Semi-Polycrystalline–Polyaniline Empowered Electrochemical Capacitor

Abstract

We report on the formation of semi-polycrystalline polyaniline, a novel electroactive polymeric material synthesized by a modified surfactant-free chemical route and its enhanced electrochemical capacitive behavior. The material exhibits uniformly arranged spindle-shaped morphology in scanning electron microscopy and well-defined crystallographic lattices in the high-resolution transmission electron microscopy images. The X-ray diffraction spectrum reveals sharp peaks characteristic of a crystalline material. The characteristic chemical properties of polyaniline are recorded using Fourier transform infrared technology and laser Raman spectroscopies. The cyclic voltammetry curves exhibit features of surface-redox pseudocapacitance. The specific capacitance calculated for the material is 551 F g⁻¹ at a scan rate of 10 mV s⁻¹. The cycle stability and the coulombic efficiency recorded at a current density of 12 A g⁻¹ exhibited good stability (90.3% and 99.5%, respectively) over 3000 cycles.

1. Introduction

There is an increasing need for developing energy storage devices to complement the exponential rise in the manufacturing and usage of portable and wearable electronic devices, such as modern mobile handsets with highly upgraded and useful functions, applications (activity trackers), smart watches, medical devices (pacemakers, insulin pumps, sensors), and so on. Newer technologies bring better and upgraded versions of these devices and appliances, which also demand upgradation of efficiency of the energy storage materials. Supercapacitors, among various and diverse electrochemical energy storage systems, provide great opportunities toward innovation and development of modern day portable electronic devices and appliances by virtue of their high power and rate capability, cyclic stability for prolonged duration, material stability and safe operations [1,2,3,4,5].

Polyaniline is a well-known conducting polymer, also known as aniline black, that can be synthesized either, by a chemical route or electrochemical methods (electro-polymerization) [6,7,8,9,10,11,12,13,14]. In recent years, it has been widely used as an electroactive material in various application, e.g., energy storage and conversion, supercapacitors, rechargeable batteries, biosensors, field-effect transistors, solar cells, corrosion protection, polymer light diodes, and so on [15,16,17,18,19,20,21,22,23]. It possesses variable oxidation states and exhibits reversible electrochemical behavior and tunable pseudocapacitive properties. In energy storage applications, it is either employed as a standalone electroactive material or in composite form with other electroactive materials, viz., carbonaceous material, metal oxides and other polymeric materials [3,5,6,15,24,25,26,27,28,29,30,31].

The electrochemical behavior and charge storage mechanisms occurring in an electroactive material, is obtained from cyclic voltammetry, galvanostatic charge–discharge curves, and electrochemical impedance spectroscopy. The charge storage process takes place via either or both mechanisms, viz., non-Faradaic reactions or formation of electrical double layer at the surface of the electrode and Faradaic or redox reactions. The mechanism of charge storage occurring in an electroactive material is classified into three main categories, viz.:

(a)

Capacitive or ideal double layer or non-Faradaic charge storage: This type of charge storage takes place due to formation of an electrical double layer at the interface of an electronically conducting phase (electrode) and an ionically conducting phase (i.e., electrolyte). The actual charges, or electrons, in excess or deficiency are accumulated on the electrode material, and the positive charges reside in the electrolyte medium at the vicinity next to the electrode. There occurs no chemical reaction (or redox reaction/Faradaic reaction), i.e., change in the oxidation state of the electroactive material (electrode material). This is a fast process and involves no chemical phase change, or no compositional change occurs and the process is highly reversible. A true capacitive or non-Faradaic charge storing electroactive material exhibits high recyclability and high cyclic stability. The cyclic voltammograms exhibit a box-type or rectangular-type profile, and the change of potential exhibits a linear dependence on time at a constant current in the galvanostatic charge–discharge (GCD) profile. Both the charging and discharging processes show a linear rise and decline with time (hence displaying a triangular profile in the GCD curves) [32,33].

(b)

Faradaic charge storage: This involves charge transfer leading to redox reactions as suggested by Faraday’s law, hence the term. The redox reaction results in phase change due to the change in oxidation state of the electroactive material. The CV curves display sharp peaks associated with theoretically explained and well-defined redox reactions taking place in the electroactive material. The GCD charge–discharge behavior is entirely different from that of non-Faradaic charge storing electroactive materials. The signature GCD profile for Faradaic charge storing material displays a characteristic plateau, or a constant or zero slope (or parallel line). The Faradaic charge storage processes are rarely reversible because a phase change or oxidation/reduction state of the material takes place. The redox processes occur at entirely different potential ranges and are highly irreversible. These kinds of electroactive materials are called battery-type materials. The redox reactions are slow and diffusion controlled and often termed as semi-infinite diffusion controlled processes [3,32,34].

(c)

Ion-insertion process: This is a diffusion-controlled process and follows slow kinetics. The charge storage depends upon intercalation/de-intercalation of cations (e.g., Na⁺, K⁺ and H⁺) in the bulk of the electroactive material [4,35,36,37]. The rechargeable batteries, e.g., aq. NiMH batteries and non-aqueous Li-ion batteries store charge through intercalation/deintercalation of H⁺ or Li⁺ ions within the crystalline architecture, and the overall kinetics is controlled by the diffusion of cations. Intercalation of ions requires a layered architecture of the material. The ions from the electrolyte move inside the layers of the material and travel through the morphological tunnels or pores, with no crystallographic changes occurring in the material [3,32,34].

Pseudocapacitor materials exhibit unique electrochemical features, which combine both current responses emerging from pure capacitive processes and bulk Faradaic processes (such as batteries). In the case of conductive polymers, it should not be confused with the terminology used for battery-type materials, as there is no phase transformation occurring during the charging process. Even if some conductive polymers display sharp redox peaks as well as large peak separations in their cyclic voltammetry profiles, those are never termed as battery-type material for the same reason, i.e., no phase transformation during the charging/discharging processes. Such materials are termed as “redox capacitors”. However, the performance evaluation of such a material resembles that of a battery material due to the similarity in electrochemical behavior [3,32,33,38]. In surface redox pseudocapacitors, the surface-controlled redox reactions are fast and reversible at the electrode surface, which occur either through intercalation, or through adsorption of electrolyte ions. This is another important feature that is distinct from the battery materials where diffusion-controlled processes are slow and semi-infinite [1,3,39,40]. The pseudocapacitor material exhibits a high rate capability similar to the double-layer capacitor materials and high-energy and high-power densities. The Faradaic charge storage behavior or redox reactions in the pseudocapacitor materials, apparently resembling that of battery materials, occur via extremely fast kinetics and are not limited by semi-infinite diffusion processes as exhibited by battery materials [3,34].

In the present study, semi-polycrystalline, a polyaniline electroactive material, has been synthesized by a modified chemical route, and its electrochemical properties have been investigated. The material exhibits chimeric domains of crystalline lattice planes in high-resolution micrographs and sharp peaks in the X-ray diffraction spectrum characteristic of a crystalline material. The method of preparation presented in the study not only provides an inexpensive means of obtaining a substantial improvement in the structure and properties of the polymeric material, but it also enhances electrochemical properties and good cyclic stability conducive to achieving a better electroactive material for charge storage applications [9,41]. Furthermore, in this context, it is important to include the exact definitions of the different charge storage mechanisms to avoid confusion in understanding the unique case of semi-crystalline polyaniline in the present work. Here, we have followed the Conway definitions to explain the electrochemical charge storing behavior of the semi-crystalline polyaniline. In the present work, the reactant molecules (polyaniline) do not move towards and away from the electrode surface and the molecules remain at their original position. Only a change in the conformation occurs during the redox reactions, which cannot be explained as a phase change. In other words, the change in the oxidation state of the polyaniline does not involve a phase change. The process is highly reversible and both the reactant and the product are still a polyaniline molecule. This is an important part of the definition of a non-Faradaic charge storing electroactive material. The change in the conformation of the polymer chain during the redox process gives rise to deviations from the ideal signature features in CV and GCD profiles, and this is explained to be due to pseudocapacitance. The morphological and geometrical restrictions and resonance stabilization in the semi-crystalline polyaniline materials are believed to be responsible for resisting conformational changes invoked by redox reactions. Only the terminal portions of the polyaniline chains are believed be available for the redox-led conformational changes and the observed features in the CV and GCD curves. This must not be confused with battery-type-characteristic features, which are purely an outcome of Faradaic reactions involving consumption of electrons and phase change of the material. The latter processes are highly irreversible (or rarely reversible). The microstructure and the overall morphology of the materials behave intricately to give rise to unique electrochemical responses. A detailed description of the formation of semi-polycrystalline polyaniline, its molecular structure, microstructure, bulk morphology, physico-chemical and electrochemical behavior, and mechanism of charge storage have been presented in the discussion section of this manuscript. From an application point of view, there have also been proposed diverse theories available in several reports, which suggest that the coexistence of amorphous and crystalline phases enhance the electrochemical behavior of the material [5,24,25,27,28,29,42,43,44].

2. Materials and Methods

2.1. Materials

Materials for synthesis, i.e., aniline (99.9%) and N-methyl 2-pyrrolidone (NMP), were procured from Sigma-Aldrich Co. (St. Louis, MO, USA) Potassium persulfate, sodium phosphate dibasic, hydrochloric acid (98%), sulfuric acid (98%), methanol and ethanol, were procured from Duksan Pure Chemicals Co., Ltd. Korea (Gyunggido, Korea).

2.2. Preparation of Semi-Polycrystalline Polyaniline

Polyaniline with crystalline properties were synthesized according to the method reported previously by the authors [9]. The polymerization reaction was initiated by dispersing 10 mL of aniline monomers in 600 mL of 1.0 N aq. HCl, followed by heating the solution up to 70 °C in a round bottomed flask fitted with a reflux condenser. Another solution was prepared by dissolving 14.8 g of potassium persulfate in 400 mL of 1.0 N aq. HCl and heating it up to 70 °C. The two hot and dilute solutions were mixed slowly, in a drop-by-drop manner, with constant stirring. Gradually, the precipitate began to appear and soon the entire solution became dark green colored due to polymerization of aniline and subsequent precipitation. The mixture solution was continually stirred for an additional duration of 2 h at 70 °C post mixing the individual solutions, and then the heating was stopped. The solution was allowed to cool to room temperature by itself, and stirring was continued overnight (14 h). After the stirring was stopped, the precipitates settled down at the bottom of the vessel, leaving behind a clear solution at the top. The supernatant layer of clear solution was decanted, and the precipitate was filtered using a Buchner funnel followed by washing with deionized water and methanol, respectively. Subsequently, the precipitate was first air dried at room temperature and then dried inside an oven at 60 °C for 12–14 h. Post drying, the precipitates were milled in an agate mortar into a uniform powder and stored in a vacuum.

3. Material Characterizations

The microstructure of the newly synthesized polyaniline was characterized by scanning electron microscope, FE-SEM (HITACHI-S4800, Tokyo, Japan), operating at an accelerating voltage of 5 kV under a vacuum of 10⁻⁴–10⁻⁶ mm Hg and field emission transmission electron microscopy (FE-TEM, FEI, Tecnai G20, TWIN, FEI company, Hillsboro, OR, USA), operating at an accelerating voltage of 200 kV. For obtaining high-resolution TEM images, a small amount of the dried polyaniline powder (0.5 g) was placed in an agate mortar, crushed and ground in excess ethanol (5 mL) until the entire consistency became homogeneous. The insoluble content was allowed to settle. The supernatant was decanted and the pellet was again ground in excess ethanol. The steps were repeated 3–4 times. This part of the experiment enabled washing out/removal of any traces of impurities or remnants of precursor components. In the final stage, a drop of the solution was cast onto a copper grid using a syringe and dried appropriately. The specimens were then observed under a high-resolution transmission electron microscope. The phase of the material was characterized by X-ray diffraction (XRD PANalytical, X-pert PRO MPD diffractometer, Philips, Eindhoven, The Netherlands), respectively. The XRD spectrum was recorded by scanning the polyaniline samples in the 2θ range from 10° to 80° by the X-ray diffractometer equipped with Cu-Kα radiation (λ = 0.154 nm). Laser Raman spectrum was recoded using LabRam HR800 UV Raman microscope (Horiba Jobin-Yvon, Paris, France) using a laser beam (λ = 532 nm).

4. Electrochemical Characterizations

The electrochemical experiments, galvanostatic charge–discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopic (EIS) analysis were carried out using an electrochemical analyzer (Potentiostat, VersaSTAT-3, Princeton Research, Oak Ridge, TN, USA) at room temperature. All the electrochemical measurements were performed in an electrochemical cell with three-electrode configuration using 1.0 M aqueous Na₂SO₄ as electrolyte. The working electrodes were prepared by drop casting polyaniline in the slurry form on nickel foam substrate within an area of 1.0 cm². The polyaniline slurry was prepared by grinding powdered polyaniline (6 mg), polyvinylidene difluoride (PVDF) (2 mg), and activated carbon (2 mg), with 2 mL of N-Methyl-2-pyrrolidone (NMP) solvent using a miniature agate mortar. The drop casting was carried out by pipetting out 100 μL using a micropipette and casting over Ni-foam (cut into dimension of 3 cm × 1 cm) at the pre-defined portion (an area of 1.0 cm × 1.0 cm) followed by drying in an oven at 60 °C for 8–10 min. The steps in the casting process were repeated and continued until all the slurry was transferred on the Ni-foam (within the defined area of 1.0 cm²). The electrodes were dried overnight at 60 °C inside the oven. The weight of the transferred material (polyaniline slurry or electro-active material with binder) was calculated by subtracting the weight of the Ni-foam before sample loading from the weight of the Ni foam after sample (polyaniline slurry) loading. Platinum wire and Ag/AgCl in 3 M KCl were used as the counter electrode and reference electrode, respectively. Cyclic voltammograms were recorded at different scan rates ranging from 10–200 mV s⁻¹ in the potential range −0.6 to 0.8 V. EIS analysis was performed in a frequency range 100 kHz and 0.1 Hz with the application of an AC perturbation of 5 mV. The galvanostatic charge–discharge curves were recorded at different current densities ranging from 2–12 A g⁻¹.

5. Results

5.1. Microstructure, Phase and Chemical Properties of Semi-Polycrystalline Polyaniline

The SEM images of the newly synthesized semi-polycrystalline polyaniline are shown in Figure 1a,b. In Figure 1a, the polyaniline appears to be a porous mass, and in Figure 1b, the material appears to be composed of spindle-shaped fibrils of approximately uniform diameter. The surface of the fibrils is not fused with one another and remains free standing. In the high-resolution TEM images (Figure 1c,d), there appear to be several patches with well-defined crystallographic lattices of polyaniline. The lattice d-spacing and pattern of distribution of crystalline domains make it apparent that the material is polycrystalline. The surface appears to be a chimeric patchwork of multiple monocrystalline domains. The polyaniline hence obtained from the modified synthesis route is termed as semi-polycrystalline polyaniline. The crystalline features seem to arise due to the incorporation of modification during the synthesis of the material.

The XRD spectrum of the semi-polycrystalline polyaniline is shown in Figure 2a. The spectrum shows sharp peaks indicating crystalline properties of the material. The peaks are assigned according to JCPDS Card no. 53-1890. The peak positions at 2θ degrees, viz., 19.2, 24.0, 25.8, 28.9, 29.2, 32.4, and 39.5 correspond to the crystallographic planes (1 1 0), (2 0 0), (0 1 2), (2 1 0), (2 1 1), (1 2 0), and (3 1 0), respectively. The FTIR spectrum of the semi-polycrystalline polyaniline in the region 500–4000 cm⁻¹ is shown in Figure 2b. The bands at 1582 and 1496 cm⁻¹ are attributed to C=C and C=N stretching vibrations, respectively. The band at 1296 cm⁻¹ is assigned to the C-N stretching vibration of the quinonoid rings. This band is also indicative of the degree of electron delocalization as an “electronic-type” bond and is characteristic of the conductive nature of the polyaniline molecule in its doped state [45]. The band at 1156 cm⁻¹ is assigned to the vibrational mode of the –NH⁺=C bond. The bands at 1113 and 821 cm⁻¹ are attributed to the C-H in plane bending and out of the plane deformation bending of the benzene ring, respectively. The bands in the FTIR spectrum are characteristic of the polyaniline molecule and confirms the successful synthesis of the polymer [41,46,47,48]. The laser Raman spectrum of the semi-polycrystalline polyaniline is displayed in Figure 2c. The spectrum contains the peaks characteristic of polyaniline, namely, the band located at 1588 cm⁻¹ corresponding to the carbon–carbon single- and double-bond stretching in the quinine-type ring. The bands at 1345 and 1491 cm⁻¹ are characteristic of C-N⁺ and C=N stretching in function of the charge localization. Accordingly, the band at 1218 cm⁻¹ corresponds to C-N stretching of the amine sites. The band at 1340 cm⁻¹ corresponds to the stretching vibrations of an intermediate bond C-N⁺, and the band appearing at 1491 cm⁻¹ corresponds to the specific C=N stretching modes. The small band at 1561 cm⁻¹ is characteristic of N-H bending vibrations, whereas C-H in plane deformation mode specific of quinonoid is observed at 1166 cm⁻¹ [6,7,8,49,50,51,52]. The product formed in the present study is polyaniline and is not aniline oligomers [13,53].

5.2. Electrochemical Capacitor Investigation

The CV curves are recorded in the working potential range of −0.6 to 0.8 V by varying the scan rates from 10 to 200 mV s⁻¹. The electrochemical performance of the semi-polycrystalline polyaniline in aqueous electrolyte (1 M Na₂SO₄) is displayed in Figure 3. Cyclic voltammograms (CV) obtained at different scan rates ranging from 10–200 mV s⁻¹ and evolution of specific capacitance with respect to the different scan rates are shown in Figure 3a,b, respectively.

The CV curves appear to be quasi-rectangular, indicating signature features of a surface-redox pseudocapacitive material [3,32]. The CV curves are apparently different from the reports on pristine polyaniline published previously, where the curves predominantly show sharp redox peaks [26]. The CV plots show a pattern of increase in the area enclosed inside the curve with an increase in the scan rates. The voltammograms display tiny or diminished peaks indicating the charging current outrunning the rate of the redox reactions occurring at the surface of the material, which in the case of polyaniline, involves both the diffusion–intercalation of H⁺–A⁻ to the active sites and change in the conformation of the molecular structure (discussed in detail in the discussion section). This is attributed to the microstructure and overall morphology of the material and sequence of events occurring at the electrode–electrolyte interface. As the scan-rate is increased, the dimension of the diffusion layer formed at the electrode–electrolyte interface shrinks, which facilitates an increase in the electrolytic ionic flux to reach at the electroactive material supported on the working electrode. This results in a greater current response at higher scan rates [26,28,29]. The specific capacitance “C_s” for the semi-polycrystalline polyaniline electrode material was calculated by using Equation (1) [28].

$$C_s=\frac{{{\displaystyle \int }}_{V_i}^{V_f}i dV}{2m\upsilon \left(V_f-V_i\right)}$$

(1)

where, ‘i’ is the voltametric current (A), ‘m’ is the mass of the electroactive material, ‘υ’ is the voltage scan rate and (V_f − V_i) is the range of applied voltage (V). The specific capacitance calculated for the material is 551 F g⁻¹ at the scan rate of 10 mV s⁻¹, which still maintained a value of ~130 F g⁻¹ at 200 mV s⁻¹. The calculated specific capacitance shows a decreasing trend with an increase in the scan rate. This is explained, as there is not enough time for the ions to penetrate deeply into the layers of the polyaniline chains (the terminal portions, explained in the discussion section) during a fast scan, resulting in less charges at the interface charges and low specific capacitances [28].

The GCD curves (Figure 4a) were plotted in the potential range of 0.0 to 0.8 V by varying the current density ranging from 2 to 12 A g⁻¹. An apparent deviation from the ideal triangular curves characteristic of capacitors indicates a pseudocapacitive charge storage mechanism occurring in the material. A substantial decrease in the charging and discharging time with an increase in the current density is apparent from the GCD curves. This is also attributed to the events occurring at the electrode–electrolyte interface. The electrolyte ions do not have sufficient time to penetrate the deep interiors in the electroactive material at a higher current density. In this condition, the electrolytic ions can have only restricted interaction with limited sites on the surface of the electrolyte and give rise to a quicker charge–discharge process. Evolution of specific capacitance with respect to the different current densities and long-term cycling stability and coulombic efficiency recorded up to 3000 continuous charge–discharge cycles at a current density of 12 A g⁻¹ is displayed in Figure 4b,c. The specific capacitance from the GCD curves for a unit surface area and coulombic efficiency have been calculated from the equations below:

$$C_{F g^{-1}}=\raisebox{1ex}{$i\times \Delta t$}\!\left/ \!\raisebox{-1ex}{$\Delta V\times m$}\right.$$

(2)

where, ‘i’ is the applied current (A), ‘Δt’ is the discharging time, and ‘ΔV’ is the working potential window. For systems displaying non-linear behavior in the GCD curves due to pseudocapacitance, an integration formula is used instead, as shown below:

$$C_{F g^{-1}}=i{\displaystyle \int }1/V\left(t\right) dt$$

(3)

$$Coulombic efficiency=\frac{\Delta t_{discharge}}{\Delta t_{charge}}\times 100$$

(4)

The specific capacitance calculated from the GCD curves shows an increasing trend with an increase in the current density, indicating that it is possible to store large quantities of electrical charges in the material. This can be explained by a combination of theories interrelating the molecular structure, microstructure, and morphological features and quantum chemical phenomenon, and this is presented in detail in the discussion section.

The newly synthesized semi-polycrystalline polyaniline electrode’s charge-transfer kinetics was studied using electrochemical impedance spectroscopy (EIS), as shown in Figure 5a–c. Here, the Nyquist plot constitutes two essential sections. The first part of the curve is the X-intercept at the higher frequency region, the electrolyte resistance, and the other immediate semicircle is the charge transfer resistance at the electrode–electrolyte interface. The properties and performance of the electroactive material are mainly dependent on pseudocapacitance, internal resistance, and charge-transfer kinetics. The Nyquist plot shows a depressed semicircular arc in the high-frequency region and a large semicircular arc in the lower frequency region. A bode plot depicting the materials response time (characteristic relaxation time) is shown in Figure 5b. An equivalent electrical circuit to fit the impedance data is shown in Figure 5c with the calculated circuit parameters. The solution resistance is 2.3 Ω cm².

6. Discussion

6.1. Formation of Semi-Polycrystalline Polyaniline via a Modified Method

Formation of polyaniline takes place by oxidative polymerization of aniline monomers. The six carbon atoms in the aniline molecule are sp² hybridized and acquire a planar geometry to form a ring-like structure. The unhybridized p-orbitals in each of the carbon atoms contain a single electron that remains perpendicular to the plane. The electrons present in the orbital lobes existing above and below the plane undergo delocalization (formation of an extended electron cloud of doughnut-like shape), also known as resonance. The nitrogen atom has a lone pair of electrons, and its geometry is interconvertible between trigonal pyramidal and planar in an isolated aniline molecule. The lone pair of electrons participates in resonance, and the geometry now becomes planar (Figure 6). The planar geometry of the nitrogen atom is stabilized in a polyaniline molecule, and the delocalization of the pi-electron cloud is now extended to a larger length along the entire length of the polyaniline chain.

The polymerization reaction and formation of the polyaniline is an exothermic process. Consequently, increasing the temperature during the reaction slows down the rate of the reaction substantially. Furthermore, an elevated temperature facilitates uniform dispersion of the reacting species, viz., anilinium ions and persulfate ions throughout the polymerization process. The slowing down of the reaction also facilitates ample space and time duration for the creation of crystal seeding with uniform structure and slow formation of subsequent crystal layers one above the other. Gradually, the entire crystallization process proceeds with a uniform growth of the polymeric chains with ordered structures. In addition, the slowed down process also abolishes the occlusion of solvent molecules within the folds of the growing polymeric chains. The latter renders the product as fluffy and amorphous. Fluffy precipitates do not settle down easily and it becomes difficult to filter the precipitate and to wash and remove the unreacted species from the product. There remain trapped plenty of occluded solvent and unreacted precursor reactant molecules even after drying of the sample, and hence, we find a broad peak in the XRD spectrum characteristic of amorphous material [9,41].

There are a number of oxidants, which can be utilized in the polymerization of aniline monomers to obtain polyaniline as the final product. The oxidation process can take place via fundamentally two different processes, viz., (a) by means of oxidants that are electron acceptors and do not contain a reactive oxygen atom and are capable of removing electron(s) or hydrogen atom(s) from the aniline monomer molecules, and (b) by means of oxidants that are oxygen donors and contain a reactive oxygen atom. Either the latter can donate an oxygen atom to the aniline molecule to yield an oxygen-containing product upon aniline oxidation, or it can remove an electron or H-atom from the aniline molecule. Some of the commonly employed electron acceptors are Fe(III), Ce(III), Cu(II), Au(III), Pt(IV), Pd(II), and Ag(I). The commonly employed oxygen donors are perphosphoric acid, peroxomonosulfuric acid, percarboxylic acids (peracetic, trifluoroperacetic, perbenzoic), etc. The oxidants, e.g., hydrogen peroxide (H₂O₂) and peroxydisulfate salts of Na⁺, K⁺ or NH₄⁺, have been reported to act as both, viz., oxygen donors and electron acceptors.

In this work, ammonium peroxydisulfate (APS) has been employed as an oxidant. The peroxydisulfate anion is one of the most efficient electron acceptors, which initiates oxidative polymerization of aniline by a two-electron oxidation mechanism resulting in formation of an aniline nitrenium cation. The polymerization of aniline monomers to form polyaniline takes place in the following steps, viz., initiation, formation of nucleates, chain propagation and elongation, and chain termination. The initiation process of aniline oxidation, region-selectivity of dimerization, and oligomerization is crucially dependent upon the type of oxidant selected and pH of the reaction medium [54,55,56,57,58,59,60,61,62]. This is due to the significant difference in the oxidizability of aniline in molecular form and in protonated form, i.e., anilinium cation, and significant differences in the redox/acid-base properties of the different reactive species. The latter is formed as a result of (a) single electron oxidation, e.g., anilinium dication radical, or aniline cation radical, or a neutral radical, and (b) two-electron oxidation, e.g., aniline di-cation, or nitrenium cation, or nitrene [56,63]. In the presence of S₂O₈²⁻ ions (APS) as an oxidant, aniline nitrenium cations, a hydrated proton and two sulfate anions are produced in the reaction medium in the initiation step via a two-electron oxidation mechanism [56]. The formation of aniline nitrenium cations in the initiation step of oxidative polymerization of aniline involving a two-electron oxidation process has been reported to be thermodynamically favorable in comparison with one-electron oxidation of aniline [56,63].

Polymerization of aniline is an exothermic process, which means that the progress of the reaction releases heat. A relatively faster synthesis can be achieved in the reaction medium at low temperatures such as zero or sub-zero degree temperatures. The amount of heat production during the reaction can be as high as to explode the reaction vessel when the concentration of the reactant molecules is high in the reaction medium [64]. In this view, the concentration of the individual reactant solutions was kept dilute, and the oxidant was added at a slow speed to avoid a sudden rise in the temperature in the reaction medium. The average temperature of the reaction mixture in the present work was maintained at 72 °C. A high temperature slows down the kinetics. The slow addition of the oxidant allows sufficient time for bond rotation at the “N-C” sites. This facilitates acquiring an equilibrium geometry during the formation of the dimer/trimer/oligomer and eventually in the subsequent growing polymer. The average pH of the reaction mixture did not show any significant change throughout the reaction in the present work. It can be assumed that this has ensured formation of a linear chain polyaniline molecule and a linear propagation of the same and rules out formation of other intermediates. The branching in the polyaniline chain has been observed when the reaction is carried out at a pH > 2.5. Linear chain formation is the major product obtained at pH < 2.5. Formation of benzidine > 20% has been reported to occur at a pH below 1.0 and approaching 50% in superacidic conditions, i.e., pH ≤ −2.0 while the formation of hydrazobenzine and azobenzine occurs at higher pH [56,57,58,59,65]. Semiempirical quantum chemical and thermodynamic studies suggest that the reaction between aniline nitrenium cation and aniline results in the formation of 4-aminodiphenylamine as a prevalent product via the most stable N-C₄-coupled intermediate. The studies on the molecular structure of polyaniline chains using modern characterization techniques, such as Raman, ESR, electron and X-ray photoelectron spectroscopy, chromatography and chemical analysis have further supported that approximately 95–98% of the polyaniline chain consists of “N-C₄” or parasubstituted (head-to-tail) monomer units orderly arranged in a polyconjugated structure [54,63,66,67,68]. Absence of or the possibly accidently formed limited presence of intermediates or molecules other than N-C₄-polyconjugated polyaniline also helps in achieving the desired crystalline material in the present work.

During the oxidative polymerization of aniline, the nitrogen atom in the aniline monomer (in initiation step) and the terminal nitrogen atoms in the growing chains act as oxidation centers. Upon formation of oligomers, only the terminal amino groups are oxidized or activated because they possess a lower oxidation potential, and this is how a linear elongation of the polyaniline chain occurs. At the time of the chain growth process, the existing polyaniline chain behaves similar to a dormant molecule, and the terminal nitrogen atom is activated by the oxidant, which adds another aniline molecule resulting in a new dormant chain with an increased molecular mass. Each incident, there occurs an addition of a nitrenium cation to the growing chain; two protons are released, which further drops the pH of the reaction medium. Furthermore, the reaction process in the present work was carried out using a mix of dilute solutions (aniline and the oxidant) with a slow drop-by-drop method. Hence, no difference in the pH was recorded until the end of the polymerization reaction. Following the formation of aniline nitrenium cations, the formation of nucleates or aniline trimers take place. The latter acts as initiation centers [68,69]. The reaction then proceeds toward an “induction period” during which aniline oligomers are formed. This is a slow process [64,70]. It is believed that slow kinetics of formation of the oligomers facilitate the linear chain to acquire an equilibrium planar geometry to form nucleation centers for growing crystalline phases. The induction period is then followed by a relatively fast and exothermic chain propagation step. It is further noted that the addition of the oxidant is slowly maintained (drop by drop). The oxidative polymerization of aniline requires the presence of oxidants throughout the reaction at every step of addition of the aniline monomer units. The oxidant molecule actively participates in every redox interaction involved in the polymerization process, and hence large amounts of oxidant are required for the synthesis of polyaniline until the last molecule of aniline is added to the growing chain, and the chain termination takes place. The slow addition of the oxidant to the reaction medium is believed to ensure slow elongation of the linear chains of polyaniline. This might have facilitated the orientation of the growing chains to acquire the equilibrium planar geometries in subsequently large numbers to give rise to the formation of crystalline phases in the matrix. The salient features appearing at different steps in the process of synthesis of polyaniline involving mechanisms of the formation of nucleates or initiation trimers and elongation of the linear polyaniline (emeraldine) chain is illustrated in Figure 7.

The linear chain molecule is conducive towards the evolution of sufficient interactions between the unhybridized pπ orbitals (pπ–pπ orbital overlap), such that a large number of individually growing linear chains of polyaniline orients and organizes themselves along other neighboring linear chains to form a highly ordered and well-defined crystalline lattice, as seen in the high resolution transmission electron microscopic (HRTEM) images. There are also amorphous phases present in the matrix as apparent in the polymer matrix. It is explained that as long as aniline monomers are present in the reaction mixture, formation of new nucleates as well as propagation of linear chains of polyaniline occur simultaneously. Further, this may result in the formation of polyaniline chains of different lengths and orientations, which may not obtain the opportunity to orient in the same direction and may eventually, end up interweaving with one another to give rise to an amorphous phase. Several patches of randomly oriented interwoven polyaniline chains and perfectly grown crystalline lattices give rise to a semi-polycrystalline matrix. This means that both kinds of phases, viz., amorphous and crystalline, are present in the same matrix. The sequence of events probably taking place in the formation of crystalline phases in the polyaniline matrix in the course of chemical oxidative polymerization of anile is illustrated and explained in the images in Figure 8a–m.

The salt form of emeraldine hydrogen chloride contains localized/delocalized polarons and bipolarons in substantial proportions to form a polaron “conduction band” [56]. For a long and linear polyaniline chain, the molecular weight must be significantly higher (manifolds of a limited portion of the polymer are shown in the diagrams). This indicates that there exist numerous intermediate oxidation states ranging from leucoemeraldine to protoemeraldine to emeraldine to nigraniline to pernigraniline [56,71]. In this regard, the inner portions of the chains are believed to be chemically stable due to (a) resonance, and (b) geometrical stability due to the π-orbital overlap and the formation of the crystalline lattices. The polyaniline chains or portions of the long chains forming crystalline lattices are stable in their equilibrium geometry and are believed to resist frequent change in the conformation because of redox reactions. Hence, it is believed that the share of the long and linear polyaniline chains susceptible to undergoing a redox reaction are the terminal portions of the chain. In the present work, the polyaniline matrix exhibits embedded nanostructured crystalline phases in different orientations. Polyaniline chains formed during conventional synthesis due to faster kinetics at low temperatures are observed to form helical structures, which quickly undergo interweaving to form fiber- or spindle-shaped macro- and supramolecular morphologies. The share of the polyaniline chain lengths embedded deep inside the fibrillar morphologies remains unaffected by redox processes. This is attributed to the geometrical restrictions at the molecular level due to π-orbital overlap, as well as morphological restriction due to extensive H-bonding and van der Waals forces of attraction, which fortify the matrix of polymeric chains in amorphous phases along with the nanostructured polycrystalline crystalline patches into spindle shapes, as can be seen in the SEM image in Figure 9.

6.2. Role of Molecular Structure and Morphological Properties of Semi-Polycrystalline Polyaniline in Its Electrochemical Behavior

Polyaniline undergoes multiple oxidation stages ranging from leucoemeraldine (yellow), protoemeraldine (light green), emeraldine (green), nigraniline (blue), and pernigraniline (violet). The interconversion of different redox states of polyaniline takes place via insertion/extraction of electrons and protons [72]. The different oxidation states of polyaniline and effects of oxidation on the conformation of the molecule are displayed in Figure 10.

Cyclic voltammetry is an efficient electrochemical technique in which current responses are measured as a function of potential varying linearly with time. The potential corresponding to reduction or oxidation of electroactive species (conducting polymer in the present work) are easily located on a CV curve. Based on the peak shapes, peak heights, and peak separation, an electroactive material can be classified as capacitor-type or a battery-type. Furthermore, the CV curves also give information about the stability of the electroactive species. The CV curves in the present work display weak or diminished (almost flattened) peaks associated with the three redox processes as mentioned above. Protons, electrons and counter-ions play important roles in the redox reactions of polyaniline. In the present work, the electrolyte (or test solution) is aqueous Na₂SO₄, which is a neutral medium. The Na₂SO₄ undergoes dissociation to yield Na²⁺ and SO₄²⁻ ions in the aqueous medium. Further, the ions undergo solvation and formation of a hydration sphere. Formation of hydration sphere(s) increases the diameter of the partially charged hydration layer around the ion and hence retards the speed of the moving ionic species in the electrolyte. Hereby, the ease in the access of protons toward the active sites in the polymer chain becomes diminished. Consequently, the conformational change (physico-chemical process) due to redox reactions occurring at the electrode becomes slow. Moreover, the geometrical and morphological restrictions and resonance stabilization resists the conformational changes in the molecule. Therefore, the oxidation and reduction processes continue to occur across a wider range of potential, resulting in a wavy appearance in the CV profiles. The wavy feature disappears completely at a higher scan rate of 1.0 V s⁻¹ indicating the charging current completely outrunning the rate of the redox reactions (Figure 11a). Conversely, the reports on CV profiles recorded in acidic electrolytes exhibit sharp redox peaks with well-defined and theoretically supported features [17,26,72,73]. A GCD curve recorded at 10 mA showing a dominance of linear variation of potential with elapsed time is shown in Figure 11b. During the charging process, the curve deviates from a linear slope for ~24.3% of the total charging time, and during the discharge process, the curve deviates from the linear slope for only 10% of the time. The remaining 90% of the time, the discharge curve shows linear variation of potential with times characteristic of a capacitive mode of discharge behavior. The feature suggests a dominance of a capacitive charge storage mechanism in the material. The deviation from the linear slope is attributed to pseudocapacitance arising from surface–redox reactions.

6.3. Determination of ‘b’ Values

The CV curves in the present work exhibit minor or tiny redox peaks in the cyclic voltammogram curves. The curves appear somewhat flattened rather than displaying sharp redox peaks. This is due to charging current outrunning the redox reactions. The specific electrochemical features exhibited by an electroactive material are analyzed from (a) response to scan rates in the CV; (b) response to a constant current in GCD curves; (c) response to alternating current in the electrochemical impedance measurements. The CV measurements at different scan rates, and the current responses offer a useful tool in differentiating quantitatively, the diffusion-controlled process and non-diffusion limited process [1,3,45,74,75]. Assuming that the total charges stored in an electroactive material are a sum of redox reactions (diffusion/intercalation of ionic species and conformational change in the molecule) and capacitive process (electrical double layer charge storage and pseudocapacitance). At a particular potential, the current (i) and scan rate (υ) hold the following relation (Power law) as below [2,3,37].

i(V) = aυ^b

(5)

The value of ‘b’ can tentatively predict the predominating mechanism of charge storage, whether it is intercalation mechanism or capacitive, and whether it is a pseudocapacitive mechanism. The b value is determined from the slope of the plot of log (i) vs. log (v). In this regard, the parameter ‘b’ has two distinctly well-defined conditions, indicated by its value, viz., b = 1.0 and b = 0.5. The ‘b’ value of 1.0 indicates a dominance of charge contribution from fast surface-controlled redox reactions accompanied with adsorption/desorption of the electrolyte ions. The current in this case is directly proportional to the scan rate (or, $i\propto \nu$). Conversely, the ‘b’ value of 0.5 indicates a diffusion-controlled redox process, and the current in this case is directly proportional to the square root of the scan rate (or, $i\propto {\upsilon }^{1/2}$) and is expressed in the following equation:

$$i=nFA{C}^{\times }{D}^{\frac{1}{2} }{\upsilon }^{\frac{1}{2}} {\left(\alpha nF/RT\right)}^{1/2}) {\pi }^{1/2} \chi \left(bt\right)$$

(6)

Here, C*, α, D, n, A, F, R, and T are the surface concentration of the electrode material, transfer coefficient, chemical diffusion coefficient, number of electrons involved in the electrochemical reaction, electrode material surface area, Faraday constant, universal molar gas constant and temperature in Kelvin, respectively. The function (bt) is a normalized current. The determination of the ‘b’ value using the “Power law” helps in identifying the type of electroactive material under investigation; either, it is a capacitor-type or a battery-type. The value ‘b’, ranging between 0.5 and 1.0, indicates a transition from the battery-type behavior to the capacitive behavior [3]. The pseudocapacitance in the present work is attributed to ion-intercalation-led redox reactions in the polyaniline chains. The ‘b’ values, determined by using the Power law, are shown in Figure 12a. The variation of the ‘b’ values ranging between 0.5 and 1.0 indicates a mixed charge storage behavior originating from both processes, viz., capacitive as well as diffusion-controlled ion-intercalation processes. Furthermore, it is possible to determine quantitatively the exact share of either of the two processes (double-layer charge storage with pseudocapacitance and diffusion-controlled/ion-intercalation process) in the entire charge storage process by using the following equation:

$$i\left(V\right)=i_{capacitive}+i_{diffusion-controlled/ion-intercalation}$$

(7)

$$\mathrm{or}i\left(V\right)=k_1\upsilon +k_2^{1/2}\upsilon$$

(8)

$$\mathrm{or}\frac{i\left(V\right)}{{\upsilon }^{1/2}}=k_1{\upsilon }^{1/2}+k_2$$

(9)

The values of ‘k₁’ and ‘k₂’ at a particular potential indicate the share of either of the two charge storage mechanisms by yielding the current values originating from capacitive and diffusion-controlled charge storage mechanisms. The slope k₁ and the y-axis intercept k₂ values are obtained from the plot of i/υ^1/2 vs. υ^1/2 (Figure 12b). After obtaining the k₁ and k₂ values, the capacitive (k₁υ) and diffusion-controlled/ion-intercalation current (k₂υ^1/2) values can be computed straightforwardly. The variation of capacitive current and diffusion/ion-intercalation current with potential is displayed in Figure 12c. It appears that the intercalation of ions led by the diffusion process dominates over the capacitive charge storage process at higher potentials. The variation of capacitive and diffusion-controlled ion-intercalation currents with scan rates is displayed in Figure 12d–e. The pattern of the curves shows that the ratio of the capacitive and intercalation current increases with an increase in the scan rates. At a higher scan rate, the capacitive current or the charging current dominates over the intercalation current. This is in accordance with the observation presented in Figure 11. It is emphasized that all the electrochemical signature characteristics should be presented in a broader range of scan rates and current densities to examine a better fit of the power law. In addition, it is important to record the CV curves at relatively slow scan rates (1–5 mV s⁻¹) to determine ‘b’ values using Dunn’s method because the CV curves recorded at a faster scan rate exhibit a sizable shift of redox peaks [2,3]. In this context, a detailed work regarding appropriate electrochemical measurements with in-depth explanation will appear in our upcoming communications. Although, the determination of ‘b’ values help in understanding the kinetics analysis and charge storage mechanisms, but still, it cannot be solely relied upon to judge the nature of the electroactive material to whether it is a capacitor, pseudocapacitor, or a battery material. The entire course of analysis and discussion must consider the basic electrochemical characteristic features first. In addition, systematic analysis of the electrochemical impedance spectroscopy must also be utilized to support the claims [1,2,3].

6.4. Electrochemical Impedance Spectrum Analysis

The impedance response is essentially determined by the resistive and capacitive properties of the charged layer formation on the surface of the electroactive material during measurement. For the ac impedance measurements in the present work, we report a formation of two capacitive loops or semicircles in the Nyquist plot. As apparent in the Figure 5a inset, the complex plane shows two semicircles, viz., a large semicircular arc in the low-frequency region and a small semi-circular arc in the high-frequency region. The shapes of the two semicircular loops arise due to the combined effects of structure and properties of the charged layers formed on the surface of the electroactive surface under investigation. The semicircles in a Nyquist plot of a pseudocapacitor material appear due to several factors, viz., (a) The presence of functional groups or dopants at the surface of the carbon materials that give rise to charge transfer events and chemisorption of ionic species (in the present case, it is believed to be a partial charge transfer during adsorption of (H⁺–A⁻)/(H⁺–A⁻) on the polyaniline surface); (b) Faradaic reactions (redox reactions, that are non-reversible) occurring in a pseudocapacitor material (in the present case, the redox reactions are highly reversible and technically non-Faradaic according to Conway definition); and (c) Interfacial impedance occurring at the current collector/active material interface [38]. The measured capacitive response in the present investigation was not found to be an ideal capacitor as indicated by depressed semicircles (i.e., the centers of the circles lying below the axis); therefore, the deviation from the ideal behavior or ideal capacitance element (C_dl, or double layer capacitance) is explained by incorporating a constant phase element (CPE). The constant phase element (CPE) is a non-intuitive circuit element employed in the electrical equivalent circuit to explain the response of a real system, which deviates from ideal system. The ideal system is represented by an arc of a semicircle in the Nyquist plot, with its center on the x-axis, whereas in the real system, the center of the semicircle lies below the x-axis and the semi-circle is termed as a depressed semicircle. This phenomenon depends upon the nature of the material surface and the electrolyte in the system. It is closely related to some inhomogeneous properties, e.g., surface roughness or surface inhomogeneity, or distribution of the values of some physical property of the system, e.g., reaction rates of the redox reactions or distribution of active sites with varying activation energies in a polycrystalline material.

Mathematically, CPE is defined as Z_CPE = A⁻¹ (iω)⁻ⁿ. Here, Z_CPE is the CPE’s impedance and A is the CPE parameter or pre-factor. The symbol A is defined as admittance (1/|Z|) at ω rad s⁻¹ (unit Ω⁻¹ cm⁻² sⁿ). The symbol i denotes the imaginary number (i = $\sqrt{-1}$), and ω is the angular frequency. Utilizing this simple equation in the impedance measurement allows the phase angle of the Z_CPE to be independent of the frequency, and it has a value of (−n × π/2) degrees. This is the reason why CPE is denoted by this name. The factor n in the equation is known as the inhomogeneity coefficient. The inhomogeneity arises due to the porous surface of the electroactive material. The value of n = 1.0 is indicative of an ideal capacitive behavior, whereas a deviation in n values from unity (i.e., lower than unity) is indicative of a deviation from ideal capacitive behavior, and we incorporate CPE. In the present study, the two n values obtained from the calculation are 0.80 and 0.82, corresponding to the two CPEs in the circuit, i.e., CPE₁ and CPE₂, respectively, which are close to unity and suggest a dominance of capacitive behavior of the material and less deviation from an ideal capacitor. The CPE can be determined from the slope of the linear part of the Bode plot, and its magnitude deviates from unity toward lower values with increasing surface roughness. The electrical equivalent circuit and the calculated circuit elements from the impedance measurements, i.e., R_s (resistance from the cell component solution and the electrode), R₁ and R₂ (corresponding to redox and charge transfer processes), CPE₁ and CPE₂ (constant phase elements corresponding to double-layer capacitance and pseudocapacitance), are shown in Figure 5c. The evaluation of the circuit elements was carried out by fitting the measured data using a simulation. The R_s comes from the total internal resistance of the cell and is an additive effect from the cell components and the electrolyte as the material surface continuously bathes in the electrolyte solution. The solution resistance of the system is governed by the conductivity of the electrolyte material used in the electrochemical experiments, and this includes the resistance of the solution existing inside the porous/layered structure of the material. The arrangement of the circuit elements in the circuit and the overall behavior mimics the actual interface behavior and explains the current and potential responses to the modulation of potential and current, respectively. The capacitive behavior or the double layer capacitance is represented by CPE₁ in the electrical equivalent circuit. The pseudocapacitance in the circuit is represented by the combination of CPE₂, R₁ and R₂ in parallel with CPE₁. Here, R₁ represents non-capacitive resistance associated with redox reactions, and R₂ represents non-capacitive resistance for discharge or desorption of ad-species. In general, in the cases where the redox reactions are truly Faradaic reactions, the R₁ and R₂ are termed as Faradaic resistances, i.e., R₁ represents Faradaic resistance associated with redox reactions that are irreversible, and R₂ represents Faradaic resistance for discharge (desorption of an adsorbed species) [32,33]. Detailed descriptions, usual meanings of the different elements of impedance measurement and the method of data fitting are reported elsewhere [32,76,77,78]. A critical role in developing supercapacitor electrodes is to have real-life applications characterized by rapid power delivery capability and long cyclic performance. As per our result concerns, we try to estimate the material’s response time (τ) from the Bode phase angle diagram, also known as the merit of a supercapacitor. As depicted in Figure 5b, we calculated the frequency (f) value at which the phase angle approaches −45°. Otherwise stated, it is the characteristic point where both real and imaginary impedance equals each other. In our case, the τ is found to be 5 s, certifying that the polyaniline-based supercapacitor electrode can handle and deliver ultrahigh-rate power.

6.5. Application of Molecular Orbital Theory for Explaining the Charge Storage Mechanism Semi-Polycrystalline Polyaniline

The polyaniline material discussed in this article is composed of highly ordered poly-conjugated linear chain molecules possessing an extended π-electron cloud above and below the plane of the chain. Depending upon the uniformity of the conjugation, the conducting polymer may behave similar to a conductor or a semiconductor or as an insulator. The conductivity as well as the charge storage properties of the polyaniline can be explained using the molecular orbital theory (Hückel’s molecular orbital theory) and band theory as a function of application of quantum chemical theory of atomic structures. The atomic orbitals overlap to form an equivalent number of molecular orbitals similar to those of individual atomic orbitals. These molecular orbitals are bonding molecular orbitals and antibonding molecular orbitals, and the electrons on the individual atomic orbitals are distributed to these molecular orbitals. The bonding molecular orbitals possess lower energy states, whereas the antibonding molecular orbitals possess higher energy states. Depending upon the energies, the orbitals in bonding and antibonding molecular orbitals are further classified into highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). According to the quantum chemical theory, the electrons in an atom can only be accommodated in well-defined energy states, and for isolated atoms, these energy states are sharp [79,80]. In a crystalline solid or a metal, the atoms sit in close vicinity to one another. In addition, they are chemically bonded such that an electron on a particular atom sees the electric field due to the presence of electrons on other atoms. Furthermore, the nature of the chemical bonds existing between the neighboring atoms implies that the electrons are on other atoms. Furthermore, the nature of the chemical bonds existing between the neighboring atom implies that the electrons can exchange their position with other atoms, and this results in a broadening of sharp atomic energy states into “energy bands” in the solid [79,81]. The molecular orbital diagram for π-orbitals in aniline and the formation of bonding and antibonding orbitals has been illustrated in Figure 13. There are six π-electrons from six carbon atoms in the aromatic ring, and the lone pair of electrons present in the nitrogen atom participates in resonance, collectively involving eight electrons in resonance. Therefore, seven p-orbitals combine to form seven molecular orbitals in which Ψ₁, Ψ₂, Ψ₃ are bonding molecular orbitals and Ψ*₄, Ψ*₅, and Ψ*₆ are antibonding molecular orbitals. There is also formation of a nonbonding molecular orbital as shown in Figure 13.

The eight electrons are occupied in three bonding and one non-bonding molecular orbitals, whereas the anti-bonding molecular remains vacant. If a large number of atomic orbitals possessing equivalent energies are there, say “N” number of interacting atomic orbitals, those will construct “N” number of molecular orbitals. In the case of metals and highly ordered crystalline solids and conducting polymers, “N” will be a large number ~ 10²² for a 1 cm³ material matrix, and such a huge number of molecular orbitals with their energy states spaced close together construct an apparently continuous band of energies, viz., a valence band and conduction band [82]. The HOMOs associate collectively to construct a “valence band” and the LUMOs associate collectively to construct a “conduction band”. The difference in the energy between these two bands is called the “band gap” (E_g) and the band gap is the range of energy that is not available to the electrons present in the material. The energy level of the electrons in a system reaching to absolute zero is called Fermi level (F_g) [79,81,83]. For a material to behave as a conductor, the two bands, viz., valence band and the conduction band, must overlap, which allows for delocalization and fast transition of the electrons. In the present work, the presence of highly ordered crystalline phases in the polyaniline matrix facilitates the overlapping of valence and conduction bands. The systematic and ordered structure of the individual poly-conjugated chains of polyaniline facilitates the delocalization of electrons due to resonance.

The conductivity as well as the charge storage behavior of semi-polycrystalline polyaniline material, in which the individual polymeric chains are long and linear highly ordered poly-conjugated molecules supercoiled in a supramolecular architecture giving rise to spindle shape morphology, can be quantitatively explained by using a simple free-electron molecular orbital model. The model involves a minimum number of elements essential for quantitatively explaining the behavior of a conducting material as well as a capacitor material for charge storage. Assuming a row of a large number of “N” atoms present in a long chain of polymer separated by a distance d, the total length of the polymer chain will be (N − 1)d. Therefore, for a large number of atoms, it will be Nd. Using the concepts from the quantum mechanical model for a free-particle in a 1D box (potential zero inside the box and infinity outside the box), the wave-function corresponding to a ladder of eigenvalues can be represented as

E_n = n² h²/8 m (Nd)²

(10)

where, ‘n’ is a quantum number and the value of n = 1,2,3, and so on; ‘h’ is Plank’s constant, and ‘m’ is the mass of the electron. Upon filling the π-electrons present in the polyaniline chain from a large number of p-orbitals (i.e., N number of p-orbitals) into the hierarchical ladder of molecular orbitals with consideration of Pauli’s exclusion principle that there cannot be two individual fermions in one quantum state, the energy of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) can be defined in the following equations (Nobel prize);

E (HOMO) = (N/2)² h²/8 m (Nd)²

(11)

E (LUMO) = (N/2 + 1)² h²/8 m (Nd)²

(12)

and, the electron will have to overcome the energy barrier ΔE, defined by:

ΔE = E (LUMO) − E (HOMO) = (N + 1) h²/8 m (Nd)²

(13)

ΔE ≈ (h²/8 md²)/N (for a large number of atoms N)

(14)

This shows that the band gap between the valence band (formed from the collective association of a large number of HOMOs) and the conduction band (formed from the collective association of a large number of LUMOs) will decrease by a magnitude of 1/N with increasing polymer chain length. The energy gap will eventually vanish for macroscopic bulk dimensions. This is further supported by Hückel’s MO theory, which explains the delocalization of π-electrons over the entire polyaniline chain and vanishingly small band gap for a substantially long chain. This condition originates due to the character of π-molecular orbitals in addition to the individual p-orbitals of all the carbon atoms present in the aromatic rings as well as the heteroatom in the poly-conjugated chain. The distribution of electron density spaces out evenly throughout the entire length of the polyaniline chain. The energy bands now no longer belong to any single atom but rather associate with the entire material, and the electron density is contributed by all the filled molecular orbitals and the bonds are all therefore predicted to be equal. As apparent from the molecular orbital diagram, the antibonding molecular orbitals remain vacant. Therefore, the orbitals in the conduction band have a huge capacity to hold electrons if it is supplied with a surplus of electrons. The accommodation of electrons in the conduction band is further facilitated by the overlapping condition of a valence band and conduction band. Hence, if extra electrons are introduced to the molecule, e.g., during the charging process in galvanostatic charge and discharge experiments, then these vacant antibonding molecular orbitals spaced over the entire conduction band are capable of accommodating the additional supply of electrons. This explains the increasing trend of specific capacitance with an increase in the current density (Figure 4b) [32,82].

The presence of crystalline phases in the polyaniline matrix enables enhanced electrochemical behavior and improved surface features conducive to a structural build-up of a double layer on its surface. The electrochemical behavior exhibits a dominance of capacitive behavior. The synthesized polymeric material is a semi-polycrystalline polyaniline, which is a conducting polymer. The crystalline lattices in the form of scattered-and/or-associated poly-crystalline zones (as apparent from the TEM images) over the polymer surfaces create sites for double-layer formation conducive of ideal capacitor requirements. This not only adds an advantage over the conventional pseudocapacitive behavior of conventionally synthesized polyaniline but also provides cyclic stability during the charge–discharge tests [3,26,28,32]. The overall events occurring at the interface and mechanism of charge storage is illustrated in Figure 14. Figure 14a shows the 2D structure of a solvated cation in aqueous medium. Na₂SO₄ dissociates completely in water to yield Na⁺ and SO₄²⁻ ions. The cations and anions thus formed are immediately surrounded by water molecules to form several layers of hydration spheres. In the schematic, two hydration spheres, primary and secondary, are shown. The oxygen and the hydrogens atoms of the water molecule become partially charged. During the charging process, the charged species move toward the electrode–electrolyte interface along with the hydration spheres as a combined structure. Figure 14b shows the formation of a double layer at the interface of the crystalline phases of the semi-polycrystalline matrix in the electrolyte. Figure 14c explains the events occurring at the amorphous phase of the polyaniline matrix. The terminal ends of the chains are the active sites for the redox reactions. The redox reactions involve the role of electrons, which move from the electrode toward the active sites of the molecule, and solvated protons, which approach as a combined structure as (H⁺–A⁻)/(H⁺–C⁺). In an acidic solution, the protons do not exist alone and combine with water molecules to form hydronium ions (H₃O⁺). Figure 14d explains the events occurring at the terminal portions of the polyaniline molecule during the redox reactions.

The pseudocapacitance in the polyaniline matrix arises due to reversible surface quinonoid reactions and partial charge transfer during electrosorption of (H⁺–A⁻)/(H⁺–C⁺) species at the N-sites of the polymer chains, resulting in oxidation of the material and change in the conformation of the molecule at the respective sites. There is no phase change of the material upon redox reaction, and the process is completely reversible. Hence, these cannot be termed as Faradaic reactions. Moreover, the redox reactions occurring in the polyaniline molecule is largely restricted at the surface of the material, as the inner portions remain dormant against conformational changes because of redox reaction. In addition, the redox reactions are highly reversible. Therefore, the pseudocapacitance observed in the electrochemical behavior is technically a non-Faradaic reaction and is termed as surface-redox pseudocapacitance [32,38].

7. Conclusions

We synthesized novel semi-polycrystalline polyaniline using a modified surfactant-free chemical route. The microstructural and phase analysis of the material exhibits well-defined properties of a crystalline material. The material shows enhanced electrochemical properties as well. The cyclic voltammograms show almost rectangular curves characteristic of capacitive behavior with a slight mix of deformation indicative of pseudocapacitive features. The specific capacitance calculated for the material is 551 F g⁻¹ at the scan rate of 10 mV s⁻¹. The life span and function of the polyaniline-based supercapacitor electrode is a crucial parameter, which gauges the long-term application in electronic gadgets. Here, the cycle stability and the coulombic efficiency are measured at a high current density of 12 A g⁻¹, exhibiting good stability (90.3% and 99.5%, respectively) over 3000 cycles. Hence, the overall results demonstrate the practicality of the developed electrode for the future supercapacitor device to power the electronics industry.