Supplementary Materials for. A bamboo-inspired nanostructure design for flexible, foldable and twistable energy storage devices

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1 Supplementary Materials for A bamboo-inspired nanostructure design for flexible, foldable and twistable energy storage devices Yongming Sun, Ryan B. Sills, Xianluo Hu,* Zhi Wei Seh, Xu Xiao, Henghui Xu, Wei Luo, Huanyu Jin, Ying Xin, Tianqi Li, Zhaoliang Zhang, Jun Zhou, Wei Cai, Yunhui Huang,* and Yi Cui* * Corresponding author. huxl@mail.hust.edu.cn (X.H.); huangyh@mail.hust.edu.cn (Y. H.); yicui@stanford.edu (Y. C.) This file include: Materials and Methods S1-S7 Figures S1-S13 Movies S1-S3 References

2 Table of contents Materials and Methods Method S1: Modulus change calculations due to porosities Method S2: Numerical simulation Method S3: Synthesis of bamboo-like carbon nanofibers Method S4: Preparation of the amorphous solid nanofibers Method S5: Materials Characterization Method S6: Electrochemical characterization of bamboo-like carbon nanofibers Method S7: Fabrication and characterization of all-solid-state supercapacitors Figures Figure S1: A typical digital image of the as-electrospun tetraethyl orthosilicate (TEOS)/ polyacrylonitrile (PAN) nanofiber web. Figure S2: Scanning electron microscope (SEM) images of the as-spun TEOS/PAN nanofibers. Figure S3: A typical digital image of a piece of carbon nanofiber web. Figure S4: Morphology and component of the products obtained at 1200 ºC before and after SiO 2 etching. Figure S5: Nitrogen-sorption analysis of the bamboo-like carbon nanofibers. Figure S6: Transmission electron microscopy (TEM) images of carbon nanofibers with tunable pore structures. Figure S7: SEM images of the products obtained at 1300 ºC. Figure S8: Electron energy loss spectroscopy (EELS) spectrum of the bamboo-like carbon nanofibers. Figure S9: X-ray diffraction (XRD) pattern of the bamboo-like carbon nanofibers. Figure S10: Survey X-ray photoelectron spectroscopy (XPS) spectrum and high-resolution C 1s spectrum (inset) of the bamboo-like carbon nanofibers. Figure S11: Raman spectrum of the bamboo-like carbon nanofibers. Figure S12: C and C plots of the as-prepared all-solid-state flexible supercapacitor. Figure S13: Structure of the as-prepared device after long-term cycling and continuous mechanical deformation operations. 2

3 Movies Movie S1: Mechanical durability test of a bamboo-like carbon nanofiber web. Movie S2: Mechanical durability test of a solid carbon nanofiber web. Movie S3: Electrochemical performance test of a prepared all-solid-state supercapacitor under continuous mechanical deformation conditions. Materials and Methods Method S1: Modulus change calculations due to porosities A number of theoretical approaches exist for estimating the change of elastic constants due to porosity. We choose to use the differential scheme, which has been shown to predict changes in modulus due to porosities over the range 0 to 70% in sintered glass well. 1 In this approach, the effect of a sequence of infinitesimal increments in porosity on the elastic constants is considered using elasticity theory, leading to a set of coupled ordinary differential equations (ODEs). The solution of these ODEs is the following pair of coupled, implicit algebraic equations, which can be solved for the bulk modulus, K, and shear modulus, G, at given porosity p: 1 G/K = ¾ + 3(1-5ν m )/[4(1+ν m )](G/G m ) 3/5 G/G m = (1-p) 2 {[2(1+ ν m )+(1-5ν m )(G/G m ) 3/5 ]/[3(1- ν m )]} 1/3 where ν is Poisson s ratio and the subscript m denotes the matrix material (carbon here). The Young s modulus of the porous material is then given by E = 9KG/(3K+G). In our case, assuming the modulus change is due to both micro- and mesopores, we have a porosity of p = 0.79, and with ν m = 0.22 we get E/E m = When assessing the effects of porosity on the mechanical properties of ceramics, a commonly used empirical relation is 2 E = E m exp(-bp), where b is a non-dimensional material parameter. For polycrystalline graphite, b has been found to fall in the range 0.6 to 3.2 for porosities up to about 30%, 2 which leads to E/E m to While there is considerable scatter, the lower bound on the range agrees with the theoretical estimate in order of magnitude. In light of these two results, we believe that the micro- and mesoporosity causes the nanofiber stiffness to be reduced to approximately 4% of its base material value. Method S2: Numerical simulation 3

4 To demonstrate the superior mechanical performance and flexibility of the bamboo-like nanofibers, a series of three-point bending simulations of different fiber geometries were conducted using the finite element code ABAQUS v6.12, with the fiber ends constrained and a upward load applied in the middle. Fibers had the following geometries: All fibers: L/r o = 33.3; Bamboo fiber: r i /r o = 5/6, w/r o = 1/3; Tubular fiber: r i /r o = (selected to give same moment of inertia as bamboo fiber), where L = length, r o = outer radius, r i = inner radius, and w = width between pores. Symmetry boundary conditions were applied at the midplanes. The right end was bonded to a rigid plate whose midnode was only allowed to translate in the x-direction (left to right). A traction distribution of the form T max cos[xπ/(2w)], x < w was applied on the underside of the nanofibers over a rectangular region of material spanning one-third of the fiber circumference; this imitates the interaction between contacting nanofibers. The material behavior was isotropic linear elastic with a Poisson s ratio of 0.3, and finite deformations were considered. Method S3: Synthesis of bamboo-like carbon nanofibers Tetraethyl orthosilicate (TEOS)/polyacrylonitrile (PAN) composite nanofibers were first electrospun on a piece of aluminium foil from a precursor solution that contained TEOS (2.4 g), PAN (average M w = , 1.8 g), and N, N-dimethylformamide (DMF, 18 ml). The voltage of 15 kv was applied over a collector distance of 12 cm and the flow rate was fixed at 1 ml h 1. A film of TEOS/PAN nanofiber web was peeled off from the collector after electrospinning. The bamboo-like carbon nanofibers were finally obtained after calcining at 1200 C for 12 h in Ar /H 2 (95/5 in volume) atmosphere and etching SiO 2 in 10 wt % HF aqueous solution. Method S4: Preparation of the amorphous solid nanofibers The solid carbon nanofibers were synthesized through electrospinning of PAN nanofibers, followed by their carbonization at 800 ºC in Ar /H 2 (95/5 in volume) atmosphere. Prior to electrospinning, 2 g of PAN (average M w = ) was dissolved in 20 ml of DMF under magnetic stirring. The as-formed homogeneous solution was loaded into a plastic syringe with a stainless steel nozzle. The electrospinning process was carried out in the electric field generated by an applied voltage of 15 kv and a collector distance of 12 cm. The flow rate of the electrospun solution was fixed at 1 ml h 1. Method S5: Materials Characterization The morphology of the carbon nanofiber webs was characterized by a field-emission scanning electron microscope (FE-SEM, FEI Sirion 200) equipped with an energy dispersive X-ray spectrometer (EDX, Oxford Instrument). Transmission electron microscopy (TEM) images and Electron energy loss spectroscopy (EELS) spectra were 4

5 recorded in a Tecnai F30 STwin microscope equipped with a Gatan image filter (GIF) and a high angle annular dark field (HAADF) detector (Fischione). Nitrogen sorption analysis was performed at 77 K using a Micrometritics ASAP 2020 analyzer. Specific surface area (SSA) was calculated with Brunauer-Emmett-Teller (BET) method, with a pressure range from 0.05 to The total pore volume was determined at a relative pressure of P/P 0 = Pore size distribution was calculated via nonlocal density functional theory (NLDFT) method by using nitrogen adsorption data and assuming a slit pore model. For estimating the porosity, we use 2 g/cm 3 as the real density of porous carbon fibers. The X-ray diffraction (XRD) patterns were collected on a X Pert PRO (PANalytical B.V., Holland) diffractometer with high intensity Cu Kα1 irradiation (λ = Å). Raman spectroscopy was performed on a Laser Micro-Raman Spectrometer (Renishaw invia) with an Ar + laser of nm excitation at room temperature. X-ray photoelectron spectroscopy (XPS) analyses were conducted with a VG MultiLab 2000 system with a monochromatic Al Kα X-ray source (ThermoVG Scientific). Method S6: Electrochemical characterization of bamboo-like carbon nanofibers Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were conducted using a conventional three-electrode configuration on an Autolab PGSTAT302N (Metrohm AG) and a CHI660D (CH Instruments). The carbon nanofiber webs were flexible and mechanically robust and thus could be directly used as electrochemical capacitor electrodes without any additional binders or conductive additives. An Hg/HgO electrode and a platinum gauze electrode served as the reference and the counter electrodes, respectively. All the electrochemical measurements were performed in a 3 M KOH solution at room temperature. The specific capacitance was calculated according to the equation: Cs = I t/m V (1) where Cs is the specific capacitance (F g 1 ), I is the discharge current (A g 1 ), t is the discharge time (s), m is the weight of the active material (g), and V is the discharge voltage (V). Method S7: Fabrication and characterization of all-solid-state supercapacitors The H 3 PO 4 /PVA gel electrolyte was prepared at 85 ºC by dissolving PVA power (6 g) in a mixed solution of H 3 PO 4 (6 g) and deionized water (60 ml). Two identical pieces of rectangular porous carbon nanofiber webs were assembled together and separated by a cellulose separator (NKK TF40, 40 µm), followed by homogeneously coating the as-prepared gel electrolyte on them. The all-solid-state supercapacitors were finally prepared after the H 3 PO 4 /PVA gel solidified. 5

6 Electrochemical properties were investigated in a two electrode configuration at room temperature using an Autolab PGSTAT302N (Metrohm AG) and a CHI660D (CH Instruments). Electrochemical impedance was carried out in the frequency range from 100 khz to 10 mhz with a potential amplitude of 10 mv. The volumetric specific capacitance of the supercapacitors could be achieved from the galvanostatic curves at different current densities using the following equations: Cv = I t/v E (2) Where Cv is the volumetric specific capacitance (F cm 3 ), I is the discharge current (ma), t is the discharge time (s), E is the potential window obtained from the discharge curve excluding the IR drop (V), and V is the volume (cm 3 ), respectively. It is worth to note that the volumetric capacitances were calculated taking into account the total volume of the devices, that includes the active material, the separator and the electrolyte. The energy density and power density of each device were calculated from the galvanostatic curves using the formulas given in Equation (3) and (4): E = 0.5C( U) 2 /V (3) P = ( U) 2 /4RV (4) where E is the volumetric energy density (Wh cm 3 ), C is the capacitance obtained from Equation (1) in F cm 3, U is the cell voltage, V is the volume (cm 3 ), P is the volumetric power density (W cm 3 ) and R is the internal resistance of the devices estimated from the voltage drop ( U drop ) at the beginning of the discharge at a constant current density (I) using the formula of R = U drop /2I. 6

7 Figures Figure S1. A typical digital image of the as-electrospun tetraethyl orthosilicate (TEOS)/polyacrylonitrile (PAN) nanofiber web. Figure S2. Scanning electron microscope (SEM) images of the as-electrospun TEOS/PAN nanofibers. 7

8 Figure S3. A typical digital image of a piece of carbon nanofiber web. Figure S4. Morphology and component of the products obtained at 1200 ºC before and after SiO 2 etching. (a, b) SEM images of the SiO 2 /carbon composite nanofibers after thermal treatment at 1200 ºC. The circles in (b) indicate the existence of SiO 2 particles in the core region of the SiO 2 /carbon composite nanofibers. (c, d) The energy dispersive X-ray (EDX) spectra of (c) the SiO 2 /carbon composite nanofibers and (d) the carbon nanofibers after SiO 2 removal. The presence of Si, O and C elements in the SiO 2 /carbon composite and the complete removal of SiO 2 in the final product are confirmed. 8

9 Figure S5. Nitrogen-sorption analysis of the bamboo-like carbon nanofibers. (a) Nitrogen adsorption-desorption isotherm. (b, c, d) Pore-size distribution curve (b), cumulative pore volume (c), and cumulative surface area (d) calculated by using a slit NLDFT model. Nitrogen-sorption analysis was carried out to investigate the pore-size distribution, pore volume, and SSA of the product (Figure S5). The representative nitrogen adsorption desorption isotherm (Figure S5a) shows the details of the low-pressure region in which micropore filling occurs, as well as the high-pressure region with slightly steep adsorption, which reveals pore condensation and a combined I/IV type adsorption-desorption hysteresis, indicating that a hierarchical porous system with micro-, meso- and/or macropores exists in the as-prepared carbon nanofibers. 3 5 The SSA of the carbon nanofibers is as high as ~1912 m 2 g 1 calculated in the range of P/P 0 by the BET model and the total pore volume is ~2.27 cm 3 g 1 evaluated at a single-point adsorption at P/P 0 = The pore volume for micro- and mesopores ~1.87 cm 3 g 1. Since there are no macropores in the shells of the carbon nanofibers, 79% porosity of the shells is estimated. Figure S5b shows the results of the pore size analysis by applying an advanced NLDFT, assuming a slit pore geometry with a regularization of Besides the micropores 9

10 located at 0.64 and 1.32 nm, the as-prepared carbon nanofibers also have continuous mesopores in the range of 2 to 50 nm and macropores in the range of 50 to over 100 nm. Figure S5c and d show the cumulative pore volume and cumulative surface area using the NLDFT model. It shows that micropores account for only 15.5% of the total volume but 50.7% of the surface area. Thus, well-defined micro-, meso- and macropores coexist in our bamboo-like carbon nanofibers, indicating a hierarchically porous structure. 10

11 Figure S6. Transmission electron microscopy (TEM) images of carbon nanofibers with tunable pore structures. Various carbon nanofibers with tunable pore structures: Carbon nanofibers prepared via heat treatment at different temperatures: (a) 600, (b) 800, (c) 1000, (d) 1100 and (e) 1300 ºC, respectively. Carbon nanofibers prepared with various TEOS/PAN mass ratios: (f) 0, (g) 1 and (h) 1.8, respectively. 11

12 Figure S7. SEM images of the products obtained at 1300 ºC. (a, b) Before (a) and after SiO 2 etching (b). One has great versatility in manipulating both the macrostructure and the microstructure of the carbon nanofiber webs. The thickness of the webs can be turned by controlling the electrospinning time. The 2D films with desired shapes and sizes can be easily selected by traditional cutting. Further structural modification and control of the carbon nanofiber webs can be achieved by varying the processing temperature and mass ratios of TEOS to PAN in the precursor. The influence of the reaction temperature and the amount of TEOS in the carbon nanofibers on the morphology and structure of the products was systematically investigated. When the TEOS/PAN nanofibers were thermally treated at a low temperature (e.g., 600 ºC, 800 ºC), ultrafine SiO 2 nanoparticles were produced. After removal of the SiO 2 particles, abundant micropores (< 2 nm) were formed and homogeneously distributed in the carbon nanofibers (Figure S6a and b). As the thermal treatment temperature increased from 800 ºC to 1200 ºC, the size of SiO 2 nanoparticles increased gradually and tended to be inlaid in the core section of the carbon nanofibers. Meanwhile, the number of these SiO 2 particles reduced accordingly. Nanopores with identical sizes and amounts were eventually generated in the products after SiO 2 etching (Figure S6c and d, and Figure 1d). A typical example has mesopores of ~20 nm in diameter in the core and meso/micropores of < 5 nm in the shell, and the structure could clearly be observed within the carbon nanofibers obtained via heat treatment at 1100 ºC (Figure S6d). When the electrospun TEOS/PAN nanofiber web was thermally treated at a high temperature of 1300 ºC, the morphology changed from straight to undulated fibers and these nanofibers greatly shrank compared to those obtained between 800 ºC and 1200 ºC. Moreover, the SiO 2 particles with sizes larger than the diameters of these nanofibers were 12

13 produced and distributed on the external surface of the nanofibers (Figure S7). Thus, compared with that synthesized at 1200 ºC, there were no macropores or mesopores in large size in the carbon nanofibers obtained at 1300 ºC (Figure S6e). Also, the macropores were not well developed if a lower or higher amount of TEOS was used. As clearly displayed in Figure S6f, the hierarchically porous carbon nanofibers cannot be obtained by heat treatment of the pure PAN nanofibers. If a precursor with a lower mass ratio of TEOS/PAN (1:1) was used, the obtained nanofibers exhibited a more thick shell (Figure S6g). Besides, for a precursor with a high mass ratio of TEOS/PAN (1.8:1), continuous SiO 2 wires, instead of evenly lined SiO 2 particles, tended to grow in the core region of the nanofibers, forming a cable-like structure. Carbon nanotubes with a thin shell would be formed finally after removal of SiO 2 (Figure S6h). Therefore, our studies represent substantial progress towards producing carbon nanofibers engineered with tunable porosity, opening diverse functionality that is advantageous for fundamental investigations and numerous, diverse applications. σ Intensity (a.u.) π Energy Loss (ev) Figure S8. Electron energy loss spectroscopy (EELS) spectrum of the bamboo-like carbon nanofibers. 13

14 Intensity (a.u.) Intensity (a.u.) (002) (100) Theta (degree) Theta (degree) Figure S9. X-ray diffraction (XRD) pattern of the bamboo-like carbon nanofibers. Intensity (a.u.) Intensity (a.u.) O-C=O C=O C O C C Binding Energy (ev) O1s C1s Binding Energy (ev) Figure S10. Survey X-ray photoelectron spectroscopy (XPS) spectrum and high-resolution C 1s spectrum (inset) of the bamboo-like carbon nanofibers. 14

15 Intensity (a.u.) D G 2D D+G 2G Raman shift (cm 1 ) Figure S11. Raman spectrum of the bamboo-like carbon nanofibers. Electron energy loss spectroscopy (EELS), Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis, and Raman spectroscopy were further used to elucidate the nature of carbon in the as-formed nanofibers. The EELS measurements (Figure S8) show a 1s-p transition, which reveals the sp 2 -hybridization for carbon at 285 ev, and the fine structure of the C-K edge also suggests a graphitic network with sp 2 -bonding. 6,7 The XRD pattern is shown in Figure S9. A large low-angle scatter is observed for the as-obtained carbon nanofibers, indicating the presence of a high density of nanopores. Two Bragg reflections centered at the 2θ angles of 24º and 44º were attributed to the (002) and (101) crystallographic planes of graphite. These results agree well with that of KOH-activated porous graphene. 8 XPS analysis shows a high C/O atom ratio of 28.2:1, indicating that most of the oxygenated functional groups in the precursor have been removed by heat treatment (Figure S10). The tail between 286 and 290 ev in the high-resolution XPS C 1s spectrum (inset of Figure S10) is due to oxygen-containing groups and energy loss shake-up features. 9,10 Meanwhile, the results of elemental analysis demonstrate the very low contents of hydrogen (1.48 wt %) and oxygen (2.03 wt % of oxygen), in agreement with the XPS results, which leads to a decreased series resistance and outstanding capacitor stability. 11 As a result, the products have a typical conductivity of S cm 1 measured by a standard four-probe method. The 15

16 disordered graphitic nature of the as-synthesized carbon nanofibers is further confirmed by Raman spectroscopy (Figure S11). The Raman band at around 1587 cm 1 corresponds to bond stretching of the sp 2 carbon (the G band), and the peak at around 1336 cm 1 is derived from the breathing mode of aromatic rings (the D band). The intense D band indicates the partial lattice defects of the graphitic carbon nanosheets compared to single graphene layer. This result is consistent with the typical Raman spectra of reduced graphene oxides Im (Z) (Ω cm 2 ) Re (Z) (Ω cm 2 ) Figure S12. Nyquist plots of the as-prepared all-solid-state flexible supercapacitor. Electrochemical impedance spectroscopy (EIS) was used to confirm the fast ion transport within the carbon nanofiber electrodes. A complex plan plot of the impedance data of the as-made device is shown in Figure S12 with an expanded view provided in the inset. The straight line is nearly parallel to the imaginary axis, which indicates that the device functionality is close to that of an ideal capacitor. 16

17 Figure S13. Structure of the as-prepared device after long-term cycling and continuous mechanical deformation operations. Typical SEM images shows that the initial structure and morphology of the as-made all-solid-state supercapacitor remain after 10,000 discharge/charge cycles at 133 ma cm -2 and in-situ mechanical property measurement (bending, twisting), indicating the good structural stability and mechanical durability. Movies Movie S1. Mechanical durability test of a piece of bamboo-like nanofiber web. In this movie a piece of bamboo-like porous carbon nanofiber membrane was repeatedly bent, folded and then recovered after being unloaded. Movie S2. Mechanical durability test of a solid carbon nanofiber web. In this movie a piece of solid carbon nanofiber membrane was slightly bent and failed at a very low bending angle. Movie S3. Electrochemical performance test of a prepared all-solid-state supercapacitor under continuous mechanical deformation conditions. In this movie an all-solid-state supercapacitor using two pieces of hierarchically porous carbon nanofiber webs as electrodes works well under repeated bending and twisting, and exhibits almost 100% capacitance retention calculated based on the CV curves. References (1) Zimmerman, R. W. Elastic moduli of a solid containing spherical inclusions. Mech. Mater. 1991, 12,

18 (2) Rice, R. W. Porosity of Ceramics (Marcel Dekker, New York, 1998). (3) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area and porosity (2nd Edn). (Academic Press, London, 1982). (4) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquérol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, (5) Groen, J. C.; Peffer, L. A. A.; Pérez-Ramı rez, J. Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater. 2003, 60, (6) Papworth, A. J.; Kiely, C. J.; Burden, A. P.; Silva, S. R. P.; Amaratunga, G. A. J. Electron-energy-loss spectroscopy characterization of the sp 2 bonding fraction within carbon thin films. Phys. Rev. B 2000, 62, (7) Reed, B. W.; Sarikaya, M. TEM/EELS analysis of heat-treated carbon nanotubes: experimental techniques. J. Electron Microsco. 2002, 51, S97 S105. (8) Zhu, Y. W.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W. W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; Su, D., Stach, E. A.; Ruoff, R. S. Carbon-based supercapacitorsproduced by activation of graphene. Science 2011, 332, (9) Leiro, J. A.; Heinonen, M. H.; Laiho, T.; Batirev, I. G. Core-level XPS spectra of fullerene, highly oriented pyrolitic graphite, and glassy carbon. J. Elec. Spec. Rela. Phen. 2003, 128, (10) Moulder, J. F.; Sticlke, W. F.; Sobol, P. E.; Bomben, K. D., Chastain, J. (Ed.), Handbook of X-ray photoelectron spectroscopy. (Perkin-Elmer Corp., Eden Prairie, 1992). (11) Pandolfo, A. G.; Hollenkamp, A. F. Carbon properties and their role in supercapacitors. J. Power Sources 2006, 157, (12) Eda, G.; Chhowalla, M. Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics. Adv. Mater. 2010, 22,