Preparation and Characterization of Polyaniline-Dodecyl Benzene Sulfonic Acid-Cadmium Oxide (PANI-DBSA-CdO) Composites: As an Electrode Material for Energy Storage Applications

doi:10.21203/rs.3.rs-4963897/v1

Earlier we have reported PANI-CdO composite electrodes which exhibited excellent supercapacitive performance at lower concentration of dopant (CdO) but failed to perform at higher concentrations. It could be considered that the limited conductivity of PANI-CdO composites at higher concentrations of dopant CdO might be associated with its poor solubility and lower dispersion into the PANI matrix. Therefore, a surfactant like dodecyl benzene sulfonic acid (DBSA) has introduced for the protonation to make long chain PANI-CdO composites that might help to enhance their electrochemical performance. Therefore, PANI-DBSA, and PANI-DBSA-CdO nanocomposites with cadmium oxide (CdO) as a dopant have been synthesized using In-situ chemical polymerization route and characterized to be used as an electrode material for supercapacitive applications. The physical and chemical properties have investigated through XRD, FTIR, and SEM analysis. The electrical properties have estimated from the obtained IV curves while electrochemical properties are tested through (CV, GCD, and EIS-develop) analysis. The cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) results of PANI-DBSA-CdO composites don’t exhibit any progress at higher concentrations of dopant CdO. However, PANI-DBSA composite electrode has exhibited a greater specific capacity of 569 F g^− 1, with maximum energy density of 12.3 Wh kg^− 1, and a higher power density of 812 W kg^− 1 at 8.33 A g^− 1 current density as compared to rest of all the samples. It also retained more than 85% of its initial capacity after the 20,000th charge–discharge cycle. Highly achieved capacity, excellent reversibility, large (energy and power) densities, and lower (ohmic, charge transfer & diffusion) resistance, suggesting PANI-DBSA composite to be a useful supercapacitive electrode material.

Polyaniline

Dodecyl Benzene Sulfonic Acid

Current Density

Power Density

Energy Density

Conducting Polymers (CPs) have extensively studied pseudocapacitive materials due to their ease of processing, low environmental mitigation, cost-effectiveness, broad potential window, high capacitance, high extrinsic conductivity, and chemically tunable redox properties. CPs can store the charge not only at the interface between the electrode and electrolyte but also in the bulk, as no phase/structural changes are involved during the charge/discharge process. Therefore, CPs possess high capacitive values, high conductivity, and low ESR values as compared to carbon-based SC materials, thereby owing to their redox storage characteristics and high surface area [1]. However, the electrode stability is limited due to mechanical stresses developed during the charge/discharge cycles of the redox reaction [2]. As the ions diffuse into the bulk, therefore slow kinetics affect the power densities, which are the main disadvantages of CPs. The CPs commonly include (PANI) polyaniline, (PTh) polythiophene, (PPy) polypyrrole, and their derivatives [3].

Polyaniline (PANI) is an eminent polymer electrode for its ease of synthesis, high conductivity, mechanical stability, large storage capacity, low cost, and environmental stability. However, the successive charging/discharging causes the degradation of PANI electrode performance practically. Researchers are trying to synthesize PANI-based composite electrodes to improve their electrochemical properties. PANI has a flexible structure resulting in better casting between two components at the nanoscale. This can be accompanied by a better synergic effect in the nanocomposite architecture. Therefore, PANI has been mixed with carbon materials and metal oxides/hydroxides [4].

The EDLCs have greater storage capability as highly porous materials such as activated carbon, graphene, and CNTs have been used, which increase the electrolyte accessibility due to their relatively high specific surface area [5]. The EDLCs possess good electrical properties due to large charge deposition at the electrode surface, good mechanical strength, large specific area, and excellent cyclic stability. Both the stability and the conductivity of activated carbon materials compromised with the increasing surface area as the material became highly porous [6].

It is considered that the high electrical conductivity and the suitable morphology of TMOs significantly impact on their electrochemical performances. Moreover, TMOs exist in two or more oxidation states are considered more suitable for electrochemical supercapacitor application. Owing to their outstanding electrical & electrochemical responses and cost-effectiveness, TMOs can be used in the preparation of advanced HECs for green energy systems. TMOs have oxygen vacancies on their surfaces that can help to enhance their surface reactivity [7]. Ruthenium oxide (RuO₂) was the first material to show pseudocapacitive behavior and possessed high specific capacity of 720 F g^− 1 and excellent cyclic retention, but it was too expansive to be commercially viable [8, 9]. In recent years, various TMO-based prepared SC electrode materials exhibited different specific capacitances such as MnO₂ [10], Fe₂O₃ [11], V₂O₅ [12], CoOx [10], NiOx [12], and so on. Among the various TMOs, CdO is a promising candidate for SC electrode material due to its pseudocapacitive characteristics, high electrical conductivity, high capacity, large surface area, ease of availability, thermal & chemical stability, two oxidation states, and low cost [13]. Cadmium Oxide has achieved high conductivity by the virtue of shallow donors, generated by intrinsic cadmium atom interstitials and the oxygen vacancies. The intrinsic dopability of CdO accompanied by tremendous Hall mobility results in excellent electrical conductivity. Thus, cadmium oxide (CdO) and its composites can be an effective low-cost SC electrode material [14].

Earlier, we have reported the synthesis and electrochemical performance of PANI-CdO composites which have exhibited remarkable electrochemical properties up to 5% concentration of CdO dopant and recommended to be used as an electrode material for supercapacitor applications [15]. However, the electrochemical properties of PANI-CdO nanocomposites are observed to be suppressed heavily under the higher (10 & 20) % concentration of CdO dopant. It could be considered that the limited conductivity of PANI-CdO composites at higher concentrations of dopant CdO may be associated with the poor solubility and the low dispersion of CdO nanoparticles into the PANI matrix. Therefore, a surfactant like dodecyl benzene sulfonic acid (DBSA) has introduced for the protonation to make long chain PANI-CdO composites. The surfactant is considered to provide some extra charge carriers that might result in an enhanced processability and more uniform dispersion of CdO nanoparticles into the PANI matrix.

In this work, PANI-DBSA and PANI-DBSA-CdO composites have been synthesized using an In-situ polymerization process, whereas their structural, morphological, and compound identification studies have investigated through XRD, SEM, and FTIR analysis, electrical properties through IV curves, and electrochemical properties tested through (CV, GCD, and EIS-develop) analysis.

2.1. Material Resources

All the materials of analytical grade were taken and consumed without any further purification except Aniline. Aniline (monomer) liquid was taken from Raidel-de-Haen and was further purified to remove the traces of polyaniline through a vacuum distillation process under reduced pressure until it became colorless. Cadmium Oxide powder was taken from chemical reagents (UNI-CHϵM), Ammonium persulfate (APS) powder was taken from Duksan Reagents, whereas, Hydrocloric acid (HCl) liquid was provided by Sacharlu.

2.2. Polyaniline (PANI) Synthesis

Polyaniline (PANI) was synthesized using In-situ polymerization method at room temperature in an open atmosphere. Aniline (monomer) to APS (oxidant) molar ratio was kept at 2:1. For this, 6.125 g APS (oxidant) and 5 ml aniline (monomer) were poured in 150 ml de-ionizing water and stirred magnetically in the open atmosphere for 10 min and then 5 ml HCl was poured dropwise into this solution for protonation. The pH of the solution was strictly maintained at 1 and stirred vigorously for an hour. The solution was left free in the open atmosphere for 12 hours to complete the polymerization process. The PANI solution was turned into greenish-black, which was then filtered and washed thrice with distilled water and once with methanol to remove HCl. Finally, the obtained paste like PANI solution was dried and minced to form powder [16].

2.3. Polyaniline-Dodecyl benzene Sulfonic Acid (PANI-DBSA) Synthesis Process

Aniline (monomer) 5 ml was poured into 100 ml de-ionizing water and stirred magnetically in an open atmosphere at room temperature. The aqueous DBSA (8.76 ml) was slowly poured into the aniline solution under continuous magnetic stirring. The APS (oxidant) 6.125 g was dissolved in 50 ml de-ionizing water in another beaker and poured into the Aniline/DBSA solution by keeping the Aniline/DBSA/APS molar ratio 2:1:1. After this, 5 ml HCl was poured dropwise into this solution by keeping its pH equal to 1 and stirred magnetically for an hour. Finally, the obtained greenish-black solution was placed in an open atmosphere for 12 hours to complete the polymerization process. The obtained Paste-like PANI-DBSA solution was filtered and washed with distilled water & acetone. Which was then dried at 70 ^oC in an oven and minced to form powder [17].

2.4. Polyaniline-Dodecyl Benzene Sulfonic Acid-Cadmium Oxide (PANI-DBSA-CdO) Nanocomposite Synthesis Process

PANI-DBSA-CdO composites were also synthesized with (10, & 20) weight percentages of CdO dopant using In-situ polymerization route. The monomer/DBSA/Oxidant molar ratio was kept at 2:1:1 and CdO was varied by (10 & 20) weight percent of PANI. For this, 5 ml Aniline (monomer), 8.76 ml DBSA, 6.125 g APS, and (0.689, & 1.378) g CdO dopant were poured into 200 ml de-ionizing water and stirred magnetically in an open atmosphere at room temperature. After this, 5 ml HCl was poured dropwise into this solution by keeping its pH equal to 1 and stirred further for an hour. Finally, the obtained greenish-black solution was kept in an open atmosphere for 12 hours to complete the polymerization process. The Paste-like (PANI-DBSA-CdO) solution was filtered and washed with distilled water and ethanol to remove HCl and other impurities (such as byproducts related to doping, oxidant traces, or oligomers of aniline). Which was then dried at 70 ^oC in an oven and minced to form powder [18, 19].

3.1. Physical Characterization

The surface morphology of prepared materials was studied using MAIA 3 TESCAN (HV 20.0 kV, SEM MAG: 5.00 kx, View Field: 41.5 µm) SEM. The structural parameters were estimated using Bruker X-ray diffractometer (D8, with specifications CuKα: λ = 1.54 Å, 2θ = 10^o-90^o, scan rate = 1^o/min: operating voltage/current = 40 kV/35 mA). For molecular structural identification and functional group analysis, the KBr pellets were scanned in (500–4000) cm^− 1 wavenumber using Bruker Tensor 27 FTIR spectrometer at room temperature. The electrical response of the prepared materials was estimated using (Keithley-2400) electrometer. The temperature was monitored and controlled using a FENWAL-AR44L1 digital bimetallic thermometer.

3.2. Electrometrical testing

The electrochemical parameters were estimated using an Auto-lab instrument (Model: CHI760E). A three-electrode cell {(comprised of Ag/AgCl reference electrode (RE), Pt counter electrode (CE), and the prepared materials working electrodes (WEs)} was used. Whereas, KOH (3.0 M concentration) was used as an electrolyte. The Ni-foam-based WEs were prepared by loading 1.2 mg of synthesized materials and dried in a vacuum oven at 70 ^oC. The CV measurements were carried out in the potential range (-0.6 to + 0.4) V at (5, 10, 20, 50, and 100) mVs^− 1 scan rate. The GCD test was carried out at current densities (8.33; 12.5; 16.7; 20.8; 25.0; 29.2; 33.3; and 41.7) Ag^− 1. The EIS measurements were carried out at potential (0.7 V) with amplitude (0.005 V_rms) whereas the frequency ranges from (10⁰ to 10⁵) Hz.

4.1. Structural Analysis

Two typical diffraction peaks at angle 2θ = 17.9^o (d = 4.9 Å), and 22.5^o (d = 3.9 Å) are observed in the XRD pattern of pure PANI, confirming its semi-crystalline nature due to repetition of benzenoid and quinoid rings in PANI chains [20]. PANI-DBSA composite XRD displayed two distinguish peaks at angle 2θ = 16.28^o (d = 5.44 Å) due to parallel periodicity to PANI chains, and at 2θ = 21.98^o (d = 4.04 Å) due to perpendicular periodicity to PANI chains [21]. XRD pattern of pristine CdO showed the characteristic peaks at angle 2θ = 32.99^o, 38.28^o, 55.25^o, 65.87^o, and 69.28^o corresponding to$\:\:\text{h}\text{k}\text{l}$ (111), (200), (220), (311), and (222) respectively [22]. The appeared sharp and narrow peaks describe the crystalline nature and the agglomeration of CdO nanoparticles. The observed reflection peaks of CdO are matched well with the reference patterns of (JCPDS file:01-073-2245, phase = Cubic (FCC), space group = Fm-3m, Space group no.225) with monoclinic structure.

XRD images of PANI-DBSA–CdO composites reflect the broad PANI peaks between angles 2θ = 15^o–25^o and all the diffraction peaks of pristine CdO. The increasing concentration of CdO content in the hybrid composites, makes CdO peaks more prominent thereby increasing their intensity and suppressing the amorphous phase of PANI peaks. A slight decrease in the lattice parameters creates lattice compression which leads to minute drift of XRD peaks towards higher angle 2θ values. Moreover, PANI-DBSA-CdO composites have a higher degree of crystallinity as compared to pure PANI and PANI-DBSA composites. This may be attributed to the ordered growth of PANI chains on the larger surface area provided by the CdO nanoparticles. The appearance of additional small peaks in the PANI-DBSA-20%CdO composite may be due to the regular re-arrangement of PANI chains in more ordered fashion in the composite [23]. The lattice constant $\:{\prime\:}\text{a}{\prime\:}$ is calculated using the relation, $\:\text{a}={\text{d}}_{\text{h}\text{k}\text{l}}\sqrt{{\text{h}}^{2}+{\text{k}}^{2}+{\text{l}}^{2}}$ for cubic structures [24], and d_hkl (d-spacing) using Bragg’s equation [25]. Whereas $\:\text{h}\text{k}\text{l}{\prime\:}\text{s}$ are the corresponding Miller indices as shown in Table 1. The percentage crystallinity is estimated using the XRD (WAXS) formula [26].

Table 1

Microstructural parameter of pure and synthesized samples.
Material	$\:\mathbf{a}=\mathbf{b}=\mathbf{c}$ (Å)	$\:\mathbf{h}\mathbf{k}\mathbf{l}$’s	d-spacing (Å) Calculated	Percentage Crystallinity
Pristine CdO	4.69	111 200 220 311 222 400	2.71 2.35 1.66 1.42 1.36 1.17	98.5
PANI-DBSA-10%CdO	4.68	111 200 220 311 222 400	2.71 2.33 1.64 1.39 1.32 1.13	75.05
PANI-DBSA-20%CdO	4.68	111 200 220 311 222 400	2.71 2.33 1.64 1.39 1.32 1.13	95.54

Table 2

Average crystallite size, lattice strain, and dislocation density are estimated from the Debye-Scherrer, Williamson-Hall, and Size-Strain plot methods.
Material	Average particle size (nm)				Lattice strain ε × 10^− 3		Dislocation Density δ × 10^− 3(nm ^− 2)
	D-S	S-P	W–H	SSP	W–H	SSP
Pure PANI	0.50	-	-	-	-	-	-
PANI-DBSA	0.32	-	-	-	-	-	108.6
Pristine CdO	28.9	28.5	29	28	0.6	0.8	1.7
PANI-DBSA-10%CdO	31	33	45	36	5.8	2	1.3
PANI-DBSA-20%CdO	34	36	43	47	6.3	3	5.3

Furthermore, four different XRD data methods have deployed to estimate the particle sizes namely (D-S) Debye- Scherrer formula [27], (S-P) Scherrer-plot [28], (W-H) Williamson-Hall plot [29], and (SSP) Size-Strain plot [27], also the results have mentioned in Table 2 above. The estimated results depicted a decrease in particle size of the synthesized composites as compared to pristine CdO, and an increase in their lattice strain and dislocation density. The increasing trend of lattice strain and the dislocation density of PANI-DBSA-CdO composites may be attributed to an improved surface-to-volume ratio [23].

4.2. Functional Group Analysis

FTIR spectra of the synthesized electrode materials are investigated in the range (500 to 4000) cm^− 1 wavenumber as shown in Fig. 3. C = C stretching vibrations are appeared at 1534 cm^− 1 and 1456 cm^− 1 owing to aromatic (quinoid and benzenoid) units, C-N stretching vibrations are observed at 1295 cm^− 1, and NH–Q–NH bonds are appeared at 1194 cm^− 1 in pure PANI [30].

PANI-DBSA IR bands are witnessed at wavenumber 2918 cm^− 1 and 2845 cm^− 1 that may be attributed to –CH₂ or –CH₃ stretching mode of vibrations, whereas the presence of C = C stretching bands (1534 cm^− 1 and 1456 cm^− 1) may be assigned to (benzenoid & quinoid) units that also confirmed the nanocomposite formation [31]. C-H in-plane IR peaks are observed at 1136 cm^− 1 whereas the R-SO₃⁻ associated IR bands are seen at 1042 cm^− 1 in PANI-DBSA composite [32, 33]. A broad C-H out-of-plane IR peak is appeared at 823 cm^− 1 and S = O stretching peak is observed at 656 cm^− 1 that may describe the existence of DBSA in the PANI matrix [34, 35]. Pristine CdO is exhibiting the Cd-O stretching mode of vibrations at 656 and 856 cm^− 1 as well as the C–H wagging mode of vibrations at 1386 cm^− 1 [36]. PANI-DBSA-CdO composites spectra contain not only all the characteristic bands of PANI and PANI-DBSA but also contain the characteristic band 1386 cm^− 1 of pristine CdO at 1211 cm^− 1. The increasing concentration of CdO in the PANI-DBSA composite results in a reduction of peak intensity that confirms the formation of the PANI-DBSA-CdO nanocomposite. The stretching vibrations of C = O are appeared at 1655.5 cm^− I which indicate the presence of over oxidized carbonyl group due to the powerful oxidant APS [37].

4.3. Surface Morphological analysis

The electrochemical properties and the kinetics of the electroactive materials can be affected by their morphologies and particle sizes. The materials with small particle sizes are considered better for electrochemical performance [38]. SEM image of pure PANI reveals the fluffy, flaky, fibrous heaps of PANI nanoparticles of indefinite shapes and sizes. Pristine CdO SEM image depicts the agglomerations of tiny crystallites that are randomly oriented in a variety of morphologies (like cylinders, cones, cuboids, and spheres) along with deep pores [22]. SEM image of PANI-DBSA composite shows immersed rods in the gel-like structure of different lengths and diameters. Whereas PANI-DBSA-CdO composites SEM images depict a random dispersion of CdO nanomaterials in the PANI-DBSA network with fibrous rods like morphology of indefinite sizes and shapes, immersed in the sauce-like architecture. The smooth, regular, and homogeneous surface of PANI-DBSA composite with rods of greater (diameter and length), gesturing a good diffusive network. Whereas PANI-DBSA-CdO composites have rough, irregular & inhomogeneous surfaces of scattered and pointed rod edges of shorter (length & diameter) along with clusters of CdO NPs, thereby providing greater voids and disrupted pathways for charge transportation. Therefore, PANI-DBSA composite can be considered to have better charge transportation due to the enlarged surface-to-volume ratio thereby exposing more binding sites for interaction [39].

Room temperature dc conductivity values can be estimated from the slope of (Current vs Voltage) characteristic curves using the relation [40]

$$\:\varvec{\sigma\:}=\frac{\mathbf{I}\mathbf{L}}{\mathbf{V}\mathbf{A}}$$

1

Where L and A are the thickness and cross-sectional area of the prepared pellets.

The obtained IV characteristic curves of pure PANI, PANI-DBSA, and PANI-DBSA-CdO nanocomposites at different weight percentages of dopant CdO at room temperature are shown in Fig. 5. The estimated conductivity values are (2.5 x 10^− 6, 9.7 x 10^− 3, 5.0 x 10^− 3, 3.5 x 10^− 3) S m^− 1 for pure PANI, PANI-DBSA, PANI-DBSA-10%CdO, and PANI-DBSA-20%CdO composites respectively. An increase in the conductivity is observed for PANI-DBSA nanocomposite as compared to pure PANI whereas it decreases for higher concentrations of dopant CdO (10 and 20)%. The increase in dc conductivity may be attributed to the development of more diffusive and paved particle pathways that may contribute to fast electronic transportation [41]. The insertion of surfactant (DBSA) into the PANI matrix organized the PANI chains network in a greater order thereby, decreasing the porosity of PANI chains and increasing the density of PANI-DBSA nanocomposite. As a consequence, the weak links between the grains of PANI are greatly improved and strongly coupled through the grain boundaries resulting in the improvement of the macroscopic conductivity. The decremented dc conductivity at higher concentrations of CdO dopant in PANI-DBSA-CdO composites may be ascribed to the blockage of conducting pathways and due to the dominant poor conductivity of metal oxides [42]. The observed enhanced conductivity of PANI-DBSA composite might refer it be a potential material for SC electrode preparation. Hence it can be concluded that the electrical performance of the prepared electrodes increases up to a certain level of CdO doping concentration into the PANI matrix and then high surface area begins to decline it under further loading of CdO content.

6.1. Cyclic Voltammetry (CV)

Electrochemical redox performances of prepared material electrodes were investigated through CV profile curves that were carried out in a potential range of (-0.6 to 0.4) V using a 3-electrode cell with KOH (3.0 M) electrolytic solution.

The CV profile curves of all the prepared electrodes describe nearly the same peak potentials and the increasing peak currents as the scan rate increases from (5 to 100) mV s^− 1 that may indicate their good capacitive and steady reversible behavior. The quasi-rectangular shapes of pure PANI CV profile curves and increasing integrated area with the increasing scan rate describe its pseudocapacitive nature and enhanced charge mobility per unit time. The CV profile curves of the PANI-DBSA and PANI-DBSA-CdO composite electrodes are close to rectangular shapes and exhibit mirror-image characteristics for both positive and negative sweep potential. These profile curves demonstrate relatively better reversibility and pseudocapacitive behavior of the composite electrodes. The synergic effect of PANI-DBSA composite results in the enhancement of PANI electrochemical performance as observed from increased redox peak current. This can be attributed to the change in morphology of PANI NPs into PANI Nanorods, short ion diffusion, transport pathways, porosity as well as high specific area. Whereas, the declination of redox performance in PANI-DBSA-CdO composites (at higher concentrations of dopant CdO) deteriorates the diffusion pathways and decreases the redox peak current. This emphasizes that the synergistic effect does not favor the weak chains of PANI-matrix to arrange themselves in long-chained polymer, also the agglomeration of large surface area CdO NPs favors the formation of small oligomers of PANI chains rather than long chains. Therefore, their electrochemical performance is greatly suppressed as the dopant concentration increases. The specific capacitance can be estimated by the relation (2) [43].

C _s = $\:\frac{\int\:\:\mathbf{i}\left(\mathbf{V}\right)\mathbf{d}\mathbf{V}}{\mathbf{m}\mathbf{k}\varvec{\Delta\:}\mathbf{V}}$ ………………………(F g^− 1) (2)

Where, ∫ i(V) dV, ΔV, and m are the area of the CV loop, potential range, and the working electrode’s active mass.

Using the above Eq. (2), the estimated specific capacitance values of pure PANI, pristine CdO, PANI-DBSA, PANI-DBSA-10%CdO, and PANI-DBSA-20%CdO composites are (143, 102, 565, 238, and 103) F g^− 1 respectively, based on CV profile curves as shown in Fig. 6. However, the enhanced CV integration area of PANI-DBSA composite as compared to pure PANI, pristine CdO, and PANI-DBSA-CdO composites may be due to the availability of more electrochemical active sites, and better charge transportation [44].

The decreasing trend of specific capacitance with the increasing scan rate indicates that the redox reactions are not completely supported by the inner redox-active centers [45]. Therefore, maximum redox-active sites are accessible only at low scan rates for the fast transportation of electrolytic ions. The fast ion transportation increases the rate of oxidation-reduction reactions resulting in higher capacitance values [46]. Therefore, the capacitance values at low scan rates are the true values.

6.2. Galvanostatic charge-discharge (GCD)

The GCD profile curves of prepared material electrodes were carried out in (3.0 M) KOH electrolyte solution at current densities (8.33, 12.5, 16.7, 20.8, 25.0, 29.2, 33.3, 41.7) A g^− 1 in (-0.6 to 0.4) V potential range as shown in Fig. 7. The symmetric and non-linear shapes of pure and synthesized material electrodes describe the reversibility and pseudocapacitive behavior of redox reactions. The slope of the GCD discharge curve is usually comprised of two portions: a portion parallel to the y-axis describes the IR drop whereas the non-linear portion describes its pseudocapacitive nature. It is obvious from our obtained GCD profile curves that the composite electrodes have smaller IR drops than pure PANI and pristine CdO therefore, offer small contact resistance. The non-linear portion of GCD profile curves confirms their pseudocapacitive nature [47].

Morphology is considered to be the measure of the redox kinetics of a material that describes whether the material is capacitive or not. The porous fibrous rod-like network of PANI-DBSA composite provides more ion diffusion trails, making it a capacitive material due to its enhanced ion transportation. For the estimation of specific capacity, energy density, and power density of the pure and prepared materials, the following equations have been employed [43].

Cs = $\:\frac{\mathbf{I}\varvec{\Delta\:}\mathbf{T}}{\mathbf{m}\varDelta\:\mathbf{V}}$ …………………….(F g⁻¹) (3)

E = $\:\frac{0.5\:\mathbf{C}\mathbf{s}\:({\mathbf{V}\mathbf{m}\mathbf{a}\mathbf{x}\mathbf{i}}^{2}-{\mathbf{V}\mathbf{m}\mathbf{i}\mathbf{n}\mathbf{i}}^{2})}{3.6}$……………..(Wh kg⁻¹) (4)

$\:\mathbf{P}=\:\frac{3600\mathbf{E}}{\mathbf{T}}\:\:\:$ ………………….(W kg ⁻¹ ) (5)

Where I, ΔT, ΔV = Vmaxi-Vmimi, and m, are the discharge current, discharge time, High-low potential limits, and deposited mass of active material, respectively.

It has been observed from the GCD profile curves that PANI-DBSA composite has the smallest IR drop and better electrochemical properties, making it the most promising material for SC electrode preparation than the rest of pure and composite materials.

The higher electrical conductivity of the PANI-DBSA composite also contributes to its excellent electrochemical performance. The Ragone plot is also considered to demonstrate the overall electrochemical performance of a superconductor. It is observed from Ragone plots that the PANI-DBSA composite has better electrochemical performance than the others.

Table 3

Electrochemical parameters of prepared samples.
Composition	Cs (F g^− 1)	E (Wh kg^− 1)	P (W kg^− 1)
Pristine CdO Pure PANI PANI-DBSA PANI-DBSA-10%CdO PANI-DBSA-20%CdO	105 143 569 248 132	1.2 1.5 12.3 5.5 1.3	607 564 812 994 375

Moreover, the PANI-DBSA composite has excellent cyclic stability, even after 20,000 cycles it retains over 85% of its redox peak area at a 5 mV s^− 1 scan rate as illustrated in Fig. 9. This reduction in capacity may be attributed to the fact that a large degree of insertion and de-insertion of ions into the PANI backbone develop defects that will reduce the charge-storing capability and the stability of the composite. Significantly, the PANI-DBSA composite exhibits excellent cyclic stability and reversibility, suggesting it to be an excellent SC electrode material.

Similar results have been published by some other researchers like PANI-DBSA-Fe₃O₄ composite electrodes possessing a specific capacity of 180 F g^− 1 and energy density of 6.33 Wh kg^− 1. Furthermore, it has 97% coulombic efficiency and 407 W kg^− 1 power density at 0.44 A g^− 1 current density [48]. PANI-NiCoO₄ nanocomposite demonstrated 439.4 Fg^− 1 specific capacity and retained almost 66.11% of it after 1000 cycles [49]. PANI-RGO-SiO₂ composite demonstrated 780 F g^− 1 specific capacity and retained almost 85% of it [50].

A careful literature review emphasizes that the electrical and electrochemical performances of the composite electrodes increase with the increasing concentration of oxide up to a certain level and then the high surface area structure begins to destroy with subsequent oxide loading [51]. Similarly in our case, the electrical and electrochemical properties are observed to improve with increasing concentration of CdO dopant in the composite (up to 5% weight of dopant CdO) as previously reported but with further CdO loading, the IR drop becomes higher, and high surface area architecture begins to destroy which results in the decrement of electrical and electrochemical properties and follow up our CV results.

6.3. Electrochemical Impedance Spectroscopy (EIS)

The capacitive performances of prepared samples were estimated through the EIS technique and the obtained Nyquist plots are shown in Fig. 10. The non-zero intercept on the impedance real axis (Z) is the quantitative measure of ohmic resistance (Rs), whereas the semicircular part is linked to the charge-transfer resistance (Rct) of all the material electrodes at a high-frequency region that may be attributed to ions exchange at the electrode/electrolyte boundary.

The smallest semi-circular diameter of the PANI-DBSA composite electrode in comparison to all other electrode materials may be attributed to its lowest Rs value at the electrode/electrolyte boundary and reflects its conductive nature. Additionally, almost all the electrode materials show a nearly vertical profile and the deviations of straight lines from 90^o at the low-frequency region correspond to Warburg diffusion resistance that may be attributed to the diffusion of protons at the electrode/electrolyte boundary. The least steep line of the PANI-DBSA electrode at the low-frequency region may be attributed to the fast adsorption of ions at the electrode surface and reveals its pseudocapacitive nature [52]. The inset equivalent circuit was used to fit Nyquist plots using Randle’s model. In contrast to the contents used here, an infinite series of simple electrical elements are required to estimate the results. Thus, the synergic effect of PANI-DBSA hybrid material results in lower (Rs, Rct & diffusion) resistances as compared to pure and higher weight% CdO composites which support our CV and GCD results and demonstrates its robustness for making stretchable, wearable, and portable supercapacitor electrodes.

In-situ polymerization route was chosen to synthesize the electrode materials. The structural (physical & chemical), and surface morphological analysis of all the pure and prepared samples were characterized through XRD, FTIR, and SEM techniques. The CV results of the PANI-DBSA composite showed a higher specific capacitance (565 F g^− 1) than Pure PANI (143 F g^− 1) and Pristine CdO (102 F g^− 1) at a 5 mV s^− 1 scan rate. The GCD results follow up the same trend as CV and showed a higher specific capacity for PANI-DBSA composite (569 F g^− 1) at 8.33 A g^− 1 with higher (12.3 Wh kg^− 1) energy density and higher (812 W kg^− 1) power density respectively. The obtained high capacity, excellent reversibility, higher (energy & power) densities, and lower (Rs, Rct, & diffusion) resistances, make it a robust material for developing stretchable, wearable, and portable supercapacitor electrodes. It has also been observed that the addition of surfactant (DBSA) in PANI-CdO nanocomposites didn’t effectively pave the diffusion pathways in the PANI-DBSA-CdO network for higher concentrations of CdO dopant and results in the blockage of charge carriers and a heavy suppression of their electrical & electrochemical performances.

Polyaniline	PANI	Pseudocapacitance	PC
Cadmium Oxide	CdO	Electrochemical Capacitor	EC
Dodecyl Benzene sulphonic Acid	DBSA	Hybrid Supercapacitor	HSC
Camphor sulphonic Acid	CSA	Conducting Polymer	CP
X-ray Diffraction	XRD	Equivalent Series Resistance	ESR
Fourier Transform Infrared Radiation	FTIR	Polypyrrole	PPY
Scanning Electron Microscopy	SEM	Polythiophene	PTh
Debye-Scherrer	D-S	Polyaniline/Nickle Hydroxide	PANI/Ni(OH)₂
Williamson-Hall	W-H	Polyaniline/Molybdenum dioxide	PANI/MoS₂
Size-Strain	S-S	Polyaniline/Molybdenum Trioxide/Graphene	PANI/MoO₃/GN
Cyclic Voltammetry	CV	Polypyrrole/Carbon nanotubes/Manganese dioxide	PPy/CNT/MnO₂
Galvanostatic Charge-Discharge	GCD	Cobalt (II) Oxide	CoO
Electrochemical Impedance Spectroscopy	EIS	Ferric Oxide	Fe₂O₃
Supercapacitor	SC	Vanadium Pentoxide	V₂O₅
Specific Capacitance	Cs	Metal Oxides	Mos
Amine Group	-NH-	Asymmetric Supercapacitor	ASSC
Transition Metal Oxide	TMO	Symmetric Supercapacitor	SSC
Manganese dioxide	MnO₂	Manganese Nickle Cobalt	MNCO
Molybdenum trioxide	MoO₃	Sulfuric Acid	H₂SO₄
Copper Oxide	CuO	Sodium Sulfate	Na₂SO₄
Zinc Oxide	ZnO	Molarity	M
Titanium dioxide	TiO₂	Nickle Molybdenum Tetroxide	NiMoO₄
Ruthenium oxide	RuO₂	Graphene Oxide	GO
Nano Particles	NPs	Joint Committee on Powder Diffraction Standards	JCPDS
Current-Voltage	IV	Wide Angle Diffraction Scattering	WAXS
Ammonium Persulfate	APS	Space Charge Limited Current	SCLC
Hydrochloric acid	HCl	Indium Tin Oxide	ITO
Platinum	Pt	Angstrom	Å
Silver/Silver Chloride	Ag/AgCl	Lattice Strain	ε
Potassium hydroxide	KOH	Dislocation Density	δ
Nickel	Ni	Polyaniline/Carbon/Titanium Nitride	PANI/C/TiN
Face Centered Cubic	FCC	Nickel Terephthalate	Ni-Tp
Wide angle X-ray scattering	WAXS	Potassium Nitrate	KNO₃
Electric Double Layer Capacitance	EDLC	Nickle Cobalt Phosphide	NiCoP
Nickel Cobaltite	NiCo₂O₄	Nickle Cobalt Sulfide	NiCo₂S₄
Farad per gram	F g⁻¹	Phosphating Cobalt Molybdate	P-CoMoO₄
Watt-hour per kilogram	Wh kg⁻¹	Polyaniline-Reduced Graphene Oxide-Silicon Dioxide	PANI-RGO-SiO₂
Watt per kilogram	W kg⁻¹	Potassium Nitrate	KNO₃
Polyaniline-Reduced Graphene Oxide-Manganese Oxide	PANI-GR-Mn₃O₄	Sodium Sulfate	Na₂SO₄
Polyaniline-SA Titanium Dioxide-Tin Oxide	PANI-SA*TiO₂-SnO₂	Sodium Hydroxide	NaOH

Author Contribution

Credit authorship contribution statementNadeem Anwar, Abdul Shakoor: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources. Haseeb Ahmad, Muhammad Irfan, Ariba Bibi: Data curation, Software, Visualization, Writing - original draft. Ghulam Ali, Niaz Ahmad Niaz: Supervision, Validation, Writing - review & editing.

Declaration of Interest statement

We wish to confirm that there are no known conflicts of interest associated with this manuscript and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

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No competing interests reported.

Preparation and Characterization of Polyaniline-Dodecyl Benzene Sulfonic Acid-Cadmium Oxide (PANI-DBSA-CdO) Composites: As an Electrode Material for Energy Storage Applications

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental route

2.1. Material Resources

2.2. Polyaniline (PANI) Synthesis

2.3. Polyaniline-Dodecyl benzene Sulfonic Acid (PANI-DBSA) Synthesis Process

2.4. Polyaniline-Dodecyl Benzene Sulfonic Acid-Cadmium Oxide (PANI-DBSA-CdO) Nanocomposite Synthesis Process

3. Measurements

3.1. Physical Characterization

3.2. Electrometrical testing

4. Results and Discussion

4.1. Structural Analysis

4.2. Functional Group Analysis

4.3. Surface Morphological analysis

5. Electrical Response (dc)

6. Electrochemical Properties for Supercapacitor Applications

6.1. Cyclic Voltammetry (CV)

6.2. Galvanostatic charge-discharge (GCD)

6.3. Electrochemical Impedance Spectroscopy (EIS)

7. Conclusion

Abbreviations

Declarations

Author Contribution

References

Additional Declarations

Status:

Version 1