2.1 │ Morphology and chemical propertis of MFC based supercapacitors
The hybrid SnO2-cellulose nanocomposite was successfully produced via hydrothermal treatment at 120 °C for 4 h. Brown and viscous composites formed after the hydrothermal treatment. The nanocomposites with an increasing amount of SnO2 (4, 8, 12 wt%) were coated on MFC thin films (1 wt%) to form flexible and thin supercapacitors. Delamination was not observed since coatings also contained MFC. More importantly, the self-standing MFC thin films retained their flexibility after coating, as shown in Fig. 1 (a) and (b). SEM images of MFC thin films before and after coating were presented in Fig. 1 (c) and (d). The surface of MFC thin film was considerably rough due to MFC entanglement. Fig. 1 (d) shows the hybrid SnO2-cellulose nanocomposites attached to the MFC thin film, revealing a flower-like structure with more active sites for charge storage to form an outstanding energy storage system. EDX results (Fig. 1 (e) and (f)) were displayed along with SEM images. The presence of SnO2-cellulose nanocomposite was confirmed by the appearance of Sn element.
Fig. 2 displays FTIR spectra of MFC, MFC-4SnO2, MFC-8SnO2 and MFC-12SnO2 thin films between 500 to 3500 cm-1. A broad peak at 3327.14 cm-1 was initiated by the stretching of -OH groups since MFC was rich in hydroxyl groups. These hydroxyl groups could improve the film capacitance upon interacting with activating agent or electrolyte. The peak obtained at 2899.01 cm-1 could be credited to CH-stretching25. Meanwhile, the band observed at 1641.42 cm-1 could be associated with the water content of the amorphous region in the MFC. C-O-C stretching in MFC also induced a peak to appear at 1029.99 cm-1. Two common peaks at 3329.14 cm-1 and 1641.42 cm-1 were observed from the FTIR spectra of MFC-4SnO2, MFC-8SnO2 and MFC-12SnO2 thin films because of the stretching and bending of additional O-H groups after incorporation SnO2. Peaks obtained within low wavenumbers (500-1000 cm-1) could be attributed to SnO226. Multiple peaks were generated at 534 cm-1, 541 cm-1, and 565.14 cm-1 due to the presence of SnO2 in the MFC-4SnO2 thin film, as shown in the inset of Fig. 2(b). The MFC-8SnO2 thin film also exhibited peaks at 572.86 cm-1 and 852.54 cm-1, which could be assigned to the Sn-O and O-Sn-O bending as well as Sn-O stretching27. The peaks appeared at 547.78 cm-1, 559.36 cm-1,852.54 cm-1, and 931.62 cm-1 for the MFC-12SnO2 thin film (inset of Fig. 2(d)) due to SnO2 hybridization on the cellulose thin film28.
The crystallinity changes of MFC thin films after incorporating SnO2 are shown in Fig. 3. The blue diffractogram represents MFC crystalization, and it exhibits a precise core peak at 2θ = 22.4 ° and two broad peaks at 2θ = 14.3 ° and 2θ = 16.1 °29. The amorphous peak of MFC was maintained in all samples at 2θ = 22.4 ° which justifies that the incorporation integration of SnO2 did not affect the crystallization of MFC. The broad amorphous peak of SnO2 at 2θ = 29.8° of (1 0 1) plane was clearly shown in the diffractogram of MFC-8SnO2 and MFC-12SnO2 samples3031. The peak at 2θ = 43.3° of (2 0 0) plane in the diffractogram of the MFC-8SnO2 sample could be associated with the Sn bond, which nearly disappeared in the other two samples32. The peak of the SnO bond could be further detected in the diffractogram of the MFC-8SnO2 sample at 2θ = 47.68 ° of (1 1 2) plane 3334, but it was less visible in the diffractogram of the MFC-12SnO2 sample. The observation could be caused by the agglomeration of SnO2 particles at a high concentration. The crystallization during hydrothermal processing was successfully attained for the MFC-8SnO2 sample, as compared to MFC-4SnO2 and MFC-12SnO2 samples.
│ Electrochemical properties and capacitance of MFC based supercapacitors
The CV analysis of MFC-4SnO2, MFC-8SnO2, and MFC-16SnO2 samples was conducted at different scan rates, ranging from 20-100 mV/s (Fig. 4). The CV curve obtained for all samples strongly suggested that SnO2-cellulose nanocomposite could be utilized for pseudocapacitive purposes. Redox peaks appeared, and oxidation curves moved towards positive potential while reduction curves moved towards negative potential. The MFC-12SnO sample exhibited a higher current response compared to other samples, indicating the best capacitive behaviour due to the highest loading of active material. The shape of the redox curve at different scan rates for all the samples was sustained even at a high scan rate. This observation confirmed the ion diffusion through porous structures and the high-rate capability of electrodes. In this work, the specific capacitance was calculated using CV data instead of galvanostatic charge-discharge (GCD) analysis. As reported by others35, the capacitance values determined using the data of CV or GCD showed insignificant deviation. Fig. 4(d) shows the specific capacitance obtained for each sample with different scan rates. At the maximum scan rate, the specific capacitances of MFC-4SnO, MFC-8SnO, MFC-16SnO samples were 101.10, 99.06 and 225.88 F/g, respectively. At the minimum scan rate, their specific capacitances increased to the range of 113.83- 486.38 F/g. This capacitance trend proved the pseudocapacitive nature of SnO2-cellulose nanocomposite. At low scan rates or current densities, the electrolyte ions had sufficient time to move across the active sites, resulting in high capacitance values. A high scan rate fastened ion movement and reduced the interaction between ions and the surface of electroactive material. The reduced interaction subsequently caused a reduction in capacitance value.
At the same time, the capacitance value was affected by the loading of electroactive materials on MFC thin films. The thickness of MFC thin films coated with SnO2-MFC nanocomposite was measured and then compared against the specific capacitance of each samplesat 50 mV/s while maintaining the volumetric capacitance. Charge accumulation through ion acceleration was highly benefited from the high loading of electractive material. Electrochemical impedence spectroscopy (EIS) was used to evaluate the electrochemical kinetics of the MFC thin films coated with SnO2-MFC nanocomposite. Nyquist plot in Fig. 5(b) displays a suppressed semicircle at high-frequency region for all the samples. The suppressed semicircle could be considered a signature of the least charge transfer resistance, indicating facile charge movement at the electrolyte/electrode interface due to excellent electrochemical capacitance36. At the low-frequency region, the oblique line was attributed to the fast ions diffusion path and charge transfer from electrolyte to the electrode interface37. As compared with pristine MFC, MFC-4SnO2, MFC-8SnO2 and MFC-16SnO2 samples shifted to the left due to the low bulk resistance. The slope of this graph (Fig. 5(b)) is directly proportional to the ion transfer rate38. Hence, the MFC-16SnO2 thin film was expected to attain a higher ion transfer rate than MFC-4SnO2 and MFC-8SnO2 thin films. A high loading of electroactive material results in a low ions diffusion resistance, as reported by others39. Thus, the MFC-16SnO2 thin film with low charge transfer resistance could be recommended as the electrode material of electrochemical energy storage systems40. In addition, cyclic stability analysis is upmost important to determine the electrode capability for real time applications in energy stroge system. The cyclic stability was observed when the MFC-16SnO2 thin film was tested for 40 cycles at themaximum scan rate of 100 mV/s in 1 M KOH. The CV curve is shown in Fig. 5(c). The specific capacitance of fabricated thin film was retained at 95% even after performing 40 cycles (Fig. 5(d), indicating the superior durability of the MFC-16SnO2 thin film. This slight reduction in stability was mainly due to the high scan rate of 100 mV/s that prevented the return of ions to their intial position during the reverse voltage scan.
│ CONCLUSIONS
In nutshell, the hybrid tin oxide-cellulose thin film was fabricated using hydrothermal and dip-coating method in this work. The obtained composite thin film with superior conductivity could be directly utilized as foldable electrodes for supercapacitors. Electrochemical testing apparently shows that integration of hybrid nanocomposites enhances the capacitive performance. Nonetheless, the XRD patterns explains the high concentration of SnO2 content in the composite thin film may tend to agglomerate and hard to disperse well to achieve miscibility which is essential factor for better supercapacitor. Thus, SnO2 content in composite thin film must be controllable. The fabricated flexible thin film achieved favorable specific capacitance of 225.88 F/g at 100 mV/s and 486.38 F/g at 20 mV/s and cyclic stability with 95% capacitance retention after 40 cycles. The flexible standalone composite thin film utilizes SnO2 as conductive filler whereby MFC was used as a main substrate and mechanical support also known as biopolymer based binder which able to offer broad future in the next generation for energy storage system application.
│ MATERIALS AND METHODS