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Yang Zhao , Jia Liu , Yue Hu , Huhu Cheng , Chuangang Hu , Changcheng Jiang , Lan Jiang , Anyuan Cao , and Liangti Qu *

Highly Compression-Tolerant Supercapacitor Based on Polypyrrole-mediated Graphene Foam Electrodes

ON

Deformation-tolerant electronic devices have attracted tremen-dous attention due to their fl exibility of integration into uncon-ventional forms of high-tech electronics while maintaining the desired levels of performance and reliability. [ 1 ] As an indispen-sable component of advanced electronics, stretchable power-source devices have been able to accommodate large strains without obvious loss of functions. [ 2 , 3 ] Of various power-source devices, supercapacitors have attracted signifi cant interest as energy storage devices due to their high power density, long cycling life, and short charging time. These advantages make them highly promising for use in electric vehicles and other high power energy sources. [ 4 , 5 ] Towards the development of lightweight, fl exible, and wearable-electronic devices, [ 6 , 7 ] achievements have been made to the fabrication of stretchable supercapacitors, [ 8 , 9 ] which possess high electrochemical capaci-tance performances even during mechanical stretch. Beyond the continuous efforts for enhancing the energy/power density of supercapacitors, however, few attempts have been dedicated to the development of compressible supercapacitors, [ 10 ] although the compression, a reverse mechanical stress in contrary to the stretching process, is one of the most infl uential factors on the electrochemical performance of the fl exible supercapacitors.

Foam-like structures with high porosity, fl exibility, and robust-ness under mechanical strain/stress have many important applications in actuators, catalytic supports, adsorption and separation, [ 11–15 ] which also have the potential as highly compres-sion-tolerant electrode materials for fabrication of compressible supercapacitors. However, it is of great challenge to synthesize porous foam-like structures with a high fl exibility, robustness and conductivity. Recently, carbon-based materials have shown potential as sponge-like structures with high porosity, structural fl exibility and large deformation. [ 11 , 15–18 ] As a two-dimensional monolayer of carbon sheet, graphene has fascinating proper-ties such as large surface areas, giant electron mobility, [ 19 ] high

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Y. Zhao, J. Liu, Y. Hu, H. Cheng, C. Hu, C. Jiang, Prof. L. QuKey Laboratory of Cluster ScienceMinistry of Education, School of ChemistryBeijing Institute of Technology, Beijing 100081, China E-mail: lqu@bit.edu.cn Prof. L. JiangLaser Micro-/Nano-Fabrication LaboratorySchool of Mechanical Engineering, Beijing Institute of TechnologyBeijing 100081, China Prof. A. CaoDepartment of Materials Science and EngineeringCollege of Engineering, Peking UniversityBeijing 100871, China

DOI: 10.1002/adma.201203578

Adv. Mater. 2013, 25, 591–595

thermal conductivity, [ 20 ] extraordinary elasticity and stiffness, [ 21 ] and has therefore been regarded as the ideal building blocks for the fabrication of 3D porous low-density macroassemblies with high electrical conductivities and large internal surface areas. [ 22–27 ] Unfortunately, the pristine 3D graphene structure tends to collapse or distortion under compression due to the rela-tively poor compressibility and springiness. As a result, although graphene has been integrated into the fabrication of capacitor electrodes, [ 28–30 ] there is still no report on the compressible super-capacitors based on the graphene electrodes. Herein, by using a representative conducting polymer of polypyrrole (PPy) as a mediator, we have developed a unique strategy for in-situ forma-tion of PPy-graphene (PPy-G) foam and demonstrated a highly compression-tolerant graphene-based supercapacitor.

PPy is one of the most important active materials for super-capacitors due to its good conductivity, facile synthesis, low cost, stability and the high redox pseudocapacitive charge storage. [ 31–34 ] For formation of in-situ polymerized PPy-G foam, we fi rst prepare the 3D graphene by hydrothermal treatment of 2 mg/mL of homogeneous graphene oxide (GO) aqueous dis-persion with 5 vol% pyrrole (Py) in a 10mL Tefl on-lined auto-clave at 180 ° C for several hours. The 3D graphene formed in this pyrrole-containing GO suspension is denoted as 3D G(Py). The hydrothermally produced 3D G(Py) subsequently acted as the working electrode in a three-electrode cell with a Pt wire as counter electrode and Ag/AgCl (3 M KCl) as reference elec-trode. After a constant potential of 0.8V was applied in 0.2 M NaClO 4 aqueous solution for polymerization of Py monomer, the 3D PPy-G foam was obtained ( Figure 1 a).

The key step to 3D PPy-G foam is to introduce the Py mon-omer into the GO aqueous suspension and form a homogeneous solution prior to the hydrothermal process. This strategy will allow the formation of 3D PPy-G framework with maximized surface areas and excellent mechanical property under compres-sion. As shown in Figure 1 a, the 3D graphene derived from Py-containing GO suspension has a 5 times volume of that formed from normal pyrrole-free GO suspension under the same condi-tion. The corresponding SEM investigation revealed that the 3D G(Py) had a looser structure with much thinner connection walls than the pure 3D graphene derived from normal GO suspen-sion (Figure 1 b,c). Consequently, the freeze-dried 3D G(Py) has a much lower density of 5–8mg/cm 3 compared with that 15–20 mg/cm 3 for freeze-dried normal 3D graphene (Figure 1 a, left). Brunauer-Emmett-Teller (BET) analyses based on low-temper-ature nitrogen adsorption measurements show that the freeze-dried 3D G (Py) has a specifi c surface area (SSA) of 463 m 2 /g, considerably larger than 186 m 2 /g of freeze-dried normal 3D graphene (Figure 1 a, left) and the BET SSA of 166 m 2 /g for freeze-dried graphene hydrogel. [ 35 ] 3D graphene framework

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Figure 1 . (a) Photographs of as-prepared normal 3D graphene, 3D G (Py) and 3D PPy-G (from left to right). The normal 3D graphene and 3D G (Py) were prepared by hydrothermal treatment of 2.0mg/ml GO aqueous sus-pension without and with a 5 vol% Py monomer, respectively. (b,c) SEM images of normal 3D graphene and 3D G(Py), respectively. (d) SEM image of normal 3D grapnene (b) after electrodeposition of PPy in the NaClO 4 electrolyte containing 5 vol% Py. (e) SEM image of 3D PPy-G. (f,g) The corresponding magnifi ed view of (d) and (e), respectively. (h) TEM image of PPy-G sheet from 3D PPy-G and (i) the corresponding high-resolution TEM image of the edge of PPy-G sheet. Inset in (h) is the selected area electron diffraction (SAED) pattern. Scale bars: b–g, 1 μ m.

Figure 2 . Raman spectra of the freeze-dried normal 3D graphene, 3D G(Py), 3D PPy-G and electropolymerized PPy fi lm.

is assembled by the strong interactions of reduced GO sheets during hydrothermal treatment. [ 36 ] The Py monomer has a typ-ical conjugated structure with electron-rich N atom, which may easily attach on the surfaces and galleries of GO sheets through H-bonding or π – π interaction. [ 37 ] Therefore, the existence of Py will effectively prevent the self-stacked behavior of GO during hydrothermal process, and accordingly increase the available GO sheets for forming large volume of 3D graphene with thin con-nection walls. Indeed, further control experiments confi rm that a

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certain amount of added Py in the low-concentration GO suspen-sion (e.g., 0.5mg/ml) will lead to the production of 3D graphene, which otherwise is not available at all (Figure S1).

To form a 3D porous PPy-G foam, the as-prepared 3D G(Py) sample was directly used as working electrode under a poten-tial of 0.8V in 0.2 M NaClO 4 aqueous solution without addi-tional Py monomer. The uniform polymerization of Py along graphene sheets was observed in Figures 1 e, g and h. The 3D porous structure is maintained well with a slightly increased wall thickness and density (ca. 40 mg/cm 3 ) due to the PPy coating (Figure 1 e). The SSA of the freeze-dried 3D PPy-G is measured to be 144 m 2 /g, which is lower than that of 3D G(Py) but still close to that of normal 3D graphene. In contrast, the polymerization of additional Py along normal 3D graphene structure will cause the serious aggregations of PPy and the sur-face porosity loss of 3D graphene (Figure 1 d,f). Figures 1 h and i show the TEM images of PPy-G sheets. As can be seen, PPy layers are uniformly coated along graphene sheets (Figure 1 i), and the electron diffraction spots of graphenes (Figure S2) have disappeared (Figure 1 h, inset). The high resolution TEM image exhibits direct evidence that a few layers of graphenes are inter-calated within the electrodeposited PPy layers (Figure 1 i). The freeze-dried PPy-G foam has an electrical conductivity of about 3 × 10 3 S/m by a four-probe method, which is 2 orders of mag-nitude higher than 3D graphene-based composites. [ 24 ]

Figure 2 shows the Raman spectra of 3D PPy-G in comparison with normal 3D graphene, 3D G(Py) and electropolymerized PPy

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Figure 3 . (a–c) The compression-recovery processes of PPy-G foam. (d–f) The surface views of PPy-G foam corresponding to the unloading-loading-unloading status in (a–c), respectively. (g–i) SEM images of the inner microstructures of PPy-G foam corresponding to the unloading-loading-unloading status in (a–c), respectively. (j) Stress-strain curves of PPy-G foam immersed in aqueous liquid at different set strains ( ε ) of 40%, 60%, and 80%, respectively. (k) Cyclic stress-strains curves of PPy-G foam at ε = 50%. Inset is the recorded deformation developed by compression for 200 cycles at ε = 50%.

fi lm. As can be seen, the 3D G(Py) repre-sents two typical bands indexed at ∼ 1334 and ∼ 1597 cm − 1 (Figure 2 b), which are attributed to the well-documented D and G bands of hydrothermally reduced GO (Figure 2 a). [ 28 ] After undergoing electropolymerization, the formed 3D PPy-G (Figure 2 c) exhibits a series of peaks at ca. 930, 1070, 1238, 1370, 1410 and 1597 cm − 1 , characteristic of elec-tropolymerized PPy (Figure 2 d), [ 38 ] indicating the successful polymerization of Py along graphene sheets. It is notable that no any perceptible peaks associated with PPy were observed for 3D G(Py) sample in Raman spectrum (Figure 2 b), suggesting that the content of PPy produced by polymerization of Py monomer during the hydrothermal process, if any, is negligibly low, and Py mainly physically distributes within the 3D-G structure. Indeed, FT-IR spectra verifi ed that the hydrothermally synthesized 3D G(Py) sample mainly contained the Py monomer rather than polymerized Py (Figure S3).

The as-prepared 3D PPy-G foam sucks up the aqueous liquid and exists in the fashion of hydrogel. The conventional freeze-drying treatment will impair its mechanic elasticity (Figure S4), while naturally drying process will cause the drastic shrinkage of 3D PPy-G (Figure S5). Although PPy-G is not elastic under dry condition, both the morphology and mechanic fl exiblity of 3D PPy-G remains well under the aqueous circumstance (Movie S1) and even organic solvents such as ethanol and acetonitrile (Figure S6). As a hydrogel, PPy-G has the capability of expansion/con-traction upon absorption/desorption of sol-vent. As shown in Figure 3 a–c, PPy-G foam can sustain large-strain deformations (e.g., ε = 50%) under manual compression and recover most of the material volume without structural fatigue within 10 s. During the

compression process, we found that the zigzag buckles without cracks were formed along the PPy-G body (Figure 3 e), while the zigzag deformation was entirely disappeared upon release of the load (Figures 3 d&f). This reversible compression-recovery process is also refl ected by the inner microstructure of PPy-G foam. As can be seen, the SEM images of the freeze-dried samples reveal that the initial 3D structure (Figure 3 g) is con-formably densifi ed while maintaining the porous confi guration under compression (Figure 3 h). Once released, PPy-G foam instantaneously recovers to this initial state without any col-lapse of 3D network within 10 seconds (Figure 3 i). BET meas-urements reveal that the freeze-dried compressive PPy-G foam has a SSA of 151 m 2 /g similar to that of unloaded counter-parts (144 m 2 /g), which indicates no signifi cantly irreversible stacking occurs between graphene sheets network. This com-pression to self-recovery process is fully reversible as long as the PPy-G foam is immersed in aqueous liquid (Movie S2).

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Compression tests of several samples at differently set strains (40% to 80%) show reproducible results in which all curves contain an approximate linear region at initial ε < 40% and an increasing slope until very high strains up to 80% (Figure 3 j). The unloading curves almost return to the origin, indicating complete volume recovery without plastic deformations. The large compression on the PPy-G foam is due to the squeeze of inherent pore-rich 3D structures, rather than the formation of heavy stacking of graphene sheets (Figure 3 h). The PPy-G foam can be further compressed to more than 90% volume reduc-tion due to its high porosity and structural fl exibility. During repeated compressions at the set strain ε = 50%, the foam can recover most of thickness and maintain a similar compres-sive stress (ca. 0.088MPa) without degradation of mechanical strength (Figure 3 k). The plastic deformation is measured to be smaller than 10% (of volume reduction) after compres-sions at a set strain of ε = 50% for 200 cycles (Figure 3 k, inset).

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Figure 4 . (a) Schematic illustration of PPy-G based supercapacitor devices. (b) CVs of the compressible supercapacitor cells based on PPy-G foam electrodes under 0% and 50% compression for one cycle. The scan rate is 30 mV/s. (c) The corresponding galvanostatic charge-discharge curves at a current of 1.5 A g − 1 under 0% and 50% compression. (d) The specifi c capacitances at different compression states for 1000 cycles.

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Impressively, the PPy-G foam shows no signifi cant strength degradation after compression at the maximum strain ( ε = 50%) for 1000 cycles (Figure S7), indicating the highly revers-ible and durable foam behavior.

In contrast, the compressive properties of 3D pure graphene prepared by the similar hydrothermal method showed no obvious elastic properties, and its structure was destroyed without recovering phenomena after being compressed at ε = 50% (Figure S8a). Even post-synthesis polymerization of Py was applied onto 3D graphene structure, which could not exhibit the foam behavior (Figure S8b) due to the 3D pore loss and uneven deposition of PPy (Figure 1 d,f). On the other hand, the 3D graphene synthesized in the presence of Py but without elec-tropolymeriazation has also presented an irreversible compress-ibility (Figure S9). Further observation shows that, unlike that PPy-G foam having compressible properties (Figure 3 b), some cracks appear on the bodies of 3D pure graphene and 3D G(Py) (Figure S10). It seems that the highly uniform deposition of PPy layers along graphene sheets with well-defi ned 3D porous graphene structures is the key to achieve the compression-tol-erant foam feature. Apart from the contribution to the capaci-tance performance, the presence of conjugated polymer of PPy could increase the strength of 3D structure via strong π – π inter-action to bear a certain extra force as demonstrated here.

To explore the application of PPy-G foam as deformable electrode materials for compressible supercapacitor, we assem-bled a two-electrode capacitor with a controlled compression state ( Figure 4 a and Figure S11). The electrochemical perform-ances were evaluated by both cyclic voltammetry (CV) and

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galvanostatic charge/discharge tests in 3M NaClO 4 aqueous electrolyte. As shown in Figure 4 b, owing to the high porosity, conductivity, and excellent mechanical strength of PPy-G foam electrodes, the CV curves of the assembled capacitor retain a rectangular shape with ideal capacitive behavior. No signifi cant change was observed in the CVs of the compressed supercapac-itors with a 50% applied strain (Figure 4 b), which is also the case for 1000 measured cycles (Figure S12a). The cycling sta-bility of the assembled supercapacitor subjected to compressive strain is also illustrated by galvanostatic charge–discharge with a constant current of 1.5 A g − 1 (Figure 4 c and Figure S12b). The specifi c capacitance calculated from the discharge slopes is ca. 350 F/g, which is much higher than that of both compact PPy fi lm (ca. 50 F/g in Figure S13) and pure 3D graphene (e.g . , 151 F/g in Figure S14, and even 220F/g in alkaline medium [ 35 ] ). These results indicate the synergetic function of PPy and 3D graphene for largely enhanced capacitance of PPy-G. The den-sity of freeze-dried 3D PPy-G is about 40 mg/cm 3 . The volume-normalised capacitance is 14 F/cm 3 for the uncompressed sample and 28 F/cm 3 for the 50% compressed one, which is much higher than that ca. 4 F/cm 3 for the normal 3D graphene supercapacitor. [ 35 ] Remarkably, the specifi c capacitances of the supercapacitor with or without compressive strain of 50% do not alter signifi cantly even up to 1000 cycles (Figure 4 d), which concludes the excellent electrochemical stability of the com-pressible supercapacitor based on PPy-G foam electrodes. In fact, the compressible supercapacitor of PPy-G foam also per-forms properly in nonaqueous electrolytes as demonstrated in 1-butyl-3-methylimidazolium tetrafl uoroborate (BMIMBF 4 )

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[ 1 ] J. A. Rogers , T. Someya , Y. G. Huang , Science 2010 , 327 , 1603 . [ 2 ] Y. G. Sun , W. M. Choi , H. Q. Jiang , Y. Y. Huang , J. A. Rogers , Nat.

Nanotechnol. 2006 , 1 , 201 . [ 3 ] J. Yoon , A. J. Baca , S. I. Park , P. Elvikis , J. B. Geddes , L. F. Li ,

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medium (Figures S15 and S16). Since the compression-recovery process involves the electrolyte desorption and absorption, the compressible supercapacitor of PPy-G foam developed in this preliminary study requires a relatively large amount of solvent, which, to some extent, decreases the device performance con-sidering the device as a whole.

It is notable that hydrogels of other materials with viscoelastic properties were reported decades ago. [ 39 ] However, the current work develops a unique strategy to form a hydrogel that rationally combined 3D graphene with PPy to achieve a remarkable com-pression tolerance, and demonstrates the concept of compressible supercapacitor based on PPy-G foam for the fi rst time. Robust 3D PPy-G foam was formed by pre-mixing Py monomer with GO for hydrothermal production of Py-containing 3D graphene, followed by direct electropolymerization. The as-formed PPy-G foam is durably tolerant to the large compressive strain without any structural collapse and loss of springiness. We further dem-onstrate that the PPy-G foam can be used as the deformable electrodes for assembly of highly compression-tolerant superca-pacitors, which perform high specifi c capacitances without sig-nifi cant variation under long-term compressively loading and unloading process. This work will benefi t the development of next generation advanced supercapacitors with tolerance to harsh conditions such as mechanical concussion and compression.

Experimental Section Preparation of 3D PPy-G foam : GO suspension was obtained as

reported in our previous papers. [ 40 , 41 ] 3D G(Py) was prepared by hydrothermal reduction of GO aqueous dispersion with 5 vol% pyrrole monomer, which was directly used as working electrode under a potential of 0.8V in 0.2 M NaClO 4 aqueous solution to form a 3D porous PPy-G foam (Figure S17 and S18).

Preparation of compressible supercapacitors : Two slices of PPy-G foam with a thickness of about 2.1 mm were separated by a fi lter paper soaked with 3M NaClO 4 aqueous electrolyte. Two Au foils in contact with the PPy-G foams act as the current collectors. The compression process was controlled by electric-drive-motor with moving speed at 0.2 mm/s. The experimental setup was illustrated in Figure 4 a and Figure S11.

More experimental details and characterizations are included in the Supporting Information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We thank the fi nancial support from the 973 project (2011CB013000) of China and NSFC (21174019, 51161120361).

Received: August 28, 2012Published online: October 18, 2012

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