KR101886871B1

KR101886871B1 - Nitrogen-doped graphene, ultracapacitor using the same and doping methode of the same

Info

Publication number: KR101886871B1
Application number: KR1020110085202A
Authority: KR
Inventors: 최장욱; 강정구; 정형모
Original assignee: 한국과학기술원
Priority date: 2011-08-25
Filing date: 2011-08-25
Publication date: 2018-08-10
Also published as: KR20130022565A

Abstract

The nitrogen-doped graphene of the present invention is characterized in that at least one of the carbon atoms is substituted with nitrogen and the nitrogen doping is comprised of a pyridine-like arrangement, a pyrrole-like arrangement or a graphite-like arrangement. In the present invention, a nitrogen doped graphene ultracapacitor was developed through a simple plasma doping process. This simple doping process not only brings about a four times greater capacitance enhancement compared to native graphene, but also greatly improves cycle life using a unique storage mechanism based on electrostatic interactions in the electrical double layer.

Description

[0001] Nitrogen-doped graphene, ultra-capacitors containing the same, and a method of manufacturing the same [0002] Nitrogen-doped graphene, an ultracapacitor using the same and doping methode of the same [

The present invention relates to a doped carbon material, an ultracapacitor including the same, and more particularly, to a doped carbon material having improved energy storage capacity and cycle life, an ultracapacitor containing the same, and a method of manufacturing the same. .

An ultracapacitor (UC) stores and emits electrical energy based on the electrostatic interactions between the electrolyte and the ions of the electrode. This interaction occurs in so-called electrical double layers (EDL) at the electrode surface. This intrinsic charge storage mechanism allows the ultracapacitors to have a high storage capacity due to 'energy smoothing' and 'momentary energy loading' and a long cycle life in various applications Have a characteristic advantage. In most cases, carbon nanomaterials such as porous carbon materials, carbon nanotubes and graphene are used as ultracapacitor electrode materials due to their high conductivity and wide surface area. However, in many applications, the capacitance of an ultracapacitor is not sufficient to be used as an independent energy reservoir, and therefore ultracapacitors are often used only as energy storage devices to assist the battery. To solve the problem of lack of capacitance, a so-called pseudocapacitor has been developed. Similar capacitors use metal oxides and conductive polymers and utilize an electrical storage principle based on redox reactions near the surface of such active materials. However, most similar capacitors are limited in their cycle life to less than 10,000 cycles because the redox reaction is not completely reversible and involves expansion-constrain of the electrodes. Accordingly, there is a growing need to newly develop an electrode material for an ultracapacitor, which is capable of greatly increasing the energy storage capacity as well as a similar capacitor and having an improved cycling life.

Therefore, the first problem to be solved by the present invention is to provide a graphene derivative which can be used as an electrode material for an ultracapacitor by greatly improving energy storage capacity and cycle life.

A second object of the present invention is to provide an ultra capacitor comprising the graphene derivative.

A third problem to be solved by the present invention is to provide a method for producing the graphene derivative.

In order to achieve the first object, the present invention provides nitrogen-doped graphene wherein at least one of carbon atoms is substituted with nitrogen.

According to one embodiment of the present invention, the nitrogen doping may consist of a pyridine-like arrangement, a pyrrole-like arrangement or a graphite-like arrangement.

According to another embodiment of the present invention, the nitrogen doping amount is preferably 1.5 to 3.0%.

In order to achieve the second object, the present invention provides an ultra-capacitor including the nitrogen-doped graphene.

According to another aspect of the present invention, there is provided a method of manufacturing nitrogen-doped graphene comprising the steps of treating graphene with nitrogen plasma and annealing at a temperature of 250 to 400 ° C.

According to an embodiment of the present invention, the step of treating the graphene with an acid by oxidizing the graphene before the step of treating the graphene with the nitrogen plasma may further comprise a step of treating the acid-treated graphene with hydrogen plasma to reduce .

According to another embodiment of the present invention, in the step of treating the graphene with the nitrogen plasma, the nitrogen plasma treatment is preferably performed at a nitrogen pressure of 10 to 30 Torr for 0.1 to 5 minutes.

In the present invention, a nitrogen doped graphene ultracapacitor was developed through a simple plasma doping process. This simple doping process not only brings about a four times greater capacitance enhancement compared to native graphene, but also greatly improves cycle life using a unique storage mechanism based on electrostatic interactions in the electrical double layer.

FIG. 1A schematically illustrates a process for producing nitrogen-doped graphene by plasma-treating graphene.
2A schematically shows an ultracapacitor structure assembled along a scanning electron microscope image showing the top of the device.
2A schematically shows an ultracapacitor structure assembled along a scanning electron microscope image showing the top of the device.
FIG. 2B shows charging and discharging curves through analysis of the constant current characteristics.
FIG. 2C is a graph showing the constant current capacity of an ultra-capacitor based on various nitrogen-doped graphenes and native graphene according to the current density.
FIG. 2 (d) shows the results of measurement of the constant current capacity of the ultracapacitors formed on the nickel and paper substrates according to the current density.
Figure 2E shows the result of cycle testing up to 10000 cycles for ultracapacitors based on nickel and paper substrates.
Figure 2f shows the non-storage capacity measured in water-soluble and organic electrolytes.
Figures 3A-3I illustrate nitrogen sequence mapping of nitrogen-doped graphene measured in a single sheet using SPEM.
Figure 4a shows the distribution of the nitrogen arrangement for the plasma treated bulk scale sample.
FIG. 4B shows charge and discharge non-storage capacities of nitrogen doped graphene ultracapacitors for different plasma durations.
Figure 4c shows the binding energy between the potassium ion and nitrogen arrangement at the basal plane and edge.

Hereinafter, the present invention will be described in detail.

The nitrogen-doped graphene of the present invention is characterized in that at least one of the carbon atoms is substituted with nitrogen and the nitrogen doping is comprised of a pyridine-like arrangement, a pyrrole-like arrangement or a graphite-like arrangement.

In the present invention, the problem of capacitance is solved by using nitrogen-doped graphene (NG) as an ultracapacitor electrode. Nitrogen-doped graphene ultracapacitors show similar capacitances as similar capacitors, but utilize a robust charging mechanism in the electrical double layer, resulting in improved cycle life, And has potential potential as an energy storage element. Furthermore, in the present invention, local N-configuration is studied in a single sheet of nitrogen-doped graphene using scanning photoemission microscopy (SPEM) to improve the electrostatic capacity I tried to explain why. This method not only provides a decisive key to improving capacitance but also shows new results associated with the N-doping process, including the possibility of N-doping in the basal plane, A clear distribution of the nitrogen-arrangement between the edges and evolution during the course of the plasma process. Nitrogen-doping is a simple but very useful process in graphene because it can handle local electronic structures and improves device performance in many applications, including biosensors, fuel cells and electronic devices. The ability to control local electronic structures in particular is known to improve ionic bonding in solutions, which can be a very useful property for high capacitance capacitive energy storage devices. For example, there are prior art for high capacitance lithium ion batteries based on nitrogen doped graphene. Considering the improvement of the ion binding ability, the present invention has developed an ultra capacitor based on nitrogen doped graphene by using a simple plasma process.

Example 1-1

Nitrogen-doped graphene was prepared by an improved Hummer method of nitrogen plasma process (Hummers, WS; Offeman, REJ Am. Chem. Soc. 1958, 80, 1339-1339 / Cote, LJ; Cruz-Silva, R .; Huang, JXJ Am. Chem. Soc. 2009, 131, 11027-11032). Briefly, first, the graphene was oxidized to the graphene oxide through an acid treatment. The dried graphene oxide was then reduced by a plasma-enhanced chemical vapor deposition process. In the reduction step, the graphene oxide is reduced by a hydrogen plasma process. The plasma process conditions were power 500 W, hydrogen pressure 4 Torr, and hydrogen flow rate 100 sccm. Subsequently, the graphene surface was treated with a nitrogen plasma after the reduction step to complete nitrogen-doped graphene. The conditions of the nitrogen plasma were a power of 500 W, a nitrogen pressure of 14 Torr, and a hydrogen flow rate of 91 sccm. Finally, the sample was annealed at 300 캜 for 3 hours to remove residual functional groups on the nitrogen doped graphene surface. FIG. 1A schematically illustrates a process for producing nitrogen-doped graphene by plasma-treating graphene. Referring to FIG. 1A, a nitrogen atom substitutes a carbon atom by a physical momentum plasma process. The inset shows the possible arrangement of nitrogen substituted by the doping process. By the plasma doping process, nitrogen atoms displace carbon atoms in the original layer of graphene and form three types of N-arrays, which are pyridine-like (N-6), pyrrole-like -like, N-5) and graphite-like (NQ) arrangements. 1B is a high-resolution transmission electron microscope image of nitrogen-doped graphene. Referring to FIG. 1B, the original layer structure was maintained unchanged during the plasma treatment. The inset shows the electron diffraction pattern of the nitrogen-doped graphene at the selected area, showing the diffraction spot at the hexagonal location, indicating that the initial honeycomb-like atomic structure of the graphene is maintained. From the image of the transmission electron microscope (TEM) of the nitrogen doped graphene as well as the selected area electron diffraction (SAED) pattern, during the plasma treatment, the original layer structure of graphene and the honeycomb It was found that the honeycomb-like atomic structure remains unchanged. In addition, the diffraction peaks of the nitrogen-doped graphene were not significantly changed in the X-ray diffraction (XRD) data as in the case of the selected area electron diffraction. In addition, the Raman spectra of nitrogen doped graphene shows a ratio of the G-band (1583-1616 cm ^-1 ) to the D-band (D-band, 1350 cm ^-1 ) as compared to the reduced graphene oxide , Which is the result of defects and N-doping produced during the plasma treatment. In addition, peak splitting and slight shifting in the G-band of nitrogen-doped graphene can confirm the nitrogen doping effect.

Examples 1-2

In order to measure the effect of nitrogen doping on the ultracapacitor performance, nitrogen doped graphene was assemble as schematically indicated in FIG. 2A and was measured by means of galvanostatic and cyclic voltammetric measurements An electrochemical test was conducted. 2A schematically shows an ultracapacitor structure assembled along a scanning electron microscope image showing the top of the device. Typical mass loading of the data shown in Figs. 2A-2F was about 1 mg / cm < ^{2 &} gt ;.

In order to fabricate an ultracapacitor device, N-methylpyrrolidone (NMP) was prepared by dissolving nitrogen-doped graphene and polyvinylidene fluoride (w: w = 9: 1) Slurry. The slurry was cast on a nickel foil and dried overnight in a vacuum oven. Potassium hydroxide (KOH) and tetraethylammonium tetrafluoroborate (C ₂ H ₅ ) ₄ NBF ₄ of 6M (Six molar) were used as water soluble and organic electrolyte, respectively.

Experimental Example 1

FIG. 2B shows charging and discharging curves through analysis of the constant current characteristics. Referring to FIG. 2B,

With pristine graphene

And the black and red lines are respectively the same as the original graphene

. The IR drop (IR drop) is also indicated.

It can be seen that the capacitance is increased by nitrogen doping since the charging and discharging period is increased. The data of Figures 2b-2e were measured in a 6M KOH electrolyte. All data shown in this figure is based on a nitrogen doped graphene ultracapacitor with a mass loading of about 1 mg / cm < ^{2 &} gt ;.

Experimental Example 2

FIG. 2C is a graph showing the constant current capacity of an ultra-capacitor based on various nitrogen-doped graphenes and native graphene according to the current density. The number in the legend is the plasma duration expressed in minutes. Referring to FIG. 2C, for all current densities, nitrogen doped graphenes exhibit several times higher capacitance than native graphenes. For example,

And the original capacitance of the graphene were 282 and 69

, And the NG value is a value comparable to that of the metal oxide / graphene and the polymer / graphene composite. In addition, enhanced capacitance by nitrogen doping leads to higher current density (> 20

), Which means that nitrogen doping does not compromise the high power capability of the graphene ultracapacitor. Even 33

The capacitance is 165

Respectively. The high power capacity is also associated with a low IR drop (IR drop) of the nitrogen doped graphene ultracapacitor,

And has a value less than ~ 15.8 mV which is the original graphene at the same current density. This trend is believed to be due to the increased electrical conductivity of nitrogen doped graphene.

Experimental Example 3

Nitrogen-doped graphene can be made in the form of a binder-free ink that can be applied to a variety of flexible substrates such as paper or fiber. Nitrogen-doped graphene can be applied to wearable or flexible ultra-capacitors, for example nitrogen-doped graphene ultra-capacitors on commercialized conductive carbon fiber, and fibers that can be worn on the wearer's arm Device. FIG. 2 (d) shows the results of measurement of the constant current capacity of the ultracapacitors formed on the nickel and paper substrates according to the current density. The inset shows that a wearable ultra-capacitor wrapped around a person's arm can store enough electricity to power the light-emitting diode. Such a wearable pouch-type ultracapacitor is formed on a nickel substrate and can have a high capacitance. If applied, the light-emitting diode device can be operated in the form of being wound around a person's arm.

Experimental Example 4

Devices formed on conductive paper can maintain high capacitance such as ultracapacitors formed on a nickel substrate at high current densities. In addition, nitrogen-doped graphene ultracapacitors can operate for more than 100,000 cycles in ultracapacitors fabricated in flat and flexible forms with improved capacitance, corresponding to as many cycle cycles as commercially available ultracapacitors. Figure 2E shows the result of cycle testing up to 10000 cycles for ultracapacitors based on nickel and paper substrates. Referring to Figure 2E, after 10,000 cycles, the ultracapacitors formed on the nickel and paper substrates exhibit an initial capacitance of 95.3 and 99.8%, respectively. It can be seen that charge and discharge processes based on electrostatic interactions remain vigorous even after a high cycle recovery time, whereby the nitrogen doped graphene ultracapacitor can function as a long term energy storage device.

Experimental Example 5

In the present invention, a nitrogen doped graphene ultracapacitor was tested in both an aqueous liquid phase and an organic phase electrolyte. Figure 2f shows the specific capacitance measured in aqueous and organic electrolytes. Referring to FIG. 2F, the non-storage capacities of nitrogen doped graphene and native graphene were compared for the two electrolytes. In all cases, the capacitance was about four times higher than that of the original graphene. When operating on 1 M tetraethylammonium tetrafluoroborate, the power density was shown up to about 8 x 10 ⁵ W / kg and the energy density was about 48 Wh / kg.

Experimental Example 6

In particular, nitrogen doped graphene ultracapacitors exhibit much higher power densities than conventional ones, which can be attributed to the NQ portion known to improve conductivity. In addition, most of the similar capacitors can function only in aqueous electrolytes, and even such aqueous electrolytes can only dissolve in extreme acidic pH conditions and therefore function only under certain acidity conditions. Conversely, the nitrogen-doped graphene ultracapacitors of the present invention can operate in all acidity ranges of aqueous solution conditions. It is believed that the plasma treatment will not significantly change the surface area of the graphene electrode since it can not change the intrinsic monolayer nature of the graphene, rather it is thought to alter the atomic configuration of the graphene carbon bond.

Experimental Example 7

Overall, the nitrogen doped graphene ultracapacitor of the present invention has specific capacitive capacitance, power capability and applicability to flexible substrates, various electrolytes, simple process and cycle life. To illustrate the improved electrostatic capacity of the nitrogen doped graphene ultracapacitor of the present invention, monolayer nitrogen doped graphene was prepared and N-arrays were analyzed using a synchrotron-based SPEM (synchrotron-based SPEM) . Using this technique, N-arrays could be measured at various locations within a single sheet of nitrogen-doped graphene, and N-arrays at the basal plane and edge were clearly identified . This measurement was possible because the diameter of the beam was about 500 nm, which is smaller than the size of a typical graphene piece (5 to 10 um in length). Lee, HJ; Kim, S .; Kim, JH; Kim, KJ; Choi, JH; Adv. Mater. 2008, 20, 3589-3591.), The same technique could be used to distinguish the number of graphene layers for native graphenes, but no information on local N-arrays has been found.

Figures 3A-3I illustrate nitrogen sequence mapping of nitrogen-doped graphene measured in a single sheet using SPEM. FIGS. 3A to 3C show the results of 1s mapping of carbon obtained using SPEM for nitrogen-doped graphene treated for 1 minute, 2 minutes and 3 minutes. And a spot where nitrogen 1s data is obtained is displayed. The size of the white point at each point represents the actual size of the SPEM beam. In Figures 3D to 3F, the nitrogen 1s signal was shown and each signal was deconvoluted with sub-peaks of 398.5, 400.1 and 401.5 eV, corresponding to N-6, N-5, and NQ, respectively do. Figures 3g-3i illustrate the proportion of each N-arrangement for three plasma durations. Referring to the figures, interesting facts can be derived from the distribution data on the apparent N-arrangement between the basal plane and the edge. First, nitrogen doping occurs even at the basal plane. This observation suggests that defects in the basal plane generated in the plasma process can initiate nitrogen doping in the basal plane. Substantially, this observation shows that the conventional expectation that nitrogen doping can occur only at the edge due to high reactivity in the thermal doping process (Wang, XR; Zhang, L .; Yoon, Y .; Weber, , HL; Guo, J .; Dai, HJ Science 2009, 324, 768-771. / Wang, XR; Tabakman, SM; Dai, HJJ Am. Chem. Soc. 2008, 130, 8152-8253) to be. Second, as the plasma process proceeds further, the N-5 ratio in the basal plane increases but the NQ ratio decreases. This tendency is due to the increase in the basal plane allowing N-5 formation without allowing NQ formation. Third, due to the same reason for the need for neighboring defects for N-5 formation, the N-5 ratio appears more at the edge than at the basal plane. During the duration of the plasma, there is a deviation between the part and the part in the N-array distribution. However, a clear N-configuration between the basal plane and the edge is observed in all samples. To verify that the SPEM data has representativeness for all the samples, we used a typical X-ray photoelectron spectroscope (XPS) measurement with a beam size of about 20 [mu] Data was obtained. 4A to 4C show the correlation between the nitrogen arrangement and the ultracapacitor and the correlation between the cations and binding energy. FIG. 4A shows the distribution of the three nitrogen arrangements measured for a bulk-scale sample plasma treated for 0.5 to 3 minutes, and the axis on the left in FIG. 4B represents the charge of the nitrogen-doped graphene ultracapacitor for different plasma durations and And the right axis shows the coulombic efficiency based on the charging and discharging non-charging capacities. Figure 4c shows the binding energy between the potassium ion and the nitrogen arrangement at the basal plane and at the edge, which was calculated by first principle density functional theory. The 'P' on the horizontal axis means pristine. These bulk-scale data are consistent with local mapping data indicating that in all plasma processes the N-5 ratio is continuously increased, the NQ ratio is steadily decreasing, and the N-6 ratio is almost unchanged. In addition, bulk-scale X-ray photoelectron microscopy data show that the nitrogen content in the nitrogen-doped graphene produced by the plasma treatment for 0.5 to 3 minutes is 1.68 to 2.51%. While the electrochemical process is governed by numerous parameters during operation of the ultracapacitor, the enhanced electrostatic capacity of the nitrogen doped ultracapacitor can be inferred based on the binding energy between the ion in the electrolyte and the N-array distribution. As the binding energy increases (on a negative scale), a greater number of ions can accumulate on the surface of the electrode, thus improving the binding energy contributes to increasing the capacitance. For this purpose, in the present invention, the binding energy of potassium ions was calculated for each N-configuration in the basal plane and edge using first principle density function theory calculations. Since the potassium ion is larger than the hydroxide ion, it is more appropriate to calculate the binding energy of the potassium ion instead of the cation. The following results were obtained through calculation. First, N-6 in the basal plane increases the capacitance for all electrolytes, because N-6 in the basal plane exhibits the greatest binding energy difference compared to the original counterpart of the graphene counterpart (counterpart) It plays an important role. As is observed in the local N-ary mapping (Figure 3g)

, The ratio of the basal plane N-6 is dominant. Thus, the plasma treatment produces the basal plane N-6, which is an essential process for manufacturing a high capacitance nitrogen-doped ultracapacitor. Second, N-5 contributes to an additional capacitance increase due to the large coupling energy at the basal plane and edge (Fig. 4c) when negatively charged in the basic condition (Fig. 2f ). Third, but with negatively charged N-5 and longer plasma durations, the resulting basal plane defects are likely to create bonds that are too large for the reversible charge / discharge process to be necessary for the reversible charge / discharge process, And may reflect substantially lower Coulombic efficiency during longer plasma durations. Fourth, the Coulombic efficiency of the device is very low in the course of 0.5 minutes of plasma, which can be attributed to ionic bonding with residual functional groups that remain unreduced in the reduction process of the graphene oxide. There have been many studies on various plasma treatments as a method for improving the capacitance of carbon nanotube ultracapacitors. However, these prior art studies are completely different from the present invention in that the plasma treatment is used to increase the area of the carbon nanotube electrode or to introduce functional groups on the surface. In the present invention, the improvement of the capacitance is mainly due to the nitrogen doping effect.

In summary, the present invention has developed a nitrogen-doped graphene ultracapacitor through a simple plasma doping process. This simple doping process not only brings about a four times greater capacitance enhancement compared to native graphene, but also greatly improves cycle life using a unique storage mechanism based on electrostatic interactions in the electrical double layer.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, . Therefore, the embodiments described in the present invention are not intended to limit the scope of the present invention but to limit the scope of the present invention. The scope of protection of the present invention should be construed according to the claims, and all technical ideas within the scope of equivalents should be construed as falling within the scope of the present invention.

Claims

Oxidizing and oxidizing the graphene;
Treating the acid-treated graphene with hydrogen plasma to reduce
Treating the reduced graphene with a nitrogen plasma; And
&Lt; / RTI > and annealing at a temperature between 250 and 400 < RTI ID = 0.0 > C. < / RTI >

The method according to claim 1,
Wherein the nitrogen doping comprises a pyridine-like arrangement, a pyrrole-like arrangement, or a graphite-like arrangement.

The method according to claim 1,
Wherein the nitrogen doping amount is 1.5 to 3.0%.

An ultracapacitor comprising nitrogen-doped graphene produced by the method of any one of claims 1 to 3

delete

The method according to claim 1,
Wherein the nitrogen plasma treatment is performed at a nitrogen pressure of 10 to 30 Torr for 0.1 to 5 minutes in the step of treating the graphene with the nitrogen plasma.