Hierarchically Structured Multidimensional Carbon Composite

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A Hierarchically Structured Multi-dimensional Carbon Composite Anchored to Polymer Mat for Super-flexible Supercapacitor Yeongdae Lee, Myung-Jun Kwak, Chihyun Hwang, Cheolwon An, Woojin Song, Gyujin Song, Suhee Kim, Soojin Park, Ji-Hyun Jang, and Hyun-Kon Song ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01417 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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ACS Applied Energy Materials

A Hierarchically Structured Multi-dimensional Carbon Composite Anchored to Polymer Mat for Super-flexible Supercapacitor Yeongdae Lee,† Myung-Jun Kwak,† Chihyun Hwang,*,† Cheolwon An,† Woo-Jin Song,‡ Gyujin Song,† Suhee Kim,† Soojin Park,‡ Ji-Hyun Jang,*,† and Hyun-Kon Song*,† †

School of Energy and Chemical Engineering, UNIST, Ulsan 44919, Korea

‡

Department of Chemistry, POSTECH, Pohang 37673, Korea

Keywords: flexible power sources, supercapacitors, three-dimensional graphene, carbonnanotube, nonwoven fabrics

ACS Paragon Plus Environment

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ABSTRACT: A carbon electrode was designed to guarantee flexibility of symmetric electric double layer capacitors (EDLCs) based on its architecture. Three different dimensional carbon materials were combined to achieve the flexibility without sacrificing high performances: highly capacitive but poorly-conductive three-dimensional graphene (3D-Gn*) as a platform for electric double layer formation, one-dimensional carbon nanotube (1D-CNT) as an electrically conductive highway and two dimensional graphene (2D-Gn) for facilitating electron communications between 3D-Gn* and 1D-CNT. From a mechanical standpoint, the 1D-CNT provided an intertangled framework to integrate the main active material 3D-Gn* and anchored the active layer to a flexible polymer matrix. Resultantly, the symmetric EDLC based on the hierarchically structured multidimensional carbon electrodes anchored to flexible substrates was operated successfully at 3 V, ensuring high energy densities even under repetitive mechanical stress conditions.


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INTRODUCTION Flexible energy storage devices are essentially required for realizing wearable devices, roll-up displays, foldable mobile phones and implantable medical applications.1-10 Flexible supercapacitors or electric double layer capacitors (EDLCs) have been developed for high power applications11-12 while flexible rechargeable batteries such as lithium ion batteries (LIBs) were preferred for high energy applications.13-17 The flexibility have been achieved by design of electrode structure: which materials are used for guaranteeing the mechanical properties; how to structure the constituents of electrodes.18 Flexible substrates including carbon cloth,19 graphene,20 and polymer cloth1, 21-23 have been extremely helpful for guaranteeing the electrode and moreover device flexibility. A strong integration of active layers to the substrates was achieved by using one-dimensional (1D) nano-threads. The nano-threads such as silver nanowires or carbon nanotubes (1D-CNTs) mechanically reinforced active layers1, 24 and anchored the layers to the flexible substrates.1 To improve the flexibility without sacrificing energy storage performances, it should be also carefully considered to develop electric conduction pathways especially through the interfaces between electrode-constituent materials. The limited energy densities of EDLCs (1 to 10 Wh kg-1 for EDLCs versus 100 Wh kg-1 for LIBs) have been improved by increasing cell potentials12 and/or by using high-capacitance electrode materials. The specific electric-double-layer capacitance depends dominantly on the surface area of active materials rather than on the material identity because the variation of capacitance per true area is limited and estimated around 10 to 20 μF cm -2 independent of materials.25 Therefore, nanostructuring carbon materials is important for realizing high capacitance.26


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Two-dimensionality of graphene and graphene-like materials (2D-Gn’s) is the benefit from the flexibility standpoint because the π-π stacking of 2D-Gn’s supports physical integrity against external bending forces.11 From the capacitance standpoint, however, the 2D-Gn stacks possibly reduce the surface area available for electric double layer formation, decreasing capacitance.27-29 To overcome the demerits of 2D-Gn assembly, three-dimensional graphene networks (3D-Gn’s) were proposed.30-32 Highly developed porous networks of 3D-Gn’s allowed ions to have an easy access to the whole surface through porous channels.11, 33-35 In this work, a composite electrode of 3D-Gn*/2D-Gn/1D-CNT without binder (named (3*+2+1)D) was presented for flexible EDLCs (Figure 1a). The 3D-Gn* was the active material of EDLC that was dominantly responsible for electric double layer formation. The 1D-CNT was used to reinforce the active composite layer and to integrate the active layer to a flexible polymer substrate by anchoring. Polyethylene terephthalate microfiber (PET-MF) mat having polyacrylonitrile nanowire (PAN-NW) back-layer on a face (PET-MF/PAN-NW) was used for the flexible substrate. The 1D-CNT played a role of electric conduction highways as well as mechanical anchors and reinforcement materials. The 2D-Gn was used to improve contact resistance developed between 3D-Gn* and 1D-CNT.

RESULTS AND DISCUSSION Multi-dimensional carbon composites. The 3D-Gn was synthesized by pyrolyzing polyvinylpyrrolidone (PVP) in the presence of silica particles as a hard template and FeCl3 as a graphitization catalyst and then removing the silica template and the catalyst. Its hierarchical pore structure having micropores, mesopores and macropores (Figure 1b and Figure S1a and b) was responsible for high surface area at 1200 m2 g-1 and large pore volume at 2.0 cm3 g-1. The 3D-Gn


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was CO2-activated to be 3D-Gn* (* indicates activated) for increasing surface area and pore volume to 1600 m2 g-1 and 2.2 cm3 g-1 (Figure S1a). Carbon atoms on edges of 3D-Gn are preferentially oxidized by CO2 to produce CO in a gas phase during the CO2 activation. Pores are formed at the sites where carbon mass is lost. Consequently, a porous and sponge-like structure guaranteeing large surface area was developed for 3D-Gn* (Figure 1b and S2a-S2d). On the other hand, the 2D-Gn was nonporous and a few graphene sheets were found overlapped (Figure 1c and S2e-S2h). More highly graphitic crystallinity of 2D-Gn than 3D-Gn and 3D-Gn*, indicated by Xray diffraction patterns and D/G ratio in Raman spectra (Figure S1c and S3), supported higher electron mobility along surface of 2D-Gn. Single-wall 1D-CNT (diameter = 3 nm) was used as network threads and anchors (Figure 1d and S2i-S2l). High Csp2/Csp3 ratios (the ratio of the basal plane to edge) of 1D-CNT (5.51) and 2D-Gn (5.12) supported the high crystallinity of 1D-CNT and the dominancy of basal plane in 2D-Gn (X-ray photoelectron spectra in Figure S4). Highly porous 3D-Gn showed low Csp2/Csp3 ratio at 3.71. The surface areas of 2D-Gn (630 m2 g-1) and 1D-CNT (12 m2 g-1) were significantly smaller than those of the 3D-Gn* (1600 m2 g-1) (Figure S5). The (3*+2+1)D composite electrodes (Figure 1a, e and f) were prepared by filtrating a mixture of 3D-Gn*, 2D-Gn and 1D-CNT in water through the PET-MF/PAN-NW membrane. The PET-MF layer faced the mixture for filtration. The 1D-CNT penetrated the PET-MF layer through the void between microfibers but further penetration of the 1D-CNT through PAN-NW back-layer was not allowed due to its small void size (Figure S6 and S7). The 3D-Gn* particles were trapped in an intertangled network of 1D-CNTs rooting from the flexible PET-MF/PAN-NW substrate. The polymer mat substrates possibly improve the mechanical strength and thus secure the flexibility of the composite electrode (Figure S8). The 3D-Gn* particles were patched with the


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2D-Gn leaflets to facilitate electron conduction between 3D-Gn*’s and between 3D-Gn* and 1DCNT (Figure S9). The resultant (3*+2+1)D composite electrode supported by the flexible substrate showed a strong integrity not allowing chipping off even when the electrode was folded up into an origami crane (Figure 1g) and peeled using 3M Scotch tape (Figure S10). It should be emphasized that neither adhesive binders nor electrically conductive current collectors were used in the (3*+2+1)D composite electrodes. The 1D-CNT played a role of binder responsible for cohesion, adhesion and three-dimensional current collectors. Accordingly, the weight was saved by replacing binder, conductive agent and metal foil current collector by the 1DCNT and the flexible substrate. At a loading of 1 mg 3D-Gn* cm-2, for example, the weight of the (3*+2+1)D composite electrode supported by PET-MF/PAN-NW was 39 % of the weight of a conventionally fabricated electrode of 3D-Gn*-loaded aluminum foil in the presence of a binder (3D*|Al) (Figure 1h; refer to mass benefit section in Supporting Information). The (3*+2+1)D composite electrode was designed to fully utilize the surface area of 3DGn* by overcoming the contact resistance between 3D-Gn* particles and between 3D-Gn* and 1D-CNT. The 1D-CNT provided the major conduction highways to the (3*+2+1)D composite electrode, which was evidenced by the high electric conductivity of 1D-CNT layer anchored to flexible substrates (295 S cm-1 in Figure 1i). The value seriously decreased to 37 S cm-1 when 3DGn* was introduced to the intertangled network of ID-CNT ((3*+1)D in Figure 1i). The 3DGn*/1D-CNT contact resistance was expected to be significantly high. However, the conductivity increased back to 93 S cm-1 after 2D-Gn was introduced to the (3*+1)D composite electrode ((3*+2+1)D in Figure 1i; 3D* : 2D = 3 : 1 in weight). The characteristic length of the 2D-Gn leaflets (width), larger than those of 3D-Gn* (dimension of solid skeleton) and 1D-CNT


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(diameter), is possibly beneficial from the conductivity standpoint. Therefore, the 2D-Gn would contribute to decreasing contact resistance by widening electron channels between two domains.

Capacitance. Coin-type cells of symmetric EDLCs having the multi-dimensional carbon composite electrodes as both electrodes were assembled and tested. 1 M tetraethylammonium tetrafluoroborate in acetonitrile was used as the electrolyte of the EDLCs. To estimate the contribution of each component to overall capacitance of the (3*+2+1)D composite, the capacitances of 1D, (2+1)D and (3*+1)D were measured (Figure S11). Then, the capacitances of 2D and 3D* were calculated by: 2D = (2+1)D – 1D; 3D* = (3*+1)D – 1D. The calculated capacitance of 3D-Gn* (or 3D*) was ~ 90 F g3D-1, significantly higher than those of 2D-Gn and 1D-CNT (2.5 to 5 F g2D-1 and 17 F g1D-1, respectively). More interestingly, the capacitance of 3D* in (3*+2+1)D was estimated at 124 or 159 F g3D-1 by (3*+2+1)D – 2D – 1D, which was even higher than the value calculated from (3*+1)D – 1D (~90 F g3D-1). The capacitance comparison confirms that the conductive 2D-Gn improved the effective utilization of 3D-Gn*. The 2D-Gn decreased the contact resistance between 3D-Gn* as the main active material and 1D-CNT as the electric highway. Therefore, the surface of 3D-Gn* could be fully utilized for forming electric double layer by the help of 2D-Gn. The conductivity improvement observed by adding 2D-Gn to (3*+1)D confirmed the decrease in contact resistance (Figure 1i). From the following paragraph, we mainly focused on the capacitance of 3D-Gn* to estimate the efficient utilization of the surface of 3D-Gn* for electric double layer formation. The component capacitance of 3D-Gn* (C3D*) was calculated by subtracting the contribution of 1D and 2D from the total capacitance. The element capacitances of 1D and 2D were fixed at 17 F g-1 and 3.8 F g-1, respectively (the average values of capacitances obtained at 20 and 200 mV s-1 in Figure S11).


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Higher capacitances were obtained from the (3*+2+1)D-based EDLC when compared with the (3*+1)D-based EDLC (Figure 2a to c, S12 and S13a for cyclic voltammograms; Figure 2d to f and S13b for chronopotentiometric curves): (3*+2+1)D versus (3*+1)D = 182 versus 114 F g1

, and 28 versus 20 mF cm-2 from cyclic voltammograms at 20 mV s-1; 225 F g-1 versus 132 F g-1 ,

and 37 versus 22 mF cm-2 from galvanostatic discharges at 0.5 A g-1, and 0.075 A cm-2; The use of 2D-Gn in the multi-dimensional carbon composites was clearly helpful to utilizing the surface area of 3D-Gn* in a more efficient way that electrons reach a whole surface of 3D-Gn* by improving contact resistance between particles. The capacity retention along repeated cycles of charge and discharge at 10 A g-1 was 98 % for (3*+2+1)D and 97 % for (3*+1)D at 10,000 cycles; 82 % for (3*+2+1)D and 73 % for (3*+1)D at 100,000 cycles (Figure 2g). The EDLC cells based on the (3*+2+1)D composite electrodes showed higher energy densities in comparison with the cells based on the (3*+1)D electrodes (Figure 2h and S13c): 57 to 70 Wh kg-1, and 10 to 12 μWh cm-2 for (3*+2+1)D; 34 to 41 Wh kg-1, and 6 to 7 μWh cm-2 and for (3*+1)D. The (3*+2+1)Dbased EDLC was superior to previously reported graphene-based supercapacitors in terms of capacitance and cycle durability (Table S1) as well as energy and power densities (Figure 2h and S13c). High capacitance of active materials on electrodes (~200 F g-1 for 3D-Gn* versus ~100 F g-1 for practical activated carbon) and high cell potential at 3 V supported by organic electrolyte were responsible for the high energy densities.

2D-Gn. The improved capacitance of 3D-Gn* achieved by the use of 2D-Gn in the 1D-CNT matrix was confirmed by the change in mass (Figure 3a and b). Ions are accumulated in electric double layer on electrode surface when potential was swept away from the point of zero charge. Therefore, capacitance is directly correlated to the change in electrode mass during a potential sweep. Gold-


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plated quartz crystal resonators coated with a mixture of carbons were used as a working electrode with the same electrolyte used in the EDLCs. Potential was swept from the rest potential found around -0.1 VAg/Ag+ (V versus Ag/Ag+) to -0.6 VAg/Ag+. Frequencies of the resonators as well as currents were measured during the potential scan. Frequency losses indicating mass gains were observed in both (3*+2+1)D and (3*+1)D composites. Tetraethylammonium ion (TEA+) was considered to form electric double layer during the negative potential sweep and therefore it would be the main species of the mass gain. After the backward potential scan, the frequency returned to the initial value at the rest potential, confirming the reversible behaviors of electric double layer formation. The change in frequency of (3*+2+1)D was about -50 Hz at -0.6 VAg/Ag+, which is nearly 30 Hz more negative than that of (3*+1)D. It indicates that more amount of TEA+ was adsorbed on the (3*+2+1)D composite electrode than on the (3*+1)D. Moreover, the mass difference between (3*+2+1)D and (2+1)D (40 ng) was much larger than the difference between (3*+1)D and 1D-CNT (19 ng) (Figure 3c and S14). Electrochemically available surface area (AEQCM) was calculated from the mass differences of the adsorbed ions under the assumption that a monolayer of spherical TEA+ covers a whole surface area of active materials. The AEQCM of 3DGn* in the presence of 2D-Gn was estimated two times as large as the AEQCM of 3D-Gn* in the absence of 2D-Gn (Figure 3c). Therefore, it is confirmed that the 2D-Gn encouraged the efficient utilization of the surface of 3D-Gn* without allowing dead area. Resultantly, capacitance of 3DGn* was improved by the use of 2D-Gn. Porous structures of the multi-dimensional carbon composites were analyzed from the viewpoint of ions responsible for electric double layer formation. Impedance spectra of capacitive processes on porous electrodes deviate significantly from that of an ideal capacitor represented by a vertical line in Nyquist plots. Two frequency dispersions are involved in the non-ideal behavior


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of porous electrodes.36-38 The first frequency dispersion is found along pore length: ions do not reach the bottom of pores at high frequencies so that a whole surface of pores is not detected by ions. The classical de Levie model describes this in-a-pore dispersion where the ideally capacitive vertical line is shifted by a 45-degree-phase-angle line at high frequencies.39 The second frequency dispersion is caused by the distribution of pore sizes. Even at the same frequency, the penetration length of ions is determined by pore size: ions detect surface at deeper location of larger pores. In this situation where pore size is distributed, the impedance even at low frequencies does not follow the ideal capacitive behavior. Instead of a vertical line (phase angle = 90 degree), a constant-phaseangle line (45 degree < phase angle < 90 degree) was obtained at low frequencies, following the 45-degree line at high frequencies like the de Levie model. The transmission line model with pore size distribution (TLM-PSD) proposed by Song et al. describes this pore-size-distribution dispersion as well as the in-a-pore dispersion.36-38 Therefore, the impedance spectra of multidimensional carbon composite electrodes were fitted by the TLM-PSD to obtain fitting parameters (Figure S15). The average pore length of (3*+2+1)D was estimated at 200 μm, which is longer than that of (3*+1)D at 130 μm (Figure 3d and e). The 2D-Gn possibly connected a dead area of 3D-Gn* in (3*+1)D electrode to electric pathways so that additional tortuous surface was involved in electrochemically available surface. Flexibility. In terms of cohesion and/or adhesion strengths, the (3*+2+1)D and (3*+1)D composite electrodes supported by the flexible substrate were compared with the conventional 3D*|Al electrode by tracing mass changes along repeated crumpling (Figure 4a). As expected, the mass decrease caused by detachment of constituent materials was significantly reduced when our proposed architecture based on the intertangled network and anchoring to the external flexible substrate was adopted. The masses of (3*+2+1)D and (3*+1)D were nearly unchanged even after


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1000 times crumpling. The PET-MF/PAN-NW as the flexible substrate was mechanically durable against the repeated abuse (the right photo of Figure 4a). However, the active mass in the active layer attached to the metal foil by the help of a binder was easily detached from the 3D*|Al electrode. The mass decrease was serious even in 50 crumpling, confirming that cohesion and adhesion strengths achieved by using binders are very limited. Moreover, the aluminum foil of 3D*|Al electrode was torn or worn after the repeated crumpling (the left photo of Figure 4a). The mechanical durability of our flexible architecture, especially (3*+2+1)D, discouraged a serious increase of resistance along the repeated folding (Figure 4b). Neither (3*+2+1)D nor (3*+1)D reached an electric failure by serious resistance development even if the (3*+2+1)D showed lower resistance and its slower increase along folding repetition than the (3*+1)D. From the morphological standpoint, on the other hand, the folding lines were clearly identified in both (3*+2+1)D and (3*+1)D composite electrodes at the 500th and 1000th folding repetition (Figure 4c to h). However, there were no cracks developed in the (3*+2+1)D (Figure 4d and e) while clear cracks were found in the (3*+1)D (Figure 4g and h). In the absence of 2D-Gn, the cracks resulted in discontinuity of carbon mass. The crack development in (3*+1)D after folding repetition explains the faster increase of resistance of the (3*+1)D along the folding cycles than that of the (3*+2+1)D (Figure 4b). The mechanically durable and therefore electrically durable properties of the multidimensional composite electrodes supported no significant changes of cyclic voltammograms of the EDLC cells based on the (3*+2+1)D even when they were bent and folded (Figure 5a). Also, there were no differences of galvanostatic cyclability between the unfolded cell, the 90-degreebent cell and the completely folded cell during 10,000 cycles of repeated charge and discharge: 92.4 % capacity retention at the 10,000th cycle and 71 % at the 100,000th cycle for the completely


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folded EDLC cell based on the (3*+2+1)D composite electrodes (Figure 5b). The cycle durability of the (3*+2+1)D under the mechanical abuse conditions was superior to those of previously reported flexible supercapacitors (Figure S16). It was demonstrated that a light-emitting diode (LED) connected to a charged EDLC cell emitted light even after the EDLC was half-folded, twice half-folded, rolled and crumpled (Figure 5c).

CONCLUSION The (3*+2+1)D composite layer assembled with a flexible polymer substrate was successfully demonstrated as a flexible EDLC electrode. Three different dimensional carbons were combined hierarchically to ensure electric pathways as well as to guarantee mechanical integrity. The (3*+2+1)D-based EDLCs were mechanically durable against bending, folding rolling and even crumpling without sacrificing energy densities. This work shows that architectural design of electrodes is important to fully utilize intrinsic capacitance or capacity of active materials and to make electrodes durable against mechanical stress.


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EXPERIMENTAL SECTION Materials. 3D-Gn was synthesized by calcining a polymeric carbon precursor in a hard template in the presence of a catalyst for graphitization (Figure S17). The detailed procedure for the 3DGn synthesis was described in our previous reports.33-35 9 g fumed silica (SiO2 as the hard template; 5 nm in size; Sigma-Aldrich) was dispersed in 45 ml aqueous solution containing 5 g polyvinylpyrrolidone (PVP as the carbon precursor) and 17.5 g FeCl3•6H2O (as the catalyst precursor). The mixture was vacuum-dried at room temperature (Step 1) and then pyrolyzed at 900 o

C for 30 min in a continuous flow of H2/Ar at 100/1000 cc min-1 (Step 2). During this pyrolysis

process, Fe ions (III) are reduced to Fe metal. Simultaneously, PVP is catalytically decomposed and diffused into the reduced Fe species covering the surface of SiO2 particles. Upon cooling, graphitic carbons are precipitated on the surface of the Fe species. Silica as well as iron species was etched from the resultant black powder by using 5 wt. % HF/HCl solution for 24 h (Step 3). 3D-Gn powder was recovered by filtering the solution, followed by washing several times. The 3D-Gn was activated at 900 oC for 15 min under flowing Ar/CO2 at 100/150 cc min-1. Temperature was ramped at 15 oC min-1 to 900 oC. Heating and cooling processes were conducted in an Ar atmosphere. 2D-Gn (XGnP graphene nanoplatelet grade C from XG science) and 1D-CNT (singlewalled CNT dispersed in water from KH Science, Korea) were used as received. Electrodes and cells. A mixture of 1 mg 3D-Gn* and 0.3 mg 2D-Gn dispersed in 1.5 ml of 0.1 wt. % 1D-CNT (aq) was vacuum-filtrated through PET-MF/PAN-NW for fabricating (3*+2+1)D electrode. The same mixture excluding 2D-Gn was used for making (3*+1)D electrode. Symmetric supercapacitor coin cells were assembled by sandwiching a commercial cellulose fiber membrane between two carbon composite electrodes supported by the flexible substrate described above. 1 M TEABF4 in acetonitrile was used as electrolyte.


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Characterization. Morphological images were obtained by using a scanning electron microscope (S-4800, Hitach, 10 kV) and a transmission electron microscope (JEM-2100, JEOL, 200 kV). Surface area and pore size distribution were measured by using nitrogen physisorption analyzer (ASAP2420,

Micromeritics

Instruments).

X-ray photoelectron

spectroscopy (K-alpha,

ThermoFisher) and Raman spectroscopy (alpha300R, WITec) were also used. Cyclic voltammograms and chronopotentiometric curves were obtained by using a potentiostat (VSP-300, Biologic). Electrochemical quartz crystal microbalance (EQCM). Active materials were loaded on goldcoated quartz crystal resonators. Nafion was used as a binder. The active-material-loading resonator was used as a working electrode. Platinum plate and Ag/Ag+ were used as a counter electrode and a reference electrode, respectively. Mass of ions participating in forming electric double layer (m) on electrodes were calculated by multiplying frequency change (∆f) by a sensitivity factor of 900 kHz resonator (1.069 ng Hz-1). Mass difference (∆m) between (3*+2+1)D and (2+1)D or between (3*+1)D and 1D was a measure of the amount of cations participating in electric double layer formation on 3D-Gn*. Electrochemically available surface area (AEQCM) was calculated from ∆m with the assumption that cation monolayer was formed on the surface of 3DGn*40-41: |∆m| AEQCM = MW

TEA+

NAvogadroπR2 / M3D

where NAvogadro = Avogadro number; M3D = loading mass of 3D-Gn* on resonator = 3.33 μg; MWTEA+ = molecular weight of TEA+ = 130.25 g mol-1; R = radius of TEA+ = 0.68 nm25.


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Figure 1. Hierarchically structured multi-dimensional composite of 3D-Gn* and 2D-Gn entangled by 1D-CNT web anchored to flexible PET-MF/PAN-NW substrate. (a) (3*+2+1)D on PETMF/PAN-NW in a schematic. (b) Porous three-dimensional graphene framework (3D-Gn*) in a SEM image, fabricated by etching of fumed silica after calcination of polyvinylpyrrolidone (PVP). (c) Two-dimensional graphene (2D-Gn) in a TEM image. (d) Single-walled carbon-nanotube (1D-


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CNT) in a TEM image. (e) (3*+2+1)D on PET-MF/PAN-NW in a SEM image. (f) Cross-sectional view of (3*+2+1)D on PET-MF/PAN-NW in a SEM image. (g) An origami crane made by folding (3*+2+1)D on PET-MF/PAN-NW. (h) Mass benefits obtained by using the current-collector-free and binder-free (3*+2+1)D|PET-MF/PAN-NW electrode architecture. m/mo = the relative mass ratio of the (3*+2+1)D|PET-MF/PAN-NW electrode architecture (m) to the conventional 3D-Gn*loaded aluminum foil architecture (3D*|Al) (mo). Loading = the areal mass density of materials loaded on substrates (PET-MF/PAN-NW substrate or aluminum foil). (i) Electric conductivities (σ) measured by 4-point probe method.


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Figure 2. Capacitance of (3*+2+1)D versus (3*+1)D. (a and b) Cyclic voltammograms (CVs). Scan rates were indicated in mV s-1. Refer to Figure S12 for the CVs at higher scan rates. (c) Scan rate dependency of capacitance. (d and e) Galvanostatic voltage profiles. Current densities were indicated in A g-1. (f) Capacitances calculated from galvanostatic discharging profiles. (g) Capacitance retention along repeated 100,000 cycles of charging and discharging at 10 A g-1. (h) Energy density versus power density in Ragone plot. The (3*+2+1)D and (3*+1)D were compared with previously reported graphene supercapacitors.42-49


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Figure 3. Electric double layer formation investigated by electrochemical quartz microbalance. (a and b) Frequency changes (∆f) during voltage sweep at 10 mV s-1. Potential was swept from open circuit potential to -0.6 V versus Ag/Ag+ and then reversed. EAg/Ag+ = potential versus Ag/Ag+ reference electrode. (3*+2+1)D or (3*+1)D was loaded on gold-coated quartz resonators by using nafion binder. (c) Mass difference (∆m) between (3*+2+1)D and (2+1)D or between (3*+1)D and 1D on the left y-axis; electrochemically available surface area (AEQCM) on the right y-axis. Mass of ions participating in forming electric double layer (m) on electrodes were calculated from ∆f. AEQCM was calculated from ∆m with the assumption that cation monolayer was formed on the surface of 3D-Gn*. (d) Electrochemical impedance spectra. Experimental data were fitted by transmission line model with pore size distribution (TLM-PSD). (e) Pore lengths of 3DGn* in (3*+2+1)D and (3*+1)D, calculated from TLM-PSD.


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Figure 4. Flexibility of (3*+2+1)D or (3*+1)D on PET-MF/PAN-NW versus 3D*|Al. (a) Mass loss on repeated crumpling. (b) Resistances developed along repeated folding. (c to h) Top views around folding lines in SEM images at 0, 500 and 1000 times folding: (3*+2+1)D on the left versus (3*+1)D on the right.


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Figure 5. Folded supercapacitors. (a) CVs of a flexible supercapacitor based on a symmetric (3*+2+1)D electrode configuration on bending at 0o, 45o, 90o, 135o and 180o. (b) Capacitance retention along charge/discharge cycles on bending at 0o, 90o and 180o. (c to e) Illumination from light-emitting diodes (LEDs) powered by supercapacitors when folded, rolled and crumpled.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Surface analysis, XRD, XPS of 3D-Gn, 3D-Gn*, 2D-Gn, 1D-Gn; SEM, TEM, AFM images of each materials and electrodes; peel test images; tensile strain-stress curves; electrochemical analysis of 1D, (2+1)D, (3*+1)D, and (3*+2+1)D including cyclic voltammetry, EQCM, TLMPSD; comparison of capacitance and durability of ours with other reported one (PDF)


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID Chihyun Hwang: 0000-0001-7469-3119 Ji-Hyun Jang: 0000-0003-4364-6605 Hyun-Kon Song: 0000-0001-7914-4186 Author Contributions Y.Lee., M.-J.Kwak, and C.Hwang. contributed equally to this work.

ACKNOWLEDGMENT This work was supported by NRF (GPF: 2016H1A2A1909427, Basic: 2018R1D1A1B07042773), MOTIE (Open Lab (KIAT): P0002068) and MOE (BK21Plus: 10Z20130011057), Korea.


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2D-Gn

3D-Gn* 1D-CNT

Polymer mat

Rolled

Crumpled


Hierarchically Structured Multidimensional Carbon Composite

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