic stability of the PANI electrode was lower than that of the AC/PANI/PVA nanocomposite electrode.     

Effect of simultaneous etching and N-doping on the surface and electrochemical properties of AC

To improve the electrical performance of activated carbon (AC)-based electric double-layer capacitors (EDLCs), the surface of AC was modified with gas phase ammonia treatment at 1073 K with different treatment times to carry out simultaneous etching and N-doping. The effects of the treatment on AC surfaces and their electrochemical properties were investigated. The specific capacitances of samples treated for 22 min were increased to 426 F/g at scan rates of 10 mV/s, which corresponded to a 76.8% increase as compared with 241 F/g of samples measured as received from the manufacturer. The increase is attributed to an increase in the specific surface area and the total pore, micro- and mesopore volumes due to the etching effect of the high-temperature ammonia gas reaction. Moreover, N-functional groups, which were introduced by the treatment, also aided to improve the electrochemical properties of the resulting AC-based electrode. Therefore, the simultaneous etching and N-doping method with ammonia gas at high temperature can easily introduce nitrogen functional groups on the AC surface. In addition, the reaction of nitrogen gas with AC can affect its specific surface area and surface pore structure, which is very effective in preparing AC for EDLCs with improved electrochemical properties.     

Synthesis of cross-linked graphene aerogel/Fe2O3 nanocomposite with enhanced supercapacitive performance

In this work, cross-linked graphene aerogel (CL-GA) and its composite with Fe₂O₃ nanoparticles (NPs) were synthesized through a one-step hydrothermal procedure by using p-phenylenediamine (PPD). Structural characterizations revealed that in the preparation of the composite PPD acts as a cross-liker and provides high surface area by decreasing restacking of graphene sheets and functions as nitrogen source simultaneously. The electrochemical characteristics of the nanocomposite were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS) and Fast Fourier transform continues cyclic voltammetry (FFTCCV). The results show that cross-linked graphene aerogel/Fe₂O₃ (CL-GA/Fe₂O₃) nanocomposite displays enhanced supercapacitive performance, where it has capacitance of 445 at 1 A g⁻¹, high energy density of 63 W h Kg⁻¹, and 89% capacitance retention after 5000 cycles in 3 M KOH. Presence of PPD considerably improved supercapacitive performance of nanocomposite as a result it could be promising material in synthesis of efficient graphene/metal oxide-based electrode material for high performance supercapacitors.     

Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process

We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets into hollow graphene spheres (HGSs), using only a GO/H₂SO₄ aqueous suspension as the starting material. Scanning electron microscope, focused ion beam scanning electron microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 μm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g⁻¹, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors.     

Synthesis and characterization of mesoporous carbon through inexpensive mesoporous silica as template

Mesoporous silica materials have been synthesized through sol–gel reaction using inexpensive sodium silicate as source of silica and low cost hydroxy carboxylic acid compounds as templates/pore forming agents. The material measured surface area of 1014 m²/g, pore diameter of 65 Å and pore volume of 1.4 cc/g when parameters like time and temperature of synthesis along with mole ratio of TA/SiO₂ were optimized. Here TA stands for tartaric acid. Carbonization of sucrose inside the pores of above silica material at 900 °C followed by removal of silica framework using aqueous ethanoic solution of NaOH gave rise to mesoporous carbon material. The resulting materials were characterized by N₂-sorption, FTIR spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, thermal analysis and cyclic voltammetry. Three dimensional interconnecting wormhole channel arrangement of mesoporous silica template leads to mesoporous carbon replica with surface area of 1200 m²/g. X-ray photoelectron spectroscopic study (XPS) of the mesoporous carbon material shows the concentration of carbon atom in the range of 97–98% with 1–2% oxygen and negligible amount of silica. The electrochemical double layer capacitance behavior of carbon material with the specific capacitance value of 88.0 F/g at the scan rate of 1 mV/s appears to be promising.     

Aerosol deposition for post-LTCC

Embedded capacitor technology is one of the most promising methods to achieve miniaturization, reduced costs and higher performance in rf wireless communication products. Much R&D on embedded capacitors has been conducted using different circuit board technologies; however, none of the circuit boards developed to date satisfy all of the requirements of embedded passive integration with high performance at low cost. Our unique method of aerosol deposition (ASD) can provide passive ceramic elements embedded in the resin substrate because it can produce ceramic dielectric film by accelerated ceramic nanoparticle aerosol bombardment at room temperature.We present our novel ASD approach to embedded capacitors in FR-4 resin and discuss the correlation between the microstructure and dielectric properties of ASD dielectric films deposited under various conditions. We confirmed that dense BaTiO₃ dielectric films with a dielectric constant of 400, tan δ of less than 2%, and a higher breakdown voltage, exceeding 80 kV/mm, could be formed on the resin substrates. The embedded capacitors on the FR-4 substrate, fabricated as a prototype using this ASD film, demonstrated a capacitance density of 300 nF/cm². We also clarified that variations in ASD film dielectric properties after thermal cycle testing between −55 and 160 °C for 2000 cycles was within 10% compared with as-deposited film.