International Journal of Electrochemistry
Volume 2019 (2019), Article ID 3409568, 7 pages
https://doi.org/10.1155/2019/3409568

Research Article

Synthesis and Electrochemical Capacitor Characterization of Novel Composite Materials with p-Type Conductive Polymer

Narges Ajami

Department of Chemistry, Payame Noor University, P.O. Box 19395-3697, Tehran, Iran

Correspondence should be addressed to Narges Ajami; n_ajami@pnu.ac.ir

Received 1 November 2018; Revised 22 November 2018; Accepted 10 December 2018; Published 1 January 2019

Academic Editor: Gerd-Uwe Flechsig

Copyright © 2019 Narges Ajami. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Considering the importance of conductive polymer nanocomposite, the present paper attempts to create a method for increasing the conductivity of poly(o-aminophenol). Nanocomposite MnO₂/poly(o-aminophenol) thin film was synthesized by using pulse potential electrodeposition technique on a graphite electrode. In this research, nanoparticles of MnO₂ are used after synthesis to prepare polymer nanocomposites in one-step. Appending of MnO₂ to polymer matrix increases the current. This current growth could be ascribed to the synergistic MnO₂ nanostructure, which presents the superior surface area and smaller particle size that is increasingly acting sites. Morphology or samples composition was investigated by the scanning electron microscope and the UV-Vis method, which clearly indicate the formation of nanocomposites. The findings show that the capacitive behavior of MnO₂-poly(o-aminophenol) is superior to poly(o-aminophenol), especially at high potential high temperatures. The results showed that MnO₂/poly(o-aminophenol) had a higher level of activity and the electron transfer capability was faster than pure polymer film. The doped MnO₂ polymer also has excellent cyclic performance and load discharge features. Additional electrochemical properties of these polymer composites were observed against pure polymer so that capacity of 645 Fg⁻¹ has been designated.

1. Introduction

Over the past years, the study of polymers has attracted much attention and has many advantages such as simple synthesis, low cost, high chemical stability, adjustable doping /doping properties, and high conductivity [1–4]. Poly(o-aminophenol) is one of the best conductivity polymers and it has been of great interest to researchers whereas it is more excellent electrical conductivity [5, 6]. They have a diversity of usages like microelectronic devices, capacitors, transistors, corrosion protection, and chemical sensors [7, 8]. Ehsani et al. [9] have fabricated POAP/1-octadecyl-3-methylimidazolium bromide composite films as active electrodes supercapacitors and reported that the supercapacitor behavior of the composite film is due to the high active surface area, charge-transfer in the polymer chain, and synergistic efficacy amongst conductive polymer and [OMD] Br. In order to improve the performance of supercapacitors, many efforts have been done to incorporate metal oxides for polymerization, making composite electrode materials. Varaprasad et al. [10] have reported that metal-oxide polymer nanocomposite films are useful for the dielectric device and supportable energy harvesting applications. MnO₂ and its derivatives have attracted particular attention and are used in various applications such as catalysts, batteries, sensors, and batteries [11, 12]. Ozan Yanik and his colleagues [13] have been investigated magnetic conductive polymer/graphene nanocomposites for supercapacitors. They were able to achieve 255 Fg⁻¹ specific capacitance values for it and they showed it has a healthy cell performance. Bahloul et al. [14] synthesized polypyrrole (PPy) deposited onto γ-MnO₂ particles. They concluded that PPy/MnO₂ composite has high specific capacitance of 141.6 F g⁻¹ compared with 73.7 F g⁻¹ for γ-MnO₂. It indicates that the improvement of the coating is due to the electronic conductivity of the polymer and an increase of the BET surface area that raises to 125m² g⁻¹ after coating, against 64 m² g⁻¹ for MnO₂. Hamid Mohammad Shiri and his colleague [15] have fabricated POAP/Dy₂O₃ films to serve as the active electrode for electrochemical supercapacitor. They found the enhanced supercapacitor behavior observed for the composite film could be attributed to synergistic effect existing between Dy₂O₃ nanoparticles and the conductive polymer. Sydulu Singu et al. [16] have reported that PPy-Ag nanoparticles nanocomposite exhibits the specific capacitance of 266.7 F g⁻¹ and the power density of 3998 W kg⁻¹ and excellent capacitance after 5000 charge-discharge cycles, which determines the potential electrode material for supercapacitor application. Poly-o-phenylenediamine/MnO2 materials were synthesized by the addition of cetyl trimethyl ammonium bromide and sodium dodecyl sulfate [17]. Research results showed a high specific capacitance (262.2 Fg⁻¹) and a good electrochemical performance, making poly-o-phenylenediamine a promising material in supercapacitor application. However, a study on the synthesis of one-stage MnO₂/poly(o-aminophenol) nanocomposite and its capacitive behavior has not been done. Poly(o-aminophenol) is a p-type conductive polymer [18] that exhibits its maximal electroactivity within the potential range -0.2 V < E < 0.5 V versus (SCE). One of the effective methods for increasing its life cycle is combination of polymer and nanomaterial for the preparation of nanocomposite coatings for various applications. Manganese dioxide is widely used for the cheap metal oxides supercapacitor applications due to abundant in nature, high porosity, highly reversible chemical adsorption/desorption, good electrochemical performance, and environment-friendly [19, 20]. It is believed that the MnO₂ electrode capacity is mainly due to reversible changes that involve the exchange of protons and/or cations with the electrolyte [21, 22]. In this research amount of specific capacity for pure MnO₂ is 203 F g⁻¹ for MnO₂. To improve electrical conductivity of the polymer and creation higher surface area, MnO₂ is mixed with poly(o-aminophenol), which increases the specific capacity.

Therefore, we proposed a one-step easy procedure for the synthesis of MnO₂ doped poly(o-aminophenol), in which doping of MnO₂ was achieved simultaneously during the electrochemical polymerization of o-aminophenol. In this paper, after synthesis of nanocomposites, the use of the MnO₂-poly(o-aminophenol) nanocomposite for capacitor applications with high performance is discussed.

2. Materials and Methods

2.1. Materials

The chemicals were prepared from Merck firm as the analytical grade. Electrochemical tests were carried out using a system three-electrode cellby a μ-Autolab potentiostat/galvanostat and a frequency response analyzer (Metrohm, Model 12/30/302, and The Netherlands). Graphite (G) with a diameter of 0.3 cm was applied as a worker electrode and platinum foil as an auxiliary electrode, while Ag /AgCl was operated as a reference electrode. The morphologies of the electrosynthesized conductive polymer obtained were investigated by field emission scanning electron microanalyzer model KYKY-EM3200.1.

2.2. Synthesis of MnO₂-Poly(o-aminophenol)

MnO₂ nanoparticles were synthesized by reduction of 0.1 M KMnO₄ aqueous solutions with ethanol by ultrasonic method. Poly(o-aminophenol) electrodeposited on the cleaned graphite disk electrode by pulse potential electrodeposition technique from an aqueous solution containing 0.02M o-aminophenol, 0.04M sodium dodecyl sulfate, and 0.6 M perchloric acid with the pulse current of 8 mA/cm², pulse time and relaxation time of 0.5 s. At the start of work, the worker electrode was polished and then rinsed with ethanol and water, respectively. The MnO₂-poly (o-aminophenol) electrode was prepared like the method of graphite, poly(o-aminophenol) electrode, but with an addition of 1% MnO₂. The MnO₂ 1% were scattered into a solution of 5 mM SDS for thirty minutes to acquire a uniform suspension. Sodium dodecyl sulfate is used as suspension additive MnO₂ particles and increase the firmness and electroactivity of the prepared films.

3. Results and Discussion

Figure 1 exhibits the SEM images of poly(o-aminophenol) and MnO₂-poly(o-aminophenol). As can be observed, the morphologies of poly(o-aminophenol) and MnO₂-poly(o-aminophenol) are completely different from each other, indicating that the presence of MnO₂ into poly(o-aminophenol) changes the microstructure of the polymer. The SEM image of the POAP coating has slightly porous structure; however, the use of MnO₂ nanoparticles seems to make the coating porous and more compact than pure polymer that present large specific surface. The micrograph shows that the POAP thin films deposited in the presence of MnO2 are very thick and present a smooth surface. Since the MnO₂ particles, alone have a more porous structure. It should be noted that the electrochemical polymerization process does not change the morphology of MnO₂ seeds, but the particle coating with POAP coating reduces the surface roughness of the particles. It is clearly seen that the surface morphology of the polymer significantly altered by adding MnO₂ nanoparticles. As is clear, MnO₂ particles are uniformly doped throughout the polymer. The distribution of MnO₂ particles in the polymer structure is well known.

(a)

(b)

Figure 1: SEM images of (a) poly(o-aminophenol) and (b) MnO₂-poly(o-aminophenol) nanocomposite electrodes after using pulse potential electrodeposition technique synthesis.

Figure 2 displays UV-Vis spectrum prepared films of the poly(o-aminophenol) and MnO₂-poly(o-aminophenol). It is observed that the shapes of UV spectra of nanocomposite and polymer are similar, but the poly(o-aminophenol) sample showed strong absorption band in the UV region. As observed by adding MnO₂, the severity of the peaks decreased because of the interaction between MnO₂ and poly(o-aminophenol), demonstrating of the MnO₂ presence in the composite film. When MnO₂ nanoparticles are dispersed in the polymer matrix, significant changes are observed in the absorption spectra, which indicate the interaction of metal-oxide nanoparticles with the polymeric chain. These results confirm the formation of MnO₂-poly(o-aminophenol) nanocomposite.

Figure 2: UV-Vis absorption spectrum of the poly(o-aminophenol) and the MnO₂-poly(o-aminophenol) nanocomposite.

The surface areas of the bare and modified electrode were gained by cyclic voltammetry method in 1 mM K₃Fe (CN)₆ solution as a probe at various scan rates. The surface area for a reversible process can be computed according to the slope of as (1) [23]:

where A is the electroactive surface area (cm⁻²), while n is the amount of electrons participated in the redox reaction. D, C, are the diffusion coefficient (cm² s⁻¹), bulk concentration of the redox probe (mol cm⁻³), and scan rate (Vs⁻¹), respectively. For a solution containing K₃ Fe(CN)₆, the surface areas of the graphite, G/poly(o-aminophenol), and G/MnO₂-poly(o-aminophenol) electrodes were computed 0.071, 0.265, and 0.326 cm², respectively. The relative standard deviation (RSD %) for three determinations is 2.1% for the graphite, 2.7% for the G/poly(o-aminophenol), and 3.2% for the G/MnO₂-poly(o-aminophenol) electrodes, indicating the good repeatability of electrodes.

Figure 3 illustrates the curves of the G/MnO2-poly(o-aminophenol) (c) electrodes obtained in 1 mM K₃Fe (CN) 6 solution containing 0.5 M KCl at various scan rates. As shown in the figure, two distinct peaks are seen for electrode. This shows the MnO₂-poly(o-aminophenol) has higher electrochemical activity and has very low variations in peak and potential difference ∆Ep. large peak potential represents the porous structure and larger specific surface area.

Figure 3: CV curves of the G/MnO₂-poly(o-aminophenol) electrode obtained in 1mM K₃Fe (CN) 6 solution containing 0.5 M KCl at different scan rates (30, 60, 90, 120, 150 mv⁻¹).

It is obvious that the redox peak current growths linearly with enhance scan rate and the redox peak currents of G/MnO₂-poly(o-aminophenol) significantly was strong. Figure 4 shows a linear dependence of anodic current for G/MnO₂-poly(o-aminophenol) electrode on the with a correlation coefficient of 0.9909.

Figure 4: Peak currents as a function of of G/MnO2-poly(o-aminophenol) electrode.

Figure 5 shows cyclic voltammogram cycles of the bare and obtained electrodes (pure poly (o-aminophenol), MnO₂-poly(o-aminophenol)). As seen, the pure poly(o-aminophenol) film shows a pair of the wide peak of about -0.1 to +0.5 V, which is associate to the redox reaction of the phenoxazine. The redox peak currents of the composite film were stronger than both electrodes. The composite shows a higher background current in the potential sweep, and there exists faradic currents, because of the existence of MnO₂.

Figure 5: Cyclic voltammograms of (a) bare, (b) poly(o-aminophenol), and (c) MnO₂-poly(o-aminophenol) electrodes obtained in 0.6 M HClO4 solution at the scan rate of 50 mVs⁻¹.

Figure 6 shows CV curves of G/poly(o-aminophenol) and G/MnO₂-poly(o-aminophenol) electrodes have been depicted in different scan rates in 0.6 M HClO4 media. As seen, two pairs of peaks become visible on the chart in CV of both electrodes. Based on CV results, with increasing scan rates, the current response of both films increases, but the current signal is low in G/poly(o-aminophenol). Clearly, with the increase in scan rate gradually, the shape of cyclic voltammograms does not deform strongly. It is clear that, the change at the peak potential of MnO₂-poly(o-aminophenol) film is smaller than that for poly(o-aminophenol). These results confirm that it has a better electrochemical reversibility and an ideal capacity compared to poly(o-aminophenol).

(a)

(b)

Figure 6: Cyclic voltammograms of G/poly(o-aminophenol) (a) and G/MnO₂-poly(o-aminophenol) (b) in 0.6 M HClO4 solution at different scan rates 10, 30, 50, 70, 90, 110, 130, and 150 mV s⁻¹.

Figure 7 shows the I–V characteristics of bare, MnO₂-poly(o-aminophenol) composite, and poly(o-aminophenol) electrodes [24]. The specimen resistance was measured using two-probe, where the I–V curves were obtained by scanning the voltage from -2 to 2 V with a step of 0.06 V. I–V curves display that the resistance of MnO₂-poly(o-aminophenol) composite has decreased compared to poly(o-aminophenol) and bare electrodes. These data show the electro-catalytic effects of MnO₂-POAP towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), in comparison to pristine POAP.

Figure 7: I–V characteristics of (a) bare electrode, (b) poly(o-aminophenol), and (c) MnO₂-poly (o-aminophenol).

The ability of electron transfer of different electrodes was studied by using electrochemical impedance spectroscopy at the open-circuit potential for investigation of a polymer matrix. Figure 8 shows the Nyquist impedance curve of the electrodes using a 1 mM redox [Fe(CN)₆]-3/-4 (1 : 1) probe with 0.5 M KCl solution. It shows that both electrodes exhibit a single semicircle at higher frequency region and a linear part in the lower frequency region. As predicted, the electron transfer (Ret), which is related to the faradic reactions in the MnO₂-poly(o-aminophenol) electrode, is less than pure polymer. This result shows that the presence of MnO₂ nanoparticles in the polymer matrix increases the conductivity of the composite electrode. It causes charge-transfer resistance decrease with MnO₂ nanoparticles. This phenomenon may be attributed to the large specific surface and faster transfer of composite electrons. It is clear that the vertical curve of MnO₂-poly(o-aminophenol) has the large slope that is close to the imaginary impedance axis. It shows MnO₂-poly(o-aminophenol) has the high conductivity or low internal resistance. This means that the results of both experiments consist of the impedance and cyclic voltammetry tests confirm each other. As shown in the diagram, both electrodes have Warburg (ZW) resistance impedance in the low-frequency region, is a result of the dependence of the ion emission/transfer frequency from an electrolyte to the electrode surface [25]. The Nyquist design data is placed in an equivalent circuit, shown in Figure 8, indicating the a bulk solution resistance Rs, a charge-transfer resistance R ct, a constant phase element for simulating the double-layer capacitance (Cdl), and a Warburg (ZW) resistance impedance.

Figure 8: Nyquist diagrams of (a) poly(o-aminophenol) and (b) MnO₂-poly(o-aminophenol) films in 1mM redox probe [Fe(CN)₆]-3/-4 (1 : 1) with 0.5 M KCl solution.

Considering the higher voltages in the cyclic voltammogram of MnO₂-poly(o-aminophenol), a larger capacity for MnO₂-poly(o-aminophenol) is predictable, and the MnO₂-poly(o-aminophenol) composite acts as a supercapacitor with a high specific capacitance. The specific capacitance of the samples can be estimated from the cyclic voltammetry curves based on this equation [26]:

which is in this equation I identify the average cathodic current, s specifies the applied potential sweep rate, and m is the mass of the active electrode. An electrode is weighing, before and after the deposition process, which yields the amount of the active material or mass of film. The specific capacitance values of the pure poly(o-aminophenol) and MnO₂-poly(o-aminophenol) nanocomposite samples are calculated as 335, and 645 F/g, respectively. The enhanced electrochemical performance of the ternary MnO₂-poly(o-aminophenol) nanocomposite electrode originates from the combined effects of the poly(o-aminophenol) and MnO₂ components.

Table 1 shows comparison of specific capacitances for different composites. Considering no reports regarding MnO₂-poly(o-aminophenol) nanocomposite, it is observed that the MnO₂-poly(o-aminophenol) nanocomposite has a better specific capacitances.

Table 1: Comparison of specific capacitances for different composites.

composite
Specific capacitance/Fg⁻¹
References

polypyrrole/MnO2
141.6
[14]

POAP/Dy₂O₃
145
[15]

poly-o-phenylenediamine/ MnO₂
262.2
[17]

poly(p-styrenesulfonate) grafted reduced grapheme oxide/polyaniline/MnO₂
425
[27]

poly-aniline/ MnO₂
210
[28]

This work
645
-

4. Conclusions

In summary, G/MnO₂-poly(o-aminophenol) electrode was made by electropolymerization of ortho-aminophenol monomer and of MnO₂. Optimum conditions for the providing of modified electrode have been obtained. With the presentation of MnO₂, the as-prepared MnO₂-poly(o-aminophenol) exhibits more capacitive behaviors than poly(o-aminophenol), especially at the high potential scan rate. In addition, the cyclic performance of the MnO₂-poly(o-aminophenol) electrode is superior to poly(o-aminophenol). Better electrochemical capacitor property was observed for these metal-oxide polymer composites against pure polymer. In addition, the results revealed that the MnO₂/poly(o-aminophenol) had higher electric conductivity, less resistance, and faster charge-transfer than the pure polymer film. Scanning electron micrographs and UV-Vis spectrum were employed for characterization of the produced composites. The composite represented excellent electrochemical capacitor property.

Data Availability

All of the data used to support the findings of this study are included in the article.

Conflicts of Interest

The author declares that they have no conflicts of interest.

Acknowledgments

I am grateful to Payame Noor University and Iranian Nano Council for supporting this work.

References

N. Ajami, A. Ehsani, F. Babaei, and R. Safari, “Electrochemical properties, optical modeling and electrocatalytic activity of pulse-electropolymerized ternary nanocomposite of poly (methylene blue) in aqueous solution,” Journal of Molecular Liquids, vol. 215, pp. 24–30, 2016.

A. Ehsan, N. Ajami, F. Babaei, and H. Mostaanzadeh, “Electrosynthesis and characterisation of poly methylene blue and its nanocomposite with ZnO nanoparticles,” Synthetic Metals, vol. 197, pp. 80–85, 2014.

N. Ajami and N. Bahrami Panah, “One-step electrosynthesis of multi-walled carbon nanotube/poly-ortho-aminophenol composite film and investigation of its electrocatalytic properties,” Journal of Nanostructure in Chemistry, vol. 3, no. 76, 2013.

V. C. Lokhande, A. C. Lokhande, C. D. Lokhande, J. H. Kim, and T. Ji, “Supercapacitive composite metal oxide electrodes formed with carbon, metal oxides and conducting polymers,” Journal of Alloys and Compounds, vol. 682, pp. 381–403, 2016.

J. Li, J. Zhao, and X. Wei, “A sensitive and selective sensor for dopamine determination based on a molecularly imprinted electropolymer of o-aminophenol,” Sensors and Actuators B: Chemical, vol. 140, no. 2, pp. 663–669, 2009.

Chen C., Sun C., and Y. Gao, “Amperometric sensor for hydrogen peroxide based on poly(aniline-co-p-aminophenol),” Electrochemistry Communications, vol. 11, no. 2, pp. 450–453, 2009.

A. Keyhanpour, S. M. Seyed, S. M. Mohaghegh, and A. Jamshidi, “Electropolymerization and characterization of polyaniline, poly(2-anilinoethanol) and poly(aniline-co-2-anilinoethanol),” Iranian Polymer Journal, vol. 21, pp. 307–315, 2012.

R. Ojani, J.-B. Raoof, and S. Fathi, “Poly(o-aminophenol) film prepared in the presence of sodium dodecyl sulfate: Application for nickel ion dispersion and the electrocatalytic oxidation of methanol and ethylene glycol,” Electrochimica Acta, vol. 54, no. 8, pp. 2190–2196, 2009.

A. Ehsani, E. Kowsari, M. Dashti Najafi, R. Safari, and H. Mohammad Shiri, “Enhanced pseudocapacitive performance of electroactive p-type conductive polymer in the presence of 1-octadecyl-3-methylimidazolium bromide,” Journal of Colloid and Interface Science, vol. 503, pp. 10–16, 2017.

K. Varaprasad, B. Diwakar S, C. Donoso, K. Ramam, and R. Sadiku, “Metal-oxide polymer nanocomposite films from disposable scrap tire powder/poly-ε-caprolactone for advanced electrical energy (capacitor) applications,” Journal of Cleaner Production, vol. 161, pp. 888–895, 2017.

G. Wang P, L. Zhang, and J. Zhang, “A review of electrode materials for electrochemical supercapacitors,” Chemical Society Reviews, vol. 41, pp. 797–828, 2012.

Z. Zhu, C. G. Wang, Q. M. Sun, W. X. Li, and Z. C. Li, “Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications,” Electrochimica Acta, vol. 56, pp. 1366–1372, 2011.

M. O. Yanik, E. A. Yigit, Y. E. Akansu, and E. Sahmetlioglu, “Magnetic conductive polymer-graphene nanocomposites based supercapacitors for energy storage,” Energy, vol. 138, pp. 883–889, 2017.

A. Bahloul, B. Nessark, E. Briot et al., “Polypyrrole-covered MnO2 as electrode material for supercapacitor,” Journal of Power Sources, vol. 240, pp. 267–272, 2013.

H. M. Mohammad Shiri, A. Ehsani, and R. Behjatmanesh-Ardakani, “Electrochemical deposition and plane-wave periodic DFT study on Dy2O3 nanoparticles and pseudocapacitance performance of Dy2O3/conductive polymer nanocomposite film,” Journal of the Taiwan Institute of Chemical Engineers, vol. 93, pp. 632–643, 2018.

S. B. Singu and K. Ro Yoon, “Mesoporous polypyrrole-Ag nanocomposites for supercapacitors,” Journal of Alloys and Compounds, vol. 742, pp. 610–618, 2018.

M. Wang, H. Zhang, C. Wang, and G. Wang, “Synthesis of MnO₂/poly-o-phenylenediamine composite and its application in supercapacitors,” Electrochimica Acta, vol. 106, pp. 301–306, 2013.

A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, and S. Kazemi, “Nanocomposite of p-type conductive polymer/functionalized graphene oxide nanosheets as novel and hybrid electrodes for highly capacitive pseudocapacitors,” Journal of Colloid and Interface Science, vol. 478, pp. 181–187, 2016.

R. Reddy N and R. Reddy G, “Synthesis and electrochemical characterization of amorphous MnO2 electrochemical capacitor electrode material,” Journal of Power Sources, vol. 132, no. 1-2, pp. 315–320, 2004.

F. Ataherian, K. Lee, and N. Wu, Electrochim Acta 55 7429, 2010.

S. Devaraj and N. Munichandraiah, “High Capacitance of Electrodeposited MnO2 by the Effect of a Surface-Active Agent,” Electrochemical Solid-State Letters, vol. 8, no. 7, pp. 373–377, 2005.

E. Raymundo-Pinero, V. Khomenko, E. Frackowiak, and F. Beguin, “Performance of Manganese Oxide/CNTs Composites as Electrode Materials for Electrochemical Capacitors,” Journal of the Electrochemical Society, vol. 152, pp. 229–235, 2005.

A. Bard J and L. R. Faulkner, LR, Wiley, Chapter 6 edition, 1980.

Y. Long and Z. Chen, Nanowires Science and Technology, INTECH, N. Gu C and N. Lupu, Eds., ISBN 978-953-7619-89-3, Croatia, 2010.

X. Li and I. Zhitomirsky, “Capacitive behaviour of polypyrrole films prepared on stainless steel substrates by electropolymerization,” Materials Letters, vol. 76, pp. 15–17, 2012.

A. I. Inamdar, Y. Kim, S. M. Pawar, J. H. Kim, H. Im, and H. Kim, “Chemically grown, porous, nickel oxide thin-film for electrochemical supercapacitors,” Journal of Power Sources, vol. 196, no. 4, pp. 2393–2397, 2011.

Y. Gui, X. Xing, and C. Song, “Fabrication of poly(p-styrenesulfonate) grafted reduced graphene oxide/polyaniline/MnO₂ hybrids with high capacitance performance,” Materials Chemistry and Physics, vol. 167, pp. 330–337, 2015.

P. M. Kharade, S. G. Chavan, D. J. Salunkhe, P. B. Joshi, S. M. Mane, and S. B. Kulkarni, “Synthesis and characterization of PANI/MnO₂ bi-layered electrode and its electrochemical supercapacitor properties,” Materials Research Bulleti, vol. 52, pp. 37–41, 2014.