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Three-Dimensional Chemically Bonded Polypyrrole/Bacterial Cellulose/ Graphene Composites for High-Performance Supercapacitors Yun Liu, Jie Zhou, Jian Tang, and Weihua Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03060 • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 1, 2015

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Chemistry of Materials

Three-Dimensional Chemically Bonded Polypyrrole/Bacterial Cellulose/Graphene Composites for High-Performance Supercapacitors Yun Liu, Jie Zhou, Jian Tang and Weihua Tang* Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China

ABSTRACT: Flexible energy storage systems have recently attracted great interest for portable electronic devices. The functionalization of graphene provides vast platform in tailoring its nanostructure and properties for energy storage via facile processing. Here we first demonstrate the development of chemically bonded graphene oxide and bacterial cellulose hybrid composite coated with polypyrrole for robust and high-efficiency supercapacitor electrodes. The as-prepared composites exhibited a highest electrical conductivity (1320 S m-1) and the largest volumetric capacitance (278 F cm-3) ever shown by carbon-based electrodes, along with 95.2% retention of 556 F g-1 gravimetric capacitance over 5000 recycling tests in asymmetric supercapacitors. Impressively, the hybrid electrode contributed a 492 F g-1 gravimetric capacitance and 93.5% retention over 2000 recycling in symmetric supercapacitors. The nanostructure and composition of the composites were found to play a crucial role for the performance of these three-dimensional, chemically bonded hybrid composite electrodes.

INTRODUCTION Flexible energy storage systems have recently attracted great interest for applications in portable electronic devices and hybrid electrical vehicles.1 Supercapacitors (also called electrochemical capacitors) have been widely explored for energy storage application because of their higher power density, cycle efficiency, wide thermal operating range, and lower maintenance cost.2-5 To date, supercapacitors can be subdivided into two classes: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs store the energy physically through the adsorption of ions on the surface of the electrodes, whereas pseudocapacitors enable electrochemical energy storage by fast redox reactions occurring between the electrode active material and the electrolyte.1 Featuring large surface area (ca. 1000 m2 g-1), activated carbon (AC) based electrodes, as the state-of-art electrodes for EDLCs, exhibit large gravimetric capacitance with electrostatic mechanism. However, the existence of large micro/macropores within AC proves to be disadvantageous for the adsorption of the electrolyte on the surface of electrodes, deteriorating the function of capacitors.5 To address this challenge, intense research efforts have devoted to graphene (GE), a carbon monolayer packed into 2D honeycomb lattice, due to high theoretical surface area (2,630 m2 g–1)6 and theoretical capacitance (550 F g-1)7 to boost the energy density of such devices. To create a large electrolyte-accessible area, porous graphene-like materials have been developed including GE foam/hydrogel,8-10 crumpled/wrinkle GE,11 graphene oxide (GO),12 and reduced graphene oxide (rGO).5 However, the lower gravimetric capacitance (100–270 F g-1 and 70–120 F g–1 with aqueous and organic electrolytes, respectively)12,7 and random restacking of GE sheets make graphene-like materials electrodes uncompetitive to AC in

practical EDLCs. To restrain the ordered stacking of GE sheets, one effective approach is using one-dimensional carbon nanotube (CNT) as a nanospacer to construct three-dimensional (3-D) hierarchical structure, which significantly improves the EDLC performance.13 Another effective approach is to combine GE with pseudo-capacitive materials (such as conducting polymers or transition metal oxides), which enables the EDLCs at the electrode/electrolyte interfaces and faradic capacitors coming from the pseudocapacitive materials simultaneously.14,15 In these GE-conductive polymer and GEmetal-oxide composites, GE acts as support matrix for the growth of the electroactive species at nanoscale, which results in a larger surface area and further enhances the electrochemical performance by increasing the electrical conductivity and mechanical stability.16-21 The exploitation of GE’s potential in pseudocapacitors significantly relies on the composites development to take full advantage of the synergistic effect of GE substrate and the electroactive components, along with morphology optimization of both spatial orientation of GE sheets and other components.15-21 Morphology well-controlled composites have been successfully developed through in-situ chemical or electrochemical polymerization, where conducting polymers (ca. polypyrrole, PPy; polyaniline, PANI) deposited or immobilized on GN sheets or bacterial cellulose (BC) nanofibers.18-24 Chemically bonded GO/CNT13 or GO/BC composites25 were further prepared by taking advantage of oxygen-containing functionalities in GO. Despite its higher pseudocapacitance than rGO,26 GO exhibits lower electrical conductivity and the remaining oxygen-containing groups may exert negative effect on the electrochemical behaviour of the electrode by reducing the cycling stability and reversibility.15

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In this report, ternary PPy/BC/GO (PBG) composites with well-defined structure have been developed by wrapping PPy onto chemically bonded BC/GO hybrid. The complementary bonding between BC and GO effective prevented the restacking of GN layers and aggregation of BC nanofibers. BC fibers attaching onto both edge and surface of GN not only act as spacers to increase the electrolyte-accessible interface, but also construct a 3D interpenetrating networks to reinforce the composite. With the coating of PPy on BC/GO skeleton, 3D electrical conduction path was constructed and the negative effect of oxygen-containing groups on GO surface was eliminated. High specific capacitance (556 F g-1 for asymmetric and 482 F g-1 for symmetric supercapacitors) and excellent recyclability (95.2% capacitive retention for asymmetric after 5000 recycles and 93.5% for symmetric supercapacitors after 2000 recycles) were first reported on such chemically bonded hybrid composites as working electrode for supercapacitors. To the best of our knowledge, we have demonstrated the highest electrical conductivity (1320 S m-1) and the largest volumetric capacitance (278 F cm-3) ever shown by carbon-based electrodes by intercalating GN sheets with BC nanofibers. RESULTS AND DISCUSSION Chemically bonded PPy@BC/GO composites. The synthesis of chemically bonded PPy/BC/GO composites is schematically illustrated in Figure 1a. By taking advantage of the oxygen-containing functionalities on GO surface, BC nanofibers were dispersed and self-assembled on the surface of GO under ultrasonication. The covalently cross-linked BC/GO hybrids were achieved by the esterification between carboxyl groups at the edge of GO and hydroxyl groups of BC (Figure 1c).25 The as-obtained BC/GO dispersion in water was then added with pyrrole (Py), which was allowed to assemble onto both surfaces of GO nanosheets and BC fibers via interactions like H-bonding and π-π interactions.20 The in-situ polymerization of Py generated the ternary PPy/BC/GO (designated as PBG) composites with welldefined nanostructure (Figures 1d-f). By varying the feeding ratios of pyrrole to BC/GO, PBG composites with different PPy contents were obtained to tune the conductivity and electrochemical properties. The weight feeding ratio of pyrrole to BC/GO explored was 1:1, 5:1, 10:1, 15:1 and 20:1. The resulting composites were thus designated as PBG1:1, PBG5:1, PBG10:1, PBG15:1and PBG20:1 (see composition in Table S1 in Supporting Information, SI), respectively. PBG composites exhibited much higher conductivity than pristine GO (54 S m−1), BC/GO (171 S m−1), pure PPy (107 S m−1), BC/PPy (77 S m-1),20 GN-PPy/CNT52 (15.3 S m-1),12 and 3D porous GN (303 S cm-1).10 And the conductivity increased with PPy content before reaching the maximum (1320 S m−1) for PBG10:1 nanocomposite (Figure 2a). This conductivity is about 23, 7, 16 and 11-fold higher than that of pristine GO, BC/GO, BC/PPy and PPy, respectively. Such high conductivity is attributed to the synergetic effect between BC/GO and PPy in forming 3D cross-linked conductive networks. The existence of maximum conductivity with PPy content may be explained with the conductive network theory. The effective conductive network formed gradually before the optimal PPy content reached to exhibit the highest conductivity. Further

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increasing the feeding ratio of Py:BC/GO (>10:1), excessive PPy aggregated to prevent the transport of effective charge along the conductive networks and resulted in decreased conductivity (further confirmed by morphology observation in Figure 4 and Figure S2 in SI).

Figure 1. (a) Synthesis scheme depicting the self-assembly of BC nanofibers on GO surface, cross-linking between BC and GO, reduction of GO, self-assembly of Py on BC nanofiber surface, and in-situ polymerization to prepare PPy/BC/GO composites. (b) SEM image of pristine GO. The cross-sectional view of SEM image of (c) cross-linked BC/GO via the covalent intercalation of GO sheets with BC, (d) a single layer of PPy/BC/GO hybrid, (e) multi-layers of PPy/BC/GO as staking and (f) PPy/BC core-sheath hybrid as linkage between stacking.

The formation of 3D cross-linked PBG composite was confirmed by Fourier transform infrared (FT-IR) spectroscopy X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). As shown in Figure 2b, pristine GO sheets exhibit the characteristic peaks for O-H stretching (3435 cm1 ), C=O stretching (1726 cm-1), C=C stretching (1615 cm-1) and C-O-C stretching (1066 cm-1).27 BC fibers exhibits a broad band for O-H stretching (3413 cm-1), a band for C-H stretching (2895 cm-1) and an intensified sharp peak for C-OC stretching (1057 cm-1).20,21,27 All characteristic absorptions of GO and BC were remained for BC/GO composite, while the intensified peaks for C=O stretching (1622, 1581 cm-1) indicate the successful cross-linking between BC and GO. For PBG10:1 composite, the peak appeared for N–H stretching vibration (3354 cm-1), indicating the full coverage of PPy layers on GO sheets and around BC fibers. Moreover, three characteristic peaks of pure PPy corresponding to C=C stretching (1513 cm-1), C–N stretching (1410 cm-1) and C–H in-plane bending (1253 cm-1) were observed for PBG10:1 composite.18-21,29 The chemically-bonded PBG10:1 composite presents a three-stage degradation profile similar to PPy but with a smaller weight loss of 36% upto 800oC (see Figure S1 in SI). The results indicate GO can remarkably improved the thermal stability of both BC and PPy. As the XRD patterns shown in Figure 2c, GO’s intrinsic peaks (2θ = 26.2o and 43.2o), corresponding to its diffraction planes (002) and (100), are observed.30 Three major peaks of


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BC locating at 14.5o, 16.9o, and 22.7o, corresponding to the (1 0), (110) and (200) diffraction planes of cellulose I, respectively, are also observed.31 The characteristic peaks of both BC (2θ = 22.7o) and GO (2θ = 26.2o) were inherited in BC/GO composite with weakened intensities. With the incorporation of PPy, the PBG composites exhibit significantly weakened intrinsic peaks of GO and BC with the increment of PPy content in the ternary composites. On the contrary, the characteristic broad diffraction peak at 24.5o for amorphous PPy was clearly observed for PBG composites. It is reasonable that the crystallization behaviours of both GO and BC were hindered accompanying with the wrapping of amorphous PPy outer layers. The Raman spectrum of 3D cross-linked PBG composites is further studied (Figure 2d). Two typical bands (indexed at 1598, 1349 cm-1) are observed for PBG composites, which correspond to the welldocumented G and D bands of the hydrothermally reduced GO.25 And the characteristics peaks of PPy (1051 and 963 cm-1) observed for PBG composites indicate the wrapping of PPy onto GO sheets and BC fibers.32

contributed to enhance the conductivity and electrochemical properties of PBG composites.

Figure 3. XPS spectra of (a) GO; (b, c) BC; (d) PPy; (e, f) BC/GO, and (g-i) PBG10:1.

Figure 2. (a) Electrical conductivity; (b) FT-IR spectra; (c) XRD patterns and (d) Raman spectra of GO, BC, BC/GO and PBG composites.

High-resolution XPS spectra revealing the chemical bonding in PBG10:1 composite is further shown in Figure 3. GO C1s exhibits a main peak at 284.5 eV for C-C and three additional peaks at higher binding energies (i.e., 285.6, 286.4 and 288.3 eV), indicating the presence of oxygen atoms bonded to carbon atoms as in C-O, C-O-C and O-C=O.33 Pure BC O1s shows two distinct peak at 528.6 (C-OH) and 531.3 eV (C-O-C) (Figure 3c).20 And PPy N1s presents three peaks at binding energies of 397.5, 399.4 and 400.5 eV, corresponding to –N=, –N– and –N+–, respectively.19 For PBG10:1 composite, the core levels of O 1s and N 1s of BC and PPy shift to higher binding energies. This may be attributed to both the shielding effect of PPy layers and robust interactions among the three components, such as π-π stacking between PPy and GO, and H bonding between the nitrogen lone pairs of PPy and the –OH groups of BC fibers. The doping level of of Cl-1 in PBG10:1 was determined to be 0.174 with the Cl/N ratio from its Cl 2p and N 1s spectra, which was higher than that of PPy (0.036),21 indicating that more ions and electrons

The morphology of novel 3D cross-linking structured PBG composites was observed using scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). Figure 1b shows the well-exfoliated GO lamellar nanosheets with clean and smooth surfaces. Upon crosslinking with BC nanofibers, the 3D chemically bonded structures can be formed via two approaches, i.e., (i) intercalating GO sheets with BC fibers in vertical direction (Figure 1c), and (ii) covalently bonding parallel-aligned GO sheets with BC networks (Figure 4a).25 The BC/GO composite with cross-linking structure in vertical direction and interconnecting linking between parallel-aligning GO sheets could be a promising scaffold for flexible matrix for new electronics. The GO nanosheets (Figure S2a) were intercalated with BC nanofibers (Figure S2b) to create loosely-stacked structure with sufficient void spaces for ion transport. Featuring an average diameter ~35 nm and a length up to dozens of micrometers, BC fibers assembled on GO surfaces to form a criss-crossed network (Figures 4b,c). Upon coating with PPy, core-sheath structured PPy/BC fibers were formed and the smooth surfaces of GO sheets were homogeneous covered with PPy layers. The lattice structure can be clearly observed to be formed by coating PPy onto parallel-aligned GO/BC sheets (Figures 4d, j-l) and GO sheets intercalated with PPy/BC in vertical direction were also seen directly (Figures 4e).The as-prepared PPy/BC/GO composites feature multi-layer stacking structure (Figures 4g-i, Figure S3). It should be noted that the aggregation of PPy (Figures S2c,d) can be effectively restrained when wrapping PPy onto BC/GO sheets form structurally-ordered PBG10:1 composite. Such 3D cross-linked PBG composites with conductive PPy wrapping structure would be promising for energy storage.



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Figure 4. SEM images showing the cross-sectional view of the covalent bonding of parallel-aligned GO sheets with BC networks (a). TEM images of BC/GO composites (b,c). SEM image showing (d) PPy/BC deposited on parallel-aligned GO sheets, (e) PPy/BC deposited on GO sheets in vertical direction, (f) single layer of PPy/BC/GO hybrid. SEM images showing multi-layers stacked structure of PBG10:1 composite in different magnifications (g,h,i), with red square showing the multi-layer stacking structure. TEM images of PBG10:1 composites (j,k), zooming-in-view of the selected region shown in (l).The yellow arrows point to GO sheets, the red points to BC nanofibers, while the white to PPy/BC core-sheath structures and the blue to PPy/BC/GO wrapping structured composite.

Electrochemical performance of PPy/BC/GO hybrid in three-electrode asymmetric supercapacaitors. The electrochemical properties of chemically bonded PBG films were compared with those of BC/GO and PPy films. The advantage of PBG composites over BC/GO and PPy is the formation of complementary cross-linking between graphene layers and BC fibers, which hampers the restacking among GO sheets and aggregation of PPy during the film fabrication process. The supercapacitive performance PBG hybrid was evaluated with galvanostatic charge/discharge profiles and cyclic voltammetry (CV) in 1 M H2SO4 (Figure S4). Typical charge/discharge curves of working electrodes based on PBG hybrids with different PPy contents were recorded at 0.4 A g-1 current density (Figure 5a). The slightly distorted triangular charge/discharge profile of BC/GO resembles the typical shape of EDLCs with good performance. For PBG hybrids, however, the curves were distorted from a symmetrical triangle shape, which is due to the existence of pseudocapacitance for PPy and the residual oxygen functional groups for GO, and a plateau region was observed at the end of charging curves, indicating the formation of pseudocapacitance. This pseudocapacitance was attributed to the

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redox of N-group in the PPy chain. The specific capacitance (Cs) could be calculated as Cs = (I×∆t)/(∆V×m), where I is the applied current (A), ∆t is the discharge time (s), ∆V is the potential window (1 V), and m the mass of active material (g). The PBG composites were found to exhibit much higher Cs than BC/GO and PPy, owing to the synergetic effect between BC/GO and PPy. For PBG hybrid electrodes, their Cs values increased with PPy content before reaching a maximum value for PBG10:1 composite, and then decreased with further increment of PPy in the composites. This behaviour agreed well with the electrical conductivity of PBG composites. The Cs of PBG10:1 composite was thus calculated to be 556 F g-1, which is 248% and 134% higher than that of BC/GO (160 F g-1) and pure PPy (238 F g-1). The theoretical specific capacitance (Ct) of PBG10:1 composite without considering the synergetic effect between BC/GO and PPy can be calculated according to Ct = CBC/GO×RBC/GO+ CPPy×RPPy,18 where C is the specific capacitance of BC/GO or PPy, and R is the mass ratio of BC/GO or PPy. The Ct of PBG10:1 composite is calculated to be ~229.6 F g-1 at 0.4 A g-1 current density. The enhanced specific capacitance (∆C=Cs-Ct,) was therefore calculated to be 326.4 F g-1. The calculated ∆C, higher than the Cs of BC/GO or PPy, reaches as high as 58.7% of that of PBG10:1. This enhancement is attributed to the synergetic effect between BC/GO and PPy, which may originate from the morphological improvement and the combination of EDLC and pseudocapacitance.18 The charge/discharge behaviour of PBG10:1 was further evaluated by gradually decreasing current density from 2.0 to 0.4 A g-1 at 0.4 A g-1 interval (Figure 5b). The almost symmetric charge/discharge profiles of PBG10:1 at high current density (ca 2.0 A g-1) are similar to typical highefficiency EDLCs. With the decrease of current density, the pseudocapacitance characteristics of PBG10:1 electrode were more clearly displayed. The Cs value of PBG10:1 composite was found to increase from 492 to 556 F g-1 with current density decreasing from 2.0 to 0.4 A g-1. The Cs of PBG10:1 composite is also much higher than that of literature reported carbaceous materials or BC based active materials (Table S2), such as PPy/BC composite (242 F g-1),8 BC/PPy/MWCNT (216.4 F g-1),23 N,P-CNF (204.9 F g-1),27 GN-PPy/CNT (211 F g-1),14 PPy/rGO (284 F g-1),34 GN/PPy (237 F g-1),35 and p-BC@MnO2 (256.7 F g-1).36 The enhanced Cs may be attributed to its structurally-defined architecture and the synergistic effects among the three components: (i) the hybrid structure with graphene intercalated by PPy-BC core-sheath layers created high electrode/electrolyte contact areas to enhance the electrochemical performance; (ii) PPy remarkably increased the pseudocapacitance of the cells; (iii) the chemically bonded structure efficiently restrained both the restacking of GO sheets and aggregation of PPy to form good interpenetrating networks. The PBG10:1 composite electrode exhibit good capacitance retentions against current densities (Figure 5c), where ~86.2% of its Cs was retained when the current density increased from 0.4 to 2.8 A g-1. The BC/GO and PPy electrodes, however, displayed only 30% and 40% retention of Cs as current density increased. This high capacitive retention over current density for PBG10:1 hybrid electrode indicated the sufficient exchange of ions between electrode and


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electrolyte even at increased current densities. The PBG10:1 composite electrode exhibited nearly 300 F g-1 higher than that of pure PPy and 400 F g-1 higher than BC/GO electrodes at the same current density (ranging from 0.4 to 2.8 A g-1), which is ascribed to the synergetic contribution of BC/GO and PPy. The capacitance retention of PBG10:1 composite electrode was further evaluated for its cycling stability by increasing current densities stepwise from 0.4 to 2.0 A g-1 at a 10-cycle interval (Figure 5d). The specific capacitance (556 F g-1) of PBG10:1 composite electrode steadily maintained at 0.4 A g-1 after 10 cycles. With current density increased stepwise to 2.0 A g-1, the hybrid electrode retained its capacitance after 10 cycles at every current density. When the current density switched back to 0.4 A g-1, PBG10:1 composite electrode still retained 99.1% of its initial Cs, indicating the good reversibility of our electrode during the charge/discharge process at varied current densities. Cycling stability at constant current density is another key consideration for supercapacitors in practical application. When the capacitors were charged and discharged for 5000 cycles at 0.4 A g-1, PBG10:1 electrode retains 95.2% of its initial capacity (Figure 5e), indicating outstanding reversibility in long-cycle charge/discharge process. The chemical bonded BC/GO also exhibits good capacitive retention (90.4%) in comparison to PPy (60.9% retention). The excellent long-term cycling stability of PBG10:1 hybrid electrode is attributed to its well-designed 3D interconnected network, where the electroactive GO layers were vertically well intercalated and chemically bonded with parallel-aligning GO sheets through PPy-wrapped BC fibers. The IPN structure protected both PPy conductive layers and BC/GO framework from collapse during repeated doping/dedoping processes, resulting in robust and enhanced electrochemical properties. The electrolyte diffusion characteristics within the hybrid electrode were evaluated by electrochemical impedance spectra (EIS) at low frequency.37 Typical Nyquist plots (Figure 5f) were recorded from 10 mHz to 100 kHz with 5 mV voltage input. The equivalent circuit of series components was simulated to EIS data shown in inset (ii), including the bulk solution resistance of the electrolyte Rs located at the semicircle with the high frequency intercept, the pseudocapacitative element Cp of PPy from the redox process, the pore resistance R1 which is probably due to the formation of ion transfer paths across the IPN. The second capacitance Cdl, which is the double layer capacitance, is parallel to the charge transfer resistance (Rct) connected to the semicircle. And in series to Rct, Warburg impedance (W) related to the sloping line reflects ion diffusion/transport. The electrochemical series resistance (ESR), estimated from the intercept of low frequency EIS with Z′ axis, was measured to be 68, 1.2, 17.2 and 9.4 Ω for PPy, GO, BC/GO and PBG10:1 electrode, respectively. The dramatically lower ESR of PBG10:1 than that of PPy and BC/GO is attributed to its conductive network to facilitate efficient electrons transfer. The ideal capacitor is characteristic for its perpendicular impedance spectrum to the real axis at low frequency.38 The nearly vertical impedance spectrum of PBG10:1 electrode confirms the fluent ion transport via the interpenetrating network (IPN) in chemically bonded PPy/BC/GO composite.

Figure 5. Electrochemical performance of various electrodes in three-electrode asymmetric supercapacitors: (a) Galvanostatic charge/discharge curves of PPy, BC/GO and PBG electrodes at 0.4 A g-1. (b) Galvanostatic charge/discharge curves of PBG10:1 electrode at varied current densities. (c) Specific capacitance of PPy, BC/GO and PBG10:1 within 0.4~2.8 A g-1 current density range. (d) Capacitance retention of PBG10:1 against current density. (e) Recycling performance of PPy, BC/GO and PBG10:1 electrode. (f) Nyquist impedance plots of GO, PPy, BC/GO and PBG10:1 electrode, with the inset showing (i) the zoom-in of the intersection part and (ii) equivalent electrical circuit from EIS data for PBG10:1.

Electrochemical performance of PPy/BC/GO hybrid in two-electrode symmetric supercapacaitor. The electrochemical performance of PBG10:1 nanocomposite has also been evaluated in two-electrode symmetric supercapacitors (Figure 6). The CV, charge/dischage and EIS were carried out in a wider potential range of -0.2 to 1.2 V in 1 M H2SO4. In symmetric supercapacitors, the specific capacitance (Cs’) of the electrodes was calculated by Cs’ = 2(I×∆t)/(∆V×m). As shown in Figure 6a, the PBG10:1 electrode exhibited more symmetric charge/discharge curves than those for three-electrode system in current density ranging from 0.625 to 5.0 A g-1, especially at higher current densities (ca. 2.5 A g-1). The Cs’ value of PBG10:1 composite was accordingly calculated to be 486, 426, 364, 322 and 286 F g-1 at current density of 0.625, 1.25, 2.5, 3.75 and 5.0 A g-1, respectively (Figure 6b). The Cs’ of PBG10:1 composite is much higher than that of other reported carbaceous materials and BC derived composites in symmetric supercapacitors (Table S2),34,39-43 owing to the structurally-defined architecture and the synergistic effects among the three components. Although the specific capacitance values of PBG10:1 in symmetric supercapacitors were lower than their counterpart in asymmetric ones, the two-electrode system are advantageous to possess wider potential window (-0.2 to 1.2 V). The PBG10:1 hybrid electrode exhibits higher ESR (16.4 Ω) in symmetric supercapacitors (Figure 6c) in comparison to asymmetric ones. The excellent electrochemical stability of PBG10:1 electrode was also demonstrated symmetric super-



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capacitors, where a high capacitance retention of 93.5% was observed after 2000 cycles at 0.625 A g-1 (Figure 6d), indicating its outstanding reversibility for long-cycle charge/discharge applications. The overall performance of PBG10:1 electrode was further compared with that of GO and PG10:1 using energy density (E) and power density (P).44 With the applied current increased from 1 to 5 mA, all three electrodes exhibited continuously increased power density but decreased energy density (Supplementary Table S3). Ragone plots of supercacitors (Figure 7a) revealed PBG10:1 exhibited much higher E values than GO and PPy/GO10:1 in asymmetric cell under the same power density. PBG10:1 afforded the highest energy density of 77.2 Wh kg-1 at 200.1 W kg-1 power density (Table S2), which is higher than that of PG10:1 (54 Wh kg-1), GO (11.1 Wh kg-1), PPy/BC membrane (39.7 Wh kg-1).45 An energy density of 68.3 Wh kg-1 still remained even at 1 kW kg-1 power density.

Figure 6. (a) Galvanostatic charge/discharge curves of PBG10:1 electrode at varied current densities in two electrode asymmetric coin cells. (b) Specific capacitance of PBG10:1 within 0.5~7.5 A g-1 current density range. (c) Nyquist impedance plots of PBG10:1 electrode, with the inset showing the zoom-in of the intersection part. (d) Recycling performance of PBG10:1 electrode, with the inset showing the structure of two-electrode symmetric supercapacitors.

Regardless the focus of gravimetric capacitance for electrodes in most literature, the volumetric capacitance (Cv) is a key consideration in the design of these electrodes for practical application. Based on the thickness (50 µm) of PBG10:1 composite films, the volume of PBG10:1 electrode was calculated with an average density of 0.5 g cm-3. According to the equation: Cv = ρCs, the Cv of this electrode was thus calculated to be 278 F cm-3 at 0.4 A g-1 (246 F cm-3 at 2.0 A g-1) in asymmetric supercapacitors, which is among the highest values ever reported for carbon-based electrodes (Table S2), such as graphene-cellulose paper (120 F cm-3),46 PNG (198 F cm-3)22 and GN-PPy/CNT (122 F cm-3 at 0.2 A g-1).14 The Cv of PBG10:1 electrode as high as 243 F cm-3 in symmetric supercapacitors is also superior to those of carbon-based electrodes in literature. Such performance may be attributed to the effective conduction pathway provided by PPy and GO,

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as well as the increased electroactive surface area with suppressed restacking of GO layers and aggregation of PPy.

Figure 7. (a) Ragone plots showing the energy density and power density for PBG10:1, PPy/GO (10:1) and GO in H2SO4. (b) Comparison of the volumertic and gravimetric capacitance of PBG10:1 electrode with other carbon and graphene electrodes. Data for other carboneous electrodes are adapted from literature.

Since the most critical criterion for producing high power and energy supercapacitors is to construct high-density carbonaceous materials with optimum structure. Tremendous research has focused on graphitic porous materials with varied densities to achieve large gravimetric capacitors. The volumetric and gravimetric capacitances of our electrode are thus compared with different carbonaceous supercapacitor electrodes in literature (Figure 7b). The as-developed PBG10:1 material exhibited a good combination of Cv and Cs in comparison to the other carbonaceous materials. For instance, graphene foam or hydrogel show rather high Cs but lowest Cv due to their low density (ca. 0.01-0.03 g cm-3).8-10 The r[GO/CNT], however, shows high Cv due to a high density (1.5 g cm-3).13 The crumpled or wrinkled graphene present low Cv of 59 and 46 F cm-3, albeit at the same density (0.5 g cm-3) as PBG10:1 hybrid11. The AC shared similar Cv and Cs with crumpled graphene, which is the dominant material electrode material for EDLC.13 Hence the chemicallybonded material as developed in the current work can be used as a good alternative for practical energy storage devices.47 CONCLUSION We have demonstrated the development of 3D crosslinked PPy/BC/GO composites with controlled layered structure for high-energy and high-power electrodes. The structurally well-defined hierarchical composites harvested an electrical conductivity as high as 1320 S m-1. Significantly, the composite electrode exhibited a mass-specific capacitance of 556 F g-1 in asymmetric supercapacitor with a capacitance retention of 95.2% over 5000 cycling. In twoelectrode symmetric supercapacitors, a mass-specific capacitance as high as 486 F g-1 was achieved together with a capacitance retention of 93.5% over 2000 cycling. Impressively, the composite electrode afforded a volumetric capacitance of 278 F cm-3 in asymmetric supercapacitors and 243 F cm-3 in symmetric supercapacitors. In a word, the wellcontrolled nanostructure and efficient composition of PBG composites played a crucial role for the excellent performance of these three-dimensional, chemically bonded hybrid composite electrodes. The great potential of our asdeveloped chemically-bonded PPy/BC/graphene hybrid in supercapacitors may open an avenue in the facile development of new biomass-derived renewable and sustainable


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electrode materials for flexible energy storage devices.

ASSOCIATED CONTENT

EXPERIMENTAL SECTION

Supporting Information. Experimental details including synthesis and characterization of the 3D chemically bonded PBG composites, data summary of composites compositions and electrochemical performance of the reported carbaneous material based electrodes in literature and our electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

Synthesis of 3D Cross-linked PBG Composites. The covalent intercalated BC/GO composite was prepared via a facile one-step esterification of GO and BC under nitrogen atmosphere at 70oC to create the cross-linking BC/GO networks. The as-obtained BC/GO composite was ultrasonicated in deionized water to get homogeneous BC/GO suspension. Inside BC/GO suspension, an in-situ oxidative polymerization of Py was performed to prepare the 3D cross-lined PPy/BC/GO (PBG) composites featuring PPy coating as conductive networks. PBG composites with different mass ratios were prepared. The weight feeding ratio of pyrrole to BC/GO was varied as 1:1, 5:1, 10:1, 15:1 and 20:1, and the resulting composites were designated as PBG1:1, PBG5:1, PBG10:1, PBG15:1 and PBG20:1, respectively. Additional details on the preparation and elemental analysis of various composites are provided in the Supplementary Information. Characterization. Chemical structure of the hybrid materials were investigated using Raman spectroscopy (RM 2000 microscopic confocal Raman spectrometer) employing a 514 nm laser beam and X-ray diffraction spectroscopy (Bruker D8 Advance diffractometer) with Cu Kα radiation (λ ≈ 1.54 Å) at 40 kV and 30 mA. Morphology of the electrode materials was observed using scanning electronic microscopy (JEOL JSM-6380LV) at an accelerating voltage of 30 kV and transmission electronic microscopy (JEOL JEM-2100) at an accelerating voltage of 120 kV. For identifying the surface chemistry of hybrid electrode, X-ray photoelectron spectrometer (Thermo ESCALAB 250) was employed by the use of Al Kα radiation (hν ≈ 1486.7 eV) as the excitation source. Conductivity Measurement. The conductivity was measured with a four-point probe technique (RTS-8, Probes Tech. Co., China) at ambient temperature (20 ± 1oC, 67% humidity). The flat surfaces of samples were prepared by pressing the composite at 15 KPa. The resistivity of sample can be calculated with ρ = 2πS (V / I ) , where S is the probe spacing (mm), I the supplied current (mA), and the corresponding voltage V (mV). Conductivity can be resulted using σ = 1/ρ. Electrochemical measurement. Electrochemical measurements were carried out on a CHI660D electrochemical workstation (Shanghai, China). For threeelectrode asymmetric cells, a platinum foil and a saturated calomel electrode (SCE, Ag/AgCl) were used as the counter and reference electrode, respectively. The working electrode was fabricated with as-prepared carbaceous materials. Typically, the composites (~ 2.0 mg) was cut out pieces and mixed with 15 wt% acetylene blank (0.45 mg) and 5 wt% polytetrafluoroethylene (PTFE, 0.15 mg) to dissolve in Nmethyl-2-pyrrolidone form a slurry. The slurry was then cast on a porous stainless steel substrate with a 1 cm2 geometric area, followed by pressing and drying under vacuum at 50°C for 12 h. For symmetric cells, two symmetric working electrodes with the same weight of PBG10:1 (1.6 mg) were pressed together separated by a Whatman filter paper to form a sandwich structure.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledged the financial support from The authors would like to acknowledge the Program for New Century Excellent Talents in University (NCET-12-0633), Jiangsu Province Natural Science Fund for Distinguished Young scholars (BK20130032), Doctoral Fund of Ministry of Education of China (No. 20103219120008), the Fundamental Research Funds for the Central Universities (30920130111006), and A Project Funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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Table of Contents

9 ACS Paragon Plus Environment


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1x1mm (600 x 600 DPI)


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Three-Dimensional, Chemically Bonded ... - ACS Publications

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