Enhanced supercapacitor performance by incorporating nickel in manganese oxide

Abstract

Nickel manganese mixed oxides (Ni_yMn_1−yO_x; 0 ≤ y ≤ 0.4) have been synthesized by in situ inclusion of nickel during the growth of manganese oxide (MnO_x). The effect of nickel concentration in MnO_x is investigated by cyclic voltammetry, current–voltage characteristics, scanning electron microscopy and N₂ adsorption–desorption analysis. Variations in electronic conductivity and specific capacitance suggest that nickel concentration in the MnO_x matrix significantly affects the supercapacitor electrode performance. At Ni/Mn ∼0.25, i.e. Ni_0.2Mn_0.8O_x, the material crystallizes into spinel NiMn₂O₄ as a prominent phase and exhibits a specific surface area (118 m² g⁻¹) with a granular morphology. Furthermore Ni_0.2Mn_0.8O_x exhibited low resistivity (2.07 × 10⁴ Ohm cm) and consequently high specific capacitance ∼380 F g⁻¹, endowing additional merits. The fabricated supercapacitor device (Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x) delivers 35 W h kg⁻¹ energy density and 3.74 kW kg⁻¹ power density with remarkably high capacitive retention ∼92% after 3000 galvanostatic charge–discharge cycles. These encouraging results show great potential in developing energy storage devices from manganese oxide based electrodes incorporating nickel in the lattice.

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1. Introduction

Electrochemical supercapacitors have attracted much attention as energy storage devices due to their promising applications in portable electronics and hybrid electrical vehicles.^1,2 Supercapacitors store charge via (a) charge accumulation at the electrode/electrolyte interface in the form of an electrical double layer (EDL) and (b) by the pseudocapacitance that arises from faradic redox transitions that go deep inside the bulk material.

Carbon materials store charge via EDL whereas transition metal oxides and conducting polymers follow redox process for charge storage.^3,4 EDL is surface dependent and therefore the resulting capacitance values are low however the charge–discharge rates are high. Due to the bulk contribution, puseudocapacitors give high specific capacitance (C_SP) whereas the response time limits their application. In order to exploit high C_SP and energy density, metal oxides have been extensively studied and the response time or charge–discharge time is modified by improving the charge collection through a support material, preferably carbon. Researchers recently focussed on mixed metal oxides as potential supercapacitor electrode materials because pristine transition metal oxides are poor conductors resulting in high equivalent series resistance with limited capacity and power density.⁵ Besides good electronic conduction, mixed transition metal oxides could offer richer redox reactions beneficial for electrochemical applications.⁶

Recently, many nanocomposites like Co₃O₄@MnO₂, Co₃O₄–Ni(OH)₂, MnO₂–NiO are attracting great interest for supercapacitors owing to their improved traits over pristine counterparts.^7–12 Various nickel and cobalt composites result in high electronic conductivity consequently enhancing the electrochemical performance.¹¹ Liu et al. synthesized high surface area spinel nickel manganese oxide for supercapacitor with high C_SP 243 F g⁻¹ at 5 mV s⁻¹. Fan et al. reported MnO₂–NiO nanoflake-assembled tubular array on stainless steel substrate by template method where ZnO is employed as in situ sacrificial template.⁹ Although high capacitance values are achieved using nano architectures, the bulk material performance and the electrode cyclability could be further improved.

In this work, we investigated the in situ inclusion of nickel in manganese oxide (Ni_yMn_1−yO_x; 0 ≤ y ≤ 0.4) lattice and tested the composite material as supercapacitor electrodes. NiMn₂O₄ crystallizes as a prominent phase when 20 wt% NiCl₂ is added to 80 wt% aqueous MnCl₂ solution for oxidative growth of manganese oxide. Ni_0.2Mn_0.8O_x exhibits highest electrical conductivity and surface area among other compositions. Proposed supercapacitor, Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x provides high energy density 35 W h kg⁻¹ with ∼92% retention of initial C_SP after 3000 galvanostatic charge–discharge cycles.

2. Experimental

2.1 Materials

Nickel chloride (NiCl₂·6H₂O), manganese chloride (MnCl₂·4H₂O), sodium hydroxide (NaOH), sodium sulphate (Na₂SO₄), isopropyl alcohol (IPA) and isooctane were purchased from Merck. Perfluorinated ion exchange resin (Nafion) was procured by Sigma Aldrich. Sodium sulphosuccinate (AOT) was used as surfactant (supplied by Alfa Aesar).

2.2 Synthesis of nickel manganese mixed oxide

Three aliquots of reverse microemulsions of water/AOT/isooctane¹³ were prepared having 0.8 M MnCl₂, 0.2 M NiCl₂ and 2 M NaOH aqueous solutions respectively. These were thoroughly mixed in a reaction vessel and stirred for 2 h. Excessive volume of IPA was added to break the miceller arrangement after completion of the reaction. The resulting precipitate was collected after washing several times with IPA and deionized water. Solid product obtained after washing was dried overnight at 80 °C. Different compositions of Ni_yMn_1−yO_x; 0.1 ≤ y ≤ 0.4 were prepared as given in Table 1 (y is molar concentration of nickel in MnO_x). MnO_x and Ni(OH)₂ were also synthesized by the same method under similar conditions.¹⁴

Table 1 Percentage of nickel in manganese oxide

Sample name	NiCl2·6H2O (g)	MnCl2·4H2O (g)	NaOH (g)	Ni/Mn (wt%)
Ni0.1Mn0.9Ox	0.48	3.56	1.60	10
Ni0.2Mn0.8Ox	0.95	3.16	1.60	25
Ni0.3Mn0.7Ox	1.43	2.77	1.60	43
Ni0.4Mn0.6Ox	1.90	2.37	1.60	65

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2.3 Preparation of electrode

10 mg of composite material (Ni_yMn_1−y O_x; 0.1 ≤ y ≤ 0.4) was ultrasonically mixed with 5 wt% Nafion in IPA. The mixture after 30 min sonication was spray deposited on 1 cm² area of the polished graphite sheets (2 × 1 cm) with the help of N₂ gas. After spray deposition, the film was dried at 80 °C in air and weighed to estimate the loading of the composite material. Loading was found to be approximately 0.3 mg cm⁻².

2.4 Characterization

Conductivity measurements were carried out on pellet (dia. 10 mm) using Keithley 2400 source meter. Perkin Elmer model 1257 X-ray photoelectron spectroscope (XPS) was employed to estimate the percentages of different metal oxide phases present in the specimen. Powder X-Ray Diffraction (XRD) measurements were performed with Bruker D8 Advance X-ray diffractometer. Microstructural investigations were carried out using Zeiss Ultra 55 Field emission scanning electron microscope (FESEM). Surface area of the samples were analyzed by N₂ adsorption–desorption isotherms using Micromeritics ASAP-2020. High resolution transmission electron microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) was conducted on Phillips Technai T-300 microscope. Electrochemical tests were performed with CHI 604D electrochemical analyzer. A piece of platinum gauze and Ag/AgCl were assembled as the counter and reference electrode respectively. Polished graphite with metal oxide/mixed metal oxide coating served as the working electrode. Cyclic voltammograms (CV) were recorded by polarizing the working electrode between 0 to 0.9 V vs. Ag/AgCl in 0.5 M aqueous Na₂SO₄ electrolyte. The charge–discharge characteristics of the supercapacitor cell were evaluated at different current density (two electrode assembly) using an Arbin instrument (model: BT2000, USA). C_SP was calculated from galvanostatic charge–discharge curve according to following equation:^3,15
where I is the current, ΔV/Δt is the change in potential with time and m is mass loading on the electrode.

The energy density (E in W h kg⁻¹) and power density (P in W kg⁻¹) were expressed as

where C is measured device capacitance, m is total mass loading on the electrodes, V is the operating potential range and R is the equivalent series resistance calculated from IR drop in galvanostatic discharge curve.

3. Results and discussion

The effect of Ni incorporation in MnO_x matrix was studied using cyclic voltammetry (CV) and current (I)–voltage (V) characteristics for Ni_yMn_1−y O_x; 0.1 ≤ y ≤ 0.4. Fig. 1 shows the variation of C_SP and electrical resistivity with different nickel percentage in MnO_x. Ni_0.1Mn_0.9O_x did not show much change in C_SP but the resistivity is slightly increased. Ni_0.2Mn_0.8O_x, compared to MnO_x shows more than twice increase in C_SP with drastic decrement in resistivity. Upon 20 wt% inclusion of Ni, C_SP of MnO_x (180 F g⁻¹) remarkably increases to 380 F g⁻¹ with one order decrease in resistivity (1.85 × 10⁵ Ohm cm of MnO_x to 2.07 × 10⁴ Ohm cm of Ni_0.2Mn_0.8O_x). With further increase in nickel content, Ni_0.4Mn_0.6O_x shows detrimental effect as C_SP decreases and resistivity increases. More importantly, drastic increment in C_SP of Ni_0.2Mn_0.8O_x is investigated by its structural and electrochemical analysis.

Fig. 1 Effect of Ni (wt%) on resistivity and C_SP of Ni_yMn_1−yO_x; 0 ≤ y ≤ 0.4.

Fig. 2 shows scanning electron micrographs and the surface area analysis using BET. The surface microstructure of MnO_x, Ni_0.2Mn_0.8O_x and Ni_0.4Mn_0.6O_x exhibit granular morphology. MnO_x in Fig. 2a, shows aggregated spherical grains of dia. ∼ 10 nm. Ni_0.2Mn_0.8O_x exhibited an increase in grain size (dia. ∼ 15 nm) (Fig. 2b). Upon increasing Ni concentration to 40 wt% (Ni_0.4Mn_0.6O_x), the composite exhibited dense microstructural features with very high aggregation (Fig. 2c). It appears that the increased concentration of Ni increases the aggregation.

Fig. 2 SEM micrographs (a–c) and N₂ adsorption–desorption isotherms (d–f) of MnO_x, Ni_0.2Mn_0.8O_x and Ni_0.4Mn_0.6O_x.

N₂ adsorption–desorption isotherm analysis of MnO_x show high specific surface area (80.5 m² g⁻¹⁾ with Type IV isotherm with H1 type hysteresis loop (Fig. 2d). Upon inclusion of 20 wt% nickel (Ni_0.2Mn_0.8O_x), surface area significantly increases to 118 m² g⁻¹. Nearly 50% enhancement in the specific surface area is attributed due to the complex pore network as in case of H2 hysteresis loop (Fig. 2e). Increasing the Ni more than 20 wt%, the surface area shows a decrease (Fig. 2f) and is attributed to the dense morphology of the composite having high Ni concentration. Pore size distribution of MnO_x (ESI S1†) demonstrate the presence of mesopores (5–20 nm) whereas in Ni_0.2Mn_0.8O_x, majority of pores fall in the optimal sizes of 2–5 nm for supercapacitors¹⁶ and thus supports the high surface area of Ni_0.2Mn_0.8O_x, consequently resulting in enhanced capacitance.

XPS analysis of MnO_x show two peaks of MnO₂ and Mn₃O₄ in Mn 2p at 641.8 eV and 642.43 eV respectively.¹⁷ Areas covered under MnO₂ and Mn₃O₄ peak are estimated to be 85% and 15% respectively (Fig. 3a) ascertaining the simultaneous presence of MnO₂ and Mn₃O₄ in MnO_x. As shown in Fig. 3b, the survey scan of Ni_0.2Mn_0.8O_x confirm the presence of Mn, Ni and O in the composite. Inset shows Ni 2p XPS which is assigned to Ni 2p_3/2 and Ni 2p_1/2 peaks with two satellites (sat.). The spin energy separation of Ni 2p_3/2 and Ni 2p_1/2 is 17.5 eV which is characteristic of Ni(OH)₂ phase.¹⁸ Mn 2p XPS spectrum of Ni_0.2Mn_0.8O_x shows a single peak at 642.4 eV (Fig. 3c), analogous to that of MnO_x ascertaining the absence of Mn₃O₄ in Ni_0.2Mn_0.8O_x.

Fig. 3 (a) Mn 2p X-ray photoelectron spectrum (XPS) of MnO_x, (b) XPS survey scan with Ni 2p core level spectra in inset (c) deconvoluted Mn 2p spectrum of Ni_0.2Mn_0.8O_x, (d) X-ray diffraction patterns of MnO_x and Ni_0.2Mn_0.8O_x, (e) XRD patterns of Ni_0.2Mn_0.8O_x powder annealed at 250 °C, 450 °C and 650 °C and (f) thermogravimetric analysis of Ni_0.2Mn_0.8O_x.

XRD analysis of MnO_x and Ni_0.2Mn_0.8O_x further supported XPS results. Fig. 3d shows XRD patterns of Manganese oxide (MnO_x) and Ni_0.2Mn_0.8O_x. Diffraction pattern of MnO_x endorses the presence of different phases i.e. MnO₂ and Mn₃O₄.¹⁹ XRD pattern of Ni_0.2Mn_0.8O_x exhibits three major diffraction peaks at 18°, 35.6° and 63.5° indexed to (111), (311) and (440) planes of spinel NiMn₂O₄ respectively.^20,21 The diffraction pattern showed relatively amorphous character with indication of the presence of some secondary phases like MnO₂, Ni(OH)₂ and NiMnO₃, interestingly there is no sign of Mn₃O₄. An important observation of the work is that in situ incorporation of 20 wt% Ni in MnO_x results in NiMn₂O₄ as prominent phase formation with complete suppression of manganese higher oxide (Mn₃O₄) phases. The suppression of Mn₃O₄ is attributed to its distorted spinel structure having Mn⁺² ions in tetrahedral sites and Mn⁺³ ions in octahedral sites.²² These Mn⁺³ ions causes Jahn–Teller distortion which is the result of departure from ideal interactions among bonding orbital and gives rise to lattice instability.²³ Ni⁺² ions can replace Mn⁺³ ions from octahedral site and such a substitution might require an equivalent amount of remaining Mn⁺³ ions to lose electrons and become Mn⁺⁴ in order to maintain the charge balance consequently resulting in NiMn₂O₄. Such a cation distribution offers lesser lattice alterations during electrochemical cycling resulting in enhanced electrochemical stability of NiMn₂O₄.⁸

Annealing carried out at different temperatures helped understanding the growth of NiMn₂O₄ by intermixing of secondary phases. Fig. 3e shows diffraction patterns of Ni_0.2Mn_0.8O_x powder after 2 h annealing at different temperatures. XRD pattern of Ni_0.2Mn_0.8O_x without any annealing treatment show three major diffraction peaks at 2θ value of 18°, 35.6°, 63.5° confirming the presence of NiMn₂O₄²⁴ with some other less intense peaks of MnO₂, Ni(OH)₂ and NiMnO₃. Annealing results in grain growth and intermixing of the secondary phases. The Ni_0.2Mn_0.8O_x sample after 250 °C annealing shows the presence of prominent NiMn₂O₄ with some secondary phases of MnO₂ and NiO and a shoulder NiMnO₃ at 36.7°. Here, Ni(OH)₂ converts to NiO which shows a peak at 61.2° corresponding to (220) plane. Further elevation in annealing temperature (450 °C) shows the crystalline XRD pattern of NiMn₂O₄ with presence of NiMnO₃. Quantification of the above XRD pattern by Xpert High Score software via Rietveld method (ESI S2†) suggest the percentage of prominent phase NiMn₂O₄ (81.3%) and NiMnO₃ (18.7%). It is assumed that the secondary phases of MnO₂ and NiO intermix and form NiMn₂O₄. The formation of NiMnO₃ could be due to dissociation of NiMn₂O₄ phase as discussed in later section. The XRD pattern of Ni_0.2Mn_0.8O_x annealed at 650 °C made this aspect clear and showed higher (44.5%) percentage of NiMnO₃.²⁵ The increase in intensity of the peaks corresponds to gradual evolution to crystalline phases.

Thermo-gravimetric analysis of Ni_0.2Mn_0.8O_x supports the phase transformation as observed in the X-ray diffraction. Due to loss of the adsorbed and/or structurally bonded water, TGA thermogram of Ni_0.2Mn_0.8O_x (Fig. 3f) shows ∼18% weight loss below 120 °C. Further weight loss ∼7% between 120–250 °C corresponds to the decomposition of Ni(OH)₂ to NiO and H₂O according to the following probable reaction:

Ni(OH)₂ → NiO + H₂O

Gradual 8% weight loss between 350–450 °C is attributed to the formation of NiMn₂O₄ and evolution of oxygen according to the reaction:²⁶

2MnO₂ + NiO → NiMn₂O₄ + 1/2O₂

Major 36% weight loss between 450–700 °C is attributed to the evaporation of MnO from NiMn₂O₄ according to the following reaction which is in agreement with the XRD results of Ni_0.2Mn_0.8O_x oxide annealed at 650 °C:

NiMn₂O₄ → NiMnO₃ + MnO(g)

As MnO is evaporated, characteristic peaks in the XRD pattern corresponds to NiMn₂O₄ and NiMnO₃ which shows the simultaneous presence of both.

TEM micrographs in Fig. 4a shows spherical particles with dia. ∼ 20 nm. Continuous lattice fringes (Fig. 4b) reveals d spacing 0.25 nm (311) of NiMn₂O₄. Three different planes (111), (311) and (440) of NiMn₂O₄ are observed in SAED pattern which are in agreement with XRD results (Fig. 4c). SEM of Ni_0.2Mn_0.8O_x (Fig. 4d) shows interconnected spherical nanoparticles forming porous channels. EDX results of spherical particles show Ni/Mn ratio ∼0.25 which is in agreement with experimental value (Fig. 4e).

Fig. 4 (a) TEM of Ni_0.2Mn_0.8O_x and (b) HRTEM shows d spacing ∼0.25 nm, (c) SAED pattern, (d) SEM and (e) EDX of Ni_0.2Mn_0.8O_x with atomic percentage of Ni : Mn ∼0.25.

The electrochemical performance of Ni_0.2Mn_0.8O_x is investigated by cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS). CV of Ni_0.2Mn_0.8O_x show much larger area indicating higher charge storage (∼380 F g⁻¹) than MnO_x (∼180 F g⁻¹) and Ni(OH)₂ (∼95 F g⁻¹), (Fig. 5a). This C_SP is higher than previous MnO₂-based nanocomposites electrodes, such as the graphene/MnO₂-based textile (315 F g⁻¹) and MnO₂ nanosheets/graphene (263 F g⁻¹).^12,27,28 Moreover, absence of any peak indicates pseudoconstant charging and discharging over whole voltammetric cycle.²⁹ The possible charge storage mechanism in Ni_0.2Mn_0.8O_x (NiMn₂O₄ prominent phase) may be explained as:

NiMn₂O₄ + Na⁺ + e⁻ → NaNiMn₂O₄

Fig. 5 (a) Cyclic voltammograms (CV) of MnO_x, Ni(OH)₂ and Ni_0.2Mn_0.8O_x at 5 mV s⁻¹, (b) Nyquist plot of Ni_0.2Mn_0.8O_x in high frequency region. Inset shows Nyquist plot of MnO_x and Ni_0.2Mn_0.8O_x, (c) effect of annealing temperature on CV response of Ni_0.2Mn_0.8O_x electrode. Inset shows variation of specific capacitance vs. annealed temperatures, (d) cycling life test of MnO_x and Ni_0.2Mn_0.8O_x at 50 mV s⁻¹ and (e and f) Mn 2p XPS spectra of MnO_x and Ni_0.2Mn_0.8O_x after 10 000 CV cycles.

Complex plane plot of Ni_0.2Mn_0.8O_x reveals resistive and capacitive response with change in frequency (Fig. 5b). The equivalent circuit for the best fit conditions is shown in the inset. High frequency region of the Nyquist plot, represented by suppressed semicircle shows internal resistance R_S 5.3 Ω and the high to mid frequency region represents RC circuit (R_CT, charge transfer resistance and C_dl, double layer capacitance). R_CT (1 Ω), lower than earlier reported values of MnO₂–NiO composite indicates intimate contact formation between the current collector and electrolyte contributing to facile charge transfer.⁹ A 45° sloped line in the mid frequency range corresponds to Warburg impedance which is related to ion diffusion resistance. Inset shows Nyquist plot of MnO_x and Ni_0.2Mn_0.8O_x exhibiting similar behaviour in high and low frequency region. Besides, the impedance line of MnO_x possess an arc in mid frequency region restricting the accessibility of ions causing low capacitance of MnO_x. More importantly, capacitive character of Ni_0.2Mn_0.8O_x attains earlier than that of MnO_x as evident from higher knee frequency below which maximum energy is easily accessible.

Effect of annealing on C_SP of Ni_0.2Mn_0.8O_x is shown by Fig. 5c. Inset shows variation of C_SP vs. annealing temperature. Ni_0.2Mn_0.8O_x has NiMn₂O₄ as prominent phase which shows high C_SP 380 F g⁻¹. It is important to mention that the samples were dried at 80 °C for 1 h. Extending this temperature to 250 °C for 2 h results in C_SP decrement to 290 F g⁻¹ which may be due to improved crystallanity of the material.³⁰ At 450 °C, single phase XRD pattern (NiMn₂O₄) shows C_SP 220 F g⁻¹ which is in close proximity to the reported values.¹² Above 450 °C, crystalline evolution takes place and the crystal lattice is rigid and not easily expanded (or contracted). This consequently retards the protonation (or deprotonation) reaction of the oxide leading to drastic decrement of C_SP at 650 °C.

Cycling life stability of MnO_x and Ni_0.2Mn_0.8O_x is explored by relatively long time cycling at 50 mV s⁻¹ (Fig. 5d). Result shows that C_SP of Ni_0.2Mn_0.8O_x shows an increment in initial cycles followed by ∼99% retention up to 10 000 cycles. Initial increment is probably attributed to activation process involving the increment in the number of available active sites allowing the trapped ions to gradually diffuse out.³¹ C_SP decay in MnO_x is found to be 50% after 10 000 cycles. This remarkable difference in cycling life stability with and without nickel in MnO_x electrode is investigated by XPS analysis before and after electrochemical cycling.

Core level Mn 2p XPS of MnO_x after redox cycling (unlike before cycling) shows intense peak of Mn₃O₄ with relatively less intense MnO₂ peak (Fig. 5e). Ni_0.2Mn_0.8O_x after electrochemical cycling shows a single peak of MnO₂ with no Mn₃O₄ (Fig. 5f). This demonstrate considerable conversion of MnO₂ to less electroactive Mn₃O₄ phase during cycling. This implicates that the decrement in charge storing capacity of MnO_x was due to the gradual formation of Mn₃O₄. It is noteworthy that in situ availability of nickel (20 wt%) in MnO_x inhibits Mn₃O₄ formation not only during growth but also during redox cycling, thereby enhancing the electrochemical performance of Ni_0.2Mn_0.8O_x.

As a demonstration of application potential of Ni_0.2Mn_0.8O_x, a symmetric supercapacitor, Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x is fabricated and examined via GCD measurements. Fig. 6a shows GCD curves of Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x at different current densities. Deviation from the linear variation of voltage with time is attributed to pseudocapacitive nature of the material. Small IR drop associated with the discharge curve ascertains low equivalent series resistance (esr) of the cell which can be calculated from the slope of linear correlation of IR drop with current density.^32,33

Fig. 6 (a) Galvanostatic charge–discharge (GCD) curves at different current densities, (b) variation of IR drop and specific capacitance vs. current density, (c) Ragone plot and (d) GCD cycling performance of Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x supercapacitor cell. Inset shows overview of last cycles.

Variation of C_SP and IR drop at different current density in Fig. 6b display lesser esr ∼0.03 Ω favouring high power delivery in practical applications making it a potential material for supercapacitor. Moreover, C_SP decreases with increase of current density as lesser surface area is accessible at higher current densities. It delivers energy density 35 W h kg⁻¹ with power density 3.74 kW kg⁻¹ at 0.5 A g⁻¹ (Fig. 6c). It still maintains an energy density 5.10 W h kg⁻¹ and power density 7.44 kW kg⁻¹ even at high current density 12.5 A g^−1.

Long term cycling performance of Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x was investigated by GCD cycling at 2.5 A g⁻¹ (Fig. 6d). Inset shows overview of charge–discharge cycles. Slight increment in C_SP during initial 1000 cycles as observed in various reports,³¹ followed by 8% decrement in C_SP in further cycles. Such a high retainability of initial C_SP after 3000 cycles endows a long term stable material for charge storage in supercapacitor.

4. Conclusions

In situ inclusion of nickel during manganese oxide (Ni_yMn_1−yO_x; 0 ≤ y ≤ 0.4) growth and its effect on structural, electrical and electrochemical properties is investigated. Upon 20 wt% addition of nickel in MnO_x (Ni_0.2Mn_0.8O_x) the composite exhibited spinel NiMn₂O₄ as prominent phase with highest surface area (118 m² g⁻¹), Consequently the superior performance in terms of highest conductivity and C_SP (∼380 F g⁻¹). Galvanostatic charge–discharge measurements of fabricated supercapacitor, Ni_0.2Mn_0.8O_x//Ni_0.2Mn_0.8O_x demonstrate high energy and power density (35 W h kg⁻¹, 3.74 kW kg⁻¹). Furthermore, it also retains 92% initial capacitance after 3000 cycles ascribing excellent cycling stability.

Acknowledgements

Authors gratefully acknowledge the financial support from DST India through project SR/S1/PC-31/2010 and Delhi University for Docotoral Research Grant. Preety Ahuja thankfully acknowledges the research fellowship from UGC India. SK Ujjain thank the INSPIRE fellowship from DST. Authors thank Dr R. Nagarajan, Department of Chemistry, University of Delhi for electrical characterisation instrumentation and Inorganic Materials and Catalysis group for the support through analysis of surface area using N₂ Sorption facility under DST(SR/S1/PC-11/2011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09027f

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