3D Porous Graphene Nanostructure from a Simple, Fast, Scalable

Apr 12, 2017 - A simple, fast, and scalable mix-and-heat process was developed for production of three-dimensional (3D) porous graphene nanostructure...
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Research Article pubs.acs.org/journal/ascecg

3D Porous Graphene Nanostructure from a Simple, Fast, Scalable Process for High Performance Flexible Gel-Type Supercapacitors Chun-Chieh Wang, Jiyuan Liang, Yi-Hsiu Liao, and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: A simple, fast, and scalable mix-and-heat process was developed for production of three-dimensional (3D) porous graphene nanostructure. The process involves only mixing and heating of starch and a graphene oxide (GO) suspension at 90 °C for 10 min to form 3D graphene monoliths, from which a three-dimensionally well-connected porous graphene nanostructure, starch/RGO, possessing a high specific surface area of 1519 m2 g−1 was obtained. The starch/RGO material was used as the electrode material to fabricate flexible, gel-type symmetric supercapacitors of outstanding capacitive performances, delivering a high energy density of 19.8 Wh kg−1 at the power density of 0.5 kW kg−1 and exhibiting an excellent high rate capability of a high power density of 9.9 kW kg−1 at the energy density of 9.6 Wh kg−1, among the highest for pristine carbon material based gel-type, symmetric supercapacitors. The cycling stability of the starch/RGO based supercapacitor was excellent, with a high specific capacitance retention rate of 80% after 8000 cycles at 10 A g−1. The starch/RGO based supercapacitor exhibited outstanding mechanical stability with a retention rate of 90% in both energy and power densities at a large bending angle of 138° and functioned well in a wide temperature range environment. KEYWORDS: Graphene monolith, Gel electrolyte, Symmetric supercapacitor, Flexible, Starch

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INTRODUCTION With the increasingly worsening situations of fossil fuel depletion and global warming, development of renewable energy sources continues to be the top priority for mankind in this century.1−3 Energy storage on the other hand is essential to the effective utilization of the produced renewable energies.4−6 Supercapacitors, because of their high power densities, short charging/discharging times, and excellent cycle life, remain one of the most important energy storage devices and continue to attract a great deal of research attention.7−9 In recent years, in addition to further boosting the energy density, cost-effective large scale production also becomes more of the relevant research consideration. Roll-to-roll processes are probably the most cost-effective way for large scale production of supercapacitors. This approach, however, requires the supercapacitor to be flexible with the use of solid or gel electrolytes.10−14 Polymer based gel electrolytes have attracted a great deal of research attention in fabrication of flexible energy conversion and storage devices such as organic solar cells and lithium ion batteries.15,16 They have also been investigated for fabrication of flexible supercapacitors, demonstrating long cycle life and ease in packaging without leakage problem.17,18 To boost the energy density of the supercapacitor, one way is to combine electrode materials that function well in separate potential windows, to form asymmetric or symmetric supercapacitors, with which the working potential window size and thus the energy density can be significantly increased.19−21 As to the electrode material, transition metal oxides are known to provide, in addition to electric double layer © 2017 American Chemical Society

capacitances, pseudocapacitances through fast superficial Faradaic redox reactions between the electrode and electrolyte ions. Two potential drawbacks of using transition metal oxides as the electrode materials are the generally poor electric conductivity and often limited choices in matching electrolytes.22,23 Porous carbons, because of their large specific surface areas, good electric conductivity, and high thermal and chemical stabilities, are widely used as the electrode material for electric double-layer capacitors (EDLC).24,25 EDLCs store electric energies through electric double layers formed by accumulating charged electrolyte ions at the electrode/electrolyte interfaces, generally possessing higher power densities but lower energy densities than pseudocapacitance dominated supercapacitors.26−28 Graphene, a two-dimensional sheet of single-atomic-layer carbon arranged in sp2 hybrid orbitals, has demonstrated extraordinary electronic, mechanical, thermal, and electrochemical properties.29,30 Graphene based materials have been extensively investigated as the electrode material for supercapacitors because of their high specific surface areas and high electric conductivity.31,32 Nevertheless, the sheet restacking and aggregation issues limit the success of the application of graphene in supercapacitors. In recent years, much research has been devoted to the development of three-dimensional (3D) porous graphene nanostructure for use in supercapacitors.33−38 These 3D porous graphene nanostructures are generally Received: March 10, 2017 Revised: April 4, 2017 Published: April 12, 2017 4457

DOI: 10.1021/acssuschemeng.7b00747 ACS Sustainable Chem. Eng. 2017, 5, 4457−4467

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prepared through physical (e.g., hydrothermal processes or selfassembling through hydrogen bonding and van der Waals forces)39−42 or chemical (e.g., sol−gel processes) routes.43−45 These methods, although successfully producing 3D porous graphene nanostructures of high specific surface areas, high porosities, and good mechanical strength, are time-consuming, taking hours of treatment times, and energy-demanding, requiring high energy input in supercritical fluid or freezedrying. Starch is one of the abundant and sustainable natural biomass compound and is best known as a common precursor for production of activated carbons (AC) because of its special composition and structure, low price, and commercial availability.46−48 ACs possess ultrahigh specific surface areas and high porosities through activation processes for micropore generation.49,50 They exhibit high capacitive efficiencies at slow charge/discharge rates, but rapid decays with increasing charge/ discharge rates because of the large mass transfer resistances encountered in its microporous structure and slow charge transport caused by the relatively poor electric conductivities.51,52 In this work, we developed a simple, fast, and scalable process for production of a 3D porous graphene nanostructure, termed starch/RGO, which is composed of reduced graphene oxide sheets glued together to form the 3D porous graphene nanostructure by the starch-derived ACs. The starch/RGO was prepared by simply dissolving starch into a GO suspension at 90 °C under vigorous stirring for 10 min to form a starch/RGO monolith, followed by thermal carbonization and chemical activation. 3D porous graphene nanostructures are often prepared by first forming three-dimensionally well-connected nanoporous graphene monoliths, followed by thermal carbonization and chemical activation. The present method requires no time-consuming hydrothermal or sol−gel chemistries and no energy-demanding supercritical fluid or freeze-drying operations to form the three-dimensionally well-connected nanoporous graphene monoliths. This is a unique advantageous feature of the present method as compared with other previously developed methods. The starch/RGO possessed a high specific surface area of 1519 m2 g−1, suitable for applications as the electrode material for supercapacitors. Flexible, gel-type symmetric supercapacitors, fabricated by using the starch/RGO as the electrode material, Ti foil as the current collector, and neutral LiCl/PVA gel as the electrolyte, functioned well in a wide potential window of 2 V and delivered a high energy density of 19.8 Wh kg−1 at the power density of 0.5 kW kg−1 and exhibited an excellent high rate capability of a high power density of 9.9 kW kg−1 at the energy density of 9.6 Wh kg−1, among the highest for previously reported pristine carbon material based, gel-type, symmetric supercapacitors. The suitability of the present starch/RGO based supercapacitor for applications in wearable electronics and harsh environment was also investigated by examining the power and energy densities of the device at bended state and in a wide temperature range. The present starch/RGO based supercapacitor exhibited outstanding mechanical stability with a retention rate of 90% in both energy and power densities at a large bending angle of 138° and exhibited optimal capacitive performances in both energy and power densities at 80 °C.

Research Article

EXPERIMENTAL SECTION

Preparation of Graphene Oxide (GO). GO was prepared with a modified Hummer’s method.53,54 Briefly, graphite powders (Alfa Aesar, 325 mesh, 0.5 g) and sodium nitrate (0.5 g) were added to concentrated sulfuric acid (23 mL) under stirring at room temperature. Potassium permanganate (3 g) was then added to the above mixture slowly with vigorous agitation so that the temperature of the suspension can be maintained at 0 °C. This step proceeded for 1 h. The suspension was then heated to 35 °C and stirred for another 1 h, forming a brown paste. An amount of 40 mL of deionized water (DI) was added into the paste carefully under stirring, and the resulting suspension was stirred for another 0.5 h. An additional 100 mL of DI water was added to the suspension, followed by a slow addition of H2O2 (30%), turning the color of the suspension from brown to yellow. The mixture was filtered and washed with DI water and collected with a centrifuge operated at 8000 rpm. The product was stored in a vacuum oven at 40 °C overnight to form the GO flakes. Preparation of Starch/RGO. GO flakes (0.16 g) were dispersed into DI water (40 mL) to form a graphene oxide suspension (4 mg mL−1) through an ultrasonication operation by using a tip-sonicator (Misonix, XL-2000) until no precipitates can be observed. Starch powders (Acros, potato starch, 4 g) were then added into the GO suspension at 90 °C under vigorous stirring until the suspension was transformed into a jelly-like monolith. It is to be noted that the ratio of starch/GO determines whether or not the starch/GO monolith can be formed and also the mechanical strength of the monolith. The formulation used here was the optimal formulation in terms of monolith quality and capacitive performances. The monolith was kept in a refrigerator overnight, followed by drying in an oven of 80 °C for 8 h to remove water before carbonization at 800 °C (with a temperature ramping rate of 5 °C min−1) under argon atmosphere for 1 h. The carbonized powders were further treated with an aqueous KOH solution (weight ratio of KOH to water of 1:1) for activation. Briefly, the carbonized powders (0.3 g) were added into the KOH solution (3 mL) under stirring, followed by drying in an oven at 100 °C. The dried powders were then thermally pyrolyzed at 800 °C in argon atmosphere for 1 h, and washed with diluted HCl (0.3 M) and DI water until solution neutrality was reached. The product was stored in an oven of 60 °C overnight for later use. For comparison purposes, starch-derived carbon, termed starch carbon, and RGO powders were also prepared. For starch-derived carbon powders, starch powders (4 g) were added into 40 mL of DI water at 90 °C under vigorous stirring until the solution was transformed into a jelly-like monolith. The rest of the procedures follows those for the starch/RGO starting from the carbonization. As for the RGO powders, GO flakes (0.16 g) were dispersed into DI water (40 mL) to form a graphene oxide suspension (4 mg/mL) through an ultrasonication operation by using a tip-sonicator (Misonix, XL-2000) until no precipitates can be observed. The suspension was then dried in an oven of 80 °C for 8 h to remove water before carbonization at 800 °C (with a temperature ramping rate of 5 °C/ min) in argon atmosphere for 1 h. The rest of the procedures follow those for the starch/RGO starting from the KOH activation. Preparation of LiCl/Poly(vinyl alcohol) (PVA) Gel Electrolyte. In a typical run, LiCl (8.4 g) and PVA (4 g) powders were added into DI water (40 mL). The mixture was stirred at 90 °C until the solution became clear, followed by natural cooling to room temperature. Fabrication of Flexible Symmetric Supercapacitors. The electrodes of the symmetric supercapacitor were fabricated by drop casting the active material (starch carbon, RGO, or starch/RGO) based suspension on Ti-foil (Alfa Aesar, 127 μm thick, 99%) of a working area of 1 × 1 cm, followed by drying at 80 °C overnight. The suspension was prepared by mixing 2 mg of active materials with binder, poly vinylidene fluoride (PVDF), at a weight ratio of 10:1. The mixture was then added into 50 μL of binder solvent, N-methyl-2pyrrolidone (NMP), to form the casting suspension. A separator (NKK, TF40, 40 μm thick) and the two electrodes were immersed into the LiCl/PVA gel electrolyte for 5 min. Then, the separator was 4458

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Figure 1. (a) X-ray diffraction patterns of RGO, starch carbon, and starch/RGO. Inset shows diffraction pattern of GO. XPS spectra of C 1s for (b) RGO, (c) starch carbon, and (d) starch/RGO. Inset of (b) shows XPS spectrum of C 1s for GO. sandwiched by the two electrodes to form the flexible symmetric supercapacitor. Electrochemical Measurements. The capacitive performances of the electrode materials and supercapacitors were characterized with a three-electrode and a two-electrode systems, respectively. A Pt wire served as the counter electrode, Ag/AgCl as the reference electrode, and LiCl/PVA as the gel electrolyte. Cyclic voltammetric (CV), galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy (EIS) were performed with an electrochemical workstation (CHI6275D, CH Instruments Inc.). The specific capacitances for single electrodes were calculated from the CV curves recorded in the three-electrode system according to the equation Cs = ∫ IdV (2νmΔV)−1. Here, I is the capacitive current (A), ν is the scan rate (V s−1), ΔV is the potential window size (V), and m is the mass of the active material (g). For the symmetric supercapacitors, the energy density (E) and the power density (P) were calculated from the charge/discharge curves of the galvanostatic measurements recorded in the two-electrode system via E = 0.5 (CsV2) and P = E t−1, respectively. Here, V is the working potential window size, t the discharging time, and Cs the specific capacitance computed according to the equation Cs = IΔt(MΔV)−1, where I is the charging/discharging current, Δt the discharging time, ΔV the working potential window size, and M the total mass of the active material on both electrodes. Characterization. An X-ray diffractometer (UltimaIV, Rigaku), equipped with Cu Kα (λ = 1.5406 Å) and operated at a voltage and current of 40 kV and 40 mA, respectively, was used to determine the crystalline structure of the GO, RGO, starch carbon, and starch/RGO samples. The C and O contents of the GO, RGO, starch carbon, and starch/RGO samples were characterized with a high resolution X-ray photoelectron spectrometer (XPS, PHI Quantera SXM, ULVACPHI). Scanning electron microscopy (SEM, JSM-6500F, JEOL) was used to characterize the morphology and microstructure of the RGO,

starch carbon, and starch/RGO. The porous structure of the RGO, starch carbon, and starch/RGO was studied with the N2 sorption/ desorption isotherms recorded at 77 K (ASAP2020, Micromeritics Instruments). The samples were degassed at 150 °C for 8 h prior to the analysis. The specific surface area was computed using the Brunauer−Emmett−Teller (BET) method and the pore size distribution was determined using the density functional theory (DFT) model.

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RESULTS AND DISCUSSION

The crystallographic structure of the RGO, starch carbon, and starch/RGO was investigated with the XRD measurements, and the results are shown in Figure 1a for examination. Also included as the inset of the figure is the XRD pattern of the GO for comparison. For the GO, the 2θ value of the dominant diffraction peak of (002) was shifted from around 26° of graphite (JCPDS 75-1621) to about 10°, indicating the presence of the oxygen-containing groups on the basal plane of the well restaked GO sheets, expanding the observed interlayer spacing from 0.34 to 0.93 nm. As for the RGO, the removal of the oxygen-containing groups shifted back the 2θ value of the (002) peak to 25°. The original diffraction peak at 10° disappeared, further signifying the complete reduction of GO to form RGO. The low intensity of the (002) peak is a result of the disorder and thinness of the restaked RGO sheets. No definite diffraction peaks can be identified for the starch carbon, which is amorphous with a low degree of graphitization. As for the starch/RGO, the diffraction pattern is similar to that 4459

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Figure 3. Nitrogen sorption/desorption isotherms for RGO, starch carbon, and starch/RGO.

higher one at 286.6 eV contributed by the oxygen-containing functional groups. The higher binding energy peak can be deconvoluted into three sub-bands located at 286.6, 287.7, and 288.6 eV attributable to epoxide or hydroxyl, carbonyl, and carboxyl groups, respectively.55 The pronounced higher binding energy peak indicates the well oxidized state of the GO. As for the other three samples, the higher binding energy peak diminishes into a shoulder of the primary peak of 284.5 eV, a result one expects for reduced graphene oxides and activated carbons. The morphology of the RGO, starch carbon, and starch/ RGO is shown in Figure 2a,b,c, respectively for comparison. The RGO appears as thick staked sheets without apparent porous structure. The starch carbon appears as aggregated particle clusters with the particles being constructed by primary particles of around 10 nm in size. As for the starch/RGO, it was composed of thin RGO sheets forming 3D porous structure as shown in Figure 2c. The combination of the starch carbon and RGO sheets can be observed with an HRTEM image (Figure 2d). The insets of Figure 2a,b,c show the photographs taken for the products before carbonization. Both the starch carbon and starch/RGO formed jelly-like products, white and dark brown in color, respectively. The RGO, however, remained a suspension without gelation. The microstructure of the RGO, starch carbon, and starch/ RGO was further characterized with N2 sorption/desorption isotherms recorded at 77 K. The three sets of isotherms are shown in Figure 3 for comparison. The isotherms for the RGO and starch/RGO can be classified as a type IV isotherm and those of the starch carbon as a type I isotherm according to the IUPAC classification.56 All three sets of isotherms exhibit rapid pick-ups in pore volumes at low relative pressures, indicating existence of micropores. The pore volume pick-up of the RGO however is much minor as compared with those of the starch and starch/RGO, implying minor amount of micropores in the RGO. In addition, the hysteresis loops of the RGO and starch/ RGO imply the presence of mesopores. With the above observations, one can conclude that the starch carbon is a microporous material and the RGO and starch/RGO contain both mesopores and micropores. This conclusion was well supported by the pore size distributions of the RGO, starch carbon, and starch/RGO presented in Figure S1. The microstructure parameters, including specific surface areas, pore volumes, and average pore size, of the three samples are summarized in Table S1 for comparison. Here, the specific

Figure 2. SEM images of (a) RGO, (b) starch carbon, and (c) starch/ RGO. (d) HRTEM image showing combination of RGO sheets and starch carbon.

of the RGO as expected because only the RGO part of the composite contributes to the diffractions. XPS was employed to analyze the residing oxygen-containing functional groups on the surfaces of the GO, RGO, starch carbon, and starch/RGO. The results for the RGO, starch carbon, and starch/RGO are shown in Figure 1b,c,d, respectively, with that of the GO included as the inset of Figure 1b for comparison. Evidently, there appear two characteristic binding energy peaks of the C 1s for the GO, with the lower binding energy one located at 284.5 eV attributable to the CC/CC of the aromatic rings and the 4460

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Figure 4. CVs of (a, d) starch carbon, and (b, e) starch/RGO electrodes at increasing scan rates in LiCl/PVA gel electrolyte over positive (a, b) and negative (d, e) potential windows. Specific capacitances of starch carbon, RGO, and starch/RGO electrodes as functions of scan rate investigated in positive (c) and negative (f) potential windows.

surface areas were determined based on the Brunauer− Emmett−Teller (BET) model, whereas the pore size distributions were characterized with the Barrett−Joyner− Halenda model (BJH) for the RGO sample and densityfunction-theory (DFT) model for the starch carbon and starch/ RGO samples. The specific surface areas were 1706, 1519, and 325 m2 g−1 and the pore volumes 0.76, 0.75, and 0.38 cm3 g−1 for the starch carbon, starch/RGO, and RGO, respectively. Evidently, using starch as the binder for the 3D nanoporous graphene nanostructure takes the advantages of the high surface area and high pore volume of the starch-derived carbon for capacitance generation. The sheet structure of the RGO, however, helped disperse the starch-derived carbon to enable

its full utilization in the starch/RGO, in contrast to the aggregated carbon nanoparticles as found in the starch carbon. As for the RGO, the lack of the micropore contributor (starchderived carbon) and apparent porous structure resulted in a much lower specific surface area and pore volume than those of the starch carbon and starch/RGO. To evaluate the capacitive performances of the starch carbon and starch/RGO electrodes, cyclic voltammetric (CV) tests were carried out using a three-electrode configuration in a neutral LiCl/PVA gel electrolyte over potential windows of 0 to 1 V (Figure 4a,b for the starch carbon and starch/RGO electrodes, respectively) and 0 to −1 V (Figure 4d,e for the starch carbon and starch/RGO electrodes, respectively). The 4461

DOI: 10.1021/acssuschemeng.7b00747 ACS Sustainable Chem. Eng. 2017, 5, 4457−4467

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the RGO and starch/RGO electrodes remains rectangular even at the high scan rate of 100 mV s−1, implying good high rate capability contributed mainly by the good charge transport ability of the electrodes. The capacitive current densities, and thus the specific capacitances, of the RGO electrode however are about 1 order of magnitude lower than those of the starch/ RGO electrode, mainly because of the much smaller specific surface area of the RGO (325 vs 1519 m2 g−1). The specific capacitances achieved by the starch/RGO electrode at 2 mV s−1 was 161 and 183 F g−1, much larger than 7 and 9 F g−1 acquired by the RGO electrode, in the positive and negative potential windows, respectively. The specific capacitance of the starch carbon electrode reached 150 and 131 F g−1 at 2 mV s−1, but drastically dropped to 18 and 8 F g−1 at 100 mV s−1 in the positive and negative potential windows, respectively, indicating its poor high rate capability. As for the starch/RGO electrode, its specific capacitance decreased only moderately from 161 and 183 F g−1 at 2 mV s−1 to 81 and 113 F g−1 at 100 mV s−1 in the positive and negative potential windows, respectively. Although the two electrode materials, starch carbon and starch/RGO, possesses about the same specific surface areas that are micropore dominated, the well-dispersed state of the starchderived carbon in the 3D porous graphene nanostructure of the starch/RGO enables the readily accessible micropores for capacitance generation. The severely aggregated state of the starch-derived carbon of the starch carbon electrode makes it very difficult to access inner micropores for capacitance generation because of the high mass transport resistances, particularly at high scan (charging/discharging) rates. The

Figure 5. Capacitive performances of starch/RGO//starch/RGO supercapacitor operated in LiCl/PVA gel electrolyte. Inset shows CVs of starch/RGO electrode recorded in three-electrode system over positive and negative potential windows.

corresponding CVs for the RGO electrode are shown in Figure S2a,b for the positive and negative potential windows, respectively. For the starch carbon electrode, the shape of the CV loops (Figure 4a,d) is rectangular at the low scan rate of 2 mV s−1, indicating good electrochemical reversibility in charging/ discharging, but deviates significantly from the ideal rectangular shape even at a moderate scan rate of 25 mV s−1. On the contrary, the shape of the CV loops (Figures 4b,e, S1a,b) for

Figure 6. CVs of (a) starch carbon//starch carbon and (b) starch/RGO//starch/RGO supercapacitors at increasing scan rates. Galvanostatic charge/discharge curves of (c) starch carbon//starch carbon and (d) starch/RGO//starch/RGO supercapacitors. 4462

DOI: 10.1021/acssuschemeng.7b00747 ACS Sustainable Chem. Eng. 2017, 5, 4457−4467

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Figure 4, and are thus expected to have a good match, serving as both the positive and negative electrodes to form a symmetric supercapacitor. The CV recorded at 25 mV s−1 over a 2 V potential window for the starch/RGO//starch/RGO capacitor is shown in Figure 5, with the separate CVs recorded in the positive and negative potential windows included as an inset to show the comparable capacitive performances of the starch/RGO electrode. The resulting CV for the starch/RGO// starch/RGO supercapacitor is in an excellent rectangular shape, implying the suitability of the starch/RGO electrode serving as both the positive and negative electrodes in the supercapacitor. The capacitive performances of the two flexible symmetric supercapacitors, starch carbon//starch carbon and starch/ RGO//starch/RGO, were evaluated by CV scans and galvanostatic charge/discharge tests. Figure 6a,b shows the high rate capability of the starch carbon//starch carbon and starch/RGO//starch/RGO supercapacitors, respectively with the scan rate increasing from 2 to 1000 mV s−1. The starch/ RGO//starch/RGO supercapacitor exhibits excellent high rate capability by maintaining the symmetric rectangular shape of the CV loops even at the extremely high scan rate of 1000 mV s−1. The starch carbon//starch carbon supercapacitor, however, exhibits poor high rate capability, with the CV loop shape deviating drastically from the ideal symmetric rectangular shape with increasing scan rate. The specific capacitance of the supercapacitor was calculated by the same equation as that for the three-electrode system, but with the mass of the active material accounting for both positive and negative electrodes. The specific capacitances of the starch/RGO//starch/RGO supercapacitor reached 36 and 17 F g−1, comparing to 30 and 1 F g−1 for the starch carbon//starch carbon capacitor, at 2 and 1000 mV s−1, respectively. This again shows the excellent high rate capability of the starch/RGO//starch/RGO supercapacitor over that of the starch carbon//starch carbon supercapacitor, with a high capacitance retention rate of 47% vs 3% at 1000 mV s−1. The results for the galvanostatic charge/discharge tests of the two capacitors are depicted in Figure 6c,d. As evident from the comparison of the two figures, the starch/RGO//starch/RGO supercapacitor gives much better charging/discharging curves, closer to the ideal symmetric triangular shape, than those of the starch carbon//starch carbon supercapacitor, better symmetry and much smaller iR drops. It is to be noted that the starch carbon//starch carbon supercapacitor failed to perform the galvanostatic charging/discharging at the high current density of 10 A g−1, whereas the starch/RGO//starch/RGO capacitor still functioned well. To demonstrate further the excellent capacitive capability of the present starch/RGO//starch/RGO supercapacitor, we compare the energy and power densities recorded with galvanostatic charge/discharge tests and achieved by the two supercapacitors with those reported for previously developed pristine carbon material based, (activated carbon, carbon aerogel, graphene films, and carbon nanotubes/graphene composites) gel-type symmetric supercapacitors in a Ragone plot shown in Figure 7a.57−63 Note that gel-type symmetric supercapacitors that are based on carbon materials doped with heteroatoms or composited with conducting polymers were not included to have a fair comparison.64−66 The starch/RGO// starch/RGO supercapacitor achieved a high energy density of 19.8 Wh kg−1 at the power density of 0.5 kW kg−1 and maintained a good energy density of 9.6 Wh kg−1 at a high power density of 9.9 kW kg−1, outperforming those achieved by

Figure 7. (a) Ragone plots (energy vs power density) of starch carbon//starch carbon and starch/RGO//starch/RGO supercapacitors compared with previously reported pristine carbon material based gel-type symmetric supercapacitors. Involved specific capacitances were determined with galvanostatic charge/discharge measurements. (b) Red light-emitting diode (LED) lightened by two starch/RGO// starch/RGO supercapacitors connected in serial.

starch carbon electrode thus can achieve high specific capacitances at low scan rates, but its capacitive performances decay drastically with increasing scan rates. Furthermore, the availability of the fast charge conducting path is also an important factor. The low degree of graphitization of the starch-derived carbon further limits the charge transport ability of the starch carbon electrode, and thus poor capacitive performances at increasing charging/discharging rates. The starch/RGO electrode, however, on the contrary, possesses three-dimensionally well-connected graphene network for the necessary fast charge transport for generation of high capacitive currents. Figure 4c,f shows the specific capacitance vs scan rate for the three electrodes over the positive and negative potential windows, respectively. Evidently, the high rate capability of the starch/RGO electrode is much better than that of the starch carbon electrode. Also evident are the outperformances of the starch/RGO electrode over the RGO electrode in specific capacitance over the whole scan rate range. The two high specific capacitance electrodes, starch carbon and starch/RGO, were further assembled into flexible, gel-type symmetric supercapacitors, starch carbon//starch carbon and starch/RGO//starch/RGO, and were tested their capacitive performances in a two-electrode system in the neutral LiCl/ PVA gel electrolyte. The two electrodes have been shown to exhibit comparable capacitive performances in the positive (0− 1 V) and negative potential (−1−0 V) windows as shown in 4463

DOI: 10.1021/acssuschemeng.7b00747 ACS Sustainable Chem. Eng. 2017, 5, 4457−4467

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Figure 8. (a) Comparison of cycling stability of starch/RGO//starch/RGO supercapacitors in aqueous LiCl electrolyte and in LiCl/PVA gel electrolyte. The capacitance data were obtained from galvanostatic charge/discharge at 0.5 A g−1. (b) Nyquist plots for starch carbon//starch carbon and starch/RGO//starch/RGO supercapacitors, with frequency scan from 100 kHz to 10 mHz and perturbation amplitude of 5 mV at open circuit potential in LiCl/PVA gel electrolyte. (c) CVs of starch/RGO//starch/RGO supercapacitor under increasing bending angles and back to flat recorded at scan rate of 100 mV s−1. (d) CVs of starch/RGO//starch/RGO supercapacitor at increasing temperatures recorded at scan rate of 100 mV s−1.

Electrochemical impedance spectroscopy was conducted to further reveal the advantages of using starch/RGO as the electrode material over starch-derived carbon. As presented in Figure 8b, the Nyquist plots exhibit two feature regions: a line obtained at low frequencies and a semiarc formed at high frequencies. The line and the semiarc are characteristic of the diffusion resistance of the electrolyte ions and the charge transport resistance, respectively. The larger the line slope, the smaller the diffusion resistance, and the smaller the semiarc, the lower the charge transport resistance. As expected, the starch carbon supercapacitor gave a much less vertical line at the low frequency region and much larger semiarc at the high frequency region than those of the starch/RGO supercapacitor, indicating much higher ion-diffusion and charge transport resistances. These results are in consistence with the structural and material characteristics of the electrode materials, inaccessible micropores and amorphous carbons for the starch carbon electrode and well-dispersed and readily accessible micropores and wellconnected conductive graphene network for the starch/RGO electrode. Flexible gel-type supercapacitors are well suited for roll-toroll large scale production to reduce drastically the manufacturing cost, and are also suitable for portable and wearable applications to largely expand their application range.67,68 To test the processability of the present flexible supercapacitor, we investigated its capacitive performances under bending. Figure

the starch carbon//starch carbon supercapacitor and all other surveyed supercapacitors. As a simple demonstration, two starch/RGO//starch/RGO supercapacitors were connected in serial to light up a red light-emitting-diode (LED) as shown in Figure 7b, demonstrating its practical application potential. The CV and galvanostatic charge/discharge curves of the two starch/RGO//starch/RGO capacitors connected in serial are presented in Figure S3, showing excellent capacitive characteristics. The cycling stability of the starch/RGO//starch/RGO supercapacitor was also investigated. We compared the capacitance retention rates of the supercapacitor in two different electrolytes: LiCl/PVA gel electrolyte and aqueous LiCl electrolyte. The results are shown in Figure 8a. Evidently, the gel-type supercapacitor outperformed the liquid electrolyte one in cycling stability, achieving an 89% capacitance retention rate vs 35% for the liquid electrolyte one after 1000 cycles of charging/discharging at 0.5 A g−1. The superior performance of the gel-type supercapacitor may be attributed to the much better solvent retention of the gel electrolyte. The cycling stability of the starch/RGO//starch/RGO supercapacitor was further tested at a much higher charging/discharging rate of 10 A g−1 for a much higher number of operation of 8000 cycles. The results are presented in Figure S4, further demonstrating the good cycling stability of the starch/RGO//starch/RGO supercapacitor with a capacitance retention rate of 80%. 4464

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ACS Sustainable Chemistry & Engineering 8c shows the CVs recorded for the starch/RGO//starch/RGO supercapacitor under increasing bending angles. The bending angle is defined in an illustration presented in Figure 8c. Photographs are also offered in Figure S5 to show the bending of the flexible supercapacitor. As evident from Figure 8c, bending at increasing bending angles up to 138° and returning to unbent state did not cause any significant variations in the CVs and corresponding capacitive performances. The energy and power densities of the bending tests were summarized in Table S2. The bending did cause decays in the energy and power densities, but only slightly. Finally, the applicability of the starch/RGO//starch/RGO capacitor in a wide temperature range was also investigated. The working temperature was controlled by oven and refrigerator. The CVs recorded for several temperatures ranging from −21 to +100 °C are presented in Figure 8d, and the derived energy and power densities summarized in Table S3 for comparison. There appears an optimal temperature, 80 °C, at which the energy and power densities reached their maximum values. This phenomenon may be explained based on the competition between ion-transport and solvent retention. Ion transport quickens but the solvent retention worsens at increasing temperatures, competition of which leading to an optimal working temperature for the supercapacitor. The above results show that the present gel-type supercapacitor has a high potential to be used in a wide temperature range environment. The leakage current and self-discharge characteristics of the starch/RGO//starch/RGO capacitor were presented in Figure S6a,b, respectively. The leakage current was maintained at 40 μA, negligible as compared with the discharge current of 2 mA, after 2 h operation. Its self-discharge characteristic was reasonably satisfactory with a remaining working potential of 0.8 V after 12 h operation.

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AUTHOR INFORMATION

Corresponding Author

*S.-Y. Lu. E-mail address: [email protected]. ORCID

Shih-Yuan Lu: 0000-0003-3217-8199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of the Republic of China (Taiwan) under grant NSC 101-2221-E-007-111-MY3 and by the Low Carbon Energy Research Center of the National Tsing-Hua University.

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REFERENCES

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CONCLUSION A simple, fast, and scalable method was developed for production of 3D porous graphene nanostructure, starch/ RGO. The starch/RGO possessed advantageous structural features, including well-dispersed, readily accessible micropores offering high specific surface areas and well-connected and highly conductive graphene network, making it a suitable electrode material for supercapacitors. The electrode fabricated from the starch/RGO functioned well in both positive (0−1 V) and negative (−1−0 V) potential windows and was used to construct flexible, gel-type symmetric supercapacitors that exhibited outstanding capacitive performances, excellent electrochemical reversibility and high energy and power densities, the best among previously reported pristine carbon material based gel-type, symmetric supercapacitors. The starch/ RGO//starch/RGO supercapacitor showed excellent mechanical stability under high angle bending and performed well in a wide temperature range. It proves to be a promising candidate for next-generation supercapacitors: cost-effective, wearable, and wide temperature applicability.

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in serial with inset showing corresponding galvanostatic charge/discharge curves; cycling stability of starch/ RGO//starch/RGO supercapacitor in LiCl/PVA gel electrolyte; photographs illustrate flexible starch/ RGO//starch/RGO supercapacitor under increasing bending angles; leakage current and self-discharge characteristics of starch/RGO//starch/RGO capacitor; table of specific surface areas, pore volumes, and average pore sizes of starch carbon, RGO, and starch/RGO materials; table of energy and power densities of starch/ RGO//starch/RGO supercapacitors at increasing bending angles; table of energy and power densities of starch/ RGO//starch/RGO supercapacitors at increasing working temperatures (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00747. CVs of RGO electrode in LiCl/PVA gel electrolyte; CV of two starch/RGO//starch/RGO capacitors connected 4465

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