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Nanotechnology for the Energy Challenge, Second Edition
In this chapter, we briefly introduce different kinds of carbon-based materials depending on their dimension. Different properties of graphene are discussed. Detailed descriptions of the synthesis of graphene-based materials are described. The chapter focuses on discussing graphene-based materials for supercapacitor application. The benefits of using graphene-based materials in the supercapacitor application are discussed. The activity of graphene-based materials toward fuel cells using different graphene-containing materials is described. The superiority of using graphene-based materials in fuel cell cathode catalysts for oxygen reduction reactions is also explained in this chapter.
Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene-based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene-based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H2) storage and high-performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)-ion batteries, Li-sulfur batteries, Li-air batteries, sodium (Na)-ion batteries, Na-air batteries, zinc (Zn)-air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage-related applications are discussed.
We report a facile approach to synthesize nanocomposites with Fe3O4 nanopaticles (NPs) attached to reduced graphene oxide (rGO) sheets by a solvothermal process, which combines the growth of Fe3O4 NPs and the reduction of GOs in one single step. These Fe3O4/rGO nanocomposites were further used to fabricate thin film supercapacitor electrodes by using a spray deposition technique without the addition of insulating binders. It was found that the Fe3O4/rGO nanocomposites showed much higher specific capacitances than that of either pure rGO or pure Fe3O4 NPs. We further carried out electrochemical characterization of the Fe3O4/rGO nanocomposites with different Fe3O4:rGO weight ratios (e.g. IFe3O4:rGO) and showed that Fe3O4/rGO nanocomposites with IFe3O4:rGO = 2.8 exhibited the highest specific capacitance of 480 F g−1 at a discharge current density of 5 A g−1 with the corresponding energy density of 67 W h kg−1 at a power density of 5506 W kg−1. These Fe3O4/rGO nanocomposites also showed stable cycling performance without any decrease in the specific capacitance after 1000 charge/discharge cycles.
A new class of multilayer films was constructed by electrostatic layer-by-layer self-assembly, using poly(sodium 4-styrenesulfonate) mediated graphene sheets (PSS-GS), manganese dioxide (MnO2) sheets, and poly(diallyldimethylammonium) (PDDA) as building blocks. UV-vis spectroscopy, field-emission scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and morphologies of the multilayer films. Capacitive properties of the synthesized multilayer film electrodes were studied using cyclic voltammetry and galvanostatic charge/discharge in 0.1 M Na2SO4 electrolyte. The specific capacitance of the ITO/(PDDA/PSS-GS/PDDA/MnO2)10 electrode reached 263 F g−1 at a discharge current density of 0.283 A g−1; moreover, this film electrode also shows a good cyclic stability and high Coulombic efficiency. Anticipatedly, the synthesized multilayer films will find promising applications as a novel electrode material in supercapacitors and other devices in virtue of their outstanding characteristics of controllable capacitance, good cycle stability, low cost and environmentally benign nature.
Recent progress in the study of graphene has triggered a gold rush for exploiting its possible applications in various areas. Graphene-containing carbonaceous materials have long been selected as electrodes in rechargeable lithium batteries. However, the understanding of the relationship between material structure and electrode performance is still poor due to the complexity of the carbon structures, which hinders the development of high performance batteries. Now it is time to focus on the structure–property relationship of carbonaceous electrodes again, but from the viewpoint of graphene.
This study demonstrated a novel nanographene platelets (NGPs)-based glucose/O2 biofuel cell (BFC) with the glucose oxidase (GOD) as the anodic biocatalysts and the laccase as the cathodic biocatalysts. The GOD/NGPs-modified electrode exhibited good catalytic activity towards glucose oxidation and the laccase/NGPs-modified electrode exhibited good catalytic activity towards O2 electroreduction. The maximum power density was ca. 57.8μWcm−2 for the assembled glucose/O2 NGPs-based BFC. These results indicated that the NGPs were very useful for the future development of novel carbon-based nanomaterials BFC device.
The chemistry of graphene oxide is discussed in this critical review. Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure. Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed. This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references).
We report the superior capacitance of functionalized graphene prepared by controlled reduction of graphene oxide (GO). In a solvothermal method, GO dispersed in dimethylformamide (DMF) was thermally treated at a moderate temperature (150 °C), which allows a fine control of the density of functionalities. Surface functionalities on graphene would enable a high pseudocapacitance, good wetting property, and acceptable electric conductivity. A specific capacitance up to 276 F/g was achieved based on functionalized graphene at a discharge current of 0.1 A/g in a 1 M H2SO4 electrolyte, which is much higher than the benchmark material. The excellent performance of the functionalized graphene signifies the importance of controlling the surface chemistry of graphene-based materials.
Highly dispersed catalysts on a conductive support, commonly platinum and platinum-based catalysts, are used as electrode materials in low-temperature fuel cells. Carbon blacks are commonly used as fuel cell catalysts supports, but their properties are not completely satisfactory. Thus, in the last years carbon black alternative materials such as nanostructured carbons, ceramic and polymer materials have been proposed as fuel cell catalyst supports. Very recently, in consideration of their high surface area, high conductivity, unique graphitized basal plane structure and potential low manufacturing cost, graphene nanosheets have been investigated as a support for low-temperature fuel cell catalysts. This paper presents an overview of graphene nanosheets used as supports for fuel cell catalysts. In particular, the catalytic activity and durability of catalysts supported on graphene are compared with those of catalysts supported on the commonly used carbon blacks and on carbon nanotubes, that is, on rolled graphene.
In this paper, graphene sheets with different reduction levels have been produced through thermal reduction of graphene oxide in the temperature range of 200–900 °C. The effects of interlayer spacing, oxygen content, BET specific surface area and disorder degree on their specific capacitance were explored systematically. The variation of oxygen-containing groups was shown to be a main factor influencing the EDL capacitor performances of the pyrolytic graphene. The highest capacitance of 260.5 F g−1 at a charge/discharge current density of 0.4 A g−1 was obtained for the sample thermally reduced at about 200 °C.
An easy novel approach which offers a fast pathway of macroscopic quantity preparation of graphene nanosheets (GS) at room temperature has been firstly proposed via lithium aluminum tetrahydride (LiAlH4) and phosphorus tribromide (PBr3) as the deoxidizer based on graphite oxide (GO). The structure of the graphene was investigated by high-resolution transmission electron microscopy (HR-TEM) indicating the GS with few layers (~ 3 layers). It is found that the GS as anode material used for lithium ion battery (LIB) has high initial coulombic efficiency (83.1%). The first discharge and charge capacity of the prepared GS were 1029.4 mAh·g− 1 and 855.1 mAh·g− 1 at a current density of 100 mA·g− 1, respectively. After 21 cycles, the reversible capacity is still kept at 760.7 mAh·g− 1. The as-prepared GS with few layers and defects could be optimized for excellent electrochemical performance as lithium ion battery anode material.
Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D hierarchical structure which significantly enhances the electrochemical capacitive performances of GS-based composites. From SEM images, inserting proper quantity of CNTs as nanospacers can effectively impede the stacking of GS and enlarge the space between GS sheets, leading to obtain a highly porous nanostructure. The specific capacitance of GS-CNTs-9-1 (326.5 F g−1 at 20 mV s−1) is much higher than that of GS material (83 F g−1). Furthermore, the energy and power densities of GS-CNTs-9-1 are respectively as high as 21.74 Wh kg−1 and 78.29 kW kg−1, revealing that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites. Therefore, the GS-CNT composites are promising carbon materials for supercapacitors.
The structure, composition, morphology, and support material significantly affect the catalytic characteristics of Pt-based nanocatalysts. Fine control of the structural and compositional features is highly favorable for the creation of new Pt-based nanocatalysts with enhanced catalytic performance and improved Pt utilization. This work reports on a systematic and comparative study of the effects of structure, composition, and carbon support properties on the electrocatalytic activity and stability of Pt-Ni bimetallic catalysts for methanol oxidation, particularly the promoting effect of Ni on Pt. Graphene-supported Pt-Ni alloy nanocatalysts were prepared by a facile, one-step chemical reduction of graphene oxide and the precursors of Ni2+ and PtCl62−. The nanocatalysts were characterized by transmission electron microscopy (TEM), ultraviolet–visible spectrophotometry (UV–vis), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The electrocatalytic characteristics of the nanocatalysts were studied by voltammetry with methanol oxidation as a model reaction to evaluate the effects of the structure, surface composition, and electronic characteristics of the catalyst on the electrochemical activity. The catalyst with a Pt/Ni molar ratio of 1:1 exhibited the highest electrocatalytic activity for the methanol oxidation reaction with greatly lowered Pt utilization. The mechanism of the promoting effect of Ni on Pt is explained based on the modification of the electronic characteristics of the surface Pt atoms (Pt 4f) by Ni atoms due to the shift in the electron transfer from Ni to Pt and the synergistic roles of Pt and nickel hydroxides on the surfaces of the catalysts. The effects of the different carbon supports (i.e., graphene, single-walled carbon nanotubes, and Vulcan XC-72 carbon) on the electrocatalytic characteristics of the nanocatalysts are investigated by Raman and XPS experiments. The results demonstrate that the graphene-supported Pt-Ni catalyst has the highest electrocatalytic activity of the three carbon materials due to abundant oxygen-containing groups on the graphene surface, which can remove the poisoned intermediates and improve the electrocatalytic activity of the catalysts.
Graphene with a Brunauer–Emmett–Teller (BET) specific surface area of 264 m2g−1 has been used as anodic catalyst of microbial fuel cells (MFCs) based on Escherichia coli (ATCC 25922). The electrochemical activities of plain stainless steel mesh (SSM), polytetrafluoroethylene (PTFE) modified SSM (PMS) and graphene modified SSM (GMS) have been investigated by cyclic voltammetry (CV), discharge experiment and polarization curve measurement. The GMS shows better electrochemical performance than those of SSM and PMS. The MFC equipped with GMS anode delivers a maximum power density of 2668mWm−2, which is 18 times larger than that obtained from the MFC with the SSM anode and is 17 times larger than that obtained from the MFC with the PMS anode. Scanning electron microscopy (SEM) results indicate that the increase in power generation could be attributed to the high surface area of anode and an increase in the number of bacteria attached to anode.
Graphene nanoplatelets have been synthesized by thermal exfoliation of graphitic oxide and nitrogen doped graphene nanoplatelets have been obtained by nitrogen plasma treatment. Graphene nanoplatelets and nitrogen doped graphene nanoplatelets have been used as a catalyst support for platinum nanoparticles for oxygenreduction reactions in proton exchange membranefuelcells. Platinum nanoparticles were dispersed over these support materials using the conventional chemical reduction technique. The morphology and structure of the graphene based powder samples were studied using X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. A full cell was constructed with platinum loaded nitrogen doped graphene nanoplatelets and the results have been compared with platinum loaded graphene nanoplatelets. A maximum power density of 440 and 390 mW cm−2 has been obtained with platinum loaded nitrogen doped graphene and platinum loaded graphene nanoplatelets as ORR catalysts respectively. Nitrogen plasma treatment created pyrrolic nitrogen defects, which act as good anchoring sites for the deposition of platinum nanoparticles. The improved performance of fuelcells with N-G as catalyst supports can be attributed to the increased electrical conductivity and improved carbon–catalyst binding.
Graphene is an emerging carbon material that may soon find practical applications. With its unusual properties, graphene is a potential electrode material for electrochemical energy storage. This article highlights recent research progress in graphene-based materials as supercapacitor electrodes. With a brief description of the working principle of supercapacitors, research progress towards the synthesis and modification of graphene-based materials, including graphene oxide, fullerenes, and carbon nanotubes, is presented. Applications of such materials with desirable properties to meet the specific requirements for the design and configuration of advanced supercapacitor devices are summarized and discussed. Future research trends towards new approaches to the design and synthesis of graphene-based nanostructures and architectures for electrochemical energy storage are proposed.
Two kinds of functionalized graphene sheets were produced by thermal exfoliation of graphite oxide. The first kind of functionalized graphene sheets was obtained by thermal exfoliation of graphite oxide at low temperature in air. The second kind was prepared by carbonization of the first kind of functionalized graphene sheets at higher temperature in N2. Scanning electron microscopy images show that both two kinds of samples possess nanoporous structures. The results of N2 adsorption–desorption analysis indicate that both of two kinds of samples have high BET surface areas. Moreover, the second kind of functionalized graphene sheets has a relatively higher BET surface area. The results of electrochemical tests is as follows: the specific capacitance values of the first kind of functionalized graphene sheets in aqueous KOH electrolyte are about 230Fg−1; the specific capacitance values of the second kind of functionalized graphene sheets with higher BET surface areas are only about 100Fg−1; however, compared with the first kind of functionalized graphene sheets, the second kind has a higher capacitance retention at large current density because of its good conductive behaviors; furthermore, in non-aqueous EC/DEC electrolyte, the specific capacitance values of the first kind sample and the second kind sample are about 73Fg−1 and 36Fg−1, respectively.
Graphene, graphene–ZnO and graphene–SnO2 films were successfully synthesized and used as electrode materials for electrochemical supercapacitors, respectively. The screen-printing approach was employed to fabricate graphene film on graphite substrate while the ZnO and SnO2 were deposited on graphene films by ultrasonic spray pyrolysis. The electrochemical performances of these electrodes were comparatively analyzed through electrochemical impedance spectrometry, cyclic voltammetry and chronopotentiometry tests. The results showed that the incorporation of ZnO or SnO2 improved the capacitive performance of graphene electrode. Graphene–ZnO composite electrode exhibited higher capacitance value (61.7F/g) and maximum power density (4.8kW/kg) as compared with graphene–SnO2 and pure graphene electrodes.
An in situ chemical synthesis approach has been developed to prepare SnO2–graphene nanocomposite. Field emission scanning electron microscopy and transmission electron microscopy observation revealed the homogeneous distribution of SnO2 nanoparticles (4–6nm in size) on graphene matrix. The electrochemical reactivities of the SnO2–graphene nanocomposite as anode material were measured by cyclic voltammetry and galvanostatic charge/discharge cycling. The as-synthesized SnO2–graphene nanocomposite exhibited a reversible lithium storage capacity of 765mAh/g in the first cycle and an enhanced cyclability, which can be ascribed to 3D architecture of the SnO2–graphene nanocomposite.
By depositing a d-wave superconductor (using proximity method) on the top of graphene grown on a substrate-induced bandgap (such as a SiC substrate), a d-wave superconductor caused by the massive Dirac electrons can be fabricated. Using the BTK theory, the tunneling conductance in a gapped graphene N/d-wave superconductor junction, where N is a normal gapped graphene, is studied. This work focuses on the influence of d-wave pairing on the conductance of the junction. In this result, for conductance G/GN plotted versus either the biased voltage V or the d-wave superconducting orientation angular α, a sharp conductance peak like an impulse function can be observed due to increasing the Dirac energy gap. The maximum conductance peak Gmax is also enhanced by increasing the electrostatic potential U in superconductor-electrode, giving rise to Gmax/GN=2 for increasing U→∞. This sharp conductance peak occurs related to the condition of eV=Δcos(2α). The unit step conductance in this junction, G/GN=2Θ(U), is found for α=π/4 and eV/Δ=0 when EF−mvF2→0. This is unlike a unit step conductance in a gapped graphene N/s-wave superconductor junction which was recently observed for eV/Δ=1.
Vertically aligned few layered graphene (FLG) nanoflakes were synthesised on silicon substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Transmission electron microscopy (TEM) shows that the structures have highly graphitized terminal planes of 1-3 layers of graphene. Raman spectroscopy revealed a narrow G band with a FWHM of similar to 23 cm(-1) accompanied by a strong G' (2D) band, with a FWHM of similar to 43 cm(-1) and an I(G)/I(G') ratio of 1, which are all the characteristics of highly crystallized few layered graphene. The FLG electrodes demonstrate fast electron transfer (ET) kinetics for Fe(CN)(6)(3-/4-) redox system with an electron transfer rate, Delta E(p), of 60 mV. Platinum (Pt) nanoparticles of similar to 6 nm diameter were deposited on as grown FLGs using magnetron DC sputtering for methanol oxidation studies. When used as electrodes for methanol oxidation, a mass specific peak current density of similar to 62 mA mg(-1) cm(-2) of Pt is obtained with a high resistance to carbon monoxide (CO) poisoning as evident by a high value of 2.2 for the ratio of forward to backward anodic peak currents (I(f)/I(b)). (C) 2011 Elsevier B.V. All rights reserved.
Graphene nanosheets are deposited on nickel foams with 3D porous structure by an electrophoretic deposition method using the colloids of graphene monolayers in ethanol as electrolytes. The high specific capacitance of 164 F g−1 is obtained from cyclic voltammetry measurement at a scan rate of 10 mV s−1. When the current densities are set as 3 and 6 A g−1, the specific capacitance values still reach 139 and 100 F g−1, respectively. The high capacitance is attributed to nitrogen atoms in oxidation product of p-phenylene diamine (OPPD) adsorbed on the surface of the graphene nanosheets. The comparable results suggest potential application to electrochemical capacitors based on the graphene nanosheets.
The use of a 2-D carbon nanostructure, graphene, as a support material for the dispersion of Pt nanoparticles provides new ways to develop advanced electrocatalyst materials for fuel cells. Platinum nanoparticles are deposited onto graphene sheets by means of borohydride reduction of H2PtCl6 in a graphene oxide (GO) suspension. The partially reduced GO-Pt catalyst is deposited as films onto glassy carbon and carbon Toray paper by drop cast or electrophoretic deposition methods. Nearly 80% enhancement in the electrochemically active surface area (ECSA) can be achieved by exposing partially reduced GO-Pt films with hydrazine followed by heat treatment (300 °C, 8 h). The electrocatalyst performance as evaluated from the hydrogen fuel cell demonstrates the role of graphene as an effective support material in the development of an electrocatalyst.
The well-documented novel performances of graphene make it a potential candidate for a wide range of utilizations, driving research into exploiting unusual properties of this wonder material. This paper reports a facile soft chemical approach to fabricate graphene−Co(OH)2 nanocomposites in a water−isopropanol system. Generally the depositing agent (OH−) and some reducing agents (such as HS− and H2S) could be produced from the hydrolyzation of Na2S in aqueous solution. Therefore utilizing Na2S as a precursor could enable the occurrence of the deposition of Co2+ and the deoxygenation of graphite oxide (GO) at the same time. Remarkably the electrochemical specific capacitance of the graphene−Co(OH)2 nanocomposite reaches a value as high as 972.5 F·g−1, leading to a significant improvement in relation to each individual counterpart (137.6 and 726.1 F·g−1 for graphene and Co(OH)2, respectively). The feeding ratios between Co(OH)2 and graphene oxide have a pronounced effect on their electrochemical activities.
Graphene has aroused intensive interest because of its unique structure, superior properties, and various promising applications. Graphene nanostructures with significant disorder and defects have been considered to be poor materials because disorder and defects lower their electrical conductivity. In this paper, we report that highly disordered graphene nanosheets can find promising applications in high-capacity Li ion batteries because of their exceptionally high reversible capacities (794−1054 mA h/g) and good cyclic stability. To understand the Li storage mechanism of graphene nanosheets, we have prepared graphene nanosheets with structural parameters tunable via different reduction methods including hydrazine reduction, low-temperature pyrolysis, and electron beam irradiation. The effects of these parameters on Li storage properties were investigated systematically. A key structural parameter, Raman intensity ratio of D bands to G bands, has been identified to evaluate the reversible capacity. The greatly enhanced capacity in disordered graphene nanosheets is suggested to be mainly ascribed to additional reversible storage sites such as edges and other defects.
A well-organized flexible interleaved composite of graphene nanosheets (GNSs) decorated with Fe3O4 particles was synthesized through in situ reduction of iron hydroxide between GNSs. The GNS/Fe3O4 composite shows a reversible specific capacity approaching 1026 mA h g−1 after 30 cycles at 35 mA g−1 and 580 mAh g−1 after 100 cycles at 700 mA g−1as well as improved cyclic stability and excellent rate capability. The multifunctional features of the GNS/Fe3O4 composite are considered as follows: (i) GNSs play a “flexible confinement” function to enwrap Fe3O4 particles, which can compensate for the volume change of Fe3O4 and prevent the detachment and agglomeration of pulverized Fe3O4, thus extending the cycling life of the electrode; (ii) GNSs provide a large contact surface for individual dispersion of well-adhered Fe3O4 particles and act as an excellent conductive agent to provide a highway for electron transport, improving the accessible capacity; (iii) Fe3O4 particles separate GNSs and prevent their restacking thus improving the adsorption and immersion of electrolyte on the surface of electroactive material; and (iv) the porosity formed by lateral GNSs and Fe3O4 particles facilitates ion transportation. As a result, this unique laterally confined GNS/Fe3O4 composite can dramatically improve the cycling stability and the rate capability of Fe3O4 as an anode material for lithium ion batteries.
Asymmetric supercapacitor with high energy density has been developed successfully using graphene/MnO2 composite as positive electrode and activated carbon nanofibers (ACN) as negative electrode in a neutral aqueous Na2SO4 electrolyte. Due to the high capacitances and excellent rate performances of graphene/MnO2 and ACN, as well as the synergistic effects of the two electrodes, such asymmetric cell exhibits superior electrochemical performances. An optimized asymmetric supercapacitor can be cycled reversibly in the voltage range of 0–1.8 V, and exhibits maximum energy density of 51.1 Wh kg−1, which is much higher than that of MnO2//DWNT cell (29.1 Wh kg−1). Additionally, graphene/MnO2//ACN asymmetric supercapacitor exhibits excellent cycling durability, with 97% specific capacitance retained even after 1000 cycles. These encouraging results show great potential in developing energy storage devices with high energy and power densities for practical applications.
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.
SEM images of round-shaped natural graphite, currently widely used as the anode active material of Li-ion batteries, show that the surface mainly consists of the basal plane, which suggests that the Li insertion/extraction reaction rate is quite limited. In contrast to this suggestion, however, the anode of commercial Li-ion batteries is capable of high rate charging/discharging. In order to explain this inconsistency, we propose that there are nano-holes in the graphene layers of the graphite allowing Li to be very easily inserted and extracted via the holes.Prior to the measurements a quantum chemical investigation was performed on the energy required for Li to pass through the hole in a graphene layer (Eact). The results showed that the Eact value is too high when the size is smaller than pyrene, but is fairly low for holes of the size of coronene, implying that Li can pass through the basal plane layer if there is a hole larger than coronene.Characterization of the rounded graphite sample and flaky natural graphite was conducted by constant-current charge/discharge cycle tests, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD revealed no appreciable difference between the rounded graphite and flaky natural graphite, in agreement with Raman data.A detailed analysis of the HRTEM results revealed the presence of a number of variously sized circular images. We believe that these are holes in the graphene layer through which Li can pass. The mechanism of formation of the holes is discussed.
Graphene has a promising role in electrode fabrication/modification for microbial fuel cell (MFC) applications but there is a lack of research on graphene in MFCs. This study has systematically investigated two types of graphene materials with very different morphologies, namely regular graphene (like flat sheets of paper) and crumpled particles (like crumpled paper balls), respectively, to modify anode and cathode electrodes in MFCs. The higher electricity generation with the crumpled graphene particles is attributed to their higher electrical conductivity in the thickness direction, their larger surface area, catalytic activities of oxygen reduction, and the open structure they pack into that facilitates mass transfer of the fuels and ions. The crumpled graphene-modified anode electrode produces the highest maximum power density (3.6 W m−3), twice that of the activated carbon-modified anode electrode (1.7 W m−3). The maximum power densities with the crumpled graphene- and flat graphene-modified cathode electrodes are 3.3 W m−3 and 2.5 W m−3, significantly higher than 0.3 W m−3 with the unmodified carbon cloth, although still lower than a platinum cathode electrode. These results have demonstrated that graphene-based materials, especially the crumpled graphene particles, can be effective electrode modifying materials for improving electricity generation in MFCs.Highlights► Microbial fuel cell performance is improved via graphene modification. ► Graphene modification increases the surface area of anode electrodes. ► Graphene modification improves catalytic ability of cathode electrodes.
Graphene-based materials are promising electrodes for supercapacitors, owing to their unique two-dimensional structure, high surface area, remarkable chemical stability, and electrical conductivity. In this paper, graphene is explored as a platform for energy storage devices by decorating graphenes with flower-like MnO2 nanostructures fabricated by electrodeposition. The as-prepared graphene and MnO2, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have been assembled into an asymmetric supercapacitor. The specific capacitance of the graphene electrode reached 245 F/g at a charging current of 1 mA after an electro-activation process. This value is more than 60% larger than the one before electro-activation. The MnO2 nano-flowers which consisted of tiny rods with a thickness of less than 10 nm were coated onto the graphene electrodes by electrodeposition. The specific capacitance after the MnO2 deposition is 328 F/g at the charging current of 1 mA with an energy density of 11.4 Wh/kg and 25.8 kW/kg of power density. This work suggests that our graphene-based electrodes are a promising candidate for the high-performance energy storage devices.
Graphene materials (GMs) as supercapacitor electrode materials have been investigated. GMs are prepared from graphene oxide sheets, and subsequently suffer a gas-based hydrazine reduction to restore the conducting carbon network. A maximum specific capacitance of 205 F/g with a measured power density of 10 kW/kg at energy density of 28.5 Wh/kg in an aqueous electrolyte solution has been obtained. Meanwhile, the supercapacitor devices exhibit excellent long cycle life along with ∼90% specific capacitance retained after 1200 cycle tests. These remarkable results demonstrate the exciting commercial potential for high performance, environmentally friendly and low-cost electrical energy storage devices based on this new 2D graphene material.
Electrochimica Acta j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a a b s t r a c t High quality graphene sheets were prepared from graphite powder through oxidation followed by rapid thermal expansion in nitrogen atmosphere. The preparation process was systematically investi-gated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and Brunauer–Emmett–Teller (BET) measurements. The morphology and structure of graphene sheets were characterized by scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The electrochemical performances were evaluated in coin-type cells versus metal-lic lithium. It is found that the graphene sheets possess a curled morphology consisting of a thin wrinkled paper-like structure, fewer layers (∼4 layers) and large specific surface area (492.5 m 2 g −1). The first reversible specific capacity of the prepared graphene sheets was as high as 1264 mA h g −1 at a current den-sity of 100 mA g −1 . Even at a high current density of 500 mA g −1 , the reversible specific capacity remained at 718 mA h g −1 . After 40 cycles, the reversible capacity was still kept at 848 mA h g −1 at the current den-sity of 100 mA g −1 . These results indicate that the prepared high quality graphene sheets possess excellent electrochemical performances for lithium storage.
Graphene is an important material for sensing and energy storage applications. Since the vast majority of sensing and energy storage chemical and electrochemical systems require bulk quantities of graphene, thermally reduced graphene oxide (TRGO) is commonly employed instead of pristine graphene. The sp(2) planar structure of TRGO is heavily damaged, consisting of a very short sp(2) crystallite size of nanometre length and with areas of sp(3) hybridized carbon. Such a structure of TRGO is reminiscent of the key characteristic of the structure of amorphous carbon, which is defined as a material without long-range crystalline order consisting of both sp(2) and sp(3) hybridized carbons. Herein, we describe the characterization of TRGO, its parent graphite material and carbon black (a form of amorphous carbon) via transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry experiments. We used the data obtained as well as consideration of practical factors to perform a comparative assessment of the relative electrochemical performances of TRGO against amorphous carbon. We found out that TRGO and amorphous carbon exhibit almost identical characteristics in terms of density of defects in the sp(2) lattice and a similar crystallite size as determined by Raman spectroscopy. These two materials also exhibit similar amounts of oxygen containing groups as determined by XPS and nearly indistinguishable cyclic voltammetric response providing almost identical heterogeneous electron transfer constants. This leads us to conclude that for some sensing and energy storage electrochemical applications, the use of amorphous carbon might be a much more economical solution than the one requiring digestion of highly crystalline graphite with strong oxidants to graphite oxide and then thermally exfoliating it to thermally reduced graphene oxide.
Hydrous ruthenium oxide (RuO2)/graphene sheet composites (ROGSCs) with different loadings of Ru are prepared by combining sol-gel and low-temperature annealing processes. The graphene sheets (GSs) are well-separated by fine RuO2 particles (5-20 nm) and, simultaneously, the RuO2 particles are anchored by the richly oxygen-containing functional groups of reduced, chemically exfoliated GSs onto their surface. Benefits from the combined advantages of GSs and RuO2 in such a unique structure are that the ROGSC-based supercapacitors exhibit high specific capacitance (similar to 570 F g(-1) for 38.3 wt% Ru loading), enhanced rate capability, excellent electrochemical stability (similar to 97.9% retention after 1000 cycles), and high energy density (20.1 Wh kg(-1)) at low operation rate (100 mA g(-1)) or high power density (10000 W kg(-1)) at a reasonable energy density (4.3 Wh kg(-1)). Interestingly, the total specific capacitance of ROGSCs is higher than the sum of specific capacitances of pure GSs and pure RuO2 in their relative ratios, which is indicative of a positive synergistic effect of GSs and RuO2 on the improvement of electrochemical performance. These findings demonstrate the importance and great potential of graphene-based composites in the development of high-performance energy-storage systems.
The performance and safety of rechargeable batteries depend strongly on the materials used. Lithium insertion materials suitable for negative and positive insertion electrodes are reviewed. Future trends, such as alternative materials for achieving higher specific charges—the Figure shows a scheme for reversible lithium storage in a high specific charge carbonaceous material—are discussed.
A detailed analysis of the thermal expansion mechanism of graphite oxide to produce functionalized graphene sheets is provided. Exfoliation takes place when the decomposition rate of the epoxy and hydroxyl sites of graphite oxide exceeds the diffusion rate of the evolved gases, thus yielding pressures that exceed the van der Waals forces holding the graphene sheets together. A comparison of the Arrhenius dependence of the reaction rate against the calculated diffusion coefficient based on Knudsen diffusion suggests a critical temperature of 550 °C which must be exceeded for exfoliation to occur. As a result of their wrinkled nature, the functionalized and defective graphene sheets do not collapse back to graphite oxide but are highly agglomerated. After dispersion by ultrasonication in appropriate solvents, statistical analysis by atomic force microscopy shows that 80% of the observed flakes are single sheets.
Graphene was electrochemically deposited on carbon cloth to fabricate an anode for a Pseudomonas aeruginosa mediatorless microbial fuel cell (MFC). The graphene modification improved power density and energy conversion efficiency by 2.7 and 3 times, respectively. The improvement is attributed to the high biocompatibility of graphene which promotes bacteria growth on the electrode surface that results in the creation of more direct electron transfer activation centers and stimulates excretion of mediating molecules for higher electron transfer rate. A parallel bioelectrocatalytic mechanism consisting of simultaneous direct electron transfer and cell-excreted mediator-enabled electron transfer was established in the P. aeruginosa-catalyzed MFC. This study does not only offer fundamental insights into MFC reactions, but also suggests a low cost manufacturing process to fabricate high power MFCs for practical applications.
We present a quick and easy method to synthesize graphene–MnO2 composites through the self-limiting deposition of nanoscale MnO2 on the surface of graphene under microwave irradiation. These nanostructured graphene–MnO2 hybrid materials are used for investigation of electrochemical behaviors. Graphene–MnO2 composite (78 wt.% MnO2) displays the specific capacitance as high as 310 F g−1 at 2 mV s−1 (even 228 F g−1 at 500 mV s−1), which is almost three times higher than that of pure graphene (104 F g−1) and birnessite-type MnO2 (103 F g−1). Interestingly, the capacitance retention ratio is highly kept over a wide range of scan rates (88% at 100 mV s−1 and 74% at 500 mV s−1). The improved high-rate electrochemical performance may be attributed to the increased electrode conductivity in the presence of graphene network, the increased effective interfacial area between MnO2 and the electrolyte, as well as the contact area between MnO2 and graphene.
Graphene nanosheets (GNSs) with narrow mesopore distribution around 4 nm were mass-produced from natural graphite via the oxidation and rapid heating processes. The effects of oxidant addition on the morphology, structure and electrochemical performance of GNSs as electrode materials for electric double-layer capacitor (EDLC) were systematically investigated. The electrochemical properties of EDLC were influenced by the specific surface area, pore characteristics, layer stacking and oxygen-containing functional group contents of electrode materials. Deeper oxidation makes graphite possess both higher specific surface area and more graphene edges, which are favorable for the enhancement of capacitive performance of EDLC. The electrodes with freestanding graphene nanosheets prepared by coating method exhibited good rate capability and reversibility at high scan rates (to 250 mV s−1) in electrochemical performances. GNS electrode with specific surface area of 524 m2 g−1 maintained a stable specific capacitance of 150 F g−1 under specific current of 0.1 A g−1 for 500 cycles of charge/discharge.
A novel high-performance electrode material based on fibrillar polyaniline (PANI) doped with graphene oxide sheets was synthesized via in situ polymerization of monomer in the presence of graphene oxide, with a high conductivity of 10 S cm−1 at 22 °C for the obtained nanocomposite with a mass ratio of aniline/graphite oxide, 100:1. Its high specific capacitance of 531 F/g was obtained in the potential range from 0 to 0.45 V at 200 mA/g by charge–discharge analysis compared to 216 F/g of individual PANI. The doping and the ratio of graphene oxide have a pronounced effect on the electrochemical capacitance performance of the nanocomposites.
A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g−1 (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s−1 compared to 115 F g−1 for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg−1 at a power density of 70 kW kg−1.
We have achieved mobilities in excess of 200,000 cm2 V −1 s−1 at electron densities of ∼2 ×1011 cm−2 by suspending single layer graphene. Suspension ∼150 nm above a Si/SiO2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, nonsuspended devices. This advance should allow for accessing the intrinsic transport properties of graphene.
A simple and effective strategy is proposed to tune the number of graphene layers by selecting suitable starting graphite, using a chemical exfoliation method. It is found that both the lateral size and the crystallinity of the starting graphite play important roles in the number of graphene layers obtained. Using artificial graphite, flake graphite powder, Kish graphite, and natural flake graphite as starting materials, ∼80% of the final products are single-layer, single- and double-layer, double- and triple-layer, and few-layer (4–10 layers) graphene, respectively, while a mixture of few-layer (4–10 layers) and thick graphene (>10 layers) is obtained when highly-oriented pyrolytic graphite is used. The smaller the lateral size and the lower the crystallinity of the starting graphite, the fewer the number of graphene layers obtained. Moreover, the graphenes obtained are of high-quality with an electrical conductivity of ∼1 × 103 S/cm. These findings open up the possibility for controlled production of high-quality graphene with a selected number of layers in a large quantity.
Graphene nanosheets (GNSs) were prepared from artificial graphite by oxidation, rapid expansion and ultrasonic treatment. The morphology, structure and electrochemical performance of GNSs as anode material for lithium-ion batteries were systematically investigated by high-resolution transmission electron microscope, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy and a variety of electrochemical testing techniques. It was found that GNSs exhibited a relatively high reversible capacity of 672 mA h/g and fine cycle performance. The exchange current density of GNSs increased with the growth of cycle numbers exhibiting the peculiar electrochemical performance.
In this study, Pt and Pt–Ru nanoparticles were synthesized on graphene sheets and their electrocatalytic activity for methanol and ethanol oxidation was investigated. Experimental results demonstrate that, in comparison to the widely-used Vulcan XC-72R carbon black catalyst supports, graphene-supported Pt and Pt–Ru nanoparticles demonstrate enhanced efficiency for both methanol and ethanol electro-oxidations with regard to diffusion efficiency, oxidation potential, forward oxidation peak current density, and the ratio of the forward peak current density to the reverse peak current density. For instance, the forward peak current density of methanol oxidation for graphene- and carbon black-supported Pt nanoparticles is 19.1 and 9.76 mA/cm2, respectively; and the ratios are 6.52 and 1.39, respectively; the forward peak current density of ethanol oxidation for graphene- and carbon black-supported Pt nanoparticles is 16.2 and 13.8 mA/cm2, respectively; and the ratios are 3.66 and 0.90, respectively. These findings favor the use of graphene sheets as catalyst supports for both direct methanol and ethanol fuel cells.
A simple and scalable method to fabricate graphene-cellulose paper (GCP) membranes is reported; these membranes exhibit great advantages as freestanding and binder-free electrodes for flexible supercapacitors. The GCP electrode consists of a unique three-dimensional interwoven structure of graphene nanosheets and cellulose fibers and has excellent mechanical flexibility, good specific capacitance and power performance, and excellent cyclic stability. The electrical conductivity of the GCP membrane shows high stability with a decrease of only 6% after being bent 1,000 times. This flexible GCP electrode has a high capacitance per geometric area of 81 mF cm-2, which is equivalent to a gravimetric capacitance of 120 F g-1 of graphene, and retains >99% capacitance over 5,000 cycles. Several types of flexible GCP-based polymer supercapacitors with various architectures are assembled to meet the power-energy requirements of typical flexible or printable electronics. Under highly flexible conditions, the supercapacitors show a high capacitance per geometric area of 46 mF cm-2 for the complete devices. All the results demonstrate that the polymer supercapacitors made using GCP membranes are versatile and may be used for flexible and portable micropower devices.
Using a simple hydrothermal procedure, cobalt oxide (Co(3)O(4)) nanowires were in situ synthesized on three-dimensional (3D) graphene foam grown by chemical vapor deposition. The structure and morphology of the resulting 3D graphene/Co(3)O(4) composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The 3D graphene/Co(3)O(4) composite was used as the monolithic free-standing electrode for supercapacitor application and for enzymeless electrochemical detection of glucose. We demonstrate that it is capable of delivering high specific capacitance of ∼1100 F g(-1) at a current density of 10 A g(-1) with excellent cycling stability, and it can detect glucose with a ultrahigh sensitivity of 3.39 mA mM(-1) cm(-2) and a remarkable lower detection limit of <25 nM (S/N = 8.5).
Microbial fuel cell (MFC) is of great interest as a promising green energy source to harvest electricity from various organic matters. However, low bacterial loading capacity and low extracellular electron transfer efficiency between the bacteria and the anode often limit the practical applications of MFC. In this work, a macroporous and monolithic MFC anode based on polyaniline hybridized three-dimensional (3D) graphene is demonstrated. It outperforms the planar carbon electrode because of its abilities to three-dimensionally interface with bacterial biofilm, facilitate electron transfer, and provide multiplexed and highly conductive pathways. This study adds a new dimension to the MFC anode design as well as to the emerging graphene applications.
Low-cost graphite submicronparticles (GSP) are employed as a possible catalyst support for polymer electrolyte membrane (PEM) fuel cells. Platinum nanoparticles are deposited on Vulcan XC-72 carbon black (XC-72), carbon nanotubes (CNT), and GSP via ethylene glycol (EG) reduction method. The morphologies and the crystallinity of Pt/XC-72, Pt/CNT, and Pt/GSP are characterized with X-ray diffraction and transmission electron microscope, which shows that Pt nanoparticles (similar to 3.5 nm) are uniformly dispersed on supports. Pt/GSP exhibits the highest activity towards oxygen-reduction reactions. The durability study indicates that Pt/GSP is 2-3 times durable than Pt/CNT and Pt/XC-72. The enhanced durability of Pt/GSP catalyst is attributed to the higher corrosion resistance of graphite submicronparticles, which results from higher graphitization degree of GSP support. Considering its low production cost, graphite submicronparticles are promising electrocatalyst support for fuel cells. (C) 2009 Elsevier B. V. All rights reserved.