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.     

Modeling Wyoming's Carbon Dioxide Pipeline Network

Abstract

A model of Wyoming's carbon dioxide pipeline network has been developed and used to analyze its capacity. The inlet to this pipeline is at Exxon Mobil's Shute Creek gas processing plant, and supercritical gas is then sent to Rangeley (Colorado), Rock Springs, Monell, Baroil and Salt Creek (all in Wyoming) fields. Along the way, the supercritical gas is cooled to a liquid. Here it is shown that the pipeline could easily accommodate a rate increase from the current 250 MMSCFD to 325 MMSCFD, utilizing the gas that is currently being vented from the Shute Creek intermediate separator, while maintaining the current inlet temperature and pressure. If new CO₂ supplies became available it would be possible to increase the carbon dioxide throughput from 250 MMSCFD to 600 MMSCFD while extending the pipeline to other oilfields such as Beaver Creek and Hartzog Draw. In certain circumstances, this high rate may produce output pressures below the required miscibility pressures and also produce phase changes in the pipeline. However, this could be easily remedied by adding relatively modest horsepower pumps at intermediate pipeline locations.     

Copper-doped cobalt oxide electrodes for oxygen evolution reaction prepared by magnetron sputtering

The Copper-doped cobalt oxide films have been prepared onto titanium support by reactive DC magnetron sputtering. The deposits are characterized by X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), Ultroviolet (UV) and scanning electron microscope (SEM). The electrochemical characteristics of deposits are explored by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). The effect of copper content in the Cu-Co oxides on the surface morphology, electrochemical and crystallographic properties has been investigated. The introduction of Cu element makes the Cu-Co oxides deposit fine and further lead to high roughness of deposits, which is helpful for the oxides’ electrochemical performance. Based on the electrochemical determination, the binary oxide electrodes exhibit better catalysis activity than that of CuO and Co₃O₄ electrodes for oxygen evolution reaction (OER). The increase OER activity has been attributed to the enlarged surface roughness and the new active sites of deposits, according to the results of EIS.     

Soft-template synthesis of vanadium oxynitride-carbon nanomaterials for supercapacitors

By using Polyvinylpyrrolidone (PVP) as the soft-template, vanadium oxynitride-carbon (VO_xN_y-C) nanomaterials were synthesized by NH₃ reduction of V₂O₅ xerogel. The powder X-ray diffraction result indicated that the VO_xN_y-C belongs to the cubic crystal system. Transmission electron microscopy (TEM) images showed that most of the VO_xN_y grains in VO_xN_y-C materials were lower than 20 nm. Electrochemical test results showed that VO_xN_y-C materials exhibited better conductivity and enhanced electrochemical properties compared to VO_xN_y materials which were synthesized only by NH₃ reduction of V₂O₅ xerogel. The maximum specific capacitance of VO_xN_y-C electrode reaches up to 271 Fg⁻¹ at 1 Ag⁻¹ which is much higher than that of VO_xN_y electrode (143 F g⁻¹). The specific capacitance of VO_xN_y-C electrode lost less than 10% as the scan rate increased 20 times from 5 to 100 mV s⁻¹, showing its superior rate capability. The VO_xN_y grains and remaining carbon were intimate contact each other in VO_xN_y-C nanocomposite which results in a better electronic conductivity, as well as the high surface area which furnishes more surface active redox sites of VO_xN_y-C nanocomposite. Therefore, VO_xN_y-C material exhibited better electrochemical performance than VO_xN_y. Cycle tests showed that restricting the potential window can improve the cycling stability of the VO_xN_y-C electrode. This cycle phenomenon may be ascribed to the use of high upper potentials which lead to the irreversible oxidation of electrode materials and the concomitant formation of soluble vanadium based species, which further result in the reduction of the active redox sites, and ultimately enhance the electrode's irreversibility in 1 M KOH.     

Carbon Dioxide Disposal via Carbonation

Abstract

Carbonation is a solidification/stabilization process. The availability of a carbon dioxide (CO₂) fixation technology would serve as insurance in case global warming causes severe restrictions on CO₂ emissions. In order to prevent rapid climate change, it will be necessary to stabilize CO₂ as carbonate by the carbonation process. Carbonation of the widely occurring mineral olivine (Mg₂SiO₄) converts CO₂ into an environmentally benign mineral magnesite (MgCO₃).     

Monograin materials for solar cells

This paper reviews results of studies on different materials and technologies for monograin layer (MGL) solar cells conducted at Tallinn University of Technology. The MGL consists of monograin powder crystals embedded into an organic resin. The MGL combines the superior photoelectrical parameters of single crystals with the advantages of polycrystalline materials, such as the low cost and simple technology of materials and layers preparation and the possibility of making devices of practically unlimited area. A main technological advantage is the separation between absorber and cell formations. The developments in the field of monograin materials of CuInSe₂, Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ and technical parameters of MGL solar cells are summarized.     

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.     

Preparation and characterization of coke resistant Ni/SiO2 catalyst for carbon dioxide reforming of methane

Silica supported Ni catalyst is highly active for the CO₂ reforming of methane but it has poor stability due to coke formation. In this work, a glow discharge plasma was applied for the decomposition of nickel nitrate on the SiO₂ support, followed by thermal calcination in air. The plasma treatment enhances the interactions between the Ni particles and the silica and significantly improves the Ni dispersion. The plasma-treated Ni/SiO₂ catalyst exhibits comparable activity to the Ni/SiO₂ catalyst prepared by the thermal method without plasma treatment. The coke resistance of the Ni/SiO₂ catalyst is significantly enhanced by the plasma treatment.     

Investigating the solid electrolyte interphase using binder-free graphite electrodes

Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF₆- and LiF₂BC₂O₄-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO₂Li) and lithium alkoxides (ROLi); Li₂CO₃ was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF₂BC₂O₄ electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF₆ electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF₂BC₂O₄ electrolyte than in the LiPF₆ electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.     

Estimating marginal CO2 emissions rates for national electricity systems

The carbon dioxide (CO₂) emissions reduction afforded by a demand-side intervention in the electricity system is typically assessed by means of an assumed grid emissions rate, which measures the CO₂ intensity of electricity not used as a result of the intervention. This emissions rate is called the “marginal emissions factor” (MEF). Accurate estimation of MEFs is crucial for performance assessment because their application leads to decisions regarding the relative merits of CO₂ reduction strategies. This article contributes to formulating the principles by which MEFs are estimated, highlighting the strengths and weaknesses in existing approaches, and presenting an alternative based on the observed behaviour of power stations. The case of Great Britain is considered, demonstrating an MEF of 0.69 kgCO₂/kW h for 2002–2009, with error bars at +/−10%. This value could reduce to 0.6 kgCO₂/kW h over the next decade under planned changes to the underlying generation mix, and could further reduce to approximately 0.51 kgCO₂/kW h before 2025 if all power stations commissioned pre-1970 are replaced by their modern counterparts. Given that these rates are higher than commonly applied system-average or assumed “long term marginal” emissions rates, it is concluded that maintenance of an improved understanding of MEFs is valuable to better inform policy decisions.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.     

Carbon materials for H2 storage

In this work a series of carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The materials used were different types of commercial carbons: carbon fibers, carbon cloths, nanotubes, superactivated carbons, and synthetic carbons (carbon nanospheres and carbon xerogels). Their textural properties (i.e., surface area, pore size distribution, etc.) were related to their hydrogen adsorption capacities. These H₂ storage capacities were evaluated by various methods (i.e., volumetric and gravimetric) at different temperatures and pressures. The differences between both methods at various operating conditions were evaluated and related to the textural properties of the carbon-based adsorbents. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. Furthermore, hydrogen storage capacity seems to be proportional to surface area, especially at 77 K. The micropore size distribution and the presence of narrow micropores also notably influence the H₂ storage capacity of carbons. In contrast, morphological or structural characteristics have no influence on gravimetric storage capacity. If synthetic materials are used, the textural properties of carbon materials can be tailored for hydrogen storage. However, a larger pore volume would be needed in order to increase storage capacity. It seems very difficult approach to attain the DOE and EU targets only by physical adsorption on carbon materials. Chemical modification of carbons would seem to be a promising alternative approach in order to increase the capacities.     

Layer-by-layer self-assembly of manganese oxide nanosheets/polyethylenimine multilayer films as electrodes for supercapacitors

Multilayer thin films of manganese oxide nanosheets (MNSs) and polyethylenimine (PEI) polyelectrolyte have been fabricated onto various substrates via layer-by-layer self-assembly technique. UV–vis absorption spectra showed that the absorbance values at the characteristic wavelength of the multilayer films increased almost linearly with the number of PEI/MNS bilayers. Field emission scanning electron microscope (FESEM) images indicated that the surface of the multilayer film was rather smooth and dense. The electrochemical performances of (PEI/MNS)_n films on indium–tin oxide (ITO)-coated glass substrates were investigated by cyclic voltammetry and constant current charge–discharge test from 0 to 0.9 V in a 2 M KCl aqueous solution. The multilayer films showed excellent electrochemical activity, high reversibility and high power density. A specific capacitance value of 288 F g⁻¹ was obtained at a current density of 1.25 A g⁻¹ for (PEI/MNS)₁₀ film in 2 M KCl aqueous solution. The specific capacitance decreased 9.5% of initial capacity over 1000 cycles at a high current density of 2.5 A g⁻¹. These good electrochemical properties could be attributed to the special microstructure of the electrode.     

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.     

Effect of NaI/I2 mediators on properties of PEO/LiAlO2 based all-solid-state supercapacitors

NaI/I₂ mediators and activated carbon were added into poly(ethylene oxide) (PEO)/lithium aluminate (LiAlO₂) electrolyte to fabricate composite electrodes. All solid-state supercapacitors were fabricated using the as prepared composite electrodes and a Nafion 117 membrane as a separator. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge measurements were conducted to evaluate the electrochemical properties of the supercapacitors. With the addition of NaI/I₂ mediators, the specific capacitance increased by 27 folds up to 150 F g⁻¹. The specific capacitance increased with increases in the concentration of mediators in the electrodes. The addition of mediators also reduced the electrode resistance and rendered a higher electron transfer rate between mediator and mediator. The stability of the all-solid-state supercapacitor was tested over 2000 charge/discharge cycles.     

Multiwall carbon nanotube-poly(4-styrenesulfonic acid) supported polypyrrole/manganese oxide nano-composites for high performance electrochemical electrodes

Poly(4-styrenesulfonic acid) (PSS) dispersed multiwall carbon nanotubes (MWCNTs) are used as a support for polypyrrole (PPy)/MnO₂ in a supercapacitor electrode. Synergetic interaction between polypyrrole and MnO₂ significantly improves the electrical properties and the mechanical stability of the electrode that yields high rate capability. The (–SO₃⁻) surface functionalities on MWCNT-PSS facilitate an ordered growth, and the molecular level dispersion of MnO₂ in PPy matrix enhances the electrode performance. As an electrochemical electrode the MWCNT-PSS/PPy:MnO₂ nano-composite exhibits 268 F g⁻¹ specific capacitance at 5 mV s⁻¹. The excellent rate capability and stability of the electrode is demonstrated by only 7% fade in the specific capacity at 100 mV s⁻¹ (compared to the available capacity at 5 mV s⁻¹) and 10% fade in the same after 5000 CV cycles. Specific capacitance of PPy:MnO₂ component in the nano-composite is as high as 412 F g⁻¹ in 0.5 M Na₂SO₄ electrolyte. Electrical conductivity of the MWCNT-PSS/PPy nano-composite is significantly improved upon inclusion of MnO₂ in molecular level dispersion.     

A new Co-ZIF derived nanoporous cobalt-rich carbons with high-potential-window as high-performance electrodes for supercapacitors

In this work, a Co-ZIF material and the derived nanoporous cobalt-rich carbons by direct carbonization of this Co-ZIF material were synthesized and used as electrode materials for supercapacitors. This ZIF material exhibited a high specific capacitance of 160.3 F g⁻¹ at 0.5 A g⁻¹, an excellent rate capability (73.72 F g⁻¹ at 10 A g⁻¹), and a good cycling stability with 100% of its initials specific capacitance after 8000 cycles. In addition, the obtained derived nanoporous carbons displayed ideal capacitor behaviors and were promising electroactive materials for supercapacitors at low current density. The nanoporous carbon obtained at 650 °C possessed a highest specific capacitance of 393 F g⁻¹ at 0.5 A g⁻¹ and a wide potential application range of −1.0–0.33 V. In addition, a symmetric supercapacitor device consisting of Z-C-650 and activated carbon exhibited a maximum energy density of 61.23 Wh Kg⁻¹ at a power density of 700 W kg⁻¹ and predicted that Z-C-650 could be used as a potential energy storage material.     

Carbon dioxide reforming of methane by solid state synthesis supported catalysts

Ni catalyst supported on MgAl₂O₄ mixed oxide was prepared by solid state synthesis, co-precipitation and wet impregnation. The mixed oxide support was synthesized by the solid state synthesis at room temperature (MgAl₂O₄^solid) and co-precipitation method (MgAl₂O₄^cop) respectively, followed by wet impregnation for Ni loading. The catalytic performances of these samples were compared in carbon dioxide reforming of methane at 700 °C. The results showed that the catalyst Ni/MgAl₂O₄^solid with mixed oxide support prepared by solid state synthesis greatly affected the properties and performance of the catalyst. The catalyst Ni/MgAl₂O₄^solid showed higher CO₂ and CH₄ conversion than the Ni/MgAl₂O₄^cop catalyst with the support prepared by conventional co-precipitation method. In addition, the BET surface area of the catalyst Ni/MgAl₂O₄^solid was three times larger than the catalyst Ni/MgAl₂O₄^cop.     

Fabrication and electrochemical characterization of two-dimensional ordered nanoporous manganese oxide for supercapacitor applications

A nanoporous manganese oxide (MnO₂) film was fabricated via a polystyrene templated electrodeposition in the solution containing MnSO₄. The nanoporous MnO₂ film obtained has been characterized by field emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge methods. The specific capacitance of 1018 F g⁻¹ was observed at a low current density of 500 mA g⁻¹. When the current density increased to 30.0 A g⁻¹, the specific capacitance of 277 F g⁻¹ remained. The high capacitance retention at high rates makes the prepared MnO₂ a promising candidate for supercapacitor applications.     

Impact of SO2 poisoning of platinum nanoparticles modified glassy carbon electrode on oxygen reduction

An extraordinary recovery characteristic of Pt-nanoparticles from SO₂ poisoning is introduced in this study. Platinum nanoparticles (nano-Pt) modified glassy carbon electrode (nano-Pt/GC) has been compared with polycrystalline platinum (poly-Pt) electrode towards SO₂ poisoning. Two procedures of recovery of the poisoned electrodes were achieved by cycling the potential in the narrow potential range (NPR, 0-0.8 V vs. Ag/AgCl/KCl (sat.)) and wide potential range (WPR, −0.2 to 1.3 V). The extent of recovery was marked using oxygen reduction reaction (ORR) as a probing reaction. SO₂ poisoning of the electrodes changed the mechanism of the oxygen reduction from the direct reduction to water to the stepwise reduction involving the formation of H₂O₂ as an intermediate, as indicated by the rotating ring-disk voltammetry. Using the WPR recovery procedure, it was found that two potential cycles were enough to recover 100% of the activity of the ORR on the nano-Pt/GC electrode. At the poly-Pt electrode, however, four potential cycles of the WPR caused only 79% in the current recovery, while the peak potential of the ORR was 130 mV negatively shifted as compared with the fresh poly-Pt electrode. Interestingly, the NPR procedure at the nano-Pt/GC electrode was even more efficient in the recovery than the WPR procedure at the poly-Pt electrode.