Internal Asymmetric Tandem Supercapacitor for High Working Voltage along with Superior Rate Performance Buddha Deka Boruah and Abha Misra* Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka, India 560012
ACS Energy Lett. 2017.2:1720-1728. Downloaded from pubs.acs.org by UNIV OF KANSAS on 01/03/19. For personal use only.
S Supporting Information *
ABSTRACT: An asymmetric tandem supercapacitor (ATSC) is fabricated for the first time by using a unique hybrid nanocomposite for a wider working voltage. The as-fabricated flexible ATSC displays a much higher working voltage of 4.5 V and hence efficient energy density as well as power density, which are 61% and 33% higher than those of the single asymmetric supercapacitor, respectively. The ATSC displays high capacitance stability over large working voltage of 4.5 V with 97% capacitance retention after 5000 charging−discharging cycles. In addition, a stable capacitive response is maintained during the flexible bending, and a constant capacitive response is maintained even at a much higher scan rate of 5000 mV/s. Moreover, the ATSC could be easily coupled with a photodetector to provide stable power during the detection of multiple illumination signals. Therefore, results suggest that ATSCs are a promising energy storage device with efficient power and energy density and a much wider working voltage of 4.5 V. introduce a wider potential window.12,13 For examples, Alshareef group reported symmetric graphene−polyaniline electrode-based STSCs with a potential window of 3 V.12 Likewise, Liu et al. reported a STSC cell of 3 V potential window based on poly(2,2-dimethyl-3,4-propylene-dioxythipohene) symmetric electrodes.14 However, to date, only symmetric SCs have been studied in tandem configuration by connecting them internally in series. In the present study we obtained a much larger potential window using stacks of asymmetric composite electrodes termed asymmetric tandem supercapacitor (ATSC). Flexible, solid-state ATSCs are fabricated for the first time based on the hybrid electrodes consisting of reduced graphene oxide (rGO)-carbon nanotubes (CNTs)-iron oxide (Fe2O3) (rGO-CNTs-Fe2O3) as negative electrodes and rGO-CNTsZnCo2O4 as positive electrodes. Superior capacitive responses of both the electrodes are obtained because of the synergistic contribution from individual material where rGO offers electrical double-layer capacitance (EDLC), CNTs increase the electrical conductivity of electrode material as well as EDLC response, and both Fe2O3 and ZnCo2O4 introduce the pseudocapacitive response in the respective negative and positive electrodes. Much higher specific capacitance is
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olid-state energy storage devices such as electrochemical capacitors or ultracapacitors commonly known as supercapacitors (SCs) have drawn significant research impact in modern scientific society because of the wide range of practical applications.1−3 Fabrication of SCs with higher working voltage is considered as a desirable approach to achieve excellent energy density and power density. Asymmetric SCs (ASCs) have been introduced to widen the potential window and are based on battery-type faradaic positive electrode (charge storage arises from the faradaic redox reactions in between the electrode and electrolyte interfaces) as an energy source and a double-layer-type as negative electrode (charge storage based on the charge separation at the electrode−electrolyte interfaces) as a power source.4−6 However, for a single ASC, the working potential window is commonly limited to only between 1 and 3 V and hence constricts both energy density and power density.7−10 In order to achieve a wider operating voltage, multiple SCs are connected externally in series with the electrical connections; however, that could result in electrical failure, increase in packing volume, unavoidable energy losses, and system resistance, which could limit their practical applications.11,12 Therefore, the fabrication of compact SCs in a single device configuration with higher operating voltage is still desirable. Recently, internal symmetric tandem SCs (STSCs) have been introduced that consist of multiple pairs of electroactive electrodes with a separator in a single device configuration to © 2017 American Chemical Society
Received: May 5, 2017 Accepted: June 30, 2017 Published: June 30, 2017 1720
DOI: 10.1021/acsenergylett.7b00379 ACS Energy Lett. 2017, 2, 1720−1728
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Figure 1. (a) Schematic illustration of the fabrication of flexible, solid-state ATSC. HRTEM images of (b) Fe2O3 and (c) ZnCo2O4. Insets show the respective TEM images. (d) XRD patterns of rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4.
S3a,b. Figure 1b depicts the high-resolution TEM (HRTEM) image of Fe2O3, where the lattice spacing of Fe2O3 is found to be 0.28 corresponding to the (022) planes, and the inset shows sizes ranging from 20 to 50 nm. Figure 1c shows the HRTEM image of ZnCo2O4, where the calculated lattice spacing of ZnCo2O4 is 0.29 nm corresponding to the (220) plane; thus, ZnCo2O4 has a cubic spinel phase structure. The sizes of ZnCo2O4 range from 20 to 80 nm (inset in Figure 1c). Moreover, panels a and b of Figure S4 present the HRTEM images of CNT and rGO, respectively. Furthermore, panels a and b of Figure S5 show scanning electron microscopy (SEM) images of both rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4 samples, respectively. The elemental compositional analysis of both the samples is provided in the Supporting Information obtained by energy dispersive spectroscopy (EDS). Panels a and b of Figure S6 are the EDS spectra of rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4, respectively, revealing the highquality as-synthesized hybrid nanocomposite samples. Moreover, panels a and b of Figure S7 show the Raman spectra of GO and rGO, respectively, where two major peaks at around 1352 and 1600 cm−1 are attributed to D and G bands, respectively. The intensity ratio of ID/IG is found to be 1.02 and 1.08 for GO and rGO, repsectively. The increase in ID/IG of rGO is mainly due to the introduction of structural defects under the reduction process which altered the structure of GO.15,16 In addition, X-ray diffraction (XRD, Rigaku Smart lab) patterns of the samples (Figure 1d) suggest that Fe2O3 has a maghemite crystal structure which belongs to the cubic system having a lattice constant of 0.835 nm, whereas ZnCo2O4 has a cubic spinel phase structure.17,18 Also, the decrease in XRD signal with the increase in 2θ angle of both the samples is mainly due to the nonuniform rGO-CNTs-Fe2O3 and rGOCNTs-ZnCo2O4 powder samples which were recorded in the powder scanning mode. Also, the low-intensity XRD peaks are due to Fe2O3 and ZnCo2O4 in the respective composites could mainly be due to the smaller particle sizes, i.e., 20−50 nm for
obtained from both negative and positive electrodes as 300 and 476 F/g at the current density of 2 A/g, respectively. ASCs of rGO-CNTs-Fe2O3//rGO-CNTs-ZnCo2O4 display not only superior electrochemical energy storage performance but also offer a much wider potential window of 4.5 V when connected internally in series, which is the highest reported value to date. ATSCs also demonstrate high energy and power densities, which are 61% and 33% higher than those of a single ASC, respectively. The outstanding cyclic retention of 97% is recorded after 5000 charging−discharging cycles of the ATSC even under mechanically deformed state and sustains much higher scan rate of 5000 mV/s. Moreover, the ATSC could easily drive a photodetector (PD) based on a heterostructure of cadmium sulfide (CdS) and zinc oxide nanorods (ZnO NRs) by offering the desired stable power for the detection of visible as well as ultraviolet (UV) illumination signals without using multiple supercapacitors. Figure 1a presents a schematic illustration of the fabricated, ATSC that was designed in a compact flexible device configuration to improve both energy density and power density, which are important factors for delivering the higher energy. ATSC consisted of three ASCs comprised of rGOCNTs-Fe2O3//rGO-CNTs-ZnCo2O4 (Figure 1a). A digital photograph shows the flexible ATSC electrode as a final fabricated device. The fabrication of highly electrochemically active SCs was obtained by depositing active electrode material on the flexible current collector by a mask-assisted spray deposition technique, and the details of fabrication processes are provided in the Supporting Information (Figure S1), and Figure S2 shows optical photographs of the prepared electrode inks at different interval of dispersion times 0, 3, 6, 9, 12, 24, 48, and 60 h. The main advantages of the spray deposition techniques provide the rapid depositions of electroactive materials onto plastic, paper, metal, etc. substrates at low temperature over large device areas. The morphology of the samples was examined using transmission electron microscopy (TEM, TITAN Themis, FEI); typical TEM images of rGOCNTs-Fe2O3 and rGO-CNTs-ZnCo2O4 are provided in Figure 1721
DOI: 10.1021/acsenergylett.7b00379 ACS Energy Lett. 2017, 2, 1720−1728
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Figure 2. Comparison plots for specific capacitance of (a) rGO-CNTs-Fe2O3 and (b) rGO-CNTs-ZnCo2O4 with previously reported negative and positive electrodes.30−44
Figure 3. CV plots of the ATSC at different scan rates ranging from (a) 50 to 1000 mV/s and (b) 2000 to 5000 mV/s, at a constant potential window of 4.5 V. (c) Digital photograph of the ATSC in bending configuration and (d) CV plots at different bending angles (0−60°) at a fixed scan rate of 500 mV/s. Schematic illustration of working mechanism of the ATSC: (e) under the chemical equilibrium discharged and (f) charged states.
citance during the charge storage process as depicted by the shape of the CV plots (Figure S8a,b). Figure S9 shows comparison CV (Figure S9a) and galvanostatic charge− discharge (Figure S9b) plots of rGO-Fe2O3 and rGO-CNTsFe2O3 electrodes over the potential window of −1 to 0 V. The specific capacitance increases from 254 to 300 F/g in the rGOCNTs-Fe 2O 3 electrode as compared to the rGO-Fe 2 O3 electrode at the current density of 2 A/g ,which is mainly due to the EDLC response as well as higher electrical conductivity of CNTs that introduce synergistic effects in the composite electrode to enhance the overall energy storage performance. The contribution from Fe2O3 in rGO-CNTs matrix for charge storage mechanism can be explained as follows, Fe2O3 + 2K+ + 2e− ↔ K2Fe2O3 in the aqua KOH electrolyte.19,20 However, no distinct pair of redox peaks due to the oxidation and reduction of Fe3+ and Fe2+/Fe is observed in the CV plots during the electrochemical energy storage process (Figure S8a).21,22 This CV response of the rGO-CNTs-Fe2O3
Fe2O3 and 20−80 nm for ZnCo2O4, as shown in SEM images (Figure S5). First, the electrochemical responses of the individual ATSC electrodes, rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4, were acquired in three-electrode configuration where Ag/AgCl was used as a reference electrode, platinum wire for counter electrode, and electrode materials for the working electrode in 6 M potassium hydroxide (KOH) electrolyte solution; the detailed measurement procedure is provided in the Experimental Section. Panels a and b of Figure S8 depict the CV plots of the rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4 electrodes, respectively, at different scan rates (1−20 mV/s) in the respective potential windows. A synergistic contribution from individual components of each electrode can be observed; for example, rGO offers the electrical double-layer capacitance, CNTs contribute in both EDLC as well as increase in the electrical conductivity of the electrode, and both Fe2O3 and ZnCo2O4 offer an excellent redox activity for the pseudocapa1722
DOI: 10.1021/acsenergylett.7b00379 ACS Energy Lett. 2017, 2, 1720−1728
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Figure 4. Charge−discharge plots of the ATSC: (a) at different current densities (0.222−0.555 A/g) in a fixed potential window of 4.5 V and (b) different potential windows (1.5−4.5 V) at a fixed current density of 0.222 A/g. Specific capacitance plots of the ATSC at (c) different current densities (0.222−0.555 A/g) and (d) potential windows (1.5−4.5 V). (e) Ragone plot of the ATSC at different current densities; inset depicts the Ragone plot at different potential windows (1.5−4.5 V). (f) Capacitance retention plot of the ATSC and inset shows a few charging−discharging cycles.
where each ASC offers a potential window of 1.5 V. During the fabrication of the solid-state ATSC, the poly(vinyl alcohol) (PVA)/KOH gel electrolyte was used, and Figure S11 shows digital photographs of the prepared PVA/KOH gel electrolyte. Figure 3a shows the CV plot of the ATSC at different scan rates ranging from 50 to 1000 mV/s. The increase in area under the CV plot with the scan rate demonstrates ideal capacitive response of the ATSC even in a much larger potential window of 4.5 V, which is wider than any reported value obtained to date; for example, a potential window of 2 V was reported in MnO2 nanowire/graphene//graphene ASC,45 1.6 V for Co2CuS4/NG//NG ASC,46 1.6 V for NCSC//CA ASC,47 1.6 V for three-dimensional nanoporous carbon//cobalt oxide ASC,48 etc. In addition, Figure 3b shows the CV plot of the ATSC at higher scan rates ranging from 2000 to 5000 mV/s. Consistence ideal symmetric nature of CV of the ATSC from the lower to the much higher scan rates up to 5000 mV/s reveals that ATSC exhibits higher rate capability than previous reports.49−52 Furthermore, Figure S12 depicts the CV plot of the ATSC at different potential windows of 1.5−4.5 V at a constant scan rate of 500 mV/s. Similarly, the identical CV shape of the ATSC in all the potential windows demonstrates that the ATSC can be operated at any potential window from 1.5 V to much higher values up to 4.5 V. CV measurements of the ATSC were also employed at different bending angles (Figure 3c depicts the digital photograph of the ATSC at bent configuration) ranging from 0 to 60° in a fixed scan rate of 500 mV/s, as depicted in Figure 3d. Therefore, the ATSC offers an excellent flexibility along with outstanding stable capacitive response at different mechanical deformations. Panels e and f of Figure 3 illustrate the working mechanism of the ATSC during discharged and charged states, respectively. Under the chemical equilibrium state, the electrolyte ions are
electrode could be due to the dominant EDLC response by surface adsorption of electrolyte ions rather than the pseudocapacitance during charge storage process, which is also observed be various groups.23−25 Likewise, the energy storage process of ZnCo2O4 in the KOH electrolyte is as follows: ZnCo2O4 + OH− → ZnCo2O4/OH + ZnCo2O4 − OH where ZnCo2O4/OH and ZnCo2O4 − OH represent the formation of electrical charge double layer and the reaction of hydroxyl ions.26−28 Moreover, the well-defined redox peaks in the CV plot of rGO-CNTs-ZnCo2O4 (Figure S8b) are mainly associated with the redox reaction related to Co(OH)2/ CoOOH of the electrode.29 Thus, both the novel composites, rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4 offer the synergistic effects in electrolytic process. The plateau nature of the charge−discharge plots of the electrodes (Figure S10a and b) confirms the involvement of both EDLC and pseudocapacitive responses during the energy storage process. The observed specific capacitance for rGO-CNTs-ZnCo2O4 is 476 F/g at the current density of 2 A/g, which is higher than that of the previously reported high-performance SC electrodes. For example, Lin et al. reported Fe2O3/ordered mesoporous carbon hybrid negative electrode with the specific capacitance of 235 F/g,30 Zhang’s group reported novel Ni(OH)2@Co(OH)2 hollow nanohexagons for high-performance SC electrode with the specific capacitance of 369 F/g at the current density of 1 A/g,31 etc. The comparison plots for specific capacitance of both the negative (Figure 2a) and positive (Figure 2b) electrodes with the reported high-performance SC electrodes demonstrate excellent performance of novel hybrid SC electrodes. CV analysis of the ATSC was carried out within a large potential window of 0−4.5 V because three ASCs are connected internally in series forming a tandem configuration 1723
DOI: 10.1021/acsenergylett.7b00379 ACS Energy Lett. 2017, 2, 1720−1728
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Figure 5. (a) Schematic illustration of PD. (a1) I−V plots and (a2) energy band diagram of the PD. (b) Schematic illustration the ATSC connected to the PD in series. (b1) Saturation and (b2) cyclic photoresponses of the PD driven by the ATSC in the configuration of Figure 4b under illumination of multiple light signals.
the potential, suggesting that electrochemical performance of the ATSC is a direct function of potential. Furthermore, a single ASC was also fabricated (rGO-CNTs-Fe2O3//rGOCNTs-ZnCo2O4), and electrochemical responses were evaluated for reference. Figure S13 shows the electrochemical performances: panels a, b, c, d, and e of Figure S13 present the CV, charge−discharge, capacitance, Ragone, and capacitance retention plots of the single ASC, respectively. Interestingly, it is noted that both the energy density and power density of the ATSC are 61 and 33% higher, respectively, than those of the single ASC as well as those of previously reported highperformance SCs.53−58 Also, Figure S14 shows a comparison Ragone plot of ATSC and ASC with previously reported SCs, which reveals high-performance as-fabricated ATSCs. The study of cyclic stability is an important parameter that defines the degradation of capacitive response of an SC. Figure 4f depicts the capacitance retention plot of the ATSC with respect to number of charging−discharging cycles, and the inset depicts a few representative charging−discharging cycles. Outstanding capacitance retention of 97% after 5000 charging−discharging cycles was observed in the ATSC, whereas 98% was noticed in a single ASC (Figure S13e). This value of ATSC is much higher than that of previously reported high-performance ASCs such as MWCNTs//MnO2/ MWCNT composite (72.3% retention after 300 cycles),59 AC//Fe 3 O 4 (82% retention after 500 cycles), 60 MnO 2 nanowire/graphene//graphene (79% retention after 1000 cycles),45 Ni−Co−S//graphene ASCs,61 and so on. Therefore, the cyclic charging−discharging analysis of the ATSC demonstrates outstanding stable capacitive performance. Furthermore, the Nyquist plot of ATSC in the frequencies ranging from 10 kHz to 0.01 Hz at 5 mV ac perturbation is shown in Figure S15. The equivalent series resistance (ESR)
distributed in the respective electrolyte matrix (Figure 3e). In the presence of external electric field (charged state), the negative (OH−) and positive (K+) electrolyte ions are transported to the respective positive (rGO-CNTs-ZnCo2O4) and negative (rGO-CNTs-Fe2O3) electrodes to form the EDLC and pseudocapacitance through the electrostatic physical adsorption processes. Similar processes also occur in the internal positive and negative electrodes during the charging and discharging states that offer a wider potential window of 4.5 V of the ATSC, as illustrated in Figure 3f. During the discharge state, electrons flow from negative to positive electrodes, neutralizing the separated electrolyte ions and maintaining the chemical equilibrium state. The charge−discharge plots of the ATSC at different current densities (0.222−0.555 A/g) and potential windows (1.5−4.5 V) are shown in panels a and b of Figure 4, respectively. Symmetric charge−discharge plot of the ATSC for both current densities and potential windows depict good electrochemical performance with the excellent reversible redox reactions. The negligible potential drop (IR) of the ATSC demonstrates an efficient charge/ion transportation, and the identical charge− discharge nature suggests the stable capacitive response from lower (1.5 V) to much higher potential windows of 4.5 V. Panels c and d of Figure 4 show the specific capacitance plots of the ATSC at different current densities (0.222−0.555 A/g) and potential windows (1.5−4.5 V), respectively. The increase in specific capacitance of the ATSC with the potential window demonstrates the enhancement of the electrochemical response. The Ragone plot of the ATSC is shown in Figure 4e, and the inset depicts the dependence of energy density and power density on the working potential window ranging from 1.5 to 4.5 V where both the energy and power densities increase with 1724
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at 250 °C for 10 min in nitrogen environment to form rGO. CNTs were synthesized by atmospheric pressure chemical vapor deposition using tolune and ferrocene as the source of carbon. The details of the synthesis procedure can be found elsewhere.69 The negative electrode was prepared by mixing 60 wt % of Fe2O3 nanopowders of size 20−40 nm, 20 wt % of rGO, and 20 wt % CNTs into 100 mL of DMF. The solution was ultrasonicated for 5 h at room temperature to form a homogeneous rGO-CNTs-Fe2O3 hybrid composite. Similarly, rGO-CNTs-ZnCo2O4 composite was synthesized as follows: ZnCo2O4 nanopowders was prepared using 2 mmol of zinc nitrate hexahydrate, 4 mmol of cobalt nitrate hexahydrate, 12 mmol of urea, and 10 mmol of ammonium fluoride; the mixture was added to 80 mL of deionized (DI) water at 120 °C for 5 h. The product was then centrifuged and dried at 200 °C for 1 h in air. Afterward, 60 wt % ZnCo2O4 nanopowder was then added into 20 wt % of rGO and 20 wt % of CNTs contained in 100 mL of DMF solution through vigorous ultrasonication to obtain rGO-CNTs-ZnCo2O4 hybrid composite. Electrochemical Measurements and Device Fabrication. The electrochemical measurements that include cyclic voltammetry (CV) and galvanostatic charge used the electrochemical workstation of CHI 660E at room temperature performing discharge and electrochemical impedance spectroscopy. The electrochemical performance of the synthesized materials of rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4 was carried out in three-electrode configurations where a platinum wire as the counter electrode, Ag/AgCl as a reference, and fabricated materials as the working electrodes in a 6 M KOH electrolyte solution. During the measurements, the working electrodes were prepared by mixing 95 wt % synthesized materials (rGOCNTs-Fe2O3 or rGO-CNTs-ZnCo2O4) with 5 wt % polyvinylidene fluoride into N-methyl-2-pyrrolidone solution. The solution was then drop casted onto the microporous threedimensional nickel foams (Goodfellow; thickness, 1.6 mm; porosity, 95%; purity, 95%; bulk density, 0.45 g cm−3; pores/ cm, 20) at 80 °C in air. The electroactive materials were deposited on the flexible stainless steel (SS) current collectors by spray deposition technique for the fabrication of flexible solid-state SC. Initially, SS substrates were cleaned with acetone, isopropyl alcohol, and deionized (DI) water and dried with nitrogen gas. The cleaned SS were masked by adhesive heat resistive kapton tape (Figure S1) and spray deposited the materials at 180 °C using nitrogen gas. Afterward, the kapton tape was removed from the sample and dried at nitrogen gas environment (200 sccm) at 250 °C for 30 min. The annealing of the electrodes in N2 environment not only increased the electrical conductivity of rGO but also provides the proper adhesion of the electroactive material with the current collector.70 As-fabricated flexible electrodes of rGOCNTs-Fe2O3 (negative) and rGO-CNTs-ZnCo2O4 (positive) were assembled on a filter paper coated with the poly(vinyl alcohol) (PVA)/KOH to form ASC. During the fabrication of solid-state ASC, the mass ratio of electroactive materials was maintained based on the charge balance principle. The preparation of the electrolyte was as follows: 4 g of PVA was dissolved into 40 mL of DI water at 95 °C under vigorous stirring; once the PVA solution became clear, 2 g of KOH was added; the same conditions were maintained for another 30 min. Afterward, samples on SS and filter papers were then dipped into the electrolyte solution for 10 min and then assembled in the asymmetric configuration. Finally, the
(intersection point on the x-axis in the high-frequency range) that includes the electrolyte resistance, resistance of the active materials, and the contact resistance between the active materials and current collectors is found to be 1.22 ohm. A smaller value of ESR suggests the efficient ion/charge transfer, good ionic conduction, and higher electrical conductivity of the hybrid composite electrodes of the ATSC. PD based on a heterostructure of CdS/ZnO was driven by the ATSC connected in series to demonstrate the direct practical implementation of the ATSC. Figure 5a depicts the schematic illustration of the PD and its current−voltage (I−V) response under light illuminations (Figure 5a1). The fabrication procedure of the PD is provided in the Figure S16. Figure S17a,b shows the microstructure of ZnO and CdS/ZnO NRs. The XRD patterns of both ZnO (Figure S17c) and CdS/ZnO NRs (Figure S17d) demonstrate that ZnO NRs are vertically aligned c-axis orientation with a hexagonal wurtzite crystal structure, whereas the coating of CdS on ZnO NRs has a hexagonal crystal structure.62,63 Figure 5a1 shows the I−V response of the PD both in the absence and the presence of UV, green, and UV−green illuminations in the voltage range of −4 to 4 V, measured using a Keithley 2611B source-meter. Under UV illumination, most of the excitons are generated by ZnO NRs, whereas only green illumination generates excitons in CdS. In the presence of both the illuminations of UV and green light, exciton generation in both ZnO and CdS subsequently leads to more photocurrent. It is observed that the current in the absence and presence of light illuminations intersect at the zero voltage (Figure 5a1); hence, driving the PD with an additional external voltage is required to separate the photogenerated excitons under light illumination. Figure 5a2 shows the schematic illustration of a possible energy band diagram of heterostructure PD.64 To operate the PD, the ATSC was connected in series (Figure 5b); panels b1 and b2 of Figure 5 are the saturation and cyclic photoresponses of the PD in the configuration of Figure 5b, respectively. Superior increase in response current (current under light illumination−dark current) under multiple light illuminations (UV, green, and UV−green) reveal that the ATSC can easily provide the desired power to detect the illuminated photon signals by the PD.65−67 The solid-state, flexible ATSC is fabricated based on the novel hybrid rGO-CNTs-Fe2O3 as negative electrode and GOCNTs-ZnCo2O4 as positive electrode. The synergistic effects contributed from each individual material of the hybrid nanocomposites of both the electrodes introduce the much higher specific capacitance of 300 and 476 F/g at the current density of 2 A/g. Therefore, the ATSC displays outstanding electrochemical performances where 61% and 33% enhancement in both energy density and power density was observed as compared to the those of the single ASC. The ATSC offers a much wider potential window of 4.5 V along with superior stable capacitance of 97% capacitance retention after 5000 charging−discharging cycles over the potential window of 4.5 V. Moreover, ATSC displays outstanding stable capacitive response at different mechanical deformations as well as maintained stable capacitance up to a much higher scan rate of 5000 mV/s. In addition, ATSC easily drives a PD by providing the stable power during the detection of multiple illumination signals.
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EXPERIEMNTAL SECTION Synthesis of Materials. GO was prepared by a modified Hummers method.68 As-synthesized GO powder was annealed 1725
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Biodegradable Supercapacitors with Ultra-High Capacitance. Energy Environ. Sci. 2017, 10, 538−545. (6) Yu, P.; Zhang, Z.; Zheng, L.; Teng, F.; Hu, H.; Fang, X. A Novel Sustainable Flour Derived Hierarchical Nitrogen-Doped Porous Carbon/Polyaniline Electrode for Advanced Asymmetric Supercapacitors. Adv. Energy Mater. 2016, 6, 1601111. (7) Qin, K.; Liu, E.; Li, J.; Kang, J.; Shi, C.; He, C.; He, F.; Zhao, N. Free-Standing 3D Nanoporous Duct-Like and Hierarchical Nanoporous Graphene Films for Micron-Level Flexible Solid-State Asymmetric Supercapacitors. Adv. Energy Mater. 2016, 6, 1600755−10. (8) Sun, G.; Ma, L.; Ran, J.; Shen, X.; Tong, H. Incorporation of Homogeneous Co3O4 into a Nitrogen-Doped Carbon Aerogel via a Facile in Situ Synthesis Method: Implications for High Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 9542−9554. (9) Niu, L.; Wang, Y.; Ruan, F.; Shen, C.; Shan, S.; Xu, M.; Sun, Z.; Li, C.; Liu, X.; Gong, Y. In Situ Growth of NiCo2S4@Ni3V2O8 on Ni Foam as a Binder-Free Electrode for Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 5669−5677. (10) Yang, J.; Yu, C.; Fan, X.; Liang, S.; Li, S.; Huang, H.; Ling, Z.; Hao, C.; Qiu, J. Electroactive Edge Site-Enriched Nickel−Cobalt Sulfide into Graphene Frameworks for High-Performance Asymmetric Supercapacitors. Energy Environ. Sci. 2016, 9, 1299−1307. (11) Gogotsi, Y. Materials Science: Energy Storage Wrapped Up. Nature 2014, 509, 568−570. (12) Chen, W.; Xia, C.; Alshareef, H. N. Graphene Based Integrated Tandem Supercapacitors Fabricated Directly on Separators. Nano Energy 2015, 15, 1−8. (13) Zhang, F.; Lu, Y.; Yang, X.; Zhang, L.; Zhang, T.; Leng, K.; Wu, Y.; Huang, Y.; Ma, Y.; Chen, Y. A Flexible and High-Voltage Internal Tandem Supercapacitor Based on Graphene-Based Porous Materials with Ultrahigh Energy Density. Small 2014, 10, 2285−2292. (14) Liu, D. Y.; Reynolds, J. R. Dioxythiophene-Based Polymer Electrodes for Supercapacitor Modules. ACS Appl. Mater. Interfaces 2010, 2, 3586−3593. (15) Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H. Reduced Graphene Oxide by Chemical Graphitization. Nat. Commun. 2010, 1, 73. (16) Das, A. K.; Srivastav, M.; Layek, R. K.; Uddin, M. E.; Jung, D.; Kim, N. H.; Lee, J. H. Iodide-Mediated Room Temperature Reduction of Graphene Oxide: A Rapid Chemical Route for the Synthesis of a Bifunctional Electrocatalyst. J. Mater. Chem. A 2014, 2, 1332−1340. (17) Fardis, M.; Douvalis, A. P.; Tsitrouli, D.; Rabias, I.; Stamopoulos, D.; Kehagias, T.; Karakosta, E.; Diamantopoulos, G.; Bakas, T.; Papavassiliou, G. Structural, Static and Dynamic Magnetic Properties of Dextran Coated γ-Fe2O3 Nanoparticles Studied by 57Fe NMR, Mössbauer, TEM and Magnetization Measurements. J. Phys.: Condens. Matter 2012, 24, 156001. (18) Luo, W.; Hu, X.; Sun, Y.; Huang, Y. Electrospun Porous ZnCo2O4 Nanotubes as a High-Performance Anode Material for Lithium-Ion Batteries. J. Mater. Chem. 2012, 22, 8916−8921. (19) Chen, J.; Xu, J.; Zhou, S.; Zhao, N.; Wong, C. − P. TemplateGrown Graphene/Porous Fe2O3 Nanocomposite: A High-Performance Anode Material for Pseudocapacitors. Nano Energy 2015, 15, 719−728. (20) Nithya, V. D.; Arul, N. S. Review on α-Fe2O3 based negative electrode for high performance supercapacitors. J. Power Sources 2016, 327, 297−318. (21) Yang, S.; Song, X.; Zhang, P.; Gao, L. Heating-rate-induced porous α-Fe2O3 with Controllable Pore Size and Crystallinity Grown on Graphene for Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 75−79. (22) Ma, Z.; Huang, X.; Dou, S.; Wu, J.; Wang, S. One-Pot Synthesis of Fe2O3 Nanoparticles on Nitrogen-Doped Graphene as Advanced Supercapacitor Electrode Materials. J. Phys. Chem. C 2014, 118, 17231−17239. (23) Fu, C.; Mahadevegowda, A.; Grant, P. S. Production of Hollow and Porous Fe2O3 from Industrial Mill Scale and its Potential for Large-Scale Electrochemical Energy Storage Applications. J. Mater. Chem. A 2016, 4, 2597−2604.
assembled SC was kept overnight under laminar airflow chamber to form solid-state flexible SC. For the fabrication of flexible, solid-state ATSC, the positive and negative electrodes were assembled alternatively in asymmetric configuration with the electrolyte-coated filter papers. During the fabrication of inner electrodes of the ATSC, rGO-CNTs-Fe2O3 was coated in one side of SS whereas rGO-CNTs-ZnCo2O4 was coated on the opposite side to form the asymmetric pattern.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00379. Schematic illustration the fabrication of the ATSC electrodes; optical photograph of the electrode inks; TEM images of rGO-CNTs-Fe2O3 and rGO-CNTsZnCO2O4; HRTEM images of CNT and rGO; SEM images and EDS spectra of rGO-CNTs-Fe2O3 and rGOCNTs-ZnCo2O4; Raman spectra of GO and rGO; calculations of specific capacitance, energy density, and power density; CV and charge−discharge plots of both rGO-CNTs-Fe2O3 and rGO-CNTs-ZnCo2O4; CV plot of the ATSC at different potential windows; CV, charge− discharge, specific capacitance, Ragone plot, and capacitance retention plots of the single ASC; comparison CV and galvanostatic charge−discharge plots of the rGO-Fe2O3 and rGO-CNTs-Fe2O3 electrodes; digital photograph of the prepared PVA/KOH gel electrolyte; comparison Ragone plot and Nyquist plot of ATSC; schematic illustration the fabrication of PD; SEM images of pristine ZnO NRs and CdS/ZnO NRs; XRD patterns of ZnO NRs and CdS/ZnO NRs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.M. thanks the Department of Science and Technology (DST) for funding the project under the fast track grant (Grant DST-1272).
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