WO2013155276A1

WO2013155276A1 - Integrated 1-d and 2-d composites for asymmetric aqueous supercapacitors with high energy density

Info

Publication number: WO2013155276A1
Application number: PCT/US2013/036134
Authority: WO
Inventors: Lixin Wang; Da DENG; K.Y. Simon Ng
Original assignee: Wayne State University
Priority date: 2012-04-12
Filing date: 2013-04-11
Publication date: 2013-10-17

Abstract

A method of creating a supercapacitor comprising sheets of graphene and metal oxide nanowires is described. In particular, managanese dioxide is chosen as it has excellent pseudocapacitative properties, but the use of other metal oxides is possible. Ideal incubation times, temperatures, concentrations, and ratios were experimentally determined. The electrochemically active compound generated is then posited on a porous metal surface, such as nickel foam, and infiltrated with an aqueous or ionic electrolyte. Supercapacitors of this construction display excellent conductivity, ability to hold charge, and cyclability.

Description

INTEGRATED 1 -D AND 2-D COMPOSITES FOR ASYMMETRIC AQUEOUS SUPERCAPACITORS WITH HIGH ENERGY DENSITY

FIELD

[0001] This invention relates generally to integrated composite materials for electrochemical energy storage devices. More specifically, this invention pertains to supercapacitors comprising graphene and a metal oxide and the manufacture thereof.

BACKGROUND

[0002] Supercapacitors, as promising electrochemical energy storage devices, are of great interest for energy storage applications due to their excellent cyclability, high power density, fast charging rate, high efficiency and easy fabrication. Supercapacitors can complement batteries in many cases where high power and extensive cycling are required. It is anticipated that next-generation supercapacitors with dramatically increased energy density could compete with batteries in the market of for example mobile electronics devices, electric vehicles, industrial equipment, military devices and throughout the energy grid. The electrochemical performance of supercapacitors is strongly dependent on the electrode materials selected. Traditionally, activated carbon having a large surface area have been employed in supercapacitors based on the mechanism of surface charge storage. Recently, a number of carbon alternative materials have been explored, such as Ru0₂, Mn0₂, Fe₃0 , Co₃0 , Ni(OH)₂, based on a pseudocapacitance mechanism involving both surface and bulk energy storage. The use of nanomaterials (such as 0-D nanoparticles, to 1 -D nanorods, and 3-D nanospheres) has further advanced the understanding of the field of supercapacitance..

[0003] An important component of a supercapacictor is the electrolyte. In the past, electrolytes with undesirable properties have been employed. Organic electrolytes used in current supercapacitors are flammable, toxic, expensive, and environmentally hazardous. Ideally, aqueous electrolytes with better ionic conductivity and lower cost would be used for next-generation large scale supercapacitors in order to enhance safety and decrease environmental impact. However, the thermodynamic voltage window of typical aqueous electrolytes (-1 .2V) in traditional symmetric capacitors significantly limit energy density that can be obtained. This issue can be addressed by introduction of asymmetric design, where high overpotentials for H₂ and 0₂ evolution at the negative electrode of an electric double layer capacitor (ELDC) and its pseudocapacitive positive electrode, respectively, could extend the effective voltage window of aqueous electrolyte to above 2V.

[0004] An increased effective voltage window is highly dependent on the material of which the electrode is made. Graphene has unique properties that can be used for just such a purpose in a supercapacitor. When combined into a hybrid structure with materials that can be used for their pseudocapacitative properties (for instance, Mn0₂), significantly improved performance can be achieved due to synergetic effects. With proper composite design and structure control, their electrochemical performance can be even further tuned.

SUMMARY

[0005] The present invention is a supercapacitor comprising at least one graphene component and at least one metal oxide component. The metal oxide component comprises a plurality of nanowires having a first end in physical attachment with the graphene component. The nanowires have an aspect ratio (length divided by width) of at least 100. The electrode of the supercapacitor further comprises a binder, such as PVDF, and the metal oxide of the supercapacitor is preferably manganese dioxide in nanowire form. The graphene/manganese dioxide structure is posited on a porous metal surface such as nickel foam. The supercapacitor also comprises an electrolyte, which is a strong base such as potassium or sodium hydroxide, or water.

[0006] In another aspect of the present invention, a method of making a electrochemically active composite is provided. The method comprises mixing graphene with a metal oxide salt and at least one nonmetallic salt, incubating the reactants at a set temperature for a set amount of time, and a drying step. According to one aspect of this invention, the metal oxide salt is potassium permanganate and the at least one nonmetallic salts are potassium sulfate and potassium persulfate. The method can further comprise a grinding step to form a powder, which is combined with a binder and coated onto a porous metal surface. Optionally included is an incubation step with a strong acid.

[0007] In yet another aspect of this invention, the invention can be a supercapacitor comprising at least one graphene component and at least one manganese dioxide component, the manganese dioxide component comprising a plurality of nanowires, the supercapacitor further comprising an ionic or aqueous liquid electrolyte, the supercapacitor having a capacitance of at least 20 F/g. The plurality of nanowires can constitute an interconnected network.

[0008] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

[0010] Figure 1 is a micrograph from scanning electron microscopy (SEM) that shows the effect of reaction time on the structure of graphene/manganese dioxide (Mn0₂) nanocomposites prepared according to the teachings of the present disclosure;

[001 1] Figure 2 is a graphical representation of the effect on specific capacitance of the reaction time of a graphene/ Mn0₂ composite material;

[0012] Figure 3 is an illustration showing the formation and growth of 1 -D

Mn0₂ nanowires on graphene forming 3-D structured composite.

[0013] Figure 4 is a micrograph from transmission electron microscopy (TEM) showing the effect of reaction temperature on the structure of a graphene/ Mn0₂ composite material;

[0014] Figure 5 is a graphical representation of the effect on capacitance of the reaction temperature of a graphene/ Mn0₂ composite material;

[0015] Figure 6 is a SEM micrograph showing the effect of precursor concentration on morphology of a graphene/ Mn0₂ composite material; [0016] Figure 7 is a graphical representation showing the relationship between precursor concentration and capacitance in a graphene/ Mn0₂ composite material;

[0017] Figure 8 is a series of X-Ray diffraction (XRD) spectra illustrating the impact of reaction temperature on composite structure;

[0018] Figure 9 is a graphical representation of the cyclability of a supercapacitor prepared in accordance with the teachings of the present disclosure;

[0019] Figure 10 is a charge/discharge curve of a graphene/ Mn0₂ composite as prepared in accordance with the teachings of the present disclosure; and

DETAILED DESCRIPTION

[0020] The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. It should be understood that throughout the description and drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0021] The present invention generally provides for the design and fabrication of integrated 1 -D and 2-D composites at nanoscale for asymmetric aqueous supercapacitors. Various experiments are carried out to optimize the formation conditions, and the fabricated materials are extensively characterized by spectroscopy and microscopy. The integration of 1 -D and 2-D materials at nanoscale into high-order 3-D structures enhances electrode conductivity, stability, and provides for easy preparation. Different aqueous electrolytes have been evaluated. The improved electrochemical performance in terms of specific energy, specific power, cycling performance are elucidated by both half-cell and full-cell electrochemical characterization.

[0022] The inventive composites are three-dimensional structures formed of two-dimensional graphene sheets interposed with one-dimensional metal oxide nanowires. When fabricated into their final form, these composites will be suitable for applications that require high-power, high-energy, low-cost electrochemical energy storage, scalable from small electronics to electrical vehicles, industrial equipment, military devices, and even power grid applications. [0023] In a preferred embodiment of the invention, the metal oxide utilized is manganese dioxide (Mn0₂), although other materials can be used in making the invention of the present disclosure, including but not limited to oxides of ruthenium (such as Ru0₂), iron (such as Fe₃0 ), cobalt (such as Co₃0 ), and nickel (such as Ni(OH)₂.) Mn0₂ will be used throughout this disclosure in order to represent a single exemplary embodiment of the invention.

[0024] The invention of the present disclosure is markedly different from currently-available symmetric supercapacitors which utilize activated carbon. The invention described here is instead made of conductive graphene and high- capacitance Mn0₂. The combination of these materials allows for construction of asymmetric, aqueous supercapacitors with high voltage.

[0025] This invention takes advantage of several unique properties of graphene. Graphene is effectively a single layer of carbon atoms packed in a hexagonal or honeycomb lattice. Because it is only one atom thick, it is extremely light relative to the surface area it creates, with 1 gram of graphene being capable of extending across 2630 square meters. Because each carbon atom in graphene is bonded to only three others instead of four, there are numerous pi bonds formed and a large number of resonance structures possible, allowing for electrons to move with ease. Furthermore, graphene is a very strong material and metals and metal oxides readily form composites with it.

[0026] For all of its advantages as listed above, graphene does have its shortcomings. One of these is the fact that it does not have an intrinsically high capacitance. However, transition metal multiple oxide compounds generally display high levels of pseudocapacitance, but with low conductivity. One such transition metal oxide is Mn0₂.

[0027] In order to exploit the high conductivity of graphene and the excellent capacitive storage properties of Mn0₂, combining the two is ideal. However, because of the low conductivity of Mn0₂, in order to see a benefit a very thin layer, in the range of several nanometers to tens of microns, must be employed. Therefore, the present invention is concerned with creating a thin film of graphene/ Mn0₂ composite material. This film can then be posited on or bonded to a metal surface that will act as a collector of electrons. Metal foams are a good surface for this purpose. In an embodiment described here by way of example only, porous nickel foam is used, but it will be appreciated that other metals or alloys, whether formed into a porous foam or not, can serve to substitute for porous nickel foam in the invention as described.

[0028] Porous nickel foam is particularly well-suited for use in this invention because of its porosity and conductivity. The pores allow the electrochemically active materials coated foam to be soaked in electrolyte and to be thoroughly infiltrated by the liquid electrolyte, providing maximum contact between the electrochemically active materials and the electrolytic molecules. This in turn permits electrons to travel with ease to the capacitive electrode store a charge.

[0029] The choice of electrolyte is important for the function of the device. Previous generation capacitors have relied upon organic electrolytes. These come with major drawbacks. They can be expensive, toxic, flammable, and proper disposal can be difficult. Another advantage of a graphene/ Mn0₂ composite is that such a composition permits the use of aqueous, rather than organic, electrolytic solution. In one embodiment of this invention, a strong base is used as the electrolyte. Both potassium hydroxide and sodium hydroxide aqueous solution have proven useful in this application. In another embodiment, water dissolved with other salts can be used as the electrolyte. Distilled water is extremely inexpensive and of course innocuous.

[0030] Graphene can be used as a scaffold for growing metal oxide structures. In general, salts of the metal oxide simply need to be combined with the graphene and allowed to commingle. The reactions are not stirred or agitated in any way. Variables that can be controlled to manipulate the outcome of the reactions include reaction time, temperature, and concentration of reagents.

[0031] In one embodiment, the following was used as a reaction scheme. The following protocol is provided by way of example only and should not be interpreted as restricting the scope of this invention. 50 mg commercial graphene is used as a starting material. It is mixed with a metal oxide salt, in this case 0.01 -1 M KMn0_4, more preferably 0.1 M KMn0₄ and with at least one nonmetallic salt, in this case 0.01 -1 M K₂S0₄, more preferably 0.1 M K₂S0₄ and 0.01 -1 M K₂S₂0₈, more preferably 0.1 M K₂S₂08. In this case the molar ratio of the three salts is preferably 1 :1 :1 , and the salts are fully dissolved in an aqueous solution. The well mixed slurry is placed into an autoclave and kept at a set temperature for a fixed amount of time. The solid product is then collected and washed using Dl water several times to remove in this case potassium ion. Then the product is dried in a vacuum drying oven under 1 10°C for 12 hours and ground, resulting in a graphene and Mn0₂ nanowire composite powder.

[0032] As mentioned above, the chief variables that can be manipulated in order to influence the structure at the nanoscale are reaction time, temperature, and the concentration of the starting materials. Turning to the figures, results of trials that altered each of these factors are shown by way of example.

[0033] Turning first to figure 1 , the effect of reaction time on the structure of graphene/Mn02 composites is displayed in a series of scanning electron micrographs. Reactions were incubated at 100°C for from 6-36 hours to produce these micrographs. Fig. 1 shows that after 6 hours of incubation, the surface morphology is limited simply to clumps of material. Nanowire character is not observed until after more than 6 hours have passed; in the example illustrated, wire shapes are evident in the micrographs taken after 24 and 36 hours have passed.

[0034] Turning now to figure 2, supercapacitors were prepared from powders that had been made from samples that had had hydrothermal reaction times that fell within the same range of 6-36 hours. Specific capacitance, in Farad per gram, was determined by a standard three-electrode protocol for supercapacitors made from material that had incubated for each interval. The three-electrode protocol was implemented as follows: graphene/Mn0₂ composites are coated on nickel foam, which is hung from a Ni wire as the working electrode into 6M KOH electrolyte.A platinum wire serves as the counter electrode and a mercury (Hg/HgO) electrode is the reference electrode. The scan rate used in cyclic voltammetry is 0.5 mV/s with a scan window of -0.1 to 0.3V

[0035] The results of this experiment show an increase in specific capacitance from the 6 to the 24 hour interval (in the case shown, from 1 .52 F/g to 28.1 F/g), but a decrease after 24 hours had passed. In the illustrated experiment, the decrease from 24 hours to 36 is drastic; the specific capacitance of the 36 hour supercapacitor is just 70% of the 24 hour device. It should be noted that the 36 hour time point still represents a suitable, though not ideal, interval for supercapacitor fabrication; however, it is reasonable to speculate that perhaps the construction of either the nanowires on an individual basis or the complex network that they form is better for energy storage after a 24 hour incubation than at 36 hours.

[0036] Figure 3 diagrams one possible mechanism by which the integration of manganese dioxide nanowires with graphene may be taking place. Permanganate anions interact with the elemental carbon of the graphene sheets in an aqueous environment and the manganese is reduced, resulting in the release of carbonate and bicarbonate ions and the formation of a bond between the manganese dioxide and the graphene sheet.

[0037] The trials illustrated in Figure 4 maintain this 24 hour interval, but now vary the temperature at which the incubation occurs. Transmission electron micrographs of the generated composites reveal that as temperature increases while time is held constant, longer and thicker wires are formed. At 80°C, the wires are poorly defined; at 140°C they have thickened and lengthened to the point where they can truly be considered nanowires; and at 220°C they are wider and longer yet.

[0038] However, one important aspect in maximizing energy storage consists of maximizing exposed surface are of each nanowire to take full advantage of the pseudocapacitive properties of the chosen metal oxide. Furthermore, conductivity in these materials is low, which is why the graphene is included in the first place. Therefore, it is important to consider the aspect ratio of the metal oxide formed. Ideally, the aspect ratio (or the measure of the length divided by the width) should be greater than 100. Nanorods, which have aspect ratios lower than 100 (and, more typically, lower than 10) will not be as effective as nanowires (aspect ratio 100 or greater) due to decreased surface area and increased sequestered volume of metal oxide. More surface area equates to more electrolyte exposure and therefore more facile electron movement through the device. A reasonable range of nanowire dimensions was found to be in the range of 10-100 nm thick, preferably 15-50 nm thick, most preferably 20-30 nm thick, with lengths in the high nanometer to micron scale length. [0039] Figure 5 provides further evidence that the thinnest possible viable wires should be created for use in these supercapacitors. In the illustrated example, Mn02 nanowires were grown for 24 hours at temperatures ranging from 80°C to 220°C and specific capacitance of supercapacitors made using them was measured by a standard three-electrode protocol. It is clear that when only nascent wires with wispy appearance as seen in Figure 4 are used (see 80 °C point) specific capacitance remains low. However, an increase of just 20 degrees to 100 °C nearly triples specific capacitance (from 8.5 F/g to 24.3 F/g), and each point at a higher temperature thereafter decreases. This shows that the temperature must be carefully controlled in order to maximize nanowire formation but to minimize thickening of the nanowires.

[0040] Turning now to Figures 6 and 7, the impact of increasing precursor material is shown. The effective amount of Mn0₂ in the three samples from the micrographs increase from 0.25 M to 0.5 M and 1 M. Because graphene is an excellent substrate for nanowire growth, the results are as expected: higher concentrations of manganese dioxide leads to greater saturation of the graphene with nanowires. Figure 7 clearly shows that specific capacitance increases as the amount of Mn0₂ in the starting material increases, to a maximum of 28.1 F/g at 1 M. It is possible that if a saturating amount of Mn0₂ was used, that there might not be sufficient interstitial space between nanowires and thus the amount of surface area in contact with electrolyte would once again decrease, but as observed here that point would come somewhere beyond 1 M.

[0041] At this point it will be necessary to further clarify the supercapacitor construction method. After the solid graphene/ Mn0₂ composite has been ground to a dry powder, a solution to a final concentration of 3% volume/volume polyvinyl difluoride (PVDF) is made in N-methyl-2-pyrrolidone (NMP) and the composite powder is dispersed. A typical mixture may be for instance 90% powder, 10% PVDF/NMP solution, but other ratios are possible.

[0042] Pre-cleaned nickel foam (about 130-150 milligrams, with a surface area of 1 square centimeter) is then submerged into the resulting slurry and it remains there for approximately 90 minutes under continued sonication. This constitutes a loading step. The loaded nickel foam is removed and incubated at 80 degrees Celsius for about 4 hours. It then undergoes a drying step at 1 10 degrees Celsius for 12 hours. This results in a thin layer of graphene/ Mn0₂ atop a bed of nickel foam, which serves as an current collector. In a typical Mn02 containing capacitor, 10 mg composite are loaded per square centimeter of nickel foam. However, different materials will result in different yields.

[0043] Figure 8 is an X-ray diffraction (XRD) spectrum of graphene/ Mn0₂ composites created at different temperatures. The characteristic pattern of peaks is conserved from 100°C to 220°C and at both temperatures tested in between, but the 80 °C spectrum differs, lacking pronounced peaks at for instance 28 degrees and 60 degrees. This is evidence of similar nanowire characteristics when incubation temperatures are at least 100 °C but also confirms that 80 °C is insufficient to generate desirable structures.

[0044] Turning now to Figures 9 and 10, the cyclability of a graphene/Mn02 supercapacitor is demonstrated. After 300 cycles, the capacitance change is only a decrease of 25.6% from the original charge as can be seen in Figure 9.

[0045] Figure 10 demonstrates that a complete charge/discharge cycle can be achieved in about 10 seconds with a graphene/Mn02 composite supercapacitor. Further data gathered (not shown) has proven that these supercapacitors can continue to be cycled over 1000 times without a catastrophic loss of function. Such robust cyclability will be crucial for applications that these capacitors will see in the field.

[0046] A further variable can increase the energy storage capacity of these graphene/Mn02 supercapacitors: an increase in overall surface area in three dimensions. To achieve this, composites are treated with a strong acid, such as nitric acid. The acid increases the porosity of the graphene, possibly by removing unbound carbons from the graphene layer. As a result, more interchange of electrolyte solution is possible between the layers of graphene (and therefore more internal surface area near the nanowires is exposed to electrolytic solution.) Specific capacitance of up to 70 F/g has been observed after acid treatment, contrasted with 28 F/g in for example Figure 2.

[0047] This example describes the methodology used to prepare a graphene/ Mn02 composite supercapacitor according to the teachings of the present disclosure. The catalyst system as prepared above was used in the various reactions and characterization tests that are described throughout this disclosure.

[0048] A person skilled in the art will recognize that any measurements described in the present disclosure are standard measurements that can be obtained by a variety of different test methods. The test methods described in the example and the throughout the disclosure represent only one available method capable of obtaining each desired measurement.

[0049] The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

CLAIMS What is claimed is

1 . A supercapacitor comprising at least one graphene component and at least one metal oxide component, the metal oxide component comprising a plurality of nanowires having a first end, attached to the graphene component.

2. The supercapacitor of claim 1 wherein the nanowires have an aspect ratio of at least 100.

3. The supercapacitor of claim 2 further comprising a binder.

4. The supercapacitor of claim 3 wherein the at least one graphene component and and least one metal oxide component are posited on a porous metal surface.

5. The supercapacitor of claim 2 wherein the metal oxide component is manganese dioxide.

6. The supercapacitor of claim 3 wherein the binder is PVDF.

7. The supercapacitor of claim 4 wherein the porous metal surface is nickel foam.

8. The supercapacitor of claim 4 further comprising an electrolyte.

9. The supercapacitor of claim 8 wherein the electrolyte is a strong base.

10. The supercapacitor of claim 9 wherein the strong base is potassium hydroxide.

1 1 . The supercapacitor of claim 8 wherein the electrolyte is water.

12. A method of making an electrochemically active composite material comprising mixing graphene with a metal oxide salt and at least one nonmetallic salt, incubating the reactants at a set temperature for a set amount of time, and a drying step.

13. The method of claim 1 1 further comprising a grinding step to form a powder.

14. The method of claim 12 wherein the powder is combined with a binder and coated onto a porous metal surface.

15. The method of claim 13 further comprising an incubation step with a strong acid.

16. The method of claim 1 1 wherein the metal oxide salt is potassium permanganate and the at least one nonmetallic salts are potassium sulfate and potassium persulfate.

17. A supercapacitor comprising at least one graphene component and at least one manganese dioxide component, the manganese dioxide component comprising a plurality of nanowires, the supercapacitor further comprising an ionic liquid electrolyte, the supercapacitor having a capacitance of at least 20 F/g.

18. The supercapacitor of claim 17 wherein the plurality of nanowires constitute an interconnected network.

19. The supercapacitor of claim 17 or 18 wherein the reactants are KMn04, K₂S0_4, and K₂S₂0₈ in a 1 :1 :1 ratio.