Porous and three-dimensional carbon aerogels from nanocellulose/pristine graphene for high-performance supercapacitor electrodes

Bio-based materials with good electrical properties produced by environmentally friendly methods have been increasingly applied in battery and supercapacitor technologies. In this work, we used 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nano�bers (TOCN) and pristine graphene (PG) as precursors to fabricate three-dimensional (3D) carbon aerogels with outstanding conductivity via freeze-drying followed by carbonization. The speci�c capacitance of the carbonized TOCN/PG (CTG) aerogels reached 134.09 F/g at a current density of 0.5 A/g. Meanwhile, a better cycling stability was achieved and the capacitance retained 98.89% after 5000 cycles. As the temperature increased to 1100°C, the electrochemical performance of the CTOCN-1100 electrode was improved signi�cantly with a speci�c capacitance of 361.74 F/g at a current density of 0.5 A/g, and the capacitance still retained as high as 99.3% after 5000 cycles. Therefore, these bio-based cellulose nano�brils are promising in the �eld of supercapacitors.


Introduction
However, the speci c capacity of the neat carbonized nanocellulose is still limited when used as supercapacitor electrodes.
Improving the electrochemical properties of nanocellulose can be carried out from two aspects.On one hand, adding high surface area llers to the nanocellulose matrix can effectively increase the speci c surface area of nanocellulose-based composites, thus improving their electrochemical speci c capacity [Yang et al. 2018;Jyothibasu et al. 2022;Liang et al. 2022].As one of the most promising twodimensional materials, graphene is widely used in the eld of energy storage devices due to its excellent electrical conductivity, outstanding mechanical properties, high speci c surface area and stable electrochemical properties [Ambrosi et al. 2014 Zhang et al. 2018].On the other hand, three-dimensional carbon assemblies are considered as ideal supercapacitor electrode materials because of their good structural stability and rich internal pore structure.Using one-dimensional nanocellulose as the structural unit to construct nanocellulose aerogel with three-dimensional structure can greatly enhance the speci c surface area and obtain rich porous microscopic morphology, which is also an effective way to endow nanocellulose with high electrochemical capacity.
In this work, TOCN/PG composite aerogels were prepared using pristine graphene (PG) exfoliated by the triethanolamine method [Chen et al. 2017] and TOCN as precursors.Then the aerogels were carbonized at 750°C and 1100°C under argon atmosphere to obtain carbonized TOCN/PG composite aerogels.The effects of different PG contents and carbonization temperatures on the structures and properties of the carbonized aerogels were investigated by recent testing techniques.The electrochemical properties of the carbonized TOCN/PG composite aerogels were tested as working electrodes for supercapacitors, and the value of the method for realizing the production of nanocellulose-based high-performance supercapacitors was explored.

Preparation of TOCN dispersions
TEMPO-oxidized cellulose (TOC) from SBKP was prepared using the TEMPO/NaBr/NaClO system [Isogai. 2021].4.34 g TOC with 81% water content was added to 150 mL deionized water, stirred magnetically for 30 min and then sonicated for 6 min to achieve a uniform TOCN dispersion in the deionized water.Finally, the undispersed TOCN was removed by centrifugation to obtain a stable dispersion of 0.5 wt.% TOCN.
Preparation of PG 0.6 g of ake graphite powder and 60 mL of triethanolamine were added in a 100 mL single-necked ask and mechanically stirred at 1000 rpm for 8 h.Then the dispersion was centrifuged at 4500 rpm for 30 min to obtain the supernatant.The supernatant was mixed with deionized water and ltered under vacuum on a 0.22 µm pore size mixed cellulose ester membrane and washed three times with deionized water.The nal ltrate was then dispersed in deionized water and freeze-dried to obtain the black PG powder.

Preparation of TOCN/PG aerogels
The PG powder was weighed in 100 g of 0.5% TOCN dispersion according to the mass fraction of PG (PG content in solid) of 5%, 10%, 15% and 20%, respectively.The mixture was stirred magnetically for 1 h and then sonicated for 4 min to disperse uniformly.The dispersion was poured into the mold and placed in HCl atmosphere for a period of time to obtain the nanocellulose-based hydrogel.It was then replaced with ethanol and tert-butanol in turn, and nally the TOCN/PG aerogel was obtained after freeze-drying for 48 h.

Preparation of working electrodes for supercapacitors
The surface of the glassy carbon electrode was polished with 50 nm diameter alumina powder before the experiment, and washed with ethanol.Then, 5 mg of the above carbonized aerogel was dispersed in 1 mL of DMF, and 5 µL of Na on solution was added dropwise to increase the adhesion of the material.1.5 µL of the above dispersion was applied to the electrode area on the surface of the glassy carbon electrode after stirring well, and when the solvent was dried, the electrode was ready for electrochemical testing.

Characterization
A Hitachi S-4800 eld emission scanning electron microscope (Japan) was used to observe the internal microscopic morphology of the samples.Raman spectroscopy was performed using a Renishaw Raman microscope (UK) with a laser source wavelength of 514.5 nm.The speci c surface area and pore distribution of the aerogel were tested by the ASAP 2020M speci c surface area and pore analyzer (Micromeritics, USA).A three-electrode system was composed of Ag/AgCl as reference electrode, platinum wire as counter electrode, carbonized aerogel material as working electrode, and 1 mol/L H 2 SO 4 aqueous solution as electrolyte.The electrode material was tested by CHI 660E electrochemical workstation (Shanghai Chenhua) for cyclic voltammetry, galvanostatic charge-discharge cycling, and impedance analysis.

Results And Discussion
Structural analysis of carbonized TOCN/PG aerogels SEM images of different carbonized aerogels are shown in Fig. 1.General carbonization usually leads to the collapse of internal network structure of aerogels [Wang et al. 2022].However, the uniform and obvious network pore structure was observed from Fig. 1a, which indicated that TOCN aerogel can still maintain its rich pore structure after carbonization at 750 ℃.This may be because the aerogel prepared by freeze-drying can avoid the shortcomings of direct heat treatment and retain open porous network.
When the carbonization temperature was increased to 1100 ℃, the pores of the aerogel became smaller and more abundant (Fig. 1b). Figure 1c showed that after the compounding of TOCN and 10% PG, PG didn't destroy the TOCN pores and still maintained a good three-dimensional network structure, while the presence of TOCN blocked the self-stacking of PG to a certain extent.However, after carbonized at 1100 ℃, the three-dimensional structure was almost invisible in Fig. 1d, showing more of a stacked graphite structure in sheets.This phenomenon may be due to the further disappearance of the characteristic functional groups on the molecular surface of the TOCN between the PG sheets after carbonized at higher temperatures.The spatial resistance between the graphene nanosheets decreased, and the PG stacking occurred due to the strong van der Waals forces between the nanosheets.This destroyed the three-dimensional network structure of the aerogel.When the content of PG increased to 20%, the distance between graphene nanosheets was further reduced, and the nanosheets were agglomerated due to the excess, destroying the pore structure of the aerogel (Fig. S1).
To investigate the internal carbon atom structure of different aerogels at different carbonization temperatures, Raman spectroscopy tests were performed on CTOCN-750, CTOCN-1100, CTG10-750 and CTG10-1100, and the results were shown in Fig. 2 S1.The results indicated that the values of I D /I G of TOCN increased signi cantly after increasing the carbonization temperature, which was due to that the higher carbonization temperature made the carbonization more complete and the characteristic functional groups on the surface further disappeared, thus increasing the surface defects.But the increasing of surface defects may exist in the form of pores and increase the speci c surface area of the material.After adding PG to TOCN, the I D /I G value decreased due to the perfect crystal structure of PG nanosheets which increased the material orderliness.For the CTG10 aerogel, the carbonized nanocellulose sandwiched between PG nanosheets played the role of hindering graphene stacking, and the increase of carbonization temperature further reduced its surface characteristic functional groups, thus decreasing the hindrance between graphene sheets, leading to the stacking and agglomeration of graphene due to strong π-π interactions between the sheets.The appearance of a large number of relatively regular graphite-like structures inside the material destroyed the internal three-dimensional structure.The I D /I G of CTG10-1100 (i.e., 0.77) was therefore much lower than that of CTG10-750 (i.e., 1.14).
To further investigate the internal microstructure of the composite aerogels, nitrogen adsorption/desorption isotherm tests were performed on CTOCN-750, CTG10-750, CTG10-1100, CTG20-750 and CTOCN-1100, respectively (Fig. 3).And the speci c surface area and pore volume of the carbonized aerogels were shown in Table S2.The speci c surface area of CTOCN-750 aerogel were 457.9 m 2 /g and pore volume were 0.79 cm 3 /g, exhibiting a more abundant pore structure.With the increase of PG content, the speci c surface area and pore volume of aerogel gradually decreased, and at the PG content of 20 wt%, the speci c surface area and pore volume of CTG20-750 aerogel decreased to 192.4 m 2 /g and 0.28 cm 3 /g, respectively, and the three-dimensional structure was substantially destroyed.After increasing the carbonization temperature to 1100°C, the speci c surface area and pore volume of CTG10-1100 were further reduced due to the further disappearance of oxygen-containing functional groups on the surface of TOCN and the reduction of PG layer-to-layer resistance, which made the self-stacking effect of graphene more obvious and disrupted the internal network structure of the material.In general, increasing the PG content and raising the carbonization temperature both decreased the aerogel speci c surface area and pore volume.The speci c surface area and pore volume of CTG10-750 were reduced but can still maintain a certain three-dimensional network structure, while CTG10-1100 and CTG20-750 aerogels decreased signi cantly, and the three-dimensional network structure was destroyed, which was consistent with the SEM results.

Electrochemical properties of carbonized TOCN/PG aerogels
Cyclic voltammetry (CV) was performed by applying an external voltage to the electrode material for cyclic scanning to obtain response current versus scanning voltage curves, thus visualizing the voltage window, reversibility and capacitive behavior of the supercapacitor response.Figure 4 showed the CV curves of the carbonized aerogels at different scan rates.It can be observed that the CV curves of the four components of CTG-750 aerogel were all symmetrical rectangular shapes, re ecting a relatively ideal double electric layer capacitance behavior.Meanwhile, there was a small symmetrical redox peak on the curves, which maybe because the carbonization treatment didn't completely remove the oxygencontaining functional groups on the surface of cellulose.Thus, the electrode material exhibited a certain pseudo capacitance effect.At different scanning rates, CV curves all presented good rectangles and symmetries.With the increase of scanning rates, the contour of curves almost didn't change, and the corresponding current increased linearly.Even at the scanning rate of 100 MV/s, CV curves still showed rectangle-like shapes.This indicated that the internal structure of CTG-750 electrode material had little resistance to charge transfer and electrolyte ion diffusion, and can work with large current.The strong π-π interaction between PG molecules made it easy to agglomerate.Although increasing the amount of PG can improve the overall electrical conductivity of the material, it also destroyed the pore structure of the material to a certain extent, thus increasing the electron and ion transport resistance.Therefore, when the PG content reached 20%, the CV curve of the material appeared obvious distortion, and the response current decreased greatly.
When the carbonization temperature increased to 1100°C, the CV curve of CTOCN-1100 exhibited an almost perfect rectangular curve (Fig. 4e) and the redox peak on the surface almost disappeared, indicating that with the increase of the carbonization temperature, the characteristic functional groups on the surface of cellulose were carbonized more completely and closer to a double-layer capacitor.However, due to the further reduction of oxygen-containing functional groups on the cellulose surface, the resistance between PG layers decreased, which made the self-stacking effect of graphene more obvious.This destroyed the internal network structure of CTG10-1100 and increased the internal resistance, thus affecting the electrochemical performance of the electrode material and obtaining a twisted CV curve (Fig. 4f).
The galvanostatic charge-discharge curves of the carbonized aerogels at different current densities were shown in Fig. 5.The mass capacitance under different current densities was calculated according to the charge-discharge curves (Fig. 6).It can be observed from the gures that the galvanostatic chargedischarge curves of all aerogel components under different current densities were relatively symmetric triangles, showing an ideal capacitance behavior.But at each moment of charge-discharge conversion, there was a small voltage drop, which was determined by the material and the internal resistance of the capacitor itself.The addition of PG can effectively reduce the resistance of the material.So with the increase of PG, the voltage drop gradually decreased.When the content of PG was over 10%, PG agglomerated due to the excess, which destroyed the internal network structure of the material and increased the ion transport resistance, making the voltage drop larger.At the current density of 0.5 A/g, the speci c capacity of CTG10-750 reached 134.09F/g, which was much higher than that of CTOCN-750 (92.65 F/g) and CTG20-750 (50.08 F/g) (Fig. 6a).In addition, with the increase of current density, the speci c capacity of all materials would be attenuated due to the internal resistance of the whole capacitor system.When the current density increased, the response speed was slow, which affected the expression of capacity.In this work, when the current density was increased to 10 A/g, CTG10-750 still had a speci c capacity of 96.3F /g and a capacity retention rate of 71.82%, which was higher than those of other components.
Compared with the properties of electrode materials carbonized at 750 ℃, the voltage drop of carbonized TOCN aerogel at 1100 ℃ was small, and the speci c capacity increased signi cantly, reaching 361.74 F/g at 0.5 A/g (Fig. 6b).While the constant current charge-discharge curve of CTG10-1100 showed a signi cant voltage drop, and the speci c capacitance value reduced by about 6 times.The above phenomenon was mainly because when TOCN was carbonized at higher temperature, the remaining uncarbonized oxygen-containing functional groups were further reduced and the electrical conductivity was increased.Meanwhile, the disappearance of oxygen-containing functional groups formed pores inside the material, providing more paths for ion transport, reducing the internal resistance and increasing the charge and ion transport e ciency.However, the conductivity of PG in CTG10-1100 didn't increase signi cantly due to the decrease of oxygen-containing functional groups inside.Combined with SEM results (Fig. 1d), PG aggregated observably due to the decrease of functional groups on cellulose surface, which destroyed the three-dimensional network structure inside the aerogel and increased the charge transfer resistance.
Electrochemical impedance spectroscopy (EIS) is an important way to study the electrochemical performance of supercapacitors.Figure 7a showed Nyquist impedance diagrams of different components CTG-750 aerogel measured at open circuit voltage in the frequency range of 0.01-100000 Hz.It can be observed from Nyquist impedance spectrum that, in the high frequency region, the intersection of the Nyquist impedance spectra with the X-axis for all materials was near 4 Ω, indicating that adding appropriate adhesive to electrode material can effectively reduce the interface contact resistance.In the intermediate frequency region, CTG10-750 exhibited a relatively small Warburg impedance, which was due to the fact that for carbonized TOCN aerogels, the addition of PG can effectively improve the electrical conductivity of the material and strengthen the charge transfer e ciency on the material surface.But PG itself was easy to stack and had limited compatibility with TOCN.With the further increase of PG addition, aggregation occurred due to the excess, which destroyed the threedimensional network structure inside the material and affected electron ion transport.
The Warburg impedance of CTOCN-1100 aerogel in the intermediate frequency region decreased a lot compared with that of CTOCN-750 when the carbonization temperature was increased to 1100°C.This is because increasing the carbonization temperature can further remove the characteristic functional groups on the surface of TOCN.On one hand, it can effectively improve the conductivity of the carbonized cellulose aerogel and enhance the charge transmission e ciency on the material surface.On the other hand, it can also enrich the pore structure inside the material, which can provide more channels for the diffusion of electrolyte ions in the electrode material, and improve the ion diffusion e ciency.The internal resistance of CTOCN-1100 aerogel material can be greatly reduced through the synergistic action.However, for CTG10-1100 aerogel, the speci c surface area and pore volume of the material were signi cantly reduced by increasing the carbonization temperature.The agglomeration effect of PG nanosheets destroyed the three-dimensional pore structure inside the material, hindered ion diffusion and increased the internal resistance.Therefore, CTG10-1100 had a large Warburg impedance.
The cycling stability results are shown in Fig. 8.The speci c capacitance of CTOCN-750 and CTG10-750 remained at the initial 98.20% and 98.89%, respectively, after 5000 cycles (Fig. 8a), indicating a good cycling stability.Combined with the SEM morphology analysis, the better cycling stability of CTOCN-750 and CTG10-750 aerogels were mainly based on the continuous porous three-dimensional network structure inside the aerogels.Figure 8b showed that the speci c capacitance retention of CTOCN-1100 electrode material reached 99.30% after 5000 cycles, which was higher than CTOCN-750 due to its higher conductivity and richer internal interlaced pore structure.However, the capacitance retention of CTG10-1100 carbonized at 1100 ℃ was reduced to 82.00%.Combined with the SEM morphology analysis, it suggested that this phenomenon was due to the further disappearance of characteristic functional groups on the cellulose surface, which made the PG nanosheets agglomerate.This destroyed the three-dimensional network structure of the aerogels, increased the internal resistance and hindered ion transport.

Conclusion
In this work, TOCN/PG carbon aerogels were obtained by gel formation, freeze-drying, and then carbonization at high temperature.With the increase of PG content, the electrochemical performance of the aerogel electrode material improved signi cantly.The speci c capacitance of CTG10-750 reached 134.09F/g at a current density of 0.5 A/g.Meanwhile, a better cycling stability was achieved and the capacitance retained 98.89% after 5000 cycles.As the temperature increased to 1100°C, the electrochemical performance of the CTOCN-1100 electrode was improved observably with a speci c capacitance of 361.74 F/g at a current density of 0.5 A/g and the capacitance retained as high as 99.3% after 5000 cycles.Owing to the outstanding electrochemical performance, low cost and eco-friendliness, the CTOCN aerogels are believed to be a promising electrode material for supercapacitors.

Declarations
Consent for publication Not applicable.
In recent years, sustainable and environment-friendly concepts have received a lot of attention in the eld of energy storage technologies.Renewable raw materials, green synthetic routes and environmentally friendly components are increasingly used in battery and supercapacitor technologies[Chio et al. 2014; Chung and Manthiram.2014; Long et al. 2014].As one of the most promising green nanomaterials[Siqueira et al. 2010; Qing et al. 2021; Ling et al. 2021; Chen et al. 2021], nanocellulose has great application value in different elds[Lao et al. 2018; Song et al. 2021; Yin et al. 2020; Zhang et al. 2014; Zheng et al. 2015; Liu et al. 2022; Song et al. 2022; Song et al. 2011; Xu et al. 2021] due to its outstanding mechanical strength, excellent exibility and large aspect ratio[Chen et al. 2020; Isogai et al. 2011; Wu et al. 2012; Wu et al. 2021].However, as an insulating material, the electrical conductivity of nanocellulose electrodes is di cult to meet the requirements of practical applications.Carbonization is an effective way to improve the electrical conductivity of biomass materials[Deng et al. 2013; Li et al. 2014; Wang et al. 2014; Gao et al. 2019; Song et al. 2019; Liu et al. 2020], and the high-temperature carbonization of nanocellulose under inert gas can effectively improve its electrical conductivity.
. Normally, the D peak in the Raman diagram indicates the disorder and the degree of defects of carbon atoms, and the G peak indicates the sp 2 hybridized structure of carbon atoms, which is caused by the symmetry and crystallinity of carbon materials.The I D /I G is generally used to indicate the degree of defects in the material.The calculated I D /I G values were shown in Table