Superelastic Graphene Aerogel/Poly(3,4-Ethylenedioxythiophene)/MnO2 Composite as Compression-Tolerant Electrode for Electrochemical Capacitors

Ultra-compressible electrodes with high electrochemical performance, reversible compressibility and extreme durability are in high demand in compression-tolerant energy storage devices. Herein, an ultra-compressible ternary composite was synthesized by successively electrodepositing poly(3,4-ethylenedioxythiophene) (PEDOT) and MnO2 into the superelastic graphene aerogel (SEGA). In SEGA/PEDOT/MnO2 ternary composite, SEGA provides the compressible backbone and conductive network; MnO2 is mainly responsible for pseudo reactions; the middle PEDOT not only reduces the interface resistance between MnO2 and graphene, but also further reinforces the strength of graphene cellar walls. The synergistic effect of the three components in the ternary composite electrode leads to high electrochemical performances and good compression-tolerant ability. The gravimetric capacitance of the compressible ternary composite electrodes reaches 343 F g−1 and can retain 97% even at 95% compressive strain. And a volumetric capacitance of 147.4 F cm−3 is achieved, which is much higher than that of other graphene-based compressible electrodes. This value of volumetric capacitance can be preserved by 80% after 3500 charge/discharge cycles under various compression strains, indicating an extreme durability.


Introduction
Recently, the great development of wearable electronics not only enriches our daily lives, but also requires the matched energy storage devices, which should withstand various high-level strains, such as bending, stretching, and compression [1,2]. Electrochemical capacitors (ECs) outstand as a promising solution due to their high power density, rapid charge rate, and long cycle life [3]. Previous reports suggest that the electrode of ECs is one of the major limiting factors in maintaining the capacity performances under high-level strains [4,5]. In the past decade, there are plentiful studies and achievements for the functional electrodes of ECs with bendable and stretchable functions, paving the way for the application in strain-tolerant energy devices [6,7]. Nevertheless, design and fabrication of compression-tolerant electrodes of ECs rarely appears, although the compressive strain is one of the most important influences for the performances of the electrodes. However, most of the conventional electrodes decrease or lost their functions due to deformation or destruction of electrode material at high compression strain [8]. For the porous and low-density materials, their intrinsic electrical and transport properties are easy to deteriorate when they are deformed at high-level compressive strains. For the highly dense materials, it is difficult to offer continuous ion pathways and facile access to storage sites when they are highly compacted. Herein, we report a ternary composite electrode with both high capacitances and high compression-tolerant ability by electrodepositing poly (3,4-ethylenedioxythiophene) (PEDOT) and MnO 2 into the superelastic graphene aerogel (SEGA) (Figure 1). SEGA as the conductive backbone contributes its high compressibility to the electrode. PEDOT as the interface layer between graphene and MnO 2 can reduce their interface resistance. And the cell walls of SEGA can be reinforced by the PEDOT coating layer, leading to the further improvement of compressibility. MnO 2 deposited on the cell walls of SEGA/PEDOT produces high pseudocapacitance for the composite electrode. We tested the recoverable compressive strains and the durability of SEGA/PEDOT/MnO 2 composite by the compression/release measurements, and characterized its electrochemical performances using the three-electrode system. In addition, we also fabricated the all-solid-state ECs based-on SEGA/PEDOT/MnO 2 electrodes to demonstrate the change of the gravimetric capacitance and volumetric capacitance of the electrodes at various compressive strains.

Preparation of Ultra-Compressible SEGA/PEDOT/MnO 2 Composite
Graphene oxide (GO) was prepared according to the modified Hummers' method. The SEGA was firstly prepared by the ice template method [9]. In a typical synthesis procedure, GO aqueous dispersion (4 mg mL −1 , 12 mL) with 100 mg L-ascorbic acid was poured into a flat-bottomed glass test tube and heated for 20 min at 95 • C to obtain the partially reduced graphene hydrogel. Then the hydrogel was treated by the freeze-thaw process in the refrigerator (−23 • C) and room temperature. Subsequently, further reduction process for the freeze-recast hydrogel was performed for 6 h at 95 • C by the initial L-ascorbic acid. Finally, the completely reduced hydrogel was dried at 60 • C for 24 h to obtain the SEGA.
The electrochemical deposition of PEDOT into SEGA was performed by the galvanostatic method using a three-electrode system, where the SEGA was used as the working electrode, a platinum electrode as the counter electrode, and a Ag/AgCl electrode as the reference electrode. Aqueous solution consisting of 0.01 M 3,4-ethylenedioxythiophene and 0.005 M sodium dodecyl sulfate (surfactant) was used as the precursor solution. And the pH value of precursor solution was adjusted by slowly adding ρ-toluenesulfonic acid (dopant) to pH = 1. The deposition was performed at a current of 1 mA cm −2 for 2 min. After the working electrode was washed with distilled water and dried at 60 • C for 8 h, the SEGA/PEDOT binary composite was obtained.
MnO 2 was subsequently deposited into SEGA/PEDOT composite. The aqueous deposition bath containing 0.01 M KMnO 4 and 0.1 M LiClO 4 was dispersed adequately under ultrasonication prior to use. Electrochemical deposition was carried out using a three-electrode system, in which the SEGA/PEDOT acted as the working electrode, a Pt foil served as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The deposition process was performed by applying a constant reduction potential of −1.2 V vs. SCE for 20 min. After washing the working electrode with distilled water and drying at 60 • C for 12 h, the SEGA/PEDOT/MnO 2 ternary composite was obtained. For comparison, SEGA/MnO 2 composite was also prepared by the same deposition conditions.
The mass contents/mass loading of PEDOT and MnO 2 in the ternary composite calculated from the weight changes of samples before and after each deposition steps were 18.8 wt %/2.1 mg cm −2 (PEDOT) and 58.6 wt %/6.5 mg cm −2 (MnO 2 ), respectively (Table S1).

Characterizations
The chemical structure of the samples was investigated by Fourier transform infrared spectroscopy (FTIR, Nicolet 520, Thermo Scientific, Waltham, MA, USA), and X-ray photoelectron spectroscopy (XPS, PHI 1600 spectroscopy, PerkinElmer, Waltham, MA, USA). The crystal structure of the composites was characterized by X-ray diffraction (XRD, Philip X' Pert Pro MPP, PANalytical B.V., Almelo, The Netherlands) using a Cu-Kα radiation (λ = 1.5418 Å). The microstructure of the samples was observed by the scanning electron spectroscopy (SEM, S-4800, Hatchi, Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS, Hatchi, Tokyo, Japan). Compression/release measurements were performed on a single-column static instrument (model 5943, Instron, Norwood, MA, USA) using a 10 N load cell and strain control mode with a strain rate of 5% per second. During the measurements, the testing samples were placed between the two flat-surface compression stages of the instrument. The dimension of compression testing samples was 5 mm × 8 mm × 8 mm.
Electrochemical characterizations including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed using the CHI660E electrochemical workstation (CH Instruments Inc., Austin, TX, USA). The electrochemical measurements of individual electrode were performed in the three-electrode system with 1 M H 2 SO 4 aqueous electrolyte. The SEGA/PEDOT/MnO 2 composite, Pt wire and Ag/AgCl (purchased from CH Instruments Inc., Austin, TX, USA) were used as working electrode, counter electrode and reference electrode, respectively. The specific capacitance (C s ) was calculated from the GCD curves according to the following equation: where I is the constant discharge current, ∆t is the discharging time, m is the total mass of the working electrode (including the SEGA, PEDOT and MnO 2 ), ∆V is the voltage drop upon discharging.
To investigate the electrochemical performances of the SEGA/PEDOT/MnO 2 electrodes under compressive conditions, we assembled the compressible all-solid-state ECs [21][22][23]. Firstly, the PVA/H 2 SO 4 gel electrolyte was prepared by mixing PVA powder, H 2 SO 4 , and deionized water according to the mass ratio of 5:4:50. Then the mixture was stirred for 40 min at 80 • C to form a clear electrolyte. Secondly, SEGA/PEDOT/MnO 2 composites with a thickness of~5 mm were immersed into the PVA/H 2 SO 4 gel electrolyte for 10 min. After evaporating of excess water, the electrolyte adhered on the surface of cell walls of the ternary composite. Then two pieces of the composites two electrodes with similar weight were placed onto two poly(ethylene terephthalate) substrates with Au (~100 nm), respectively. One piece of porous separator (model 3501, Celgard, Charlotte, NC, USA) was also infiltrated with PVA/H 2 SO 4 gel electrolyte. The compressible all-solid-state ECs were obtained by assembling the as-prepared two electrodes with identical or very close weight sandwiched with the separator under pressure (~1 N). Finally, the device was kept at 45 • C for 24 h to remove excess water in the electrolyte.
The gravimetric capacitance (C g ) and volumetric capacitance (C vol ) of the SEGA/PEDOT/MnO 2 electrodes in the compressible ECs were calculated from the GCD curves using the following Equations (2) and (3): where I is the constant discharge current, ∆t is the discharging time, m is the total mass of two electrodes, ∆V is the voltage drop upon discharging, ρ is the density of the SEGA/PEDOT/MnO 2 composite under various compressive strains.

Results and Discussion
The composition of the SEGA/PEDOT composite was firstly studied by FTIR (Figure 2a). For PEDOT, the peak at 1352 cm −1 originates from C-C and C-C stretching vibrations of the quinoid structure in the thiophene ring, peaks at 1201 and 1083 cm −1 are attributed to C-O-C bond stretching, and peaks at 975 and 837 cm −1 are corresponding to the C-S bond in the thiophene ring [24,25]. And the spectrum of SEGA/PEDOT consists of similar functional groups present in PEDOT. But the peak corresponding to C=C and C-C stretching vibrations in thiophene ring is red-shifted to 1311 cm −1 , and the peaks corresponding to C-O-C bond stretching are red-shifted to 1180 and 1052 cm −1 , which is attributed to the π-π interactions between graphene and PEDOT [26]. In addition, the peak at 1738 cm −1 corresponding to -C=O/-COOH stretching peak of GO disappears, indicating that the reduction process removes most of the carbonyl groups. XPS was performed to characterize the chemical structure of SEGA/PEDOT/MnO 2 composite. Compared with the XPS spectra of SEGA, the full spectra of SEGA/PEDOT/MnO 2 composite reveals additional S 2p, S 2s, Mn 2p and Mn 3s peaks (Figure 2b), indicating the presence of PEDOT and MnO 2 . As shown in Figure S1, the Mn 2p region consists of a spin-orbit doublet of Mn 2p 3/2 with a binding energy of 653.5 eV and Mn 2p 1/2 with a binding energy of 642.1 eV, which are characteristic peaks of a mixed-valence manganese system (Mn 4+ and Mn 3+ ) [6,20]. The high-resolution XPS O 1s spectra (Figure 2c Figure 3 shows the SEM images of as-prepared SEGA. It can be seen that SEGA possesses the macro-porous, honeycomb-like and oriented cellular structure at both cross-section view and vertical-section view (Figure 3a,b). The graphene sheets are tightly packed and well oriented in parallel manner to build the cellar walls of SEGA (Figure 3c,d). As reported in the previous literatures [9,[32][33][34], the honeycomb-like and oriented cellar structure enhances the mechanical strength of the graphene cells and bring the graphene aerogel ultra-compressible ability. In addition, the cell dimension in SEGA is as large as hundreds of micrometers, which is in favor of the impregnation of precursor solution during the subsequent deposition processes. After the electrochemical deposition of PEDOT, the microstructure of the SEGA/PEDOT binary composite was observed. As shown in Figure 4a, the highly porous, honeycomb-like, and oriented cellular structure of SEGA is well inherited without any collapse after the deposition of PEDOT. As displayed in the SEM images at high magnification (Figure 4b), it can be found that PEDOT do not change the surface morphology of graphene cell walls significantly. And the thickness of the graphene cell walls is just slightly increased after the deposition of PEDOT, which is attributed to the relatively low mass content of PEDOT (18.8 wt %) in the composite. After electrochemical deposition of MnO 2 , the honeycomb-like and oriented cellular architecture of the SEGA is still preserved well (Figure 4c). It can be found that abundant pompon-like MnO 2 spheres attached on the cell walls of the ternary composite (Figure 4d), which is significantly different from the smooth surface of cell walls in SEGA (Figure 3d) and SEGA/PEDOT composite (Figure 4b). In the SEM images at high magnification ( Figure S2), we can see that the pompon-like spheres with diameter~0.5 µm are composed of a quantity of MnO 2 nanoflakes. These spheres are homogeneously distributed on the graphene cell walls even at the inner portion of the composite, which is attributed to that macroporous structure and large cell dimension of the composite enable the fast flux and uniform penetration of precursor solution into the interior zone. EDS mapping results of C, S and Mn (Figure 4e-g) also confirm the homogenous distribution of PEDOT and MnO 2 on the graphene cell walls.  Figure 5a. As reported in the previous literatures, graphene aerogel with honeycomb-like and oriented cellular structure can present ultra-compressible ability [9,16]. It can be found that, SEGA can bear the maximum compressive strain of 90% (stress = 54 kPa) without any plastic deformation (Figure 5a). After the deposition of PEDOT, the unloading curves of SEGA/PEDOT composite at high compressive strain also can return to origin without producing residual strain (Figure 5a). It is worth noting that the maximum recoverable strain of SEGA/PEDOT composite reaches 95% (stress = 113 kPa), which is higher than that of SEGA. And the compressive stress of SEGA/PEDOT composite is higher than that of SEGA at the same strains (Figure 5a). This phenomenon is attributed to the physical reinforcement of the graphene cell walls by the uniform coating of PEDOT. The PEDOT coating layers tightly adhere on the graphene cell walls due to the strong π-π interaction between PEDOT and graphene sheets. Upon loading, the load is effectively transferred between the graphene skeleton and the PEDOT coating layers. This unique structure can help to relax the local stress and dissipate the micro-crack energy. Similar mechanisms of 3D graphene reinforced by polymer have also been mentioned in previous literatures [35][36][37].
After the deposition of MnO 2 , SEGA/PEDOT/MnO 2 composite shows the similar stress/strain curve with SEGA/PEDOT composite (Figure 5a). And SEGA/MnO 2 also shows the similar stress/strain curves with SEGA (Figure 5a). This indicates that the deposition of MnO 2 does not significantly influence the mechanical performances of the backbones. As shown in the real-time photos of the compression-recovery process of SEGA/PEDOT/MnO 2 composite (Figure 5b), the ternary composite can be squeezed into a pellet under manual compression and recover the volume without structural fatigue. This compression-tolerant ability of the SEGA/PEDOT/MnO 2 composite is also reflected by the inner microstructure of the composite. As can be seen from Figure S3a, the initial ordered cellar structure is conformably densified while keeping the continuous configuration under compression. Once released, the SEGA/PEDOT/MnO 2 composite rapidly recovers to the initial state without any collapse of the ordered cellar structure ( Figure S3b). And MnO 2 spheres are still tightly attached on the cell wall surface of SEGA/PEDOT without obvious peel-off after the compression-release process (Figure 3c,d). The cycle stability of the compressible ability of SEGA/PEDOT/MnO 2 composite was also studied. As shown in Figure 5c, the first compression cycle is different from the subsequent ones showing the higher stress at the same strains. The hysteresis loop for the 10th cycle shrinks significantly compared to the first one. Since the 10th cycle, the shrink of the stress/strain curves becomes unremarkable. The decrease of the loading curves is attributed to the irreversible damage occurred in partial region of the porous structure. During the first compress-release process, the pressure leads to the bending of cell structures followed by the breakage of a few interconnected cells. For the subsequent cycles, the loading curves are gradually overlapped with each other, indicating that less and less breakdown occurred in these cycles. This phenomenon is also reported in the previous literatures [1,9,16,38]. Although there is a little failure happened during the compress-release process, the total strain was lost for only about 5% and tended to be stable after the 10th cycles ( Figure S4). In addition, the maximum compressive stress of SEGA/PEDOT/MnO 2 composite only slightly decreases to 84% of the original value (stress = 110 kPa) after 1000 compression-release cycles (Figure 5d). Therefore, the SEGA/PEDOT/MnO 2 composite is ready for the compressible electrode materials of compression-tolerant ECs. The electrochemical performances of SEGA/PEDOT/MnO 2 ternary composite were firstly investigated in the three-electrode system by CV and GCD tests. Figure 6a compares the CV curves of SEGA, SEGA/PEDOT, SEGA/MnO 2 , and SEGA/PEDOT/MnO 2 electrodes at 20 mV s −1 . The CV responses of SEGA/MnO 2 and SEGA/PEDOT/MnO 2 are much larger than that of SEGA and SEGA/PEDOT, indicating that MnO 2 is mainly responsible for the high current density and enlarged area. Compared with SEGA/MnO 2 , CV curve of SEGA/PEDOT/MnO 2 is relatively rectangular in shape and exhibits a mirror-like replication below and above the zero line, indicating the higher electrochemical performance of SEGA/PEDOT/MnO 2 . GCD curves of the different electrodes at 1 A g −1 are shown in Figure 6b. Compared with SEGA and SEGA/PEDOT electrodes, SEGA/MnO 2 and SEGA/PEDOT/MnO 2 electrodes show obvious voltage drops (iR drops) at beginning of discharge, which is attributed to the high resistance of MnO 2 phase. And the discharge curve of SEGA/PEDOT/MnO 2 electrode has less iR drop (0.03 V) than SEGA/MnO 2 electrode (0.08 V), indicating that PEDOT reduces the interface resistance between graphene and MnO 2 . In agreement with the CV results, the GCD curve of SEGA/PEDOT/MnO 2 composite holds the longest discharge time, with a specific capacitance increased from 65 F g −1 for SEGA to 97 F g −1 for SEGA/PEDOT, 353 F g −1 for SEGA/MnO 2 , and 483 F g −1 for SEGA/PEDOT/MnO 2 , respectively. This improvement of the specific capacitance for the ternary composite is attributed to the synergistic effect of the three components. In the SEGA/PEDOT/MnO 2 composite, the SEGA provides continuously conductive network; MnO 2 significantly improves the specific capacitance; and the middle PEDOT layer reduces the interface resistance between graphene and MnO 2 . EIS test was performed to further characterize the SEGA/PEDOT/MnO 2 composite electrode. As shown in Figure 6c, the Nyquist plots of different electrodes are all composed of a typical semicircle in the high frequency region and a straight line in the low frequency region. The diameter of the semicircle corresponds to the charge-transfer resistance of the electrodes. It can be seen that, SEGA/PEDOT/MnO 2 shows a significant decrease of charge-transfer resistance than that of SEGA/MnO 2 , which verifies that the middle PEDOT coating layer improves the electron transport from MnO 2 to graphene. The rate capacity and cycle stability of the ternary composite electrode were also studied. As the current density increases from 1 to 20 A g −1 , the specific capacitance of SEGA/PEDOT/MnO 2 composite electrode has 81.6% retention of its initial value (Figure 6d), indicating a good rate capability. And its specific capacitance preserves 91.1% after 1000 GCD cycles at current density of 1 A g −1 (Figure S5), showing an excellent cycling stability.  As shown in Figure 7c, the ECs based on SEGA/PEDOT/MnO 2 electrodes show the gravimetric capacitance of 343 F g −1 at original state, and retain 97% of this value at 95% compressive strain (332 F g −1 ). The gravimetric capacitance values of SEGA/PEDOT/MnO 2 electrodes with/without compression are comparable to that of other graphene-based compressible electrode materials (Table 1). It worth noting that the volumetric capacitance of SEGA/PEDOT/MnO 2 electrodes is dramatically improved after 60% strain, and finally reach maximum value of 147.4 F cm −3 under 95% strain (Figure 7c), which is much higher than other graphene-based compressible electrodes (Table 1). The remarkable improvement of volumetric capacitance results from almost unchanged gravimetric capacitance and significant increased density of the SEGA/PEDOT/MnO 2 electrodes under high compression. When the electrodes undergo 95% compressive strain, the density of the electrodes is 20 times the original value, and the gravimetric capacitance declines by only 3%. According to the Equation (3), the volumetric capacitance of SEGA/PEDOT/MnO 2 electrodes at compressive strain of 95% is 19.4 times that of them at uncompressed state.
The EIS of the ECs based on SEGA/PEDOT/MnO 2 electrodes was also characterized (Figure 7d). The Nyquist plots consist of a typical semicircle in the high frequency region and a straight line at low frequency. The ECs show similar Nyquist plots in original and compressed states (strains of 30%, 60% and 95%), verifying the compression-tolerant ability. In order to study the reversible compressibility and durability of the compressible ECs with SEGA/PEDOT/MnO 2 electrodes, cycle stability was demonstrated by GCD at 1 A g −1 . Under both static (constant compressive strain) condition and dynamic (repeated compression/release) condition, there is only slight fluctuation of capacitances ( Figure 7e). For long-term durability of ECs, the compressive strains of 0%, 30%, 60%, and 95% are each varied at 500 charge/discharge cycles and finally, recovered to a fully relaxed state (Figure 7f). The original volumetric capacitance of ECs is preserved by 80% after 3500 charge/discharge cycles with various compressive strains. In addition, for realizing the practical function of compression-tolerant ability, three compressible all-solid-state ECs were integrated into one unit and interconnected together in series. After charging for 10 min, the resultant integrated device can light up a red-light-emitting diode and works well during the compression/release process (Figure 7g-i).

Conclusions
For acquiring the compressible electrodes with both ultra-compressible ability and high specific capacitance, PEDOT and MnO 2 were successively deposited into SEGA. Compression tests show that the maximum compressive strain of SEGA/PEDOT/MnO 2 composite reaches 95%. The gravimetric capacitance of the SEGA/PEDOT/MnO 2 electrodes without compression is 343 F g −1 , and can retain 97% under 95% compressive strain. Resulting from the invariant of gravimetric capacitance of SEGA/PEDOT/MnO 2 electrodes under compression, the volumetric capacitance of the ternary composite electrodes reaches 147.4 F cm −3 at 95% strain. This ternary composite paves a new way for advanced applications of ultra-compressible electrodes in the area of compression-tolerant ECs.
Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/12/1353/s1. Table S1. Mass content and mass loading of the materials in different composites. Figure S1. XPS spectrum of Mn 2p of SEGA/PEDOT/MnO 2 composite, Figure S2. SEM images of SEGA/PEDOT/MnO 2 composite at high magnification, Figure S3. SEM images of SEGA/PEDOT/MnO 2 composite corresponding to the loading status and unloading status, Figure S4. A part of the stress-strain curves (strain ≤ 10%) of SEGA/PEDOT/MnO 2 composite during the measurement of the cycle stability at a set strain of 95%, Figure S5. Cycle stability of SEGA/PEDOT/MnO 2 composite in the three-electrode system.