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. 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 sp2 hybridized structure of carbon atoms, which is caused by the symmetry and crystallinity of carbon materials. The ID/IG is generally used to indicate the degree of defects in the material. The calculated ID/IG values were shown in Table S1. The results indicated that the values of ID/IG of TOCN increased significantly 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 specific surface area of the material. After adding PG to TOCN, the ID/IG 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 ID/IG 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 specific surface area and pore volume of the carbonized aerogels were shown in Table S2. The specific surface area of CTOCN-750 aerogel were 457.9 m2/g and pore volume were 0.79 cm3/g, exhibiting a more abundant pore structure. With the increase of PG content, the specific surface area and pore volume of aerogel gradually decreased, and at the PG content of 20 wt%, the specific surface area and pore volume of CTG20-750 aerogel decreased to 192.4 m2/g and 0.28 cm3/g, respectively, and the three-dimensional structure was substantially destroyed. After increasing the carbonization temperature to 1100°C, the specific 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 specific surface area and pore volume. The specific 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 significantly, 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, reflecting 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 oxygen-containing 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 figures that the galvanostatic charge-discharge 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 specific capacity of CTG10-750 reached 134.09 F/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 specific 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 specific 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 specific capacity increased significantly, reaching 361.74 F/g at 0.5 A/g (Fig. 6b). While the constant current charge-discharge curve of CTG10-1100 showed a significant voltage drop, and the specific 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 efficiency. However, the conductivity of PG in CTG10-1100 didn’t increase significantly 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 efficiency 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 three-dimensional 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 efficiency 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 efficiency. The internal resistance of CTOCN-1100 aerogel material can be greatly reduced through the synergistic action. However, for CTG10-1100 aerogel, the specific surface area and pore volume of the material were significantly 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 specific 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 specific 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.