Ru oxide supercapacitors with high loadings and high power and energy densities

Supercapacitors with very high energy and power densities have been constructed with hydrous ruthenium oxide powder prepared by a sol–gel method and annealed at 110 °C. Novel features of the capacitors, which improve their performances, are the use of a carbon fibre paper support, a Nafion separator, and Nafion as a binder. 1 M sulfuric acid was employed as the electrolyte. The performances of the supercapacitors were characterized by cyclic voltammetry, impedance spectroscopy and constant current discharging. The interfacial capacitance increased linearly with increasing ruthenium oxide loading to at least 50 mg cm⁻² on each electrode. The gravimetric capacitance of the Ru oxide measure by impedance reached 742 F g⁻¹ (9.66 F cm⁻²) at a loading of 13.0 mg cm⁻², and an interfacial capacitance of 34.9 F cm⁻² (682 F g⁻¹) was obtained at 51.2 mg cm⁻². The average effective series resistance was 0.55 Ω, the electronic resistance of the electrodes was negligible, and their ionic resistances were <0.42 Ω. The average power density for full discharge at 1 A cm⁻² for supercapacitors with 10 mg cm⁻² Ru oxide increased by 39% when 5% Nafion binder was added. The maximum average power density for full discharge was 31.5 W g⁻¹ while the maximum energy density was 31.2 Wh kg⁻¹. At a 1 mA discharge rate a specific capacitance of 977 F g⁻¹ of Ru oxide was obtained.     

An asymmetric anthraquinone-modified carbon/ruthenium oxide supercapacitor

An asymmetric supercapacitor with improved energy and power density, relative to a symmetric Ru oxide device, has been constructed with anthraquinone-modified carbon fabric (Spectracarb 2225) as the negative electrode and Ru oxide as the positive electrode. The performance of the supercapacitor was characterized by cyclic voltammetry and constant current discharging. Use of the anthraquinone-modified electrode extends the negative potential limit that can be used, relative to Ru oxide, and allows higher cell voltages to be used. The maximum energy density obtained was 26.7 Wh kg⁻¹ and an energy density of 12.7 Wh kg⁻¹ was obtained at a 0.8 A cm⁻² discharge rate and average power density of 17.3 kW kg⁻¹. The C-AQ/Ru oxide supercapacitor requires 64% less Ru relative to a symmetric Ru oxide supercapacitor.     

Performance and stability of electrochemical capacitor based on anthraquinone modified activated carbon

A series of high surface area activated carbon powders modified with various loadings of electroactive anthraquinone groups was obtained by the spontaneous reduction of the corresponding in situ generated diazonium derivative on activated carbon. The diazotation and grafting reactions are fast and efficient and by varying the stoichiometry of these reactions the grafting amount can be controlled. With appropriate reaction conditions, the attachment of anthraquinone groups allows to double the capacitance of the modified carbonaceous material (195 F g⁻¹) compared to the unmodified carbon (100 F g⁻¹) due to the contribution of the redox reaction of grafted anthraquinone molecules. Long time galvanostatic charge-discharge cycling experiments were performed for composite electrodes prepared using modified carbons having two different AQ loadings (e.g. 6.7 and 11.1 wt.%). Following 10 000 charge/discharge cycles, only a 17% loss of the faradaic capacitance was observed for these two carbons. Thus, this hybrid bifunctional material appears to be an excellent candidate for application as active electrode in electrochemical capacitors.     

Hollow nickel-cobalt sulfide nanospheres cathode hybridized with carbon spheres anode for ultrahigh energy density asymmetric supercapacitors

There has been increasing attention to the transition metal sulfides because of their unique property such as high electrical conductivity. Moreover, hollow sphere structures can provide excellent performance in electrochemical energy conversion applications due to their rich holes volume and unique properties. In this study, nickel-cobalt sulfide （Ni-Co-S） hollow spheres are successfully synthesized by two step methods of hard template method and hydrothermal method. Benefiting from the hollow sphere structure with optimum size and mesoporous surface, the Ni-Co-S exhibits an excellent specific capacity of 819.9 C g^?1, and has good stability in the charge-discharge cycles. In addition, uniform size and monodisperse carbon sphere (CS) that synthesized by a simple self-assembly method also shows excellent electrochemical properties. Furthermore, the fabricated Ni-Co-S//CS asymmetric supercapacitor shows energy density of 65 Wh kg^?1 at a power density of 850 W kg^?1 and outstanding cycling stability with a capacity retention of 82.8% after 10,000 cycles, demonstrating that as-prepared Ni-Co-S hollow spheres and carbon spheres possess promising potential as electrode materials for high-performance asymmetric supercapacitors.     

Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors

Graphene nanosheet/carbon nanotube/polyaniline (GNS/CNT/PANI) composite is synthesized via in situ polymerization. GNS/CNT/PANI composite exhibits the specific capacitance of 1035 F g⁻¹ (1 mV s⁻¹) in 6 M of KOH, which is a little lower than GNS/PANI composite (1046 F g⁻¹), but much higher than pure PANI (115 F g⁻¹) and CNT/PANI composite (780 F g⁻¹). Though a small amount of CNTs (1 wt.%) is added into GNS, the cycle stability of GNS/CNT/PANI composite is greatly improved due to the maintenance of highly conductive path as well as mechanical strength of the electrode during doping/dedoping processes. After 1000 cycles, the capacitance decreases only 6% of initial capacitance compared to 52% and 67% for GNS/PANI and CNT/PANI composites.     

Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes

Graphene nanosheets (GNs) dispersed with SnO₂ nanoparticles loaded multiwalled carbon nanotubes (SnO₂-MWCNTs) were investigated as electrode materials for supercapacitors. SnO₂-MWCNTs were obtained by a chemical method followed by calcination. GNs/SnO₂-MWCNTs nanocomposites were prepared by ultrasonication of the GNs and SnO₂-MWCNTs. Electrochemical double layer capacitors were fabricated using the composite as the electrode material and aqueous KOH as the electrolyte. Electrochemical performance of the composite electrodes were compared to that of pure GNs electrodes and the results are discussed. Electrochemical measurements show that the maximum specific capacitance, power density and energy density obtained for supercapacitor using GNs/SnO₂-MWCNTs nanocomposite electrodes were respectively 224 F g⁻¹, 17.6 kW kg⁻¹ and 31 Wh kg⁻¹. The fabricated supercapacitor device exhibited excellent cycle life with ∼81% of the initial specific capacitance retained after 6000 cycles. The results suggest that the hybrid composite is a promising supercapacitor electrode material.     

Optimization of power and energy densities in supercapacitors

Supercapacitors use nanoporous electrodes to store large amounts of charge on their high surface areas, and use the ions in electrolytes to carry charge into the pores. Their high power density makes them a potentially useful complement to batteries. However, ion transport through long, narrow channels still limits power and efficiency in these devices. Proper design can mitigate this. Current collector geometry must also be considered once this is done. Here, De Levie's model for porous electrodes is applied to quantitatively predict device performance and to propose optimal device designs for given specifications. Effects unique to nanoscale pores are considered, including that pores may not have enough salt to fully charge. Supercapacitors are of value for electric vehicles, portable electronics, and power conditioning in electrical grids with distributed renewable sources, and that value will increase as new device fabrication methods are developed and proper design accommodates those improvements. Example design outlines for vehicle applications are proposed and compared.     

Facile preparation and electrochemical characterization of cobalt oxide/multi-walled carbon nanotube composites for supercapacitors

A series of cobalt oxide/multi-walled carbon nanotube (Co₃O₄/MWCNT) composites are successfully synthesized by a facile chemical co-precipitation method followed by a simple thermal treatment process. The morphology and structure of as-obtained composites are characterized by X-ray diffraction, scanning electron microscopy, and N₂-adsorption/desorption measurements, and the electrochemical properties are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). For all Co₃O₄/MWCNT composites, MWCNTs are well dispersed in the loosely packed Co₃O₄ nanoparticles. Among them, the Co₃O₄-5%MWCNT composite exhibits the highest specific surface area of 137 m² g⁻¹ and a mesoporous structure with a narrow distribution of pore size from 2 to 10 nm. Because of the synergistic effects coming from Co₃O₄ nanoparticles and MWCNTs, the electrochemical performances of pure Co₃O₄ material are significantly improved after adding MWCNTs. The Co₃O₄-5%MWCNT composite shows the largest specific capacitance of 418 F g⁻¹ at a current density of 0.625 A g⁻¹ in 2 M KOH electrolyte. Furthermore, this composite exhibits good cycling stability and lifetime. Therefore, based on the above investigation, such Co₃O₄/MWCNT composite could be a potential candidate for supercapacitors.     

Enhanced Coulombic efficiency and power density of air-cathode microbial fuel cells with an improved cell configuration

Single chamber air-cathode microbial fuel cells (MFCs) that lack a proton exchange membrane (PEM) hold a great promise for many practical applications due to their low operational cost, simple configuration and relative high power density. One of the great challenges for PEM-less MFC is that the Coulombic efficiency is much lower than those containing PEM. In this study, single-chamber PEM-less MFCs were adapted by applying a J-Cloth layer on the water-facing side of air cathode. Due to the significant reduction of oxygen diffusion by the J-Cloth, the MFCs with two-layers of J-Cloth demonstrated an over 100% increase in Coulombic efficiency in comparison with those without J-Cloth (71% versus 35%) at the same current density of 0.6 mA cm⁻². A new cell configuration, cloth electrode assembly (CEA), therefore, was designed by sandwiching the cloth between the anode and the cathode. Such an MFC configuration greatly reduced the internal resistance, resulting in a power density of 627 W m⁻³ when operated in fed-batch mode and 1010 W m⁻³ in continuous-flow mode, which is the highest reported power density for MFCs and more than 15 times higher than those reported for air-cathode MFCs using similar electrode materials. This study indicates that the Coulombic efficiency and power density of air-cathode MFCs can be improved significantly using an inexpensive cloth layer, which greatly increases the feasibility for the practical applications of MFCs.     

Waste sugar solution polymer-derived N-doped carbon spheres with an ultrahigh specific surface area for superior performance supercapacitors

Waste sugar solution (WSS), a waste by-product of manufacturing vitamin C, contains abundant waste acids and organics. In this work, a N/O-enriched copolymer was synthesized via a facile polymerization via the hydrogen bonding of O-containing functional groups and melamine and the crosslinking of aldehyde groups. Subsequently, N-doped carbon spheres were prepared by a typical carbonation/activation method. Remarkably, benefiting from an ultrahigh specific surface area (3612 m²/g) and rich heteroatom content (4.3% for N, 8.8% for O), the carbon spheres deliver a high specific capacitance of 387 F/g at 50 mA/g and 283 F/g at 5 A/g with 6 M KOH in two-electrode system. The assembled symmetric electric double-layer capacitor exhibits high energy density of 10.83 Wh/kg at 11.10 W/kg. This research provides a facile method for preparing N/O-doped carbon spheres by WSS, and confirms the excellent electrochemical performance of WSS-derived carbons in energy storage applications.     

The preparation and performance of calcium carbide-derived carbon/polyaniline composite electrode material for supercapacitors

Calcium carbide (CaC₂)-derived carbon (CCDC)/polyaniline (PANI) composite materials are prepared by in situ chemical oxidation polymerization of an aniline solution containing well-dispersed CCDC. The structure and morphology of CCDC/PANI composite are characterized by Fourier infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM) and N₂ sorption isotherms. It has been found that PANI was uniformly deposited on the surface and the inner pores of CCDC. The supercapacitive behaviors of the CCDC/PANI composite materials are investigated with cyclic voltammetry (CV), galvanostatic charge/discharge and cycle life measurements. The results show that the CCDC/PANI composite electrodes have higher specific capacitances than the as grown CCDC electrodes and higher stability than the conducting polymers. The capacitance of CCDC/PANI composite electrode is as high as 713.4 F g⁻¹ measured by cyclic voltammetry at 1 mV s⁻¹. Besides, the capacitance retention of coin supercapacitor remained 80.1% after 1000 cycles.     

Synthesis and characterization of tin oxide/carbon aerogel composite electrodes for electrochemical supercapacitors

Two types of carbon aerogel-based functional electrodes for supercapacitor applications are developed. To improve the electrochemical performance of the electrodes, carbon aerogels are doped with pseudocapacitive tin oxide either by impregnating tin oxide sol into resorcinol–formaldehyde (RF) wet gels (Method I), or by impregnating tin tetrachloride solution into carbon aerogel electrodes (Method II). The electrodes are heat-treated to 450 °C in air to activate the electrode surface and complete the oxidation of tin-precursors in the network structure of the aerogel. The effects of different impregnation methods on the physical/electrochemical properties of the composite electrodes are investigated. Microstructural and compositional variations of the electrodes with tin oxide doping are also examined by scanning electron microscopy and energy dispersive X-ray analysis. The tin oxide/carbon aerogel composite electrodes synthesized by both methods have similar specific capacitances (66–70 F g⁻¹). Composite electrodes synthesized via Method II showed better cyclic stability compared with electrodes synthesized via Method I.     

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.     

Life cycle GHG assessment of fossil fuel power plants with carbon capture and storage

The evaluation of life cycle greenhouse gas emissions from power generation with carbon capture and storage (CCS) is a critical factor in energy and policy analysis. The current paper examines life cycle emissions from three types of fossil-fuel-based power plants, namely supercritical pulverized coal (super-PC), natural gas combined cycle (NGCC) and integrated gasification combined cycle (IGCC), with and without CCS. Results show that, for a 90% CO₂ capture efficiency, life cycle GHG emissions are reduced by 75–84% depending on what technology is used. With GHG emissions less than 170 g/kWh, IGCC technology is found to be favorable to NGCC with CCS. Sensitivity analysis reveals that, for coal power plants, varying the CO₂ capture efficiency and the coal transport distance has a more pronounced effect on life cycle GHG emissions than changing the length of CO₂ transport pipeline. Finally, it is concluded from the current study that while the global warming potential is reduced when MEA-based CO₂ capture is employed, the increase in other air pollutants such as NO_x and NH₃ leads to higher eutrophication and acidification potentials.     

Future carbon regulations and current investments in alternative coal-fired power plant technologies

We analyze how uncertain future US carbon regulations shape the current choice of the type of power plant to build. Our focus is on two coal-fired technologies, pulverized coal (PC) and integrated coal gasification combined cycle technology (IGCC). The PC technology is cheapest—assuming there is no need to control carbon emissions. The IGCC technology may be cheaper if carbon must be captured. Since power plants last many years and future regulations are uncertain, a US electric utility faces a standard decision under uncertainty. A company will confront the range of possible outcomes, assigning its best estimate of the probability of each scenario, averaging the results and determining the power plant technology with the lowest possible cost inclusive of expected future carbon related costs, whether those costs be in the form of emissions charges paid or capital expenditures for retrofitting to capture carbon. If the company assigns high probability to no regulation or to less stringent regulation of carbon, then it makes sense for it to build the PC plant. But if it assigns sufficient probability to scenarios with more stringent regulation, then the IGCC technology is warranted. We provide some useful benchmarks for possible future regulation and show how these relate back to the relative costs of the two technologies and the optimal technology choice. Few of the policy proposals widely referenced in the public discussion warrant the choice of the IGCC technology. Instead, the PC technology remains the least costly. However, recent carbon prices in the European Emissions Trading System are higher than these benchmarks. If it is any guide to possible future penalties for emissions in the US, then current investment in the IGCC technology is warranted. Of course, other factors need to be factored into the decision as well.     

Supercapacitors accumulating energy harvesting from stacked sediment microbial fuel cells and boosting input power for power management systems

Supercapacitors, are commonly connected to the sediment microbial fuel cell (SMFC) and then serves as the input source for the power management system (PMS). To compare and analyze functions of supercapacitors in SMFC energy harvesting, PMSs (PMS I and PMS II) are powered by SMFC stack or charged supercapacitors as the input source. Tests indicate that the charged supercapacitor results in a higher input power and a larger output power. In addition, the overall efficiency of PMSs is rarely affected by the capacitance, but the initial voltage of the supercapacitor. By charging supercapacitors connected in parallel and then discharging them in series, the overall power efficiency of PMS II is increased from 44.33% to 69.52%. In conclusion, supercapacitors firstly storing SMFC energy is beneficial to provide sufficient energy, resulting in an improved PMS performance. Further, these results can be useful and informative to PMS design for efficiently harvesting and utilize MFC energy.     

Carbon-coated fluorinated graphite for high energy and high power densities primary lithium batteries

The electrochemical performances of fluorinated graphite have been improved by coating a uniform carbon layer on commercial CF_x (x = 1) powder used as cathode material in lithium battery. In comparison with the cell using un-coated CF_x as cathode, the cell using carbon coated CF_x cathode has a higher energy density and higher power density, particularly at higher discharge current rates (1C above). This is because the conductive carbon coating provides the exterior connectivity between particles for facile electron conduction, resulting in high rate performance.     

A hybrid PEMFC/supercapacitor device with high energy and power densities based on reduced graphene oxide/Nafion/Pt electrode

Proton exchange membrane fuel cells (PEMFCs) possess high energy and low power densities, while supercapacitors are characterized by high power and low energy densities. A hybrid PEMFC/supercapacitor device (HPSD) with high energy and power densities was proposed and fabricated for the first time using a reduced graphene oxide/Nafion/Pt electrode in this study. The reduced graphene oxide (rGO) was a capacitive material, and Pt was used as the electrocatalyst. Nafion ionomers adsorbed onto the rGO sheets surface and connected the rGO sheets and the electrolyte (Nafion membrane), thus increasing the utilization rate and specific capacitance of rGO. During the half-cell tests, the rGO/Nafion/Pt electrode exhibited better pulse discharge and galvanostatic discharge performance than the conventional Nafion/Pt electrode. Due to the unique synergy of electrochemical reaction current and capacitance current during the discharge process, the HPSD exhibited a higher power density (26.2 kW kg⁻¹) than the PEMFC (23.9 kW kg⁻¹). The energy density (12.7 kWh kg⁻¹) exhibited by HPSD was close to that of the PEMFC (13.5 kWh kg⁻¹). Therefore, the concept of HPSD is to create a new method for developing next-generation electrochemical devices with high energy and power densities.     

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.     

Techno-economic appraisal of fossil-fuelled power generation systems with carbon dioxide capture and storage

Carbon capture and storage (CCS) facilities coupled to power plants provide a climate change mitigation strategy that potentially permits the continued use of fossil fuels whilst reducing the carbon dioxide (CO₂) emissions. This process involves three basic stages: capture and compression of CO₂ from power stations, transport of CO₂, and storage away from the atmosphere for hundreds to thousands of years. Potential routes for the capture, transport and storage of CO₂ from United Kingdom (UK) power plants are examined. Six indicative options are evaluated, based on ‘Pulverised Coal’, ‘Natural Gas Combined Cycle’, and ‘Integrated (coal) Gasification Combined Cycle’ power stations. Chemical and physical CO₂ absorption capture techniques are employed with realistic transport possibilities to ‘Enhanced Oil Recovery’ sites or depleted gas fields in the North Sea. The selected options are quantitatively assessed against well-established economic and energy-related criteria. Results show that CO₂ capture can reduce emissions by over 90%. However, this will reduce the efficiency of the power plants concerned, incurring energy penalties between 14 and 30% compared to reference plants without capture. Costs of capture, transport and storage are concatenated to show that the whole CCS chain ‘cost of electricity’ (COE) rises by 27-142% depending on the option adopted. This is a significant cost increase, although calculations show that the average ‘cost of CO₂ captured’ is £15/tCO₂ in 2005 prices [the current base year for official UK producer price indices]. If potential governmental carbon penalties were introduced at this level, then the COE would equate to the same as the reference plant, and make CCS a viable option to help mitigate large-scale climate change.