LETTER pubs.acs.org/NanoLett
Ultrathin Planar Graphene Supercapacitors Jung Joon Yoo,†,‡,^ Kaushik Balakrishnan,†,^ Jingsong Huang,§ Vincent Meunier,*,§,z Bobby G. Sumpter,§ Anchal Srivastava,†,|| Michelle Conway,† Arava Leela Mohana Reddy,† Jin Yu,‡ Robert Vajtai,† and Pulickel M. Ajayan*,† †
Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States Department of Material Science and Engineering, KAIST, Daejeon, Republic of Korea § Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States Department of Physics, Banaras Hindu University, Varanasi, India
)
‡
bS Supporting Information ABSTRACT: With the advent of atomically thin and flat layers of conducting materials such as graphene, new designs for thin film energy storage devices with good performance have become possible. Here, we report an “in-plane” fabrication approach for ultrathin supercapacitors based on electrodes comprised of pristine graphene and multilayer reduced graphene oxide. The in-plane design is straightforward to implement and exploits efficiently the surface of each graphene layer for energy storage. The open architecture and the effect of graphene edges enable even the thinnest of devices, made from as grown 1-2 graphene layers, to reach specific capacities up to 80 μFcm-2, while much higher (394 μFcm-2) specific capacities are observed multilayer reduced graphene oxide electrodes. The performances of devices with pristine as well as thicker graphene-based structures are examined using a combination of experiments and model calculations. The demonstrated all solid-state supercapacitors provide a prototype for a broad range of thin-film based energy storage devices. KEYWORDS: Graphene, supercapacitor, in-plane geometry, single-layer graphene, multilayer graphene
C
arbon-based materials have been researched extensively as electrodes in electrical energy storage devices such as supercapacitors and batteries, owing to a unique combination of properties such as high surface area, lightweight, good electrical conductivity, compatibility with other materials, and controlled pore size distribution.1-8 Control of structure and morphology is key for carbon-based electrodes to allow the effective permeation of the electrolyte to establish electrical double layers (EDLs) in supercapacitors.8,9 The advent of new forms of carbon materials such as high quality graphene sheets (single layer to a few layers) with superior electrical properties have allowed for the development of new engineered carbons for energy storage.2,10-14 Owing to their large in-plane conductivities graphene films are expected to play a crucial role in the development of electrodes for a variety of energy applications such as photovoltaics15-18 and supercapacitors.19 These materials have recently been used in supercapacitor devices to replace conventional carbon electrodes and have shown very good performance.20-24 Most recently, supercapacitor devices composed of curved graphene as electrodes along with ionic liquids have demonstrated a record performances in terms of the energy density.24 Typically, graphitic carbon-based materials are randomly oriented with respect to the current collectors in a conventional stacked geometry in supercapacitors. In such cases, the electrolyte ions are often limited from penetrating far inside the graphitic planes (Figure 1a and Supporting Information Figure S1a,b). This lowers the complete utilization of the electrochemical surface area r 2011 American Chemical Society
of graphene layers and consequently limits the extent of the EDL formed at the interface. In two-dimensional (2D) systems such as thin-film supercapacitors, a modification in the architecture can be made to suit the material. Most recently, Miller et al.25 demonstrated the utilization of the vertically oriented graphene electrodes26 grown using plasma-enhanced chemical vapor deposition methods onto Ni-substrates toward use in the ac-line signal filtering of 120 Hz, which is expected to have an immense impact on the power management of future electronic devices especially using ultrathin materials (size factor). Furthermore, these vertically oriented graphene electrodes have already shown reasonable performance as electrochemical supercapacitors and are expected to show very high capacitance values when fabricated using the coilbased device geometry, as high as 14 900 F.26 Here, we demonstrate the utilization of pristine and multilayer graphene as electrodes in an “in-plane” device geometry for use in supercapacitors. The main features of the design are shown in Figure 1b and Figure S1c,d (Supporting Information). The 2D in-plane design takes advantage of the atomic layer thicknesses and flat morphology of graphene and is ideal for 2D devices. The favorable in-plane design demonstrated here offers new opportunities for the electrolyte ions to enhance interaction with all the carbon layers (Figure 1a), leading to a full utilization of the high surface area offered by the graphene layers. Furthermore, the 2D design also allows for exploiting the Received: September 22, 2010 Published: March 07, 2011 1423
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Figure 1. (a) Schematic depiction of the stacked geometry used for the fabrication of supercapacitor devices reported here. Graphitic carbonbased materials are randomly oriented with respect to the current collectors in such a stacked geometry. Here we show the unfavorable limit with all graphene layers parallel to the current collectors. In the case of the stacked (conventional) geometry the electrochemical surface area is incompletely utilized, because some of the regions are inaccessible to the electrolyte ions. (b) Schematic depiction of the operating principle in case of the in-plane supercapacitor device utilized for the performance evaluation of graphene as electrodes. The new architecture presents the added benefit of increased ability of the electrolyte to percolate into the layers of graphene to allow for full utilization of the electrochemical surface area. A more detailed graphical illustration for the mechanism of operation is provided in Figure S1 (Supporting Information).
unique electrochemical properties of graphene edges along with the basal planes of graphene.27,28 The starting hypothesis of this work is that the 2D in-plane design allows for the maximization of the available electrochemical surface area, in addition to the possibility of extreme miniaturization of device thickness (e.g., single layer graphene devices). In this study we have utilized both pristine graphene (G) synthesized using the chemical vapor deposition method (CVD),13 and multilayer graphene films (reduced multilayer graphene oxide, RMGO) prepared using chemical reduction of the graphene oxide (GO) films obtained through layer-by-layer (LBL) assembly (see Figure S2 and Materials and Methods in Supporting Information). Specifically, the G and RMGO films were prepared on copper foil and quartz substrates, respectively (see Materials and Methods in Supporting Information). Unlike the conventional stacked geometry used in supercapacitors, wherein the graphene layers have random orientation with respect to the current collectors, in the new in-plane design a large conductive planar sheet of graphene (2D) is isolated into two electrodes by physically creating a micrometer-sized gap via a scission on the graphene layer (Figure S3, Supporting Information). Current collectors are then produced by sputtering gold on external edges of two electrodes (Figure S3, Supporting Information). In the final design step, the electrolyte is spread across the active electrode surface and the micrometer-sized gap that allows for charge separation and mobility (of the ions) across the two electrodes (Figure 2a, and Figure S3, Supporting Information). Here, we demonstrate the proof of concept for
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Figure 2. Device fabrication and material characteristics. (a) Schematic depiction of the device fabricated using the concept of 2D in-plane supercapacitors. The various stages of in-plane 2D graphene device fabrication are illustrated in the Figure S3 (Supporting Information). (b) A prototype flexible supercapacitor device based on RMGO developed using the new in-plane geometry. (c) Optical image of the large area single-layer graphene transferred onto a SiO2 substrate after growth on Copper foils. The TEM image of the single-layer graphene is shown in the inset (also see Supporting Information). From the TEM image of the single-layer, we observe that roughly 40% of the area in the G show open hole structures, allowing for more electrolyte ions to percolate efficiently into the G film and consequently maximize the electrochemical surface area available. (d) A real-time photographic image of the multilayer graphene oxide (GO) film obtained by the LBL (left) and the RMGO film (right) obtained by subsequent chemical reduction using hydrazine. A notable change in the color of the films along with decreased electrical resistance after reduction clearly indicate significant reduction of the oxide content and restoration of the conjugated π-rich network within the layers of the graphene. (e) SEM image of inner layers of the RMGO film after cleaving the top surface using scotch tape. It is evident that stacked layers with continuous contact leading to uniform films are indeed formed by the layer-by-layer method.
all solid-state 2D in-plane supercapacitors by using a polymer-gel (PVA-H3PO4) electrolyte over the graphene electrodes. Polymer-gel electrolytes have previously been used to yield stable solid-state supercapacitor devices.29 The polymer-gel electrolyte shows very stable performance in the operating range of 1 V (since water is used as solvent in preparation of the electrolyte, the voltage range is closer to that for an aqueous electrolyte) in which the new graphene-based devices are tested. The polymergel electrolyte serves as both an ionic electrolyte and a separator, thereby making the new design rather unique. The devices fabricated are compact, ultrathin, flexible, and optically transparent (Figure 2b). RMGO films with the 2D in-plane structure were produced on a quartz substrate by the LBL self-assembly and subsequent chemical reduction (Materials and Methods, Supporting Information). A photographic image of the typical GO film obtained by LBL assembly of graphene oxide and poly(ethyleneimine) (PEI) and the RMGO film obtained by subsequent chemical reduction of the GO film with hydrazine is shown in Figure 2d. The SEM (Figure S4a, Supporting Information) and AFM surface images (Figure S4c, S5, Supporting Information) of the RMGO indicate a smooth and uniform surface. X-ray photoelectron spectroscopy (XPS) measurements on the RMGO films show dramatic reduction of the oxygen content in comparison to the original GO films obtained by LBL self-assembly (Figure S6, Supporting Information). The inner1424
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Figure 3. The electrochemical properties of the graphene electrodes tested using the in-plane geometry. (a) Cyclic voltammetry curves obtained at different scan rates for RMGO. (b) Cyclic voltammetry curves obtained at different scan rates for G. (c) Galvanostatic charge-discharge curve of RMGO supercapacitor measured at constant current density of 176 mA g-1. (d) Cyclic stability obtained by performing charge-discharge of the RMGO supercapacitor over 1500 cycles. It is apparent that the materials retain good stability over large number of charging-discharging cycles. Also, some of the prototype devices retested ca. nine months of their creation retained their original performance. Both these tests imply the long-term stability of these all solid-state devices created here using the new in-plane design. A capacity of 35 and 3.333 μF were calculated using the CD curves for RMGO and G, respectively (see Table 1).
layers of RMGO films in a partially peeled-off area by adhesive tape (Figure 2e and Figure S4b in Supporting Information), clearly show the boundaries of deposited RMGO layers, indicating that a flat-layered structure was successfully formed without any agglomeration. Height measurements performed using AFM (Figures S4c and S5, Supporting Information) reveal a film thickness of ∼10 nm for the RMGO films implying a relatively large number of layers. As will be clear later in the discussion based on theoretical estimates, about 21 layers of graphene are present in a 10 nm thick RMGO film, yielding an interlayer spacing of ca. 5 Å for the RMGO films. Pristine graphene films (G) (∼1 cm 1 cm) were grown on copper foils at 950 C by a CVD method using hexane as a carbon source under Ar/H2.13 The G film (Figure 2c, Figure S7, Supporting Information) shows single-layer characteristics in >90% of the area and only some regions display 2-3 layers of graphene as can be inferred from the edge structure and Raman spectroscopy (Figure S7, Supporting Information).13 This information is essential for the overall evaluation of the performances from the G in-plane devices (see Supporting Information). The as-grown G films were transferred from Cu to quartz substrates for device fabrication.
The electrochemical performance of the graphene-based 2D design was analyzed using cyclic voltammetry (CV) and galvanostatic charge/discharge (CD). The specific capacitance was extracted from both the CV curves and the galvanostatic CD curves. The CV curves of the G and RMGO device were measured with various scan rates in the range of 1-100 mVs-1. As shown in Figure 3a,b, CV curves display nearly rectangular shape even at very high scan rates, indicating that an efficient EDL is established in both of the graphene-based electrodes. Galvanostatic CD curves (Figure 3c) for RMGO were obtained at a constant current density of 176 mA g-1 (i.e., 281 nA cm-2). As seen in Figure 3c, the CD curve is close to a triangular shape, confirming the formation of an efficient EDL and good charge propagation across the two electrodes. The supercapacitor devices comprised of RMGO or G show no degradation in performance after 1500 cycles (Figure 3d). The specific capacitance was calculated by integrating the area under the CV curves and by measuring the slope of the galvanostatic CD curve. The measured mass for each electrode in RMGO device is 0.283 μg. For the RMGO electrode, assuming a symmetric capacitor, the gravimetric capacitance of the active material is calculated from the device capacitance as (2 35 μF)/(0.283 μg) = 247.3 F g-1. 1425
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Table 1. Performance Evaluation and Comparison of the G and RMGO 2D “In-Plane” Supercapacitors device propertiesa
electrode propertiesb specific capacity
mater
method
G
CVD
RMGO
LBL
N
T (nm)
1 21f
capacitance (μF)
mass (μg)
geometrical area (cm2)
0.283
0.085 2.1
35
μF cm-2 d 80
80
247
394
19
0.1 0.835
3.333 10
F g-1 c
μF cm-2 e
a
N = number of layers; T = thickness of the electrode. The capacitance values are reported for the best performance obtained using the CD curves with current density of 281 nAcm-2 for RMGO and 630 mAcm-2 for G. b Electrode capacitance converted from the device capacitance assuming a symmetrical capacitor. c Normalized by the electrode mass. d Normalized by one electrode’s geometrical area. e Normalized by one electrode’s interface area. f Calculated using the mass, the geometrical area, and the specific area of one side of graphene (1310 m2 g-1).
These correspond to the best values of up to 250 F g-1 obtained for graphene based supercapacitors to-date (using aqueous electrolytes).22 The normalized capacitance by the geometrical area is (2 35 μF)/(0.08465 2.1 cm2) = 394 μF cm-2, higher than 300 μF cm-2 reported previously.23 From the mass and the geometrical area of one electrode, we evaluate the specific geometrical area of one electrode as (0.08465 2.1 cm2)/(0.283 μg) = 62.8 m2 g-1. The geometrical area of one graphene layer is 1310 m2 g-1, according to the C-C bond length of 1.42 Å. Therefore, the number of layers in the RMGO electrode is N = 1310 m2 g-1/62.8 m2 g-1 = 20.9 ≈ 21 layers. Since the thickness of the RMGO sample is 10 nm (AFM, Figures S5 and S7, Supporting Information), the interlayer separation is 10 nm/(N - 1) = 5 Å. This distance is slightly larger than the ionic size of 4.8 Å for H2PO4-, HPO42-, and PO43- ions.30 The interlayer distance of ∼5 Å in the RMGO material synthesized from the reduction of graphite oxide is large enough to accommodate counterions. At this interlayer distance, the potential energy surface of graphite is far away from the global minimum at an interlayer distance of 3.35 Å. This is in sharp contrast with the case in graphitic carbons (interlayer distances of ca. 3.35 Å) where it is indispensable to activate the electrode materials at elevated voltage in order to expand the interlayer distance for counterion intercalation.31 Assuming only one side of each graphene layer in the RMGO electrode contributes to the surface area accessible to charge storage (Figure 1b), for an interface area of 1310 m2 g-1 per graphene layer, the interface area of RMGO should be 1310 m2 g-1 as well. Hence, the capacitance of RMGO normalized by the accessible interfacial surface is (247 F g-1)/(1310 m2 g-1) = 18.9 μF cm-2. This value is much closer than the geometrical area normalized value of 394 μF cm-2 to the limiting double-layer capacitance32 of 16 μF cm-2. In spite of the different electrolyte used in this study (PVA/H3PO4) compared to the 1 M H2SO4 used in ref 32 the comparison is still valid since both aqueous H3PO4 and H2SO4 electrolytes yield almost the same double layer capacitance.33 Of course, there is a difference between the PVA/H3PO4 polymer-gel electrolyte and an aqueous H3PO4 electrolyte. However, we expect that the use of a polymer-gel electrolyte does not affect the dielectric constant or the double layer thickness because it contains water in the matrix. On the other hand, the fact that the computed value of 18.9 μF cm-2 is higher than 16 μF cm-2 could be ascribed to the contribution from the graphite edges. It is a well-accepted experimental phenomenon that area-normalized capacitance of the basal plane of graphite is lower than that of edges.1,34 Furthermore, assuming a symmetric capacitor, the voltage drop across each electrode is 0.5 V. Thus, the charge density on the RMGO interface is 18.9
μF cm-2 0.5 V = 5.9 10-3 e/Å2. This would translate to a unit area of 169.5, 339.0, and 508.5 Å2 for each H2PO4-, HPO42-, and PO43- ion, respectively. For H2PO4-, this in turn gives a distance of 14 Å between neighboring counterions, assuming a 2D hexagonal close packing of counterions. This distance is much larger than the ion size of 4.8 Å (which may be larger if a solvation shell is considered), indicating that the interface is far from reaching surface saturation.35 This is an important finding as it shows there is no reason for the RMGO layers to expand wider in order to utilize both sides of each graphene for charge storage. RMGO films thinner than 10 nm had negligible weight and therefore obscure gravimetric consideration of the figures of merit. It is noted here that in addition to the EDL contribution to the capacitance values, there could also be small pseudocapacitance contributions arising from any possible redox reactions during the charge discharge cycles, especially as aqueous electrolytes are used, however the shape of the CV curves and the nonexistence of any spurious peaks in these curves suggest that the pseudocapacitance contributions should be minimal. The mass of G electrode is very small and prevents its accurate evaluation. Therefore, a more appropriate metric for the characteristic figure of merit should be based on the performance per geometrical area instead of the gravimetric value. The CV and CD curves for G are shown in Figure 3b and Figure S8 (Supporting Information), respectively. The in-plane devices consisting of G electrodes show excellent charge propagation as observed by the nearly rectangular CV curves and triangular CD curves. The normalized capacitance by the geometrical area is 80 μF cm-2 [(2 3.333 μF)/(0.1 0.835 cm2)]. We have chosen to focus on the areal capacitance and not on the gravimetric capacitance value for the G device due to the ambiguities in mass determination. The overall performance evaluation of the devices using both RMGO and G is shown in Table 1 and Table S1 (Supporting Information). The specific capacitance of the RMGO device per geometric area was calculated to be ca. 390 μF cm-2, about 5 times higher than the value of 80 μF cm-2 obtained for G. This is understandable as more layers are stacked in the RMGO device. It is obvious that the new geometry would also provide a pathway to store more charge per unit area by increasing the number of layers in the device. To further test the validity of the superior performance from the in-plane device geometry, stacked devices (conventional geometry) were fabricated from the RMGO electrodes comprised of the same area of the electrodes containing similar thickness (amount) of the polymer-gel (see Supporting Information for the fabrication details of the stacked device). The specific capacity of the stacked device was found to be 140 μFcm-2, 1426
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Nano Letters which is about 3 times lower than those obtained for the in-plane devices (see Table S2, Supporting Information). This again highlights the fact that favorable placement of the electrodes, electrolyte and the current collectors in an in-plane device geometry allows for maximizing the offered active electrochemical area by the multilayer graphene surface. In summary, we have demonstrated a simple fabrication approach for making an original graphene supercapacitor geometry based on the 2D in-plane design that achieves high capacitive energy storage characteristics. This new, ultrathin design allows for the formation of an efficient electrical double layer and most importantly allows for utilization of the maximum electrochemical surface area (both the carbons in the basal planes and along the edges). From a practical standpoint, this device geometry can be easily extended to other thin-film based supercapacitors, and adapted to various structural and hybrid designs for energy storage devices.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental methods and materials, additional analysis of the materials, detailed AFM analysis of the multilayer RGO films, and calculations of the figures of merit for pristine graphene. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: (P.M.A.)
[email protected]; (V.M.)
[email protected]. Present Addresses z
Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180-3590.
Author Contributions ^
These authors contributed equally to this work.
’ ACKNOWLEDGMENT P.M.A. acknowledges the support from Rice University startup grants. K.B. and J.J.Y. extend gratitude to Professor Bruce Weisman, Rice University, and his group for allowing access to their microbalance. Some parts of this research were funded through Advanced Energy Consortium (AEC, BEG 10-02). J.J.Y. acknowledges the support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0000862). J.H., V.M., and B.G.S. acknowledge support from the Laboratory Directed Research and Development Program of ORNL, the Division of Materials Science and Engineering, Basic Energy Sciences, U.S. Department of Energy and the Center for Nanophase Materials Sciences, sponsored by the Scientific User Facilities Division, U.S. Department of Energy. A.S. acknowledges the support from Department of Science and Technology (DST), India, under BOYSCAST fellowship.
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