Edge-riched graphene nanoribbon for high capacity electrode materials

Accepted Manuscript Title: Edge-riched graphene nanoribbon for high capacity electrode materials Authors: Yunjie Ping, Yupeng Zhang, Youning Gong, Bing Cao, Qiang Fu, Chunxu Pan PII: DOI: Reference:

S0013-4686(17)31689-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.08.051 EA 30055

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

6-6-2017 2-8-2017 8-8-2017

Please cite this article as: Yunjie Ping, Yupeng Zhang, Youning Gong, Bing Cao, Qiang Fu, Chunxu Pan, Edge-riched graphene nanoribbon for high capacity electrode materials, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.08.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Revised Manuscript Journal: Electrochimica Acta Manuscript ID: EO17-3218

Edge-riched graphene nanoribbon for high capacity electrode materials

Yunjie Ping1, Yupeng Zhang2,3* , Youning Gong1, Bing Cao1, Qiang Fu1,4, Chunxu Pan1,4*

1 School of Physics and Technology, and MOE Key Laboratory of Artificial Microand Nano-structures, Wuhan University, Wuhan, 430072, China 2 College of Electronic Science and Technology, Shenzhen University, Shenzhen 518000, China 3 Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia 4 Center for Electron Microscopy, Wuhan University, Wuhan, 430072, China

*Author to whom correspondence should be addressed. E-mail: [email protected] (Y. Zhang), [email protected] (C. Pan); Tel: +86-27-68752481 ext. 8168;

Highlights 

The graphene nanoribbon has been successfully synthesized by longitudinal unzipping of carbon nanotubes with oxidants KMnO4.



Compared with graphene oxide and carbon nanotubes, graphene nanoribbon shows the largest capacitance up to ~202F/g at a scan rate of 5 mV/s.



The importance of the location of functional groups and the importance of the edge structure.

 The pseudo-capacitance material should have high electron transfer and rapid ion diffusion.

Abstract：Carbon materials have attracted great attention for their diversified applications in supercapacitors, and different structures of carbon have been reported to exhibit dissimilar electrochemical properties. In the past, activated carbons, carbon nanotubes (CNTs), carbon nanofibers and graphene have been shown to have excellent electrochemical performances, but it still remains a problem on how to improve the capacitance of carbon-based materials effectively from the viewpoint of their giant commercial potential. Noticing that connecting chemical groups to carbon can provide large pseudo-capacitance, we hereby demonstrated that the position of the chemical groups also plays an important role in the pseudo-capacitance. In our work, we synthesized graphene nanoribbon (GNR), graphene oxide (GO) and functional MWCNTs and showed that GNR has larger capacitance (calculated to be 202 F/g at a

scan rate of 5 mV/s) and energy density compared to CNTs and GO when using as electrode materials. Furthermore, the supercapacitor device based on as-synthesized GNR exhibits excellent cycle stability and rate capability which evident is potential in high performance supercapacitor. Revealing the source of the capacitance, we found that though GNR has less oxygen-containing groups, it has larger pseudo-capacitance than GO and CNTs due to the remarkable edge-riched structure with high activity in electrochemical reactions. This finding highlights the importance of edge structure in carbon-based pseudo supercapacitor and suggests a new insight for the development of pseudo-capacitance electrode materials. Keywords: graphene nanoribbon; graphene oxide; carbon nanotubes; supercapacitor

1. Introduction Supercapacitor, one of the energy-storage devices, which can provide larger power densities, longer cyclic lives than batteries and also much larger energy densities than capacitor, has attracted considerable attention due to its special application in the situation where needs high energy in short times [1-3].

Electrode

materials are the key component of supercapacitor, and determine main performance. Among them, RuO2 seems to have the greatest

capacitance. However, the high

prices and scarcity of this resource limit its application severely. Relatively, carbonaceous materials are the

preferred chosen electrode materials for

supercapacitors due to their high surface area, good conductivity, high stability and low cost [4-5]. It is noteworthy that there is a close relationship between the structure

and the capacitance of carbon materials, as shown in Figure 1. Activated carbon, one porous structure carbon, the most popular electrode material in the supercapacitor industry, exhibits capacitance from 20-100 F/g [6-7]. Carbon nanotubes (CNTs), one-dimensional tubular structures, have been reported to exhibit capacitance ranging from 4-135 F/g [8-9], Recently, graphene, two-dimensional carbon, has been proven to have larger capacitance ranging from 100-200 F/g [10-12] as it possesses larger surface area of over 2600 m2/g [13] and superior electrical conduction. To further enhance the capacitive performance, the combine of CNTs, graphene oxide (GO) and graphene [14-21]

or the activation of graphene [22] have also been reported.

The capacitance of supercapacitor for carbon materials contains two parts: electric double layer capacitance (EDLC) and pseudo capacitance. The EDLC based on the specific surface area which could be utilized by the electrolyte ions, and the pseudo capacitance comes from the additional faradaic reactions with the surface functional groups containing carboxyl, hydroxyl and epoxy [23-24]. As pseudo capacitance is much larger than EDLC, it is a research focus how to increase the pseudo capacitance [25]. For graphene, large surface area provides it large EDLC, but the unavoidable aggregation decreases the effective surface area undoubtedly which caused the reported capacitance of graphene is much lower than the theoretical one (~500 F/g) [26]. Due to the chemical tolerance of graphene, there is nearly no pseudo capacitance for pure and integral graphene and only small pseudo capacitance for reduced graphene oxide. Hence, there is still much room for further improving the capacitance of graphene. Recent report [27] revealed that GO may be a better choice

than graphene because oxygen-containing functional groups were introduced and carbon black added as conductive additive improves its poor conductivity . This research provided a new direction for the development of graphene electrode material and also showed the importance of atom groups bonded to the carbon. It is well-known that the edges and defects are highly sensitive areas in graphene. With a larger electron density, the edges usually present special properties [28-30]. GNR is constructed by “cutting” graphene and thus it possess large amount of edges compared to graphene. Due to the special edge structure, it has distinctive properties. Such as by applying electric field or chemical decoration, half-metallicity GNR could be got from the semiconducting GNR [31]. Hence, more and more attention has been given to synthesizing or utilizing GNR [32-35]. Compared to graphene, the dimension of GNR decreases into quasi-one dimension which makes it impossible to aggregate, and the edges with oxygen-containing functional groups may provide large amounts of pseudo capacitance and also improve the wettability. Based on the above considerations, it may be a better choice as the electrode material for supercapacitors. However, to our known, there is little research on the electrochemical properties of GNR; hence, it is necessary and urgent to study the electrochemical properties of GNR. In this paper, we synthesized GNR and presented an investigation on it as the electrode material for supercapacitors.

To further understand the unique effect of the

edge structure, GO and functional MWCNTs were also synthesized and tested. For functional MWCNTs, it can be viewed as rolled up graphene sheet, only plane

region exist, and the functional groups most prefer located in the plane

region with

defects; For GO, the plane

region and edge

region coexist, but the plane region is

much larger than the edge

region. Hence, the functional groups prefer located in the

in-planeregion with defects, as shown in Figure 2 the yellow ring region; While for GNR, the most dominant composition is edge

region, the functional groups prefer

located in the edge region. The results indicated that GNR possesses very good electrochemical properties and has larger capacitance than GO and functional MWCNTs. More importantly, the results revealed that the position of the atoms groups on the carbon materials plays an important role in contributing to the pseudo-capacitance and only when they located in the edge area, they can have great contributions to the

pseudo-capacitance. Our

work may give a new insight for synthesizing and utilizing carbon-based supercapacitors

and other energy-storage devices.

2. Experimental Section 2.1. Synthesis of GNR, GO and functional MWCNTs The graphene nanoribbon was synthesized by longitudinal unzipping of carbon nanotubes with oxidants KMnO4, as reported by D. V. Kosynkin [36]. 150 mg MWCNTs was suspended in 150 ml concentrated sulphuric acid followed by treatment with 750 mg KMnO4 for 1 h at room temperature (22 °C) and 1 h at 70 °C. After purified by dialysis and isolation, the resulting nanoribbons were highly soluble in water. Then the solution was centrifuged and dried, waiting for the fabrication of electrode. GO was prepared from natural graphite by a modified Hummers method [37] The functional MWCNTs were prepared by treating MWCNTs with nitric acid and

sulfuric acid 3:1 (v/v) for 12 h at room temperature. Then, the mixture was washed with deionized water until pH 6.5-7.0, and dried at 70 °C for 12 h. 2.2. Characterizations Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL 2011 transmission electron microscope at an acceleration voltage of 200 kV. Fourier transformation infrared (FT-IR) spectra were obtained on a Perkin-Elmer Spectrum one FT-IR spectrometer at a resolution of 4 cm−1 with an HgCdTe detector. XPS measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an excitation source of Mg-Ka ¼ 1253.6 eV. The Brunauere-Emmette-Teller (BET) specific surface area of the samples was measured by using low-temperature sorption of nitrogen (BET, JW-BK, JWGB, China). 2.3. Electrochemical Property Measurements The working electrode was prepared by coating the active material onto nickel foam (10 mm x 10 mm) followed by pressing with a pressure of 0.5 tons. While the active material was the mixture of GNR/GO/CNTs, carbon black and PDFE binder with a mass ratio of 7:2:1. Before the coating, the mixture was ground in a mortar using a pestle, and the grinding time was 1 h. Then, the samples were uniformly distributed upon the Ni foam and allowed to dry in a vacuum oven for 2 h. The end loading of active material for each electrode was 3~5 mg/cm2. At last, all of these electrodes were studied by cyclic voltammetry (CV),

galvanostatic charge/discharge

(GCD) and electrochemical impedance spectroscopy (EIS) measurements on a commercial electrochemical workstation with a three-electrode system. Electrodes made from carbon materials worked as the working electrode, Ag/AgCl electrode as the reference electrode, and platinum network electrode as the counter electrode, the electrolyte is the 6 M KOH solution. The specific capacitance values of the three electrodes were calculated from the −0.2

CV curves using the following equation: Cs=

∫−1.1 𝐼(𝑉)𝑑𝑉 mv(Va−Vc)

, where I (V) is the reduction

current, Va − Vc is the potential window, m indicates the mass of the active electrode

material, and v indicates the scan rate. Specific capacitance could also be calculated from the GCD curves, using the following equation: Cs=

I∗Δt

, where I is the

m∗ΔV

discharge current, ∆t is the time for a full discharge, m indicates the mass of the active material, and ∆V represents the voltage change after a full charge or discharge. EIS measurements were made in the frequency range of 0.1–100,000 Hz by applying an AC voltage with 5 mV perturbation. Energy density was derived from the CV curves using the following equation: Ed= 1/2 C (ΔV)2, where C is the specific capacitance of the active material, and ∆V is the voltage range of one sweep segment. Power density Ed

was calculated from the following equation: Pd= , where Ed is the energy density, Δt

and ∆t is the time for a sweep segment.

3. Results and Discussions The GNR was synthesized by longitudinal unzipping of MWCNTs with oxidants KMnO4, as reported by D. V. Kosynkin [36].The morphology and nanostructure of the GNR were characterized by TEM and AFM observations, as shown in Figure 3a, 3b. The width of the GNR was several tens of nanometers, and the length of the GNR was a few microns which meet the basic characteristics of nanoribbons. In addition, the edge of the GNR was full of defects which can be evidenced by the TEM images as well as the process of chemical oxidation and unzipping (longitudinal opening the closed carbon-carbon structure by introducing oxygen). From the height profiles, we could clearly see that the thickness of the GNR was in the range of 2-3 nm and there might be a layer distribution in individual GNR as indicated by the blue arrow. More structure information of the GNR was supplied in Supporting Information (see supporting Figure S1). In addition, the BET test was performed to evaluate the

specific surface area of the samples. The specific surface area of the GNR (439 m2/g) is larger than GO (405 m2/g) and CNTs (237 m2/g), which indicates that the surface area may be a factor in the electrochemical properties. In order to understand and confirm the surface atom groups of GNR, FTIR and XPS measurements were carried out. Figure 3c and Figure 3d show the FTIR spectra and C1s XPS spectra of GNR separately. In FTIR spectra, the spectrum illustrates C-OH at 1410 cm-1, C-O (epoxy and alkoxy) at 1059 cm-1, C=O at 1640 cm-1, while the band from 3200 to 3400 cm-1 may

be from the stretching of OH or intercalated

water [38]. The result of FTIR indicates that many oxygen containing functional groups are bond to GNR. In the XPS spectra, the C1s core level peak can be resolved into four components centered at 284.5, 286.4, 288.4 and 289.9 eV. According to the reports on carbon materials [39], the strongest peak at 284.5 eV is attributed to C=C/C-C bond; the second strongest peak of 286.4 eV should be C-O bonds; the peak at 288.4 eV and 289.9 eV are C=O and COOH groups respectively. From the XPS results, we could clearly see that: although there are so many oxygen-containing groups in the GNR, the most component of the GNR is carbon (71.6% C, 26.6% O) which are different from GO (46.1% C, 43% O). And compared to GO, GNR seems to have less functional groups (see supporting information Figure S2). The electrochemical performances of GNR, GO and functional MWCNTs were measured by compressing the active materials onto a Ni foam to support and testing in a three-electrode system. Figure 4 show the CV curves, GCD curves and their capacitive behavior with different conditions. Although all of the three electrodes

show typical capacitive behavior, we could clearly see that, compared to traditional electrode materials CNTs, GO shows larger capacitance; compared to GO, the GNR shows an even larger capacitance which was confirmed by the larger current density in CV curves as well as longer discharge time in GCD curves. The average specific capacitance of the GNR was calculated to be ~202F/g at a scan rate of 5 mV/s and 130 F/g at a high scan rate of 50 mV/s which was much larger than the GO (134 F/g at 5 mV/s and 97 F/g at 50 mV/s) or CNTs (90 F/g at 5 mV/s and 60 F/g at 50 mV/s). It indicates that GNR possess greater

capacitance either at low scan rate or high scan

rate, thus GNR is more appropriate for the high speed supercapacitors. Figure 5 show the CV curves with different scan rates, GCD curves with different current density and cyclability of GNR with a current density of 4 A/g for 1500 cycles. With the scan rate increasing, the shape of CV curves basically remains rectangle, which indicates the electrode has good rate capability. With the current density increasing, no obvious “IR Drop” was observed, which

suggests the

electrode has small resistance. There was no obvious capacitance decrease observed over 1500 cycles of charge and discharge at a current density of 4 A/g (96% remained), which indicates the electrode has good cyclability. From the inset picture, we could find that after 1000 cycles, there is only 0.2 s in the reduction discharge time for the electrode which is consistent with the good cyclability, and the “IR Drop” decreases from 112 mV to 75 mV. The reason for this phenomenon is the contact between electrolyte and electrode is better after some cycles, and it is easier for ion to diffuse in the electrode material. The good rate capability comes from the highly

reversible redox reaction of the functional groups. Moreover, the EIS was measured to investigate the performance of electrochemical capacitors. In the Nyquist plots, the GNR electrode showed a smaller quasi-semicircle and a more vertical straight line than the GO and CNTs electrode, which suggests the lower charge transfer resistance (Rct) and the better capacitive performance, as shown in Figure 6. Figure 7 present the relation between power output and energy density. We could clearly see that GNR possess larger power density as well as larger energy density. Thinking about the capacitance, the cyclability, the rate capability, the energy and power density, GNR has shown great potential in high performance supercapacitors. A pure EDLC behaves as typical rectangular in the CV curve

and straight line

in the GCD curve, but the deformation of the CV curves or the GCD curve is due to the defects and functional groups on the surfaces of the electrode materials [40-41] such as >C-OH>C=O + H++ e, >C=O + eC-O-. When we take account the source of the capacitance, we could find that all of the three curves show offset from rectangle and the voltage-time curves

don’t show ideal linear shapes, indicating the

presence of pseudo-capacitance. However, from the size of the deviation, the GNR should have much more pseudo-capacitance than CNTs or GO. Although the existence of the oxygen-containing groups has been confirmed by FTIR and XPS analysis, It is really interesting that why GO has more oxygen-containing groups but less pseudo-capacitance and CNTs show the least pseudo-capacitance. Just as discussed before, only plane

region exists for CNTs, and plane

region coexists for GO where the plane

region and edge

region is much larger than the edge

region.

While for GNR, the most composition is edge located in the edge

region, the functional groups prefer

region. Although the plane structure of graphene shows super

electron transfer in the air,

it seems to show slow electron/ion transfer in the

solution due to low electron density [42-43]. Hence, even electrochemical reaction occurs among these groups in the defects area of the plane, the electron released by the reaction cannot be transferred effectively to the nickel electrode due to the discontinuities of the defects. While the reaction with no electronic transfer cannot contribute to pseudo-capacitance but only cause the accumulation of electrons. This may be the reason for the low pseudo-capacitance of CNTs. While for GNR, the most functional groups are located in the edge plane, where the areas show large electron density and can be seen as continuous defects, the electrons released

can be passed

away quickly which would have a great contribution to the capacitance. Plane and edge region coexist in GO, so the electron released by the reaction could not get fully utilizing which caused the pseudo-capacitance of GO is less than GNR. In addition, due to the fast electron transfer, there is little electron accumulation, which makes those atom groups live a longer life.

4. Conclusions The GNR has been successfully synthesized by longitudinal unzipping of MWCNTs with oxidants KMnO4. Compared with GO and CNTs, GNR shows the largest capacitance up to ~202F/g at a scan rate of 5 mV/s, and the large capacitance

comes from the pseudo-capacitance generated in the edge

region. The

electrochemical results suggest the importance of the location of functional groups and highlight the importance of the edge structure. The results also indicate that the supporter of the pseudo-capacitance material should have high electron transfer or rapid ion diffusion. This finding suggests a new insight for the development of pseudo-capacitance electrode materials.

Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 11174227) and Chinese Universities Scientific Fund.

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Figure Captions Figure 1 The capacitive performance for different carbon structure. Figure 2 The atom groups distribution for different carbon structure. Figure 3 Morphology and structure of the GNR (a) TEM image of individual GNR (b) Atom force microscope image of GNR on Si/SiO2 substrate with height profiles (c) FTIR transmittance spectra of GNR (d) XPS spectra of GNR. Figure 4 Electrochemical capacitance of GNR, GO and functional CNTs in 6 mol/L KOH aqueous electrolyte. (a) CV curves for GNR, GO and CNTs at 10 mV/s. (b) Average specific capacitance of GNR, GO and CNTs at various scan rates. (c) GCD curves for GNR, GO and CNTs at 50 mA/g. (d) Average specific capacitance of GNR, GO and CNTs at various current densities. Figure 5 Electrochemical capacitance and cyclability of GNR in 6 mol/L KOH aqueous electrolyte. (a) Scan rate studies for GNR at 5, 10, 20 and 50 mV/s. (b) Current density studies for GNR at 0.5, 2.5, 4 and 8 A/g. (c) Cyclability of GNR with a current density of 4 A/g. The inset in panel C is the Charge-discharge curves before and after 1000 cycles. The right graph is the local high magnification of the inset graph. Figure 6 Nyquist plots of GNR, GO and CNTs Figure 7 Ragone plots of GNR, GO and CNTs derived from the CV curves at different scan rate.

Figure 1 The capacitive performance for different carbon structure.

Figure 2 The atom groups distribution for different carbon structure. .

Figure 3 Morphology and structure of the GNR (a) TEM image of individual GNR (b) Atom force microscope image of GNR on Si/SiO2 substrate with height profiles (c) FTIR transmittance spectra of GNR (d) XPS spectra of GNR

Figure 4 Electrochemical capacitance of GNR, GO and functional CNTs in 6 mol/L KOH aqueous electrolyte. (a) CV curves for GNR, GO and CNTs at 10 mV/s. (b) Average specific capacitance of GNR, GO and CNTs at various scan rates. (c) GCD curves for GNR, GO and CNTs at 50 mA/g. (d) Average specific capacitance of GNR, GO and CNTs at various current densities.

Figure 5 Electrochemical capacitance and cyclability of GNR in 6 mol/L KOH aqueous electrolyte. (a) Scan rate studies for GNR at 5, 10, 20 and 50 mV/s. (b) Current density studies for GNR at 0.5, 2.5, 4 and 8 A/g. (c) Cyclability of GNR with a current density of 4 A/g. The inset in panel C is the Charge-discharge curves before and after 1000 cycles. The right graph is the local high magnification of the inset graph.

Figure 6 Nyquist plots of GNR, GO and CNTs

Figure 7 Ragone plots of GNR, GO and CNTs derived from the CV curves at different scan rate.