In-situ synthesis of flexible nanocellulose/carbon nanotube/polypyrrole hydrogels for high-performance solid-state supercapacitors

Nanocellulose has become one of the most attractive matrix materials for flexible supercapacitors, owing to the high surface area, good mechanical properties and environmental friendliness. Herein, we developed electrode materials with high capacitance and mechanical flexibility through the in-situ synthesis of polypyrrole (PPy) in TEMPO-oxidized cellulose nanofibril (TOCN)/sulfonated carbon nanotubes (SCNT) composite hydrogels. The TOCN/SCNT/PPy composite hydrogels were thus obtained via a bifunctional Fe3+ in-situ oxidation, showing high specific capacitance of 5299 mF/cm2 at a current density of 1 mA/cm2. Furthermore, the assembled symmetric TOCN-40SCNT-PPy solid-state supercapacitor exhibited outstanding capacitance of 375 mF/cm2 and electrochemical stability with 163.2 % capacitance retention at a current density of 1 mA/cm2 for 2500 cycles. These nanocellulose/carbon nanotube/polypyrrole hydrogels are thus promising in the fields of flexible solid-state supercapacitor with superior electrochemical performance.


Introduction
As a new type of energy storage device, supercapacitors have received extensive attention due to their unique power density and energy storage density characteristics differing from traditional storage devices. Supercapacitors have high power density, fast charge/discharge and ultralong cycling stability (Hou et al. 2010; Yu et al. 2011), which may bring some innovative products to electronic industrial equipment and mobile electronic devices. Considering the fact that electrode materials and dielectric materials in supercapacitors can be replaced by exible and bendable soft materials, the application of supercapacitors in the eld of mobile electronic devices (such as electronic skin) have great application prospects.
Cellulose is a natural polymer material, which could be used as a structurally reinforced exible substrate or backbone in supercapacitors (Li et  ). Dispersing these nanophase electrode materials uniformly in the TOCN network can provide extended ion transmission channels and facilitate ion transmission, which provides the supercapacitor with excellent charging and discharging rates. In addition, this also can reduce the volume change of electrode materials in many pseudocapacitive reactions. In order to eliminate the in uence of internal collapse and performance degradation caused by the volume changes, (Naoi and Simon 2008;Yan et al. 2014) (for example, the redox reaction of conductive polymers involves the doping and dedoping of counter ions), cellulose and electrode materials were could compounded together to improve the cycling stability of supercapacitors.
Carbon nanotubes (CNTs) have the advantages of low density, regular 1D tubular structure, good mechanical properties, excellent electrochemical stability and high speci c surface area, thus they are widely used in the preparation of supercapacitor electrode materials (Shao et al. 2015). However, the hydrophobicity of CNTs makes it very easy to aggregate, resulting in inability to uniformly disperse CNTs.  (Merlet et al. 2012). Polypyrrole (PPy) is the most studied conductive polymer used in energy storage applications. The monomer is light in weight, and the theoretical speci c capacity at a doping ratio of 25% is 100 mAh/g (Naoi and Simon 2008). PPy obtained by chemical or electrochemical polymerization is combined with various substrates to prepare a conductive polymer-based electrode material (Novák et al. 1997).
Herein, a novel hybrid exible supercapacitor is design and fabricated. The formation of TOCN/SCNT/PPy hydrogel used the TOCN as the matrix to construct the 3D porous framework, sulfonated carbon nanotubes (SCNT) as the ller and the situ synthesis of PPy in the substrate. The hybrid hydrogel was formed by crosslinking of ferric salt and polymerization of pyrrole through ferric salts too (oxidative initiation). The negative charges carried on the surface of SCNTs grafted with paminobenzenesulfonic acid electrostatically repel each other, so that they can be uniformly dispersed in water, hindering the agglomeration of CNTs. The TOCN/SCNT/PPy hydrogel presents notable electrochemical performance, and the assembled TOCN/SCNT/PPy aerogel-based all-solid supercapacitor possesses an excellent capacitance retention.

Materials
Softwood bleached kraft pulp (SBKP) was from Nippon Paper Industries (Tokyo, Japan). NaClO was bought from Aladdin (Shanghai, China). TEMPO and NaNO 2 were purchased from Sigma-Aldrich Corporation (Saint Louis, USA). Multiwalled carbon nanotubes (MCNTs) were purchased from Shenzhen Nano Co., Ltd (China). Pyrrole, NaBr, NaOH, and other chemical reagents were of analytical grade and obtained from Sinopharm Chemical Reagent Co. Ltd (China).

Preparation of TOCN dispersion
TOCN was prepared from SBKP by the TEMPO-oxidized method (Isogai et al., 2011;Wu et al., 2020). In brief, 0.2 g NaBr, 0.032 g TEMPO and 200 g deionized water were added into a beaker. Then wet SBKP (water content 80%, dry weight 2 g) was dispersed in the mixture under stirring, and 5.65 ml NaClO solution (1.77 mol/L) was added to the suspension. The pH was adjusted to 10 by adding 0.5 M NaOH.
The obtained TEMPO-oxidized pulp was washed with water. Then it was re-dispersed in deionized water, sonicated and centrifuged to obtain the TOCN dispersion.
Preparation of sulfonated carbon nanotubes 20 ml of a 1 mol/L sodium nitrite solution was added to a mixed solution of 20 ml of 5 wt% NaOH solution and 3.4 g of p-aminobenzenesulfonic acid. Then, the above solution was transferred to an ice water bath and 1 mol/L hydrochloric acid was added. When the solution became yellowish, a diazonium salt solution was obtained. Subsequently, 0.4 g of carbon nanotubes were added to the diazonium salt solution prepared above, stirred for 24 h and washed three times with deionized water until the ltrate was colorless. Finally, the products on the lter paper were dried in a vacuum oven at 60 ℃ to obtain sulfonation carbon nanotubes and coded as SCNTs.

Preparation of TOCN/SCNT/PPy hydrogel working electrodes
Different amounts of SCNTs were added to the 0.5 wt% TOCN dispersion followed by stirring for 1 h, sonicating for 4 minutes and removing the air bubbles. Then, the above mixture was poured into a plastic mold inserted with carbon cloth, and 1 ml of 0.5 mol/L Fe(NO) 3 solution was added and allowed to stand for 12 h to obtain the TOCN/SCNT hydrogels, which were denoted as TOCN-40SCNT and TOCN-50SCNT with the SCNT content of 40 wt% and 50 wt%, respectively. Finally, the prepared TOCN/SCNT hydrogels were placed in a pyrrole atmosphere for 12 h to obtain TOCN/SCNT/PPy hydrogels, which were labeled as TOCN-40SCNT-PPy and TOCN-50SCNT-PPy. In this process, pyrrole was polymerized on the surface of nanocellulose under the action of Fe 3+ . The TOCN/SCNT/PPy composite hydrogels prepared above were then cut into thickness of 1 mm and area of 1×1 cm 2 . They were directly placed in 1 mol/L H 2 SO 4 electrolyte (in a three-electrode system) for electrochemical testing.
Fabrication of TOCN/SCNT/PPy aerogel based solid-state supercapacitor in two-electrode system The water in the above obtained TOCN-40SCNT-PPy hydrogels was replaced by ethanol and t-butanol. After the solvent was completely replaced, the alcohol gels were frozen in liquid nitrogen, and freeze-dried for 48 h to obtain TOCN-40SCNT-PPy aerogels. The TOCN-40SCNT-PPy aerogels prepared above were pressed into composite aerogel lms of 10 mm× 10 mm under a pressure of 1 MPa. Then the aerogel lms were immersed into the polyvinyl alcohol (PVA)/H 2 SO 4 gel electrolyte (6 g PVA and 6 g H 2 SO 4 was dissolved into 60 mL deionized water at 85 ℃) for 12 h as the working electrode.

Analysis
The peak of each functional group in the composite was observed in the infrared spectrum measured by the Fourier infrared spectrometer (Nexus, USA). The crystal structure of the composite was analyzed by Xray powder diffractometry (XRD) (D8 Advance). The chemical environment of carbon and nitrogen in the composite was analyzed and tested by X-ray photoelectron spectroscopy (XPS) (V G Multilab 2000). The cross section of the aerogel was scanned with a eld emission scanning electron microscope (SEM) (Hitachi S-4800), and the cross section was obtained by freezing and brittle fracture with liquid nitrogen. The polypyrrole content in the composite gel can be calculated from the nitrogen content measured by the elemental analyzer (EA) (CHNS). The speci c surface area and porosity of the solid supercapacitor were tested on the sample with the speci c surface area and porosity analyzer (BET) (ASAP 2020M) with the adsorbed gas as N 2 and temperature as 40°C.

Electrochemical Measurements
The electrochemical properties were measured by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests by CHI 660E. The areal capacitance (C A , mF cm − 2 ), energy density (Ea, µW h cm − 2 ) and power density (Pa, µW cm − 2 ) were calculated by GCD test using the following equation: where I is the charge-discharge current, Δt is the discharge time, S is the electrode area, ΔU is the potential window subtracting IR.

Results And Discussion
Characterization of TOCN/SCNT/PPy   Fig. S2 show XRD patterns of TOCN, SCNT, TOCN/SCNT and TOCN/SCNT/PPy aerogels. TOCN aerogel exhibits typical peaks at 14°, 16°, and 23°, corresponding to the (110) and (200) crystal planes of native cellulose I. SCNT powder shows two diffraction peak at 26° and 44° owing to (002) and (100) crystal planes of carbon nanotubes. It reveals that the natural cellulose crystal structure of TOCN was preserved in the process of TEMPO oxidation and SCNT could maintain the crystal structure of carbon nanotubes well. The TOCN/SCNT/PPy composite aerogels possessed partial characteristic diffraction peaks of TOCN and SCNT, the (100) crystal plane of SCNT were not obvious, and the diffraction peak at 26° of SCNT was widened. It indicated that TOCN could hinder the agglomeration of SCNT to a certain extent.
To further study the detailed chemical structure, the element compositions of TOCN/SCNT/PPy were analyzed by XPS and shown in Fig. 3. The C 1s peak is tted into four peaks for TOCN-40SCNT-PPy, with the peaks at 284.6, 286.1, 287 and 287.7 eV corresponding to C-C/C=C, C-O/C-N, C=O and O-C=O, respectively (Fig. 3a). These peaks are mainly derived from oxygen-containing functional groups on SCNTs and TOCN, which make the composites exhibit good hydrophilic properties and excellent rewetting property in an aqueous electrolyte, increasing the utilization rate of the electrochemically active material.

Microstructure of TOCN/SCNT/PPy
SEM images of the TOCN-40SCNT-PPy and TOCN-50SCNT-PPy aerogels are shown in Fig. 4 to study their morphology. The TOCN/SCNT/PPy aerogels exhibited three-dimensional network structures, which were formed by random orientation and interconnection of one-dimensional nano brils, and rich pores were present between the nano brils. The brils were distinctly observed because TOCNs were entangled with the SCNTs, and a uniform layer of conductive PPy was deposited on the surface of TOCNs or SCNTs (Fig.  4a-d). Furthermore, it could be clearly observed from the TEM image in Fig. 4e that relatively thick SCNTs were present between TOCNs, and PPy was deposited on both surfaces to form a three-dimensional structure with conductivity. It is consistent with the results of SEM.
In order to further study the internal microstructure of the composite aerogels, nitrogen adsorption/desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution of TOCN-40SCNT-PPy and TOCN-50SCNT-PPy aerogels were performed in Fig. 5. The speci c surface area and pore volume of TOCN-40SCNT-PPy aerogel are 334 m 2 /g and 0.76 cm 3 /g, respectively, indicating an abundant pore structure. When the content of SCNT in the electrode was increased, the shape of the nitrogen adsorption/desorption isotherm remained unchanged, indicating that the three-dimensional structure with pores was maintained inside. Compared with TOCN-40SCNT-PPy, TOCN-50SCNT-PPy possessed a lower speci c surface area of 300 m 2 /g and a smaller pore volume of 0.71 cm 3 /g. The decrease in speci c surface area may be attributed to the fact that the speci c gravity of the active material such as sulfonated carbon nanotubes and polypyrrole in the electrode were too large, and partial packing occurred to lower the theoretical speci c surface area of the material (Fig. 5a). In addition, the pore sizes of the two different components aerogels were mainly distributed between 2-10 nm, re ecting that the aerogels mainly possessed mesoporous structures (Fig. 5b). Elemental analysis and the relevant PPy content of TOCN/SCNT/PPy aerogel are shown in Table 1. It could be seen from the table that when the amount of SCNT was small (e.g. TOCN-40SCNT-PPy), the C content was relatively small. By calculating the mass fraction of N, the content of polypyrrole in the TOCN-40SCNT-PPy aerogel was 17.55 wt%, which was slightly higher than the content of polypyrrole in TOCN-50SCNT-PPy (i.e. 14.28 wt%). Therefore, when the content of SCNT in cellulose is different, there is no signi cant in uence on the content of PPy in the whole aerogel.
Electrical characteristics of TOCN/SCNT/PPy 40SCNT-PPy had almost no change in the shape of the curve as the scanning rate increased, and it exhibited a rectangular-like shape even at a large scanning rate of 100 mV/s, indicating that the internal structure of TOCN-40SCNT-PPy was bene cial to the reversible diffusion of electrolyte ions and could work at high current. Therefore, the TOCN-40SCNT-PPy hydrogel-based supercapacitor exhibited better capacitance performance than the TOCN-50SCNT-PPy. In general, a suitable amount of active material facilitates the formation of a continuous conductive three-dimensional network in a cellulose-based supercapacitor. However, excessive active ller will form local accumulation, which causes a signi cant decrease in capacitance and conductivity. Fig. 7 and Fig. S4 show the areal speci c capacitances of various TOCN/SCNT/PPy hydrogels at different current densities estimated based on the GCD tests. At a current density of 1 mA/cm 2 , the area speci c capacitance of the TOCN-40SCNT-PPy hydrogel-based supercapacitor was 5299 mF/cm 2 , which was higher than in previously reported studies (Wu et al. 2020); while the area speci c capacitance of TOCN-50SCNT-PPy at the same current density was 1781 mF/cm 2 (Fig. 7b). Because the proportion of SCNT in TOCN-50SCNT-PPy was larger, the microstructure was denser and the diffusion of electrolyte ions inside the material became di cult. The TOCN-40SCNT-PPy hydrogel with an appropriate amount of active material had a thin active material layer, which improved the diffusion of the electrolyte, and could fully realize the electrochemical performance of the SCNT and PPy. In addition, as the charge-discharge current density increased, the area capacitance of the all electrodes decreased. This was due to the electrolyte ions in the depth of the pores didn't participate in the reaction under higher charge-discharge current density, and the capacity of electrolyte could not be fully expressed. When the current density was increased to nearly 10 mA/cm 2 , the decay of the material capacity approached a plateau state, indicating that the effect of electrolyte ion diffusion had little effect on the area capacitance of the material (Fig.   7c). Additionally, TOCN-40SCNT-PPy hydrogel had a high energy density of up to 471 μWh/cm 2 at a power density of 568 μW/cm 2 (Fig. S5) Electrochemical impedance spectroscopy (EIS) was used to analyze the transport behavior of electrons and ions (Fig. 8). The equivalent series resistance of the TOCN/SCNT/PPy hydrogel supercapacitor was approximately 2 Ω, indicating that the interfacial impedance of the hydrogel was small. In the intermediate frequency region, the Warburg impedance of TOCN-50SCNT-PPy hydrogel was much lower than that of TOCN-40SCNT-PPy, because the pore structure of TOCN-40SCNT-PPy hydrogel was relatively loose, which is bene cial to the diffusion of electrolyte ions. In addition, in the low-frequency region, the Nyquist plot of TOCN-40SCNT-PPy approximates a vertical line, indicating a perfect capacitive behavior.
The cyclic stability of TOCN-40SCNT-PPy was investigated by consecutive GCD cycling at a current density of 5 mA cm -2 as shown in Fig. 9, which still had a capacitance retention of 82 % over 2000 cycles. On the one hand, the exible matrix of nanocellulose could inhibit the molecular chain damage of PPy caused by volume changes in the charge and discharge cycles; on the other hand, the good ratio of the three components promotes the formation of the conductive porous three-dimensional network structure. Therefore, the hydrogel has good cycle performance.
In order to explore the practical application of the TOCN/SCNT/PPy electrode for exible supercapacitors, a symmetric all-solid-state supercapacitor was prepared. The electrochemical performance of the TOCN-40SCNT-PPy solid-state supercapacitor was characterized in a two-electrode system with PVA/H 2 SO 4 as electrolyte and separator. The CV curves of the TOCN-40SCNT-PPy solid-state supercapacitor exhibited approximately rectangular-like shape, and it was well maintained even at 100 mV/s scan rate, indicating a good reversibility and quick charge-transfer capability (Fig. 10a). The TOCN-40SCNT-PPy solid-state supercapacitor possessed a high areal speci c capacitance of 233 mF/cm 2 at a current density of 0.5 mA/cm 2 (Fig. 10b). The carboxyl group on the surface of TOCN and the sulfonic acid group on the surface of SCNT are negatively charged, and electrostatic repulsion is generated to uniformly disperse both in water and hinder the formation of SCNTs. Using Fe 3+ as a cross-linking agent, TOCN and SCNT were entangled by coordination. The addition of SCNT could improve the conductivity and stability of the network structure, but the speci c capacitance was poor. Therefore, PPy was introduced into the network, which deposited on the surface of the TOCN through hydrogen bonding, further improving the overall conductivity and electrochemical properties of the material.
The TOCN-40SCNT-PPy solid-state supercapacitor exhibited a steep oblique line in the low-frequency region in EIS curve, indicating the ideal capacitive response. The intercept of the curve at high frequency was 4 Ω, which represented the equivalent series resistance was about 4 Ω. And the smaller Warburg impedance was also observed in the intermediate frequency region, owing to the hydrophilicity of the TOCN and easy ion transmission in the robust porous network of TOCN-40SCNT-PPy gel (Fig. 10c). The electrochemical stability of the TOCN-40SCNT-PPy solid-state supercapacitor was tested at a current density of 1 mA/cm 2 for 2500 cycles (Fig 10d). It is noticeable that the capacitance of the TOCN-40SCNT-PPy solid-state supercapacitor increased by 63.2% after 2500 charge/discharge cycles, and the maximum speci c capacity in the cycles was up to 375 mF/cm 2 . The excellent cycling stability of the solid-state supercapacitor may be attributed to the superior 3D porous network structure and the PPy lled in a 3D network structure was gradually activated in the charge and discharge cycles.

Conclusions
In this work, we demonstrate an electrode material with high capacitance and mechanical exibility for supercapacitors by in-situ synthesis of PPy in TOCN matrix and SCNT. A facile fabrication process was achieved, in which TOCN/SCNT/PPy hydrogels were prepared via a bifunctional Fe 3+ in-situ oxidiation method and the hydrogels were converted to aerogels by solvent-exchanging and freeze-drying. The TOCN-40SCNT-PPy hydrogel exhibited a high speci c capacitance of 5299 mF/cm 2 at a current density of 1 mA/cm 2 , probably resulting from a large surface area, 3D network porous structure and su cient electrical conductivity of the composites. Furthermore, the assembled symmetric TOCN-40SCNT-PPy based solid-state supercapacitor exhibited outstanding capacitance of 375 mF/cm 2 and electrochemical stability with 163.2% capacitance retention at a current density of 1 mA/cm 2 for 2500 cycles. This got bene t from the PPy covered on the surface of the TOCN-SCNT-PPy aerogel gradually activated in the charge and discharge cycles. The employing of the exible nanocellulose substrate provides prominent opportunity by solving the problem of poor cycle stability of PPy based supercapacitors.