Triethanolamine–ethoxylate (TEA-EO) assisted hydrothermal synthesis of hierarchical β-MnO₂ nanorods: effect of surface morphology on capacitive performance

Analytical grade reagents (KMnO₄ and MnSO₄·H₂O) were purchased from Qualigen SD. Fine-Chem Ltd India. Tri-ethanol amine ethoxylate was purchased from Venus chemicals, Goa, India. Ethanol and double distilled water were used as a solvent for all the experiments. Graphite sheets (∼250 μm thick), was purchase from Nickunj Eximp Entp., India.

Synthesis of hierarchical nanostructured MnO₂ have been carried out by adding 2 g KMnO₄ and 0.8g MnSO₄·H₂O in 50 ml double distilled water to a 5 ml solution of tri-ethanolamine ethoxylate(TEA-EO) under magnetic stirring. The obtained mixture was stirred magnetically at ambient temperature for 30 min to obtain homogenous mixture. Further the mixture was sifted to Teflon-lined stainless steel autoclave (capacity 250 ml). After sealing the autoclave properly it was heated at different temperatures for 12 h. The hydrothermal reaction temperature was conducted at 100 °C and 120 °C overnight to obtained two types of samples. After treatment autoclave was cooled up to room temperature to collect the prepared precipitate. Sample was collected using 4–5 times centrifugation (5000 rpm up to 5 min). It was washed with ethanol and using deionized water several times to maintain pH 7. Afterward, the synthesized precursors were calcined at 300 °C in the presence of air for 6 h to get the final product. Temperature was increased with scanning rate of 1 °C per min. The resulting samples synthesised at 100 °C and 120 °C are named S1 and S2, respectively.

Graphite sheet (area = 1 cm²) as current collector (substrate) was used to fabricate the working electrode was fabricated. In the whole process acetylene black worked as conducting agent and polyvinylidene fluoride (PVDF) worked as binder. The ratio of active material, conductive additive and binder was fixed as 90:5:5. To achieve the homogeneous mixture said materials were initially grinded in a mortar pestle. Obtained mixture was further dispersed in dimethyl formamide (DMF) forming a slurry, then coated on graphite sheet and dried at 60 °C. The weight of active mass loaded on the graphite electrode was measured by sensitive electrical balance. The deposited mass of β-MnO₂ on the current collector was estimated using weight balance of least count 0.01 mg. In different electrodes the mass of β-MnO₂ was found to be in the range of 0.8–1.1 mgcm⁻².

Elemental composition, crystal structure and surface morphologies, of as-synthesized β-MnO₂ nano-rods samples were studied using transmission electron microscope (FEI TECNAI G2), x-ray diffractometry (Bruker AXS, D8 advance) operating at 20 kV and 40 mA with CuKa radiation (λ = 1.5418 Å) and field emission scanning electron microscopy (Carl Zeiss, Ultra plus). A detailed study of the β-MnO₂ structure of samples synthesized at 100 °C and 120 °C was obtained by Rietveld refinement at 2θ (5°–95°) with slow scan rate (0.01880°) on 4788 points. Both the sample's data were refined by the same file available at Crystallography Open Database (COD-9009081). The simulated pattern was done by use of FULLPROF program. X-ray Photoelectron Spectroscopy measurements were performed using XPS analyser (Perkin Elmer model 1257), while FT-IR data obtained by FT-IR analyser (Nicolet NEXUS Aligent 1100).

Electrochemical workstation (CHI 680) was used to carry out electrochemical studies. For carrying out cyclic voltammetry three electrode system was adopted. Prepared electrode material was used as working electrode and Ag/AgCl was used as a reference electrode. For aqueous system most popular counter electrode is platinum which is chosen as counter electrode in present study also. Electrochemical studies were performed using 1 mol l⁻¹ Li₂SO₄ and 1 mol l⁻¹ Na₂SO₄ electrolyte with said electrode system within the potential window 0 V–0.8 V. Impedance spectroscopic studies was carried out within the frequency range 1 Hz to 100 kHz at open circuit potential and 5 mV AC perturbation.

For the developed prototype supercapacitor device, the values of specific capacitance (Cs) (Fg⁻¹), energy density E (WhKg⁻¹), power density P (kW kg⁻¹) and coulomb efficiency (η) (retention capacity) were calculated by using equations (1)–(4), respectively [2].

$$\begin{eqnarray}&&{C}_{s}=\displaystyle \frac{1}{mv\left({V}_{c}-{V}_{a}\right)}\displaystyle {\int }_{{V}_{a}}^{{V}_{c}}I(V)dv\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&E=\displaystyle \frac{0.5\times {C}_{s}({V}_{\max }^{2}-{V}_{\min }^{2})}{3.6}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&P=\displaystyle \frac{E\times 3600}{{t}_{d}}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\eta =\displaystyle \frac{{t}_{d}}{{t}_{c}}\end{eqnarray} \tag{ 4 }$$

where m is the mass of electrode material deposited over graphite sheet (gcm^–1), I(v) (mA) is the response current per unit area of the MnO₂ electrode. V is known as scan rate and (V_c − V_a ) is measured in (V) is called the operational potential window. I_d and t_d is known as discharge current and discharge time respectively however V_a and V_c are known as anodic current and cathodic current respectively.

Phase and purity of synthesized β-MnO₂ samples S1 and S2 were analysed by x-ray diffractometry. Figure 1(a) depicts the XRD pattern of both the samples in the 2θ range of 20° to 80°. As shown in figures 2(a) and (b), the resulting XRD pattern is well fitted by Rietveld refinement analysis with space group P42/mnm [43]. XRD patterns of both the samples show intense diffraction peaks. The characteristic peaks at 28.696°, 37.389°, 42.852°, 56.710°, 59.423°, 64.924° and 72.486° are assigned to (110), (101), (111), (211), (220), (002) and (112) planes of β-MnO₂ with lattice dimensions a = b = 4.3985Å and c = 2.8701Å [44]. Obtained spectral data are compared with JCPDS data file no. 81-2261. The high intense peaks in XRD patterns reveal the formation of high crystalline nanostructure. The arrangement of atoms in the crystal structure of sample S1 was confirmed by the models as depicts by figures 2(c) and (d), which clearly confirm that as-synthesized β-MnO₂ materials have a tetragonal crystal structure [45].

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Bond bonding and oxidation states of β-MnO₂ samples were investigated using XPS measurements. Figure 3(a) shows the survey scan of XPS spectrum relating to β-MnO₂ prepared at 100 °C in a wide range of binding energy (0–1100 eV). The spectrum revealed the typical characteristic peaks of Mn2p, C1s and O1s. In full XPS scan no impurity peaks are founds, which revels high compositional purity of prepared sample. Further, in this spectrum, the C1s peak is relatively intense and probably arises from the existence of carbon in atmosphere of analyser, which has a fix binding energy (B.E) (284.8 eV) and used to calibrate the whole spectra. However, the core level spectra of Mn2p and O1s were also acquired in a confined range as depicted in figures 3(b) and (c). The spin–orbit doublet peak was observed in core level Mn2p spectra as shown in figure 3(b) at B.E of 642.2 eV & 653.9 eV, corresponding to Mn²⁺, Mn³⁺ and Mn⁴⁺ oxidation states of Mn2p_3/2 and Mn2p_1/2core levels, respectively. The four-valent manganese (Mn⁺⁴) is present in the prepared sample is confirmed by the above results [46]. The characteristic core level XPS spectra of O1s was also acquired and depicted in figure 3(c). This core level HR-XPS spectrum is broad and deconvoluted into four sub peaks positioned at 527.75 eV, 527.98 eV, 528.35 eV and 529.67 eV corresponding to various oxidation state of oxygen, endorsing the formation of Mn–O, OH, C–O & C=O groups, respectively. To the end, XPS results revealed that the prepared sample has a less oxygen deficient area, which would be accountable for good electrochemical activities. Figure 3(d) depict the Raman spectra of investigated β-MnO₂ sample synthesis at 100 °C in frequency region 200 to 1000 cm⁻¹. Three bands are recognised at 538, 651 & 758 cm⁻¹ as fundamental modes of tetragonal β-MnO₂ and agrees with the literature [47, 48]. The peak of stretching mode vibrations of Mn–O is observed at 651 cm⁻¹. Nevertheless, the two peaks located at 538 and 758 cm⁻¹ can be assigned to the deformation modes of the metal-oxygen chain of Mn–O–Mn in the MnO₂ octahedral lattice [49, 50].

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The FESEM images of synthesized samples S1 and S2 are shown in figures 4(a), (b) and (d), (e), respectively. The rod-like structure of both the samples can be seen clearly. The average length and diameter of nanorods of sample S1 is 200–500 nm and 20–50 nm respectively. Nanorods of sample S2 show lower diameters. Further, EDX spectra of the as-synthesized samples S1&S2 are shown in figures 4(c) and (f). This results show the elements of manganese and oxygen are uniformly distributed in the samples.

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Further investigations of sample S1 nanostructures were analysed by TEM. TEM images of figures 5(a) and (b) reveal the smooth surfaces with uniform diameters of nanorods. Additionally, there is not any noticeable dislocation or defect in the nanorods. Moreover, the high resolution TEM image shown in figures 5(c) and (d)) display the lattice fringes with 3.12 Å & 2.406 Å the d-spacing corresponding to (110) and (101) crystal planes β-MnO₂ in the SAED pattern [51]. The lattice fringes indicate the (110) growth direction and high purity of as-synthesized nanorods relating to the separation with neighbouring lattices due to (101) crystal plane. A single nanorod is demonstrated with its growth direction and dominant crystal fact in figure 5(e). This result anticipates the improvement in employment ratio of MnO₂ electrode for better electrochemical behaviour facile to deintercalation [52, 53].

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To characterize the functionality of as-synthesized nanostructured β-MnO₂ samples, FTIR analysis was illustrated and performed in figure 6. As from the FTIR spectrum, the main absorption bands were found at 3431.2, 2931.4, 1631.2, 1385.1, 1118.8 and 707.1 cm⁻¹. The absorption peaks 3431.2 cm⁻¹ represents O–H stretching of water molecules and 2931.4 cm⁻¹ corresponds to C–H stretching in residual TEA–ethoxylate that may be present [52]. The absorption peak at 1631.2 cm⁻¹ was assigned to O–H group binding vibration [53]. The principle bonds at 1118.8 and 1385 cm⁻¹ are due to C=O and C=C stretching [54]. The main absorption characteristic peaks at 529.2 cm⁻¹, which is corresponds to Mn–O stretching modes of the octahedral layers in the β-MnO₂ nanostructure [55].

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The electrochemical (EC) study of as fabricated β-MnO₂ electrodes is carried out by three electrode arrangement, in this arrangement β-MnO₂ as working electrode, platinum as counter electrode & Ag/AgCl as reference electrode has been used. In this study, we have compared the electrochemical behaviour of β-MnO₂ electrodes in two different electrolytes, namely 1M L⁻¹ Li₂SO₄ and 1 M L⁻¹ Na₂SO₄, within the voltage window 0 V to 0.8 V. During electrochemical analysis the manganese oxide electrodes undergo reversible transformation between Mn³⁺ to Mn²⁺ found to stable in this particular range of potentials [56]. To provide excellent electrochemical performance, the selection of a suitable electrolyte for a supercapacitor device is a very important step. For manganese-based electrode supercapacitors H₂SO₄, Na₂SO₄, KCl, KOH, K₂SO₄, LiSO₄ electrolytes are commonly used [11, 57]. Considering the migration speed of hydrated ions in electrode's inner pores and in bulk of electrolyte, Na₂SO₄, and Li₂SO₄ liquid solution are found to be more suitable electrolytes for β-MnO₂ electrode [58, 59]. Thus we have recorded cyclic voltammeter (CV) curves of electrodes S1 and S2 in Na₂SO₄ & Li₂SO₄ electrolytes at scan rate (10 mVcm⁻²) and constant electrolyte concentration (1 ML⁻¹) (figures 7(a) and (b)). We observed that both the electrodes show the best electrochemical performance in Li₂SO₄ than Na₂SO_4. Previous reports also suggested that the electrolyte Li₂SO₄ is best suited for MnO₂ [60–62]. Hence, further electrochemical analysis of β-MnO₂ electrodes S1 and S2 have been analysed in (1M L⁻¹) Li₂SO₄.

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CV curves of β-MnO₂ nanostructured S1 and S2 electrodes in 1 mol l⁻¹ Li₂SO₄ electrolyte at scan rates 10, 20, 50 and 100 mVcm⁻² are represented in figures 8(a) and (b), respectively. It shows nearly rectangular shape CV curve at higher and lower scan with broad peak in the cathodic and anodic direction. The rectangular CV curve conforms the formation of electric double layer and resulting capacitance via non-faradaic mechanics. This is formed due to surface absorption of electrolyte cations (Li⁺ or Na⁺) on nanostructured β-MnO₂ electrode, as demonstrated by the reaction (5) [56, 63, 64]. Furthermore, the broad peak in the anodic and cathodic direction of the CV reveals the oxidation and reduction reaction of MnO₂ surface. Such redox reaction occurs at β-MnO₂ electrode due to the intercalation of cations from the electrolyte (Li⁺ or Na⁺) via reduction and de-intercalation upon oxidation (reaction (6)). The capacitance results via redox reaction at β-MnO₂ electrode and has a pseudocapacitive nature [56]. Hence, as-synthesised β-MnO₂ electrodes demonstrates a mixed behaviour of the ideal pseudocapacitive behaviour EDLCs [8, 65].

$$\begin{eqnarray}&&{(\beta -{{\rm{MnO}}}_{2})}_{{\rm{surface}}}+{{\rm{Li}}}^{+}+{{\rm{e}}}^{-}\leftrightarrow {(\beta -{{\rm{MnO}}}_{2}^{-}{{\rm{Li}}}^{+})}_{{\rm{surface}}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\beta -{{\rm{MnO}}}_{2}+{{\rm{Li}}}^{+}+{{\rm{e}}}^{-}\leftrightarrow {\rm{MnOOLi}}\end{eqnarray} \tag{ 6 }$$

By CV curve, the obtained specific capacitance (Cs) from equation (1) at different scan rates is shown in table 1. The highest Cs for sample S1 and S2 is found to be 462 and 289 Fg⁻¹, respectively in 1M Li₂SO₄. The high Cs provided by electrode S1 can be due to 1D nanostructure, mesoporous of structure and high surface area of β-MnO₂. The CV curves of sample S1 and S1 is given in figures 8(c) and (d) for electrolyte Li₂SO₄ respectively.

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The obtained values of specific capacitance higher as previously reported data relating to β-MnO₂ pseudocapacitors (table 2). Samal et al observed a capacitive behavior of layered δ-MnO₂ which shown the specific capacitance 518 Fg⁻¹ at 3 Ag⁻¹ in a 3M KOH with a cyclic stability of 83.95% after 2000 cycles [66]. The present study found higher cyclic stability as discussed in recent literature in table 2. Punnusamy et al synthesized polymorphs MnO₂ of different 2 × 2, 1 × 1 tunnel structure and simulated by DFT to confirm higher B.E [67]

GCD analysis of β-MnO₂ electrodes S1 and S2 was performed in 1M L⁻¹ Li₂SO₄ electrolyte at constant current density of 50 mAcm⁻². Figures 9(a) and (b) represents the GCD curves of sample S1 & S2 in 1 mol L⁻¹ Li₂SO₄ and Na₂SO₄ electrolytes. The resulting curves were attributed to redox reactions. As in figures 9(a) and (b) the triangular shape of charge discharge curves reveals the ideal supercapacitive behavior of β-MnO₂ electrode, whereas the similarity between charging and discharging curves indicates a low IR drop (i.e. high electrical conductivity) [74]. More specifically, the outline of the GCD curve is nearly symmetrical for both the samples, revealing the faradaic and as well as the pseudocapacitive charge storage behaviour with high redox reversibility within the potential window [1]. From GCD curves the maximum specific capacitance for electrodes S1 and S2 was found to be 437.5 Fg⁻¹ and 278.1 Fg⁻¹ at 50 mAcm⁻² current density respectively. The obtained Cs by GCD is resembled with the results of CV measurements. Furthermore, the maximum energy density of sample S1 and S2 was found to be 9.72 and 6.18 W h kg⁻¹, respectively at a power density of 25 kWkg⁻¹. To assess the rate capability of β-MnO₂electrodes, CV analysis of β-MnO₂ electrodes (S1 and S2) was performed at higher scan rates. Figure 9(c) represents Cs of β-MnO₂ electrodes S1 and S2 at various scan by CV. The shape of all the CV performance is preserved from lower to higher scan rate, demonstrating the ideal pseudocapacitive behaviour as well as superior rate capability of as-fabricated β-MnO₂ electrodes [1]. As shown in figure 9(c), the Cs of the as fabricated electrodes decreases at scan rate increase. Such decrease in Cs is due to the incomplete redox transition at the inner active sites at higher scan rates [42]. Figure 9(d) demonstrated the variation of capacitance retention over the cycle number at a scan rate of 100 mVcm⁻². For β-MnO₂ S1 electrodes the retention of Cs was found out to be 90.26% over 2000 CV cycles.

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After the electrochemical measurement the electrode (S1) is again characterized by XPS. The different spectrum given in figures 10(a)–(f) for reduced and oxidation MnO₂ electrodes presents Mn 2p at 642, and O 1s at 530 eV. The C 1s at 285 eV, S 2p at 164 eV and Na 1s at 1171.6 eV are associated due to presence of carbon, graphite sheet, and Na₂So₄ electrolyte. Raman spectrum is also described after electrochemical measurements of electrode (S1) as in figure 10(g) in the frequency range 400 to 900 cm⁻¹. The Mn–O vibration starching mode is found as the main Mn peak at 635 cm⁻¹. Two more peaks are also found at 576 and 760 cm⁻¹ which can be attributed to Mn–O–Mn in the MnO₂ octahedral lattice that deformate the modes of the Mn–O chain.

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Nyquist plots (NP) relating to the electrodes S1 and S2 in1ML⁻¹ Na₂SO₄ and 1ML⁻¹ Li₂SO₄ electrolytes are shown in figures 11(a) and (b), respectively. In general, a NP is separated into low & high frequency regions. The low and high frequency region are corresponds to capacitive behaviour and charge transfer resistance respectively. Figure 11(a) presents the Nyquist plot relating to the electrodes (S1 and S2). The intercept of NP on the x-axis gives equivalent series resistance (Rs), which is due to the contact resistance of the active material and the current collector, the intrinsic resistance of the active material and electrolyte [75]. The values for each component obtained by the equivalent fitting method are given in table 3. From Nyquist plot, the equivalent series resistance (Rs) for three electrodes cell comprising sample S1 and S2 are found out to be 5.4 Ω and 11.9 Ω in Li₂SO₄ electrolyte, and 12.12 Ω and 15.25 Ω in Na₂SO₄ electrolyte, respectively.

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