Shaped Controlled Polyindole decorated Vanadium Selenide nanosheets: A worthy approach using Hydrotrope for asymmetric supercapacitors

Polyindole (PIN) has been one of the rising promising conducting polymers of this decade, attracting researchers’ attention worldwide. This can be attributed to better redox activity and physicochemical stability. Several techniques had reported synthesis of PIN in organic and aqueous media. However, it has been always reported as di�cult. Challenges such as mixed morphology, irregular particle size and undesired agglomeration are among several outcomes’ researchers face. Hence, for the �rst time we present hydrotopically engineered chemical oxidative polymerisation of indole in the presence of Vanadium selenide (VS) in aqueous media. A hydrotrope is a compound that enhances the aqueous solubility of organic compound. The resulting nanocomposite was comprised of decorated polyindole over self-assembled 1D vanadium selenide, giving a 3D sheet like structure, as evident from FESEM studies. The results also highlighted uniform decoration of PIN over the background of VS. Besides, the physicochemical interaction between PIN and VS had been validated using Fourier transform infrared (FTIR) and XRD analysis. Furthermore, electrochemical studies through cyclic voltammetry, galvanic charging discharging techniques for the nanocomposite revealed improved speci�c capacitance of PV (w.r.t VS) as evidenced from the higher voltametric output current than VS. Electrochemical impedance also corroborated the superior charge transfer at lower frequencies, suggesting the real time applications of the nanocomposite. Lastly, to compliment the electrochemical performance, the nano composite was also used in fabrication of asymmetric super capacitor, which was used to illuminate LED and power a digital stopwatch to augment the conductivity and real time charge storage ability of the as synthesised nanocomposite. To the best of our knowledge, synthesis of PV nano composite using hydrotrope – tetra n-octyl ammonium bromide in aqueous medium can be a promising electrode material for fabrication of super capacitor, resulting in a synergistic enhancement of vanadium selenide using PIN.


Introduction
Recently, polyindole (PIN) has been one of the most preferred conducting polymers by researchers worldwide. 1,2This can be attributed to its intriguing physicochemical, remarkable redox activity and environmental stability. 3,4As a result, they have been synthesised using several polymerisation techniques including, electrochemical oxidation, 1 chemical oxidative, 5 interfacial, 6 DNA template, 7 and emulsion. 8Polymerisation of indole (PIN) has also presented several advantages including ease of synthesis, and lower processing costs.Despite this, majority of researchers have termed the synthesis of PIN as di cult.These di culties include pre and post polymerisation challenges including low yield, 1 inseparable impurities, and irregular morphology. 9For example, while crystallinity of indole is reduced during polymerisation using established techniques in the presence of organic solvents, the porosity along with dimensional irregularity is also hugely affected and can be evident through multiple reports by researchers 9,10 .For instance, Erdonmez et al. (2020), synthesised PIN using chemical oxidative polymerisation using chloroform as solvent, resulted granular structure and irregular morphology. 4Apart from this, irregular size PIN particles increase internal resistance thereby affecting overall electrical conductivity.This can be attributed to inability of electrolytic ions to e ciently transport through the irregular PIN particles.The ion transport can be affected by multiple factors such as crystal structure and porosity. 11,12Apart from organic solvents, which are costly and environmentally unfriendly, PIN is also reported to be synthesised in aqueous medium in the presence of surfactant, 13,14 however they also suffered from mixed morphologies which resulted in limited electrochemical performance. 15Surfactants, are chemical compounds and have been explored for controlling morphological features (tubular, and spherical) through micelle stabilisation.For instance, Unal et al. (2013) reported effect of anionic surfactant SDS in obtaining homogenous sponge-like morphology of PIN/ TiO 2 nanocomposite.However, the particulate geometry of the synthesised nanocomposite featured an average particle size of 0.6 microns. 13In addition to this, thermal stabilities were found to be reduced, and were attributed to decomposed PIN chains owing to the removal of SDS molecules.In another instance, Phasuksom et al.
(2016) were able to obtain smaller PIN particles with the help of SDS, however, the SDS remained in the nal product, as con rmed by FTIR analysis, suggesting the inseparability of surfactant during post polymerisation processing. 16Post polymerisation remnants increase post polymerisation processing, which in turn negatively affect the electrochemical performance.All these reports raise a need to nd an alternate technique which could remediate above issues thereby enhancing overall electrochemical performance.Our previous work, 17 for the rst time introduced the use of hydrotrope -tetra n-octyl ammonium bromide (TOAB), a class of TBAB in terms of structural integrity for synthesis of polyindole in aqueous media.A Hydrotrope is a chemical compound that enhances aqueous solubility of an organic compound. 18The enhanced aqueous solubility had been attributed to the co-operative solubilisation, which improves the interfacial interaction between inorganic-organic with aqueous medium. 19rthermore, its role as phase transfer catalyst and encapsulating agent was also established, owing to which, nano-dimensioned PIN was obtained without any traces of hydrotrope.The hydration effect reduced overall reaction kinetics thereby bringing uniform morphology throughout the material.This was attributed to the regioselective complexes between the TOA --Indole, TOA --APS, and TOA --Br.
Although, pristine PIN has been reported to exhibit better electrochemical properties, they have been incorporated with other inorganic metal for enhanced performance.Consequently, several PIN based nanocomposites, owing to their synergistic gains have been explored with several inorganic metal-oxides, hydroxides, metal sulphides and selenides.For instance, Raj et al. (2015) reported PIN-Co 3 O 4 nanocomposite to have enhanced electrochemical properties (such as speci c capacitance, rate capability and cycling stability) and attributed to the synergistic effect between Co 3 O 4 and PIN.Similarly, the charge storage performance parameter, de ned by the real component (behaviour of the dielectric function in lower frequency region) was found to decrease (lower, the better) with increasing concentration of WO 3 in PIN/WO 3 nanocomposite, which was attributed to hierarchical interconnectivity network offered by PIN. 4 In another instance, 20 the stability of RuO 2 in the ternary composite SWCNT/RuO 2 /PIN was found to increase owing to the protective nature of PIN in terms of electrochemical stability.Similarly, owing to the physicochemical interactions (such as electrostatic) in nanocomposites (involving hydroxides and sulphides with conducting polymers such as polyaniline, polythiophene etc) resulted in minimal metallic particle agglomeration along with improved electrochemical activity.However, unlike metal oxides and sulphides, transition metal selenides (TMS) offer lower bandgap and weak metal selenium linkage, which can be effectively utilised to enhance redox performance even more.Further, with respect to EDLC, the increased layer spacing also facilitates rapid ion diffusion.These bene ts along with its low cost, abundancy, and rich electrochemical activity has been led TMS to be deployed in majority of applications including catalysts, 21,22 water splitting, 23,24 oxygen/hydrogen evolution reactions (OER/HER) 25,26 .8][29] However their stability has severely limited their electrochemical applications. 27r instance, TMS suffer sever volume expansion on extended cycles and exhibit poor conductivity in energy storage devices.Hence, to remediate this issue, they have been preferred with reinforcing polymer materials such as conducting polymers (CPs).For e.g., polypyrrole (PPy) coating over CoSe 2 has been reported to enhance electrochemical performance of the nanocomposite through high temperature selenization. 30Other conducting polymers such as with TMS comprised of Co, Ni, Mn has also been explored however to our knowledge, we could not nd polyindole based TMS nanocomposites, especially Vanadium selenide (VS).VS has been focussed a lot recently owing to its layered structure and its wide array of applications including energy storage.Apart from this, vanadium also features a low ionisation energy, facilitating easy volunteering in a chemical reaction.Very few studies have featured VS in regard to electrochemical performance, 31 however electrochemical properties of conducting polymer based VS have seldomly explored.As a result, herein we report for the rst time, in-situ hydrotopically engineered one pot synthesis of polyindole decorated vanadium selenide (PV) in aqueous medium.VS was synthesised using facile one step hydrothermal technique, and its monoclinic phase was con rmed using XRD technique.PV nanocomposite was then subjected to physicochemical characterisation such as FTIR, FESEM, which validated the decoration of PIN over VS.In addition to this, electrochemical analysis of the nanocomposite over VS through galvanic charging discharging studies revealed enhanced speci c capacitance, energy and power density of 318.75 F/g, 102 Wh/kg and 0.8 W/kg, respectively.Furthermore, to complement real time charge storage ability, the as synthesised nanocomposite along with activated carbon was used in fabrication of asymmetric supercapacitor device.The device was connected in series with DC source and was able to illuminate LED, suggesting the conductive nature.Additionally, the device was also connected in series with digital stopwatch which successfully powered for at least 15 minutes.

Materials and chemicals:
Chemicals used herein were of analytical grade.Tetra n-octyl ammonium bromide (TOAB), Vanadium pentoxide (VA) and Oxalic acid (OA) were purchased from Spectrochem and used as it is.Indole (IN, 99%) ammonium persulfate (APS) and Selenium powder (SE) was procured from Merck.For this work Double distilled water (DDW) was used all the time.Electrochemical analysis was carried out in a conventional 3electrode electrochemical workstation, CHI 660 A. Freshly synthesised PV and VS was mounted on the tip of graphite rod and used as working electrode.Pt wire and Ag/AgCl electrode (saturated KCl) was used as counter and reference electrode, respectively.The tip of the working electrode was thoroughly polished using abrasive paper of 1200 mesh, degreased with 1:1 (ethanol: acetone) mixture followed by washing using DDW and nally drying in hot air oven at 70°C.All electrochemical analysis were carried out at room temperatures.

Methods:
Synthesis of V 2 Se 9 nanosheets: Vanadium selenide V 2 Se 9 nanosheets (VS) were synthesized using facile one step hydrothermal technique.In a typical approach, 0.5 g of VS and SE were added to 50 ml DDW in a beaker and was then magnetically stirred.To this, 0.3 g of OA was gradually introduced to the stirring solution.Once the homogeneity was achieved, the solution was autoclaved and heated in a hot air oven at 230°C for 18 hours.After su cient cooldown, the crude VS was subjected to centrifuge and ltration followed by washing with distilled water and ethanol.Post this, the synthesised VS dried in hot air oven at 75°C was stored for further analysis.

Synthesis of PIN-V 2 Se 9 nanocomposite
Polyindole decorated V 2 Se 9 nanocomposite (PV) was synthesized through in-situ chemical oxidative polymerisation technique.In brief, molar ratio of indole (IN) to APS (oxidant) was conserved at 1:2.In beaker A, IN along with 25 mg TOAB was added to 50 ml of DDW and was gradually stirred.In beaker B, 50 ml of DDW along with 8 mg of VS was ultrasonicated for 15 minutes and was transferred to the burette.This solution was then gradually added to beaker A in a dropwise fashion.The beaker A, now consisting of IN, TOAB and VS was kept at low temperature i.e., at 5°C in a refrigerator.In a different beaker C, APS was dissolved in 50 ml of DDW and transferred to burette for further reaction.Following this, an ice-bath was prepared, to which beaker A was placed and continuously stirred, above which a burette containing APS solution was placed rmly.To initiate polymerisation, APS was slowly transferred to beaker, ensuring no agglomeration takes place.A colour change from milky white to stone black was observed.The polymerisation was allowed 8 more hours, followed by 8 hours of refrigeration at low temperature.After, this, the beaker was brought at room temperature and washed with DDW and ethanol to remove unreacted product and impurities.Finally, the precipitate was dried in hot air oven at 60°C and stored for further analysis.Synthesis of material is shown schematically in Fig. 1.

Electrochemical analysis
Electrochemical analysis was performed over conventional three electrode system, with aq.1M KCl as electrolyte.Working electrode was prepared as follows: The tip of the graphite rod was thoroughly polished using silica slurry followed by 30 minutes of ultrasonication and subsequent washing using distilled water (DDW) and nally drying in hot air oven at 80°C.Two samples, viz.VS and PV (0.1 mg each) was ultrasonically dispersed in 100 µL of ethanol.Later, the samples were drop casted over working electrode i.e., graphite rod.A voltage window of 0.8 V ( -0.2 V to 0.6 V) was maintained throughout the analysis.Cyclic voltammetry was performed at various scan rates viz.5, 10, 25, 50, 100, 250 and 500 mV/s.Galvanic charging discharging was performed at 0.45, 1.25, 3.5 and 4.75 A/g current densities.Electrochemical impedance spectroscopy was performed at open circuit potential of 25 mV and between 0.1 Hz to 100 kHz.The speci c capacitance (in F/g) was determined through CV and GCD using following equations.
Where, i/m is current density (in A/g), for a given ΔV potential window and Δt at discharge time (s).
To complement the real time charge storage of the synthesised nanocomposite, a binder free supercapacitor device with activated carbon (AC) was fabricated.The device featured two identical waste TLC sheet (T1, T2 each 30 x 25 mm 2 ) were thoroughly cleaned with acetone, washed several times, and were used as current collector.PV and AC were thoroughly sonicated in ethanol and then drop casted carefully over T1 and T2 respectively.Both electrodes were kept in hot air oven for drying.For separator, a Whatman lter paper of grade 42 was used, and was wetted using CMC-Sodium sulphate electrolyte solution and assembled between T1 and T2.Whole con guration was then insulated by plastic adhesive to protect electrolyte and electrode with su cient contact windows.

Results and Discussions
Mechanistic approach towards in-situ VS templated PIN The role of Hydrotrope -TOAB has been essentially explored as a phase transfer catalyst and to enhance aqueous solubility by addressing organic inorganic compatibility.In addition, it also serves as an effective encapsulating agent.Keeping these roles in mind, TOAB has been opted as hydrotrope to engineer facile integration of VS and PIN.Initially, TOAB is added to the DDW, which effectively reduces its surface tension, in turn improves the organic inorganic compatibility with indole.Schematically, this can be understood as partly broken water structure (as seen in Fig. 1), where IN monomers are sparsely distributed within the aqueous vicinity but surrounded by intercepting hydrophobic claws of TOAB.At this stage, solution is then composed of TOA + and Br -owing to cooperative solubilisation 19 .Following this, the TOAB molecules spontaneously arrest IN monomer on account of electrostatic interaction (van der Waals force) and hydrotropically encapsulate to form indole shuttles.To this, when VS solution is introduced drop wise though burette, indole shuttles anchor themselves over the surface of VS, making them a nano template sheets, throughout the solution.Now when, APS is introduced drop wise it readily dissociated in ammonium and persulfate (NH 4 + and SO 4 * -) which are pulled electrostatically towards the core through the cationic head of TOAB i.e., TOA + .This occurs spontaneous at all anchored sites, effectively converting shuttles into microreactors, where radical cations are generated through simultaneous extraction of lone pair of electrons by sulphate radical.Following this, a coupling occurs at 2,3 sites of IN resulting in dimer and deprotonation.Consequently, the chain propagates and terminates following deprotonation, marking successful polymerisation.The generated PIN chain experiences the attractive electrostatic pull by the templated surface, PIN own inherent nature to wound around itself and reduced kinetics imparted by TOAB in water emulates the warp beam.However, this beam is electrostatically limited within the hydrophobic claws, which restricts the decoration limited to nanosheet and should explain the su cient coating.Consequently, the nal morphology features the uniform decoration of PIN over VS nanosheets.The discussion is in good agreement with physico chemical characterisation such as FTIR, FESEM and XRD which concurs the presence of PIN over the nanosheets of VS and has been discussed in upcoming sections.

FTIR analysis of PV nanocomposite
The physico-chemical interaction between PIN and VS nanosheets was identi ed by Fourier transform infrared spectroscopy (FTIR).Figure 2 features FTIR spectra of PV nanocomposite.As evident from the spectra, intense vibrations at 3397 cm -1 and 2925 cm -1 has been assigned to N-H stretching and C-H antisymmetric bending, respectively.The peaks are broad and signify the polymerisation of IN monomer.
Further, the deform vibration observed at 1616 cm − 1 has been associated with N-H, suggesting the polymerisation site at 2,3 in IN monomer. 32In addition, peaks at 1450 cm -1 , 1112 cm -1 , and 744 cm -1 has been observed owing to the characteristic vibrations of benzene ring, oxidised PIN and C-H out of plane bending vibrations of C-H accordingly. 33With respect to VS, peaks at 619 cm -1 was assigned to V-Se bond. 34In addition, additional vibration observed at 3214 cm -1 can be assigned between hydrogen bonding between oxidised PIN and VS nanosheet.This appearance also implies improved aqueous compatibility and can be attributed to the cooperative solubilisation imparted by TOAB 19 .The successful hydrogen bonding also improves the hydrophilic ability of VS nanosheets and their stability.Furthermore, absence of additional peaks related to TOAB suggests its non-invasive nature and further corroborates its role as phase transfer catalyst.The characteristic bands with respect to V-Se and PIN as evidenced by the spectra attest to the decoration of PIN over VS nanosheet.

XRD analysis
The synthesis of 1D Vanadium selenide (V 2 Se 9 ) nanosheets (VS) and Polyindole decorated Vanadium nanoshets (PV) was analyzed and investigated using X-ray diffraction technique (XRD) as shown in the Fig. 3.At rst, VS was synthesised through a simple one step hydrothermal route 230°C.Formation on VS can be con rmed as it takes place only under temperatures of 320°C, were in agreement with Oh et al. 35 In addition to this, the XRD spectra revealed the monoclinic phase for 1D V 2 Se 9 (JCPDS card 01-077-1589).This attests to the fact the 1D VS had experienced self-assembly leading to a 3D nanosheets.In addition to diffraction peaks at the characteristic diffracted peaks of VS at 2 ~28.92 ° were indexed to the (202) plane. 31The XRD spectra reveals that PV is decorated over VS.The PIN orientation with their aromatic ring is proposed to be perpendicular to the sheets, and is in agreement with previous report 36 , and is also in agreement with FESEM analysis, presented in next section.Another, important change that has been observed is the intensity of the diffraction.The sharp intensity of the diffraction of VS can be owed to the crystalline nature of the material.Furthermore, after interaction of PIN occurs between the VS, the crystalline nature of the nanocomposite is reduced, and as a result the intensity of the peaks is reduced signi cantly in PV.Furthermore, presence of most of the VS peaks in PV suggests uniform decoration of PIN.
FESEM analysis: The morphological study was conducted on PV nanocomposite to investigate the decoration of PIN over VS. Figure 4 (a) features the FESEM image of PV nanocomposite and its (b) magni ed micrograph.Figure 4 (a) presents uniform decoration of PIN over sheet like formation of VS.This can be attributed to uniform adsorption of indole over the VS nanosheets prior to polymerisation.Post polymerisation, the uniform coating is just su cient and agrees with FTIR and XRD studies.The 1D VS, which selfassembled to form 3D nanosheets during polymerisation is also evident from the micrographs, as seen in Fig. 4(b).This can be attributed to the encapsulation effect imparted by TOAB-driven microreactor.In addition to this, the micrograph also reveals interconnected nanosheets through PIN chains, thereby improving speci c capacitance.It is worth to be noted from FESEM images, that the texture of the nanosheet is less than 20 nm, which further suggests the role of hydrotrope TOAB in limiting reaction kinetics.

UV Vis Analysis
The UV-Vis analysis of PV nanocomposite has been featured in Fig. 5 (a).The nanocomposite revealed enhanced visible light absorption and can be attributed to the VS templated PIN nanosheets.Oh et al.
(2018) reported the bandgap for 1D V 2 Se 9 as 0.73 eV.In this work, the corresponding Tauc plot featured a bandgap energy of 1.1 eV, which enhanced the nanocomposite to operate at higher voltage, and can be attributed to protective nature of PIN.

Electrochemical Studies:
The electrochemical studies of PV and VS were studied through three techniques viz.cyclic voltammetry (CV), galvanostatic charging discharging and electrochemical impedance spectroscopy.

Cyclic Voltammetry (CV) analysis
CV technique was performed within a potential window − 0.2 V to + 0.6 V Vs Ag/AgCl at increasing scan rates (viz.5, 10, 25, 50, 100, 250 and 500 mV/s).Figure 6 (a) features typical cyclic voltammograms of PV.As per the gure, the enclosed area increased with increase in scan rate.This can be attributed to higher rate of diffusion, facilitating more electrolyte ions reaching at the electrode electrolyte interface.
Also, as few ions participate in the charge transfer reaction, resulting in higher current. 37Furthermore, the higher diffusion is also a result of synergistic involvement of PV along with VS, in contrast to VS alone.This further allowed the nanocomposite to withstand a higher current.Better synergistic performance is also subject to uniform decoration of PIN over VS nanosheets, which agrees with FESEM, FTIR and XRD results.The highest speci c capacitance was found to be 267.65 F/g at 5 mV/s which reduced with increasing scan rate.The comparative CV response of the PV with respect to VS has been illustrated in Fig. 6 (a) at 5 mV/s.A sharp reduction and oxidation peak can be observed in PV at 0.35 V and 0.45 V, respectively.The reduced peaks can be attributed to the insertion of electrolyte ions between the nano spherical PIN and VS sheets, whereas the extraction of electrolyte ions within the same results in oxidative peak.The e cient insertion and removal of ion can be understood through minimal IR drop featured in GCD plot, as shown in Fig. 6(c) This can also be attributed to improved conductivity of PV for enhanced ion transportation, along with no agglomerates.

Galvanic charging discharging (GCD) analysis
The GCD for PV nanocomposite was performed at different current density 0.45, 1.25, 3.5 and 4.75 A/g and shown in Fig. 6C.
Speci c capacitance and energy-power density of the PV nanocomposite were found to be 318.75F/g, 102 Wh/Kg and 0.8 W/kg respectively and has been illustrated in Fig. 7 (a).Also, as evidenced from GCD plot, the IR drop is improved in comparison to other conducting polymer-based nanocomposites where organic solvents and surfactants are used.The speci c capacitance of 318.75 F/g is also higher with respect to other TMDs such as MoS 2 . 38The huge improvement can be attributed to uniform decoration of PIN over VS.Furthermore, the speci c capacitance behaviour with respect to increasing current density is also linear and stable, suggesting the protective nature of PIN and stability of VS nanosheets, which keep the nanocomposite from failing. Figure 7(b) represents relationships between energy and power density.There exists a non-linear relationship which gets stable at higher current density.Higher energy density of 102 Wh/kg can be attributed to the interconnected network of PIN @ VS, owing to the conducting highway offered to the charge transport and faradaic reaction.Also, speci c capacitance retention was found to be 90.6% at 0.45 A/g for synthesised PV.An asymmetric supercapacitor composed of PV and activated carbon was also fabricated and explored for real time application.
Step by step process of device fabrication, their charging followed by illumination of LED with digital watch are also shown in Fig. 8.

Electrochemical Impedance spectroscopy (EIS) study
EIS analysis enables us to understand charge transfer and its kinetics.Hence it is essential to subject the as synthesised PV to EIS analysis.A typical EIS spectrum features a high frequency and a low frequency region.A high frequency region features a semi-circle and signi es the charge transfer resistance, R ct .It primarily refers to the resistance faced by the charge to reach the graphite rod through the coated material.When a current is passed, the coated material experiences solution resistance and Rct.Conventionally, both R s and R ct should be as low as possible since ion mobility de nes the electrochemical performance.As evident from the Fig. 8

Conclusion
Uniformly decorated PIN over VS was successfully synthesized through Hydrotrope assisted in-situ chemical oxidative polymerisation of indole in the presence of VS, in aqueous mode at room temperature.
The synthesised nanocomposite was characterized by physicochemical (such as FTIR, XRD) and morphological tests, revealing the VS as template for PIN decoration.The as synthesised material was also subjected to electrochemical studies revealing better electrochemical performance and charge storage ability.Further, to complement the electrochemical performance, the nanocomposite material was used to fabricate an asymmetric supercapacitor device with activated carbon as well.The device demonstrated conductive nature and powered a digital stopwatch for at least 10 minutes, suggesting the superior charge storage ability.Because of the real-time application concurring the experimental studies, the nanocomposite synthesised through hydrotrope assisted in-situ polymerisation can be expected to be a potential electrode material in energy storage applications.

Figures
Page 14/  Schematic illustration for synthesis of PV.
Figure 2 FTIR spectra for as synthesized PV.

Figure 7 (Figure 9 Fig 8 .
Figure 7 , the PV featured a low R s and R ct of 4.84 Ω and 2 Ω respectively.The lower values can be attributed to interconnected PIN decorated VS nanosheets.RO is grateful for the project nancial support to DST SERB (EEQ/2018/000595).SSV is thankful to SERB for project fellowship.SSV also acknowledges CU Jharkhand for research fellowship and proving infrastructure.
Con ict of Interest:All authors of this manuscript declare no con ict of interest.