Exploring the potential of poly (caprolactone) and guar gum biodegradable blend film: an investigation for supercapacitor

A biodegradable polymer electrolyte comprising poly (caprolactone) (PCL) and guar gum (GG) doped with lithium perchlorate (LiClO4) was investigated for its application in supercapacitors. The films’ thermal properties, surface morphology, and tensile strength were determined to understand the interaction between the blend system and the salt. Scanning electron microscopic images showed a network of GG channels across the polymer matrix. A unique combo of THF/water as solvent was used for this study as they bring out relaxation in GG segments and compatibility between GG and PCL. The blend polymer electrolyte (BPE) was characterized using conductivity, dielectric, and biodegradation studies. Supercapacitors were fabricated, and electrochemical studies were performed. The optimized BPE was used to fabricate supercapacitors, producing a specific capacitance of 125 F g−1. The time constant was measured at 0.8 s, and a consistent cyclic pattern was observed during galvanostatic charge/discharge studies with 96% Coulombic efficiency. This novel amalgamation of polymeric films holds immense promise for supercapacitor applications.


Introduction
Supercapacitors are electrochemical energy storage devices that store and release energy much faster than batteries.They have a high power density, which makes them ideal for applications that require short bursts of power, such as hybrid electric vehicles and renewable energy storage.Supercapacitors also have a longer cycle life than batteries, making them a more sustainable option for energy storage.Blend polymer electrolytes (BPEs) are essential in supercapacitor applications because they have high ionic conductivity, good mechanical properties, flexibility, chemical stability, and environmental friendliness.These properties make BPEs suitable for various applications, including hybrid electric vehicles, renewable energy storage, and power grid stabilization.
BPEs have high ionic conductivity, enabling the rapid and efficient transport of ions, which is crucial for supercapacitors needing fast charge and discharge capabilities.BPEs also possess good mechanical properties, making them resistant to damage, a vital characteristic considering supercapacitors are often exposed to vibration and shock.Additionally, BPEs are flexible, facilitating easy fabrication tailored to specific applications.They exhibit chemical stability, making them resistant to corrosion or degradation, a crucial factor for supercapacitors needing to endure harsh environments.Furthermore, BPEs can be crafted from environmentally friendly materials such as polymers and organic solvents [1][2][3].A blend of poly (vinyl alcohol)poly (styrene sulphonic acid) demonstrated a specific capacitance of 40 F g −1 , primarily due to Grotthus-type proton movement, where a proton rushes from H 3 O + to a hydrogen-bonded water molecule [4].In a biodegradable biopolymer electrolyte system based on a polyvinyl alcohol-chitosan blend, a disruption in the crystalline region of the film can be observed with the existence of Lithium acetate (LiOAc) and ethylene carbonate (EC).The inclusion of EC promoted the ion's dissociation, thus enhancing the amorphous region of the electrolytes, which is beneficial for ionic conduction.The supercapacitor performed excellently with an average specific capacitance of 130 F g −1 [5].In another study, ceramic composite polymer electrolyte made from poly (vinylidene fluoride) (PVDF)/poly (ethylene oxide) (PEO)/ Lithium Lanthanum titanate (LLTO), incorporating LLTO affects the PVDF/PEO polymer backbone by improving the polymer segmental motion.This CPE-based hybrid supercapacitor possessed a high specific capacitance value of 182 F g −1 [6].To maintain the porosity and high-level electrolyte in the separator, a Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (Pluronic P123) was blended with Poly(vinylidene fluoride-cohexafluoropropylene) (PVdF-HFP).P123 has an amphiphilic structure with a polar, water-soluble group attached to a nonpolar, water-insoluble hydrocarbon chain.When prepared by the phase inversion approach, a homogeneous pore with higher pore density was obtained [7].Similar to this concept, we have used the hydrophobic aspect of PCL and the hydrophilic part of GG in a common THF/water solvent, a unique approach for blending the polymers.Utilizing environmentally friendly materials is crucial, especially in applications where environmental impact is a concern.The current use of synthetic polymer electrolytes raises concerns since they are not recyclable or biodegradable.Biodegradable polymer electrolytes, under controlled conditions, can offer similar performance to synthetic ones.After use, the metal salt can be extracted, and the polymer electrolyte can undergo biodegradation.Hence, we chose guar gum as the electrolyte.
Guar gum (GG), belonging to the galactomannans family, is derived from the seeds of Cyamopsis tetragonolobus.GG, a non-ionic polysaccharide, boasts distinct features such as biodegradability and the ability to form a viscous solution in cold water.Due to these properties, it finds applications in various fields, such as petroleum extraction, stabilizers, and binding agents in ice cream, frozen foods, beverages, explosives, and textiles.However, its use as a polymer electrolyte is intermittent.
Poly (caprolactone) (PCL) is prepared from ε-caprolactone through ring-opening polymerization.It promotes biodegradation, tensile strength, impact resistance, and easy processability [8].Its low glass transition temperature of about −60 °C makes it versatile, finding applications in biomedical and drug delivery systems [9].PCL conducts ions effectively, a crucial requirement for charge transport in supercapacitors.Nevertheless, its relatively low dielectric constant results in lower capacitance than other electrolytes.Moreover, PCL has a high vapor pressure, leading to potential evaporation and electrolyte degradation.Despite these challenges, PCL remains a promising material for supercapacitor electrolytes [10].
Our current focus is enhancing PCL electrolyte properties to render them more suitable for this application.Usually, PCL forms hard film and is only soluble in organic solvents like tetrahydrofuran (THF), which shows low toxicity and is known as a renewable solvent that can be produced from bio-derived chemicals.GG curls up in water in the presence of salts, forming aggregates of polymer strands.Hence, our approach introduces a highly polar solvent, THF [11], which can break the intermolecular hydrogen bonding and relax GG segments.The relaxation pools of GG allow the rigid PCL strands to pass through it, reducing its hardness.Furthermore, blending PCL with GG improves its ionic conductivity, and the dielectric constant represents a novel approach.In this study, a biodegradable film comprising PCL and GG was prepared.The film is flexible and doped with an optimized concentration of LiClO 4 , determined based on achieving the highest ionic conductivity without salt agglomeration.Subsequently, a supercapacitor was fabricated using the film exhibiting the highest ionic conductivity.Its stability and electrochemical performance were thoroughly evaluated.

Material and methods
A viscous solution of poly (caprolactone) (Merck: average M n 80,000) was prepared by mixing 5% by weight in tetrahydrofuran (THF) at 60 °C for 2 h until a clear solution was obtained.A 10% (w/v) guar gum (Sigma Aldrich) solution in water was mixed to get a viscous liquid.The stock solutions were mixed in the ratios 80:20, 70:30, 60:40, and 50:50.An optimized amount of 0.1 wt% of LiClO 4 (Merck) was introduced into the blend mixture and stirred for 24 h.The solution was cast onto petri plates and dried to obtain the film.Figure 1 shows a schematic drawing of the blend polymer electrolyte film preparation using the solution casting method.The reproducibility of films with almost thickness is about 1% error.
The BPE was characterized using electrochemical impedance spectroscopic studies at different temperatures.Comprehensive analysis of films involved utilizing thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) conducted on the SDT Q600 V20.9 Build 20 instrument.The DSC and TGA data were reproducible with 0.2%.The experiments were conducted within a temperature range of 20 °C-700 °C under a nitrogen atmosphere.The BPEs, which were freeze-dried, were placed in a nitrogen environment.Subsequently, the films were exposed to high vacuum, and SEM images were captured using the ZEISS EVO18 SEM.The tensile strength of biodegradable blend films was determined by Instron 3360 following ASTM D638.The test was reproducible with a 0.1% error due to a change in the thickness of the BPE.
A supercapacitor was fabricated using activated carbon (AC) coated on a stainless steel (1 × 1 cm) electrode using a binder solution containing poly (vinylidene fluoride) and N-methyl pyrrolidone (8:1:1).The weight of AC varies for different fabricated supercapacitors during optimization.Hence, the specific capacitance will have an error difference of 0.2%.The BPE was packed between two AC-coated electrodes.CV, EIS, and galvanostatic charge/discharge experiments were performed using the Biologic SP50e instrument.

Results and discussion
Figure 2(a) shows the TGA thermograms of blend samples.Initially, a slight decrease in weight, approximately 1% of the total mass, occurred at 101 °C, attributed to the elimination of moisture content.Subsequently, a significant weight reduction of around 30% was observed at 200 °C, resulting from the rupture of bonds within the guar gum and the formation of intermolecular bonds between GG and PCL.Ultimately, the residual weight underwent decomposition at 400 °C.
To further elucidate the results of TGA, DSC thermograms were analyzed for PCL/GG blend samples (figure 2(b)).The glass transition temperature (T g ) remained almost the same, with a 2 °C variation among blend mixtures in the 30 °C-40 °C range.Lower T g values indicate more segmental motion at ambient conditions; hence, BPEs show higher ionic conductivity.Endothermic peaks were observed between 201 °C-210 °C, indicating the change in intermolecular interactions in the blend matrix.Exothermic peaks were seen after 400 °C, indicating the decomposition of the blends.The blend containing a high GG content exhibited an endothermic peak lower than that of the enriched PCL film.Figure 3 shows the SEM images of PCL/GG biodegradable blend films, revealing uniformly dispersed tubular networks.The tubules exhibited lengths ranging from approximately 7 to 12 μm, with a diameter of 3 μm.These dimensions play a significant role in facilitating the binding and interconnection of the blend polymer electrolyte.The porous and hollow tubular structure provides a softness property, enabling the transport of Li + ions.
The PCL/GG biodegradable blend films exhibited an ultimate tensile strength of 30 ± 1.8 MPa and an elongation factor of 80 ± 2%.Biodegradation of the blend films was carried out according to ASTM standards (D 638 IV and D570-98) through soil burial testing.Degradation was monitored for 60 days, and by the end of the 50th day, 95% of the film had degraded.Characterization was not possible after 50% degradation in the soil on the 24th day, and on the last day, only a trace amount of the film was noticeable.This indicates that PCL provides mechanical support to jelly guar gum, where they bond through weak hydrogen bonding between hydroxyl groups in GG and ester 'O' in PCL.The synergistic effect of this bonding resulted in a tubular structure in the blend matrix.Consequently, the blend polymer electrolyte (BPE) is impermeable, soft, flexible, and possesses sufficient tensile strength for use in supercapacitors.
Figure 4 illustrates the ionic conductivities of the BPEs at various temperatures.Showing a linear trend, the plot suggests adherence to the Arrhenius-type equation.This implies that ionic transport occurs within a viscous medium, with no formation of phase transitions or domains caused by the doped lithium ions [12].Notably, at 323 K, the ionic conductivity reaches 8.5 × 10 -4 S cm −1 , while at 298 K, it stands at 4.3 × 10 −4 S cm −1 for samples with a high content of GG.The elevation in temperature introduces segmental motion, facilitating the hopping of Li + ions along the blend polymer chains and consequently enhancing conductivity at elevated temperatures [13].Notably, the blend containing 50% GG exhibits greater conductivity than other blend compositions.This phenomenon underscores the plasticizing effect of GG within the blend, a finding similar to our previous study on GG/PVA blend electrolytes.

Supercapacitor studies
The high-conductivity BPE, with a 50:50 ratio of GG to PCL, was employed to create a supercapacitor.Figure 5 showcases the cyclic voltammetry (CV) responses for a symmetrical carbon-carbon supercapacitor, demonstrating diverse scan rates.Notably, a distinct rectangular window on the graph signifies the presence of double-layer capacitance at the interface.To determine the supercapacitor's specific capacitance, the equation C = (2ΔI)/(ΔV × m) was utilized, with ΔV representing the voltage scan rate, m being the electrode mass, ΔI standing for the average current, and C signifying the specific capacitance [14].Remarkably, at a scan rate of 5 mV s −1 , a specific capacitance of 125 F g −1 was achieved.This remarkable capacitance value can be ascribed to electrode polarization at the electrode-electrolyte interface due to ions within the tubules.Table 1 depicts the comparison of specific capacitance of various similar biopolymer electrolyte-based supercapacitors.Significantly, the cyclic voltammogram pattern remained unaffected by varying scan rates, underscoring the exceptional reversibility of the supercapacitor amid diverse voltage fluctuations.
Figure 6 portrays the Nyquist plot that elucidates the supercapacitor's characteristics.It discloses the identification of the double-layer capacitance (C dl ) through the resistance in the high-frequency section [19].Notably, the calculated C dl value was established at 4 mF cm −2 .This unique capacitance attribute results from   the interface's microtubular structure, which facilitates the creation of both a double layer and a diffuse layer, each exhibiting substantial capacity.To delve deeper, it is imperative to conduct further investigations into binding materials capable of modifying ionic conductance and influencing double-layer capacitance.Such exploration will enable a more comprehensive understanding of these dynamics.Figure 7 demonstrates the Galvanostatic charge-discharge (GCD) studies conducted across various current densities: (a) 0.5 mA g −1 , (b) 1 mA g −1 , (c) 1.5 mA g −1 , (d) 2 mA g −1 , and (e) 2.5 mA g −1 .These experiments were conducted within a potential window of 1 V. Notably; all current measurements exhibit capacitive behavior marked by triangular shapes.Specifically, at a current density of 0.5 mA cm −1 , the chargedischarge cycles displayed initial values of 108 F g −1 and 102 F g −1 for the first and 2000th cycle, respectively, accompanied by a Coulombic efficiency of 96%. Figure 7(i) offers insight into the relationship between specific capacitance and cycle count [20].The equivalent series resistance (ESR) registered a value of 81 Ω, alongside an IR drop of 0.02 V. Regarding energy and power densities, the specific energy density achieved 25.65 W kg −1 , while the specific power density reached 198.7 Wh kg −1 .The marginal initial charge decline can be attributed to stored charge at the interface in conjunction with the characteristics of the activated carbon material.

Mechanism
Based on the analytical and electrochemical characterization of the BPE sandwiched between AC electrodes, we suggest the probable mechanism and the role of each component in the system.The nature of the GG polymer segment is that it curls in the presence of inorganic salts and forms tubular-like aggregates due to excessive intersegmental hydrogen bonding.Hence, this leads to poor mechanical and ionic conductivity.The use of PCL, which has high mechanical strength but poor film-forming ability, can dissolve only in organic solvents.The THF solvent has the ability to form a miscible system with water due to its high polarity and also dissolves PCL effectively.During the mixing of PCL/THF and GG/water, the THF/water tends to break the cross-linking of GG segments and make them uniform tubular channels.These channels help in ionic movements across the BPE matrix, which helps improve the specific capacitance of the supercapacitor.Moreover, THF concentration should not exceed 4% in the system as it will break further bonds, and the gel system may lose its structure [11].It is made sure the THF is removed via evaporation during film formation up to 0.5% residue.The PCL polymer can penetrate the GG and form a compatible blend polymer system without any phase separation.Thereby enhancing the mechanical strength while maintaining the high ionic conductivity of the BPE system.In figure 8, the PCL is shown to provide mechanical support and oxygen-rich sites for probable Lewis acid-base interaction with the LiClO 4 .The GG tubules offer a channel for Li-ion movement through and around them as they possess oxygen-rich groups in their segments.The AC provides a surface that can accommodate these tubules and, hence, the easy accessibility of ions near the electrode/ electrolyte interface region.The Li ions movement slows down when it hops to the PCL segment and fastens as it jumps to the GG segments.Hence, overall energy density is relatively higher than similar reported works of literature.

Conclusion
A novel combination of GG/PCL, involving a blend of natural and synthetic materials, was prepared.This blend exhibited unique tubular and matrix structures, enhancing the movement and storage of Li ions.A high conductivity of 10 −4 S cm −2 at 323 K was observed when doped with Li salt.Moreover, using this highly conductive blend, the fabricated supercapacitor showed a specific capacitance of 125 F g −1 at 5 mVs −1 .The cyclic stability was evident for up to 2000 GCD cycles.The future scope of this work is to explore its potential applications in other energy storage devices, as well as possible pathways for performance optimization.The performance optimization may be done by using materials with high surface area such as graphene, MXene [21] etc., or by adding suitable filler such as carbon dots, carbon nanotubes etc., to enhance the connectivity between BPE and electrode.Moreover, large scope for exploring compatibility, miscibility studies of the blend polymers in amalgamation of highly polar organic and aqueous solvents.Hence, the combination of GG and PCL leads to the development of a new BPE for supercapacitors.

Figure 1 .
Figure 1.Schematic drawing showing the method of preparation of blend biodegradable polymer electrolyte.

Figure 3 .
Figure 3. SEM images of PCL-guar gum blend polymer film in different regions.

Figure 4 .
Figure 4. Variations of conductivities of blend compositions of BPE at different temperatures.