Free-standing composite hydrogel films for superior volumetric capacitance

High volumetric capacitance is vital for the development of wearable and portable energy storage devices. We herein introduce a novel simple route for the fabrication of a highly porous, binder-free and free-standing polyaniline/reduced graphene oxide composite hydrogel (PANi/graphene hydrogel) as an electrode with a packing density of 1.02 g cm -3 . PANi played critical roles in gelation, which include reduction, crosslinking, creation of pseudocapacitance and as a spacer preventing graphene sheets from stacking. The composite hydrogel film delivered a volumetric capacitance of 225.42 F cm -3 with a two-electrode supercapacitor configuration, which was enhanced to 592.96 F cm -3 in a redox-active electrolyte containing hydroquinone. This new strategy will open a new area for using conducting polymer derivatives in the development of flexible graphene electrodes towards many applications such as batteries, sensors and catalysis. High volumetric capacitance is vital for the development of wearable and portable energy storage devices. We herein introduce a novel simple route for the fabrication of a highly porous, binder-free and free-standing polyaniline/reduced graphene oxide composite hydrogel (PANi/graphene hydrogel) as an electrode with a packing density of 1.02 g cm (cid:1) 3 . PANi played critical roles in gelation, which include reduction, crosslinking, creation of pseudocapacitance and as a spacer preventing graphene sheets from stacking. The composite hydrogel ﬁ lm delivered a volumetric capacitance of 225.42 F cm (cid:1) 3 with a two-electrode supercapacitor con ﬁ guration, which was enhanced to 592.96 F cm (cid:1) 3 in a redox-active electrolyte containing hydroquinone. This new strategy will open a new area for using conducting polymer derivatives in the development of ﬂ exible graphene electrodes towards many applications such as batteries, sensors and catalysis.


Introduction
Recently, exible electrodes have received increasingly more attention because of their extensive applications in bendable, wearable and portable electronic devices, especially exible energy storage devices such as supercapacitors and batteries. 1,2 Supercapacitors or ultracapacitors are advantageous over batteries due to their high power density, fast charge-discharge rates, long cycle life, safety, cost-effectiveness and environmental friendliness. Graphene, a at monolayer of sp 2 hybridized carbon atoms, is considered as the most promising candidate for exible supercapacitor electrodes. 3,4 But graphene sheets just stack themselves leading to graphite-like behaviour, and an effective solution to this problem is to use polymers as spacers between the sheets. [5][6][7][8][9][10][11] Mechanically robust graphene hydrogels hold promise for the development of exible electrodes, because they demonstrate high exibility, low ion diffusion resistance yet large ionaccessible surface areas. These gels were initially developed by a hydrothermal treatment of graphene oxide (GO) suspension, and they showed a specic capacitance of 175 to 190 F g À1 . 12,13 To further increase the capacitance, Xu et al. prepared a graphene hydrogel lm using a hydrothermal process followed by pressing with a gold-coated polyimide substrate, and the lm was then functionalized with hydroquinone; the functionalization improved the specic capacitance from 190 to 441 F g À1 . 14,15 Recently, the treatment appears to have been replaced by chemical reactions at an elevated temperature with no pressurized containers involved. 16 For instance, a graphene hydrogel fabricated by the reduction of GO with L-glutathione showed a specic capacitance of 157.7 F g À1 . 17 Another hydrogel was fabricated by crosslinking GO with ethylenediamine, subsequently reduced by hydrazine, delivering a specic capacitance of 232 F g À1 . 18 Synergistic effects on capacitance were observed between conducting polymers, graphene and carbon nanotubes, leading to a high specic capacitance of 498 F g À1 . 19 However, it is unknown whether conducting polymers can form hydrogels; if so, what will be the capacitance.
Volumetric capacitancea paramount metric in real industrial applicationsis indeed far more important than gravimetric capacitance for portable devices in need of as much energy storage as possible in rather limited space. 20 A compressed holey graphene oxide framework, which was hydrothermally prepared in the presence of H 2 O 2 , showed a gravimetric capacitance of 298 F g À1 and a volumetric capacitance of 212 F cm À3 in an organic electrolyte. 21 Yang et al. prepared a chemically reduced, free-standing graphene oxide hydrogel lm by a capillary compression process, which exhibited a volumetric capacitance of 255.5 F cm À3 . 22 The highest volumetric capacitance of 376 F cm À3 to date was reported by Tao et al. who developed a highly dense graphene by evaporation-induced drying of a graphene hydrogel. 23 The current study introduces a new strategy for the development of a free-standing hydrogel composite lm consisting of polyaniline (PANi) and reduced graphene oxide sheets, where PANi plays multi-roles including reduction, crosslinking, as a spacer to wedge open stacked graphene sheets, and an additive for pseudocapacitance. As noted before, GO can be partially reduced during the in situ polymerization of aniline [24][25][26] or during its contact with some conjugated polymers such as poly(m-phenylenediamine) to produce a composite through a redox reaction. 27 A dopant poly(2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPA)) is selected for PANi because it has extraordinary stability under high oxidizing potentials and also it improves water accessibility to PANi. 28 The obtained freestanding hydrogel lm electrodes have high exibility and an interconnected porous structure, and they deliver the highest gravimetric (C wt ¼ 580.52 F g À1 ) and volumetric (C vol ¼ 592.96 F cm À3 ) capacitance values among all the reported porous carbon-based supercapacitors to date.
Synthesis of polyaniline: poly(2-acrylamido-2-methyl-1-propanesulfonic acid) The synthesis of a compound containing polyaniline and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (denoted as PANi-PAMPA) was carried out according to a previous report. 28 In brief, 1.65 g of PAMPA was dissolved in 107.14 ml of deionized water, into which 0.74 g of aniline monomers was added, followed by stirring for 1 h at room temperature. APS (4.6 g) was dissolved in 7.14 ml of deionized water. Aerwards, these two batches of solutions were purged with nitrogen gas for 1 h. Then the APS solution was added dropwise at 1.25 ml min À1 to the aniline-PAMPA solution to start the polymerization lasting 24 h. Finally, 200 ml of acetone was used to precipitate the PANi-PAMPA composite. The resulting polymer was washed several times with acetone and dried under vacuum at room temperature giving 93% yield.

Synthesis of graphene oxide
Graphene oxide (GO) was prepared as per a previous study. 29 In brief, graphite powder (0.5 g) was mixed under stirring with a mixture of 60 ml H 2 SO 4 and 6.6 ml H 3 PO 4 , and then 3 g of KMnO 4 was slowly blended with the mixture within 1 h in an ice bath. Subsequently the reaction system was transferred to an oil bath, with the temperature raised to 50 C and stirring for 24 h. Aer that, the reaction was lied for cooling down to room temperature and poured into a mixture of 200 ml ice and 1 ml H 2 O 2 (30%). The produced GO was centrifuged and washed with 1 M HCl solution and deionized water, respectively, to remove the residual metal ions and acids. The nal GO paste was dried at 60 C for 48 h for the following synthesis.

Synthesis of a polyaniline/graphene hydrogel lm
A polyaniline (PANi)/graphene hydrogel was prepared according to the following steps. 60 mg GO (90 wt%) and 7 mg PANi-PAMPA (10 wt%) were dispersed, respectively, in 10 ml and 5 ml of deionized water, both with ultrasonication for 1 h. The two batches of solutions were stir-mixed, followed by sonication for another 1 h. Subsequently the mixture was heated at 95 C for 30 h with no stirring to form a PANi/graphene hydrogel. The hydrogel was added into 60 ml deionized water with vigorous stirring, turning the hydrogel monolith into many tiny pieces. Next, the resulting suspension was ltered through a PTFE membrane to form a hydrogel lm. The lm was easily peeled off from the lter paper aer drying under vacuum at room temperature for 4 h. The composites with over 10 wt% of PANi obviously showed lower mechanical strength, and thus the 10 wt% composite hydrogel was used throughout this study. The hydrogel lm of $7 mm in thickness was then washed several times with deionized water, and was immersed again in water for 24 h to remove unreacted materials, producing the end producta PANi/graphene hydrogel composite lmfor the following application.

Supercapacitor fabrication and electrochemical characterization
The composite lm was cut into small pieces, each of which has a surface area of 1 cm 2 ; these pieces were dipped into electrolytes overnight which can be either 1 M H 2 SO 4 or a mixture of 1 M H 2 SO 4 and 0.4 M hydroquinone (HQ) (H 2 SO 4 /HQ). Two identical lms were attached to Pt foil as current collectors, and then the electrodes were sandwiched by a piece of lter paper as the separator. All electrochemical measurements, such as cyclic voltammetry (CV), galvanostatic charge/discharge (CD) and electrochemical impedance spectroscopy (EIS), were performed by a two-electrode conguration using a CHI 660E electrochemical workstation. The CV testings were done between À0.2 and 0.8 V at different scan rates from 5 to 100 mV s À1 . The CD measurements were carried out in a voltage window between 0 and 0.8 V at 0.4-10 A g À1 . The EIS spectra were recorded from 100 kHz to 10 mHz at an open circuit potential with an AC voltage amplitude of 5 mV. All measurements were performed at room temperature.

Results and discussion
PANi/graphene composites Fig. 1 illustrates our simple method for the preparation of a polyaniline (PANi)/graphene hydrogel lm. We adopted graphene oxide (GO) in this study because it has shown great potential in the development of energy storage devices 8,10,11,[30][31][32][33][34] and other applications. 7,9,35 When the GO suspension was mixed with the solution of PANi and poly(2-acrylamido-2-methyl-1propanesulfonic acid) (denoted PAMPA), an electrostatic interaction was immediately created between the negatively charged carboxylic groups of GO and the positively charged nitrogen atoms of both PANi and PAMPA (Fig. S1a † contains the polymers' morphology); leaving the mixture at 95 C for 12 h changed the colour from brown to black. While PAMPA may crosslink PANI, the PANi-PAMPA solution has the following roles: (i) reducing GO and (ii) interleaving graphene sheets as a spacer; this would produce an impressive pseudocapacitance that is to be discussed later. Heating the mixture at 95 C for 30 h created a cylindrical shape of the PANi/graphene hydrogel (Fig. 2a). The hydrogel morphology may consist of particulates, nanotubes and nanobers, as shown in the ESI Fig. S1; † Fig. S2 † contains images of the hydrogels. The hydrogel used in the following measurement contains 99.33 wt% H 2 O, 0.07 wt% PANi and 0.60 wt% graphene. The internal micro-porous network of the freeze-dried hydrogel is clearly seen in Fig. 2b and c.
We report below a new method for the fabrication of a hydrogel composite lm. The cylindrical PANi/graphene hydrogel obtained from the last step was sheared into many tiny pieces in deionized water by vigorous stirring, producing a colloidal suspension; by ltration and drying, we created a exible and free-standing PANi/graphene hydrogel lm (Fig. 2d). This ltration process contributes to a higher volumetric capacitance because it created a lm of controllable lower thickness ($7 mm) for supercapacitor electrodes than the original composite hydrogel. The cross section and surface of the lm are shown in Fig. 2f, h and S3, † all of which reveal orderly layered graphene sheets with a porous structure. Fig. 2f shows polymeric particles embedded between the graphene layers providing exactly more interspace and high accessibility to the electrolyte ions.
Raman scattering was used to elucidate the structural integrity of GO and its composite hydrogel (Fig. 3a). The Raman spectra displayed two peaks, a D band at 1348 cm À1 relating to the conversion of sp 2 to sp 3 -hybridized carbon and a G band at 1590 cm À1 corresponding to the vibration of a sp 2 -hybridized carbon. The I D /I G intensity ratio provides a major tool to determine the degree of disorder and the information of sp 2 domains. Aer the PANi-PAMPA treatment, the I D /I G intensity ratio increases from 1.03 for GO to 1.11 for a PANi/graphene hydrogel, indicating the role of PANi in reducing GO.  37 The reduction of GO by PANi was clearly proved by the disappearance of the diffraction at 10.6 in the hydrogel XRD pattern, in agreement with the Raman analysis. However, a broad diffraction is found at 24.7 for the hydrogel in Fig. 3b corresponding to an interlayer spacing of 0.36 nm which relates to a graphite-like structure (0.33 nm); the broadness implies a wide variety of interlayer spacing which may be caused by inserted polymer particles (Fig. 2f).
High-resolution XPS was used to investigate the compositions of GO and its composite hydrogel (Fig. 3c). The hydrogel survey spectrum shows an N1s peak centred at 399 eV corresponding to the secondary amine groups of PANi indicating its presence. The GO reduction by PANi increased the C/O ratio from 2.52 for GO to 5.27 for the gel (aer deducting the contribution from PANi, the C/O ratio is 4.74 for the reduced GO). The reduction led to an increase in sp 2 hybridized carbon atoms on the graphene sheets, which enhanced the p-p interaction between graphene and PANi facilitating the electron transfer between graphene and PANi. Fig. 3d contains the graphs of thermogravimetric analysis (TGA) of PANi, GO and the dried PANi/graphene hydrogel which  is an aerogel. For PANi, the weight loss up to 100 C is due to the elimination of adsorbed water molecules, and the largest weight loss at 200-500 C should be attributed to the decomposition of low molecular weight PANi. GO displays low thermal stability with three stages of thermal degradation. The weight loss at $100 C is likely due to the removal of physically adsorbed water molecules (stage 1), and the rapid degradation at $200 C is attributed to the removal of oxygen-containing functional groups (stage 2), and the carbon skeleton of the GO decomposed at elevated temperatures between 550 and 650 C leading to a total weight loss of 74.54% (stage 3). The dried hydrogel lm has far better thermal stability than both PANi and GO, as shown by the lm weight loss of 28.63% at 600 C.
It is worth noting that the design of our hydrogel composite lm electrodes provides an effective solution to the problem of graphene stacking, where PANi nano-entities, by p-p interaction with graphene sheets, work as a spacer inserted between the graphene sheets; this leads to a high interfacial area and excellent electrolyte diffusion rate throughout the network. Moreover, the produced hydrogel lm has high exibility (Fig. 2d) and high electrical conductivity (5600 S m À1 ), eliminating the need for polymer binders or conductive additives that are electrochemically inert with low capacity.
Electrochemical performance of PANi/graphene composite hydrogels in 1 M H 2 SO 4 The electrochemical performance of the as-prepared PANi/graphene hydrogel lm electrodes was rst investigated in 1 M H 2 SO 4 . The cyclic voltammetry (CV) curve of PANi/graphene hydrogel lms is shown in Fig. 4a, where quasi-rectangles are seen with very weak reversible anodic and cathodic peaks, indicating pseudocapacitance mainly contributed by PANi or residual oxygen-containing functional groups on graphene sheets.
To gauge the effect of PANi, we replaced PANi with 40 ml of ethylenediamine (EDA) as the reducing agent to produce a control samplean EDA/graphene hydrogel lm. In Fig. 4a, the characteristic peaks for pseudocapacitance disappear in the EDA/graphene hydrogel with a lower integrated area; and this demonstrates the role of PANi to enhance the electrode capacitance.
The CV curves of PANi/graphene hydrogel electrodes at different scan rates are displayed in Fig. 4b, showing the capacity of the composite hydrogel lm to present competitive electrochemical behaviour in a wide range of scan rates. A linear relationship is observed between the capacitive current peaks and the scan rates, conrming that the electrochemical behaviour was mainly determined by the diffusion of H + into the porous structure of the hydrogel electrodes (Fig. S4a †). Fig. 4c contains the galvanostatic charge/discharge (CD) curves of the hydrogel lm at different scan rates; the deviation from linearity is due to the pseudocapacitance from PANi. The long charge and discharge time is attributed by the incorporation of EDLC and Faradic capacitance, respectively, from graphene sheets and PANi. Fig. 4d shows the gravimetric (C wt ) and volumetric (C vol ) capacitances of the supercapacitor (density ¼ 1.02 g cm À3 ) at different current densities, calculated from charge/discharge curves by the following equations: where I is the applied constant current (A), DE Dt is the slope of the discharge curve, m is the total mass of the two electrodes (g), and V is the total volume of the two electrodes. The hydrogel lm electrodes display a gravimetric (C wt ) capacitance of 223.82 F g À1 at 0.4 A g À1 , which is higher than the chemically reduced graphene gel lm (170.60 F g À1 ) but lower than the holey graphene framework (310.00 F g À1 ). 21,22 On the other hand, the calculated volumetric capacitance based on the packing lm density (1.02 g cm À3 ) was 225.42 F cm À3 . As the value is lower than the reported capacitance values such as 255.5 F cm À3 for the graphene gel lm 22 and 300 F cm À3 for carbon nanotube-graphene bres, 38 we improved our electrode performance by selecting an electrolyte mixture.

Superior performance of the hydrogel composite lm
Since hydroquinone (HQ) was reported as an effective redoxactive electrolyte providing additional redox reactions, 39 we added 0.4 M HQ into 1 M H 2 SO 4 to produce a mixed electrolyte. Using the mixed electrolyte obviously increased the integrated area of cyclic voltammetry (Fig. 5a). In Fig. 5b, the electrolyte shows the longest CD time, where the deviation from the nearly symmetric triangular shape conrms the contribution from HQ. Fig. 5c contains the gravimetric and volumetric capacitances (C wt and C vol ) of the hydrogel lm electrodes in the mixed electrolyte (H 2 SO 4 /HQ) at different current densities, which demonstrate remarkably far higher performance (C wt ¼ 580.52 F g À1 and C vol ¼ 592.96 F cm À3 ). As shown in Table S1, † our C vol is 197% higher than the highest 300 F cm À3 reported to date for exible supercapacitors. 38 Similarly, the areal capacitance per footprint of the device increases from 78.89 mF cm À2 (in H 2 SO 4 ) to 207.53 mF cm À2 (in H 2 SO 4 /HQ) (Fig. S4e †). Thus, our composite hydrogel lms are promising for the development of exible supercapacitors.
Electrochemical impedance spectroscopy (EIS) is essential to understand the electrochemical response of our composite lm electrodes. Fig. 6a shows Nyquist plots of the hydrogel electrodes tested, respectively, in H 2 SO 4 and in the mixed electrolyte (H 2 SO 4 /HQ). It is apparent that the sample in H 2 SO 4 /HQ shows a more obvious semicircle in the high-frequency region (inset in Fig. 6a). By extrapolating the straight portion of the graph, we  obtained an equivalent series resistance (ESR) of 1 U for the electrodes in H 2 SO 4 . By contrast, the sample in H 2 SO 4 /HQ presents a much lower ESR at 0.78 U due to the high electrical conductivity of the electrolyte. 40,41 At low frequency, the hydrogel in H 2 SO 4 /HQ demonstrates a more vertical line than the hydrogel in H 2 SO 4 , corresponding to the lower diffusion resistance of ions and more capacitance from the HQ's redox behaviour.
In Fig. 6b we made the Ragone plot with the average volumetric energy density (E vol ) and the average volumetric power density (P vol ) based on the total volume of electroactive materials in the two electrodes by using the following equations; For the hydrogel electrodes in H 2 SO 4 , the average volumetric energy and power density values are, respectively, 5.1 Wh L À1 and 2427.4 W L À1 . In case of the H 2 SO 4 /HQ electrolyte, much higher values are obtained at 13.2 Wh L À1 and 2636.9 W L À1 , corresponding to the gravimetric energy and power density at 12.9 Wh L À1 and 2581.6 W kg À1 .
The platinum current collector is 100 mm in thickness, which can be readily bent. The composite hydrogel lm supercapacitor with H 2 SO 4 /HQ as the electrolyte shows excellent exibility and can be bent to a large angle with no damage to the structural integrity. While all the previous electrochemical measurements were performed on at electrodes due to the convenience of fabrication and handling, Fig. 6c shows the CVs of the hydrogel electrodes in H 2 SO 4 /HQ with different bending conditions at 100 mV s À1 . It is seen that bending does not pose any effect on the capacitance, indicating excellent mechanical stability of our hydrogel electrodes, and this implies potential applications for many portable devices.
The long-term cyclic stability of the hydrogel lm electrodes was studied by charge/discharge testing at a current density of 10 A g À1 in both H 2 SO 4 and H 2 SO 4 /HQ electrolytes for 5000 cycles (Fig. S4d †). The electrodes in H 2 SO 4 /HQ reveal lower cyclic stability (70% of cyclic retention) than that in H 2 SO 4 (87.5%), and this may be due to the extensive redox reactions in the redox electrolyte. 40,41 In spite of these, the capacitance in the redox electrolyte is 70% higher than that in H 2 SO 4 . The rst and last ve cycles of the hydrogel electrodes in H 2 SO 4 /HQ (inset Fig. 4Sc †) are nearly identical, indicating good reversibility.
A number of supercapacitor units are oen connected either in series or parallel to make a tandem cell; this is a traditional way to enhance the ability of the supercapacitor to supply a variety of devices in practical applications. In this study, three composite hydrogel lm supercapacitors with H 2 SO 4 /HQ as the electrolyte were assembled in series to power a red light emitting diode (LED). The operating potential window was thus extended from 0.8 V for a single device to 2.4 V for the assembled cell (Fig. 6d), and there is no change on the discharge time, revealing the high capability of the cell. Charging the cell at 2.4 V for 30 seconds provided sufficient power to light up the LED for over 6 minutes (inset in Fig. 6d).

Conclusions
To summarize, we developed a simple method for the fabrication of a free-standing conducting polymer/graphene composite hydrogel lm. The conducting polymer polyaniline (PANi) played a number of key roles in the fabrication. The composite lm has a porous and orderly layered structure where polymeric particles were found between the layers; these particles worked as stiff nano-spacers providing plenty of electrochemical interface for electrolyte ion interactions. When the hydrogel lm was used as a exible supercapacitor electrode in a redox active electrolyte, it revealed the highest gravimetric and volumetric capacitances at 580.52 F g À1 and 592.96 F cm À3 , respectively, at a current density of 0.4 A g À1 . Furthermore, the hydrogel electrodes provided a maximum volumetric energy density of 13.20 Wh L À1 with an average volumetric power density of 124.08 W L À1 .