Supercapacitor Electrodes with Remarkable Specific Capacitance

Research Article www.acsami.org

Supercapacitor Electrodes with Remarkable Specific Capacitance Converted from Hybrid Graphene Oxide/NaCl/Urea Films Yi Zhao,† Jinzhang Liu,*,† Bin Wang,† Jiangbo Sha,† Yan Li,† Dezhi Zheng,‡ Mojtaba Amjadipour,§ Jennifer MacLeod,§ and Nunzio Motta§ †

School of Materials Science and Engineering, Beihang University, Beijing 100191, China School of Instrumentation Science and Optoelectronics Engineering, Beihang University, Beijing 100191, China § School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane 4001, QLD, Australia ‡

S Supporting Information *

ABSTRACT: A novel approach to improve the specific capacitance of reduced graphene oxide (rGO) films is reported. We combine the aqueous dispersion of liquid-crystalline GO incorporating salt and urea with a blade-coating technique to make hybrid films. After drying, stacked GO sheets mediated by solidified NaCl and urea are hydrothermally reduced, resulting in a nanoporous film consisting of rumpled N-doped rGO sheets. As a supercapacitor electrode, the film exhibits a high gravimetric specific capacitance of 425 F g−1 and a record volumetric specific capacitance of 693 F cm−3 at 1 A g−1 in 1 M H2SO4 aqueous electrolyte when integrated into a symmetric cell. When using Li2SO4 aqueous electrolyte, which can extend the potential window to 1.6 V, the device exhibits high energy densities up to 35 Wh kg−1, and high power densities up to 104 W kg−1. This novel strategy to intercalate solidified chemicals into stacked GO sheets to functionalize them and prevent them from restacking provides a promising route toward supercapacitors with high specific capacitance and energy density. KEYWORDS: graphene, liquid crystals, supercapacitors, nitrogen-doping, specific capacitance high-temperature vapor phase growth,9 Hummer’s method, which converts graphite powder into monolayer graphene oxide (GO) sheets, is a mature synthesis technology that has the potential for mass production of reduced GO (rGO). However, for supercapacitors, the challenge is that the specific capacitance of rGO is affected by its chemical nature, i.e., oxygen residues, defects, and higher electrical resistance when compared to CVD graphene. Furthermore, rGO sheets tend to restack during the reduction process due to π−π interactions, which reduces the specific surface area as well as the specific capacitance. The past decade has seen considerable progress toward supercapacitors derived from GO. In 2008, Ruoff et al. for the first time reported graphene-based supercapacitors, in which the electrode material was hydrazine-monohydrate-reduced GO and the specific capacitance was around 100 F g−1 when using KOH aqueous electrolyte.10 In 2013, Duan et al. reported the use of 3D graphene hydrogel films as supercapacitor electrodes, showing 196 F g−1 at 1 A g−1, measured in 1 M H2SO4 aqueous solution.11 In 2014, the same group reported their work on using holey GO to make hydrogel films as electrodes, exhibiting 310 F g−1 at 1 A g−1 in 6 M KOH aqueous electrolyte.12 In addition to constructing 3D graphene frameworks and making

1. INTRODUCTION Supercapacitors are an emerging energy storage technology that are competing with Li-ion batteries in the global market. The supercapacitor combines the properties of traditional batteries and capacitors in a single component. They can be fully charged in seconds and can release energy quickly when required. Moreover, the long cycling life and low-cost material of supercapacitors make them economical components for energy storage systems. Pseudocapacitors store energy via redox processes; their electrode materials include metal oxides and conductive polymers.1,2 Pseudo-supercapacitors have the potential to provide very high specific capacitance, but the cycling life is relatively short and this technology has not been commercialized.3Electrical double layer (EDL) capacitors store energy via physical adsorption and desorption of ions; commercial supercapacitors based on activated carbon are of this type. Supercapacitor electrodes are required to have high specific surface area, and in this regard, graphene has been considered as an ideal material to replace porous activated carbon. Graphene, a two-dimensional monolayer of carbon atoms with high specific surface area (2765 m2 g−1) and good electrical conductivity, has been widely used in the development of next-generation supercapacitors.1,4,5 Among the various methods for producing graphene, including wet chemical reaction (Hummer’s method),6 liquidphase ultrasonic exfoliation,7 electrochemical exfoliation,8 and © 2017 American Chemical Society

Received: April 28, 2017 Accepted: June 13, 2017 Published: June 13, 2017 22588

DOI: 10.1021/acsami.7b05965 ACS Appl. Mater. Interfaces 2017, 9, 22588−22596

Research Article

ACS Applied Materials & Interfaces

of GO. On the other hand, urea has a reducing effect on GO. The intended function of urea is for N-doping of graphene, but excess urea additive makes the solution less viscous due to its reduction effect on GO,27 rendering the solution unsuitable for blade-coating. The bladecoated film is dried and hydrothermally reduced. If too much NaCl and urea have been added, the final graphene film is too brittle to be handled by a tweezer. Adding more urea has only a minor effect on increasing the content of N-doping in final rGO film. The three solutions in glass vials were ultrasonicated for 3 h, and then allowed to sit for 3 days to allow the spontaneous formation of LCs. For bladecoating of these viscous solutions by an automatic film-coater (Figure S1a), the gap between the blade and substrate was set at 800 μm. This determines the thickness of the freshly coated hydrogel film. From a practical point of view, the higher the mass loading of graphene in the electrode film, the higher the capacitance of the device. If the gap between the blade and substrate is set beyond 1 mm, the soft hydrogel film would not maintain a flat surface and significant variations in thickness would occur due to gravity. Therefore, for blade-coating the viscous solution, we determined an optimal gap of 800 μm. After coating, the hydrogel film was immersed into an acetone bath for coagulation, followed by a drying process in air. This process is analogous to that of making graphene fibers using the LC-GO solution and a syringe injection method.26 Using the dried films as substrates, the blade-coating process was repeated to increase the mass loading of graphene, which is necessary for increasing the device capacitance. In this work, all films were coated twice and their GO mass loadings were equivalent. The dried film was covered by a glass slide and put into an autoclave filled with diluted ammonia solution (200 mL). The pH value of the aqueous solution was adjusted by controlling the amount of ammonia. We hydrothermally reduced films in ammonia solutions at different pH values, but at identical temperature (180 °C) and duration (3 h). The optimal pH value corresponding to the highest capacitance of rGO films was found to be 11 (Figure S1b). After the reduction process, rGO films were immersed into DI water to remove residues. To measure the areal mass loading of graphene, a piece of rectangular-shaped film was freeze-dried and weighed using an analytical balance with a resolution of 0.01 mg. In this work, the areal densities of rGO films were around 0.5 mg cm−2. 2.2. Characterization Methods. An FE-SEM (Zeiss Supra55) was employed to image the N-doped rGO films. Raman spectra were collected using a Horiba Jobin-Yvon HR800 Raman spectroscope with a 514 nm wavelength laser. XPS analysis was performed on an Omicron Scienta XPS instrument using a Mg Kα X-ray source (300 W, 1253.6 eV) and a 125 mm hemispherical electron energy analyzer. XRD spectra were collected on a D/MAX-2500 X-ray generator equipped with Cu Kα (λ = 1.5405 Å) radiation. Brunauer−Emmett− Teller (BET) analysis was performed on a micromeritics ASAP 2460 instrument. 2.3. Fabrication of Supercapacitors and Electrochemical Measurements. Symmetric cells were assembled using two identical rectangular rGO films (1.5 cm by 2 cm) as electrodes. A 100 μm thick porous membrane was squeezed in between, and two Au-coated plastic films were used as current collectors. The overlapping area of two electrodes was about 2.25 cm2. Two aqueous solutions, 1 M H2SO4 and 2 M Li2SO4 (pH 2.0), were used as electrolytes for different operating voltage windows. For electrochemical tests using a three-electrode system, a single electrode film soaked in water was transferred onto a gold-coated plastic film and mechanically pressed. When immersed into the aqueous electrolyte, the graphene film remained attached to the current collector without any binder. A Pt foil and a Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the counter and reference electrodes, respectively.

holes in graphene sheets to improve the capacitance, doping nitrogen in graphene sheets provides an additional mechanism to help store more charges.13 In theory, N-doping can improve the quantum capacitance of graphene.14 In addition, redox reactions between N−H/N−O bonds and ions from electrolyte contribute pseudocapacitance to the EDL supercapacitor.15 Pseudocapacitors based on conducting polymers16,17 or metal oxides18,19 store charge through the redox reactions between the electrode material and electrolyte. Normally, the coating of redox material is prepared as thin as possible to render high specific capacitance (F/g). For N-doped graphene, the introduced N−O bonds in graphene sheets preserve the intrinsic advantages of the lightweight and atomically thin graphene, which are key factors for achieving high specific capacitance. However, most reports on N-doped graphene have not demonstrated a significant improvement of capacitance. Jiang et al. prepared N-doped holey rGO sheets and bound them together to measure the capacitance in 2 M H2SO4, achieving 276 F g−1 at 1 A g−1.20 Han et al. prepared N-doped graphene aerogels as supercapacitor electrodes, showing ∼220 F g−1 at 1 A g−1 in 1 M H2SO4.21 Though much effort has been expended, the specific capacitance of rGO electrodes remains in the range between 200 and 300 F g−1 for most studies in this field. A technological breakthrough to improve the specific capacitance of rGO-based supercapacitors to approach the theoretical capacitance of pristine graphene (550 F g−1) is highly desired. This goal has important practical connotations, because GO has the potential for mass production and meets the requirements for industrial production of graphene supercapacitors. GO has some additional characteristics that can be advantageous for supercapacitor electrode processing. Due to electrostatic interactions, GO sheets in water solution can be spontaneously aligned to form liquid crystals (LCs) when the concentration is high enough, normally above 2.0 mg mL−1.22−24 Domains of aligned GO sheets exhibit birefringence, which can be directly observed by the polarized optical microscopy (POM). Previously, we combined the viscous LCGO solution with a blade-coating technique to make graphene films for supercapacitors, and found that the trapped water in naturally dried LC-GO films has the function of facilitating the removal of oxygenated groups during the hydrothermal process, improving the specific capacitance of rGO films.25 In this paper, we report a novel strategy for making N-doped rGO films with remarkably improved gravimetric and volumetric specific capacitances, by exploiting an LC-GO hydrogel incorporating salt and urea. This facile and efficient approach is demonstrated by a comparison study and the effects of NaCl and urea on the morphology, N-doping, and the specific capacitance of rGO films is investigated.

2. EXPERIMENTAL SECTION 2.1. Preparation of rGO Films. The GO was prepared by a modified Hummers’ method that has been reported elsewhere, using expanded graphite powder (EG, 50 mesh, Qingdao Jingrilai Graphite Co., Ltd.) as the precursor. The GO aqueous solution was centrifuged to increase the concentration to 8 mg mL−1 and sat for over 1 week to form LCs. Using this LC-GO dispersion, we prepared three different solutions in this work. S1: GO; S2: GO−NaCl, and S3: GO−NaCl− urea. The concentration of GO in each solution was set at 3 mg mL−1, and the weight ratio of GO:NaCl:urea was set to be 1:5:5. We used this optimal ratio because the concentration of NaCl can influence the interspace between aligned GO sheets in LC domains.26 Excessive NaCl could make the dispersion unstable and lead to agglomerations

3. RESULTS AND DISCUSSION 3.1. The Influence of NaCl and Urea Additives on LCGO Aqueous Solution. The viscosity of the LC-GO aqueous solution is related to the concentration of GO. However, dissolving NaCl or urea into this solution can modify the 22589


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Figure 1. POM images of three aqueous solutions with GO LCs. (a) S1: GO; (b) S2: GO and NaCl; (c) S3: GO, NaCl, and urea. Each solution contains GO at the concentration of 3 mg mL−1. (d) Photographs of the blade-coated hydrogel film on a glass substrate using the S3 solution. The upper and lower photos were taken when the film was freshly coated and naturally dried, respectively. (e) The hydrothermally reduced film as a supercapacitor electrode.

Figure 2. SEM images of rGO films derived from three different mixtures. (a) F1: GO; (b) F2: GO and NaCl mixture; (c) F3: GO−NaCl−urea mixture. (d)−(f) Cross-sectional SEM images of the three rGO films shown in (a)−(c), respectively. (g) Evolution process for the F3 sample from hydrogel to rGO film. The inset illustrates the cross-linking between two GO sheets via coattraction with Na+.

polarizers would exhibit bright textures if the solution contained GO LCs. Figure 1a−c shows POM images of S1, S2, and S3 solutions, respectively, with the two polarizers crossed and a light source behind. In Figure 1a, the bright regions correspond to domains of GO LCs that allow polarized light to pass through the front polarizer. The bright patterns in Figure 1b,c are charged, for the Zeta potential of our GO solution is −64 mV. The LC-GO dispersion is stabilized by electrostatic repulsion between GO sheets, and the interspace between aligned GO sheets in LCs, originally about 60 nm, can be altered by dissolving salt into the solution.26 Sodium ions create a screening effect on the charged GO surface, weakening the electrostatic repulsion and shortening the interspace

viscosity. Sodium ions have a gelation effect to the LC-GO solution. When only urea is added, it will partially reduce the GO,27 changing the solution color from yellow to dark brown and making the solution less viscous. In this work, we studied three aqueous solutions. S1: only GO; S2: GO + NaCl; and S3: GO + NaCl + urea. The concentration of GO in each solution was set at 3 mg mL−1. NaCl and urea were, respectively, dissolved at 15 mg mL−1. The viscous LC-GO aqueous dispersion gelled after adding NaCl and urea, making it suitable for balde-coating. We used a POM to observe the LC characteristics of the three solutions. For optical observation, a drop of solution was squeezed by two glass slides with a gap around 150 μm. The solution layer between two crossed 22590


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Figure 3. (a) Raman spectra of three different rGO films, F1, F2, and F3. (b, c) High-resolution XPS spectra for C 1s and N 1s regions of the F3 rGO film, respectively. The inset in (c) illustrates different locations of N atoms doped in a graphene sheet.

illustrated for the F3 film in Figure 2g. In the LC-GO hydrogel containing Na+, Cl−, and urea molecules, the outer GO sheets of an LC domain are stripped and cross-linked due to the electrostatic interaction with Na+. The inset in Figure 2g illustrates the cross-linking of two GO sheets by bridging with Na+. The blade-coated hydrogel film was initially coagulated in acetone to partially extract water and then naturally dried before implementing the hydrothermal reduction. NaCl and urea were condensed and combined with the stacked GO sheets as the water evaporated. Solid NaCl has a good affinity with graphene, as NaCl crystals have been used as substrates for growing few-layer graphene.29 Solid grains on the film surface vanished during the hydrothermal reduction process, making the rGO film surface rough. Meanwhile, tiny grains trapped among stacked GO sheets have the function of rumpling the rGO sheets and facilitating N-doping. In our study, NaCl leads to GO sheets cross-linked and rumpled, and the decomposition of urea favors N-doping. With the removal of oxygenated groups, dangling carbon bonds would interact with Na+ or NH4+ ions for stabilization, and defective graphene rumpled due to the incoporation of these ions. Our XRD analysis of asprepared rGO films shows no graphite peaks (Figure S5b), indicating the prohibition of restacking between rumpled graphene flakes. The high roughness of the rGO film favors electrolyte infiltration and benefits the capacitive performance. However, N-doping also plays an important role in improving the capacitance. The quality of as-prepared rGO graphene films was evaluated by Raman spectroscopy and XPS. In Raman spectra (Figure 3a), two peaks centered at 1350 and 1584 cm−1 are attributed to the well-documented graphitic band (G) and the disordered band (D), respectively. Typically, the ratio of ID/IG is used to determine the defect level in the graphitic structure of carbonaceous materials. The ID/IG ratios of F1, F2, and F3 samples are 1.01, 1.05, and 1.10, respectively. The increase of the D band correlates with nitrogen dopants in graphene sheets, as the N content of the three films has the same order, with F3 > F2 > F1. XPS measurements reveal the atomic percentages of nitrogen in the three samples, F1: 2.3 at %; F2: 3.2 at %; F3: 6.7 at %. Results of XPS elemental quantitative analysis for the three samples are summarized in Table 1. The high-resolution spectra of C 1s and N 1s regions of the F3 film are shown in Figure 3b,c, respectively. The C 1s peak is deconvolved into five components, corresponding to C−C, C− O, C−N, CO, and COOH bonds, respectively. The N 1s peak is divided into four components corresponding to pyridinic (N-6), pyrrolic (N-5), graphitic (N-Q) nitrogens, and N−O bonds, respectively. From these subpeaks, it can be

between aligned GO sheets. An uneven distribution of sodium ions over the surface of a GO sheet could distort the basal plane, making the LCs curved and entangled. Therefore, GO sheets peeled off LC domains are cross-linked to form networks without the birefringent feature. Previously, the gelation effect of metal ions, such as Ca2+, Mg2+, Cu2+, Cr3+, Fe3+, etc., in GO solution was reported.28 However, it was found that monovalent ions, e.g., K+, Li+, and Ag+, cannot induce GO gelation. In our case, sodium ions intercalated into GO LCs are pinned on surfaces of GO sheets due to strong electrostatic interactions. For an LC domain, the outer GO sheets would be partially peeled off and cross-linked with those stripped from other GO-LCs through electrostatic interactions. Possibly, one COO− in a GO sheet can be linked to a CO− group from an adjacent GO sheet, via COO−−Na+−CO−. The gelled solution is suitable for blade-coating. In principle, films with areas of square meters can be prepared, depending on the size of the blade-coater. Figure 1d shows a freshly coated hydrogel film made using GO−NaCl−urea solution, before the coagulation and drying procedure. After hydrothermal reduction, the film is black in color and has detached off the substrate, as shown in Figure 1e. Three rGO films, named as F1, F2, and F3, derived from bladecoated S1, S2, and S3 solutions, respectively, were prepared for studies to reveal the influence of additives on the specific capacitance of rGO films. 3.2. Morphological and Structural Characterization. The three samples, F1, F2, and F3, were hydrothermally reduced under identical conditions, using an autoclave filled with diluted aqueous ammonia solution. As an initial step, the optimal pH value of the solution for maximizing the specific capacitance was determined (Supporting Information). FESEM was employed to study the morphology of the freezedried rGO films. Plane-view SEM images of the three samples, F1, F2, and F3, are shown in Figure 2a−c, respectively. Correspondingly, their cross-sectional view images are displayed in Figure 2d−f, respectively. The F1 sample derived from LC-GO solution consists of densely packed graphene sheets and has ridges in the smooth surface (Figure 2a,d). Due to the NaCl additive, the rGO sheets in the F2 and F3 films are rumpled, and both films have rough surfaces and a wavy texture within the stacked sheets. Graphene sheets are known to suffer from π−π interaction-induced restacking that is a technological hurdle in improving the specific capacitance. This restacking is largely prohibited in these rumpled GO sheets, as indicated in our XRD analysis (Figure S5). The F3 sample, which was obtained by reducing the film containing GO, NaCl, and urea, has a rougher surface and less compaction as compared to the F2 film. The evolution process from hydrogel to rGO film is 22591


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compressed graphene film remained attached to the current collector via van der Waals adhesion. The cycling stability of the electrode was studied by repeatedly charging and discharging at 5 A g−1 for 5000 cycles, over a total duration of about 9 days. As shown in Figure 4d, the capacitance retention after 2000 cycles was 95%, and it decreased to 89% after 5000 cycles. 3.4. Electrochemical Characterizations of Symmetric Supercapacitors. The three films, F1, F2, and F3, were used as electrodes to fabricate symmetric supercapacitors with 1 M H2SO4 aqueous solution as electrolyte, and their electrochemical performances are compared in Figure 5. CV loops measured at 5 mV s−1 on the three devices are shown in Figure 5a. The CV curves of our devices are close to rectangular shape, indicating good capacitive behavior. By integrating the entire CV loop, the specific capacitance of one electrode can be calculated using C = S/vmΔV, where S is the area of the CV loop, v is the voltage scan rate, m is the graphene mass of one electrode, and ΔV is the potential window. At 5 mV s−1, the specific capacitances of F1, F2, and F3 films are calculated to be 240, 284, and 423 F g−1, respectively. Galvanostatic charge/ discharge (CD) tests on the three devices at different current densities were also performed, and three CD curves at 1 A g−1 are shown in Figure 5b. The higher the capacitance, the longer the time span of a CD cycle. The inset in Figure 5b shows Nyquist plots of the three devices, from which we can conclude that equivalent series resistances are in the narrow range of 5− 10 Ohm. The low internal resistance indicates a capability for providing high maximum powder output, and also accounts for the small IR drop in CD curves at high current density, as shown in Figure S9. Using CD curves at different current densities, we calculated the gravimetric specific capacitances for each device, and their values are plotted in Figure 5c. The trend of specific capacitance of the three samples is consistent with that of nitrogen content, F3 > F2 > F1. However, in terms of specific area, the order is F3 > F1 > F2. To measure the BET specific surface area, we shredded the three films and obtained

Table 1. XPS Quantitative Elemental Analysis of Three rGO Films and Their Specific Capacitances at 1 A g−1 content after reduction (at %) film samples

content before reduction

C

N

O

capacitance (F/g, in 1 M H2SO4)

F1 F2 F3

GO GO + NaCl GO + NaCl + urea

81.3 84.0 79.6

2.3 3.2 6.7

16.4 12.8 13.7

225 290 425

concluded that the pyrrolic N is dominant, which is important to the capacitance improvement of graphene.13 3.3. Electrochemical Characterizations Using the Three-Electrode System. The role of nitrogen-doping in increasing the capacitance of graphene is mainly contributed by redox-related pseudocapacitance of oxidized pyrrolic (N-5) nitrogen.15 The F3 film that has the most nitrogen content was used to observe the redox peaks in cyclic voltammetry (CV) curves measured from a three-electrode cell. In this configuration, the F3 film is pressed against a Au-coated plastic film to form the working electrode. The other two electrodes were a Pt foil as the counter electrode and a Ag/AgCl reference electrode (Figure S6). The electrolyte was 1 M H2SO4 aqueous solution. Figure 4a shows CV loops at different scan rates. The slower the voltage scan rate, the more pronounced the redox hump. The redox occurs at ∼0.5 V in the charging process. Using galvanostatic charge−discharge (CD) curves, shown in Figure 4b, we obtained specific capacitances at different current densities, as shown in Figure 4c. A specific capacitance as high as 479 F g−1 at a current density of 1 A g−1 was achieved. When the current density is increased to 20 A g−1, the capacitance retention is 68%. For this graphene film, the capacitance drops quickly with increasing current, which is attributed to its compactness because we mechanically pressed the free-standing film onto a Au-coated plastic film for the three-electrode electrochemical test. When immersed in the electrolyte, the

Figure 4. Electrochemical performance of the F3 film in a 1 M H2SO4 aqueous solution using a three-electrode cell. (a) CV loops. (b) Galvanostatic CD curves. (c) Specific capacitances calculated from CD curves versus current density. (d) Cycling stability of the cell at a CD current density of 5 A g−1. 22592


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Figure 5. (a) CV curves measured at 5 mV s−1 from three different symmetric cells using F1, F2, and F3 films as electrodes, respectively. (b) Galvanostatic CD curves of the three devices. The inset shows Nyquist plots of the three devices. (c) Comparison of gravimetric specific capacitances of F1, F2, and F3 electrodes versus different current density. (d) Specific capacitance calculated from CV curves of the device based on F3 films versus voltage scan rate.

Figure 6. Electrochemical performances of symmetric cells using the F3 films as electrodes and different electrolytes. (a) CV curves of two cells using 1 M H2SO4 and 2 M Li2SO4 aqueous electrolytes, respectively. (b) CD curves of the cell with Li2SO4 aqueous electrolyte at different current densities. The CD curve at 1 A g−1 from the cell with H2SO4 electrolyte is plotted for comparison. (c) Capacitance retention of the symmetric capacitor after continuously charged and discharged at 5 A g−1 for 5000 cycles. (d) Comparison of gravimetric specific capacitances of two devices using H2SO4 and Li2SO4 aqueous electrolytes, respectively, versus current density. (e) Comparison of volumetric specific capacitances of the two devices. (f) Phase change vs frequency for the two devices with H2SO4 and Li2SO4 aqueous electrolytes, respectively.

capacitances of 290 and 228 F g−1, respectively. In terms of the capacitance retention at 20 A g−1, F2 and F1 devices show 70% and 59%, respectively. By integrating the CV loops of the device based on F3 films, we calculated the specific capacitance versus voltage scan rate, as shown in Figure 5d. These capacitance values are quite close to those deduced from CD curves. The F1 film is inferior due to the compact structure of the flat, tightly packed rGO sheets, as shown in the SEM image in Figure 2d. The F3 film has not only the best structure for

nitrogen adsorption−desorption isotherms for each of them (Figure S8). These freeze-dried graphene films show high specific surface area, F1: 1162 m2/g; F2: 1150 m2/g; and F3: 1236 m2/g. The F3 film shows the highest capacitance of 425 F g−1 at 1 A g−1, and it decreases to 315 F g−1 (74% retention) when the current density is increased to 20 A g−1. The good rate capability is attributed to the porous structure of electrodes, allowing ions to diffuse quickly among the rumpled graphene sheets. At 1 A g−1, the F2 and F1 films show 22593


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ACS Applied Materials & Interfaces electrolyte infiltration but also the highest content of N-doping due to the urea additive. Generally, N-doping can enable a redox process that increases the capacitance. However, redox humps in the CV loops in Figure 5a are absent. This is likely because a symmetric cell is electrically equivalent to two serial capacitors. When one electrode reaches the potential at which electrons can reduce N−H bonds, nitrogenated groups in the opposite electrode could be in the oxidation process to offset the current. 3.5. Extending the Potential Window for High Energy and Power Densities. We assembled symmetric supercapacitors using the F3 films as electrodes and 2 M Li2SO4 aqueous solution as electrolyte. Two CV loops measured at 20 mV s−1 in H2SO4 and Li2SO4 aqueous electrolytes, respectively, are compared in Figure 6a. Figure 6b shows galvanostatic CD curves in the voltage window of 0−1.6 V. For comparison, the CD curve at 1 A g−1 from the cell using 1 M H2SO4 aqueous electrolyte is plotted as well. The cyclic lifetime of the device with Li2SO4 aqueous electrolyte was assessed by running the CD test at 5 A g−1 for 5000 cycles, and the result is shown in Figure 6c. After 1000 cycles, the capacitance retention is 93%; after 5000 cycles, the capacitance retention decreased to 86%, which is only slightly lower than that in H2SO4 aqueous electrolyte (89%; see Figure 4d). Specific capacitance values versus current density, corresponding to Li2SO4 and H2SO4 aqueous electrolytes, are compared in Figure 6d. At 1 A g−1, the capacitance of the F3 film in Li2SO4 electrolyte is 396 F g−1, which is about 93% of the value in H2SO4 (425 F g−1). By using the packing density of our electrode film, 1.63 g cm−3 (Figure S7), we converted the gravimetric capacitance into volumetric capacitance, as shown in Figure 6e. At 1 A g−1, the volumetric specific capacitances corresponding to H2SO4 and Li2SO4 electrolytes are 693 and 645 F cm−3, respectively. The dependence of phase change against frequency for devices using H2SO4 and Li2SO4 aqueous electrolytes, respectively, are shown in Figure 6f. In H2SO4 solution, the phase angle reaches −84°, close to the ideal value of −90°. When using Li2SO4 aqueous electrolyte, which extends that voltage window, the phase angle reaches −79°, which is good compared to other work.15 Graphene-based supercapacitors have been intensively studied, and the technologies for preparing graphene electrodes are various. For a better comparison, state-of-the-art results published in recent years are listed in Table 2. Our rGO film derived from the GO−NaCl−urea mixture outperforms most other graphene electrodes, in terms of both gravimetric and volumetric specific capacitances. Notably, the F3 film in our work is comparable to PANI-graphene composite electrodes in gravimetric capacitance. PANI (polyaniline) is a representative conducting polymer being intensively studied for pseudosupercapacitor applications because it provides Faradaic capacitance much higher than the EDL capacitance of carbon materials.37,38 However, pseudocapacitor materials, including metal oxides and conducting polymers, generally suffer from structural degradation and more than 10% capacity loss after hundreds of cycles charging/discharging.2,18 In 2014, Liu et al. reported the use of a composite paper consisting of rGO, PANI, and cellulose fibers for flexible supercapacitors, showing 464 F g−1 at 1 A g−1 in H2SO4−PVA gelled electrolyte.39 In 2016, Li et al. infiltrated PANI hydrogel into spongy graphene films to make supercapacitor electrodes, achieving a gravimetric capacitance of 457 F g−1 and a volumetric capacitance of 572 F cm−3, at 5 A g−1 in 1 M H2SO4 aqueous electrolyte.40 This

Table 2. Comparison of Gravimetric and Volumetric Specific Capacitances of our F3 Films with Other State-of-the-Art rGO-Based Electrodes Reported in Recent Years scan rate (A g−1)

C (F g−1)

C (F cm−3)

materials

electrolyte

N-doped rGO film

1 M H2SO4

1

425

693

year

holey rGO framework11 graphene hydrogel10 KOH-activated GO30 liquid-mediated Gr film31 N-doped Gr aerogels (N 8.4 at %)21 N-doped holey graphene32 N-doped 3D Gr (N 7.7 at %)33 3D N-doped GrCNT networks34 F and N codoped carbon microspheres35 N-doped microporous carbon microspheres36

6 M KOH

1

310

220

1 M H2SO4

1

190

6 M KOH

1

265

185

2016

1 M H2SO4

0.1

203

256

2013

1 M H2SO4

0.2

223

6 M KOH

0.5

250

397

2016

6 M KOH

0.5

334

437.5

2016

6 M KOH

0.5

180

1 M H2SO4

0.2

270

521

2015

1 M H2SO4

0.1

297

287

2015

this work 2014 2013

2015

2013

volumetric capacitance is among the few top-tier results in PANI-based pseudocapacitors. In our work, at 5 A g−1, the F3 film exhibits 385 F g−1 in gravimetric capacitance and 628 F cm−3 in volumetric capacitance when using H2SO4 aqueous electrolyte. Compared to the polymerization methods of PANI, our method for making graphene electrodes has the merit of nontoxcity, low cost, and facile production. In performance, despite the long cycling life and high specific capacitance, one important advantage of our rGO films is that the voltage window can be extended by changing electrolytes to magnify the energy density, whereas the charging voltage of pseudocapacitors based on PANI or other metal oxides is normally below 1 V. Energy and power densities of the F3-film-based symmetric cells are given in the Ragone plot in Figure 7. When using the

Figure 7. Ragone plot of gravimetric energy and power densities for the F3 films in symmetric cells with different electrolytes, in comparison with some reported carbon-based supercapacitors. 22594



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Li2SO4 aqueous electrolyte, the energy density of our device is in the range of 23−35 Wh kg−1, comparable to that of a Ni-Cd battery.41 For comparison, energy and power densities of conventional electrochemical batteries and supercapacitors based on other materials, including the composite of Ndoped rGO and PANI,16 S and P codoped graphene,42 functional pillared graphene,43 3D graphene hydrogel,11 porous activated carbon,44 and rGO/MnO2−rGO/MoO3 as anode− cathode,45 are marked in Figure 7. Note that the x-axis is the average power density. The maximum power density is inversely proportional to the internal resistance of the device, PMax ∝ (ΔV)2/R, where ΔV is the operating voltage window, R is the internal resistance of the device that can be deduced from the IR drop, which is a voltage drop at the initial stage of discharging curve. It is worth mentioning that the IR drop of our device is extremely low even at high discharge current (Figure S9), indicating the ability to output high peak power.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jinzhang Liu: 0000-0003-4788-3560 Jennifer MacLeod: 0000-0002-2138-8716 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “The Fundamental Research Funds for Central Universities” through Beihang University.

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REFERENCES

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4. CONCLUSIONS We report a facile and efficient route toward high-specificcapacitance rGO films by exploiting an LC-GO hydrogel incorporating salt and urea. This hydrogel is combined with a blade-coating technique to make dry and compact films, in which aligned GO sheets are mediated by solidified NaCl and urea. During the hydrothermal reduction process, NaCl has the function of rumpling the graphene sheets to prohibit restacking, and urea facilitates N-doping that further improves the capacitance. We carried out a comparison study to determine optimal conditions for maximizing the capacitance and found that the morphology and amount of urea-derived insoluble residues in the rGO films are related to the pH value of the solution filled in the autoclave. The complete decomposition of urea derivatives leads to the maximum content of N-doping in graphene as well as the highest capacitance of the electrode film. For a symmetric cell with H2SO4 aqueous electrolyte, a high gravimetric specific capacitance of 425 F g−1 at 1 A g−1, corresponding to a record high volumetric capacitance of 693 F cm−3, was obtained. In Li2SO4 aqueous electrolyte, which extends the potential window to 1.6 V, the specific capacitance also remained high, at 396 F g−1, and high energy densities up to 35 Wh kg−1 were achieved. Our study demonstrates a green approach to improving the specific capacitance of rGO-based supercapacitors, paving the way for large-scale production of high-performance supercapacitors derived from mass-produced GO.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05965. Calculations of the specific and volumetric capacitances, energy density, and power density; determining the optimal pH value for hydrothermal reduction of films; photograph of blade-coater; dependence of specific capacitance on pH value; SEM images; evolution of urea during the hydrothermal process; XPS spectra; XRD patterns; three-electrode electrochemical test; determining the packing density of graphene films; BET analysis; N2 adsorption−desorption isotherms; IR drop in CD curves (PDF) Movie showing three GO solutions (AVI) 22595


Research Article


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Supercapacitor Electrodes with Remarkable Specific Capacitance

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