Investigation on the electrochemical performance of hausmannite Mn₃O₄ nanoparticles by ultrasonic irradiation assisted co-precipitation method for supercapacitor electrodes

ABSTRACT

Spinel type manganese oxide (Mn₃O₄) nanoparticles were synthesized by ultrasonic irradiation assisted co-precipitation method using PEG and NaOH. The formation of tetragonal hausmannite phase (Mn₃O₄) with spinel structure was confirmed from XRD and FTIR studies. The average crystallite size of Mn₃O₄ nanoparticles was determined from Scherrer formula (39.9 nm) and Williamson–Hall relation (36.5 nm). The presence of elements, the overall oxidation state of manganese (Mn²⁺, Mn³⁺ and Mn⁴⁺) and chemical composition of these nanoparticles were determined from Mn 2p, Mn 3s and O 1s spectra using XPS. The spherically shaped morphology of these nanoparticles was analysed by SEM. The expected chemical composition of Mn₃O₄ nanoparticles was analysed by EDS. The electrochemical performance of these Mn₃O₄ nanoparticles was studied by CV, CP and AC impedance analysis. The maximum specific capacitance of Mn₃O₄ nanoparticles was found to be 296 F/g at a current density of 1 A/g². Nyquist plot shows lower value of resistance that indicates the good electrical conductivity of the Mn₃O₄ electrode, which accounts for the improved electrochemical performance of the supercapacitors. Hence, the use of surfactant and the presence of ultrasonic irradiation in the synthesis process plays a dominant role in the electrochemical performance of Mn₃O₄ supercapacitors.

KEYWORDS:

• Spinel
• Mn₃O₄ nanoparticles
• oxidation state
• electrochemical performance
• electrode

1. Introduction

In recent years, high performance electrical energy storage materials are needed in electrical vehicles, batteries and power electronics. Supercapacitor is one of the energy storage devices; it can store the energy by diffusion/surface redox process in the electrode materials. Mn₃O₄ is a promising electrode material for supercapacitor due to the availability of multiple oxidation states. It is also applied in a variety of fields such as solid oxide fuel cells (SOFCs), sensors, supercapacitor and lithium-ion batteries [1,2]. Mn₃O₄ can also be used as an active catalyst for the oxidation of carbon monoxide and methane or the selective reduction of nitrobenzene [3,4]. Among the various types of spinel system, Mn₃O₄ material belongs to normal spinel having the general formula of AB₂O₄ with divalent manganese (Mn²⁺) occupying in the tetrahedral (A) sites and trivalent manganese (Mn³⁺) occupying in the octahedral (B) sites i.e. Mn²⁺[Mn³⁺₂]O²⁻₄. Bulk Mn₃O₄ exhibits a tetragonal Jahn–Teller distortion at the trivalent manganese (Mn³⁺) site at high temperatures with I4₁/amd space group [5].

Based on the practical applications; Mn₃O₄ can be synthesized by different methods. Chen et al. have synthesized three different morphologies of Mn₃O₄ (nanoparticles, nanorods and nanofractals) through chemical liquid homogeneous precipitation method by controlling the dripping speed of NaOH solution [6]. WS Seo et al. [7] have synthesized Mn₃O₄ nanoparticles by thermal decomposition of a single precursor [Mn(acac)₂] in oleylamine under an inert atmosphere. The average size of 10 nm Mn₃O₄ was synthesized through esterification process by Li et al. using alcohol and Mn(Ac)₂,4 H₂O [8]. Mn₃O₄ hexagonal nanoplates and nanoparticles were synthesized by Ahmed et al. via hydrothermal oxidation process at low temperature and a solvothermal oxidation method [9]. Ganesh Kumar et al. have synthesized pure single-phase spinel-tetragonal-Mn₃O₄ nanoparticles under open-air conditions using manganese salts [10]. Mn₃O₄ nanowires were obtained via solvothermal treatment of c-MnOOH in polypropylene glycol for 24 h [11]. By polyol method, Mn₃O₄ nanoparticles were obtained using Mn(CH₃COO)₂ as precursors and diethylene glycol [12]. Also, high intensity ultrasonic irradiation method was reported for the synthesis of Mn₃O₄ nanoparticles with an average size of 4 and 15 nm by Rohani et al. [13,14]. Some researchers are also focused on preparing 1-Dimensional nanomaterials through ultrasonic irradiation method for their unique properties [15]. 1-D Mn₃O₄ nanorods were obtained by Yang et al. using high temperature calcinations of c-MnOOH [16].

Recently, researchers are paying much attention towards simple, effective and low temperature solution method for the preparation of pure phase nanomaterials. Among the wet-chemical method, ultrasonic irradiation assisted co-precipitation method is facile and attractive method for the synthesis of nanomaterial in low temperature medium due to the efficient mixing and faster mass transfer of the reactants under ultrasound [17]. In addition to this method, the good morphology of nanomaterials is obtained. Ultrasonic assistance synthesized nanomaterials are more active in photocatalysis due to the high surface area and particle size [18].

Stabilization has gained special attention and is achieved using surfactants as protective molecules, which prevent the agglomeration on the surface of the nanoparticles [19]. Among these molecules, polyethylene glycol (PEG) has been widely used as both size controller and capping agent to protect nanoparticles from agglomeration. The stabilization of metal colloids, the size and shape of nanomaterials depend strongly on the solution concentration of PEG.

In this present investigation, PEG works both as size controller and capping agent for the synthesis of spinel type manganese oxide (Mn₃O₄) by ultrasonic-assisted co-precipitation method using metal nitrate as a starting precursor. The novelty of present work is to understand that the preparation conditions of this spinel type Mn₃O₄ material have potential impact on the electrode parameters such as phase, morphology and cation distribution thereby controlling the electrochemical performance of Mn₃O₄ supercapacitors. Hence, we made an attempt to prepare Mn₃O₄ spinel material via ultrasonic-assisted co-precipitation method. The phase identification of these manganese oxide nanoparticles was characterized by X-ray diffraction (XRD) and Fourier transfer infrared spectroscopy (FTIR) studies. The X-ray photoelectron spectroscopy (XPS) was used to analyse the presence of elements, oxidation states and chemical composition of the synthesized nanoparticles. Morphological nature and elemental analysis of the samples were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). For supercapacitor application, the samples were characterized by cyclic voltammetry (CV), chronopotentiometry (CP) and AC impedance analysis. To the best of our knowledge, there is no report about the ultrasonic-assisted co-precipitation synthesis of spherically shaped Mn₃O₄ nanoparticles under low temperature for supercapacitor application.

2. Experimental

2.1. Synthesis

Pure spinel type manganese oxide nanoparticles were prepared by ultrasonic irradiation assisted co-precipitation method. Stoichiometric amounts of manganese nitrate (Mn(NO₃)₂·4H₂O) (Alfa Aesar 98% purity), was dissolved in about 20 ml of distilled water. Then, few grams of polyethylene glycol (PEG) and NaOH were added dropwise into a nitrate solution. The aqueous solution was maintained at pH = 10 while stirring for 4 h at a temperature of 60°C. The mixed solutions were aged for 10 h before sonication. The prepared solution was transferred into an Erlenmeyer as a reaction flask and placed in an ultrasonic water bath. Then, the solutions were sonicated for one hour at a power of 5 W/cm². The obtained precipitates were washed with de-ionized water and acetone several times; dried at 120°C. The resulted dried powder was grounded to form fine powder. All experiments were carried out in ambient conditions under air atmosphere. The schematic diagram represents the synthesized Mn₃O₄ nanoparticles are shown in Figure 1.

Figure 1. Schematic diagram of synthesized Mn₃O₄ nanoparticles.

2.2. Characterization and instrument

X-ray diffraction measurements for the as-synthesized powders were carried out by Bruker D2 Phaser Powder X-ray diffractometer with a CuK_α source (λ = 1.5418 Å). X-ray diffraction profiles were obtained for 2θ ranging from 10° to 80° with a scanning rate of 0.02/min. FTIR measurements were analysed by BRUKER ALPHA spectrophotometer using opus 6.5 (version) software. For recording FTIR spectra, the powder was mixed with KBr and then pressed into a disc of 1 mm thickness. FTIR measurements were carried out at room temperature in the range of 400–1400 cm⁻¹. The chemical states of elements present in the synthesized powders were analysed by X-ray photoelectron spectroscopy (XPS) using Kratos Analytical Axis Ultra DLD with Al Kα1 radiation. For wide spectrum, the energy of an X-ray photon of 1.486 keV with pass energy of 160 eV was used and narrow scan spectra of Mn 2p, Mn 3s and O 1s were recorded in the binding energy range from 0 to 1200 eV using pass energy of 40 eV. All XPS spectra were recorded in vacuum below 5 × 10⁻¹⁰ mbar. The XPS spectra were collected using the combination of electrostatic and magnetic lens for an analysed area of (700 × 300 µm). Surface charging effects were minimized using a charge balance operating at 3.6 and 1.8 V maintained as filament bias. During XPS analysis, sample charging was neutralized using an electron flood gun. The C 1s signal from adventitious hydrocarbon at 284.8 eV was used as an energy reference to correct for charging. All data processing was carried out using CasaXPS software package (version 2.2.14). A Shirley-type background subtraction was used as a baseline for all peaks, and curve fitting using an 80% Gaussian/20% Lorentzian function. The surface morphology and elemental analysis were done using scanning electron microscope (SEM) Quanta FEG 250 operated at 20 kV and Energy Dispersive X-ray Spectrometry (EDS) EDAX-TSL. The ultrasonic equipment used for the synthesis of Mn₃O₄ nanomaterials was a PCI81 Analytics ultrasonic bath with tank dimension of 300 × 150 × 100; overall dimension of 330 × 175 × 250, operating voltage of 230 V. The electrochemical performance test was carried out by three-compartment cell employing as-prepared working electrode, Ag/AgCl as reference electrode and platinum wire as counter electrode. The 3 M KOH solution was used as an electrolyte for electrochemical measurement using CHI 661 C electrochemical workstation with DELL personal computer for data acquisition and potential control. The Cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance studies were carried out.

2.3. Preparation of working electrodes

In the typical procedure, 85 wt.% of electroactive material (Mn₃O₄) was mixed together with 5 wt.% of conducting graphite and 10 wt.% of polyvinylidene fluoride until it becomes a homogeneous powder. To this mixture, a few drops of 1-methyl-2-pyrrolidinone were added to make the paste. The resulting paste was coated onto nickel foil (1 × 1 cm) collector, then dried at 80°C for 4 h. The mass of the electroactive material was approximately taken to be 0.2 mg measured in Shimadzu analytical balance.

3. Results and discussion

3.1. X-ray diffraction (XRD) studies

The phase identification of the synthesized powder was examined by X-ray diffractometer. Figure 2 shows the XRD pattern of synthesized Mn₃O₄ nanoparticles obtained by co-precipitation method. It shows that all the diffraction peaks of (101), (112), (200), (103), (211), (004), (220), (105), (312), (303), (321), (224) and (400) planes were well matched with ICDD no.080-0382 and it can be attributed to the tetragonal hausmannite phase of Mn₃O₄ spinel structure. No other extra/impurity diffraction peaks were detected in this XRD pattern. This implies the purity of the synthesized powders which is further confirmed from FTIR and XPS studies. The broadening of diffraction peaks indicates the small size of the crystallites within the resulting powders. The average crystallite size (d) of the synthesized Mn₃O₄ powders was calculated from Scherrer formula using the equation [20],(1) $d=k\lambda /(\beta \cos \theta ),$(1) where d is the average crystallite size, θ is the diffraction angle, λ (0.154 nm) is the wave length of X-ray beam, k is the constant (0.9 assuming nanoparticles are spherical in shape) and β is the full-width half maximum (FWHM). It is observed that the average crystallite (d) size of the as-synthesized Mn₃O₄ nanoparticles is 39.9 nm. However, in comparison with the literatures, this value is in good agreement with the reported value of 25 nm by thermal decomposition method [21], 30 nm by simple precipitation method [22], 22 nm by sonochemical synthesis respectively [23].

Figure 2. Powder X-ray diffraction (XRD) pattern of the synthesized Mn₃O₄ nanoparticles.

The average crystallite size (d) and the micro strain in the synthesized Mn₃O₄ nanoparticles were estimated by using Williamson–Hall relation [24],(2) $\frac{K}{d}+\frac{4\epsilon \sin \theta }{\lambda }=\frac{\beta \times \cos \theta }{\lambda }.$(2) Here ε is the micro strain developed due to imperfection in crystals. The Williamson–Hall (W–H) equation incorporates the Scherrer formula of average crystallite size and the micro strain terms. Figure S1 shows the W–H graph of Mn₃O₄ nanoparticles by plotting βcosθ versus 4sinθ for all the diffraction peaks. The W–H plot is a straight line in which the reciprocal of an intercept gives the average crystallite size and the slope gives the micro strain. The average crystallite size and micro strain obtained from W–H plot are mentioned in Table 1. From the table, it is evident that the average crystallite sizes obtained from both results are in good agreement with each other. The micro strain value obtained from the slope of W–H plot is positive, it gives the relaxation strain. This clearly indicates that surfactant and the presence of ultrasonic irradiation induces strain which leads to the formation of spherically shaped nanoparticles as observed in SEM [25].

Table 1. XRD parameters of synthesized Mn₃O₄ nanomaterials.

Parameters	Mn3O4 nanoparticles
d, nm (Scherrer formula)	39.9
d, nm (W–H relation)	36.5
ε (× 10^−3)	13.75
δ × 10^−3 (Lin-nm^−2)	0.75

The dislocation density (δ), defined as the length of dislocation lines per unit volume, gives the amount of defects in a crystal. The dislocation density (δ) was calculated using the formula [26](3) $\delta =1/d^2.$(3) The dislocation density (δ) value obtained from the above equation is noted in Table 1. The table shows the lower value of dislocation density. This means that use of surfactant and the presence of ultrasonic irradiation in the synthesis of Mn₃O₄ nanoparticles, reduces the dislocation density (δ). This clearly indicates that surfactant and the presence of ultrasonic irradiation enhances the crystallinity of Mn₃O₄ nanomaterials, making it a potential candidate for optoelectronic devices [27].

3.2. Fourier transfer infrared spectroscopy (FTIR) studies

FTIR analysis was carried out to examine the presence of phase of Mn₃O₄ nanoparticles. Figure S2 shows the FTIR spectra of the prepared Mn₃O₄ spinel nanoparticles in the wavelength region of 400–4000 cm⁻¹. It is known that spinel type Mn₃O₄ comes under the normal cubic structure in which Mn²⁺ ion occupies in tetrahedral (A) site and Mn³⁺ ion occupies in octahedral (B) sites of Mn²⁺[Mn³⁺₂]O²⁻₄. The FTIR spectra band at 462.5 cm⁻¹ corresponds to the vibration of trivalent manganese ions in the octahedral coordination [28]. The vibration band at 507.4 cm⁻¹ was attributed to the distortion of Mn–O in an octahedral (B) site [29]. The strong band at 622.3 cm⁻¹ was assigned to the stretching vibration mode of Mn²⁺–O at tetrahedral (A) site [30,31]. The absorption band at 1300–1600 cm⁻¹ is due to C–H bending vibration and adsorbed water molecules on the nanoparticles. The broad band observed in the range of 3000–3500 cm⁻¹ indicates the stretching vibrations of H₂O molecules on the surface of samples. These FTIR observations confirm that no additional/impurity phase was formed which further supports the XRD and XPS results.

3.3. X-ray photoelectron spectroscopy (XPS) studies

The overall oxidation state of Mn ions and chemical composition of the synthesized Mn₃O₄ nanoparticles was examined by X-ray photoelectron spectroscopy (XPS). Figure 3 shows the XPS survey spectrum of manganese oxide nanoparticles, the presence of Mn, O and C elements were detected. No other elements were identified; it indicates the purity of the samples which is further supported from XRD and FTIR results. The data of atomic ratio obtained from survey spectrum of Mn₃O₄ nanoparticles are nearly close to 3/4 agreeing with the expected chemical composition of normal spinel structure of synthesized Mn₃O₄ nanoparticles.

Figure 3. XPS (a) Survey spectrum, (b) deconvolution spectra of Mn 2p, (c) deconvolution spectra of Mn 3s and (d) deconvolution spectra of O 1s for Mn₃O₄ nanoparticles.

Figure 3(b) shows the high-resolution XPS spectra of Mn 2p peak. Due to the occurrence of spin orbit splitting, Mn 2p peak gives rise to a doublet with the two possible states 3/2 and 1/2 having different binding energies at 641.84 and 653.4 eV with energy splitting of 11.6 eV, which is similar with the previous report on XPS spectrum of Mn₃O₄ [32]. Generally, Mn₃O₄ has normal spinel structure with Mn having different oxidation states which can be present in the tetrahedral sites (Mn²⁺) and octahedral sites (Mn³⁺) with the chemical composition of Mn²⁺[Mn³⁺₂]O²⁻₄. In order to determine the different oxidation states of Mn ions, we can deconvolute the Mn 2p peak by Gaussian–Lorentzian function. The Mn 2p_3/2 peak consists of two components with binding energies values of 641.5 and 643.1 eV, which corresponds to binding energies of Mn²⁺ and Mn⁴⁺ species [33]. Meanwhile, this species at binding energies of 641.5 and 643.1 eV are attributed to MnO and MnO₂. Theoretical calculation represents 40 wt% of MnO₂ is in Mn₃O₄ and remaining 60 wt% is in Mn²⁺. The area ratio (i.e. molar ratio) of Mn⁴⁺ versus Mn²⁺ is almost 1:2 which agree well with theoretical oxide compositions of MnO₂–2MnO.

The formation of manganese oxide (Mn₃O₄) was further confirmed from splitting of Mn 3s spectra. This splitting of spectra is proportional to the remaining electron in the 3s orbital and the other unpaired electrons, which all have parallel spins [32]. Figure 3(c) shows the XPS Mn 3s spectra of Mn₃O₄ nanoparticles. In the Mn 3s spectra, a 2p_3/2-2p_1/2 doublet at 83.7 and 89.23 eV is observed, and the splitting width (5.53 eV) agrees with an earlier report on Mn₃O₄ [34].

The change in the oxidation state of manganese during redox reaction was also confirmed by analysing the O 1s core level XPS spectra. Figure 3(d) shows the high-resolution O 1s XPS spectra for Mn₃O₄ nanoparticles. It shows that the O 1s spectra can be fitted into three components at 529.63, 530.49 and 531.47 eV, which are related to the Mn–O–Mn, Mn–OH and H–O–H bonds respectively. The variation in the relative intensity of the Mn–O–Mn and Mn–OH signals indicates the change in the oxidation state of manganese. The formula used to determine manganese oxidation state from the intensities of the Mn–O–Mn and Mn–OH signals is given by the equation [35],$\text{Oxidation state}=\frac{\left[\left(\text{IV}\ast \left(\text{SMn}-\text{O}-\text{Mn}-\text{SMn}-\text{OH}\right)\right)+(\text{III}\ast \text{SMn}-\text{OH})\right]}{\text{SMn}-\text{O}-\text{Mn}}.$where S is the bond corresponding to the signals from the O 1s spectra. From the equation, the obtained mean manganese oxidation state is found to be 2.99 (∼3) for Mn₃O₄ nanoparticles. Therefore, the overall oxidation states of manganese are Mn²⁺, Mn³⁺ and Mn⁴⁺. The growth mechanism of Mn₃O₄ nanoparticles can be represented as follows;$\text{M}{\text{n}}^{2+}+\text{2O}{\text{H}}^{-}=\text{Mn(OH}{)}_2,$$\text{Mn(OH}{)}_2+{\text{O}}_2=\text{MnOOH}+\text{Mn(OH)2}=\text{(M}{\text{n}}^{2+}\text{, M}{\text{n}}^{3+})\text{Precursor,}$

$\left(\text{M}{\text{n}}^{2+}\text{, M}{\text{n}}^{3+}\right)\text{Precursor}=\text{M}{\text{n}}_3{\text{O}}_4\text{nanoparticles}.$ 3.4. Morphology studies

Field emission scanning electron microscope (FE-SEM) is used to decipher the surface morphology of the synthesized Mn₃O₄ nanoparticles. The SEM micrographs of Mn₃O₄ nanoparticles are shown in Figure 4(a–c). It is observed that the particles are nanosized have spherical morphology and are slightly agglomerated. The reason behind the formation of spherical shape and agglomeration of nanoparticles in the presence of ultrasonic irradiation can be explained by three reasons:

1. Use of surfactant in the synthesis process induces strain which leads to the formation of spherically shaped nanoparticles as discussed in XRD studies.

2. PEG acting as a capping agent has strong adsorbed on the surface of Mn₃O₄ particles with coordination bonds. This strong capping agent on the surface of nanoparticles hinders the particle growth. By applying the ultrasonic waves can led to an irreversible desorption of capping agents (PEG) from the surface of nanoparticles by cavitational collapse [36,37]. Keeping them from excessive growth and leading to the formation of nanoparticles. For this reason, the spherical shape of particles can be observed in the presence of ultrasonic irradiation.

3. Aggregation of Mn₃O₄ nanoparticles results in particle size in the range of <100 nm and this is the cause for agglomeration.

Figure 4. FE-SEM morphology and EDX spectra of Mn₃O₄ nanoparticles.

3.5. Energy dispersive X-ray spectroscopy (EDS) analysis

Energy dispersive X-ray spectroscopy is a useful analytical technique to analyse the chemical elements and compositions in the sample. EDS spectra for the synthesized spinel manganese oxide nanoparticles is shown in Figure 4(d). Form the EDS result confirms the presence of Mn and O as elementary components in the synthesized Mn₃O₄ nanopowders. The data of atomic ratio obtained from EDS spectrum of the synthesized nanoparticles are listed as an insert (table) of Figure 6(d). The obtained values are nearly close to 3/4 agreeing with the expected chemical composition of normal spinel structure of synthesized Mn₃O₄ nanoparticles. Based on the EDS observations, it is confirmed that no impurity or additional elements were present which further supports the XRD and XPS results.

3.6. Electrochemical properties

3.6.1 Cyclic voltammogram analysis

The supercapacitor behaviour of synthesized Mn₃O₄ nanoparticles at room temperature was studied by cyclic voltammetry (CV). The CV curve of Mn₃O₄ nanoparticles electrode in 1M aqueous Na₂SO₄ electrolyte solution at different scan rate of 5, 25, 50, 75 and 100 mV s^-1are shown in Figure 5. It could be clearly observed that all the curves show no redox peaks in the range of 0–0.55 V (vs. SCE). Furthermore, a CV curve shows rectangular under various scanning rates. This indicates the synthesized Mn₃O₄ electrode has excellent ideal capacitive behaviour at room temperature. This agrees with the previous reports [9,35,38].

Figure 5. Cyclic voltammetry curves of prepared Mn₃O₄ electrode material recorded at scan rate of 5–100 mV s⁻¹.

3.6.2 Galvanostatic charge–discharge analysis

The electrochemical properties of synthesized Mn₃O₄ nanoparticles at room temperature were also studied by galvanostatic charge–discharge tests. Figure 6 shows the cyclic performance and corresponding charge–discharge curves of Mn₃O₄ nanoparticles at current density between 1 and 10 A/g². The specific capacitance was calculated from the charge–discharge curves using the relation [39](4) $\text{SC}=\text{I}\mathrm{\Delta }t/\mathrm{\Delta }{V}_{\mathrm{m}},$(4) where SC is the specific capacitance, I is the current, Δt is the discharging time, ΔV is the potential drop during discharge and m is the mass of the active electrode material.

Figure 6. Charge-discharge curves of the prepared electrode materials at different current densities of 1–10 A g⁻².

An SC of 296 F/g for the prepared Mn₃O₄ electrode material is obtained at a current density of 1 A/g². The obtained SC value is in good agreement with those reported by Hao Jiang et al. for Mn₃O₄ nano-octahedrons prepared via hydrothermal route using EDTA (322 F/g) [35], Mn₃O₄ nanoparticles (diameter 10 nm) synthesized by Lu Wang et al. through ultrasonic-assisted route of whose SC of 262.1 F/g at a current rate of 0.4 Ag⁻¹ [40] and microwave-assisted reflux-synthesized Mn₃O₄ nanoparticles by Kalimuthu Vijaya Sankar et al. of SC 94 F/g in 6 M KOH electrolyte solution [41].

Figure 7 shows the specific capacitance versus scan rate for prepared Mn₃O₄ electrode materials at a different scan rate of 5–100 mV s⁻¹. Insert figure shows the specific capacitance versus applied current. From Figure 7(insert), specific capacitances calculated from cyclic voltammetry (CV) are in agreement with those calculated from galvanostatic charge–discharge technique. Moreover, from both studies, specific capacitance values are found to decrease in the range of 234–78 F/g from CV and 296–95 F/g from galvanostatic charge–discharge. This decreasing behaviour of SC may be due to low scan rates as there is sufficient time for diffusion of ions into the host Mn₃O₄ material and maximum utilization of active species available but at high scan rates only surface ions participate in the redox reaction and agglomeration is observed from FESEM images. Generally, the specific capacitance of the materials depends on high crystallinity, as it enhances the ionic mobility of the charge carriers [42]. To define the material capability for long and high-power deliverance, a long charge–discharge characteristic has been carried out. Figure 8(a) shows the long cycling profile of the electrode Mn₃O₄ material carried out for over 1000 cycles at 3 A/g in in 3 M KOH solution. The Columbic efficiency (η) of the electrode Mn₃O₄ material was calculated using the formula [39](5) $\eta =\left(t_d/t_c\right)\ast 100\mathrm{\%},$(5) where t_d is the discharge time and t_c is charging time from charge–discharge curves. Figure 8(a, b) shows the electrode Mn₃O₄ material realizes the maximum columbic efficiency of 84% as expected from its nearly symmetrical charge–discharge curves and strong cyclic stability during long cycle periods, resulting in reasonable reversibility characteristics.

Figure 7. Specific capacitance vs. scan rate for prepared Mn₃O₄ electrode materials at different scan rate. Insert figure showing the decrease in specific capacitance with increase in applied current.

Figure 8. (a) Cycle stability for the electrode Mn₃O₄ material recorded at the current density of 3 A g⁻¹ up to 1000 cycles and (b) Continuous charge-discharge cycles.

3.6.3. Electrochemical impedance spectral analysis

The chemical kinetics of electron and ions at the electrode in the electrode–electrolyte interface was examined by electrochemical impedance spectroscopy. Figure 9 shows the electrochemical impedance plot or Nyquist plot drawn real part Z′ against the imaginary part Z″ for Mn₃O₄ electrode materials in the frequency range of 1–10⁵ Hz. Incomplete semicircle near high frequency region depicts the lower resistance of Mn₃O₄ electrode materials and diffusion controlled rate kinetics of the redox process. The diameter of the semicircle gives the measure of charge transfer resistance (R_ct). Negligible diameter implies the lower R_ct resistance of the material. Hence, lower resistance i.e. high conductivity of Mn₃O₄ electrode material makes it a better candidate of supercapacitor application as less energy will be wasted during the charge–discharge process.

Figure 9. Nyquist plot for prepared Mn₃O₄ electrode materials. Insert figure shows the impedance at higher frequency region.

4. Conclusion

In summary, ultrasonic irradiation assisted co-precipitation method has been used to successfully synthesize Mn₃O₄ nanoparticles using PEG and NaOH. The XRD studies show that the as-synthesized nanoparticles possess hausmannite phase Mn₃O₄ with spinel structure. The average crystallite size obtained from Scherrer formula (39.9 nm) and Williamson-Hall relation (36.5 nm) are in good agreement with each other. The lower value of microstrain and dislocation density is due to the use of surfactant and the presence of ultrasonic irradiation in the synthesis process and hence increases the crystallinity of Mn₃O₄ nanoparticles. The formation of spinel Mn₃O₄ nanoparticles was also confirmed from FTIR analysis in the band region of 450–650 cm⁻¹. The presence of elements (Mn and O), chemical composition and oxidation states of manganese (Mn²⁺, Mn³⁺ and Mn⁴⁺) were determined from Mn 2p, Mn 3s and O 1s spectra using X-ray photoelectron spectroscopy studies. The surface morphology of the sample shows spherical with slightly agglomerated assembly of nanoparticles and the elemental analysis reveals that the sample contains Mn and O elements, respectively. The investigation of the electrochemical performance of spinel Mn₃O₄ nanoparticles was performed by cyclic voltammetry (CV), chronopotentiometry (CP) and AC impedance analysis. The maximum SC of Mn₃O₄ nanoparticles was found to be 296 F/g at a current density of 1 A/g². Nyquist plot shows lower value of R_ct which indicates the good electrical conductivity of the Mn₃O₄ electrode, which accounts for the improved electrochemical performance of the supercapacitors.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

R. Tholkappiyan http://orcid.org/0000-0001-6729-5227