MODIFIED LIGNIN PVA MEMBRANE FOR FLEXIBLE SUPERCAPACITORS

Abstract

Modified lignin PVA membrane for flexible supercapacitors explains a polymer matrix host that consists of a bio-polymer blend of a synthetic polymer PVA and alkali lignin which is a biopolymer. This blend can be used as a good polymer host for solid polymer electrolyte as it shows conductivity and most importantly it has the advantage of being sustainable and easy production due to the abundance availability of lignin. This polymer host with good ionic conductivity even in absence of salt, plasticizer and fillers, with best electrochemical stability and importantly economically beneficial with no environmental pollution used in the manufacture of supercapacitors.

Specification

Description:MODIFIED LIGNIN PVA MEMBRANE FOR FLEXIBLE SUPERCAPACITORS
FIELD OF INVENTION
The present invention relates to a modified lignin PVA membrane for flexible supercapacitors and method for said membrane fabrication thereof. More particularly the present disclosure recites an optimum blend composition as a membrane for supercapacitor applications where the membrane has been prepared by solution casting method, which can also be possible to be made with other methods like electrospinning, hot pressing etc. and said membrane can be utilized in any type of battery and different type of supercapacitor applications.
BACKGROUND ART
The demand for green energy solutions has intensified the exploration of bio-based materials, emphasizing eco-friendly and renewable resources. The research works on supercapacitors, and batteries are moving towards flexible devices, for which the solid polymer electrolytes becomes the centre of attraction. The liquid electrolyte conforms to the shape of the container, making it unsuitable for use in flexible devices. Therefore, polymer electrolytes are the only viable option. But polymer electrolytes being ionically insulating in nature, need to be doped with salts, fillers and plasticizers in order to have an ionic conductivity suitable for energy devices, adding to the cost of the device. So, what is required is a polymer host with good ionic conductivity even in absence of salt, plasticizer and fillers, with best electrochemical stability and importantly economically beneficial with no environmental pollution. Such a polymer matrix has been invented by experimental approach. The biodegradable synthetic polymer poly (vinyl alcohol) has been reported by many authors for its usability in energy devices like batteries and supercapacitors and is preferred in several commercial applications. But PVA is a synthetic polymer with very poor or almost nil ionic conductivity.

The present invention overcomes the very poor ionic conductivity of PVA by blending it with lignin, where the blend made with 1 : 0.5 ratio of PVA : lignin was found to have an ionic conductivity of 3.75 × 10−7 Scm−1 which is much greater than conductivity of pure PVA [1]. By blending lignin and PVA to make the host polymer matrix, the solid polymer electrolyte becomes easily biodegradable and cost effective, due to the abundance of lignin in nature. Moreover, the tensile strength test of the current polymer matrix indicates that, it is better than that of the pure PVA film, and that of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) as reported by Harmonie Michelot et al. [2], making it more suitable for flexible device applications. Also, water being used as the solvent, makes the making of this electrolyte a greener approach.

Another significant advantage of this invention is that 1:0.5 ratio of PVA : lignin blend membrane has a breakdown voltage of 2.263 V which is higher than some plasticized, salt incorporated, and filler added solid polymer electrolyte. Higher values for dielectric constant are required for efficient energy storage. In this invention, we have a high dielectric constant in the range of 5 × 104 for the best blend ratio. This is greater than the dielectric constant of pure PVA film and other salt incorporated or plasticized PVA based solid polymer electrolyte. This invention also shows significant response when tried as an EDLC device with specific energy density and power density of 17 mWh/g and 28 W/kg, respectively for a high current density of 50 mA/g. The cyclic stability was performed for 1000 cycles and it showed retention of 92.14% of its initial capacity. This Invention shows a significant importance even without the incorporation of any salt additives, plasticizers or filler, therefore there is a potential use of this polymer matrix host for high efficiency energy storage devices.
References
[1] A. Alakanandana, A. Subrahmanyam, J. Siva Kumar, Structural and Electrical Conductivity studies of pure PVA and PVA doped with Succinic acid polymer electrolyte system, Materials Today: Proceedings 3 (10) (2016) 3680-3688. doi:10.1016/j.matpr.2016.11.013.
URL https://linkinghub.elsevier.com/retrieve/pii/S2214785316303868

[2] H. Michelot, B. Stuart, S. Fu, R. Shimmon, T. Raymond, T. Crandell, C. Roux, The mechanical properties of plastic evidence bags used for collection and storage of drug chemicals relevant to clandestine laboratory investigations, Forensic Sciences Research 2 (4) (2017) 198-202. doi:10.1080/20961790.2017.1335459.
URL https://academic.oup.com/fsr/article/2/4/198-202/6780946

SUMMARY OF INVENTION
Existing polymer host matrices for solid polymer electrolytes in the field of supercapacitors often rely on salt additives for a good conductivity, as a result of which the stability of the membrane decreases which is very crucial for supercapacitor and battery application. It can be understood that the conductivity of solid polymer electrolyte is largely influenced by the nature and properties of the polymer host. This invention introduces a polymer matrix host that consists of a bio-polymer blend of a synthetic polymer PVA and alkali lignin which is a biopolymer and the second most abundant compound after cellulose. This blend can be used as a good polymer host for solid polymer electrolyte as it shows conductivity and most importantly it has the advantage of being sustainable and easy production due to the abundance availability of lignin.

In this venture, The inventors herein have developed a polymer host in the form of a membrane by utilizing the concept of blending the synthetic polymer with alkali lignin. The resultant membrane was found to show good ionic conductivity even in the absence of salt additives, and when tried for device application, it was found to show good electrochemical stability which is crucial for good energy device performance. Additionally, the blending of the membrane was done through an easy and scalable solution casting process.

Advantageous effect of Invention:
1. We have relatively good ionic conductivity considering there are no salt additives, plasticizers or fillers.
2. The method adopted for the preparation of the membrane is simple and scalable.
3. Due to the abundance of lignin in nature, it is sustainable.
4. Lignin helps in increasing the amorphous nature of the membrane which enhances the ionic conductivity of PVA.
5. It shows high dielectric constant.
6. It has good cyclic stability when tried as an EDLC device.
7. The use of lignin makes this invention more sustainable and cost effective.
8. Transforming lignin from a mere waste by-product of bio-refineries and pulp mills into something useful drastically cuts down the waste these industries produce. Since Lignin is a carbon rich polymer, by using it to create durable products, the carbon contained within it is effectively sequestered. This will help in preventing the carbon from being released into the atmosphere as CO2, which would definitely occur if lignin was burned for energy or allowed to decompose in landfills, thereby contributing to greenhouse gas emissions. This becomes a boon for not only less trash ending up in our landfills, saving precious land space, but also less methane released as waste breaks down, making our planet a little greener and cleaner.
9. Utilizing lignin as a raw material for new products promotes the efficient use of resources. It exemplifies the principles of a circular economy, where materials are kept in use for as long as possible, extracting maximum value while in use, then recovering and regenerating products and materials at the end of each service life.
10. Lignin and its modified counterparts can be used as a reinforcement material for biopolymer composite production, highlighting its potential as a sustainable resource.
11. The use of lignin exemplifies the principles of a circular economy, where the materials are kept in use for as long as possible, extracting maximum value while in use, then recovering and regenerating products and materials at the end of each service life. Additionally, if a stage is reached where lignin-based material needs to be disposed, due to its inherent property, it is biodegradable and hence, products that are made from lignin are more likely to break down and decompose in the environment compared to synthetic polymers. It has been reported that lignin-based water-soluble polymer show bio-degradation. The biodegradability of lignin and modified lignin is attributed to its natural origin and the presence of specific microorganisms, such as bacteria and certain fungi which are capable of degrading it in the environment. This will have a very low long-term impact on the environment.
12. The use of lignin and lignin-based product plays a role in carbon neutrality as it maintains a balanced carbon footprint and it is still ongoing research and is supported by studies on the use of lignin derived materials for CO2 capture and carbon footprint reduction.
13. Particularly, the use of aqueous supercapacitors is gaining attention, owing to the high safety nature of the aqueous medium (water-based electrolyte). But again, the water-based electrolytes cannot lead to flexible devices. In this sense, the PVA- Lignin system described by us in this patent is prepared using water as the solvent, with no threatening or flammable organic contents. The thermal stability of the membrane developed is studied by Thermogravimetric Analysis (TGA) and it showed that the first stage of decomposition for the lignin added membrane is slowly decaying as compared to the pure PVA membrane which is quite rapid. Additionally, the maximum weight loss in the first stage occurs at 110˚C for lignin added membrane whereas the maximum weight loss in the first stage of decay for pure PVA membrane is maximum at 99˚C. Overall, in the temperature range from 200˚C to 500˚C the overall weight loss of pure PVA membrane was 82.36% whereas for the lignin added membrane (PVA: lignin in the ratio of 1:0.5) the overall weight loss in the same range of temperature was only 51.19%. This shows that there is an improve in the thermal stability of the material.
14. The bottleneck of the aqueous supercapacitors is that, they cannot be made wearable as water is used as the electrolyte. But here, the water-soluble electrolyte are flexible for any safer wearable supercapacitors.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 illustrates images for electrolyte films in accordance with the present invention;
Figure 2 illustrates CV curves for different compositional ratio of electrode for scan rate (2a)-50 mV/s and (2b)-100 mV/s in accordance with the present invention;
Figure 3 illustrates Mean Thickness of the films in accordance with the present invention;
Figure 4 illustrates XRD for pure PVA film in accordance with the present invention;
Figure 5 illustrates XRD deconvolution spectrum for the polymer electrolyte films PL1-PL8;
Figure 6 illustrates XRD deconvolution spectrum for PL9 polymer electrolyte in accordance with the present invention;
Figure 7 illustrates FESEM images for (a). PL1, (b). PL2, (c). PL3, (d). PL4, (e). PL5, (f). PL6, (g) PL7, (h). PL8 and (i) PL9 in accordance with the present invention;
Figure 8 illustrates FESEM images for PL9 at different magnifications in accordance with the present invention;
Figure 9 illustrates FT-IR spectrum of pure PVA film in accordance with the present invention;
Figure 10 illustrates FT-IR spectrum of Lignin in accordance with the present invention;
Figure 11 illustrates FT-IR spectrum of PL1 - PL9 in accordance with the present invention;
Figure 12 illustrates FTIR Spectra of PL1 to PL8 in the range (a) 1413 − 1428 cm−1 and (b) 1567 − 1574 cm−1 in accordance with the present invention;
Figure 13 illustrates TGA curves of (a) Pure PVA, (b) PL7, (c) PL8 and (d) PL9 in accordance with the present invention;
Figure 14 illustrates Derivative of TGA curves of (a) Pure PVA, (b) PL7, (c) PL8 and (d) Figure 15illustrates Stress-strain curve for PL8 in accordance with the present invention;
Figure 16 illustrates Mean Tensile Strength, Young's Modulus, Thickness, and elongation at break in accordance with the present invention;
Figure 17 illustrates Nyquist plot of PL1 to PL8 in accordance with the present invention;
Figure 18 illustrates Influence of lignin concentration on conductivity in accordance with the present invention;
Figure 19 illustrates Dielectric constant (Є') as a function of frequency for different lignin concentration in accordance with the present invention;
Figure 20 illustrates Dielectric loss (𝜖′′) as a function of frequency for different lignin concentration in accordance with the present invention;
Figure 21 illustrates Current density versus potential for PL8 in accordance with the present invention;
Figure 22 illustrates CV for PL8 system at different scan rates in accordance with the present invention;
Figure 23 illustrates CV Schematic diagram for EDLC cell in accordance with the present invention;
Figure 24 illustrates Specific Capacitance from CV curves versus scan rate in accordance with the present invention;
Figure 25 illustrates Charge-discharge curve vs current density at 50 mA/g current density in accordance with the present invention;
Figure 26 illustrates Specific capacitance and Retention vs number of cycles at 50mA/g Current density in accordance with the present invention.
DETAILED DESCRIPTION

The present invention herein discuss on creating a solid polymer matrix that will serve as a good base for the development of solid polymer electrolyte by investigating the different ratio of blend between PVA and lignin, and finally the particular composition ratio was selected for a solid polymer matrix that will serve as a base for solid polymer electrolytes. The said particular ratio showed optimum performance in dielectric constant, degree of crystallinity, and also mechanical strength. This blend ratio can also serve as a host for further improvement by the incorporation of salts, fillers and plasticizers, to subsequently fabricate high performance solid polymer electrolyte.

The prior art approach of use of organic solvents is eliminated, and herein water is only used as solvent which is economically viable and environmentally friendly method. In present approach the applicants have not intended to make a gel electrolyte and have made a solid polymer electrolyte at ambient temperature, and no kind of freezing has been made.

The present invention utilizes more lignin per gram of PVA as compared to prior art. The higher lignin content in our formulation not only maximizes the utilization of lignin, which is plentiful waste material, but also reduces the dependency on PVA. Given that the production of PVA involves substantial costs related to raw materials, energy consumption, processing, and compliances with environmental regulations, our approach is more cost effective and sustainable, as more lignin is utilized per gram of PVA. By using more lignin and less PVA, the herein disclosed method supports the conversion of greater amount of lignin into useful materials, thereby minimizing waste and contributing to a greener environment.

The herein disclosed specification focuses on achieving a polymer matrix being a good polymer matrix for a specific blend combination with high dielectric constant (higher than some solid polymer electrolyte with salts and fillers) and high stability over many charge-discharge cycles witch can withstand high temperature much better than hydrogel electrolyte. Due to its soft and flexible nature, hydrogels are not that easy to integrate into certain devices, requiring additional support structures or containment strategies. On the other hand SPEs, being solid and rigid, are easier to handle and integrate into various device architectures without the need for additional containment measures.

The characterization studies were done by XRD, FTIR, FESEM, TGA, tensile tests, and EIS for analysis. The blend membrane demonstrated noteworthy properties, including a good tensile strength of 32.848 MPa. Notably, the blend showcased promising electrochemical behaviour, marking its potential for environmentally conscious energy storage solutions, detailed through electrochemical studies like linear sweep voltammetry (LSV), Cyclic voltammetry (CV) and Galvanostatic charge-discharge studies (GCD) analyses, revealing a breakdown voltage of 2.263 V, a specific capacitance of 1.86 F/g at 1 mV/s scan rate by CV studies, and exceptional stability over 1000 cycles at 50 mA/g of current density with a retention of 92.14 % of its initial capacity by GCD studies.

Experimental data:
Materials
Polyvinyl Alcohol with CAS no. 9002-89-5 was purchased from Fisher Scientific and alkali lignin, low sulfonate content was purchased from Sigma Aldrich. Distilled water was used as a solvent and the all the chemicals were used with no further purification.

Sample preparation procedures
Through an extended amount of experimentation and optimization, the membrane fabrication employed a refined methodology, specifically engineered to augment both the electrochemical functionality and mechanical robustness of the membrane. The steps of the process are delineated as follows:
1. Initial solution preparation: The procedure begins with a precise dissolution of 1 gram of PVA into 30 mL of distilled water. The selection of distilled water ensures the purity of the solvent to prevent any ionic contamination.
2. Stirring of Homogenization: The solution is subjected to continuous stirring with a rate of 350 revolutions per minute (rpm) for 24 hours at room temperature, so as to facilitate a complete dissolution of PVA.
3. Ultrasonic Dispersion: After PVA has dissolve completely, the solution is allowed to undergo ultrasonication for 15 minutes to promote the deagglomeration of polymer chains, enhancing the uniformity of the solution, after which an additional stirring is followed to ensure thorough mixing.
4. Addition of lignin, alkali: Lignin alkali (low sulfonate) content is then introduced to the homogenized solution of PVA in 0.04 g, 0.06 g, 0.08 g, 0.1 g, 0.4 g, 0.3 g, 0.4 g, 0.5 g and 0.6 g for sample code PL1, PL2, PL3, PL4, PL5, PL6, PL7, PL8, and PL9 as shown in table 1 respectively, which is then followed by another 24- hour stirring period to achieve a fully integrated polymer blend.
5. Enhanced Ultrasonic Treatment: The blend then undergoes a two-stage ultrasonic treatment, initially for 5 minutes, to further improve the dispersion. After which, the solution is stirred again but at a higher rate than before, at 480 rpm for 2 hours to ensure optimal molecular entanglement. Subsequently, a 10-minutes sonication in a cold-water bath was performed to achieve a solution with desirable viscosity characteristics.
6. Bubble Removal: The solution is then placed in vacuum, where the bubbles are sucked out through an air pump. This step was taken in order to remove the bubbles or potential bubble formation during heating which adversely affect the film integrity and ensures a smooth and uniform membrane. This step is very significant as it is evidenced by different experimentation technique to achieve a good uniform membrane.
7. Casting Preparation: A plastic petri dish, which is chosen due to its ease of membrane detachment compared to glass alternatives, is cleaned and preheated to 40˚C in a hot air oven. This step has proven to be very crucial for facilitating the subsequent casting process.
8. Film Casting: The polymer blend solution is gently and slowly casted into the preheated petri dish, allowing the solution to evenly spread across the surface on its own. The dish is then heated for 24 hours to evaporate the solvent and form the membrane.
9. Post-Casting Treatment: After the solvent evaporation, the membrane undergoes further drying in a vacuum oven at 40˚C for 3 hours to remove any residual solvent, ensuring the membrane's structural integrity and performance.
10. Cooling and storage: After the drying stage is completed, the petri dish is pulled out of the oven and then placed inside a zip lock and is allowed to cool down to room temperature. After which, the membrane is carefully peeled off from the petri dish and stored in a zip lock bag to preserve its quality until further use.
The images of the electrolyte films are shown in Fig 1.

Sample Code PVA (in grams) Lignin (in grams)
PL1 1 0.04
PL2 1 0.06
PL3 1 0.08
PL4 1 0.1
PL5 1 0.2
PL6 1 0.3
PL7 1 0.4
PL8 1 0.5
PL9 1 0.6

Table 1: Sample design of PVA: Lignin based SPEs.
(PL8 was found as best based on performance)
In order to make the device, the membrane chosen from blend i.e PL8 has to be sandwiched between the electrodes. For this purpose, the electrodes are made as below.

Electrode preparation
The electrodes were prepared by a composition of charcoal activated carbon as active material, carbon black as conductive agent, and polyvinylidene fluoride (PVdF) as binder. Investigation was done for three different compositional ratios of these components: 80:10:10, 83:10:7, and 85:10:5, respectively, with 15 mL of N-methyl-2-pyrrolidone (NMP) serving as the solvent for the slurry preparation. Prior to selecting the best ratio, electrodes were prepared on a graphite substrate with 3 mg of active mass and cyclic voltammetry of the three compositional ratios was performed in a three-electrode system with KOH as the electrolyte. The electrochemical analysis revealed that the slurry composition of 83:10:7 exhibit the highest capacitance, as evidenced by the largest integrated area under the cyclic voltammetry (CV) curve shown in figure (2a) and (2b), hence, the compositional ratio of 83:10:7 was adopted. The CV's were done at two scan rates to check if the same trend exists.

For electrode fabrication meant for device, a methodical approach was employed to ensure a uniformity and optimal electrochemical performance. Initially, polyvinylidene fluoride (PVdF) and N-methyl-2-pyrrolidone (NMP) solvent were combined in a small borosilicate glass bottle. This mixture was stirred vigorously in order to achieve a homogeneous PVdF-NMP solution. Subsequently, charcoal activated carbon and carbon black were thoroughly mixed using mortar and pestle in order to create a fine and uniform particulate distribution. This powdered mixture was then gradually introduced into the PVdF-NMP solution, then the container was tightly sealed in order to prevent moisture adsorption by the particles and was allowed to stir continuously until a thick, homogeneous black slurry was obtained, suggesting a successful integration of all components. The slurry was then uniformly coated onto an aluminium sheet, which had been sanded previously in order to enhance adhesion and electrical contact. The coating process was facilitated using a doctor blade technique, which will allow precise control over the thickness of the electrode. After the coating has been done, the electrodes were subjected to a two-stage drying procedure so as to remove any residual solvent and also ensure structural integrity. Initially, the coated electrodes were placed in a hot air oven and heated at 80˚C for 24 hours. This step is crucial for evaporating the bulk of the solvent. Then the second stage of heating was done, where the electrodes were transferred to a vacuum oven and dried at 50˚C for an additional 4 hours for a complete removal of NMP. Upon completion of these steps, the electrodes were fully prepared and conditioned for device fabrication and subsequent electrochemical testing.

Electric double layer capacitor (EDLC) fabrication and characterization
The fabrication of the EDLC cell was done by sandwiching the highest conductivity prepared polymer electrolyte PL8 between two identical activated carbon-based electrodes (ratio optimized under electrode preparation).
Stainless Steel blocking electrode were used as current collector and the cyclic voltammetry and galvanostatic charge discharge were analyzed using the CH Electrochemical Analyzer/Workstation with instrument Model CHI6008E.

Structural and morphological studies of membranes (Results and discussions)
Thickness of membranes (films)
The thickness pf the films was measured using a digital micrometre screw gauge with a resolution of ±1 µm. 10 thickness measurements were taken randomly on each sample and a mean value was calculated. The mean value was used in the calculations.

The film thickness is also one of the important parameters that will influence the ionic conductivity. It has been shown that the thicker the films the lower will be the ionic conductivity. This is because there will be a larger diffusion distances for the ions, and as the potential is applied the ions will be moving but in the process of moving, they will be met with resistance form the medium, which for larger distances will lower the ionic movement. The thickness of the film will also influence the energy density of the device as a whole. Thicker electrolyte will add to the energy density of the device because they can store more ions but as stated earlier, due to the greater thickness, it will limit or slow down the ion transport which will affect the power density of the device as a whole.
Therefore, optimizing between the energy density and the power density is crucial.
The film thickness was measured at 10 random positions and the mean for each film was calculated. It is to be noted that the mean thickness of the films is taken for the necessary calculation in this study. In Fig. 3, the mean thickness of each film has been shown with their corresponding standard deviation.

X-Ray diffraction (XRD) investigation of membranes
The X-Ray Diffraction (XRD) data for all the samples were obtained using the BrukerAXS X-ray diffractometer with operating current of 30 mA and voltage of 40 kV to explore the nature and complexation between PVA and lignin. CuK X-rays of wavelength of 1.5406 were used for scanning the materials and the glancing angles ranged from 10◦ to 80◦ and the diffraction step size was 0.01◦.

The applicants herein employ XRD in order to investigate the structure and crystallinity of the prepared membranes. Figure 4 shows the XRD spectrum of pure PVA film. We clearly see that the XRD pattern of the pure PVA film revealed a strong crystalline reflection at 2θ = 19.44◦ and 2θ = 23.23◦ which are very close to those being reported in literature, which represent those reflection from (101) and (200) from a monoclinic unit cell. The intense peak observed in PVA inferring to its semi-crystalline nature which are mainly due to the presence of strong intramolecular and intermolecular bonds among the PVA polymer chains. The XRD deconvolution of pure PVA and PL1-PL8 are shown in figure 4 and figure 5 respectively. The deconvolution technique allows for the determination of a material's microcrystalline characteristics. In this method, the amorphous and crystalline peaks are separated using the known algorithm and then the percentage of crystallinity can be calculated by using equation 1.
Pc = AC /AT * 100 ……(1)
where AC represents the area of crystalline peaks and AT represents the combined area of crystalline and amorphous peaks. One can see that the degree of crystallinity of pure PVA is about 40.35%.
It can be seen through XRD deconvolution for PL1 to PL8 in Fig. 5 that, as lignin was added, another two broad peaks appeared on the samples around 2θ = 12◦ and 2θ = 42◦ as shown in figure which is in the vicinity of 2θ = 11◦ and 2θ = 40◦, this could be due to the interaction of lignin with PVA that may cause phase separation which in turn affect the crystallization behaviour of PVA. Table.2 shows that the PL8 has very low degree of crystallinity, that is, it becomes more amorphous. But pure PVA, PL1 to PL7, and PL9 are crystalline than the PL8. This shows that the PL8 is the best membrane for conductivity.
Sample Code Degree of crystallinity (%)
Pure PVA 40.35
PL1 29.07
PL2 26.34
PL3 22.23
PL4 20.19
PL5 19.80
PL6 19.40
PL7 17.89
PL8 13.81
PL9 18.96
Table 2: Degree of crystallinity through XRD deconvolution analysis showing PL8 to be more amorphous in nature.
It is also observed that as the concentration of Lignin increases, the crystalline peaks of PVA start to decrease which is dominantly visible for PL7 and PL8. But as we increase the lignin concentration further, we can see that in PL9 as shown in Fig. 6, the crystallinity increase as there are many sharp peaks that are introduced which is a characteristic attribute for a complete phase transition (this is also supported by FESEM analysis) for PL9 as shown in figure 6. This means that PL8 is the maximum concentration of lignin that will exhibit a homogeneous membrane.

Morphology analysis by Field Emission Scanning Electron Microscopy (FESEM)
The surface morphology analysis of the materials was performed using the HR-FESEM (Jeol) with acceleration voltage of 5 kV with image resolution of 10 µm and 1000× magnification.

The field emission scanning electron micrographs were taken at a 1000× magnification for the samples to support our XRD observations and the micrograph are shown in Fig. 7. It was observed that at lower concentration of the lignin i.e., for PL1, PL2, and PL3, the FESEM micrographs showed a seemingly smooth surface, however it is to be noted that the non-uniform white entities that are observed can be attributed to undissolved PVA particles being present in the blend. We can infer that these samples exhibit mostly homogeneous morphology which also show that there is good miscibility between PVA and lignin. For sample PL4, it is observed that irregular domain starts to emerge, this can indicate that phase separation has started to take place. Although these domains are not highly pronounced as can be seen from the image, they suggest that there is a slight reduction in the blend homogeneity compared to those samples with lower concentration of lignin. For PL5, the FESEM micrograph reveals distinct domains which indicates a further phase separation in the blend and could be regions where PVA retains some crystalline structure. They appear brighter in the FESEM images because crystalline regions are known to have higher electron density compared to amorphous areas.

As the lignin concentration is increased from PL6 to PL8, the micro structures grow in size and they progressively take on a hollow circular shape like structure that can be considered pores. This observation suggests that the degree of phase separation in the blend is more pronounced with the hollow circular grains indicating the formation of domains with different material compositions. This increasing phase separation disrupts the regular arrangement of the polymer chains of the semi crystalline PVA lead as a result of which the polymer chains might have more freedom to move leading to an amorphous like structure. Among the samples, PL8 exhibits the largest hollow circular pores, due to maximum phase separation as lignin concentration is increased and evidently, by de- convoluting the XRD spectrum it has been found that PL8 has the lowest degree of crystallinity as shown in table 2 due to the highest degree of disruption of PVA polymer chains by the presence of lignin. The increase in amorphous regions due to phase separation can influence the thermal, mechanical and electrical properties of the film.

In contrast to the previous samples, PL9 shows a distinct behaviour. From XRD analysis it was observed that the PL9 sample showed an increase in the degree of crystallinity as compared to PL8 which can be confirmed by electrochemical studies, where conductivity of PL9 is less than that of PL8. This increase in the degree of crystallinity can be due to the lignin concentration crossing the limit of homogeneity condition for the blend, this is supported by the FESEM micrograph shown in Fig. 8. The hollow circular shape observed in the previous samples is no longer maintained in PL9. Instead, the micro- structure exhibits a rectangular strand-like structure that appears in some region to be intertwined with each other as shown in Fig. 8.


Fourier transform infrared spectroscopy (FT-IR) analysis
In order to look for any probable interaction between distinct chemical groups, all the samples were subjected to FTIR spectroscopy. The FTIR spectrum were obtained using a PerkinElmer Spectrum 2 FT-IR Spectrometer between 400 and 4000 cm−1.

One can see that the FT-IR spectrum of pure PVA film in Fig. 9 shows a strong and broad band that is centered at 3310 cm−1 which is present for all the samples and is attributed to be the stretching vibration of hydroxyl (OH) group due to strong hydrogen bonding within the polymer. The observed small band at 2941 cm−1 can be assigned to asymmetric stretching vibration of aliphatic CH2 (methylene) groups which is commonly found in organic compounds and at 2854 cm−1 can be assigned to CH stretching. Comparing the spectrum of Polyvinyl Alcohol (PVA) from reported literature and our own, we also found that in addition to the functional groups expected in PVA, there is a stretching vibrational band of C--O at 1715 cm−1 which is attributed to the carbonyl functional groups and a few percentage of terminal acetal, keto or ketal groups may also be present which has persisted upon hydrolysis from polyvinyl acetate or it can be that there was oxidation reaction during preparation and processing.

The band observed at 1428 cm−1 is associated with the C-H bending, however this specific frequency can fluctuate depending on the specific PVA sample. The band observed at 1247 cm−1 can be attributed to the C-O stretching vibration of secondary hydroxyl groups segment. The band that is observed at 1089 cm−1 and 1024 cm−1 can be assigned to the crystallization sensitive bands which can be referred to C-O stretching vibration which is also consistent with that reported in literature.

The FTIR spectrum of lignin sample in Fig. 10 is showing a broad band in the range 3330 to 3420 cm−1 which verifies the presence of hydroxyl group OH stretching vibration of phenolic hydroxyl and alcoholic hydroxyl groups (aromatic and aliphatic OH groups).

The two small peaks at 2928 cm−1 and 2832 cm−1 are related to CH2 and CH stretching vibration of aromatic Methoxyl groups and are in methyl and methylene groups of side chains. it is observed that there are no absorption bands of lignin 2600 cm−1 to 1750 cm−1. The band at 1628 cm−1 and and 1580 cm−1 are caused by aromatic ring vibrations (C--C) stretching that are present in the lignin. The absorption observed in 1113 cm−1 and 1022 cm−1 are related to the stretching vibration of of the C-O in the aliphatic ethers and aliphatic alcohols respectively. These observations are consistent with that reported in literature.

The FTIR spectrum for polymer blends of PVA and lignin is shown in Fig. 11 In the region of hydroxyl stretching (3000 cm−1 to 3700 cm−1), we saw that PVA film showed a wide absorption band focused at (OH) 3299 cm−1. When the blend was formed, this band was shifted to a lower wavenumber in the range 3290 cm−1 to 3295 cm−1 for all the samples. This suggests that a new hydrogen bond is being formed either within the PVA or between PVA and lignin. As we go from PL1 to PL9, we observed an increasing trend in the C-O stretching frequency in of the range 1244 − 1264 cm−1 which suggest that interaction has taken place in the vicinity. The blue shift in the C−O wavenumber from 1244 cm−1 to 1264 cm−1 as shown in Fig. (12a) can be influenced by the altered hydrogen bond, which suggest that the bond length has decreased as the concentration of lignin is increased which further suggest that the bond strength has increased. This shows that there is a strong interaction between lignin and PVA.

The stretching frequency in the range 1088 − 1092 cm−1 corresponds to C-O-C group and PL7 and PL8 displayed the highest frequency amongst the other samples. on a broader scale, there are two regions that are showing the most change in the transmittance curve, they are those regions from 1413−1428 cm−1 and those from 1567−1574 cm−1. The signals appearing in the range 1413−1428 cm−1 as shown in Fig.(12a) corresponds to C- H bending frequencies which is shifted towards lower frequency as we increase the lignin concentration, with PL7 and PL8 showing the sharpest signal amongst all the samples which suggest that there is an increase in hydrogen bonds. The increased in intensity suggest that there are many hydrogen bonds in the blend however, the red shift as shown in Fig. (12a) suggest that the hydrogen bonds are weak due to low wavenumber that further suggests the bond length is longer hence less energy is needed to break those bonds. This would explain the electrical behaviour observed by electrochemical impedance spectroscopy as shown in Fig. (18).

The signals appearing in the range, 1567−1574 cm−1 as shown in Fig. (12b), correspond to the stretching vibration of the C--N or C--C bond in the aromatic structures which shows maximum signal in PL8. Since lignin is an aromatic polymer the presence of C--C is to be expected but aromatic carbon-carbon double bonds are quite stable and there is a slim chance for them to take part in the chemical reaction so the only reason that can explain for the peak in the 1567 − 1574 cm−1 region is the formation of the π − π stacking interaction as shown in literatures. These π − π stacking interaction play a crucial role in facilitating charge transport and enhancing the ionic conductivity of the material by creating a more ordered and interconnected polymer network which in turn enhances the mobility of charge carriers within the material which leads to the higher overall conductivity.

From the FTIR spectrum of all the sample we can say that there is an effective hydrogen bonding between the hydroxyl groups of PVA and lignin. However, due to the red shift, it can be suggested that though there are many hydrogen bonds, they can be easily disrupted when a sufficiently small potential is applied, which will explain the conductivity of the film. The disruption of hydrogen bond can lead to increased segmental motion of the polymer chains. The strong C-O bond as shown by the blue shift in Fig. (12a) from 1244 − 1264 cm−1 together with the hydrogen interaction can improve the overall performance of the blend.

Thermal studies using Thermogravimetric analysis (TGA)
To study the thermal stability of the materials, thermogravimetric analysis (TGA) analysis was done by using the PerkinElmer TGA 4000 thermal analyser instrument in the temperature range of 25 to 750◦C with a scan rate of 10◦C/min under nitrogen atmosphere.

TGA is used to determine the safe operation of energy devices and can also be used to study and investigate the thermal stability and degradation of pure PVA, PL7 and PL8 and PL9. The samples of approximately 10 mg were heated from 25◦C to 750◦C at a scan rate of 10◦C / min in nitrogen atmosphere at a flow rate of 19.8 mL per minute. Fig. 13 shows the TGA curves of pure PVA, PL7, PL8 and PL9.

From our TGA measurement, the pure PVA sample showed a reduction in weight of 10.55% in the first stage of heating which started at about 32◦C and the maximum weight loss occurred at around 99◦C as observe from the derivative of the curve shown in Fig. 14, this can be attributed to the weight loss due to evaporation and moisture removal and the removal of any volatile substances adsorbed or bound to the PVA sample. This is called the process of dehydration. The second stage of weight loss was observed of about 67.12% starting from 219◦C with the maximum weight loss at 336◦C. The weight loss is this region is very significant which suggest the decomposition of the PVA polymer side chains. With increasing in the temperature, the PVA polymer as a whole start fragmenting into smaller molecular segments as a result of which there is a release of volatile decomposition products which is typical for organic polymers undergoing thermal degradation. The third stage suffered a weight loss of 16.29% starting from 433◦C with its maximum weight loss at 446◦C. This may be attributed to the decomposition of the main Polymer chains of PVA. The last stage started from 566◦C with a weight loss of 2.65%. In this stage, since the weight loss is very small this suggest that the remaining residue of PVA may have undergone additional decomposition and with high temperature there is also a release of carbon dioxide. From the observations, it can be stated that there was a loss of a total of about 75% in the range of 200◦C − 450◦C which mainly is the decomposition of PVA.


The TGA results for the polymer blend PL7, PL8 and PL9 showed that the degradation occurs relatively in the lower temperature but the overall weight loss shows less than that of pure PVA film. For PL7, the weight loss due to increase in temperature started occurring at 32◦C of approximately 14.25% which can be attributed to water or moisture removal with its maximum rate of weight loss at around 97◦C. The second showed the highest weight loss of 47.33% starting from 180◦C with its maximum rate at around 286◦C and this can be attributed to the decomposition of the main polymer chains of the polymer blend. The third phase began at around 400◦C with a weight loss of 12.67% owing to the degradation of the side chains in the blend and any other volatile gaseous that is generated with the breaking of the chains with a maximum rate at around 450◦C. Then it is observed that there is no significant weight loss till it reaches 750◦C.

For PL8, the weight loss due to water or moisture removal started at 32◦C of approximately 14.81% with its maximum rate of weight loss at around 110◦C. The second stage of weight loss started from 203◦C with a significant weight loss of about 43.87% owing to the degradation of the main polymer chains of the polymer blend, with its maximum rate of weight loss at around 289◦C. The third stage of weight loss occurred from 418◦C of about 10.80% owing to the degradation of the side polymer chains of the blend and the release of any volatile gaseous, with its maximum rate at around 450◦C. For PL9, the weight loss due to water or moisture removal started at 32◦C of approximately 14.3% with its maximum rate of weight loss at around 110◦C. The second stage of weight loss started from 203◦C with a significant weight loss of about 44.16% owing to the degradation of the main polymer chains of the polymer blend, with its maximum rate of weight loss at around 282◦C. The third stage of weight loss occurred from 418◦C of about 10.67% owing to the degradation of the side polymer chains of the blend and the release of any volatile gaseous, with its maximum rate at around 449◦C.
On comparing the total weight loss of the blended films and the pure PVA film, we take the overall loss of weight in a given range of temperature that is from 200◦C to 500◦C and we found out that the blended films are more resistant to thermal degradation despite having lower maximum weight loss temperatures and we compare the residual left after the analysis. In the given temperature range, the weight loss for pure PVA is 82.36%, for PL7 is 56.82%, for PL8 is 51.19% and for PL9 is 52.18%. Therefore, PL8 showed the best thermal stability from among the polymer blend.

Tensile strength test for best membrane
The tensile test was done only for the sample with the highest conductivity according to ASTM D638- type V standard. The tensile curve, including tensile strength, elastic modulus, and elongation break of the film was obtained using the Universal Testing Machine (UTM) (AGS-X series 5 kN, Shimadzu, Japan). With a rate of 18 mm/min the tensile test was repeated three times for the same sample.

Tensile strength test is a mechanical test that is used to determine the ability of the material to withstand a tensile stress or load. In the context of the solid polymer electrolyte, the tensile test is done by clamping the polymer electrolyte at both ends and is subjected to a uniaxial tensile load using a tensile testing machine. Then the load is applied gradually until the sample breaks or ruptures. Tensile strength in this study is done for the sample with best conductivity. The stress-strain curve for PL8 is shown in Fig.15, being repeated three times.


The thickness of the sample under study was measured and the mean of the thickness is shown in table 3. From the stress-strain curve, the mean of tensile stress, the young's modulus and the elongation at break was calculated which are shown in Table 3 and Fig. 16.


Sample Thickness (µm) Tensile Stress (MPa) Young's Modulus (MPa) Elongation at break (%)

PL8 121
132 32.130
33.325 0.105
0.086 206.958
264.719
126 33.089 0.092 247.769
Average 126.333 32.848 0.094 239.815
Standard Deviation
4.496
0.517
0.00819
24.242

Table 3: Mean and standard deviation of mechanical properties.

As shown in table 3, the mean tensile strength of the polymer electrolyte under study is 32.848 MPa which is greater than the tensile strength of the pure PVA film reported by Melbi et al with a value of 24 MPa and is certainly greater than tensile strength of low- density polyethylene (LDPE) and high-density polyethylene (HDPE) plastic bags as reported by Harmonie Michelot et al. This implies that lignin increases the tensile strength of the film as a whole. It is important for solid polymer electrolyte to have good tensile strength because it affects the overall mechanical properties of the composite, ionic conductivity and the interfacial capacitance with the electrode.
Electrical properties of the membranes by the Electrochemical impedance spectroscopy (EIS) studies
The impedance properties of synthesized electrolytes were initially studied using CH Electrochemical Analyzer/Workstation with instrument Model CHI6008E to measure the real (Zr) and imaginary (Zi) components of impedance. The frequency range was 1 Hz to 1 MHz, and the test was done at ambient temperature. The polymer electrolytes were cut into a 1 × 1 cm2 cross section and were held between two stainless steel blocking electrode which were under spring pressure for this measurement.

The impedance spectra exhibit two distinct regions, a high frequency region which takes the form of a semicircle which is due to the ion movements in the electrolyte and a low frequency region taking the form of a spike which is due to the motion of the charges or ions at the blocking electrode. The plot of the real and imaginary impedance will give us the Nyquist plot, and it can consist of (i) a depressed semicircle. (ii) a tilted spike or (iii) a depressed semicircle with a tilted spike. The bulk resistance of the system has a direct influence on the conductivity and is directly related to the semicircles. The intersection of the high frequency side semicircle with the real axis will give the value of the bulk resistance, therefore it can be said that the diameter of the semicircle represents the sum of the polarization resistance and the bulk resistance. Whenever an AC electrical field is applied across the membrane, if ions do present in the membrane, they will diffuse across the membrane and an accumulation at the electrode/electrolyte interface will take place. The stainless-steel electrode has a property where it blocks the ions from passing through it, hence this will give a different signature in the impedance spectra which can be found out by tracing the impedance measured through EIS.

The estimation of the bulk resistance in this paper is done by two approaches, i.e., through graphical approach and through Equivalent electrical circuit (EEC) fitting and is found that they are in agreement with each other and their values are shown in Table 4.

Sample Code Graphical Bulk resistance (Ω) EEC Bulk resistance (Ω) Graphical Conductivity (Scm−1) EEC
Conductivity
(Scm−1)
Pure PVA 5.14 × 1010 5.15 × 1010 1.10 × 10−12 1.10 × 10−12
PL1 4.76 × 107 4.71 × 107 2.82 × 10−10 2.85 × 10−10
PL2 1.61 × 107 1.59 × 107 6.22 × 10−10 6.22 × 10−10
PL3 8.26 × 106 8.23 × 106 1.28 × 10−9 1.28 × 10−9
PL4 7.15 × 106 7.25 × 106 1.79 × 10−9 1.79 × 10−9
PL5 5.84 × 106 1.38 × 107 2.36 × 10−9 2.36 × 10−9
PL6 1.57 × 106 1.5 × 106 1.06 × 10−8 1.06 × 10−8
PL7 2.08 × 105 1.95 × 105 7.02 × 10−8 7.02 × 10−8
PL8 3.25 × 105 3.13 × 104 3.75 × 10−7 3.75 × 10−7
PL9 3.32 × 105 3.23 × 105 6.67 × 10−8 6.67 × 10−8
Table 4: Conductivity calculated from graphical and EEC fitting approach. The mean conductivity is plotted against lignin concentration as shown in Fig. 18.
It can be seen that PL8 from all of the characterization showed the most promising result. So, we have restricted our studies for PL1 to PL8.

Dielectric Properties
The higher the real part of the dielectric constant, the greater is the polarizability of the membrane which means more storage capacity.

The dielectric constant and the dielectric loss for all the membranes are shown in Fig. 19 and 20, respectively. For all the membranes, the dielectric constant is relatively larger in magnitude than that of dielectric loss, which means that the membranes can store more energy than it can dissipate. The membrane PL8 has the best dielectric constant making it the best membrane for energy storage.

One can see that PL8 with 0.5 grams of lignin showed the highest dielectric constant. The reason being that the dielectric constant (𝜖′) and the dielectric loss (𝜖′′) are influenced more by the amorphous phase present in the system, which confirms that PL8 being the most amorphous sample is also showing highest conductivity.

Linear sweep voltammetry (LSV) study to determine the potential stability of the membrane -
The determination of the degradation potential or the potential window of the solid polymer electrolyte is necessary because it will give us the maximum electrochemical stability. A cell was constructed as SS|PL8|SS, where SS stands for stainless steel, and the LSV was recorded at a sweep of 10 mV s−1 at room temperature using the CH Electrochemical Analyzer/Workstation with instrument Model CHI6008E.

Fig. 21 shows the LSV response of the highest conducting PL8 membrane. It is seen that in the range of 0 to approximately 1.9 V the current seems to be steady and constant without any substantial increment in the current. As potential reaches 2.263 V, the current increases substantially which is due to the decomposition of the polymer electrolyte in particular at the electrode's surface. So, it can be concluded that the decomposition or degradation potential of the SPE can be approximately 2.263 V.

DEVICE PERFORMANCE : Electric Double Layer Capacitor (EDLC)
EDLC is a type of supercapacitor made by PL8 membrane sandwiched between two optimized activated carbon electrodes
Electrochemical studies to analyze the device
Cyclic voltammetry (CV) and galvanostatic charge discharge (GCD)
The cyclic voltammetry and the galvanostatic charge discharge analysis was done using the CH Electrochemical Analyzer/Workstation with instrument Model CHI6008E
Cyclic Voltammetry analysis
It has been observed that the PVA-Lignin polymer blend shows conductivity without the addition of any salt, therefore a case study has been done to observe its response to cyclic voltammetry and galvanostatic charge discharge experiment. The basic schematics of the cell construction are shown in Fig. 23. The CV response of the device is shown in Fig. 22. As seen from the CV response, the shape is approximately rectangular especially in the lower scan rate and there is an absence of the redox peaks which further suggest that the device is having (electric double layer capacitor) EDLC behaviour with no pseudo capacitor behaviour for energy storage. It is a well-known concept that the faradaic charge storage mechanism involves intercalation/deintercalation process whereas the non-faradaic charge storage mechanism only involves the accumulation of the charges at the interfacial region.


In our study, it can be said that both the electron and the ions are accumulated at the electrode and electrolyte interface. This accumulation of charges at the interface will cause a double layer which gives rise to storage of energy in the form of potential energy. In the present system, a non-ideal rectangular shape is observed, this can be due to influence of the porosity of the electrode and the internal resistance factor. The non-ideal rectangular shape observed in this system resembles more like a leaf like shape.

The corresponding specific capacitance (Csc) is obtained and is shown in Fig. 24 and the highest capacitance was found to be 1.86 F/g and was seen to decreased in value as the scan rate increases as it is the characteristics of an EDLC.

Galvanostatic charge discharge analysis
Galvanostatic charge discharge analysis had been done for current 50 mA/g. Fig. 25 displays the sample's charge-discharge behaviour for four cycle at the highest current density of 50 mA/g, noting a longer initial charging duration compared to subsequent cycles due to potential electrolyte incomplete wetting in electrode pores and solid electrolyte interface (SEI) formation on the electrode, both increasing resistance to ion transport and charge transfer. The almost straight discharge curve, indicative of capacitive traits, suggests double layer charge storage.

It is also important to study the cyclic stability of the system, that is, to allow the system to undergo a charge and discharging process over a long cycle and measure the retention. The Specific capacitance and Retention for 50 mA/g current density with respect to number of cycles is shown in Fig.26

It has been observed that the percentage of retention for 50 mA/g is high and retaining about 91.14% of its initial capacity even after 1000 cycles. From GCD, it can be concluded that the film has a good stability even for high number of cycles. The amount of energy that can be stored by the EDLC per unit mass is called specific energy and it is measured in watt-hours per kilogram (Wh/kg). Therefore, higher specific energy means the EDLC can store more energy per unit mass. The amount of energy that can be delivered by an EDLC for a given time is called specific power and it is measure in watt per kilogram (W/kg). It can be inferred that higher the specific power, higher will be the energy that can be delivered by the device in a given amount of time. The value of the specific energy and the specific power for the last cycle of 50 mA/g current density is found to be 17.72 mWh/kg and 27.73 W/kg, respectively.

Industrial Applicability: The polymer blend membrane has application in energy devices like supercapacitor, and battery.

While the present invention has been described with reference to a specific preferred embodiment, it will be apparent that various modifications and changes could be made to this embodiment without departing from the scope of the invention. The above- mentioned description are provided to serve the purpose of clarifying the aspects of the invention, and it will be apparent to one skilled in the art that they do not serve to limit the scope of the invention. By way of example, the total assembly is customized. The above-mentioned assembly example is just one of the many configurations that the mentioned components can take on. All modifications and improvements have been incorporated herein for the sake of conciseness and readability but are properly within the scope of the present invention.
, Claims:1. A modified lignin PVA membrane for flexible supercapacitors comprising of :
a blend composition of 1 : 0.5 ratio of PVA : lignin as a membrane;
wherein the said composition ratio is selected for a solid polymer matrix that serve as a base for solid polymer electrolytes.
2. The modified lignin PVA membrane for flexible supercapacitors as claimed in claim 1, wherein said membrane is configured to be sandwiched between the activated carbon electrodes, any metal oxide electrode, metal sulfide electrodes and combination thereof.
3. The modified lignin PVA membrane for flexible supercapacitors as claimed in claim 1, wherein said membrane is configured for operational with of battery and /or supercapacitor applications.
4. The modified lignin PVA membrane for flexible supercapacitors as claimed in claim 1, wherein said membrane further includes fillers, salts, plasticizers for the improving the device performance.
5. A method for fabrication of modified lignin PVA membrane for flexible supercapacitors comprising the steps of:
• preparing an initial solution with a precise dissolution in the ratio of 1 gram of PVA into 30 mL of distilled water;
• providing continuous stirring of the solution for homogenization so as to facilitate a complete dissolution of PVA;
• performing ultrasonic dispersion after dissolution of PVA in order to promote the deagglomeration of polymer chains and enhancing the uniformity of the solution, after which an additional stirring is followed to ensure thorough mixing;
• adding of lignin, alkali (low sulfonate) content to the said homogenized solution of PVA in followed by another 24- hour stirring period to achieve a fully integrated polymer blend.
• performing enhanced two stage ultrasonic treatment, where first stage sonication in done in order to further improve the dispersion, after which, the solution is stirred again but at a higher rate than before, in order to ensure optimal molecular entanglement and thereafter performing second stage sonication in a cold-water bath in order to achieve a solution with desirable viscosity characteristics;
• performing bubble removal by placing the same in vacuum, where the bubbles are sucked out through an air pump;
• preparing a casting in a plastic petri dish, after cleaning and preheating to 40˚C in a hot air oven;
• performing film casting from said polymer blend solution onto the preheated petri dish by allowing the said solution to evenly spread across the surface on its own and thereafter the dish is heated for 24 hours in order to evaporate the solvent and form the membrane;
• performing post-casting treatment by undergoing further drying of the said cast membrane in a vacuum oven in order to remove any residual solvent, ensuring the membrane's structural integrity and performance; and
• cooling said cast membrane after the drying stage is completed, thereafter petri dish is pulled out of the oven and placed inside a zip lock and is allowed to cool down to room temperature; and
• peeling off the said cooled membrane off from the petri dish and thereby storing the same in a zip lock bag to preserve its quality.
6. The method for fabrication as claimed in claim 5, wherein the blend composition of 1 : 0.5 ratio of PVA : lignin is present in said membrane; wherein the said composition ratio is selected for a solid polymer matrix that serve as a base for solid polymer electrolytes.

7. The method for fabrication as claimed in claim 5, wherein the said step of stirring for homogenization is carried out at a rate of 350 revolutions per minute (rpm) for 24 hours at room temperature; and wherein under second stage of enhanced ultrasonic treatment, the stirring is carried out again at a rate of 480 rpm for 2 hours to ensure optimal molecular entanglement.

8. The method for fabrication as claimed in claim 5, wherein said ultrasonic dispersion is carried out for 15 minutes for dissolving PVA; wherein under first stage of enhanced ultrasonic treatment, the ultra sonification is carried out again for 5 minutes in order to improve dispersion; and wherein under second stage of enhanced ultrasonic treatment, the ultra sonification is carried out again for 10 minutes in cold water bath.
9. The method for fabrication as claimed in claim 5, wherein during post-casting treatment the said membrane undergoes further drying in a vacuum oven at 40˚C for 3 hours in order to remove any residual solvent,
10. The method for fabrication as claimed in claim 5, wherein the said membrane further includes fillers, salts, plasticizers for the improving the device performance.

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