Influence of Zn doping on the electrochemical capacitor behavior of MnO₂ nanocrystals

Abstract

Herein, we suggest a simple chemical precipitation method for the preparation of bare and different levels of Zn doped MnO₂ nanoparticles as electrodes for supercapacitors. The structure and chemical composition of the products were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. The morphologies of the undoped and doped products were analyzed by a scanning electron microscope (SEM) and a field emission transmission electron microscope (FE-TEM). The surface areas and pore volumes of the products were determined from N₂ adsorption–desorption isotherm curves and the results reveal that MnO₂ doped with Zn yields a smaller particle size, higher specific surface area, and a larger pore volume than those of pure MnO₂. The capacitance behavior of the products was analyzed by cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy. The results of the capacitance behavior reveal the improved capacitance performance for MnO₂ on Zn doping. Especially, among the doped products, MnO₂ doped with 0.125 M Zn gives the high specific capacitance of 620 F g⁻¹ at 10 mV s⁻¹. The present work may open up a new path for the improvement of pseudocapacitance behavior of manganese oxide by Zn doping.

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1. Introduction

With the rising concerns regarding global warming and the fuel crisis, there has been a growing demand for sustainable energy based on renewable high energy conversion and storage technologies.¹ In recent years, electrochemical capacitors have represented a new type of energy storage device. They have the advantages of high power density, environmental friendliness, high safety, convenient operation over a wide temperature range, and they show tremendous flexibility complementing lithium-ion batteries.^2–4 Specific capacitors are broadly classified into two categories, electrical double layer capacitors (EDLCs) and pseudocapacitors, depending on the nature of the storage mechanism.^5,6 Compared to other storage devices, supercapacitors have some attractive advantages.⁷ Overall, the research results show that the good performance of electrochemical capacitors usually results from the high specific area and the highly reversible redox reaction of the electrode materials.⁸ Pseudocapacitors are being widely investigated because of their large specific capacitance and high-energy properties. Since pseudocapacitance arises from the redox reaction of electroactive materials, transition metal oxides⁹ and conductive polymers^10,11 with several oxidation states are considered as promising electrode materials for pseudocapacitors. The MnO₂ electrodes have large open tunnels, which can favor the storage of alkaline cations (e.g., Na⁺, K⁺, and L⁺) and exhibit higher capacitance. Recently many studies have focused on the development of manganese dioxide based materials because of their electrochemical behavior, low cost, and environmental compatibility.¹² Doping with transition metal ions is another frequently employed method to modify the properties of functional materials. As a result of the confinement of electronic states and the tendency for the dopants to occupy the sites in the crystalline structure, the doping of Mn-based nanostructures may induce new phenomena not found in bulk materials.¹³ Doping with the element zinc has commercial viability due to its low cost, abundance and environmental compatibility. Further, ZnO is expected to follow a similar reaction mechanism as MnO₂. Therefore, manganese oxide hybrid materials with a large specific surface area and high electrical conductivity are expected. In general, the electrical conductivity of the MnO₂ based materials can be improved by doping some conductive materials into manganese oxides.^14,15 Nagamuthu et al. studied the effect of Ag incorporation on the supercapacitor behavior of Mn₃O₄/AC nanocomposites. They achieved a specific capacitance of 180 F g⁻¹ at 10 A g⁻¹ with a higher energy density of 81 W h kg⁻¹.¹⁶ Byung Chul Kim et al. predicted enhanced electrochemical properties of cobalt doped manganese dioxide nanowires.¹⁷ They observed the higher specific capacitance of 415 F g⁻¹ at a constant discharge current density of 0.2 A g⁻¹. A maximum specific capacitance of 218 F g⁻¹ was achieved by Dubal et al. on 2 at% Fe doped MnO₂.¹⁸ The aim of the present study is to determine the supercapacitive properties of MnO₂ with zinc doping. Here, for the first time, we report a systematic study of the effect of Zn doping on the supercapacitive properties of MnO₂ by a chemical precipitation method. Structural, morphological and electrochemical studies of the synthesized nanoparticles were conducted.

2. Materials and methods

2.1 Chemicals

All chemical reagents were purchased in refined grade (E. Merck) and used without further purification. All solutions were prepared in deionized water. Manganese acetate tetra hydrate [C₄H₆MnO₄·4H₂O], zinc acetate [C₄H₆ZnO₄·H₂O] and potassium permanganate (KMnO₄) were used as precursors.

2.2 Synthesis of the pristine and Zn doped MnO₂ nanocrystals

For the synthesis of Zn doped MnO₂, 6.12 g (0.5 M) of manganese acetate tetra hydrate [C₄H₆MnO₄·4H₂O] was dissolved in 30 mL of deionized water and stirred vigorously by a magnetic stirrer. Then, zinc acetate [C₄H₆ZnO₄·H₂O] of a selected molarity (0.00, 0.025, 0.05, 0.075, 0.1 and 0.125 M) prepared in 20 mL deionized water was mixed drop by drop. Further, 5.92 g (1 M) of potassium permanganate in 50 mL of deionized water was added drop by drop to the above mixture. The entire mixture was stirred vigorously using a magnetic stirrer. After 5 hours of stirring, a dark brown colored precipitate of the precursor MnO₂ was obtained. The obtained precursor was washed and then dried in a hot air oven at 80 °C for 6 hours to evaporate water and organic material to the maximum extent. Finally, the obtained product was annealed in a muffle furnace at 400 °C for 3 hours. The annealed powders were pulverized to fine powders using an agate mortar for further characterization. A similar method of preparation, without the addition of Zn, was used to synthesize the undoped MnO₂ nanocrystals.

2.3 Material characterization

The crystalline phase and particle size of pure and Zn doped MnO₂ nanoparticles were analyzed by X-ray diffraction (XRD) which was carried out at room temperature using an X’PERT-PRO diffractometer system (scan step of 0.05° (2θ) and a counting time of 10.16 s per data point) equipped with a Cu tube for generating Cu K_α radiation (λ = 1.5406 Å) as an incident beam in the 2-theta mode over the range of 10–80°, operated at 40 kV and 30 mA. The Fourier transform infrared (FTIR) spectra were obtained using a SHIMADZU-8400 FTIR spectrometer in the range of 4000–400 cm⁻¹. The morphology of the product was observed by scanning electron microscopy (SEM; JEOL-JSM-56100) operating under a 20 kV accelerating potential and confirmed by field emission transmission electron microscopy (FE-TEM: JSM2100F JEOL). Multipoint N₂ adsorption–desorption experiments were carried out on a Micromeritix (ASAP 2020) analyzer, using the BET gas adsorption method, at 77 K.

2.4 Electrochemical performance investigation

Cyclic voltammograms (CVs) were achieved using a CHI 660C electrochemical analyser. A conventional three-electrode cell was used, including an Ag/AgCl (saturated KCl) electrode as the reference electrode, a Pt wire serving as a counter electrode, and a glassy carbon electrode coated with the prepared powder as a working electrode. The glassy carbon electrode (GCE) was sequentially polished with alumina powder before the experimental performance. To fabricate a working electrode, 0.5 mg of the prepared powder was dissolved in 500 µL of distilled ethanol and sonicated for 30 min. Further, 2 drops of this solution were pipetted onto the surface of the GC electrode and dried for 15 min at room temperature. After the evaporation of ethanol, the GC electrode was coated with a layer of the prepared sol, 0.1 mL of 5% Nafion solution and dried for 15 min to form a membrane on the top. A potential window in the range of −0.3 to 0.7 V was used in all the measurements. The CV measurements were performed at scan rates of 10, 20, 30, 40, 50, 70, 90 and 100 mV s⁻¹. GV charge–discharge measurements were conducted at various current densities of 10, 20, 30, 40, and 50 A g⁻¹ for evaluating the specific capacitance, power density and energy density.

3. Results and discussion

3.1 X-Ray diffraction

The diffraction features of undoped and Zn doped MnO₂ nanocrystals are shown in Fig. 1. The diffraction pattern of MnO₂ exhibits the pure phase of α-MnO₂ with the tetragonal structure of I4/m space group according to the standard card (JCPDS no. 44-0141). At low concentrations of Zn doping (≤0.05 M), the δ-phase of MnO₂ started to appear in addition to α-phase, with a slight shift in the diffraction patterns to the lower angle side.¹⁹ However, at higher concentrations of doping (>0.05 M), the (001) plane of δ-MnO₂ dominates with an intensity reduction in the (211) plane of α-MnO₂. The crystallite size of MnO₂ estimated from Scherrer’s formula is 9.6 nm, whereas the sizes are 4.3, 4.1, 3.7, 7.0 and 5.6 nm for MnO₂ doped with 0.025, 0.05, 0.075, 0.1 and 0.125 M of Zn, respectively.

Fig. 1 X-Ray diffraction patterns of undoped and different levels of Zn doped MnO₂ nanocrystals.

3.2 Functional group analysis

To know the presence of functional groups in the prepared samples, FTIR spectra were recorded in the range of 4000–400 cm⁻¹ and the results are presented in Fig. 2. The appearance of the broad absorption at around 3404 cm⁻¹ may be due to stretching vibrations of hydroxyl groups. The presence of bands at 1632 and 1402 cm⁻¹ can be ascribed to bending vibrations of the hydroxyl groups combined with Mn atoms. Apart from these, the intense bands observed at around 573 cm⁻¹ should be related to the Mn–O vibrations in MnO₆ octahedra. From the above discussions, it is an acceptable fact that MnO₂ in the present form has some bound water.

Fig. 2 FTIR spectra of undoped and different levels of Zn doped MnO₂ nanocrystals.

3.3 Morphology analysis

To better understand the effect of doping on the morphology of MnO₂, SEM micrographs were recorded for undoped and doped MnO₂. As shown in Fig. 3a and b, all the undoped product exhibits the presence of spherical particles with little agglomeration, whereas the doped products show the presence of spherical particles with a high degree of agglomeration. The high degree of agglomeration could be due to the non uniform dispersion of Zn into the MnO₂ lattice. In order to confirm the morphology of the doped product, FE-TEM measurements were made. As shown in Fig. 4a, the FE-TEM image of the MnO₂ reveals the presence of both spherical particles and nanorods. However, the FE-TEM image of the MnO₂ doped with Zn exhibits spherical particles with little agglomeration (Fig. 4b). The EDX spectrum of the Zn doped MnO₂ is shown in Fig. 3c which confirms the presence of Mn, Zn, and O in the doped product.

Fig. 3 SEM images of (a) pure MnO₂, (b) MnO₂:Zn (0.125 M) and (c) the corresponding EDX spectrum.

Fig. 4 FE-TEM image of (a) MnO₂ and (b) MnO₂:Zn (0.125 M).

3.4 Surface area analysis

Fig. 5a and b show the N₂ adsorption–desorption isotherm curves, displaying a classic H₃ hysteresis loop for the nanocrystals of MnO₂ and Zn doped MnO₂. The microporous properties of these materials are reflected by the N₂ adsorption in the low partial pressure area. The steep adsorption branches at high partial pressure are indicative of a large external surface area.²⁰ The mesopores are attributed to the interstitial space between the nanocrystals of these materials. In order to explain the role of Zn as a dopant, the BET measurement was recorded and the obtained results are present in Table 1. Pure MnO₂ has a specific surface area of 29.86 m² g⁻¹ with a pore volume of 0.234 cm³ g⁻¹. However, on doping with 0.125 M of Zn, the surface area of MnO₂ is increased to 46.47 m² g⁻¹ with a pore volume of 0.385 cm³ g⁻¹. The increased pore volume can enhance the formation of micropores accessible for the electrolyte, resulting in an increased capacitance of MnO₂ on Zn doping.

Fig. 5 (a) Nitrogen adsorption and desorption isotherms and the corresponding pore size distribution curve (inset) of the MnO₂ nanocrystals. (b) Nitrogen adsorption and desorption isotherms and the corresponding pore size distribution curve (inset) of Zn (0.125 M) doped MnO₂ nanocrystals.

Table 1 Surface area and pore structure parameters of MnO₂ and different concentrations of Zn doped electrodes of MnO₂

Samples	Specific capacitance (F g^−1)	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
MnO2	210	29.864	0.234
MnO2:Zn (0.025 M)	205	28.164	0.175
MnO2:Zn (0.05 M)	211	31.310	0.218
MnO2:Zn (0.075 M)	356	39.059	0.298
MnO2:Zn (0.1 M)	403	41.263	0.325
MnO2:Zn (0.125 M)	620	46.470	0.385

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3.5 Electrochemical studies (three electrode configuration)

3.5a Effect of Zn doping concentration. The electrochemical performance of the pure and Zn doped nanostructure electrodes were evaluated using cyclic voltammetry and galvanostatic charge discharge studies. Fig. 6a shows the typical CV curves of the pure and Zn doped MnO₂ modified GCE electrode in a 0.5 M KCl aqueous electrolyte over the voltage range −0.3 to 0.7 V and at a constant scan rate of 10 mV s⁻¹. The quasi-rectangular shape of the CV curves indicate the pseudocapacitance nature of all the electrode materials.²¹ The current density and integrated area in the CV curves for the Zn doped MnO₂ electrodes are significantly much higher those for the MnO₂ electrode, demonstrating that the presence of the Zn nanoparticles can improve the electrochemical performance. From the CV curves, the specific capacitance was calculated using the following equation.²²

Fig. 6 Electrochemical performance: (a) CV curves of the undoped and doped MnO₂ at 10 mV s⁻¹, (b) CV curves of MnO₂:Zn (0.125 M) at different scan rates, (c) plot of specific capacitance vs. Zn content, and (d) plot of specific capacitance at different scan rates.

At the scan rate of 10 mV s⁻¹, the estimated specific capacitance of the pristine MnO₂ is 210 F g⁻¹, whereas the Zn doped MnO₂ electrodes exhibit specific capacitances of 205, 211, 356, 403, and 620 F g⁻¹ for 0.025, 0.05, 0.075, 0.1 and 0.125 M respectively. Recently Chin-Yi Chen et al.²³ reported an enhanced specific capacitance of 230 F g⁻¹ for ZMO (Zn added Mn₃O₄) by the spray pyrolysis technique. Fig. 6b shows the variation in the specific capacitance of the prepared electrodes versus the Zn content in the binary oxide. The increase in the specific capacitance of MnO₂ with Zn content was clearly observed. When the ratio was less than 0.075 M concentration, the specific capacitance of the doped electrodes increased very slightly (211–352 F g⁻¹), perhaps the electrochemical properties of the doped electrodes were not yet optimal under such a ratio. When the concentration of the doping was 0.125 M, the specific capacitance of the doped electrodes increased to the maximum (620 F g⁻¹) at a scan rate of 10 mV s⁻¹, which is higher than pure MnO₂ (210 F g⁻¹). At low levels of Zn incorporation (0.025 and 0.05 M), as a result of low pore volume, MnO₂ was retained with its low specific capacitance. However, at higher concentrations, the increased pore volume can improve the capacitance values (Table 1). On the other hand, at 0.125 M of Zn doping, the electrode has more active sites that can yield a higher specific capacitance. The enhanced specific capacitance may be attributed to synergic effects of Zn and the pristine component. These results are much better than the reported C_s values for other nanostructure electrode systems such as graphene/MnO₂ (315 F g⁻¹),²⁴ CFP/MnO₂ (200 F g⁻¹)²⁵ and Zn/Mn₂O₃ (414 F g⁻¹).²⁶

3.5b Effect of scan rate. Fig. 6c shows representative CV curves for the MnO₂:Zn (0.125 M) electrode at different scan rates from 10 to 100 mV s⁻¹. At the scan rates of 10 to 50 mV s⁻¹, all the CV curves are quasi-rectangular and symmetric in shape, indicating a fast reversible Faradaic reaction and pseudocapacitve behavior. However, when the scan rate was increased further, the CV curves deviated from the quasi-rectangular shape. In addition, at high scan rates, the specific capacitance decreased.²⁷ It is well known that the charge storage mechanism of amorphous α-MnO₂ is mainly a surface process, which includes adsorption–desorption and insertion/extraction of proton and alkali cations. The process can be expressed as:

(ZnMnO₂)_surface + K⁺ + e⁻ = [KZnMnO₂]_surface

ZnMnO₂ + K⁺ + e⁻ = [ZnMnO₂]

Obviously, the diffusion of the cations (i.e., K⁺) into MnO₂ surface or bulk and the transfer of electrons have a great influence on the rate of charging–discharging capacitance. Higher scan rates result in a smaller available capacitance due to the reduced diffusion time.²⁸

By using eqn (1), we have calculated the specific capacitance of the modified electrodes for all the scan rates, and a graph has been drawn between specific capacitance values and scan rates (Fig. 6d). The specific capacitance of the MnO₂ electrodes is 210 F g⁻¹ at a scan rate of 10 mV s⁻¹; it is obvious that Zn doped MnO₂ electrodes show a higher performance than MnO₂. The 0.125 M Zn doped electrode achieves a higher specific capacitance of 620 F g⁻¹ at a scan rate of 10 mV s⁻¹, which is 45% higher than that of the MnO₂ electrode. At a higher scan rate (100 mV s⁻¹), MnO₂ exhibits the specific capacitance of 48 F g⁻¹, but it is 127 F g⁻¹ for MnO₂:Zn (0.125 M). In general, at higher scan rates, the charge transfer is found to be low because ions access a limited part of the active material that limits the charge transfer and diffusion rate of the electrolyte in the electrode materials and result in lower capacitance.²⁹

3.5c Charge–discharge studies. Current density is another criterion used to evaluate the rate capability, the power density and the energy density for the use of supercapacitors in power applications. Fig. 7a shows galvanostatic charging–discharging (GCD) curves of the pure and various levels of Zn incorporated MnO₂ electrodes at a current density of 10 A g⁻¹. The linear voltage versus time profiles, the symmetric charge–discharge characteristics and quick I–V response represent good capacitive characteristics of the product.³⁰ The increase in the charging time represents the higher specific capacitance. The specific capacitance has been evaluated from the charge–discharge curves, according the following equation.³¹where, C_s is the specific capacitance (F g⁻¹), i is the specific current (A), Δt is the discharge time (s), m is the mass of the active material (g), and ΔV is the potential window (V). The specific capacitance value obtained from discharge curve for MnO₂ is 200 F g⁻¹, whereas, it is 188, 207, 231, 290, and 550 F g⁻¹ respectively, for 0.025, 0.05, 0.075, 0.1 and 0.125 M of Zn doped MnO₂ electrodes.

Fig. 7 Galvanostatic charge–discharge curves: (a) CV curves for undoped and doped MnO₂ at 10 A g⁻¹, (b) charge–discharge curves for MnO₂:Zn (0.125 M) at different current densities, (c) plot of current density vs. specific capacitance, and (d) Ragone plot of the MnO₂:Zn (0.125 M) electrode.

The best specific capacitance of the electrode was further analyzed by the capacitance retention and energy density. The GCD curves of the MnO₂:Zn (0.125 M) electrodes at different current densities (10–50 A g⁻¹) are shown in Fig. 7b. The charge–discharge curves display a symmetric shape; indicating that the composite has outstanding supercapacitive behavior and there is a highly superior reversible Faradaic reaction between K⁺ and the nano MnO₂ matrix.²⁹ The specific capacitance and capacitive retention of the MnO₂ and MnO₂:Zn electrodes are presented in Fig. 7c. At 50 A g⁻¹, the MnO₂:Zn (0.125 M) electrode exhibits a reduced capacitance, whose value is 42% of its value at 10 A g⁻¹. The effect of the current density on the specific capacitance may be caused by the transport of effective ions into active materials. Higher concentration polarization at the large current density allows the charging process to reach completion in a short time. At the same time, protons in the vicinity of the electrode–electrolyte interface would be exhausted, thus retarding the redox transitions of electroactive species at a high current density. This is due to larger access to the electroactive surface by OH⁻ ions at low current densities, but at higher current densities, diffusion limits the OH⁻ ions movement, leading to a decrease in the capacitance.^32,33 The higher utilization of the electrode material is predictable in surface redox processes. The electrochemical utilization of the electrodes can be calculated from the following relation³⁴

where C_s is the specific capacitance (F g⁻¹), ΔV is the potential window (V), M is the molecular weight of MnO₂ (86.94 g), and F is the Faraday constant. The Z values of current densities 10, 20, 30, 40 and 50 A g⁻¹ are 0.546, 0.328, 0.182, 0.118 and 0.081, respectively. At a current density of 10 A g⁻¹, the higher content of electronegative sites (0.546) involved in the redox process improves the specific capacitance to 550 F g⁻¹.

Specific energy and specific power are the two key factors for evaluating the power applications of electrochemical supercapacitors. A good electrochemical supercapacitor is expected to provide high energy density or high specific capacitance at charging–discharging rates (current densities). Fig. 7d shows the Ragone plot for the MnO₂:Zn (0.125 M) composite electrode. The energy and power density are calculated from the following equations.³⁵

P = E/t (5)

where P, C_s, ΔV, t, and E indicate the average power density (W kg⁻¹), specific capacitance based on the mass of the electroactive material (F g⁻¹), the potential window of the discharge (V), discharge time (s) and average energy density (W h kg⁻¹) respectively. As the galvanostatic charge–discharge current density increased from 10 to 50 A g⁻¹, the specific energy decreased from 76 to 15 W h kg⁻¹ whereas, the specific power increased from 166 to 782 W kg⁻¹.

3.6 Impedance analysis

Fig. 8a and b shows a Nyquist plot of the MnO₂ and Zn doped MnO₂ electrodes in a 0.5 M KCl electrolyte solution. These measurements were carried out at room temperature in the frequency range of 0.01 Hz to 10 kHz at the open circuit potential, with an AC amplitude of 5 mV. The ion diffusion and effective electron transfer in the MnO₂ and Zn doped MnO₂ electrodes were further confirmed by EIS measurements. The equivalent circuit diagram used for the fitting of the EIS data is presented in the inset of Fig. 8a. In the fitting circuit, R_s and R_ct are the solution and charge-transfer resistance, respectively. C_dl is the double layer capacitance, and Z_w is the Warburg impedance. The impedance spectra of the all the electrodes are similar in form, with a quasi-semicircle at high frequency. The linear part corresponds to the Warburg impedance (W), which is described as the diffusive impedance of the OH⁻ ion within the electrode. In addition, the semicircle of the Nyquist plot corresponds to the Faradaic reaction and its diameter represents the interfacial charge transfer impedance (R_ct).³⁶ The R_s and R_ct calculated from the fitting of the experimental impedance spectra are presented in Table 2. The overall resistance values of the electrodes are a combination of the R_s with the R_ct values, which were found to decrease with increases in the Zn content in the electrodes, confirming the influence of the Zn ion on the improvement of the conductivity of the electrode. The charge transfer resistance of the MnO₂:Zn (0.125 M) electrode is 109.2 Ω, which is smaller than that of the MnO₂ electrode (275 Ω). The lower charge transfer resistance (R_ct) enhances the rate capacity of the MnO₂:Zn (0.125 M) electrode due to the addition of zinc, which maintains the low internal resistance of the electroactive material, resulting in lower polarization with higher electrochemical activity. From the results, the low electron transfer resistance is also responsible for the enhanced electrochemical performance.³⁷ This study also confirms that the MnO₂:Zn (0.125 M) electrode is the most suitable material for high rate supercapacitors.

Fig. 8 (a) Nyqusit plot of (a) MnO₂ with the electrical equivalent circuit (inset), and (b) different concentrations of the Zn doped MnO₂ electrodes.

Table 2 The calculated values of R_s and R_ct through the fitting of the experimental impedance spectra based on the proposed equivalent circuit in Fig. 8a

Samples	R s (Ω)	R ct (Ω)
MnO2	109.51	275
MnO2:Zn (0.025 M)	96.61	266.67
MnO2:Zn (0.05 M)	94.61	136. 6
MnO2:Zn (0.075 M)	93.52	120.56
MnO2:Zn (0.1 M)	92	115.56
MnO2:Zn (0.125 M)	88.36	109.81

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4. Conclusion

With the aim of tailoring the electrochemical properties of MnO₂, we have doped Zn in different proportions into the host material through a simple chemical precipitation method. The results of the XRD reveal the appearance of the δ-phase besides to α-MnO₂ at low levels of Zn incorporation. However, at higher levels, δ-phase domination has been observed. The FE-TEM analysis of MnO₂:Zn (0.125 M) confirms the significant change in the morphology of MnO₂. The electrochemical measurements imply that MnO₂ doped with 0.125 M of Zn is a promising candidate for high rate supercapacitor applications owing to its higher specific capacitance and power density.

References

1. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
2. X. Y. Wang, W. G. Huang, P. J. Sebastian and S. Gamboa, J. Power Sources, 2005, 140, 211–215.
3. M. Toupin, T. Brousse and D. Bélanger, Chem. Mater., 2002, 14, 3946–3952.
4. L. Athouël, F. Moser, R. Dugas, O. Crosnier, D. Bélanger and T. Brousse, J. Phys. Chem. C, 2008, 112, 7270–7277.
5. B. Babakhani and D. G. Ivey, J. Power Sources, 2010, 195, 2110–2117.
6. Y. T. Wang, A. H. Lu and W. C. Li, Microporous Mesoporous Mater., 2012, 153, 247–253.
7. W. F. Wei, X. W. Cui, W. X. Chen and D. G. Ivey, Chem. Soc. Rev., 2011, 40, 1697–1721.
8. J. Rajeswari, P. S. Kishore, B. Viswanathan and T. K. Varadarajan, Electrochem. Commun., 2009, 11, 572–575.
9. J. P. Zheng and T. R. Jow, J. Electrochem. Soc., 1995, 142, L6–L8.
10. J. K. Chang and W. T. Tsai, J. Electrochem. Soc., 2003, 150, A1333–A1338.
11. D. Belanger, X. Ren, J. Davey, F. Uribe and S. Gottesfeld, J. Electrochem. Soc., 2000, 147, 2923–2929.
12. F. Fusalba, P. Gouerec, D. Villers and D. Belanger, J. Electrochem. Soc., 2001, 148, A1–A6.
13. Y. J. Mai, J. P. Tu, X. H. Xia, C. D. Gu and X. L. Wang, J. Power Sources, 2011, 196, 6388–6393.
14. C. Zhao, F. Feng, X. Wang, R. Liu, S. Zhao and Q. Shen, Powder Technol., 2014, 261, 55.
15. G. Wang, R. Chen, Y. Zhou, H. Wang and J. Bai, J. Nanopart. Res., 2014, 3, 16.
16. S. Nagamuthu, S. Vijayakumar and G. Muralidharan, Dalton Trans., 2014, 43, 17528–17538.
17. B. C. Kim, C. J. Raj, W.-J. Cho, W.-G. Lee, H. T. Jeong and K. H. Yu, J. Alloys Compd., 2014, 617, 491–497.
18. D. P. Dubal, W. B. Kim and C. D. Lokhande, J. Phys. Chem. Solids, 2012, 73, 18–24.
19. M. Huang, Y. Zhang, F. Li, Z. C. Wang, Z. Alamusi, N. Hu, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 4518.
20. J.-G. Wang, Y. Y. Z.-H. Huang and F. Kang, Electrochim. Acta, 2012, 75, 213–219.
21. Y. Zhao and P. Jiang, Colloids Surf., A, 2014, 44, 232–239.
22. J.-W. Wang, Y. Chen and B.-Z. Chen, Met. Mater. Int., 2014, 20, 989–996.
23. C.-Y. Chen, H.-W. Chang, S.-J. Shih, C.-Y. Tsay, C.-J. Chang and C.-K. Lin, Ceram. Int., 2013, 39, 1885–1892.
24. M. Kim, Y. Hwang and J. Kim, J. Power Sources, 2013, 239, 225–233.
25. G. H. Yu, L. B. Hu, M. Vosgueritchian, H. L. Wang, X. Xie, J. R. Mc Donough, X. Cui, Y. Cui and Z. N. Bao, Nano Lett., 2011, 11, 2905–2911.
26. Y. S. Luo, J. Jiang, W. W. Zhou, H. P. Yang, J. S. Luo, X. Y. Qi, H. Zhang, D. Y. W. Yu, C. M. Li and T. Yu, J. Mater. Chem., 2012, 22, 8634–8640.
27. C.-Y. Chen, H.-W. Chang, S.-J. Shih, C.-Y. Tsay, C.-J. Chang and C.-K. Lin, Ceram. Int., 2013, 39, 1885–1892.
28. A. Sahoo and Y. Sharma, Mater. Chem. Phys., 2015, 49–150, 721–727.
29. S. L. Kuo and N. L. Wu, J. Electrochem. Soc., 2006, 153, A1317–A1324.
30. S. Nagamuthu, S. Vijayakumar and G. Muralidharan, Energy Fuels, 2013, 27, 3508–3515.
31. Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao and E. Xie, J. Am. Chem. Soc., 2013, 71, 174–182.
32. J.-G. Wang, Y. Yang, Z.-H. Huang and F. Kang, Electrochim. Acta, 2011, 56, 9240–9247.
33. S. I. Cho and S. B. Lee, Acc. Chem. Res., 2008, 41, 699–707.
34. H. Pang, B. Zhang, J. Du, J. Chen, J. Zhang and S. Li, RSC Adv., 2012, 2, 2257–2261.
35. S. Vijayakumar, S. Nagamuthu and G. Muralidharan, ACS Appl. Mater. Interfaces, 2013, 5, 2188–2196.
36. A. L. Reddy, M. M. Shaijiumon, S. R. Gowda and P. M. Ajayan, J. Phys. Chem. C, 2010, 114, 658–663.
37. J. Yan, Z. J. Fan, T. Wei, W. Z. Qian, M. L. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.

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