Patent Application
20180072573
The present invention relates generally to a method of producing high quality graphene. The method is particularly suitable for producing engineered graphene particles and flakes.
Graphene is one of the most exciting materials being investigated not only due to intense academic interest but also with potential applications in mind. Graphene is the “mother” of all graphite forms; including 0-D: bucky balls, 1-D: carbon nanotubes and 3-D: graphite. Electronic and Raman spectra of carbon nanotubes and graphene differ significantly, even though carbon nanotubes are formed through the rolling of graphene sheets. Graphene exhibits significantly different physical properties than that of carbon nanotubes, such as electrical conductivity, thermal conductivity and mechanical strength. Graphene has fascinating properties, such as anomalous quantum Hall effect at room temperature, an ambipolar electric field effect along with ballistic conduction of charge carriers, tunable band gap, and high elasticity. The lack of a suitable environmentally innocuous, high volume or “bulk” manufacturing method for the production of high-quality graphene restricts graphene for use in commercial applications.
Conventionally, graphene is defined, is a single layer two-dimensional material, but bi-layer graphene, with more than two but less than ten layers, is also considered “few layer graphene” (FLG). FLG is often visualized as 2D stacking of graphite layers, which start to behave like graphite if there are more than ten layers. Most investigations of physical properties of graphene are performed using mono-layer pristine graphene obtained either by micro-mechanical cleavage or by chemical vapor deposition (CVD). However, producing bulk quantities of graphene using these methods is still a challenging task.
Several non-limiting applications of graphene, include being an active ingredient in polymer composites, interconnect applications, transparent conductors, energy harvesting and storage applications. Non-limiting examples of such applications include batteries, supercapacitors, solar-cells, sensors, electrocatalysts, electron field emission electrodes, transistors, artificial muscles, electroluminescence electrodes, solid-phase microextraction materials, water purification adsorbents, organic photovoltaic components and electromechanical actuators.
One of the widely used methods for the bulk production of graphene type materials is known as “Hummer's” or “Modified Hummer's” method. This process generates heavily hydrophilic functionalized graphene materials, known as graphene oxide. Hummer's method relies on the use of aggressive oxidative steps to achieve exfoliation of graphite powder. The resulting flakes are either highly defective graphene or graphene oxide, which needs to be further processed to produce graphene from graphene oxide. Graphene oxide is an electrically insulating material, unlike graphene which is electrically conductive. Graphene oxide is not suitable for a vast majority of applications. Typically, thermal or chemical reduction is necessary to restore, at least in part, the π-electrons of graphene from highly insulating phase graphene oxide. An additional limitation and negative side effect of employing the Hummer's method is that the method results in very large quantity of acidic waste.
There have been efforts over the past few years to develop an environmentally safe, scalable synthetic method for the bulk-production of high-quality graphene. Methods include solvent- and/or surfactant-assisted liquid-phase exfoliation, electrochemical expansion, and formation of graphite intercalated compounds. The electrochemical exfoliation method of graphite sheet/block production has shown significant promises in the scientific community because it is an easy, quick, and environmentally benign manner of bulk producing of high-quality graphene.
There are two kinds of well-known electrochemical exfoliation processes, “anodic” and “cathodic”. The anodic process seems to be the most efficient in terms of yield of the final product, but creates substantial amount of defects/functionalization of the resulting graphene material during the course of the exfoliation process. On the other hand, a cathodic process results in much higher quality graphene material, but the yield needs to be significantly improved for high volume manufacturing.
In the anodic process, highly pure graphite sheets/blocks/rods are used as the working electrode (anode) and metals or conductors are used as counter cathode (cathode) (
In a cathodic process, highly pure graphite sheet/block/rod is used as the working electrode (cathode) and metals or other conductors are used as a counter electrode (anode) (
The electrochemical exfoliation process is divided into two steps: first there is intercalation of suitable ions between the graphite inter-layers through electrostatic interactions and then a second step that generates various gases and leads to production of few-layered graphene flakes from swelled/expanded bulk graphite under electrochemical biasing conditions. There is a need to improve this method so that the process is more environmentally friendly while producing high yields, which can be suitable for large scale manufacturing.
It is therefore an object of the current invention to provide an improved method for electrochemical graphene production.
It is an object of the current invention to provide higher quality graphene, with fewer defects than previous methods.
It is another object of the current invention to enable engineered graphene products.
It is another object of the current invention to provide an environmentally benign method of producing graphene.
It is yet another object of the current invention to provide less effluent in the graphene production method.
It is yet a further object of the current invention to provide non-hazardous effluent, consumables, and chemicals in the electrochemical graphene production method.
It is another object of the current invention to allow for scalability and high volume manufacturing capability.
It is yet another object of the current invention to allow for process monitoring, automation and continuous production of high quality graphene.
It is yet another object of the current invention to provide a low cost method of producing high quality graphene.
It is yet a further object of the current invention to provide a method of tailoring the dimensions of high quality graphene.
To that end, in one embodiment, the present invention relates generally to a method of making high quality graphene comprising the steps of:
wherein the high quality graphene has characteristics that are engineered for targeted applications.
In another preferred embodiment, the present invention relates generally to an electrochemical cell for making graphene flakes comprising:
The present invention discloses a simple, environmentally benign, scalable production method involving electrochemical exfoliation (both anodic as well as cathodic) of graphite. High quality graphene materials can be produced with multiple exfoliating ions which enables engineering of end flakes for targeted applications. The characteristics that can be engineered include size, aspect ratio, edge definition, surface functionalization and number of layers.
In this invention, a combination of exfoliating ions is used, which enables greater control in both kinetics and tailoring the features of graphene materials (
All the previous methods have generally focused on a single species of exfoliating ions. This approach of using multiple exfoliating ions enables engineering of end graphene flakes for targeted applications. A particular strength of this method is its benign nature leading to fewer defects in the end product. This is due to use of less corrosive/aggressive reaction media.
In comparison, a widely used process, namely the Hummer's method relies on use of aggressive oxidative steps to achieve exfoliation. The resulting flakes are either highly defective graphene or graphene oxide, which needs to be further processed to produce graphene from graphene oxide. Further, Hummer's method produces much smaller flakes than the method presented herein. Another major limitation, and often a stumbling block, of the Hummer's method is the resulting very large quantity of acidic waste. A major advantage of the present method is that it does not use acid. Furthermore, much smaller quantities of reaction media are employed in the current invention.
The present method results in much larger graphene flakes with far fewer defects and far less oxidation compared to previous methods.
Another key benefit of the present invention is that it can be continuous and amenable to automation. This feature enables subsequent processing steps to be added, thereby enabling the production of engineered particles ready for targeted end applications.
A key feature of this approach is to generate the exfoliating ions through use of appropriate salts in aqueous media. The current invention results in a gentler (less aggressive) media. It is an electrochemical process that can be implemented at ambient temperature. These features result in an overall low cost and a greener process.
The method has remarkable advantages over other methods described in prior art that use, for example, ionic liquids, acidic media, and molten metal salts. The present method can be implemented either in aqueous media or acid media or a combination thereof.
A second key feature of the inventive approach is the use of multiple exfoliating ions in the same process. Prior described methods have generally focused on a single species of exfoliating ions. This method of using multiple exfoliating ions enables engineering of end flakes for targeted applications. In using this method, it enables use exfoliating ions of different sizes in order to control the graphene flake dimensions (Thickness, Lateral Dimensions) as well as the kinetics of the exfoliation process. The results of using a combination of exfoliating ions were both surprising and unexpected.
A third key feature of the current method is to vary the ratio of the exfoliating ions mixture. This enables control the kinetics of the exfoliating process.
A fourth key feature of our approach is the possibility of changing the polarity as a part of the process to engineer a particular or a set of properties. This feature provides substantial flexibility to the overall process.
Another key feature of this method is that the duty cycle can be varied for the electrochemical process. This is another key to optimizing the method as well as being able to engineer attributes and properties of the graphene particles and flakes for targeted applications.
In the case where both electrodes are fabricated from carbon materials, the electrical potentials can be applied in pulse mode by alternately changing the polarity of the electrodes from positive to negative or vice versa. The duty cycle (changes the electrodes polarity) can be selected or optimized for a particular solvent and electrolyte mixture. Furthermore, this configuration of both carbon electrodes can be used in static mode, where the polarity is fixed and not changed. Anode-cathode pairs can be configured as an independent circuit, or be connected in series, or in parallel configurations.
However, it is emphasized that the use of multiple exfoliating ions, ratios of these ion mixtures and flexible duty cycles and changes in polarity may also be beneficially employed in other approaches that use molten liquid salts, acids and solvent media. This method is particularly well suited for the use of flexible, multiple steps to further enhance or improve the graphene particles and flakes for targeted end applications
The electrochemical cell for producing graphene flakes includes a graphene producing working electrode and another electrode, called counter electrode, which is an inert electrode that is stable in the electrolyte containing solvent.
The electrochemical cell for high volume manufacturing can be fitted with multiple working and counter electrodes and can be connected in series or in parallel fashion. Furthermore, this multiplicity of cathode-anode configurations can be independent circuits. Additionally counter electrode or working electrode positions can be parallel, coaxial or in alternating comb fashion.
The electrochemical device that supplies electrical potential either in static (solely positive or solely negative), potential sweep, or pulse mode that is alternately changing the polarity of electrodes from positive to negative, or vice versa after a fixed duty cycle.
The electrochemical cell is additionally fitted with an external cooling/heating jacket for cooling or heating solvents. Furthermore some other heating device can be employed, such as hot plate or microwave system to achieve the same effect (heating or cooling).
The working electrode that is used to produce graphene flake or particles is manufactured from pyrolytic graphite, natural graphite, synthetic graphite, intercalated carbon materials, carbon fiber, carbon flakes, carbon platelets, carbon particles or used processed or manufactured graphite sheets. Furthermore, the working electrode can be produced from carbon powder or flakes compressed together to form sheets, rods or pellets etc.
The counter electrode is an inert conducting metallic or nonmetallic electrode that is stable in the electrolyte containing solvent. The counter electrodes can be made from, metals such as platinum, titanium, high quality steel, aluminum, or from a nonmetal conductor, such as graphite or glassy carbon, etc.
This method is particularly well suited for the use of flexible, multiple steps to further enhance or improve the graphene particles and flakes for targeted end applications using a preprocessed graphite or carbon electrodes. The electrode may be chemically pretreated by electrochemical treatment, thermal treatment, sonication treatment, or by plasma treatment in a suitable choices of solvents/electrolytes/acids/bases and inorganic compounds or in air or in vacuum.
For a separate cell design, an electrochemical graphene producing configuration can be used, where both electrodes are carbon based. Both of these working and counter electrodes can be fabricated from any number of carbon materials. Examples of suitable carbon materials are carbon or graphite based materials, such as pyrolytic graphite, natural graphite, synthetic graphite, intercalated carbon materials, carbon fiber, carbon flakes, carbon platelets, carbon particles, or manufactured graphite sheets. Furthermore, the working electrode can be produced from carbon powder or flakes compressed together to form sheet, rods or pellets etc.
In the case where both electrodes are fabricated from carbon materials, the electrical potentials can be applied in a pulse mode that is alternately changing the polarity of the electrodes from positive to negative or vice versa. The duty cycle (changes the electrodes polarity) can be selected or optimized for a particular solvent and electrolytic mixture. Furthermore, this configuration of both carbon electrodes can be used in static mode, where the polarity is fixed and not changed.
The benefits of alternating polarity are higher graphene production rate and also enabling either or both of the electrodes to be cleaned or conditioned thereby providing a superior process. This configuration will produce more consistent and higher quality graphene along with higher yields. The applied voltage range is from 0.01 to 200 V, more preferably 1-50 V, most preferably 1-30 V.
The temperature of the electrolytic solution is less than 100° C. or more preferably less than 90° C. or most preferably below 85° C.
The process can be operated in continuous mode or in batch mode. The electrical potential can applied in several ways, such as constant voltage level throughout the duration of the process, a potential ramp to constant voltage level, a potential sweep between two voltage levels, an alternating mode with various duty cycles, or any combination of the above.
The electrolyte mixture in the electrochemical cell can be an aqueous solution, organic solvent mixture, or a mixture of organic solvent and aqueous solution containing electrolytes. This electrolyte mixture can have cations and anions of varying sizes in varying ratios. Examples of cations include Na+, K+, Li+, NR4+ (R=solely hydrogen or solely organic moiety or mixture of hydrogen and organic moiety) or combinations thereof. Examples of anions include sulphates along with other anions of various sizes, such as Cl−, OH−, NO3−, Po43−, ClO4−, or mixtures thereof. The electrolyte solution can also contain radical scavengers or in-situ radical generating chemicals (e.g. (2,2,6,6-tetramethylpiperidin-1-yl) oxyl or (2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl and similar materials) that can play a key role in improving and maintaining the quality of graphene.
Graphene flakes are separated from electrochemical bath using filtration, centrifugation, or decantation. Separation of graphene flakes in slurry from the top of the electrochemical bath, or bottom surface by sequential or continuous removal in a continuous fashion, makes this method especially suitable for continuous manufacturing process.
During the electrochemical process, graphene typically floats on top of the reaction media. This is fortuitous and a very useful feature as it allows the graphene being produced to be siphoned from the top of the reaction media to the next tank, making it suitable for a continuous flow process.
For production of graphene flakes in a batch process, securing carbon electrode(s) with an electrolyte permeable membrane or fastening carbon electrode (s) using a flexible electrolyte permeable membrane, such as cellulose dialysis membranes, polycarbonate membranes and muslin cloth could also be used. Such electrodes (i.e. located in an isolating membrane enclosure), after electrochemical exfoliation in an appropriate mixture of solvent and electrolyte mixture for fixed amount of time, are separated from the bath for subsequent processing of the graphene. The same electrode assembly can be sonicated in an appropriate solvent bath to produce graphene. Graphene produced by this method can be separated using filtration, centrifugation, or decantation.
Graphene particles after separation can be repeatedly cleaned with dilute acidic water, distilled/deionized water, and alcohols, such as, ethanol, methanol, isopropanol, or acetone. Wet graphene particles can be dried in air, in vacuum, in inert atmosphere, in hydrogen atmosphere, in hydrogen and argon mixed gas environment or any other mixed gas environment, by applying heat from 30-200° C. for several hours or as needed to achieve the required property.
Electrochemically produced graphene can be further post-processed using air milling, air jet milling, ball milling, rotating-blade mechanical shearing, ultrasonication, solvothermal, sonochemical, acoustic, chemical treatment, heat treatment in presence of hydrogen, inert atmospheres, vacuums, plasma treatment or a combination thereof. Chemical treatment methods include treatment of graphene particles with different reducing agents, such as sodium borohydride, hydrazine hydrate, ascorbic acid, or bubbling hydrogen gas in a suitable solvent with or without applied temperature and mechanically stirring.
Graphene is a material with a unique combination of properties with potentially very large number of applications. Many of these applications will require graphene to be tailored with a specific combination of properties. Furthermore, producing high quality and consistent graphene in appropriate quantities is critical. The electrochemical set-up and method for the production of tailored graphene materials that is suitable for both lab-scale and high volume manufacturing (HVM) has been achieved by the current invention. This method additionally produces less effluent than other methods described in prior art. This method is uniquely suited to enable tailoring and optimization of graphene properties. The following non-limiting examples are provided to describe the current invention.
GO was prepared by using a modified Hummers' method. In a typical reaction, ˜50 ml conc. H2SO4 was added to ˜1 g of NaNO3 followed by stirring in an ice bath for ˜15 min. 1 g of natural graphite powder was then added to it and stirred for ˜15 min. After this step, 6.7 g KMnO4 was added to it very slowly while stirring in an ice-bath and it was stirred for ˜30 min. The ice bath was then removed and it was then kept at 40° C. for ˜for ˜30 min. 50 ml D.I. H2O was added to it very slowly to it while stirring. The inside temperature in the beaker increased to ˜110° C. and at that temperature it was again stirred for ˜15 min. 100 ml of warm H2O was then added to it at last followed by 10 ml of 30 vol % H2O2. The reaction stopped and it was allowed to cool down to room temperature. The final product was isolated via centrifugation and washed with D.I. H2O several times to remove all the acidic waste and other water soluble unreacted stuffs. Finally, it was washed with acetone with ˜3-4 times for drying purpose and kept in an oven at 60° C. for drying. The final product was weighed. The average yield was ˜1.5 g. Shift of (002) peak of Graphite in PXRD pattern towards lower angle around 2θ˜10-11° (
The typical Raman spectrum of example 1, as seen in
In a typical reaction, 1 g of solid pre-exfoliated graphite oxide (prepared via Modified Hummers' method) was dispersed in 0.5 L of D.I. H2O through ultra-sonication for ˜2 h.˜0.5 ml N2H4.H2O was then added to it. It was then refluxed at ˜80° C. overnight, while stirring. The color became brown to black on the next day and the final product settled down at the bottom of the flat-bottom flask. The final product was then isolated through filtration and washed several times with D.I. H2O and then washed with acetone for drying purposes. The final supernatant pH was around ˜6 and it was then kept in an oven for final drying at ˜60° C.; weighed then. The weight of the final product was ˜0.5 g. In
Example 3 was procured from a commercial supplier, having average flake diameter of ˜15μ with 6-8 layers for our external benchmarking purpose. The PXRD pattern of example 3 given in
The graphite sheet was procured to use as an electrode for the electrochemical exfoliation method from a commercial supplier. The PXRD pattern of example 4 in
A cell was assembled having above mentioned commercially available graphite sheet as anode/working electrode (Anodic process) and Ti as cathode/counter electrode in a 1000 ml capacity acrylic polymer container having rectangular cross-section. In all the examples D.I. H2O was used as solvent media and 10 V static potential was applied for a fixed duration, less than 24 hours, more preferably less than 12 hours, and most preferably less than 6 hours (
The electrolyte was used in this example was (NH4)2SO4. After 2:30 h duration, the exfoliated product was isolated by decanting the excess solvent followed by filtration. The final product was then thoroughly washed with suitable solvents. It was then weighed and used for further characterization and analysis. The average weight of the final product is around ˜0.8 g (Table 1).
The PXRD pattern of example 5 (
The electrolyte used in this example was a mixture of (NH4)2SO4 and NaNO3. After a 2:30 h duration, the exfoliated product was isolated by decanting the excess solvent followed by filtration. The final product was then thoroughly washed with suitable solvents. It was then weighed and used for further characterization and analysis. The average weight of the final product is around ˜2.2 g (Table 1).
In
The corresponding Raman spectrum is shown in
This sample was obtained from example 6 and was added to D.I. H2O and then stirred for ˜10 min for proper mixing. Then NH4.H2O was added to it and refluxed with stirring ˜55° C. for ˜18 h. The final product was then thoroughly washed with suitable solvents. It was then weighed and used for further characterization and analysis. The average weight of the final product is ˜0.4 g.
In
The Raman spectrum of example 7 is shown in
The electrolyte used in this example was a mixture of (NH4)2SO4 and Na3PO4.10H2O. After 2:30 h, the exfoliated product was isolated by decanting the excess solvent and followed by filtration. It was then thoroughly washed with suitable solvents. It was then weighed and used for further characterization and analysis. The average weight of the final product is around ˜1.0 g (Table 1).
The PXRD pattern of example 8 (
The electrolyte used in this example only contains Na3PO4.10H2O. After 2:30 h, the final product was isolated by decanting the excess solvent and followed by filtration. It was then thoroughly washed with suitable solvents. It was then weighed and used for further characterization and analysis. The average weight of the final product is around ˜0.5 g (Table 1).
Lack of long-range order along z-direction in example 9 was evident from the PXRD pattern as seen in
The effects of varying ratios of multiple exfoliating ions on the characteristics of final graphene materials have been demonstrated in this disclosure. The corresponding samples have been named as example 6 and 10-12 respectively for the case when, exfoliating ions are (NH4)2SO4 and NaNO3. Corresponding TGA curves in air as well as from Raman spectra are shown in
The kinetics of the exfoliation process are highly dependent on the nature and the varying ratio of multiple exfoliating ions. This phenomenon is reflected by the variation in yield of the graphene materials produced under similar processing condition as can be seen in Table 1. For comparison, examples 13-15 show very kinetically sluggish processes, when non appropriate mixtures of exfoliating ions are used.
Ternary mixtures of multiple exfoliating ions have been used for the production of graphene materials as demonstrated in this disclosure. The corresponding samples have been described in examples 16 and 17. The details of these processes have been given in Table 1. The characteristics of these final graphene materials could be engineered by this strategy which is evident from corresponding comparative TGA curves in air (
Stepwise exfoliation using multiple exfoliating ions have been used for the production of graphene materials as demonstrated in this disclosure. The corresponding samples have been described in examples 18 and 19. The details of these processes have been given in Table 1. The characteristics of these final graphene materials can be engineered by this method which is also evident from corresponding comparative TGA curves in air and from the Raman spectra shown in
Different graphene materials can be produced by post heat treatment of the as prepared graphene materials. To demonstrate the effect of post heat treatment, the sample produced in example 5 was heat treated at 550° C. and 1000° C., respectively, in N2 environment. The corresponding samples have been named examples 20 and 21 respectively. The characteristics of these final graphene materials can be engineered by this approach which is evident from corresponding comparative Raman spectra, PXRD and TGA curves in air, as shown in
(2,2,6,6-tetramethylpiperidin-1-yl) oxyl or (2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl, (commonly known as TEMPO), has been utilized as a radical scavenger to see the effect on the quality of final graphene material and has been presented in this disclosure. The corresponding sample has been described as Example 22 as seen in Table 1. Comparative TGA curves in air of Examples 5 and 22, as well as Raman spectrum of Example 22 sample are shown in
