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