Morphology Effect of Vertical Graphene on the High Performance of

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Morphology effect of vertical graphene on its high performance of supercapacitor electrode Yu Zhang, Qionghui Zou, Hua Shao Hsu, Supil Raina , Yuxi Xu, Joyce B. Kang, Jun Chen, Shaozhi Deng, Ningsheng Xu, and Weng P. Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12652 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 6, 2016

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ACS Applied Materials & Interfaces

Morphology effect of vertical graphene on its high performance of supercapacitor electrode †

†

‡

‡

‡

Yu Zhang , Qionghui Zou , Hua Shao Hsu , Supil Raina , Yuxi Xu§, Joyce B. Kang , Jun Chen†, Shaozhi Deng†*, Ningsheng Xu†, Weng P. Kang‡* †

State Key Laboratory of Optoelectronic Materials and Technologies, and

Guangdong Province Key Laboratory of Display Material and Technology, School of Microelectronics, Sun Yat-sen University, Guangzhou, 510275, People’s Republic of China ‡

Deptartment of Electrical Engineering and Computer Science, Vanderbilt

University, Nashville, TN 37212, USA §

Department of Macromolecular Science, Fudan University, Shanghai, 200433, China

Keywords: vertical graphene, supercapacitor, morphology, orientation, ion diffusion, surface area.

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ACS Paragon Plus Environment


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Abstract

Graphene and its composites are widely investigated as supercapacitor electrodes due to their large specific surface area. However, the severe aggregation and disordered alignment of graphene sheets hamper the maximum utilization of its surface area. Here we report an optimized structure for supercapacitor electrode, i.e. the vertical graphene sheets, which have a vertical structure and open architecture for ion transport pathway. The effect of morphology and orientation of vertical graphene on the performance of supercapacitor is examined using a combination of model calculation and experimental study. Both results consistently demonstrate that the vertical graphene electrode has a much superior performance than that of lateral graphene electrode. Typically, the areal capacitances of vertical graphene electrode reach 8.4 mF/cm2 at scan rate of 100 mV/s; this is about 38% higher than that of lateral graphene electrode and about 6 times higher than that of graphite paper. To further improve its performance, a MnO2 nanoflake layer is coated on the surface of graphene to provide a high pseudocapacitive contribution to the overall areal capacitance which increases to 500 mF/cm2 at scan rate of 5 mV/s. The reasons for these significant improvements are studied in detail and are attributed to the fast ion diffusion and enhanced charge storage capacity. The microscopic manipulation of graphene electrode configuration could greatly improve its specific capacitance, and furthermore, boost the energy density of supercapacitor. Our results demonstrate that the vertical graphene electrode is more efficient and practical for the high performance energy storage device with high power and energy densities.

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1. INTRODUCTION Supercapacitors are an attractive technology for electric energy storage because of their high cycle life and high power density. However, the low energy density of supercapacitors, which is still an order of magnitude lower than that of batteries,1 limits their adoption in diverse applications. To pursue high energy performance of supercapacitors, one of the techniques is to increase the surface area of the electrode in order to obtain larger charge storage capacity. Porous materials such as active carbon,2 carbon nanotube3,4 have already been investigated. Graphene is another potential electrode material,5,6 because of its large specific surface area,7 excellent electrical conductivity8 and chemical stability.9,10 To further increase its surface area, several types of graphene, such as 3D stacked graphene, 11 graphene oxide composite12 and chemically modified graphene flake,13 have been constructed and demonstrated high specific capacitance and high frequency response performance.14,15 However, in addition to increasing the surface area, it is also critical to utilize the total surface area for maximizing its functionality which is not achieved in most of the cases. For example, the aggregation of the graphene stacks in solution can reduce the effective surface area. 16 , 17 The random arrangement of graphene hampers the transport pathway for the electrolyte ions to sufficiently penetrate inside the graphene plane thus decreases the storage capacity.18 Graphene sheets that are vertically oriented on electrode surface are ideal for the supercapacitor electrodes.19,20,21,22 They grow perpendicularly on the substrate and the distance between each sheet is large enough (several ten to hundred nanometres) for the electrolyte to reach the bottom of graphene sheets.23 Thus, in these cases the high surface area is fully utilized and electrolyte ions can interact with all graphene sheets sufficiently. In literature, Randin et.al have revealed that the capacities on the edge plane of pyrolytic graphite electrode contributed more than on the basal plane. 24

The morphology and orientation of electrode strongly affects the performance of

the supercapacitor. The vertical graphene electrode consists plenty of edge plane, thus this effect could be greatly magnified. However, it is not yet systematically investigated to what extent it will affect its electrochemical performance. In this paper 3



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we adopted vertical graphene sheets as electrode surface layer of the supercapacitor in order to study how to use them for maximizing the current density of the electrode. The effect of orientation and shape of electrode structure on its electrochemical properties is investigated using a combination of model calculation and experimental study to reveal more about its underlying mechanism. The optimized supercapacitor performance is revealed.

2. EXPERIMENTAL METHODS Vertical graphene Fabrication Vertical graphene sheets were grown on graphite paper substrate without any metal catalysts using microwave plasma chemical vapour deposition (CN-CVD 100, ULVAC). First, a silicon substrate was placed on a cathode plate in a quartz chamber. A H2 flow of 100 sccm was introduced into the chamber and plasma was generated by applying a microwave power of 500 W. A DC bias was applied between the anode and cathode plate to enhance the plasma energy. Then, a CH4 flow of 8 sccm was injected into the chamber, the pressure was maintained at 220 Pa, and the DC bias was raised to -200 V to grow vertical graphene. The mechanism of vertical growth is due to a sheath electric filed between the plasma bulk and the substrate. This built-in sheath electric field polarized the graphene nucleation center and has a dipole force on the graphene growth direction. The build field direction is perpendicular to the substrate, therefore, the graphene grows in a vertical direction. The detailed mechanism has been reported in our previous work at Ref 25. After typically 20 minute of growth, the system was cooled down to room temperature in vacuum. The sheet density and height of vertical graphene sheet can be controlled through varying the growth time and the concentration of CH4. MnO2 Coating Controlled chemical deposition of MnO2 on the graphene sheets was achieved by directly immersing the graphene in acidic KMnO4 solution. The acid in the solution is H2SO4. The reaction between carbon and acidic KMnO4 solution26 is as follow: 3C(s) + 4KMnO4(aq) + 2H2SO4(aq) → 4MnO2(s) + 2K2SO4(s) + 2H2O(aq) + 3CO2(g) 4


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The product of the reaction, K2SO4, is freely soluble in water. The thickness of MnO2 layer is modified by increasing the temperature of the acidic KMnO4 solution and the immersion time. (See details in support information) Electrochemical Measurement Electrochemical characterization was performed in a flat cell in 3-electrode configuration with an Hg/Hg2Cl2 reference electrode and a graphite rod as the counter electrode. A 0.5 mol/L Na2SO4 aqueous solution is utilized as the supporting electrolyte. Cyclic voltammograms were recorded in a 0-0.8 V potential window at different scan rate. The capacitance was calculated by using area under the curve and incorporating that into equation 1. C (VF − Vi ) = q =

1

Vf

ϑ∫

Vi

I (V ) dV

(1)

The capacitance increased with decrease in scan rates. The areal capacitance equals to C/A. A is the effective area of electrode (0.20cm2).

3. RESULTS AND DISCUSSION Few layer graphene is vertically grown on substrate by using microwave plasma chemical vapour deposition method. It can be grown on any substrate which is sustainable to high temperature processing. In this work pyrolytic graphite paper with a thickness of 500 µm is used as the substrate which is made from a highly oriented graphite polymer film. The schematic illustration and morphology of as-grown vertical few layer graphene sheets is shown in Fig. 1. All the graphene sheets stand vertically (Fig. 1b) on the substrate and their top edges can be observed in the top view of SEM image (Fig. 1c). They are distributed uniformly on the substrate and the gap between each sheet is around several tens to hundreds of nanometres. The thickness of graphene sheet is less than 1 nm, and there exists only 2-3 graphitic layers which can be seen in the HRTEM image (Fig. 1d). The height of graphene sheet is several tens of micrometres which could be modulated by varying the growth time. Raman spectrum confirms the formation of graphene sheets (inset of Fig. 1d). The intensity ratio of 2D peak (2706 cm-1) and G peak (1580 cm-1) is 0.94,

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representing the typical characteristic of few layer graphene. The D peak (1350 cm-1) is strong which is due to amorphous carbon and large amounts of graphene edges’ signal. An electroactive MnO2 thin layer can be deposited on the surface of graphene sheets by the chemical reaction of carbon and KMnO4, catalyzed by H2SO4 in a hot bath. As shown from the SEM image in Fig. 1e-f, the MnO2 layer adhered conformally on the graphene sheets. The MnO2 layer was characterized by TEM and XPS spectroscopy (See details in supporting information)

a

b

c

d

e

f

Figure 1. (a) Schematic of vertical graphene/MnO2 hybrid electrode. (b) Tilt view and (c) top view SEM image of vertically oriented graphene sheets. (d) HRTEM image of graphite layers at edge of a graphene sheets and the inset is Raman spectra of vertical graphene sheets. (e) Tilt view and (f) top view SEM image of the graphene sheets with MnO2 coating. Scale bar 100 nm.

The vertical shape of graphene has great benefits for its use as a supercapacitor electrode. It contributed to three main aspects: the increased surface area, open structure for ion exchange, and the electric field enhancement for electron storage. The total surface area of vertical graphene electrode is estimated from its SEM image. We counted the sheet number from the SEM image and divided by the area of the image to get the sheet density. Then, we pictured and measured the length and height of graphene sheet in 10 locations of a sample. The thickness of graphene sheet is 6


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around 1nm which could be measured in the TEM image (Fig. 1d). So, the surface area of a graphene sheet could be calculated. The total surface area is the product of single sheet surface area and sheet density. The results showed that its sheet density is about 2×106 sheet/mm2, and its typical parameters are 2-5 µm in length and 20-40 µm in height. So the increased surface area of vertical graphene is as large as 800 times of planar graphene. This value of surface area is not as large as that of carbon nanotube which is reported as 2014 times with the same height of 40 µm.4 However, it should be noted that the aggregation of vertical carbon nanotubes after being immersed in a chemical solution would seriously reduce its surface area.27 In contrast, vertical graphene sheet has a stable mechanical structure which can maintain its vertical structure without aggregation in solution. As shown in Fig. 1f, the vertical graphene keeps its origin vertical structure after MnO2 deposition in chemical solution. Thus, the maximum utilization of the surface area is maintained. The micro-structure of an electrode could affect its capacitance. De Levie had adopted a model to reveal the influence of surface roughness of electrode on its electrochemical properties in 1963 which would be the first report of micro-structure on its properties.28 Here, the vertical graphene electrode we fabricated is full of micro-structures which effect should be considered on its electrochemical properties. We performed a simulation analysis to evaluate the morphology effect of vertical graphene electrode on the capacitance properties. The flux of ions under the influence of both the ionic concentration gradient and the electric field can be described by a continuum model based on the Nernst-Planck equation 2,

∂ci = ∇ ⋅ ( Di ∇ci + mi z i Fci ∇φ + cu) + Ri ∂t

(2)

Where ci is the concentration of species i (mol/m3); Di is its diffusion coefficient (m2/s); mi is its mobility (mol·m2(s·V·A)); zi is its charge; F is the Faraday’s constant (C/mol); ф is the electric potential; u is the velocity vector (m/s). The first term relates to the ion flux due to ionic concentration diffusion. The second term relates to the ion flux due to the electric field. The third term relates to the ion flux due to transport by bulk fluid motion. Ri is its reaction term. 7



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To simulate the current of the voltammetric wave of the vertical graphene electrode, we focus on the electrode kinetic and concentration-dependent effects,29 and assume that the resistance of the solution is low, no convection and no chemical reaction of the electroactive substances. So the electric field, convection and the reaction term of equation 2 are negligible, and the equation 2 changes to,

∂ci = ∇ ⋅ ( Di ∇ci ) ∂t

(3)

Assuming that it has equilibrium diffusion properties. At steady-state, equation 3 reduces to equation 4:

∇ ⋅ ( Di ∇ci ) = 0

(4)

So a parametric sweep can be used to assemble a voltammogram under a quasi-static approximation. The current density of the electrode can be described by the Butler-Volmer equation 5.

α A nF

j = j 0 [exp(

RT

η ) − exp(−

α C nF RT

η )]

(5)

Where j0 is the exchange current density (A/m2), η is the activation overpotential (V, defined as η = E – Eeq, E is the electrode potential, Eeq is the equilibrium potential), αA is the anodic charge transfer coefficient; αC is the cathodic charge transfer coefficient; n is the number of exchanged electrons; F is the Faraday’s constant; R is the universal gas constant; T is the absolute temperature (K). The electrode surface boundary condition is subjected to the Faraday’s laws of electrolysis. The triangular waveform for the voltage is set from -0.4 V to 0.4 V. The scan rate is set at 50 mV/s. The total electrode current is a surface integral of the current density with the electrode area. To probe the orientation effect of graphene sheet electrode, three types of graphene electrodes were designed. They are vertical graphene, lateral graphene and bulk graphite, as shown in Fig. 2a-c. They have the same length, height and current collector area. The simulation results of cyclic voltammetry (Fig. 2d) show that both the vertical and lateral graphene electrodes have a larger current than the bulk graphite. 8


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It is because the surface area of graphene sheets is larger than the bulk. Furthermore, the vertical graphene electrode has a 58% larger current and current density than that of lateral graphene. It means that the orientation of electrode has the effect on its voltammetry performance.

a

d

b

c

e

Figure 2. The 2D cross-section characteristic concentration profiles with (a) vertical electrodes, (b) horizontal electrodes, and (c) bulk electrodes. (d) The simulation of current-voltage curves for the three different electrode structures. (e) Schematic illustration of ion diffusion direction based on two morphologies of graphene electrodes

Vertical graphene has an open exchange channel for the ion exchange in a shortest straight path. On the other hand, the ions have to follow a more complex route to access the bottom of lateral graphene, as shown in the schematic picture of Fig. 2e. Due to the screening effect, the driving electric field in the gap is much smaller than on the surface, which makes it even harder for the ions to reach the bottom. In the vertical graphene electrode, the ions could go through the gap perpendicular to the horizontal plane and reach the bottom without being captured in between. In case of the lateral graphene electrode, the ions enter the gap at an inclined angle, and have little chance to get to the bottom without being blocked. Therefore, the vertical structure benefits the ion direct transport and exchange process and minimizes the ion kinetic losses. That could result in high speed and high efficiency

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for ion exchange and current collection. As compared to other porous materials such as foam and stacked structure, vertical graphene could be an optimal structure for supercapacitor electrode. Besides the advantage of the orientation, the atomically sharp vertical graphene also creates a large field enhancement which benefits the current collecting capability. As mentioned in the Nernst-Planck equation 2, to evaluate the effect of electric field, the solution resistance term should be considered, while the electrode kinetic and concentration-dependent terms should be neglected. Under these conditions, the effect of electric field is not negligible for the current distribution on the surface structure of electrode. So the equation 2 can be described as follow,

∇ ⋅ (mi z i Fci ∇φ ) = 0

(6)

The net current density can be described as follow,

i = −F ∑ zi mi Fci ∇φ 2

(7)

Where i is the current density vector (A/m2). The boundary conditions for the primary current distribution assume that the potential of electrode surface is constant. The geometry of the vertical and lateral graphene electrode on the substrate is shown in Fig. 3 inset. The results in Fig. 3 show the dimensionless current density (defined as the current density divided by the average current density) distribution at the different shapes of sheet electrodes. The dimensionless current density on a vertical sheet is 33 at the corner of the top edge and 10 at the middle of the top edge. However, it is only 0.5 on the plane of the graphene. As for the three laterally stacked sheets (Fig. 3b), the dimensionless current density of the top sheet is 10, 4 and 10-8 (corner, edge and plane) and that of the bottom sheet is only 0.4, 0.1 and 10-9. The results demonstrate that the current density is much larger along the top edge of the vertical graphene sheet than the planar surface, and has a maximum value at the two corners of graphene sheets. That is due to the high electric field enhancement at the edge and corners. Therefore, more ions could be accumulated at the edge and contribute more for the total charge storage capacity. However this effect is much smaller at a laterally stacked sheet. In addition, 10


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the high electric field intensity facilitates the ionic migration and increases the ion exchange speed.

a

b

Figure 3. Dimensionless current density distribution on (a) vertical sheet electrode. (b) lateral sheet electrode.

To examine the simulation results, we fabricated three types of graphene electrodes: a vertical graphene, a lateral graphene and a bare graphite paper substrate as shown in Fig. 4a-c. The lateral graphene sample was fabricated by intentionally squeezing the vertical graphene down into lateral structure as shown in Fig. 4b. The capacitance using each type of graphene electrode with the same MnO2 coating are measured and compared as shown in Fig. 4d and e. The areal capacitance of vertical structure is 38% higher than the lateral structure which proves that the more efficient ion exchange path and greater field enhancement affects the capacitance of graphene sheets. The capacitance of graphite substrate is 6 times smaller than the vertical graphene electrode which strongly supports the role of the high surface area of the graphene sheets on the capacitance. The Nyquist plot (Fig. 4f) showed that the resistance of vertical, lateral and plane graphene electrodes are 17.0, 19.9 and 20.2 Ω. The vertical structure has lower resistance than that of the other two structure. The rate capability (Fig. 4g) showed that the areal capacitance of vertical graphene has the better capacitance retention of 82% than the lateral graphene and plane graphene when the scan rate increased. It is believed that the faster ion diffusion in the vertical graphene helps to keep its capacitance. 11



a

b

c

e

f

g

300

200

2

d

Cs (mF/cm )

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50 mV/s 100 mV/s

82%

100 71% 77%

0 Vertical

Lateral

Plane

Figure 4. Tilt view SEM image of (a) vertical graphene sheets, (b) lateral graphene sheets, and (c) thick graphite paper. Scale bar is 1µm. (d) Cyclic voltammetry curves, (e) areal capacitances, (recorded at 50 mV/s), (f) Nyquist plot (recorded at 5mV, 0.05 Hz to 0.1 MHz) and (g) rate capability using these three kinds of graphene electrode.

The electrochemical performance of the supercapacitor using vertical graphene electrode is shown in Fig. 5. The cyclic voltammetry (CV) curves, recorded at the scan rates ranging from 0.05 to 5V/s (Fig. 5a), show a rectangular shape until the scan rate is higher than 5V/s, and demonstrate a good high speed response. The areal capacitance is 8.8 mF/cm2 at the scan rate at 50 mV/s (Fig. 5b) with capacitance retention of about 50%, when the scan rate increased from 0.05 to 3 V/s (Fig. 5c). The galvanostatic charging-discharging of this sample at current density of 0.5 mA/cm2 is shown in Fig. 5d. The corresponding areal capacitance was 9.3 mF/cm2. The detailed

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effect of height, density of vertical graphene on the capacitance is shown in Supporting Information.

a

b

c

d

Figure 5. (a) CV curves recorded at different scan rate. (b) CV curve recorded at 0.05V/s. (c) Areal capacitances retention as a function of scan rate. (d) Galvanostatic charge-discharge curves at current density of 0.5 mA/cm2, using as-grown vertical graphene sheets.

To further improve its capacitance, a layer of pseudocapacitive MnO2 is coated on the surface of vertical graphene. The CV curves of MnO2/graphene hybrid electrodes are recorded at the scan rate from 5 to 100 mV/s (Fig. 6a). A high areal capacitance value of 500 mF/cm2 at the scan rate at 5 mV/s is obtained. Fig. 6b shows that the capacitance retention of graphene/MnO2 electrodes is 49% even at 100 mV/s. The galvanostatic charging-discharging of this sample at current density of 2.5 mA/cm2 is shown in Fig. 6c and an areal capacitance of 510 mF/cm2 was recorded. The corresponding ESR is 14.5 Ω which should be further reduced to improve the performance of supercapacitor. The cycling stability of the sample was tested at a scan

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rate of 100 mV/s for 1000 cycles. The capacitance of the sample exhibits no reduction during the cycle (Fig. 6d).

a

b

c

d

Figure 6. (a) CV curves recorded at different scan rate; (b) Areal capacitances retention as a function of scan rate; (c) Galvanostatic charge-discharge curves at current density of 2.5 mA/cm2; (d) Cycle performance of the graphene/MnO2 electrode at a scan rate of 100 mV/s for 1000 cycles, using vertical graphene sheets with MnO2 coating.

4. CONCLUSIONS In summary, we have investigated the relationship between electrode structure and capacitive performance in case of graphene sheets with different orientations, Also, a high capacitance value of 500 mF/cm2 has been achieved by utilizing vertical graphene sheets covered with pseudocapacitive MnO2 coating by taking advantage of its high areal surface and porous vertical structure. Simulation results demonstrate that the orientation and morphology of graphene sheets strongly impact the ion transport capability and the charge storage capacity. Besides increasing the surface area,

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manipulating the microscopic morphology of graphene electrode could maximize the utilization of surface area and greatly improve the specific capacitance. Experiment results showed that the areal capacitance of vertical graphene electrode is 38% higher than that of lateral graphene electrode and 6 times higher than that of graphite paper. By combining the advantage of high surface area, fast ion diffusion and enhanced charge storage, vertical graphene could be an optimal electrode for high performance supercapacitor with both high energy and power densities.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Identification of MnO2 coating using HRTEM, EDS Mapping and XPS. Effect of MnO2 thickness on its capacitance of graphene/MnO2 hybrid electrode. Effect of sheet density and height of vertical graphene on its capacitance of graphene/MnO2 hybrid electrode.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51290271, 51102287 and U1134006), the National Key Basic Research Program of China (Grant No. 2013CB933601, and 2013YQ12034506), the Fundamental Research Funds for the Central Universities，the Science and Technology Department of Guangdong Province, and the Science & Technology and Information Department of Guangzhou City. We acknowledge Prof. Y.X. Tong and Prof. X.H. Lu for providing 15



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the electrochemical measurement facilities.

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Table of Contents Graphic 58x38mm (300 x 300 DPI)


Morphology Effect of Vertical Graphene on the High Performance of

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