Manipulation of Nanoplate Structures in Carbonized Cellulose

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C: Energy Conversion and Storage; Energy and Charge Transport

Manipulation of Nanoplate Structures in Carbonized Cellulose Nanofibril Aerogel for High-Performance Supercapacitor Zhen Zhang, Lei Li, Yan Qing, Xihong Lu, Yiqiang Wu, Ning Yan, and Wen Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06058 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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The Journal of Physical Chemistry

Manipulation of Nanoplate Structures in Carbonized Cellulose Nanofibril Aerogel for High-Performance Supercapacitor Zhen Zhang1, Lei Li1, Yan Qing1*, Xihong Lu2, Yiqiang Wu1*, Ning Yan3, Wen Yang1 1

School of Materials Science and Engineering, Hunan Provincial Collaborative Innovation

Center for High-efficiency Utilization of Wood and Bamboo Resources, Central South University of Forestry and Technology. 2

MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of

Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University. 3 Faculty

of Forestry, University of Toronto, Toronto, Canada.

*Corresponding Authors Yan Qing Affiliation: School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, Hunan 410004, China Email address: [email protected] Phone number: +86 731 85623301 Fax: +86 731 85623301 Yiqiang Wu Affiliation: School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, Hunan 410004, China Email address: [email protected] Phone number: +86 731 85623989 Fax: +86 731 85623989

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The internal structure of carbon aerogels (CAs) is a key factor affecting their electrochemical performance and applications. In this study, the morphology of CAs and their effects on electrochemical properties were investigated via the control of the carbonization temperature. CAs were prepared by freeze-drying combined with high-temperature carbonization using wood cellulose as the raw material. The nanoplate and nanofiber structures of CAs were manipulated via the control of the carbonization temperature. Results revealed that the volume shrinkage as well as independent nanofibers of CAs increase with temperature, but the nanoplate structure decreases. The nanofiber and nanoplate structures affect the specific surface area (SSA) and electrochemical performance of carbon materials. At a temperature of 800 °C, the highest SSA and total pore volume of CAs were observed, with excellent electrochemical stability (after 5000 cycles of cyclic voltammetry testing, it can still keep 89.43% of the original specific capacitance), and the specific capacitance up to 172.7 F g−1. Finally, a symmetrical supercapacitor device was assembled using the CAs, which exhibited a desirable energy density of 4.5 Wh kg−1. And the results obtained from this study can be conducive to the applications of nanoscale CAs in energy storage.


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1. Introduction Supercapacitors (SC) have attracted increasing attention in recent years due to their satisfactory energy density, power density, and durable service life1. In the light of their energy storage mechanisms, SC can be categorized into two groups2, i.e., double-layer capacitor and pseudo-capacitor, respectively. Carbon materials have been extensively studied as double-layer capacitors electrode materials, including graphene3, porous activated carbon4, carbon nanotubes5, and carbon aerogels (CAs)6. Among these carbon materials, CAs as a new type of nanoporous carbon material, due to their high specific surface area (SSA), large pore volume, and good conductivity, has become an ideal electrode material for SC7, 8. During the last decade, cellulose has been used to prepare CAs due to its high regeneration ability, non-toxic nature, and good biodegradability9,

10.

Zu11 has reported high SSA for

lignocellulosic CAs obtained by the pyrolysis of nanocellulose aerogels via freeze-drying or supercritical CO2 drying. The CAs demonstrated application potential for use in SC electrode materials. Tian12 has prepared CAs from cellulose without further activation, which is assembled into a two-electrode symmetric SC, and this SC exhibits a comparatively high specific capacitance (up to 160 F g1 at 0.2 A g1). Wang13 has used bacterial cellulose (BC)-prepared CAs that exhibit excellent electrochemical performance in lithium-ion batteries. However, the high SSA and high conductivity of CAs can’t always affect the electrochemical performance of carbon materials14. Thus, it is imperative for carbon materials to have a reasonable internal structure for use as SC electrodes. Zuo15 has prepared sheet-like BC-derived CAs containing nickel sulfide on BC by the in-situ growth method. The CAs electrodes reportedly exhibit excellent capacitance performance,



good rate capacitance retention, and enhanced cycling stability. Li16 has prepared CAs with a pearly network structure derived from an organic sol–gel process on porous carbon materials. With a spherical particle size of less than 100 nm, the CAs exhibits excellent specific capacitance (183.6 F g1). Zhang17 has prepared CAs with the reciprocal stacking lamellae derived from a trifunctional benzoxazine monomer. Super-fluffy CAs comprising abundant lamellae exhibit a large SSA, with a mesoporous structure comprising several diffusion paths for the transport of electrolyte and lots of reactive sites between the electrode and electrolyte. Hence, the CAs structure demonstrates excellent rate capability and durable cycling stability. However, to the best of our knowledge, few systematic studies have reported the influence of the carbonization temperature on the structure and electrochemical performances of CAs. The CA internal structure is well known to be regulated by the precursor concentration. In this study, the nanoplate and nanofiber of CAs, as well as their effects on electrochemical properties via the control of the carbonization temperature were examined. Rhim18 has investigated the thermophysical properties of microcrystalline-cellulose-derived carbon materials at different temperatures. In addition, the relationship between the temperature and thermal conductivity, specific heat, thermal diffusivity of carbon materials is better understood. However, the effect of different temperature on the internal structure and electrochemical performances of carbon materials has not been discussed. In this study, the change in internal structure of CAs and its effect on electrochemical performance via the control of the carbonization temperature were examined. First, lignocellulose powder was dissolved in a sulfuric acid solution, followed by centrifugation, dialysis, and high-pressure homogenization, affording a cellulose nanofibril (CNF) hydrogel.


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Then, high-SSA CAs were synthesized by the pyrolysis of CNF aerogels, which were prepared using the freeze-dried CNF hydrogel. The microstructures and electrochemical performance of the CAs at different carbonization temperatures were examined. Compared with those of CAs obtained at low temperature, the CAs obtained at high temperature exhibited considerably higher SSA (658.53 m2 g−1) and capacitance (172.7 F g−1). In particular, CAs exhibited durable long-term cycling stability (after 5000 cycles of cyclic voltammetry testing, it can still keep 89.43% of the original specific capacitance). The as-obtained CAs were incorporated as electrode materials into an SC device, which possessed favorable power density and energy density. Accordingly, CAs as electrode materials demonstrate immense application prospects in SC. 2. Experimental section 2.1 Materials and instruments Lignocellulose powder mainly consists of cellulose, was purchased from Mississippi State University; polyvinylidene fluoride (PVDF), sulfuric acid (H2SO4), potassium hydroxide (KOH), and acetylene black (Sinopharm Chemical Reagent Corp.). Tubular furnace (YQL1100-80). Electrochemical workstation (CHI660E A15004) 2.2 Preparation of the CNF hydrogel The preparation of CNF hydrogel had been reported in our previous work. The preparation method in this work is detailed in Zhang’s work19. 2.3 Preparation of CNF aerogels and CAs The method of preparing CNF aerogels was the same as our previous work. In order to obtain CAs, the CNF aerogels were put into a tube furnace for high temperature carbonization and



nitrogen is the protective gas. At room temperature, the CNF aerogels were heated to a specified temperature at a heating rate of 5°C min-1 and continuous carbonization for 2 hours at this temperature. Importantly, the CNF aerogels were subjected to carbonization at 400°C, 600°C, 800°C, and 1000°C, which will be hereafter referred to as CA-400, CA-600, CA-800, and CA-1000, respectively. 2.4 Characterization The morphology and structure of the samples were characterized by field emission scanning electron microscopy (FESEM, Quanta 250 FEG).The sample density was measured by the formula ρ = M/V, where ρ, M, and V represent the density, mass, and volume of the samples, respectively. The SSA and pore size distribution of the samples were obtained by the Brunauer–Emmett–Teller (BET) method (BELSORP-max2). Sample compositions were analyzed by Thermogravimetric analyzer (TGA-1250C), X-ray diffraction (XRD, D8 FOCUS), Raman spectroscopy (Renishaw inVia), and Fourier transform infrared spectroscopy (FTIR, Bruke D8 Advance). 2.5 Electrochemical measurements The electrochemical performance of the samples was measured by a three-electrode system using an electrochemical workstation, in which the electrolyte was 6 M KOH solution. The CAs, PVDF, and acetylene black were ground into slurry according to the ratio 80:10:10 and then coated on the foam nickel substrate (1 × 1 cm2). After drying under vacuum at 65°C, the working electrode was obtained. Carbon rod and Ag/AgCl2 electrode were counter electrodes and reference electrodes in three-electrode system, respectively. Cyclic voltammetry (CV) and galvanostatic


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charge/discharge (GCD) were carried out in the electrochemical workstation. The specific capacitance of the sample was calculated according to the discharge time in GCD curve and formula C = (I△t)/(m△V), where I, △t, m, and △V represent the current, discharge time, mass of the sample on the working electrode, and discharge voltage, respectively. 3. Results and Discussion 3.1 Structure and morphology of aerogels and CAs

Fig. 1 Schematic of the CAs synthesis. The SEM images in Fig. 2 revealed the morphology of CNF aerogels and CAs. Lignocellulose powder exhibited a large diameter and disordered distribution without the formation of a network structure (Fig. 2a). Cellulose nanofibrils were prepared by acid hydrolysis using lignocellulose powder, followed by the self-assembly into CNF aerogels (Fig. 2b). The fiber diameter decreased, and a three-dimensional (3D) network structure was



formed, comprising interlaced fibers, with some fibers embedded in the lamellar structure. The fiber diameter was in the range of 10–500 nm. Several lamellar structures and a long fibrous structure were observed because of the self-assembly of the hydroxyl groups in cellulose via hydrogen bonding during freeze-drying. It can be seen from the illustration in Fig. 2b, the lamellar structure is formed by the self-assembly of CNF into a dense network structure. Compared with that of the aerogels, the fiber structure number of CAs increased, and the lamellar structure decreased because of the removal of the oxygen and hydrogen atoms of cellulose in the lamellar structure under high temperature during pyrolysis.

Fig. 2 SEM pictures of (a) lignocellulose powder, (b) CNF aerogel, (c) CA-400, (d) CA-600, (e) CA-800, and (f) CA-1000. The skeletal densities of the CNF aerogel, CA-400, CA-600, CA-800, and CA-1000 samples were 5.62, 2.70, 6.19, 6.39, and 7.44 mg cm3, respectively (Fig. 3a). The shrinkage of CAs increased with the carbonization temperature (Fig. 3b). After pyrolysis at different temperatures, CAs exhibited considerable linear shrinkage of 59.23%, 73.12%, 78.93%, and


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84.04%, respectively. The shrinkage phenomenon was bounded up with the loss of water and the release of H, C, and O atom during carbonization process.

Fig. 3 (a) Densities of the different samples. (b) Volume shrinkage of different CAs (c) N2 adsorption/desorption isotherms of the different samples. (d) Pore size distributions of the different samples. Fig. 3c shows the N2 adsorption/desorption isotherm curves of the CNF aerogel and CAs. From the results obtained from the isotherm curve, CAs are mesoporous materials. The N2 adsorption capacity and porosity of the CAs increased along with the carbonization temperature increasing. The SSA of the CNF aerogel, CA-400, CA-600, CA-800, and CA-1000 were 85.12, 213.76, 352.47, 658.53, and 576.24 m2 g−1, respectively. The SSA increased because of the decrease in the lamellar structure and the increase of fibers with the increase in the carbonization temperature. Fig. 3d shows the pore size distribution of the different samples. With the increase in the carbonization temperature, the number of mesopores increased (2–50 nm), eventually leading to the increase in the SSA of CAs. The



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pore diameter distribution of different samples were estimated using the BJH method (Fig. 3 b). The sample CA-800 exhibited a wider size distribution (5–25 nm) in the mesoporous region by comparison with CA-400, CA-600, and CA-1000. Besides, the pore volume of the CNF aerogel, CA-400, CA-600, CA-800, and CA-1000 were 0.095, 0.155, 0.214, 0.432, and 0.356 cm3 g−1, respectively. From Table 1, the SSA, pore volume, and specific capacitance of CAs in this study were comparable to those of previously reported carbon materials. Table 1 The SSA, total pore volume, and specific capacitance of some typical CAs. Specific capacitance (F g-1)

SSA

Total pore volume

(m2 g1)

(cm2 g1)

(Testing conditions)

CA-FD

418

0.75

72 (0.5 A g−1)

11

CA-SCD

892

1.80

187 (0.5 A g−1)

11

PCNs

381

0.37

74 (2 mV s−1)

20

CNFs

510

0.74

161 (2 mV s−1)

20

PCN/CNF

1037

1.04

261 (2 mV s−1)

20

Aerogel

85.12

0.095

-

This study

CA-400

213.76

0.155

1.1 (0.5 A g−1)

This study

CA-600

352.47

0.214

100.8 (0.5 A g−1)

This study

CA-800

658.53

0.432

172.7 (0.5 A g−1)

This study

CA-1000

576.53

0.356

223.2 (0.5 A g−1)

This study

Sample

Ref.

3.2 Chemical evolution of cellulose nanofibril CAs TG and DTA analysis revealed the mass loss and decomposition of CAs during pyrolysis(Fig.


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4a). The mass loss of CAs mainly occurred in two regions. The first mass loss occurred at ~100C, mainly corresponding to the dehumidification of cellulose. The second mass loss occurred at ~250C, mainly corresponding to the depolymerization of cellulose21. The most rapid mass loss rate was observed at 300–400C, corresponding to the loss of organics and decomposition of cellulose. At ~500C, a small mass loss occurred because of the removal of excess hydrogen atoms from cellulose. As the continuous increase in the carbonization temperature to ~800C, the quality of cellulose CAs tended to be stable.

Fig. 4 (a) TG and DTG curves of the CNF aerogel. (b) FTIR patterns of the different samples. (c) XRD patterns of the different samples. (d) Raman patterns of CAs. Fig. 4b shows the FTIR spectra of aerogels and CAs. After carbonization at 400°C, the C=C stretching band was observed near 1600 cm1, and the C=O stretching band was observed near 1700 cm−1. The band observed at 1100 cm1 corresponded to the H–C–H wagging vibration, indicative of the dominance of deoxidation and hydroxylation in the process at 400°C22, 23. The hydroxyl groups of the neighboring hydroxyl groups afforded the aromatic



ring skeleton C=O bond and aromatic ketone C=C bond. At 600°C, the -OH peaks of cellulose near 3340 cm−1 decreased further, indicative of continuous dehydrogenation and deoxygenation. The peak intensity observed near 1700 cm1 also decreased, and the peak corresponding to the C=C double bond observed near 1600 cm1 was still clearly visible, albeit with weakened intensity because the carbon network formed by the aromatic ring structure increased with temperature. With the continuous increase in the temperature to 800°C and 1000°C, peaks near 1700 and 1600 cm1 were barely appeared in the curve spectra, indicative of the further increase in the number of carbon networks. During carbonization, the aromatic skeleton disappeared with the increase in the carbon network structure to a certain extent. The loss of the cellulose functional groups in FTIR corresponded to the mass loss in the TG curve. XRD was employed to research the crystalline structure of the CNF aerogel and CAs (Fig. 4c). The main diffraction peaks for the CNF aerogels were observed at 16.7° and 22.6°, corresponding to the (1 1 0) and (2 0 0) crystallographic planes, respectively18, 24; these peaks are typical for the cellulose I structure. The crystallinity of original cellulose significantly decreased after cationization, which was related to the destruction of hydrogen bonds and the low crystallization degree of cellulose. Peaks observed at 43.4° in the XRD curve pattern of CA-1000, corresponding to the (1 1 0) planes of graphite, were broadened, indicative of the graphitization of CAs. Fig. 4d shows the Raman spectra of CAs. The D-band peaks at 1323 cm1 represents the defects and disorder carbon structure, G-band peaks at 1593 cm1 represents graphitic carbon structure. The defect quantity of graphitic materials was examined by the integrated intensity ratio ID/IG25, 26. Owing to the incomplete carbonization


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of CA-400 and CA-600, only the ID/IG ratios of CA-800 and CA-1000 were calculated to be 1.037 and 1.058, indicative of the effective increase in the defect quantity of cellulose nanofibril carbon materials at high temperatures. 3.3 Electrochemical properties of CAs

Fig. 5 (a) GCD curves of different CAs at a current density of 0.5 A g−1. (b) CV curves of different CAs at a scan rate of 50 mV s−1. (c) Nyquist plots of different CAs. (d) Optical photographs of the CNF membrane, CM-800, CNF aerogel, and CA-800. (e) Comparison of GCD curves between CA-800 and CM-800. (f) GCD curves of CAs prepared from different nanocellulose concentrations. The electrochemical properties of the samples were measured by a three-electrode system using an electrochemical workstation, in which the electrolyte was 6 M KOH solution. Fig. 5a shows the GCD curves of four different sample electrodes. On the basis of the GCD curves, the calculated values for the specific capacitance of CA-400, CA-600, CA-800, and CA-1000 samples were 1.1 F g−1, 100.8 F g−1, 172.7 F g−1, and 223.2 F g−1 (at a current density of 0.5 A g−1) , respectively. Fig. 5b presents the CV curves of different CA samples at



a scan rate of 50 mV s−1. It can be seen that the CV curves of the CA electrode were rectangular. With the increase in the carbonization temperature, the area formed by the CV curve of the CA electrodes gradually increased, the specific capacitance also increases clearly. CA-400 was only partially carbonized; hence, capacitance is relatively low. A excellent specific capacitance of 223.2 F g−1 for CA-1000 was observed, corresponding to the partial graphitization of cellulose at high temperature. Fig. 5c shows the results obtained from electrochemical impedance spectroscopy (EIS) for different CA electrodes. Low resistivity aided in the increase of the CA specific capacitance. The series resistance (Rs) was obtained by intersecting the Nyquist curves in the high frequency region with the abscissa axis (inset of Fig. 5c)27. The Rs values for CA-400, CA-600, CA-800, and CA-1000 electrodes were calculated to be 3.79, 2.50, 1.14, and 0.54 Ω, respectively. Its low resistance revealed that the electrolyte ions migrate well in CAs materials. In view of the above description, the electrochemical performances of CA-800 and CA-1000 electrodes were mainly analyzed herein. To examine the effect of the lamellar and fiber structures on the electrochemical performance of CAs, the CNF membrane carbon material (CM-800) with more lamellar structure was especially prepared at 800°C (Fig. 5d). Fig. 5e shows the GCD curves of CA-800 and CM-800 electrodes: The reversibility and specific capacitance of CA-800 were superior to those of CM-800 because the SSA and active sites of CA-800 were considerably greater than those of CM-800. The 3D network structure not only provide a large number of active sites for electrostatic attraction, but also provide an efficient, unique route path for the migrated ion transport. The CAs can continuously transmit electrons and electrolyte ions to the


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material surface in a stable manner. Hence, CA-800 exhibits excellent electrochemical performance28, 29. Simultaneously, CAs with different internal structures can be obtained by controlling the cellulose nanofibril solution concentration. Hence, CAs are prepared by using cellulose nanofibril solutions of 0.2 wt%, 0.5 wt%, and 0.8 wt%, respectively, which were hereafter referred to as 0.2 wt% CA-800, 0.5 wt% CA-800, and 0.8 wt% CA-800. Fig. 5f shows the GCD curves. Excellent electrochemical performance was observed for 0.5 wt% CA-800, which was related to its uniformly dispersed fiber structure.

Fig. 6 GCD curves for (a) CA-800, and (b) CA-1000 at current densities from 0.25 to 5 A g−1. (c) Specific capacitances of CA-800 and CA-1000 at current densities from 0.25 to 5 A g−1. (d, e) CV curves of CA-800 and CA-1000 at different scan rates. (f) Cyclic life diagram of CA-800 and CA-1000 samples for 5,000 CV curves. Fig. 6a and Fig. 6b show the GCD curves for CA-800 and CA-1000 electrodes at different current densities (from 0.25 to 5 A g−1). With increasing current density, the triangular area formed by the GCD curves continuously decreased, as well as the specific capacitances for the CA-800 and CA-1000 samples. The specific capacitances for the CA-800 and CA-1000 samples calculated at 0.5 A g−1 were 172.7 and 223.2 F g−1, respectively. However, the GCD



performance for the CA-1000 sample was not stable under a low current density due to the increase in the number of micropores. Fig. 6c shows the specific capacitances of CA-800 and CA-1000 at different current densities. Fig. 6d and Fig. 6e show the CV curves for CA-800 and CA-1000 sample electrodes at different scan rates. The specific capacitance of both electrodes at scan rates of 5 to 100 mV s−1 were 257.6 to 121.4 F g−1 and 334.7 to 204.6 F g−1, respectively. The specific capacitance of both CA electrodes increased with the decrease of scan rate, due to the ion transport in the porous electrode structure lagged behind the potential change, and the internal resistance of the charge carriers significantly retarded the capacitive function of the CA electrode30. To further examine the electrochemical properties of wood cellulose nanofibril CAs, the cycle life values for the CA-800 and CA-1000 electrodes were measured on the electrochemical workstation. The cycle life for both electrodes were examined by repeat CV tests at the scan rate of 100 mV s−1. After 5,000 cycles of testing, the specific capacitance values for the CA-800 and CA-1000 sample electrodes were retained at the initial 89.43%, and 92.40%, respectively. Clearly, the cycling stability of these two electrode materials was relatively similar. The excellent electrochemical constancy of the CAs was related to its interconnected nanostructure. The multidimensional network structures could offer a short path and stable transport of electrolyte ions to the internal electrode material.


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Fig. 7. (a) CV curve, (b) GCD curve, (c) Nyquist plots and equivalent circuit of CA-SC. (d) Power and energy densities of CA-SC. Finally, the CA-800 electrode was assembled into a simple supercapacitor device (denoted as CA-SC) and tested. Square-like CV curves for CA-SC were observed at different scan rates (Fig. 7a), indicating excellent capacitive behavior for CA-SC. The GCD curves at different current densities revealed that CA-SC possessed excellent electrochemical properties (Fig. 7b). The Rct value of CA-SC was 2.73 Ω, which shows the CA-SC has excellent conductivity (Fig. 7c). Fig. 7d shows the energy and power densities of the CA-SC. CA-SC exhibited an energy density of 4.5 Wh kg−1 at a current of 2.3 mA. In addition, the maximum power density of the device reached 1.75 kW kg−1 at a current of 16.1 mA. These energy and power densities were greater than those of other carbonaceous SCs reported previously31-38. 4. Conclusions High-SSA CAs were prepared by freeze-drying combined with high-temperature carbonization using wood cellulose nanofibril. Moreover, the influence of different



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carbonization temperatures on the pore structure and electrochemical properties of CAs was investigated. Results revealed that the pore volume and SSA of CAs increase with temperature at the experimental temperature, and the specific capacitance of CAs also considerably increases. At a carbonization temperature of 800 °C, a high SSA of 658.53 m2 g−1 was observed. Specific capacitance values for CA-800 and CA-1000 were 172.7 F g−1 and 223.2 F g−1 at a current density of 0.5 A g−1 in a 6 M KOH solution. The capacitance values for CA-800 and CA-1000 exhibited capacity retention values of 89.43% and 92.40% after 5,000 cycles of GCD tests, indicative of good cycling stability. In addition, CA-SC exhibited an energy density of 4.5 Wh kg−1 at a current of 2.3 mA. Hence, wood cellulose nanofibril CAs demonstrate good prospects as supercapacitor electrode materials. Acknowledgement This work was financially supported by National Natural Science Foundation of China (31530009, 31500476), National Key Research and Development Program of China (2017YFD0600804), (2016QNRC001),

Young

Outstanding

Elite

Scientists

Innovative

Sponsorship

Youth

Training

Program Program

by of

CAST

Changsha

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Manipulation of Nanoplate Structures in Carbonized Cellulose

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