Superelastic Pseudocapacitors from Freestanding MnO2-Decorated

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Superelastic Pseudocapacitors from Freestanding MnO2 Decorated Graphene-Coated Carbon Nanotube Aerogels Yepin Zhao, Maxwell Patrick Li, Siyuan Liu, and Mohammad F. Islam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06210 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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ACS Applied Materials & Interfaces

Superelastic Pseudocapacitors from Freestanding MnO2 Decorated Graphene-Coated Carbon Nanotube Aerogels Yepin Zhao, Maxwell P. Li, Siyuan Liu, and Mohammad F. Islam* Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213-3815, USA KEYWORDS: compressible pseudocapacitor, carbon nanotubes, graphene, MnO2, volumetric capacitance

ABSTRACT: In recent years, the demand for emerging electronic devices has driven efforts to develop electrochemical capacitors with high power and energy densities that can preserve capacitance under and after recovery from mechanical deformation. We have developed superelastic pseudocapacitors using ≈ 1.5 mm thick graphene-coated single-walled carbon nanotube (SWCNT) aerogels decorated with manganese oxide (MnO2) as freestanding electrodes that retain high volumetric capacitance and electrochemical stability before, under, and after recovery from 50% compression. Graphene-coated SWCNT aerogels are superelastic and fatigue-resistant with high specific surface area and electrical conductivity. Electrodeposition of MnO2 onto these aerogels does not alter their superelasticity with full shape recovery even after 10,000 compression-release cycles to 50% strain. Total (utilized) gravimetric capacitances of these aerogels before compression are similar to those under and after recovery from 50%

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compression over a wide range of scan rates with capacitances reaching 98 F/g (468 F/g), 106 F/g (522 F/g), and 128 F/g (626 F/g) at a scan rate of 2 mV/s, respectively. These gravimetric capacitances are preserved even after 10,000 compression-release cycles to 50% strain. Further, 50% compression of these aerogels increases the volumetric capacitance from 1.5 F/cm3 to 3.3 F/cm3. Before compression, the lifetime performances of these aerogels remain largely stable with capacitance degrading by only ≈ 14% over the first 2,000 charge-discharge cycles and remains constant for further 8,000 cycles. Under 50% compression, capacitance displays a similar trend over 10,000 charge-discharge cycles. After recovery from 10,000 compressionrelease cycles to 50% strain, the aerogels show slightly greater capacitance loss of ≈ 28% over the first 2,000 charge-discharge cycles and an additional ≈ 10% loss over the subsequent 8,000 charge-discharge cycles. Finally, substantially higher gravimetric capacitance is achieved through greater MnO2 deposition, facilitated by large porosity of these aerogels, albeit at a loss of capacitance retention upon compression. These capacitors display the feasibility of coating graphene-coated SWCNT aerogels with various pseudocapacitive materials to create superelastic energy storage devices.


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INTRODUCTION Recent advances in technologies to meet the ever-growing demand for newer consumer products have brought a great deal of attention towards electrochemical capacitors (ECs).1-3 Products such as smartphones, wearable electronics, and electric vehicles require ECs to be lightweight with high gravimetric and volumetric energy and power densities.1-2 Furthermore, for use in flexible electronics, ECs need to be mechanically deformable under external applied stresses (stretching, bending, and/or compression) without any degradation of electrochemical performances.4-5 Compressible ECs, in particular, have an additional benefit that their volumetric capacitance can be enhanced through compression of the electrodes. Consequently, significant research has been directed towards development of freestanding, lightweight, superelastic, and thick electrodes that withstand mechanical deformations and reduce or eliminate inactive yet heavy EC components such as current collectors and polymer binders.6-9 Electric double layer capacitors (EDLCs), one type of ECs, operate by storing charges in the electric double layer that allows for rapid charge-discharge and high power densities.1-3 Most utilize foams or aerogels with high surface area materials tailored from activated carbons, graphene, and carbon nanotubes (CNTs) as electrodes.1-3,6-7,10-13 Despite the tendency for graphene and CNTs in foams to bundle or restack upon compressive strain, many of these electrodes have been engineered to be elastomeric. Superelastic, compressible EDLCs fabricated using these type of electrodes retain capacitance fully over many compression-release cycles and demonstrate volumetric capacitances under 90% compression as high as 5 F/cm3 using aqueous electrolytes and 18 F/cm3 using room temperature ionic liquid electrolytes.6 Pseudocapacitors, the other type of ECs, employ Faradaic reactions on or near the surface of redox-active materials such as transition metal oxides or redox polymers to store charges.1-3


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Although the Faradaic reactions are slower than charge adsorption-desorption, the gravimetric capacitance of pseudocapacitors typically far exceeds that of EDLCs, albeit with substantially lower stability over a comparable number of charge-discharge cycles.1-4,14-18 Electrodes in pseudocapacitors are typically fabricated by depositing redox-active materials on electrically conducting, porous scaffolds with high surface area. A majority of pseudocapacitors utilize transition metal oxides because of their high intrinsic gravimetric capacitance with manganese oxide (MnO2) being the most researched material due to its low-cost, environmental friendliness, and ease of synthesis.1,3,14,17,19-29 Pseudocapacitors with MnO2 have achieved total (i.e., normalized to mass of entire electrode that combines masses of redox-active material and scaffold) and utilized (i.e., normalized to mass of redox-active material) gravimetric capacitances of 600 F/g and 1,380 F/g, respectively.14,17-20,30-33 A majority of these electrodes, however, range from 340 nm–100 µm in thickness, which is much thinner than the hundreds of micrometers to several millimeter thickness required for practical applications, increasing the complexity of EC device design and further restricting their usage in flexible electronics.17,31-32 Unfortunately, superelastic pseudocapacitors with MnO2 and other transition metal oxides that retain their performances upon mechanical compression and subsequent recovery as well as after repeated compression-release and charge-discharge cycles are limited,16 likely because metal oxides are inherently brittle. As such, transition metal oxides can fracture during chargedischarge cycling induced volumetric expansion-contraction as well as during mechanical deformation, leading to detachment from the underlying scaffolds and ultimately capacitance degradation.1-2,14,19 For example, compressible pseudocapacitors, fabricated from superelastic CNT foams decorated with iron oxide (α−Fe2O3), retain gravimetric capacitance of 296 F/g after 1,000 compression-release cycles to 50% strain.16 Further, these pseudocapacitors retain ≈ 90%


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of their gravimetric capacitance under 70% compressive strain, but lose 20% of their capacitance after 200 charge-discharge cycles and 40% over 1,000 cycles. Additionally, volumetric capacitance before and under 70% compression as well as lifetime stability under compression or after recovery from compression-release cycles were not examined. Alternatively, superelastic pseudocapacitors utilizing redox polymers supported on superelastic scaffolds have demonstrated comparable gravimetric capacitance that is retained after many compression-release cycles but still suffer from poor charge-discharge stability.1-3 This class of pseudocapacitors also exhibit high volumetric capacitance through compression of the electrodes.4 For example, Pseudocapacitors fabricated from superelastic CNT foams coated with polypyrrole, a redox polymer, have been able to sustain 1,000 compression-release cycles to 50% strain with complete capacitance retention of 310 F/g, and display a volumetric capacitance of 18.2 F/cm3 under 50% compression.4 However, the capacitance degrades by ≈ 20% and ≈ 40% after 1,000 and 5,000 charge-discharge cycles, respectively.4 To overcome these shortcomings, metal oxides and redox polymers have been integrated with porous superelastic scaffolds to increase gravimetric capacitance with substantially expanded charge-discharge stability, but stability across compression-release cycles showed negligible improvements.15 For instance, pseudocapacitors based on superelastic CNT sponges coated with MnO2 and polypyrrole exhibit a gravimetric capacitance of 320 F/g, retaining ≈ 90% of this gravimetric capacitance over 1,000 charge-discharge cycles before compression and reaching a volumetric capacitance of 16 F/cm3 with 50% compression.15 Although the electrodes were able to recover their original dimensions after two compression-release cycles to 50% strain, capacitive performances after recovery have not been reported.


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Herein, we report fabrication of superelastic pseudocapacitors that maintain stable gravimetric and volumetric capacitances across broad scan rates along with a long stable lifetime even under 50% compression using freestanding graphene-coated single-walled carbon nanotube (SWCNT) aerogels decorated with MnO2 as electrodes. SWCNT aerogels comprise of individual SWCNTs shaped into a porous, isotropic network with an open-cell structure.10,34-37 Graphene-coated SWCNT aerogels are fabricated by coating nearly all the junctions between SWCNTs and most of the struts in SWCNT aerogels with 2–5 layers thick ≈ 3 nm long graphene nanoplates.6,38-41 We have recently demonstrated high-rate EDLC performance of SWCNT and graphene-coated SWCNT aerogels, which have ultrahigh porosity.6,10 Furthermore, graphene-coated SWCNT aerogels fully recover without plastic deformation or degradation of electrical conductivity after compression of ≥90% and over thousands of compression-release cycles,38,40-41 which in turn allows for high volumetric capacitances upon 90% compression.6

We decorated these

superelastic aerogels with MnO2 and characterized the mechanical properties including compressibility and recovery from 10,000 compression-release cycles to 50% strain using dynamic mechanical analysis (DMA). Next, we measured their gravimetric (total and utilized) and volumetric capacitances before, under, and after recovery from 50% compression through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) cycles, and electrical impedance spectroscopy (EIS). The gravimetric capacitance was again examined after the aerogels underwent 10,000 compression-release cycles to 50% strain utilizing above mentioned electrochemical characterization techniques. Finally, the lifetime stability was examined over 10,000 charge-discharge cycles before compression, under 50% compression, and after recovery from 10,000 compression-release cycles to 50% strain. MATERIALS AND METHODS


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Preparation of MnO2 Decorated Graphene-Coated SWCNT Aerogels. Dispersing of SWCNTs in water, fabrication of SWCNT hydrogels, and the subsequent coating of SWCNT hydrogels with graphene are reported elsewhere.6,10,34-35,38-45 We used CoMoCAT SWCNTs (CG200; SouthWest NanoTechnologies Inc.) that have manufacturer reported diameters of 1.0 ± 0.3 nm and lengths of ≈ 1 µm. The graphene-coated SWCNT aerogels had cylindrical shapes with diameters of ≈ 8.65 mm and thicknesses of 1.5 mm, and had a mass of 1.16 mg, corresponding to an average density of ≈ 13.2 mg/mL. For electrodeposition of MnO2 onto graphene-coated SWCNT aerogels, aerogels were first infiltrated under vacuum with an electrolyte solution of 100 mM sodium sulfate (Na2SO4) and 100 mM manganese acetate tetrahydrate (Mn(C2H3O2)2⋅4H2O) in deionized water (resistivity 18.2 MΩ·cm with total oxidizable carbon 2 nm) in the aerogel (Figure S7, Supporting Information). However, majority of pore diameters remain between 2–20 nm (i.e., mesopores), which are highly suitable for facile ion transport through the network during charge-discharge.

Figure 3. Nitrogen adsorption-desorption isotherms of graphene-coated SWCNT aerogels before and after MnO2 decoration.

The structural integrity of graphene-coated SWCNTs and the interactions between MnO2 and the graphene-coated SWCNTs before and after charge-discharge cycles were characterized through Raman spectroscopy and a comparison with those from MnO2 and SWCNTs (Figure S8,


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Supporting Information). The Raman spectra from MnO2 decorated graphene-coated SWCNT aerogels display identifiable features associated with SWCNTs and MnO2. The intensity ratio ID/IG between the SWCNT D- and G-bands can be used to characterize damage or structural defects in SWCNTs as a result of MnO2 electrodeposition. The G-band, located at ≈ 1590 cm-1, is characteristic of graphitic carbon and quantifies the amount of sp2-hybridized carbon bonds in the aerogels. The D-band, located at ≈ 1300 cm-1, quantifies the amount of sp3-hybridized carbon bonds in the aerogels. Note that a comparison between the spectra from SWCNT and graphenecoated SWCNT aerogels indicates that graphene coating does not damage SWCNTs.6,38,40-41 The ID/IG before (0.17) and after (0.21) MnO2 decoration as well as after charge-discharge cycles (0.20) are similar, establishing that MnO2 electrodeposition and Faradaic reactions occurring during charge-discharge do not damage the underlying SWCNTs. Additionally, the radial breathing modes (RBMs), features exclusive to SWCNTs, are still present in the spectra following MnO2 decoration and charge-discharge cycles, indicating SWCNTs remain intact through the decoration, compression-release cycles, and charge-discharge cycles. A pronounced feature in the Raman spectra located at ≈ 600 cm-1 from graphene-coated SWCNT aerogels after MnO2 decoration is associated with Mn–O lattice vibrations. The same feature is present in the spectra from MnO2 powder but not in that from graphene-coated SWCNT aerogels.14-15,33 Further, this feature remains in the spectra after charge-discharge cycles, both before compression and after recovery from 10,000 compression-release cycles to 50% strain, confirming the presence of MnO2 albeit with reduced intensity due to a reduction in MnO2 amount following charge-discharge cycles. Notice that the location of the characteristic Raman signature associated with Mn–O does not show any shift after deposition on graphene-coated SWCNTs as well as after the aerogels undergo any compression-release and/or charge-discharge


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cycles, suggesting that MnO2 does not strongly interact with the underlying graphene-coated SWCNTs and may be susceptible to detachment during repeated compression. This may also explain the decrease in the MnO2 content in the aerogels after 10,000 compression-release cycles to 50% strain and subsequent 10,000 charge-discharge cycles, as revealed in EDS and XRD measurements. The chemical structure and interactions between MnO2 and graphene-coated SWCNTs were further evaluated by analyzing the carbon (C) 1s, manganese (Mn) 2p, and oxygen (O) 1s core level spectra measured using XPS. Deconvolving of the C 1s spectrum from graphene-coated SWCNT aerogels before MnO2 decoration leads to three peaks with two dominant peaks at 284.0 and 285.1 eV arising from the C=C and the C–C bonds of the graphene-coated SWCNTs, respectively (Figure S9a, Supporting Information). The last peak at 286.2 eV represents C–O bonds, suggesting the presence of CO groups at defects on graphene and/or SWCNTs. The C 1s spectrum retains all three deconvolved peaks after MnO2 decoration (Figure S9b, Supporting Information). The primary 2p3/2 and 2p1/2 peaks at 642.1 and 653.6 eV, respectively, in the deconvolved Mn 2p spectrum from aerogels after MnO2 decoration result from the Mn–O (Mn4+) bonds (Figure S9c, Supporting Information) and identify the dominant MnO2 oxidation state. Finally, the O 1s spectrum of the same aerogels can be fitted into three peaks at 529.7, 531.6, and 532.7 eV that can be attributed to Mn–O–Mn, Mn–O–H, and C–O bonds, respectively (Figure S9, Supporting Information). The absence of any bonding between MnO2 and graphene-coated SWCNTs indicates that the metal oxide nanoparticles are physically adhered to the underlying graphene-coated SWCNTs, and corroborates Raman measurements. Mechanical Characteristics of MnO2 Decorated Graphene-Coated SWCNT aerogels. We assessed mechanical characteristics of graphene-coated SWCNT aerogels after MnO2 decoration


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by measuring their compressive stress versus compressive strain. The underlying graphenecoated SWCNT aerogels are superelastic at least up to 90% strain.6,38,40-41 The aerogels retain their superelasticity after MnO2 deposition for up to 50% compression, completely recovering to original dimensions when the load is removed (Figure 4a), but show 48% plastic deformation when compressed to 80% strain (Figure S10, Supporting Information). The Young’s moduli, calculated from the linear regime (compressive strain ≤ 7%), before (0.21 MPa) and after (0.2 MPa) MnO2 decoration are very similar, indicating that electrodeposition of MnO2 does not substantially affect the junctions between graphene-coated SWCNTs that dictate the mechanical properties of this type of aerogels.35,38 We also examined fatigue resistance of the aerogels after MnO2 decoration by subjecting them to 10,000 compression-release cycles to 50% strain (Figure 4b). Note that the underlying graphene-coated SWCNT aerogels display fatigue resistance at least till 60% strain for more than 2,000 cycles.38 After MnO2 decoration, the aerogels recover their original shapes and dimensions even after 10,000 compression-release cycles to 50% strain, however, display some softening with the Young’s modulus decreasing at greater compressionrelease cycles.


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Figure 4. Mechanical characteristics of MnO2 decorated graphene-coated SWCNT aerogels. (a) Compressive stress versus compressive strain curves before and after MnO2 decoration. Inset showing photographs of MnO2 decorated graphene-coated SWCNT aerogels under 0% (left) and 50% (right) compression. (b) The compressive stress versus compressive strain curves for the 1st through the 10,000th compression-release cycle to 50% strain demonstrate that the aerogels are fatigue-resistant. Electrochemical Performances. The high SSA, abundant mesopores and fatigue-resistant superelasticity make MnO2 decorated graphene-coated SWCNT aerogels highly suitable as superelastic pseudocapacitor electrodes. We measured electrochemical characteristics of these aerogels using a symmetric, parallel plate setup to assess their performances as real devices (Figure S1, Supporting Information). We began by CV characterization of these aerogels before, under, and after recovery from 50% compression (Figure 5a). Before compression, the CV curves from these aerogels display a symmetric, rectangular shape over a broad scan range of 2– 200 mV/s, indicating an ideal capacitive behavior originating from fast and reversible Faradaic reactions even at a high scan rate (200 mV/s) and high electrochemical stability (Figure S11,


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Supporting Information). Total gravimetric capacitance, calculated using total mass of an aerogel, is 98 F/g at a scan rate of 2 mV/s and gradually decreases to 35 F/g at 200 mV/s (Figure 5c). The electrochemical properties of the aerogels were further examined using GCD measurements (Figure 5b). The symmetrical GCD profiles corroborate high reversibility of the redox reactions and rapid ion transport through the aerogels. Further, the negligible voltage drop upon discharging, which signifies internal resistance (IR), indicates high electrical conductivity of the MnO2 decorated graphene-coated SWCNT aerogels likely due to high electrical conductivity of the underlying graphene-coated SWCNT aerogels. To evaluate their superelastic pseudocapacitive properties, the electrochemical properties of the aerogels were next determined under and after recovery from 50% compression with the same electrochemical characterization techniques. The electrochemical performances remain similar as demonstrated by nearly identical CV and GCD curves to those before compression, demonstrating the robustness of these aerogels and their ability to operate under compression (Figure 5a and 5b). This also indicates that these aerogels still maintain an open porous structure under compression allowing ions to diffuse freely, completely retaining their electrochemical performances. The total gravimetric capacitance under 50% compression (106 F/g) is similar to that before compression, but increases to 128 F/g after recovery from 50% compression at a scan rate of 2 mV/s (Figure 5c). The interconnections between the MnO2 nanoparticles, particularly at the junctions between graphene-coated SWCNTs, possibly break during compression, exposing the interfaces and creating new redox sites after recovery from compression. Further, since the capacitance originates from Faradaic reactions occurring at MnO2, we also calculated the utilized gravimetric capacitance using only the mass of the electrodeposited MnO2. These values are 468 F/g, 522 F/g, and 626 F/g before, under, and after recovery from 50% compression, respectively,


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at a scan rate of 2 mV/s (Figure 5c). Lastly, we exploited nearly identical electrochemical properties of these aerogels under 50% compression to fabricate pseudocapacitors with high volumetric capacitances, calculated by normalizing the capacitances with the volume of the aerogels. The volumetric capacitances reach 3.3 F/cm3 under 50% compression from 1.5 F/cm3 for uncompressed aerogels at 2 mV/s scan rate (Figure 5d). A table summarizing the performance and characteristics of compressible pseudocapacitors is presented in Table S1 which also highlights the limited number of compressible pseudocapacitors utilizing metal oxides.


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Figure 5. Electrochemical performance of MnO2 decorated graphene-coated SWCNTs aerogels before, under, and after recovery from 50% compression. (a) Representative cyclic voltammograms at 100 mV/s scan rate. (b) Galvanostatic charge-discharge curves at 1 A/g current density. (c) Total and utilized gravimetric capacitances and (d) volumetric capacitances over scan rates of 2–200 mV/s. We also assessed the electrical and ionic transport characteristics of the aerogels before, under, and after recovery from 50% compression through EIS measurements (Figure 6a). The characteristic Nyquist plots, generated from the EIS measurements, show that the aerogels possess a low series resistance of 1.97 Ω before compression that becomes even smaller to 1.24 Ω under 50% compression, likely due to an increase in the contact area between graphene-coated


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SWCNTs at the junctions as well as a shortened path length for ion transport. After recovery from 50% compression, the aerogels display similar Nyquist plot as that before compression but with a slightly lower series resistance of 1.74 Ω, suggesting that any MnO2 that might have entered into the junctions during synthesis, which would reduce electrical transport through the junction, likely was expelled during compression. We magnify the high frequency range of the Nyquist plots in the inset of Figure 6a to highlight low series resistance for three different deformation states. The real and imaginary capacitances of the aerogels with respect to frequency are shown in the Bode plot in Figure 6b. Under 50% compression, the real capacitance curve shifts to a higher frequency indicating greater capacitance retention at increased chargedischarge rates which is in agreement with CV and GCD tests. The time constant for chargedischarge of the aerogels under 50% compression, determined from the inverse of the frequency at the maximum imaginary capacitance, is shorter than in uncompressed states (i.e., before compression and after recovery from 50% compression), indicating faster charge-discharge rates, a feature that is also in agreement with CV and GCD tests. We imagine that under 50% compression, the shorter ion transport path length within the aerogels at any frequency combined with smaller series resistance arising from larger contact area between graphene-coated SWCNTs at the junctions generate faster charge-discharge rates. After recovery from 50% compression, the real and imaginary curves shift back to a lower frequency and is similar to that before compression but display a slightly greater capacitance which is also observed with CV and GCD tests (cf. Figure 6b with Figure 5a and 5b).


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Figure 6. Frequency response of MnO2 decorated graphene-coated SWCNT aerogels before, under, and after recovery from 50% compression. (a) Nyquist plots at frequencies between 1 Hz and 1 kHz. The inset magnifies high frequency regime to reveal low series resistance. (b) Bode plots show the real and imaginary capacitances of the aerogels.

Next, we examined the electrochemical performance of MnO2 decorated graphene-coated SWCNT aerogels after mechanical deformation by subjecting the aerogels to 10,000 compression-release cycles to 50% strain and subsequently performing similar electrochemical tests as shown previously. The CV curves display similar and quasi-rectangular shape with a slight tilt at higher voltages indicating aerogels retain their capacitance despite vigorous mechanical deformation albeit with an increase in internal resistance (Figure 7a). The total


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gravimetric capacitances after 10,000 compression-release cycles to 50% strain are nearly identical over 5–200 mV/s and slightly smaller (≈ 19%) at 2 mV/s to those before compression (Figure 7b). The small reduction at low scan rates is likely due to detachment of a small fraction of MnO2 from graphene-coated SWCNTs during extraordinarily large compression-release cycling (Figure S4 and S5, Supporting Information). GCD and EIS measurements of the aerogels resemble those before compression, further demonstrating the robustness of these MnO2 decorated graphene-coated SWCNT aerogels (Figure S12, Supporting Information).

Figure 7. Electrochemical stability of MnO2 decorated graphene-coated SWCNT aerogels after recovery from 10,000 compression-release cycles to 50% strain. (a) Representative cyclic voltammograms from electrodes before compression and after 10,000 compressionrelease cycles to 50% strain at 100 mV/s. (b) Corresponding total gravimetric capacitances over scan rates of 2–200 mV/s. Since volumetric expansion-contraction associated with charge-discharge cycling frequently degrades structural integrity of MnO2 and cause delamination from underlying scaffold, ultimately reducing the electrochemical performance, we examined the electrochemical stability of these pseudocapacitive aerogels before compression, under 50% compression, and after


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recovery from 10,000 compression-release cycles to 50% strain over 10,000 charge-discharge cycles at a rate of 1 A/g (Figure 8). Before compression, total gravimetric capacitance deceases by ≈ 14% over the initial 2,000 charge-discharge cycles and remains constant for the following 8,000 cycles. This measurement is indicative of the mechanical stability of MnO2 on the superelastic graphene-coated SWCNT aerogel scaffold and demonstrates the feasibility of these pseudocapacitive aerogels in real-world applications. Under 50% compression, these aerogels display similar performance as that of aerogels before compression. Capacitance displays an initial drop of ≈ 16% after 1,000 cycles and gradually decreases retaining ≈ 74% of its original capacitance after 10,000 charge-discharge cycles. After recovery from 10,000 compressionrelease cycles to 50% strain, these aerogels display slightly greater capacitance loss of ≈ 28% over the initial 2,000 charge-discharge cycles and an additional ≈ 10% loss over the subsequent 8,000 charge-discharge cycles. This loss in capacitance correlates to the significant decrease in the amount of MnO2 remaining in the aerogels as observed in EDS measurements (Figure S6, Supporting Information). Because MnO2 content in aerogels before charge-discharge cycling but after 10,000 compression-release cycles is similar to that of as-prepared aerogels, we postulate that the vigorous deformation creates extensive cracking in MnO2 and partial delamination from the graphene-coated SWCNT aerogel scaffold. Charge-discharge cycling induced volumetric expansion-contraction exacerbates delamination, leading to capacitance loss. However, the lifetime stability of these aerogels after compression-release cycling is substantially better than other redox-active metal oxide based compressible pseudocapacitors that lose 20% of their capacitance after 200 charge-discharge cycles and 40% over 1,000 cycles.16


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Figure 8. Capacitance retention of MnO2 decorated graphene-coated SWCNT aerogels before compression, under 50% compression, and after 10,000 compression-release cycles to 50% strain at a current density of 1 A/g over 10,000 charge-discharge cycles. We also present the gravimetric and volumetric energy and power densities of MnO2 decorated graphene-coated SWCNT aerogels calculated from total gravimetric and volumetric capacitances (Figure 9a and 9b, respectively) before, under, and after recovery from 50% compression.1,10 The gravimetric energy density versus power density curves are similar before, under, and after recovery from 50% compression with a maximum energy density of ≈ 2.5 Wh/kg and power density of ≈ 0.9 kW/kg. As observed with volumetric capacitances, the maximum volumetric energy and power densities increase when the aerogel is under 50% compression from ≈ 0.035 Wh/L and ≈ 11 W/L, respectively, to 0.075 Wh/L and 30 W/L, respectively. Finally, because graphene-coated SWCNT aerogels are highly porous, the amount of MnO2 deposited onto the underlying graphene-coated SWCNT aerogels can be tuned by varying the deposition time but leads to a reduction in superelasticity and changes in electrochemical responses before, during, and after mechanical deformations, particularly at high loadings.


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Nevertheless, we have been able to load ≈ 33 wt% of MnO2 on graphene-coated SWCNT aerogels that show small changes in CV curves under and after recovery from 50% compression in comparison to that before compression (Figure S13, Supplemental Information). Total gravimetric capacitance of these aerogels reaches 200 F/g at 2 mV/s before any compression (Figure S14, Supplemental Information) with substantial increase in both total gravimetric and volumetric energy and power densities (S15, Supplemental Information). Unfortunately, the gravimetric capacitances as well as energy and power densities greatly reduce under compression, likely because MnO2 is more prone to detachment from the underlying graphenecoated SWCNTs at such high loadings under 50% compression.

Figure 9. Energy density versus power density of MnO2 decorated graphene-coated SWCNT aerogels normalized by (a) total mass of the aerogels and (b) volume of the aerogels before, under, and after recovery from 50% compression.

CONCLUSIONS We have developed compressible, freestanding pseudocapacitive electrodes that can maintain capacitance under 50% compression and after recovery from 10,000 compression-release cycles


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to 50% strain over 10,000 charge-discharge cycles. We fabricated these electrodes by electrodepositing MnO2 onto a thick (1.5 mm), superelastic graphene-coated carbon nanotube aerogel that does not alter the superelasticity of the underlying scaffold, and the resultant aerogels fully recover to their original dimensions after 10,000 compression-release cycles to 50% strain. The well-dispersed MnO2 decoration also does not modify the open porous network consisting mostly of mesopores of the underlying scaffold, even with 10,000 compressionrelease cycles to 50% strain, which enables facile ion transport necessary for ECs. The gravimetric capacitances of these MnO2 decorated graphene-coated SWCNT aerogels before compression are similar to those under and after recovery from 50% compression while the volumetric capacitances are enhanced upon compression. Furthermore, the aerogels do not lose their gravimetric capacitances even after 10,000 compression-release cycles to 50% strain over scan rates of 5–200 mV/s and exhibit ≈ 19% decrease at 2 mV/s, likely because a small fraction of MnO2 detaches from graphene-coated SWCNTs during repeated compression. These aerogels also display exceptional stability over 10,000 charge-discharge cycles even with 10,000 compression-release cycling in contrast to other reported results.16 For example, before compression, the aerogels lose only ≈ 14% of their capacitance during the initial 2,000 chargedischarge cycles with no drop over the next 8,000 cycles. Under 50% compression, capacitance retention shows similar trend over the same charge-discharge cycles. Even after recovery from 10,000 compression-release cycles to 50% strain, the aerogels show only ≈ 28% capacitance loss over the first 2,000 charge-discharge cycles and an additional ≈ 10% loss over the remaining 8,000 charge-discharge cycles. The large porosity and SSA of graphene-coated SWCNT aerogels38 enable grater MnO2 loading, substantially increasing the capacitance albeit with a loss of capacitance retention upon compression. Lastly, both SWCNT aerogels and graphene-coated


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SWCNT aerogels are mechanically robust to withstand deposition or coating of broad types of materials such as TiO2, B2O3, polymers, and enzymes.36-37,39,46 As such high performance, superelastic ECs can be devised with these aerogels by hierarchically coating them with diverse redox-active oxides and polymers. ASSOCIATED CONTENT Supporting Information: Supporting figures: (1) schematic of “home-built” parallel-plate setup; (2) SEM and TEM images of graphene-coated SWCNT aerogels; (3-5) SEM and corresponding EDS mapping of MnO2 decorated graphene-coated SWCNT aerogels after 10,000 charge-discharge cycles, after 10,000 compression-release cycles, and after 10,000 compressionrelease and 10,000 charge-discharge cycles; (6) EDS spectra of MnO2 decorated graphene-coated SWCNT aerogels on its surface and interior, before and after 10,000 charge-discharge and 10,000 compression-release cycles; (7) BJH pore diameter distribution for graphene-coated SWCNT aerogels before and after MnO2 decoration; (8) Raman spectra of graphene-coated SWCNT aerogels before and after MnO2 decoration; (9) XPs spectra from graphene-coated SWCNT aerogels before and after MnO2 decoration; (10) Compressive stress versus compressive strain curves of MnO2 decorated graphene-coated SWCNT aerogels to 50% and 80% strain; (11) CV curves of MnO2 decorated graphene-coated SWCNT aerogels before compression; (12) GCD and Nyquist plots of MnO2 decorated graphene-coated aerogels after 10,000 compression-release cycles; (13) CV curves of MnO2 (33 wt%) decorated graphenecoated SWCNT aerogels before, under, and after recovery from 50% compression; (14) Total gravimetric capacitance of MnO2 (33 wt%) decorated graphene-coated SWCNT aerogels versus scan rates (mV/s); (15) Energy density versus power density plots of MnO2 (33 wt%) decorated


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graphene-coated SWCNT aerogels. Supporting table: (1) Comparison of compressible pseudocapacitors. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *M.F.I.: Phone: (412) 268-8999 Fax: (412) 268-7596 E-mail: [email protected] Author Contributions M.F.I. developed and designed the project. Y.Z. and M.P.L. prepared the samples, performed the experiments, and collected the data. S.L. performed XPS measurements. M.F.I. provided technical and conceptual advice. Y.Z., M.P.L., S.L., and M.F.I. analyzed the data. M.F.I. wrote the manuscript with input from M.P.L. ACKNOWLEDGMENT This work was supported by the National Science Foundation through grant CMMI-1335417. We acknowledge the use of the Carnegie Mellon Materials Characterization Facility at Carnegie Mellon University (supported by grant MCF-677785) for SEM imaging and XRD. We also thank Y. Wei and M. Skowronski for assistance with the XPS measurements. REFERENCES (1) Simon, P.; Gogotsi, Y., Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845-854. (2) Zhang, L. L.; Zhao, X. S., Carbon-Based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38 (9), 2520-2531.


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TABLE OF CONTENTS FIGURE


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Superelastic Pseudocapacitors from Freestanding MnO2-Decorated

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