Self-Assembled, Redox-Active Graphene Electrodes for High

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Letter pubs.acs.org/JPCL

Self-Assembled, Redox-Active Graphene Electrodes for HighPerformance Energy Storage Devices Tianyuan Liu,† Reza Kavian,† Inkyu Kim, and Seung Woo Lee* George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Graphene-based materials have been utilized as a promising approach in designing high-performance electrodes for energy storage devices. In line with this approach, functionalized graphene electrodes have been self-assembled from the dispersion of graphene oxide (GO) in water at a low temperature of 80 °C using tetrahydroxyl-1,4benzoquinone (THQ) as both the reducing and redox-active functionalization agent. We correlated the electrochemical performance of the electrode with surface oxygen chemistry, confirming the role of THQ for the reduction and redox-active functionalization process. The assembled graphene electrodes have a 3D hierarchical porous structure, which can facilitate electronic and ionic transport to support fast charge storage reactions. Utilizing the surface redox reactions introduced by THQ, the functionalized graphene electrodes exhibit high gravimetric capacities of ∼165 mA h/g in Li cells and ∼120 mA h/g in Na cells with high redox potentials over ∼3 V versus Li or Na, proposing promising positive electrodes for both Li and Na ion batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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have pseudocapacitive (surface redox) reactions. Recently, reversible surface redox reactions between Li ions and the oxygen functional groups on the nanocarbons, including CNT and graphene, have been observed at a high redox potential of ∼3.2 V versus Li.27−29 On the basis of these surface redox reactions, oxygen-functionalized carbon materials have been employed as positive electrodes in Li cells for high-power batteries or hybrid capacitors.22,29−31 The electrochemical performance of these functionalized carbon electrodes strongly depends on their physical and chemical structures. Binder-free 3D porous nanostructured electrodes showed enhanced electrochemical performance compared to that of the composite electrodes including polymer binder owing to increased electrochemically active surface area.29 In addition, the oxidized CNT and CNT/graphene composite electrodes showed that surface oxygen chemistry can greatly influence the electrochemical performance.27,30 Therefore, the assembly of 3D nanostructured electrodes with a fine-tuning of surface oxygen chemistry is a crucial design factor in developing highperformance pseudocapacitive carbon electrodes. In this study, we present a simple synthetic route to assemble a functionalized graphene electrode using tetrahydroxyl-1,4benzoquinone (THQ), a renewable and redox-active molecule, as both the reducing and functionalization agent (Scheme 1). THQ exhibits a strong reducing ability, enabling the fast selfassembly of 3D functionalized graphene from the dispersion of GO in water at a low temperature of ∼80 °C under

anocarbons, such as a carbon nanotube (CNT) and graphene, have become rising electrode materials for electrochemical energy storage devices, such as electrochemical capacitors (ECs) and rechargeable batteries.1−7 This is owing to their superior physicochemical properties, including high surface area, high electrical conductivity, high mechanical strength, and chemical stability.8,9 In particular, the scalable and low-cost synthesis methods of graphene materials from graphene oxide (GO) using solution-based processes open up a new opportunity to develop large-scale graphene-based electrodes.10,11 Graphene-based electrodes have been intensively studied for electrochemical double layer capacitors (EDLCs),12−16 one type of EC, which store charge by electrolyte ion adsorption (double layer capacitance) on high-surface-area conductive carbon materials.17 However, graphene sheets can be readily restacked via the strong π−π interaction during the electrode fabrication and subsequent drying process,18 eventually decreasing the available surface area for electrochemical reactions. Accordingly, research efforts have been focused on preventing the restacking of graphene sheets by designing 3D nanostructured electrodes,16,19−23 which can maximize the electrochemically active surface area. Although these graphenebased EDLCs represent state-of-the-art high-power energy storage devices, their energy density utilizing double layer capacitance is significantly lower than that of Li ion batteries employing intercalation reaction mechanism in the bulk of active materials. A promising approach to increase the energy density of carbon-based electrodes is incorporating heterogeneous atoms into the electrodes, such as nitrogen24,25 or oxygen,26 which © XXXX American Chemical Society

Received: November 1, 2014 Accepted: December 2, 2014

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Scheme 1. Reduction and Functionalization Mechanism of GO by THQa

a The mixture of GO and THQ in water was heat-treated at 80 °C, during which H+ released from THQ could reduce the oxygen functional groups on the surface of the GO. The remaining THQ and oxidized THQ (C6O6Hx) could adsorb on the surface of the reduced GO by π−π interaction, forming functionalized graphene.

Figure 1. (a) Digital images of the mixture of GO (1 mg/mL) and THQ (3 mg/mL) in water (left) and the assembled functionalized graphene hydrogel (right). (b) X-ray photoelectron spectroscopy (XPS) wide scan survey of the GO film and the functionalized graphene electrodes. (c) XPS C 1s spectra of the GO film (top) and the functionalized graphene electrodes obtained from mixtures of GO and THQ with different concentration ratios (GO/THQ = 1:1 (middle) and 1:4 (bottom)). (d) Low-magnification and (e) high-magnification scanning electron microscope (SEM) images of the 3D functionalized graphene obtained from the mixture of GO and THQ (GO/THQ = 1:3).

atmospheric pressure. In addition, the surface oxygen chemistry of the graphene electrodes can be manipulated, and additional redox-active oxygen can be incorporated onto the surface of graphene by simply controlling the concentration of THQ. We correlate the surface oxygen chemistry with the gravimetric capacity of the electrodes assembled at different THQ concentrations, confirming the introduction of the surface redox reactions into the electrodes by THQ. The functionalized graphene electrodes can be employed as positive electrodes in

both Li and Na cells, storing Li up to a gravimetric capacity of ∼165 mA h/g and Na up to ∼120 mA h/g due to the surface redox reactions between oxygen functional groups and cations. The functionalized graphene electrode showed excellent cycling stability up to 10 000 cycles. As THQ can be produced in high yield from renewable myo-inositol, a kind of natural compound that widely exists in plants,32−34 the simple THQ-based 3D graphene assembly process opens a new pathway to synthesize 4325

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Figure 2. (a) Comparison of the steady-state cyclic voltammetry (CV) scans of the GO film and functionalized graphene electrodes with different concentration ratios of GO and THQ in Li cells, in 1 M LiPF6 in a mixture of EC and DMC (3:7 volume ratio). The voltage window of the CV was 1.5−4.5 V versus Li at a scan rate of 1 mV/s. (b) Instantaneous specific capacitance of the electrodes at 3.4 V versus Li as a function of the concentration ratio of GO and THQ. Current-density-dependent galvanostatic charge and discharge curves of (c) the GO film electrode prepared by the vacuum filtration process and (d) the functionalized graphene electrodes assembled at concentration ratios of GO/THQ = 1:4 in the voltage range between 1.5 and 4.5 V versus Li. Before each discharge and charge process, the cells were held at a constant voltage of 4.5 or 1.5 V versus Li, respectively, for 30 min, and the gravimetric current density was controlled from 0.05 to 100 A/g. The thicknesses of the electrodes were ∼10 μm for the GO electrodes and ∼107 μm for GO/THQ = 1:4. (e) Gravimetric capacity and oxygen atomic percentages of the electrodes as a function of the concentration ratio of GO and THQ. GO/THQ = 1:0 indicates the GO film. (f) Gravimetric charge and discharge capacities of the functionalized graphene electrode (GO/THQ = 1:4) and its coulombic efficiency as a function of cycle number up to 10 000 cycles. Charge and discharge capacities were measured at 0.1 A/g, once every 100 cycle up to 1000 cycles and once every 500 cycle between 1001 and 10 000 cycles, after a voltage holding process for 30 min. Within these measurements at 0.1 A/g, the Li cells were cycled at an accelerated rate of 10 A/g. The figure inset compares the charge and discharge curves for the 1st and the 10 000th cycle at 0.1 A/g.

large-scale, high-performance positive electrodes for both Li and Na ion batteries. Functionalized graphene hydrogel was self-assembled by heating a mixture of GO (1 mg/mL) and THQ (1−4 mg/mL) in deionized water at 80 °C for 3 h under atmospheric pressure without any disturbance (Figure 1a). The hydroxyl groups in THQ can generate H+ ions in water, and the released H+ ions react with the oxygen functional groups, such as hydroxyls, epoxide, and carbonyl groups, on the GO surface, enabling the deoxygenation (reduction) of GO.35 The reduced graphene sheets have hydrophobic properties, and therefore, they tend to interact with each other through the hydrophobic and π−π interaction.14,36,37 When the concentration of GO is high enough (>1 mg/mL), the reduced graphene sheets can self-

assemble into a 3D structure (Figure 1a). The functionalized graphene electrode was prepared by removing water within the graphene hydrogel using a freeze-dryer. The surface oxygen chemistry of the functionalized graphene electrodes were determined by X-ray photoelectron spectroscopy (XPS) analysis (Figures 1b,c and S1, Supporting Information). XPS wide scan survey peaks revealed that the functionalized graphene electrode prepared from a mixture of GO and THQ with a 1:1 concentration ratio (GO/THQ = 1:1) had a significantly reduced atomic ratio of oxygen to carbon (O/C = 0.12) compared to that of a pristine GO film (O/C = 0.42) (Figure 1b), indicating that THQ is an efficient reducing agent. High-resolution C 1s spectra of the electrodes were fitted with an sp2-hybridized graphitic carbon peak (284.5 eV) and 4326

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showed a 3D interconnected conductive network structure assembled by the partial stacking of the graphene sheets (Figure 1d and e). The hierarchical porous structure was observed with the pore size ranging from tens of nanometers to tens of micrometers (Figure 1d and e), which can facilitate ionic transport and increase electrochemically accessible surface areas. In contrast, the GO film obtained by a vacuum filtration process showed a densely packed layered structure (Figure S4, Supporting Information). Thus, the 3D porous microstructure of the functionalized graphene electrodes can provide continuous electronic and more effective ionic transport pathways within the electrode, supporting fast surface redox reactions on the oxygen functional groups. The introduction of redox-active oxygen functional groups of THQ into the graphene electrodes can be confirmed by investigating surface redox reactions using cyclic voltammetry (CV). Figure 2a shows the steady-state CV scans of the electrodes in Li cells as a function of the concentration ratio of GO and THQ in the solution. The graphene electrode prepared from the 1:1 concentration ratio of GO and THQ (GO/THQ = 1:1) showed a small redox peak at around ∼3.2 V versus Li (Figure 2a) on top of double layer capacitance, which is similar to that found for previously reported oxidized CNT and CNT/graphene composite electrodes (Figure 2a).27−29 The surface redox current and redox potential region were found to increase gradually as the concentration of THQ increased (Figure 2a). The functionalized graphene electrode prepared from the 1:4 concentration ratio of GO and THQ (GO/THQ = 1:4) showed the enhanced redox current from several redox peaks in a broad range of 2.5−4.3 V versus Li, which can be attributed to the multiple redox reactions of the adsorbed THQ or oxidized THQ (C6O6Hx). In contrast, the CV scan of the GO electrode prepared by vacuum filtration exhibited smaller current density with negligible redox peaks compared to those of the graphene electrodes. To estimate the introduced redox-active oxygen functional groups from THQ into the electrodes, the instantaneous capacitances at 3.4 V versus Li were plotted as a function of the concentration ratio of GO and THQ (Figure 2b). The instantaneous capacitance gradually increased with higher THQ concentration from ∼90 F/g of GO up to 277 F/g of the graphene electrode (GO/ THQ = 1:4), confirming the role of THQ for introducing redox-active oxygen functionality into the electrodes. Galvanostatic charge and discharge tests of the electrodes in Li cells revealed that the capacities at a slow rate of 0.05 A/g increased with increasing THQ concentration (Figures 2c,d and S5, Supporting Information). The capacity of the GO electrode was ∼94 mA h/g, while the capacities of the functionalized graphene electrodes progressively increased from ∼102 mA h/g for GO/THQ = 1:1, to ∼117 mA h/g for GO/THQ = 1:2, to ∼165 mA h/g for GO/THQ = 1:4 (Figures 2c,d and S5, Supporting Information). The functionalized graphene electrodes also showed significantly improved rate capability compared to that of the GO film. The capacity of the GO film at 1 A/g was ∼25 mA h/g (27% of the capacity at 0.05 A/ g), whereas the capacities of the functionalized graphene electrodes at 1A/g were 78 mA h/g for GO/THQ = 1:2 (66.5% of the capacity at 0.05 A/g) and 91 mA h/g for GO/THQ = 1:4 (55% of the capacity at 0.05 A/g), respectively. The significantly enhanced rate capability of the functionalized graphene electrodes can be attributed to their favorable chemical and physical structure, which can facilitate electronic and ionic transportation to support fast electrochemical

oxygenated carbon groups (C−O: hydroxyl or epoxide group peak at 286.4 ± 0.1 eV; CO: carbonyl group peak at 288.2 ± 0.1 eV; and COOH: carboxylic group peak at 290.2 ± 0.1 eV).38 C 1s spectra of the functionalized graphene electrode (GO/THQ = 1:1) had a significantly reduced hydroxyl or epoxide (C−O) peak and partially decreased carbonyl (CO) peak relative to sp2-hybridized carbon (sp2-C) peak compared to those of the GO film, showing the recovery of a conjugated carbon structure during the reduction process (Figure 1c). In addition, the O/C ratio of the functionalized graphene electrodes was found to increase from 0.12 to 0.25 as the concentration of THQ increased from GO/THQ = 1:1 to 1:4 (Figure 1b). This trend is supported by high-resolution C 1s spectra, which showed the increased intensities of both C−O and CO in the functionalized graphene electrodes as the THQ concentration increased (Figures 1c and S1, Supporting Information). We postulate that excess THQ (C6O6H4) or oxidized THQ (C6O6Hx)39,40 molecules can adsorb on the reduced graphene sheets via π−π interaction,14,41−43 incorporating oxygen functional groups in the electrodes prepared with high THQ concentration. Compared to other reducing methods,44,45 which could only decrease the oxygen level of GO, this reducing process via THQ provides a versatile platform to control the atomic oxygen to carbon ratio of the reduced graphene in a wide range of 0.12−0.25. In addition, the Raman spectra of the GO film and reduced graphene electrodes showed that the intensity of the D band (∼1350 cm−1) to G band (∼1575 cm−1) increased after a reducing process via THQ (Figure S2, Supporting Information), which is a consistent trend with other chemical reducing methods.37,46 Previous work has shown that hydroquinone (C6O6H4, HQ) has a similar reduction and functionalization mechanism of GO.14,37,47 However, THQ showed several advantages over HQ. First, THQ showed much higher reducing ability than that of HQ. Figure S3a (Supporting Information) compares the reducing ability of THQ and HQ for the self-assembly process of reduced GO in water. Functionalized graphene hydrogel was assembled when the mixture of GO (1 mg/mL) and THQ (1 mg/mL) was kept at 80 °C for 2 h, while no obvious change was observed for the mixture of GO (1 mg/mL) and HQ (1 mg/mL), indicative of stronger reducing power of THQ than HQ (Figure S3a, Supporting Information). The aggregation of GO (1 mg/mL) was even initiated with THQ (2 mg/mL) at a lower temperature of 50 °C (Figure S3b, Supporting Information). THQ can release H+ ions from four hydroxyl groups in aqueous solution, forming anions such as C6H2O62− and C6O64−.40 The ionization constants of THQ for releasing the first two H+ are pK1 = 4.8 and pK2 = 6.8 in aqueous solution,40 which are much lower than pK1 = 9.9 of HQ.48 Accordingly, the mixture of GO and THQ showed lower pH (2.82) than that of the mixture of GO and HQ (3.38), indicating that more H+ exists in the THQ solution. In addition, the adsorbed THQ or oxidized THQ (C6O6Hx) on the graphene sheets has four more oxygen donor atoms compared to HQ or benzoquinone (the final oxidization state of HQ), which can adopt more oxidation and reduction states for surface redox reactions. Furthermore, THQ can be easily prepared from renewable myo-inositol, a natural compound that widely exists in plants.33,39,40,49 For electrochemical tests, a small part of the functionalized graphene was pressed into a film electrode. The microstructure of the functionalized graphene film electrode was characterized by a scanning electron microscope (SEM). The electrodes 4327

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reactions. The functionalized graphene electrodes have higher electrical conductivity than that of GO film (Figure S6, Supporting Information) owing to the recovery of a conjugated sp2 structure. The electrical conductivity of the functionalized graphene electrodes was ∼13 S/cm, while it was impossible to measure the conductivity of an insulating GO film due to the detection limit of the four-point probe measurement. In addition, the 3D hierarchical pore structure (Figure 1d and e) of the functionalized graphene electrodes can provide a faster ion diffusion pathway and larger electrochemically accessible surface area compared to those of GO films having the 2D layered structure.50 The gravimetric capacities of the electrodes were plotted with their oxygen atomic percentage (Figure 2e). The functionalized graphene electrodes showed a strong correlation between the oxygen percentage and the gravimetric capacity, while the GO electrode showed the highest oxygen percentage (29.7%) but the lowest capacity (∼94 mA h/g) compared to those of the graphene electrodes. This comparison indicates that THQ can replace electrochemically inactive oxygen groups on the surface of GO with redox-active oxygen functional groups on the surface of the reduced graphene via the reduction and concomitant functionalization process. It is interesting to note that the functionalized graphene electrodes assembled with THQ showed higher capacity (∼165 mA h/g) than those (120−135 mA h/g) of oxidized CNT or CNT/graphene composite electrodes having similar oxygen percentages (15− 17%).27,30 Because the oxygen functional groups of the oxidized CNT or CNT/graphene composite electrodes were incorporated into the electrode by a chemical oxidation process using strong acids,27,30 such an increase of the capacity for the functionalized graphene electrodes can be attributed to the enhanced redox-active oxygen functionality introduced by THQ molecules. In addition, the functionalized graphene electrodes showed excellent cycling stability in Li cells. Galvanostatic charge and discharge cycle test showed that after 10 000 cycles, the capacity at 0.1 A/g retained 122 mA h/ g, which was 87.4% of the initial capacity, and the coulombic efficiency was maintained at 96.7−100% during the cycling test (Figure 2f). In addition, the energy efficiency of the functionalized graphene electrodes was in the range from 74−81% at 0.05 A/g (Figure S7, Supporting Information). Electrochemical performance of the functionalized graphene electrodes was also evaluated in Na cells. The CV scan of the graphene electrode (GO/THQ = 1:3) showed redox peaks in a broad potential range of 2−4.2 V versus Na that are ascribed to the surface redox reactions between oxygen functional groups introduced by THQ and Na ions (Figure 3a). It is worth pointing out that the surface redox reactions on the functionalized graphene electrodes are mainly centered at a higher potential region from 3 to 4.2 V versus Na, which is significantly higher than that (2−3 V versus Na) of the oxidized carbon foam electrodes,51 which may be attributed to the different chemical structures of oxygen functional groups in the graphene and carbon foam electrodes. The gravimetric capacity of the electrode (GO/THQ = 1:3) was ∼120 mA h/g at a slow rate of 0.05 A/g and ∼76.5 mA h/g at 1 A/g (64% of the capacity at 0.05 A/g) in a Na cell, showing a comparable gravimetric capacity as well as rate capability to those measured in Li cells (Figure 3b). In summary, functionalized graphene electrodes were assembled from the aqueous solution of GO in water at a low temperature of ∼80 °C within a short time of ∼3 h under

Figure 3. (a) The steady-state CV scan of the functionalized graphene electrode (GO/THQ = 1:3) in a Na cell in the voltage range between 1.3 and 4.2 V versus Na at a scan rate of 1 mV/s in 1 M NaPF6 in a mixture of EC and DMC (volume ratio 3:7). (b) Current-densitydependent charge and discharge curves for the graphene electrode (GO/THQ = 1:3). The current density was controlled from 0.05 to 25 A/g in a voltage window of 1.3−4.2 V versus Na. Before each discharge and charge process, the cells were held at a constant voltage of 4.2 or 1.3 V versus Na for 30 min, respectively. The thickness of the electrode was ∼225 μm for GO/THQ = 1:3.

atmospheric conditions. A renewable molecule, THQ was used as both a reducing and functionalization agent, which can replace electrochemically inactive oxygen groups on the surface of GO with redox-active oxygen functional groups. The surface oxygen chemistry of the graphene electrodes and corresponding surface redox reactions can be controlled by the concentration of THQ relative to GO. Functionalized graphene electrodes delivered high gravimetric capacities of ∼165 mA h/ g in Li cells and ∼120 mA h/g in Na cells with high redox potentials, which can be employed as positive electrodes for both Li and Na ion cells. In addition, these functionalized graphene electrodes with high gravimetric capacities and redox potentials may be employed as positive electrodes for hybrid Li or Na capacitors by combining with intercalation-type negative electrodes.52−54 The functionalized graphene electrodes, assembled by a simple aqueous process, provide a new opportunity to develop low-cost, large-scale, and high-performance positive electrodes for both Li and Na ion batteries or hybrid capacitors.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental methods and Figures S1−S7, showing XPS C 1s spectra of the functionalized graphene electrodes, Raman spectra of the pristine GO film and the functionalized graphene electrodes, digital images of the mixture of GO and THQ, the mixture of GO and hydroquinone, and aggregates of reduced GO in a mixture, SEM image of the GO film, current-densitydependent galvanostatic charge and discharge curves of the functionalized graphene electrodes, electrical resistance measurements, and gravimetric charge and discharge curves of the 4328

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functionalized graphene electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

T.L. and R.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.W. L. acknowledges the financial support of the startup fund from the Georgia Institute of Technology. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation.

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