Ionic transport kinetics and enhanced energy storage in the electrode/poly(N-vinyl imidazole) interface for micro-supercapacitors

The detailed understanding and control of ionic transport pathways in the electrode/electrolyte interface is vital for realizing micro-scale energy storage devices and formulating adequate design principles. A planar device geometry with nanostructured thin solid polymer electrolyte (SPE) and potassium hydroxide (KOH) incorporated poly(N-vinyl imidazole) (PVI) is demonstrated for micro-supercapacitors (MSCs). The adsorption/desorption kinetics of ionic charges in the interfacial regime of ITO/PVI–KOH has been investigated for electrical double layer capacitance (EDLC) characteristics. A single-cell of ITO/PVI–KOH/ITO planar MSC shows the large variation in volumetric capacitance and capacitance retention characteristics when the thickness of PVI–KOH approaches the characteristic nanoscale. Moreover, ITO/PVI–KOH/ITO planar MSC consisting of five series-cells exhibits the maximum operating cell voltage of 5.0 V with maximum volumetric energy and power density of 0.056 mW h cm−3 and 6.89 mW cm−3, respectively. The electrochemical properties of planar MSC have been systematically studied so as to confirm how the anions and cations are separated at electrode/electrolyte interfaces by means of an electromotive force. Significantly, the hydrated PVI enables charge migration and separation of cations and anions at the electrode/electrolyte interfaces.


Introduction
With increasing demand for wearable and portable electronic devices, micro/nanoscale power systems have received intense attention due to their ultra-high power density and high rate capability. [1][2][3][4][5] Miniaturization and integration of micro-power systems such as micro-supercapacitors (MSCs), microbatteries and piezoelectric energy converters with on-chip devices offer great potential for future portable electronics. [6][7][8] Among various power systems, MSCs have attracted considerable attention because of their high capacitance, high energy density and large endurance characteristics. [8][9][10][11] In MSCs, electrical energy is stored at the electrode/electrolyte interface either by the reversible adsorption/desorption of charges (nonfaradaic) or rapid redox reaction of charges (faradaic). 12,13 Till now, many reports available to elucidate the high performance supercapacitance characteristics by specically modifying the properties of electrode materials. [14][15][16][17][18] But, the supercapacitance characteristic not only depends on the electrode materials, but also relies on electrolyte properties and new device architecture. [19][20][21] In recent years, structurally conned solid polymer electrolyte (SPE) offers many advantages in fabricating MSCs without modifying the properties of electrode. [22][23][24] Connement of SPE lm to the characteristic nanoscale can reveal variation of inter-polymer chain interaction, which inuences the ionic conductivity and stability characteristics. Furthermore, supercapacitance characteristics become more pronounced when the SPE lm conned to certain nanoscale due to the synergistic effect of shorter ionicdiffusion pathways and low equivalent series resistance (ESR). [25][26][27][28] Herein, we demonstrated a poly(N-vinyl imidazole) (PVI)based planar MSCs on indium tin oxide (ITO) coated exible PET substrate through a micro-edging of ITO lms. A planar device architecture consists of thin ITO electrodes separated by potassium hydroxide (KOH) salt-incorporated PVI electrolytic lm (ITO/PVI-KOH/ITO) has been fabricated. In order to understand the interfacial interaction between imidazole group of PVI and KOH for supercapacitance characteristics, planar MSCs were fabricated with various PVI-KOH lm thicknesses. PVI-KOH with thicker lm shows higher ionic conductivity that led to the higher volumetric capacitance of planar MSC. In contrast, the reduction of PVI-KOH lm thickness decreases the ionic conductivity as well as the volumetric capacitance. Consequently, a single cell of ITO/PVI-KOH/ITO planar MSC with $121 nm of PVI-KOH reveals higher volumetric capacitance (128 mF cm À3 ) and 92% capacitance retention less than 1400 continuous cycles. But, the reduction of PVI-KOH lm thickness to the characteristic nanoscale ($28 nm) remarkably enhances the capacitance retention ($95%) for more than 3500 continuous cycles. This signicant deviation in capacitance characteristic between thick and ultra-thin PVI-KOH lms has attracted and enabled us to investigate the effect of polymer lm thickness on EDLC formation mechanism and stability characteristics. To quantitatively analyse the performance characteristics of EDLC, planar MSC consisting of ve series-cells also fabricated. Such multi-cell planar MSC exhibits extended operation voltage (5.0 V) with maximum volumetric energy and power density of 0.056 mW h cm À3 and 6.89 mW cm À3 , respectively. These results explain the importance of connement effect on solid polymer electrolyte for supercapacitance characteristics.

Results and discussion
Structural and morphological characteristics of prepared PVI-KOH complex The structural investigations of the prepared pure PVI and KOH incorporated PVI thin lms were carried out using various techniques. Fig. 1 shows the X-ray diffraction (XRD) patterns of both pure PVI and PVI-KOH thin lms. The two major diffraction peaks found at 10.6 and 21.28 (2q) correspond to the PVI polymer. 29 By taking into account of the normalized peak intensity, two distinct peak in the pure PVI may be due to existence of crystalline ordering. 29,30 In PVI-KOH thin lm, the large variation in the peak prole is observed. Especially, four additional peaks are found in PVI-KOH lm that mostly arise when the KOH loading exceeds the threshold limit. 31 As compared to the pure PVI lm, the decrease in peak intensity with large peak broadening mainly due to the amorphous nature of PVI-KOH lm.
To further understand the structural characteristics of the prepared PVI and PVI-KOH lms, the Raman spectroscopy was conducted (Fig. 2). In the pure PVI lm, the observed peak at 748 cm À1 corresponding to the stretching vibration of C-N bond that connects the imidazole group to the vinyl moiety and the backbone of the aliphatic chain. The major peak at 1083 cm À1 is attributed to the C-C-C bending vibration of the aliphatic chain. The C-H bending with C]N stretching of imidazole group showed a peak at 1235 cm À1 . The peak at 1292 cm À1 arises due to the stretching of chain and ring breath. The peaks at 1348 and 1509 cm À1 and 1604 cm À1 can be because of the stretching of ring, combined stretching vibrations of C-C and C]N bond and characteristic C]C stretching vibration, respectively. 32 But, in the PVI-KOH lm, large peak broadening that cause disappearance of some normal modes, which reveals the amorphous nature of the lm. 33,34 It also suggests that the incorporation of large concentration of KOH in the PVI polymer signicantly affects the PVI polymer structure. 31 Fourier transform infrared (FTIR) study was also performed on a prepared pure PVI in order to understand the vibrational properties of polymer under ambient condition. From FTIR spectra, the C-H and N-H stretching vibrations of PVI are observed, as shown in Fig. 3. The broad peak at $3397 cm À1 arises by the presence of bound water molecules in the PVI. The peak at $3110 cm À1 corresponding to C-H vibrations of the imidazole ring. The observed peak at $2953 cm À1 is due to the C-H stretching vibration of the polymer backbone, suggesting the polymer formation. The peak at $1659 cm À1 is ascribed to the characteristic of C]C stretching vibration of PVI. There are three major vibrational modes ($1499, $1281, and $1230 cm À1 ) corresponding to the imidazole group of PVI. The stronger peak at $1089 cm À1 is assigned to the in-plane   bending of the azole C-H group. Emergence of peaks at $914, $825 and $747 cm À1 are majorly attributed to the bending vibrations of heterocycles of PVI. The characteristic peak found at $664 cm À1 is corresponding to the imidazole ring-puckering vibration. 32 These results suggest that the FTIR studies are in good agreement with the Raman spectroscopy.
The topography of the microscopic analysis of PVI-KOH thin lms were analyzed using both eld-emission scanning electron microscopy and scanning probe microscopy. Fig. 4 shows the FESEM images of the PVI-KOH thin lm ($121 nm) measured in the ITO/PVI-KOH/ITO planar MSC device. It is clearly seen that the KOH salt is distributed randomly throughout the surface of the lm. However, the PVI-KOH showed smooth surface with large-scale polymer alignment pattern (Fig. 4a). Fig. S1a-c in ESI † shows the AFM images of PVI-KOH planar MSC device with various thicknesses. All the lms exhibit smooth surface with roughness at nanoscale. It was observed that the surface roughness of the lm slightly increases with an increase in lm thickness, which mostly arises due to the distribution of KOH salt in the matrix polymer. Fig. S1d (ESI †) represents the measurement technique adopted to nd the thickness of the thin PVI-KOH lm.

Electrochemical performance of PVI-based planar MSC
Electrochemical properties of ITO/PVI-KOH/ITO planar MSCs were investigated using two-electrode cell conguration. Cyclic voltammetry (CV) studies were performed on a single-cell planar MSC with various scan rates (10-300 mV s À1 ) and constant potential window of 0-1 V. To understand the interfacial interaction between polymer chains (PVI) and metal salts (KOH) on EDLC characteristics, PVI-KOH lm thicknesses were varied in the planar MSC device conguration. Fig. 5a represents the CV curves of the planar MSC measured at constant scan rate (50 mV s À1 ) with various PVI-KOH thicknesses ranging between $28 nm and $121 nm. The CV plots of thickness dependent planar MSC exhibited a quasi-rectangular behavior, indicating the formation of an electrical double layer at electrode/ electrolyte interfaces. It is obvious that the characteristic CV curve of planar MSC decreases with the decrease of lm thickness. Also, the scan rate dependent CV measurement was conducted to evaluate the thickness dependent planar MSC on supercapacitance characteristics. Fig. S2 (ESI †) shows the CV curves of the thickness dependent planar MSCs as function of scan rates. The CV curve retained their quasi-rectangular prole in all the scan rates, which demonstrates that the PVI-based planar MSCs have an ideal capacitance characteristics and good rate capability. The volumetric capacitance (C V ) of planar MSC was estimated by taking into account of CV plot. Fig. 5b shows the estimated C V curves of thickness dependent planar MSC. Signicantly, the planar MSC with a PVI-KOH thickness of $121 nm revealed higher C V of 128 mF cm À3 at 10 mV s À1 . The decrease of PVI-KOH thickness to $28 nm showed drop of CV to 43 mF cm À3 at 10 mV s À1 . Furthermore, the C V decreased with an increase in scan rates in all the cases and exhibited a minimum value of 2.9 mF cm À3 at 300 mV s À1 for a PVI-KOH thickness of $28 nm.
Galvanostatic charge-discharge (GCD) behavior of planar MSC was investigated as function of PVI-KOH thickness. Fig. 5c represents the GCD plots of thickness dependent planar MSCs with various current densities, which is essential to estimate the charge-discharge characteristics at specied time constant. The charge-discharge rate at around 50 s is observed for all the planar MSCs. Also, the current density decreases with decrease in PVI-KOH thickness in order to obtain similar chargedischarge rate. Fig. S3 of ESI † shows the GCD plots of thickness dependent planar MSCs with various current densities. The single-cell of thickness dependent planar MSC exhibited a GCD pattern close to triangular shape at all current densities, suggesting an effective electrical double layer formation and high charge propagation across the electrodes.
To evaluate the cycling characteristics of the fabricated planar MSC device, the GCD studies were carried out according to the device capability. In Fig. 5d, the planar MSC device with higher PVI-KOH lm thickness ($121 nm) exhibited lower cycling performance of # 1400 continuous cycles and $92% capacitance retention. It was observed that, both the cycling performance and capacitance retention increases with the reduction of PVI-KOH thickness to certain characteristic nanoscale. The planar MSC with ultra-thin PVI-KOH ($28 nm) showed higher cycling performance of $3500 continuous cycles and large capacitance retention ($95%), as shown in Fig. 5d.
To understand the ionic transport characteristics of the planar MSC device, the electrochemical impedance spectroscopy (EIS) was conducted. The Nyquist plot shows ( Fig. 5e) a semicircle with vertical response in the low frequency region, which corresponds to the ideal capacitance behavior. The Nyquist plot was further analysed by tting with equivalent circuit model, as shown in Fig. S4 of ESI. † From Nyquist plot, the intercept in the real axis is modelled by a series resistor (R S ), and the semicircle is modelled by a constant phase element (CPE 2 ) with a parallel resistor (R p ). The vertical response is assigned to be CPE 1 . The obtained standard error was as low as 7% for the tting parameters. Each CPE is characterized by capacitance C 1 and C 2 , respectively. Here, a single CPE, designated as CPE 1 is due to the electrode polarization, exhibiting vertical response at low frequency region. Presence of large number of ions in the PVI interface that are easily attracted by the nonblocking ITO electrode, resulting in the formation of EDL. Furthermore, the impedance response in the medium frequency range is modelled by an R p /CPE 2 , which corresponds to the bulk behavior of the PVI-KOH electrolyte. The ionic conductivity of the planar MSC is estimated to be between 10 À3 and 10 À4 S cm À1 with various PVI-KOH thicknesses. To quantitatively analyze the ionic transport characteristics of planar MSC, three different devices were used in each thickness (error bar plot is shown in Fig. 5f).
In order to estimate the performance characteristics of MSC, the ITO/PVI-KOH/ITO device with ve series cell conguration also fabricated. For this study, we used a relatively thick PVI-KOH electrolyte ($121 nm) to fabricate the ve series-cells because of its higher ionic conductivity and higher volumetric capacitance. Fig. 6a represents the CV plots of the device studied at higher cell voltage of 5.0 V with various scan rates (10-300 mV s À1 ). It is obvious that the CV plot exhibits quasirectangular behavior in all the scan rates even at higher cell voltage, demonstrating an ideal EDLC characteristic with high rate capability. Moreover, the maximum volumetric capacitance of the ve series cell planar MSC was found to be 18.5 mF cm À3 at low scan rate (10 mV s À1 ). The device exhibits decrease in volumetric capacitance with an increase in scan rate, as shown in Fig. 6b. The charge-discharge behavior of ve series cell planar MSC was studied at higher cell voltage (5.0 V) with various current densities, as shown in Fig. 6c. Signicantly, the device exhibits typical charge-discharge pattern in all the current densities. It infers that the PVI-KOH based planar MSC can adopt to work in a wide potential window for practical applications. The Nyquist plots (Fig. 6d) show the ionic conductivity of ve series cell planar MSC, which remains nearly same aer all the electrochemical measurements relative to that of the pristine device.
The performance of the planar MSC was further demonstrated using Ragone plot, as shown in Fig. 6e. Ragone plot is essential to understand the relationship between energy density (E V ) and power density (P V ) by GCD prole. The PVI-KOH based single-cell of planar MSC shows maximum volumetric energy and power density of 0.023 mW h cm À3 and 1.64 mW cm À3 , respectively. But, the ve-cell planar MSC exhibited the maximum volumetric energy and power density of 0.056 mW h cm À3 and 6.89 mW cm À3 , respectively. It infers that the PVI-KOH electrolyte conned to certain nanoscale can give excellent device characteristics, which are comparable to the previously available MSC devices (Table S1 †). To conrm further on the supercapacitance characteristics, ve-cell planar MSC was connected with multimeter as load and estimated the discharging effect aer one complete charging ($5.0 V). Fig. 6f shows the galvanostatic charging and discharging proles (inset represents the corresponding photograph of multimeter display) of the planar MSC. It illustrates that the planar MSC discharges certainly aer one complete charging (the corresponding real-time movie is shown in Movie S1, ESI †). This characteristic phenomenon of the ITO/PVI-KOH/ITO planar MSC comprising a nanostructured lm revealed excellent EDLC characteristics, which promise for their use in practical applications.
It is speculated that the effect of polymer connement on the ionic transport at electrode/electrolyte interface has strong impact on the EDLC characteristics. The inclusion of ionic charges separated at the respective interfaces, in which the K + ions at the negative ITO/PVI interface and OH À ions at the positive ITO/PVI interface, gives rise to the generation of emf voltage (V emf ) within the device. 35 The inset of Fig. 7a shows the equivalent circuit model of the planar MSC under emf generation. Owing to the partial electronic component of the device, the theoretical V emf cannot directly measured. By taking into consideration of the ionic resistance R i , and electronic resistance R el , the V cell can be obtained from the following relation, 35,36   The R el of the device is very high (>10 6 U) due to the insulating nature of PVI-KOH. As consequence, the behavior of V cell becomes closely associated with the V emf . In EDLC, the formation of V emf can be described by the contribution of Nernst potential V N and a diffusion potential V d . Hence, the total V emf in the EDL device consists of 36,37 where V 0 is the standard potential, k B is Boltzmann constant, e is charge of electron, T is absolute temperature and t ion is the mean ion transfer number ( t ion ¼ t K + + t OH À). 36 Fig. 7a shows the time dependent V cell of both thick ($66 nm) and relatively thin ($28 nm) PVI-KOH based planar MSCs at pristine state. Fig. 7b and c show the V cell of the $66 nm and $28 nm PVI-KOH based planar MSC devices, measured under various scan rates (10 to 200 mV s À1 ). The measurement was carried out in two steps: initially, a positive cyclic sweep was performed as 0 V / 1 V / 0 V at certain sweep rate. Then, the V cell was measured as a function of time, under an open circuit condition. In both cases, gradual increase in V cell for more than 200 s was observed because of the equilibration of ionic charges (K + and OH À ) at the respective electrode/electrolyte interfaces. It was observed that the increase in scan rate decreases the V cell for both cases; because, the ionic charge separated at the electrode/ electrolyte interfaces is a rate limiting process. However, the relatively thick PVI-KOH ($66 nm) based planar MSC showed higher V cell , which was almost one order of magnitude larger than that of the thin device. This can be attributed to the inuence of ionic contribution at interfaces. To gain further insight into the ionic charge distribution in the interfaces, the mean ionic concentration (C ion ) was calculated from cyclic sweep measurements. The increase in cyclic sweep rate changes the intensity and area of CV curve, suggests that the ion transfer controlled reaction may happen at the electrode/electrolyte interfaces. The difference in current density can be evaluated from the Randles-Sevcik equation, 38,39 where J is the current density, z is number of electrons that assumed to be one, C ion is the ionic concentration, a is the ion transfer coefficient, 38 D is the diffusion coefficient 39 and n is the scan rate. It is essential to mention that the concentrations of both K + ions and OH À ions are contributing equally to C ion in eqn (3). Based on this assumption, C ion was estimated from the cyclic scan and device geometry. 38,39 Fig. 7d represents the scan rate dependent C ion for both thin and relatively thick PVI-KOH based MSC devices. It was found that the contribution of C ion in thick planar MSC is higher than that of thin device. This can be attributed to the inuence of ionic contribution at interfaces, resulting higher V cell increases over time (Fig. 7b). Also, it is essential to mention that the cyclic scan rate can majorly inuence the ionic contribution in EDLC. At small scan rate, large concentration of ionic species can be separated at the respective electrode/electrolyte interfaces due to the longer scan cycle. Whereas, the large scan rate signicantly reduces the contribution of ionic carriers at electrode/electrolyte interfaces, resulting the decrease in C ion . Based on these results, we infer the charge migration and separation in the PVI structure. The PVI has large number of imidazole pendant groups. Out of two nitrogen atoms in the imidazole ring, one nitrogen atom has a lone pair of electrons that are not contributing to the aromaticity. Consequently, that lone pair of electrons are available for hydrogen bonding formation and metal ion coordination. Since the PVI itself does not have any hydrogen bond donor, therefore, it has an inherent tendency to associate with large amount of water molecules through hydrogen bonding (Fig. S5, ESI †), which results PVI as hygroscopic in nature.
Our FTIR data also supports well with observation of broad OH stretching signal at $3397 cm À1 (vide supra) due to the PVI bound water molecules. Consequently, it is anticipated that the PVI bound water molecules in PVI-KOH lm facilitates OH À / OH 2 hydrogen bond and K + /OH 2 coordination bond mediated ion transport. This assertion was further validated by RH dependent impedance spectroscopy studies. As shown in Fig. S6 of ESI, † the variation of environmental humidity around the PVI-KOH lm signicantly alters the R ct values. It is apparent from Fig. S6, † higher relative humidity level lowers the R ct implying the incorporation of more number of water molecules in PVI-KOH lm under humid condition that in turn allows facile ion transport. The plausible mode of ion-transport is shown Fig. 8. According to the hard-so acid-base (HSAB) theory, potassium ion is a hard Lewis acid that prefers to make coordination bond with hard Lewis base oxygen of water molecules than the relatively so base nitrogen of imidazole ring. Not violating HSAB theory, in biology, while both N and O donors are prevalent in proteins, K + is being transported across the membrane along the carbonyl oxygen of amide backbone in the membrane proteins (e.g., KcsA (K channel of streptomyces A) through K + /O]C coordination bonding mode). 40 The previously reported molecular mechanics calculations reveal that potassium ions display an average coordination number of 6.6 in KcsA membrane channel and 6.2 in bulk water. 40 According to these information the pristine state was drawn and shown in Fig. 8, where K + is represented as K + (H 2 O) 6. This ion migrates in PVI lm through breaking and forming of potassium ion-water molecule (K + /OH 2 ) coordination bonds as shown in Fig. 8a (blue arrow). Whereas, the hydroxide ion migration occurs in long range via breaking and forming of hydrogen bonds with water molecules present in PVI-KOH lm through the well-known Grotthuss-like mechanism. 41 Since hydrogen bonding in water is a three dimensional network, we have shown only representative hydrogen bonds for clarity. Previous studies have shown that OH À is a hypercoordinate species having four hydrogen bonds, HO À (H 2 O) 4 . 42 Hence, we assumed that OH À ion in pristine state is a hypercoordinate ion with three hydrogen bonds and one coordination bond with K + (H 2 O) 6 ion (Fig. 8a) 4 , as shown in charge separated state (Fig. 8b). The hydrogen bonding facilitates long range as well as fast HO À ion migration in the PVI structure, which plays crucial role in determining charge separation and energy storage characteristics of the ITO/PVI-KOH/ITO planar MSC device.

Conclusions
In summary, the ITO/PVI-KOH/ITO planar MSC devices with nanostructured PVI-KOH lms are demonstrated on the EDLC formation mechanism and stability characteristics. The distribution of ionic charges and the corresponding electric double layer formation in the electrode/electrolyte interface were systematically studied using thin and relatively thick PVI-KOH in the planar device conguration. Single-cell of ITO/PVI-KOH/ ITO planar MSC with a PVI-KOH thickness of $121 nm revealed higher volumetric capacitance (128 mF cm À3 at 10 mV s À1 ) and 92% capacitance retention (#1400 cycles). But, the connement of PVI-KOH thickness to the characteristic nanoscale ($28 nm) remarkably enhanced the capacitance retention ($95%) for more than 3500 continuous cycles. This signicant deviation in EDLC characteristics contribute to the detailed understanding of the connement effect of MSC devices not only for the use of EDL based energy storage applications but also for EDL based transistor applications.

Materials and methods
Synthesis of poly(N-vinyl imidazole). 1-Vinylimidazole, azoisobuty-ronitrile (AIBN) and toluene were used as received from spectrochem chemical, India. In a typical reaction, 1-vinyl imidazole (0.1 mol) was mixed with toluene (40 mL) in a two neck round bottom ask tted with a reux condenser. Then, AIBN as initiator was subsequently added into the above reaction mixture. The reaction mixture was kept at constant stirring at 70 C for 4 h under nitrogen atmosphere. The resultant polymer was washed several times with acetone. Then, the polymer was dried at 40 C under vacuum for 6 h. The synthesized PVI was further conrmed using NMR analysis. Fig. S7 (ESI †) represents the 1 H NMR spectrum of the prepared PVI. The 1 H NMR signals for PVI is observed between the range 8 ppm and 1 ppm. The broad peaks observed between 6.6-7.4 ppm correspond to the imidazole protons. The broad peaks centred at 2.0 ppm and 3.0 ppm are ascribed to the polymer backbone of CH 2 and CH groups, respectively. 32 Fabrication of solid polymer electrolyte-based exible planar MSC Poly(N-vinyl imidazole) (PVI)-based planar structure of ITO/PVI-KOH/ITO devices was fabricated using ITO-coated (thickness $130 nm) on exible PET substrates. For planar device, two opposing ITO with a gap distance between ITO electrodes of $3 mm was carefully edged using diluted hydrochloric acid (HCl). Then, the edged ITO/PET substrate was cleaned with isopropyl alcohol and distilled water. To fabricate ITO/PVI-KOH/ITO planar MSC, homogeneous PVI-KOH solution was spin-coated on the substrate. The 0.1 g of PVI was dissolved in 5.0 mL of distilled water and then constantly stirred for 30 min at 90 C to obtain a homogeneous solution. Separately, 50 wt% potassium hydroxide (KOH) salt was dissolved in 2.5 mL distilled water. Aer that, KOH solution was added into the homogeneous PVI solution. The resultant mixture of PVI-KOH was ultrasonicated for 15 min at room temperature. Finally, 150 mL of PVI-KOH homogeneous solution was spin-coated (Milman spin coater, India) on the ITO-edged PET substrate. The This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 45019-45027 | 45025 prepared PVI-KOH lm thickness was found to be $121 nm using atomic force microscopy. Similarly, various thicknesses of the PVI-KOH lms ($66 nm, $28 nm) were prepared for planar MSCs by modifying the polymer to solvent ratio.

Characterization
All the electrochemical characteristics of the planar MSC devices were performed with a two-electrode conguration. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) studies were carried out by Metrohm Autolab potentiostat/galvanostat instruments. The ionic conductivity (s) of thin lm was estimated using the following relationship, where, d is the distance between the ITO electrodes, R ct is the resistance value obtained directly from EIS measurement, l is the length of the ITO electrode, and t is the lm thickness. The CV measurements were conducted with various scan rates (10 to 300 mV s À1 ) and specic potential window (0 to 1 V for single cell and 0 to 5.0 V for ve series-cells). To study the GCD behavior, different charge-discharge current densities were used for charging/discharging of the planar MSC device. Furthermore, in GCD studies, two different voltage windows such as 0-1.0 V for single cell and 0-5.0 V for ve series-cells were used. The CV curves were used to estimate the volumetric capacitance (C V ) of the device, using the following relation, where, I(t) is the current measured during C V testing, t is the time, V is volume of the device, and DV is potential range (¼1.0 and 5.0 V for single and ve-series cell planar MSC device, respectively). The volumetric energy density (E V ) and the power density (P V ) of the planar MSCs were estimated from the CV plots for a scan rate in the range of 0.01-100 V s À1 by the following relations, where, Dt (in seconds) is the discharge time. Also, the volumetric capacitance of planar MSC devices was estimated by GCD proles using the relation where, i, Dt, V, and DV are the current, discharge time, volume of device, and potential window of the cell, respectively. The prepared PVI polymer was characterized using X-ray diffractometer (XRD; X'Pert PRO, PANalytical, Cu Ka, l ¼ 0.15406 nm). Raman spectra for both pure PVI and PVI-KOH thin lms were recorded using Laser Raman Microscope (Horiba Jobin Yvon-LabRAM HR Evolution). The FTIR study was done using FTIR spectrometer Bruker tensor 27. The polymerization process was further demonstrated using 1 H NMR analysis using FT NMR spectrometer BrukerAvance III HD 400 MHZ. The surface morphology and thicknesses of the PVI-KOH lms were observed using both eld emission scanning electron microscope (FESEM) (Hitachi FE-SEM S-4800) and scanning probe microscope (Agilent technologies 5500 series) analysis.

Conflicts of interest
There are no conicts to declare.