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High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co3O4 Composite Aerogel Electrodes Debasis Ghosh, Joonwon Lim, Rekha Narayan, and Sang Ouk Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07511 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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ACS Applied Materials & Interfaces

High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co3O4 Composite Aerogel Electrodes AUTHOR NAMES Debasis Ghosh, Joonwon Lim, Rekha Narayan, Sang Ouk Kim*

AUTHOR ADDRESS National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Materials Science and Engineering, KAIST, Daejeon 34141, Republic of Korea

Keywords: graphene aerogel; nanomaterials; energy storage; energy density; flexible devise

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ABSTRACT Modern flexible consumer electronics require efficient energy storage devices with flexible free standing electrodes. We report a simple and cost effective route to graphene based composite aerogel encapsulating metal oxide nanoparticles for high energy density, free standing binder-free flexible pseudocapacitive electrodes. Hydrothermally synthesized Co3O4 nanoparticles are successfully housed inside the microporous graphene aerogel network during the room temperature interfacial gelation at Zn surface. The resultant 3D rGO-Co3O4 composite aerogel shows mesoporous quasi-parallel layer stack morphology with a high loading of Co3O4, which offers numerous channels for ion transport and 3D interconnected network for high electrical conductivity. All solid state asymmetric pseudocapacitors employing the composite aerogel electrodes have demonstrated high areal energy density of 35.92 µWh/cm2 and power density of 17.79 mW/cm2, accompanied by excellent cycle life.

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Introduction Supercapacitor, also known as electrochemical capacitor or ultracapacitor, is the energy storage device under intensive research attentions in recent days. The electrodes of electrochemical capacitors store electrical charges either by electrical double layer formation (electrical double layer capacitor) or by faradaic reaction (redox capacitor) at the electrode/electrolyte interface.1 While the former ensures high power density, the later may offer high energy density, hence it is considered ideal to synergistically hybrid these two features in a single device structure. In this regard, graphene and graphene based composites have been of enormous research interest.

Graphene and chemically modified graphene or heteroatom doped graphene based materials hold great promise for energy storage and conversion owing to its large surface area, high mechanical flexibility, chemical/thermal stability, high electrical/thermal conductivity, excellent catalytic activity, and so on.2-5 Unfortunately, the π-π stacking tendency among graphene sheets commonly diminishes the available surface area and restricts the electrolyte ion diffusion. The practical utilization of graphene for energy storage requires highly porous robust 3D network configuration to maximize the available surface area for charge storage, while maintaining the electrical connectivity.

Transition-metal oxides or hydroxides such as MnO2, RuO2, NiO, Co3O4, Ni(OH)2 and Co(OH)2, may offer large charge capacitance arising from their fast surface redox reactions.611

However, their overall performances for electrode applications are generally limited by the

low electrical conductivity, which significantly deteriorates the power response. Recent researches demonstrate remarkable progress in the power performance of the pseudocapacitive materials by forming porous network structures or composites with carbon 3



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materials. However, the random agglomeration and uncontrollable distribution of the pseudocapacitive inorganic materials still remain significant challenge.12-14

In this work we report a novel approach for reduced graphene oxide(rGO)/pseudocapacitive nanomaterials based 3D composite aerogels with uniform spatial distribution and minimal agglomeration of inorganic nanoparticles. Spontaneous reductive assembly of rGO porous network is induced at metal (Zn) surface immersed in acidified aqueous dispersions of graphene oxide liquid crystals.15 The rGO aerogel can effectively house pseudocapacitive metal oxide (Co3O4) nanoparticles inside its 3D cage during the interfacial gelation and greatly improve the electrochemical performance. The composite aerogel maintains highly porous network throughout the 3D architecture resulting in numerous ion diffusional channels that facilitates the capacitance of composite electrodes. All solid state asymmetric pseudocapacitors employing the composite electrodes were fabricated and demonstrated remarkable performance in terms of energy density, power density as well as cycle life.

Experimental section Synthesis of Co3O4 nanoparticles: A hydrothermal method was utilized for the synthesis of Co3O4 nanoparticles. In a typical synthetic procedure, 35 ml 0.05M CoSO4, 7H2O (Alfa Aesar) metal precursor solution was mixed with 2 ml 28% ammonia solution in a 100 ml Teflon sealed autoclave, kept at 180˚C for 24 h and allowed to cool down to room temperature. The black precipitation was collected and washed several times with D.I. water and vacuum dried. Synthesis of reduced Graphene oxide aerogel: Graphene oxide was synthesized following the modified Hummers method. rGO aerogel was synthesized following a redox process, where GO was spontaneously reduced and grown at the Zn surface in a mild acidic 4


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suspension of GO.15 The acid stock solution was prepared by diluting conc. HCl with DI water with a ratio of 1:19 (V/V). Briefly, 10 ml of GO suspension (3 mg/ml) was acidified with 10 µL of dilute HCl stock solution. A clean zinc foil was immersed in the acidified GO suspension for 5 h for the interfacial gel at Zn surface. The resultant gel structure was washed with a stream of water to remove the physisorbed GO sheets and was treated with the acid stock solution for the isolation from Zn surface. For the removal of Zn impurities, the gel was further immersed in stock HCl solution for 4 h, followed by washing with D.I. water to remove acidic impurity. Finally, the gel was freeze dried to obtain free standing flexible rGO aerogel.

Synthesis of rGO-Co3O4 composite aerogel: For the synthesis of rGO-Co3O4 composite aerogel the as prepared Co3O4 nanoparticles were first dispersed in D.I. water by ultrasonication for 1 h (concentration: 3 mg/ml). The solution was settled down for 5 min and the top homogeneous suspension was taken using a pipette and thoroughly mixed with equal volume of GO suspension (6 mg/ml). 10 ml of the CO3O4/GO mixed suspension was used for the interfacial gelation to form composite gels. An excessive amount of acid stock solution (50 µL) was used in order to compensate the slow gelation rate in the presence of foreign particles.

Result and Discussion Schematic diagram of the interfacial gelation mechanism of rGO-Co3O4 composite aerogels is shown in Fig. 1. The standard reduction potential of Zn/Zn2+ (E0 = -0.76 V vs SHE) is lower than the reduction potential of rGO/GO couple (E0 = -0.4 V vs SHE at pH 4).16 Hence in acidic medium, Zn can spontaneously form Zn2+ ion, and the released electron can reduce GO to form rGO. Moreover, the negatively charged GO is attracted by the positively charged 5



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Zn surface and facilitates a tight contact with the metal. The subsequent reduction process induces layer-by-layer gelation to form 3D rGO aerogels. The two half reactions can be written as: 15

Zn2+ + 2e- → Zn (E0 = -0.76 V vs SHE) GO + H+ +e- → rGO + H2O (E0 = -0.4 V vs SHE)

Hence, the overall reaction can be written as: GO + Zn + H+ → rGO + Zn2+ + H2O

This reduction of GO on Zn surface allows an easy and cost effective procedure that exclude the use of any hazards chemical (like hydrazine or sodium borohydride) and the need to dispose of the byproducts. Moreover, since the rGO gel grows over Zn surface one can engineered the shape of the rGO gel by choosing different architecture of Zn metal,15 which is not yet a successful practice with other reduction procedure like thermally reduction or electrochemical reduction.

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Figure 1. Schematic diagram of the formation of rGO-Co3O4 composite gel: (a) Digital image of rGo-Co3O4 mixed suspension; (b) Interfacial gelation of rGo-Co3O4 mixed suspension at Zn surface; (c) Digital image of rGo-Co3O4 gel formed at Zn surface and (d) a representative composite gel showing mechanical flexibility.

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Fig. 2a indicates the XRD pattern of the as-prepared Co3O4, GO and rGO aerogel. The XRD pattern of the Co3O4 is well-matched with JCPDS card no. 073-1701 with a little additional phase of β Co(OH)2 (JCPDS card 30-443). In the XRD pattern of rGO aerogel, the sharp peak at 2θ =11.13˚ of GO, corresponding to (002) plane disappears and a new broad peak appears at ~2θ = 23.6˚ confirming the successful reduction of GO by interfacial gelation. The room temperature reduction by interfacial gelation was further confirmed by Raman spectra shown in Fig. 2b. The G (1594 cm-1) and D bands (1341 cm-1) in the Raman spectra of GO are downshifted in both rGO and rGO-Co3O4 composite aerogel with an increased ID/IG ratio of 1.83 and 1.75, respectively from the initial ID/IG ratio of 1.02 for GO. The enlarged ID/IG ratios indicate the increase of sp2 hybrid C atoms.17 Thermogravimetric analysis (TGA) of the as-prepared rGO aerogel and rGO-Co3O4 composite aerogel measured under atmospheric conditions is compared in Fig. 2c (temperature range: 50-800 ˚C; heating rate: 10 ˚C/min). The TGA plot of pure rGO aerogel shows a significant weight loss of 95.81% with the noticeable decomposition temperature range from 479-711 ˚C. The little residual weight can be attributed to the almost complete combustion of rGO. For the rGO-Co3O4 composite aerogel, the total weight loss was 64.1% with the major decomposition range from 430-630 ˚C. From the difference of the weight loss of the rGO and rGO-Co3O4 composite aerogel, the weight composition of Co3O4 in the composites was determined to be 31.7%. The surface area of the as prepared rGO-Co3O4 composite aerogel was determined by the methylene blue absorption method,18 using an initial methylene blue solution of concentration 10.66 mg/L and a composite aerogel with mass of 2 mg. The surface area was determined to be 289 m2/g.

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Figure 2. (a) XRD pattern of as-prepared Co3O4 nanoparticles, GO powder and rGO aerogel; (b) Raman spectra of GO powder, rGO and rGO-Co3O4 composite aerogel; (c) TGA analysis of rGO and rGO-Co3O4 composite aerogel; (d) UV-Visible absorption spectra of methylene blue solution before and after absorption by rGO-Co3O4 composite aerogel for 4 days for surface area measurement.

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SEM image of the as-prepared CO3O4 (Fig. 3a) indicates a nanoscale globular morphology with an average particle dimension from 35 to 50 nm. Cross-sectional SEM images (Fig. 3bc) of the rGO aerogels verifies a well-ordered quasi parallel stacking of graphene sheet and high mesoscale porosity. The SEM images in Fig. 3(d, e, f) indicates the successful formation of rGO-CO3O4 composite aerogels, where the Co3O4 nanoparticles were well-decorated inside the 3D rGO network without significant agglomeration. Use of a high concentration HCl to accelerate the gelation of GO in the presence of foreign particles resulted in a thicker gel. Noteworthy that the accelerated gelation rate does not significantly influence the morphology of quasi parallel assembly of rGO sheets and maintains well-ordered mesoporous morphology.

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Figure 3. SEM image of Co3O4 nanoparticles (a); cross-sectional SEM images of rGO aerogel (b, c) and rGO-Co3O4 composite aerogel (d, e); high magnification micrograph of rGO-Co3O4 composite aerogel showing the spatial distribution of Co3O4 nanoparticles (f).

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Electrochemical Characterization All the electrochemical characterizations were carried out by means of two electrode configuration. The Co3O4 electrodes were prepared by mixing the nanoparticles (70 wt%) with carbon black (20 wt%) and PVDF (10 wt%) in ethanol and the homogeneous paste was coated at the surface of Ni current collectors. The electrochemical testing of the Co3O4 and rGO-Co3O4 electrodes were carried out in a symmetric two electrode configuration in 6M KOH. The rGO//rGO symmetrical supercapacitor was fabricated using electrodes of the same dimension (1×1 cm2) separated by a Whatman filter paper in 6M KOH electrolyte. The aerogel papers were directly used as electrodes with Ni current collector for the necessary current transport to the potentiostat. Ag paste was used for the contact of the electrode with current collector. Asymmetric pseudocapacitor was fabricated using the rGO aerogel as negative and rGO-Co3O4 composite aerogel as positive electrodes with whatman filter paper as separator. 6M KOH was used as supporting electrolyte. For the fabrication of all solid state asymmetric supercapacitors, rGO aerogel (area: 1 × 1 cm2, thickness: ~320 µm, density: ~0.6 mg/cm2) was used as negative electrode and rGO-Co3O4 composite aerogel of the same area (1×1 cm2, thickness: ~420 µm, density ~0.96 mg/cm2 ) was used as the positive electrode. In a typical procedure the electrodes were pretreated with 6M KOH solution for overnight and air dried. PVA gel was prepared by dissolving 4 g PVA in 60 mL water at 85 °C with continuous stirring until the solution became clear. The air-dried rGO and rGO-Co3O4 electrodes were covered by the thin layers of PVA gel and were immersed in excess 6M KOH solution (50 mL) for 24 h. The two electrodes were then pasted on each other and dried under ambient condition. The schematic diagram of the fabrication of the solid state device is shown in Fig. S1. The gel electrolyte acts as electrolyte as well as separator between the two electrodes. PET was used as flexible substrate to support the flexible electrodes. Ni current collector was used for the necessary current transport to the potentiostat. The electrochemical 12


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characterizations were carried out, including cyclic voltammetry, galvanostatic charge discharge and electrochemical impedance spectroscopy.

Comparative CV plots normalized with the electrode area of as-prepared Co3O4 and rGOCo3O4 electrodes at 10 mV/s scan rate are shown in Fig. 4a. The effective working potential for the redox capacitor was chosen considering the voltage of reversible conversion between different cobalt oxidation state.19 The CV plot of pure Co3O4 electrode symmetric assembly shows a non-rectangular behavior due to the faradaic redox reaction occurring at the electrode. By contrast, the CV plot of rGO-Co3O4 electrode symmetric assembly shows quite rectangular behavior with a small but well resolved redox peak pair, indicating the combined contribution from both double layer capacitance and pseudocapacitance. The Co3O4 electrode showed moderated areal capacitance due to the poor electrical conductivity of metal oxide. Moreover, the use of binder and conducting agent creates dead volume, resulting in a decreased energy density. The areal capacitance per electrode and energy density calculated from the CV plot were 183.2 mF/cm2 and 4.06 µWh/cm2, respectively, at 10 mV/s scan rate. In the binder-free rGO-Co3O4 electrode, the synergistic interaction between the highly conductive porous 3D rGO aerogel and well-dispersed pseudocapacitive Co3O4 nanoparticles resulted in a synergistic areal capacitance of 326 mF/cm2, with an improved energy density of 7.24 µWh/cm2 per electrode. The pseudocapacitance at Co3O4 arises from the following redox reaction in KOH electrolyte:19

Co3O4 + OH- + H2O ↔ 3CoOOH + e-

(1)

CoOOH + OH- ↔ CoO2 + H2O + e-

(2)

Since the redox active metal oxides are poorly electro-conductive, their performance at high 13



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scan rate is comparatively low, as obvious from the large deviation from rectangular shape of the CV plots at high san rate (Fig. 4b). For these types of materials only a very thin layer can ensure a better performance, but results in poor areal capacitance. By contrast, the symmetric assembly of the rGO-Co3O4 electrodes retains its symmetrical CV plots with almost same positive and negative current response even at high scan rates (Fig. 4c). The high rate capability results from the high ion diffusion within the microporous aerogel electrode accompanied by the high electrical conductivity.

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Figure 4. (a) Comparative CV plots of Co3O4 and rGO-Co3O4 electrodes in symmetric assembly at 10 mV/s scan rate within the potential range of 0-0.4V; (b) CV plots of Co3O4 at 2, 10, 20 and 30 mV/s and (c) CV plots of rGO-Co3O4 electrodes at different scan rates of 2, 10, 20, 30, 50, 100, and 200 mV/s scan rate.

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Noteworthy that as Co3O4 shows reversible redox behavior within the potential range of 0-0.4 V, the effective energy density is reasonably low due to the poor cell voltage. To increase the cell voltage of a supercapacitor, an asymmetric electrode assembly of electrodes having different working potentials were employed. The practice with pure rGO aerogel as a symmetric supercapacitor electrode showed an effective working potential of 0 ─ (-1)V providing a cell voltage of 1 V. It is reasonable to use the said working potential for EDLC capacitor in aqueous electrolyte to avoid electrolysis, a phenomenon occurring at higher potential (~ 1.23 V). The CV plots of the rGO//rGO symmetric supercapacitor at different scan rates of 2-100 mV/s are shown in Fig. 5a. All the CV plots show close rectangular behavior illustrating supercapacitive behavior. The rectangular shape CV plot even at 100 mV/s indicates its high rate performance. GCD plots (Fig. 5b) of the rGO//rGO also show almost triangular behaviors with very low potential drop at the initial voltage region, also supporting its supercapacitive behavior and low ESR as supercapacitor electrode. The maximum areal capacitance obtained from the rGO//rGO symmetric supercapacitor was 76.5 mF/cm2 at 2 mA/cm2 current density. The maximum areal energy and power density obtained from the rGO//rGO symmetric supercapacitor was 10.3 µWh/cm2 and 11.57 mW/cm2, respectively at 2 mA/cm2 and 30 mA/cm2 current density. The as prepared rGO//rGO symmetric supercapacitor also showed high specific capacitance of 63.7 F/g at 2 mA/cm2 (1.67 A/g mass specific current) density. In order to analyze the cycle life performance of the as prepared rGO//rGO symmetric supercapacitor, the GCD test was continued to 8000 GCD cycles at 40 mA/cm2 current density. High capacitance retention of 94.8% was achieved at the end (Fig. 5c).

Inspired from the decent electrochemical performance of the rGO aerogel in the negative 16


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potential (0- (-1)) V and rGO-Co3O4 in the positive potential region (0-0.4 V), an asymmetric pseudocapacitor was fabricated using the rGO aerogel as negative electrode and rGO-Co3O4 as positive electrode with 6 M KOH as supporting electrolyte. By the coupling of the positive (rGO-Co3O4) and negative (rGO) electrode material with opposite potential window, the as constructed aqueous asymmetric pseudocapacitor (AAP), rGO-Co3O4//rGO showed unprecedented performance with high cell voltage of 1.4 V. The CV plots of the AAP at different scan rates (Fig. 5d) show a close rectangular behavior at low scan rate of 2 mV/s, which was consistent even at high scan rate of 30 mV/s. The redox peaks corresponding to the pseudocapacitive Co3O4 is not so prominent in the CV plots, indicating the redox reaction to occur at a pseudoconstant rate.20 The CV plots at different scan rates show a rapid current response on potential sweep. The near rectangular nature of the CV plots combining the similar current response during the positive and negative scan indicates excellent reversibility of the electrodes owing to the fast ionic diffusion in the electrolyte. The GCD plots (Fig. 5e) of the as-fabricated AAP also support the CV analysis of excellent reversibility by exhibiting almost linear GCD plots with high Coulombic efficiency close to 100%. The maximum areal capacitance obtained from the GCD plots was 200.6 mF/cm2 at 2 mA/cm2 current density, which was ~2.6 times higher than that of the rGO//rGO symmetric supercapacitor. On increasing current density, the areal capacitance showed a gradual decay, still the AAP retained high capacitance of 119.8 mF/cm2 at high current density of 30 mA/cm2 indicating its excellent rate capability. For normal composite electrodes of redox active materials with porous carbon having random pores, the full utilization of the active materials is restricted at high current due to the restricted ion diffusion in random pores. The high rate capability of the rGO-Co3O4//rGO pseudocapacitor can be explained by considering the unique quasi parallel stacking of the rGO sheet in the composite gel that results in open porous structure to facilitate the ion transport resulting in high utilization of both the rGO and Co3O4 even at 17



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high current density. The maximum energy density of 51.82 µWh/cm2 was achieved at a power density of 1.35 mW/cm2. At high current density of 30 mA/cm2 the AAP exhibited energy density of 20.94 µWh/cm2 at a higher power delivery rate of 16.83 mWh/cm2. Considering the mass of the two electrodes, the specific capacitance of the AAP was calculated to be ~128.6 F/g at 2 mA/cm2 (1.28 A/g mass specific current) current density. The cycle life plot of the as-fabricated AAP (Fig. 5f) shows an initial decrease in capacitance followed by a gradual increase up to 6000 consecutive GCD cycles at 80 mA/cm2 current density. The initial decrease of capacitance followed by an increase can be attributed to the slow activation of the electrode materials at high current density. The AAP retained 95.5% of initial capacitance at the end.

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Figure.5 (a) CV and (b) GCD plots of rGO//rGO symmetric supercapacitor; (d) CV and (e) GCD plots of rGO-Co3O4//rGO asymmetric pseudocapacitor in 6 M KOH electrolyte; cycle life plot of the (c) rGO//rGO symmetric supercapacitor and (f) rGO-Co3O4//rGO asymmetric pseudocapacitor.

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Based on the excellent performance of as-fabricated AAP, all solid state asymmetric pseudocapacitor (SAP) was fabricated by replacing the aqueous electrolyte with PVA/KOH gel electrolyte. The redox peak pair in the CV plots of the SAP (Fig. 6a) at different scan rates is associated with the Co2+/Co4+ redox couple. In the CV plots of the SAP, there is a little deviation from the close rectangular nature of CV plots for the AAP. This deviation is associated with the increased resistance in the gel polymer electrolyte. The higher resistance in SAP compared to that of the AAP can also be seen from the EIS analysis (Fig. 6h). However, the GCD plots (Fig. 6b) does not show noticeable fluctuation from the linear behavior. The GCD plots maintained close mirror image symmetry of the charging plot to the discharging plot at each current density, indicating excellent reversibility in the charge storage of electrode materials. The maximal areal capacitance obtained from the SAP was 136.6 mF/cm2 at 2 mA/cm2 current density, that lead to the areal energy density of 35.92 µWh/cm2 at the power density of 1.38 mW/cm2. However, the calculated capacitance was lower than that of AAP. This decrease is associated with the increased ESR in the PVA/KOH gel electrolyte. At a higher current density of 30 mA/cm2, the performance of the SAP was still remarkable with the areal capacitance of 77.4 mF/cm2, energy density of 15.12 µWh/cm2 and power density of 17.79 mW/cm2. The gravimetric capacitance was calculated to be ~87.6 F/g at 2 mA/cm2 (1.28 A/g mass specific current) current density.

The cycle life performance of SAP (Fig. 6c) shows an increasing trend of capacitance during the initial 450 cycles. This should be due to the partial exposure of the active materials to the PVA/KOH gel polymer electrolyte during the initial cycles. Along with the cycle number, the electrochemically active surface area increased and enlarged the capacitance.21 After the initial activation process, the SAP showed stable behavior up to 6000 consecutive GCD cycles without any noticeable capacitance decay. The GCD plots maintained ~99-100% 20


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coulombic efficiency throughout the GCD cycles. The variation of areal capacitances for rGO//rGO, AAP and SAP as a function of current density is shown in Fig. 6d. Along with current density, all three systems showed a gradual decay of capacitance. This is reasonable as the high current causes a rapid potential reversal that restricts the access of the inner pores of the active materials. As-fabricated rGO//rGO, AAP and SAP retained 54.9%, 59.7% and 55.7% of their initial capacitances, respectively along with the current density increase from 2 to 30 mA/cm2.

Ragone plot comparison of rGO//rGO, AAP and SAP is shown in Fig. 6e. The areal energy density delivered by the as-fabricated SAP was higher than many other previously investigated graphene based flexible supercapacitors, such as graphene/cellulose paper flexible supercapacitor (15 µWh/cm2 (three in-series units), PVA/H2SO4 ),22 all-graphene core–sheath microfibers (0.105 µW h cm−2, PVA/H2SO4)23 Graphene Quantum Dots//MnO2 solid state microsupercapacitor (0.414 µWh cm−2,EMIM-NTF2-4 wt % FS ),24 MnO2/graphene/carbon fiber //porous graphene hydrogel wrapped copper wire (18.1 µW h cm−2, PAAK/KCl),25 graphene/carbon nanotube core-sheath fibres based (3.84 µWh cm−2, PVA/H2SO4),26 all graphene coaxial fiber supercapacitor (17.5 µWh cm−2 , H2SO4/PVA ),27 Graphene/polypyrrole solid state fiber supercapacitor (6.6 to 9.7 µWh cm−2, PVA/H2SO4 ),28 Carbon nanotube films loaded GO/PEDOT-CNTs (4.4 µWh cm−2, PVA/H3PO4).29

The variation of specific capacitance with mass specific current density for the rGO//rGO, AAP and SAP has been shown in Fig. 6f. The maximum specific energy of the rGO//rGO, AAP and SAP was calculated to be 8.6 Wh/kg, 34 Wh/kg and 23 Wh/kg, respectively, at respective specific power of 885.7 W/kg, 882.3W/kg, and 820.5 W/kg. The rGO//rGO, AAP and SAP also retained respective high specific energy of 2.9 Wh/kg, 13.4 Wh/kg and 9.7 Wh/kg at high specific power of 9.61, 10.79, and 21



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11.4 kW/kg, respectively. The performance of our as fabricated asymmetric supercapacitor was higher than or somewhere comparable to many other similar asymmetric supercapacitor such as, MnO2/CNF//Bi2O3-CNF (11.3 Wh/kg, 1M Na2SO4 electrolyte),30 RGO–RuO2//RGO–PANi (26.3 Wh/kg, 2 M H2SO4 electrolyte),31 AC//MnO2 (28.4 Wh/kg at 150 W/kg, 0.5 mol/L K2SO4 electrolyte.)32 GF/CNT/MnO2//GF/CNT/Ppy (22.8 Wh/kg at 860 W/kg 0.5 M Na2SO4 electrolyte)33, RuO2–IL-CMG//IL-CMG (19.7 Wh/kg, at power density 0.5 kW/kg, PVA/H2SO4 electrolyte),34 GNR//GNR–MnO2 (29.4 Wh/kg at a power density of 12.1 kW/kg, PAAK/KCl electrolyte)35 TiN@GNS//Fe2N@GNS (15.4 Wh/kg and power density of 6.4 kW/kg, PVA//LiCl solid electrolyte)36

In order to understand the various resistive parameters involved with the electrochemical behaviors, the EIS analysis of the as-fabricated rGO//rGO, AAP and SAP were carried out and represented in the form of Nyquist plot in Fig. 6h. All three curves show a similar characteristic with a starting semicircle in the high frequency region, followed by a straight line in the low frequency region. While rGO//rGO and AAP show comparable solution resistances (Rs) (~ 3.8 Ohm), SAP shows a bit higher value (~ 6.6 Ohm). The charge transfer resistance (Rct) measured from the initial semicircle radius was determined to be 2.2, 6.6 and 17 Ohm, for rGO//rGO, AAP and SAP, respectively. It is notable that the increased Rs and Rct of the SAP over the AAP should be attributed to the poor ion diffusion of PVA. The decreased capacitance of SAP compared to AAP is a consequence of the increased ESR. The post semicircle straight line indicates the diffusion behavior of electrolyte ions within electrode and is an indicative of capacitive performance of all three electrodes.

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Figure. 6 (a) CV plots at different scan rate and (b) GCD plots at different current densities of rGO-Co3O4//rGO SAP; (c) cycle life plot of the SAS and the initial few GCD cycles are shown inset at 40 mA/cm2 current density; (d) Variation of areal capacitances as a function of current densities, (e) plot of areal energy vs. areal power density of rGO//rGO, AAP and SAP (f) variation of specific capacitances as a function of mass specific current and (g) plot of specific energy vs. specific power of rGO//rGO, AAP and SAP; (h) complex plane impedance 23



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spectra of the as-fabricated rGO//rGO, AAP, and SAP. (i) Digital photograph of a representative SAP showing its flexibility; (j) CV plot of a representative SAP (1.6 cm × 1 cm ) before bending (L = 1.6 cm) and under being (L = 1 and 0.5 cm).

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In order to demonstrate the mechanical flexibility of all solid state pseudocapacitor, CV analysis was carried out for the representative SAP of dimension (1.6 cm × 1 cm) at 20 mV/s scan rate in different bending conditions (Fig. 6j). No remarkable deviation was detected from the CV plots while changing the distance of two ends of SAP from 1.6 to 1 and 0.5 cm under bending. This confirms the stable electrochemical performance of the SAP under mechanically deformed states and thus establish its potential suitability for flexible device applications.

Conclusion Room temperature interfacial gelation of rGO from graphene oxide liquid crystal aqueous dispersion has been successfully employed for the well-distributed encapsulation of pseudocapacitive Co3O4 nanoparticles within 3D mesoporous rGO network. The resultant mesoporous composite aerogels facilitated ion transport throughout their enormous pore channels during charge and discharge, while ensuring robust electrical connectivity through the interconnected 3D network structure. Binder-free all solid state asymmetric supercapacitors were fabricated utilizing the pure rGO aerogel as negative electrode and rGO-Co3O4 composite aerogel as positive electrode, which accomplished high areal energy density of 35.92 µWh/cm2 and high power density of ~17.79 mW/cm2 in addition to excellent cycle life. Our scalable reliable route to graphene based composite structures based on interfacial gelation holds great promise for various energy storage/conversion applications, such as batteries, fuel cells, water splitting as well as the immediate supercapacitor applications demonstrated in this work.

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ASSOCIATED CONTENT

Supporting Information The Supporting Information contain: equations for the calculation of electrochemical analysis result, schematic diagram of flexible devise fabrication, CV plots of rGO//rGO, rGOCo3O4//rGO in 6M KOH and rGO-Co3O4//rGO in PVA/KOH electrolyte at high scan rates and variation of specific capacitance as a function of scan rate. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions S.O.K., D.G, conceived the idea and designed the experiments. D.G principally performed the experiments: materials synthesis, characterization and fabrication of electrodes. S.O.K. supervised the entire project. D.G, R.N, J.L together performed some of the characterizations of the materials. D.G, S.O.K., co-wrote the paper and all author participated in discussions.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was financially supported by the National Creative Research Initiative (CRI) Cente r for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061) and Nano Material Technology Development Program (2016M3A7B4905613) through the National 26


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Research Foundation of Korea(NRF) funded by the Ministry Science, ICT& Future Planning

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Table of Content High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co3O4 Composite Aerogel Electrodes Debasis Ghosh, Joonwon Lim, Rekha Narayan, Sang Ouk Kim* Department of Materials Science & Engineering, KAIST, Daejeon 305-701, Korea

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High Energy Density All Solid State Asymmetric ... - ACS Publications

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