Introduction

In the 21st century, there is an urge to develop novel new energy storage technology to meet the demand of highly sustained energy storage systems. Interest on the development of low cost, high energy density and power density electrochemical energy storage systems compatible with nuclear, solar and wind energy resources are growing tremendously particularly on supercapacitors and batteries1,2,3, former one satisfy all the requirements. Whereas, conventional capacitors deliver high power, but lower energy density which instigates the search for long-term cyclic stability, operating safety, environmental friendly, low cost and can deliver high power and energy density supercapacitors4,5,6,7. The prominent features of supercapacitors are long-term cyclic stability, operating safety, environment friendly, and can deliver high power density. The supercapacitors can be classified in to two types such as; (i) EDLC (electrical double layer capacitors) and (ii) pseudocapacitors which is differentiated by their charge-storage mechanism8,9. In EDLC charge is stored by rapid adsorption/ desorption at the electrode/electrolyte interface, there is no redox reaction occurs in the electrode materials but the specific capacitance is very lower which depends on the electrode active surface area, whereas the surface of the electrode materials will undergo a reversible redox (Faradaic) reaction in the pseudocapacitors during the charge – discharge processes particularly in the metal oxide or conducting polymer electrode materials. EDLC offers lower capacitance which necessitate to identify a suitable pseudocapacitor materials (transition metal oxides) that can able to store more charges for longer period without noticeable energy loss10,11.

The transition metal oxides have received great attention towards pseudocapacitor materials owing to their variable oxidation states and ease of preparation. Among the various metal oxides, RuO2. XH2O is a well-known pseudocapacitor with good performance and have high specific capacitance (760 F g−1) with enhanced cycle-life. However, the low porosity, toxicity, less abundance of its raw materials and rapid decrease in power density at high charge- discharge current limit, restrict its glassine applications12,13. A large number of electrochemical studies are focused in finding a low-cost metal oxide as a replacement for RuO2. Recently, enormous effort has been focused on transition metal compounds containing nickel, cobalt, manganese with different morphologies. Wang and his co-workers synthesized interlinked multiphase Fe-doped MnO2 nanostructures with electrochemical properties14. Chen et al. have synthesized NiCo2O4@NiWO3 nanowire arrays that can serves as an electroactive material for super capacitors and it delivers a high capacity retention and long-term cycle stability15. Zhang et al. fabricated 3D Co3O4–Ni3(VO4)2 heterostructured nanorods on nickel foam possessing improved electrochemical properties for supercapacitor electrodes and exhibits good comprehensive electrochemical performance16. These transitional electrode materials with high specific surface area have been widely used as pseudocapacitor applications17,18,19. In general, aforementioned reports illustrate the use of transition metal oxides, sulphides and hydroxides as the supercapacitors electrode materials, only a few electrodes with metal carbonates are reported20,21,22,23,24,25,26. However, to the best of our knowledge, first time we have reported the synthesis of Mn0.75Ni0.25CO3 nano/sub-microspheres and applied as electrode materials for solid-state asymmetric supercapacitor (ASC) applications, since it is environment friendly, and can be easily fabricated with capacitance performance.

In this work, we have adopted a facile template-free co-precipitation method assisted with sodium bicarbonate to synthesize the shape-controlled Mn1-xNixCO3 (x-0, 0.20, 0.25 and 0.30) nano/sub-microspheres. The crystalline structure, functional groups and morphology of the as-prepared materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. The electrochemical studies show that the Mn0.75Ni0.25CO3 nano/sub-microspheres exhibits a higher specific capacitance of 364 F g−1 at a current density of 1 A g−1, high rate capability and superior cyclic stability. In addition, the solid-state ASC with Mn0.75Ni0.25CO3//GNS configuration was fabricated device using Mn0.75Ni0.25CO3 nano/sub-microspheres and graphene nanosheets (GNS) as positive and negative electrodes, respectively. The solid-state ASC Mn0.75Ni0.25CO3//GNS device exhibits a high energy density of 25 Wh kg−1 with a power density of 499 W kg−1. The fabricated Mn0.75Ni0.25CO3//GNS configuration is a potential system for commercial applications.

Results and discussion

The X-ray diffraction (XRD) analysis was employed for the crystallite and phase structural characterization of the samples. Figure 1a shows the XRD patterns of the as-prepared Mn1-xNixCO3 (x = 0.0, 0.20, 0.25 and 0.30) nano/sub-microspheres. As can be seen from the spectra, the MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres samples show the diffraction peaks of (012), (104), (110), (113), (202), (018), (116), (122) and (300) planes which are corresponding to the reflection located at 2Ɵ values of 24.25, 31.36, 37.52, 41.42, 45.18, 51.48, 51.68, 59.18 and 67.70 respectively27. All the diffraction peaks are related to the pure rhombohedral phase of MnCO3 structure with the space group of R3-c (JCPDS card No. 44-1472) and the peak sharpness and broadness show the highly crystalline and nano/sub-micron nature of the particles. The MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres samples does not shown any new peak associated with other phases such as Mn(OH)2, MnNi(OH)2, Ni(OH)2, NiCO3 or metallic Mn and Ni based phases in the samples. As the Ni content increases, the peak intensity decreases due to the generation of charge imbalance arises by the Ni ratio. The average crystallite size of MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres samples was calculated by using the Debye-Scherere’s equation which are 70 nm 65 nm 50 nm and 43 nm respectively28. The data shows that the presence of Ni ions in Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 prevented the growth of crystal grains and slows down the motion of a grain boundary due to the interruption on a movement of the grain boundaries by Zener pinning29. The smaller crystallite size and phase pure of the Mn0.75Ni0.25CO3 nano/sub-microspheres are expected to offer high electron transport at electrode/electrolyte interface for high power applications.

Figure 1
figure 1

(a) XRD patterns and (b) FT-IR spectra of the MnCO3, Mn0.80Ni0.20CO3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres.

The Fourier transform infrared spectroscopy (FT-IR) is usually employed as an additional probe to find the organic and inorganic functional species present in the samples. The as-prepare MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres samples were characterized by the FT-IR spectroscopy in the range of 4000–450 cm−1 and the respective spectra are depicted in Figure 1b. The MnCO3 nano/sub-microspheres still contain water molecules since OH and CO2 molecules have the property of chemisorption on to the MnCO3 surface when they are exposed to the atmospheres. The Mn1-xNixCO3 samples displayed a broad peak at ~3450 cm−1, which is attributed to the O-H stretching vibration and a band at ~1623 represents the bending vibration of water molecules present in the Mn1-xNixCO3 nano/sub-microspheres. These two vibrational modes show the residual water and hydroxyl groups on the nano/sub-microspheres. The broadness of the peak at 3458 cm−1 is decreased slowly on Ni indicating the water molecules adsorption diminished. The band in the range of 450–750 cm−1 is attributed to the MnCO3 rhombohedral sites; MnCO3 stretching vibration peak appeared at ~530 is the characteristic peak for the Mn-C-O (MnCO-Mn2+ with CO32−) and the other peaks at 1026 cm−1 is attributed to the C-O stretching vibration of CO32− ion and the 2353 cm−1 peak is related to carbon dioxide30,31. The intensity of characteristic peak for the Mn-C-O appeared at ~530 cm−1 is increasing on the addition of 20% Ni, then on the 25% addition, the peak stabilized and the other peak at 414 cm−1 is decreasing and increasing by the consequent additions, these characteristics are the indication of the of the formation of Mn0.75Ni0.25CO3 homogenously. These FT-IR spectra as well as the XRD patterns indicate that the framework and Ni does not interfere with the MnCO3 structure.

The morphology of the as-prepared MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres were investigated by scanning electron microscopy presented in Figure 2(a-h). The MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres are highly uniform and homogenously distributed with an average diameter size of 430 to 470 nm. The formation of nano/sub-microspheres morphology is possible in the co-precipitation method as it takes place in the carbonate medium (digestion of carbonate solution with a base and the precipitation process simultaneously). Under these circumstances, the carbonate solution controls the particles agglomeration and leads to the formation of thermodynamically stable sphere shape of MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres. The smaller Ni2+ ion (70 pm) with the bigger (81 pm) Mn2+ ion lattice, contracts the crystals, thus smaller crystallites are formed XRD, FT-IR and SEM results are corroborated one another. Surface roughness is also increases with increasing the Ni2+ level, which attracts more electrolyte and expected higher capacitance and faster exchange of e- on its surface.

Figure 2
figure 2

SEM images of (a, b) MnCO3, (c, d) Mn0.80Ni0.20CO3, (e, f) Mn0.75Ni0.25CO3 and (g, h) Mn0.70Ni0.30CO3 nano/sub-microspheres at low and high-magnifications.

The TEM micrographs for MnCO3 and Mn0.75Ni0.25CO3 nano/sub-microspheres at different magnifications are shown in Figure 3(a-f) and these images are in moral agreement with the SEM micrographs in terms of the nanospheres morphology and aggregation nature. The sizes of the nano/sub-microspheres are 430 nm (MnCO3) and 450 nm (Mn0.75Ni0.25CO3). Figure 3c,f show the lattice fringes of MnCO3 and Mn0.75Ni0.25CO3 nano/sub-microspheres, demonstrate that the lattice distance is 7.4 Å (MnCO3) and 7.2 Å (Mn0.75Ni0.25CO3) agrees well with (104) planes and the inset images of both are the MnCO3 and Mn0.75Ni0.25CO3 well well-defined spots arranged in circular rings confirms the poly crystalline formations. Both the nanosphere surface possesses pores, which can offer an effective electron and ion transport consequently supports for the improved electrochemical performance for the supercapacitor applications.

Figure 3
figure 3

TEM images of (a–c) MnCO3, (d–f) Mn0.75Ni0.25CO3 nano/sub-microspheres at low and high-magnifications.

Additionally, the elemental chemical composition and electronic state of the Mn0.75Ni0.25CO3 was determined using X-ray photoelectron spectroscopy (XPS) technique shown in Figure 4(a–d) and the survey spectrum is provided in Figure S1 (representative). As shown in Figure 4a, the binding energy peaks at 641.9 eV and 654.5 eV are assigned to Mn 2p3/2 and Mn 2p1/2 respectively, which coincide with the Mn2+ state of Mn in the Mn0.75Ni0.25CO3 samples32,33. The convolution of Ni 2p peaks at binding energy positions of 855.2 eV and 873.1 eV attributed to the Ni 2p3/2 and Ni 2p1/2 respectively and other two satellite (shake-up process) peaks appeared at 862.5 eV and 880.2 eV are correspond to the Ni 2p3/2 and Ni 2p1/2 respectively, as shown in Figure 4b. The main peaks and satellite peaks emerged for the Ni 2p region are owing to the presence of Ni2+ state in the Mn0.75Ni0.25CO3 samples34,35. The high resolution spectra of C 1 s was convolute into three binding energy peaks and shown in Figure 4c. Three main binding energy peaks at 284.6 eV, 285.5 eV and 287.5 eV can be assigned to the characteristics bands of C-Mn, C-O and C-OO, respectively, indicating the presence of carbonate36,37,38. The O 1 s spectra (Figure 4d) show that the peak appeared at 532.3 eV is ascribed to surface adsorption of the material oxygen and the other peak at 530.0 eV is typical characteristic peaks of the metal – oxygen bonds32,33,34. Thus, the XPS data revealed the presence of Mn, Ni, O and C without other impurity elements, which is consistent with the XRD results.

Figure 4
figure 4

High-resolution XPS spectrum of the Mn0.75Ni0.25CO3 nano/sub-microspheres (a) Mn2p spectrum, (b) Ni 2 P spectrum, (c) deconvoluted C1s spectrum and (d) O 1s spectrum.

Electrochemical Characterization

The electrochemical performances of the as-prepared MnCO3, Mn0.80Ni0.20Co3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres electrodes were investigated in a three-electrode system. Figure 5a compares the cyclic voltammetry (CV) curves of various electrodes tested at a scan rate of 20 mV s−1 with the potential window of 0.0 V–1.0 V Vs SCE in 1 M of Na2SO4 electrolyte. These CV curves are quasi-rectangular shape, indicating an ideal electrical double layer capacitance with no O2 or H2 gas evolution peaks and the area of the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode are larger than that of the other nanospheres electrodes. The superior electrochemical property of Mn0.75Ni0.25CO3 nano/sub-microspheres electrode is due to the synergistic effects of Ni and Mn elements in the Ni-Mn-CO3 solid solution with proper Ni ratio spherical morphology and smaller crystallites size with pores natures. Hence, CV curves of the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode measured at diverse scan rates ranging from 10 mV s−1 to 100 mV s−1 are measured and illustrated in Figure 5b and for the other three electrodes presented in Figure S2. Thus, denote that the shapes of the CV curves are retained well as the scan rate increases, indicating rapid electronic and ionic transportation at the electrode/electrolyte interface. The charge storage mechanism of Mn0.75Ni0.25CO3 is explained based on the intercalation/de-intercalation mechanism as follows:

$${{\rm{Mn}}}_{0.75}{{\rm{Ni}}}_{0.25}{{\rm{CO}}}_{3}+{{\rm{Na}}}^{+}+{{\rm{e}}}^{-}\leftrightarrow {{\rm{NaMn}}}_{0.75}{{\rm{Ni}}}_{0.25}{{\rm{CO}}}_{3}$$
(1)
Figure 5
figure 5

Electrochemical performance of the synthesized pristine MnCO3, Mn0.80Ni0.20CO3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres electrode samples in 3-electrode cell: (a) Comparison CV curves at 20 mV s−1, (b) CV curves of the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode at diverse scanning rates (10 to 100 mV s−1) between 0.0 and 1 V, (c) Comparison GCD curves of the nano/sub-microspheres electrodes at 1 A g−1, (d) GCD curves for the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode at diverse current density (1 to 9 A g−1), (e) specific capacitance of the nano/sub-microspheres electrodes at various current densities and (f) cycling stability of the Mn0.75Ni0.25CO3 nano/sub-microspheres measured at 5 A g−1 for 7500 cycles, 1st and 7500 charge-discharge cycles (insert)..

During the charging process, the Na+ ions from the electrolyte intercalates into Mn0.75Ni0.25CO3 matrix and release one electron. On the other hand, during discharging Na+ ions are de-intercalated from Mn0.75Ni0.25CO3 matrix and diffuse into the electrolytic solution.

Galvanostatic charge/discharge (GCD) measurements are further evaluated in the potential window range between 0.0 V and 1.0 V to assess the performance of various nano/sub-microspheres electrodes at 1 A g−1 (Figure 5c). The superior performance was obtained for the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode and interrogated at various current densities from 1 A g−1 to 9 A g−1 (Figure 5d). The charge/discharge curves for MnCO3, Mn0.80Ni0.20Co3, and Mn0.70Ni0.30CO3 electrodes are presented in Figure S3. The specific capacitance of the various nano/sub-microspheres electrodes is calculated by using the following formula,

$${{\bf{C}}}_{{\bf{s}}}=\frac{i\Delta t}{m\Delta v}$$
(2)

Where, Cs is specific capacitance (F g−1), i is the constant current (A), ∆t is the discharge time (s), m is the total mass of the active material, and ∆v is the potential window (V)39. In our charge - discharge studies, the specific capacitance contribution from the nickel foam current collector was ignored. The specific capacity value of MnCO3, Mn0.80Ni0.20CO3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres electrodes is 280, 295, 364 and 315 F g−1 respectively at 1 A g−1. Higher specific capacitance value was obtained for Mn0.75Ni0.25CO3 electrode owing to its higher surface area and stabilized structure. Whereas, at 30% Ni, disintegration (Mn0.70Ni0.30CO3) of crystal structure reduces the capacitance. The specific capacitance of Mn1-xNixCO3 (x-0, 0.20, 0.25 and 0.30) nano/sub-microspheres electrodes with different current densities are presented in the Figure 5e. Among the all samples, Mn0.75Ni0.25CO3 exhibits higher specific capacitance than the other fabricated electrodes in this work and also with Mn0.75Ni0.25CO3 previously reported values (Table 1).

Table 1 Compression of specific capacitance and capacity retention with other Mn-based materials reported in the literature.

The specific capacitance of the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode (364 F g−1; 23.07% %) is higher than that of pristine MnCO3 (280 F g−1 at 1 A g−1) in 1 M of Na2SO4 electrolyte. The specific capacity retention of the Mn0.75Ni0.25CO3 is slowly decreased (280 F g−1) by increasing the current density and retained about 78% at 9 A g−1 current density. Whereas, the MnCO3 nano/sub-microspheres electrode exhibits only 66% specific capacitance retention of at 9 A g−1. The specific capacitance of Mn based metal oxides electrodes are shown in Table 1. The significant specific capacitance retention is offered only by the Mn0.75Ni0.25CO3 nano/sub-microspheres.

The practical performance is highly important for the application of any energy systems, particularly in hybrid electric vehicles and renewable energy storage systems. Hence, the long-term cycle stability studies for the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode was measured over 7500 continuous charge-discharge cycles at a specific current of 5 A g−1 in 1 M of Na2SO4 electrolyte (Figure 5f) and it retains about 96% capacitance retention even after 7500 cycles. Within the test voltage window, the intercalation and de-intercalation processes of the guest ions are taken place significantly structural in the meso-structural electrodes. Further, there were no structural changes in the electrode observed, as it showed very stable cycle life.

The electrochemical impedance spectra (EIS) of the MnCO3, Mn0.80Ni0.20CO3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres electrodes were taken in the frequency range from 100 mHz to 100 kHz and the results are shown in Figure S4. The Nyquist plot of nano/sub-microspheres electrode showed a semicircle at the high frequency region and a straight line at the low frequency region. An equivalent circuit was fitted by using Zview software and the charge-transfer resistance (Rct) of the MnCO3, Mn0.80Ni0.20CO3, Mn0.75Ni0.25CO3 and Mn0.70Ni0.30CO3 nano/sub-microspheres electrodes is 17.59, 6.45, 5.51 and 9.81 ohm, respectively. Evidently, the lower resistance value is obtained for Mn0.75Ni0.25CO3 nano/sub-microspheres electrode, which result better electrochemical performance. The fast transfer of charged species between the electrode and electrolyte is confirmed by the lower Rct values of the Mn0.75Ni0.25CO3 nano/sub-microspheres electrode, which could lead to good electrochemical performance. The synergism between Ni and Mn give rise to good specific capacitance and cycle life25,40.

Electrochemical performance of solid-state asymmetric supercapacitor

To further evaluate the practical applicability of the Mn0.75Ni0.25CO3 nano/sub-microspheres, we assembled solid-state ASC device using Mn0.75Ni0.25CO3 nano/sub-microspheres as the positive electrode, graphene nanosheets as the negative electrode and PVA- Na2SO4 as the solid-state electrolyte (Figure 6a). For the fabrication of solid-state ASC, GNS was used as negative electrode, its phase and morphology were examined (Figure S5) and the specific capacitance of GNS is 115 F g−1 at 1 A g−1, calculated from charge discharge curves in Figure S6a. Figure S6b shows, the CV curves of GNS and Mn0.75Ni0.25CO3 nano/sub-microspheres electrodes, whose stable voltage windows were identified as −1.0 V to 0.0 V and 0.0 V to 1.0 V, respectively. The solid-state ASC device exhibits a capacitive behaviour with almost rectangular CV curves with no obvious redox peak at various operating potential window between 0.0 V and 2.2 V at 50 mV s−1, resulting spectra are presented in Figure 6b. The effect of scan rates was evaluated for a solid-state ASC device in the voltage window from 0 to 2.0 V at various scan rates from 10 to 100 mV s−1 and presented in Figure 6c. The ASC device CV profile of cell has remained relatively rectangular shape even at a higher scanning rate of 100 mV s−1 without any other distraction in the double layer behaviour. This electrochemical characteristic offered by the fabricated electrode/electrolyte interfacial phenomenon are expected to offer good charge / discharge properties with higher rate capability characteristics.

Figure 6
figure 6

(a) Schematic illustration of the solid-state asymmetric supercapacitor configuration, (b) CV curves of Mn0.75Ni0.25CO3//GNS solid-state ASC measured at different potential window at a scan rate of 50 mV s−1, (c) CV curves of the solid-state ASC measured at different scan rates (10 to 100 mV s−1), (d) GCD curves at different current densities (0.5 to 9 A g−1), (e) Calculated specific capacitance of solid-state ASC at different current density (0.5–9 A g−1), (f) long-term cyclic stability of solid-state ASC over 7500 cycles at 3 A g−1, 1st and 7500 charge-discharge cycles (insert)..

The galvanostatic charge-discharge (GCD) curves at different current densities are shown in Figure 6d and it can be seen that the potential of the charge-discharge profile indicates the higher discharge time. The discharge curves at different current density of 0.5, 1.0, 2.0, 3.0, 5.0, 7.0 and 9.0 A g−1, are providing the specific capacitance of 46, 42, 36, 32, 27, 24 and 22 F g−1, respectively (Figure 6e). Furthermore, 87.7% capacitance retention was achieved even after 7500 charge-discharge cycles at the higher current density of 3 A g−1 (Figure 6f), testifying its high rate capability. This capacity retention capability is possibly due to synergistic effect mainly related to the stable nano/sub-microspheres morphology and structure of electrode materials build during the synthesis of the precursor by sodium bicarbonate, where the nanoparticles surface roughness provides more active surface area, less resistance for electron/ion transport at the electrode/electrolyte interface and stable phase structure. The electrochemical impedance spectra were taken for the ASC device before and after 7500 cycles, there was negligible variation in the charge transfer resistance and solution resistance values was noted, due to the stable electrode and electrolyte configurations, as shown in Figure 7a. Herein, we have provided the energy density and power density of Mn0.75Ni0.25CO3//GNS ASC in the Ragone plot (Figure 7b), these two parameters characterize the performance of a ASC, which can be calculated using the following equations:

$${\rm{E}}=[{\rm{Cs}}\times {(\Delta V)}^{2}]/2\times 3.6$$
(3)
$${\rm{P}}={\rm{E}}\times 3600/\Delta {\rm{t}}$$
(4)

Where E, Cs, ∆V, P and ∆t are the energy density (Wh kg−1), specific capacitance (F g−1), discharge potential (V), power density (kW kg−1) and discharge time (s), respectively23,41. The energy density of the fabricated ASC decreases from 25 to 12.22 W h kg−1 as the power density increases from 499 to 15840 W kg−1 (The ASC device of Mn0.75Ni0.25CO3//GNS can be used to light the red-light emitting diode (LED) after being charged (insert Figure 7b)). Nanostructured Mn0.75Ni0.25CO3 nano/sub-microspheres solid-state ASC exhibits superior electrochemical performance; due to their definite size effect, high surface area, low density, and permeation of Na+ ion in the Mn0.75Ni0.25CO3 nano/sub-microspheres electrodes. There are few literature reports on the fabrication of Mn based ASC electrode materials available and their performance are compared (Table 2). Most importantly, at lower current density of 0.5 A g−1, 499 W kg−1 power density was observed and at higher current density of 9 A g−1, 15540 W kg−1 power density was obtained, this value is enough to meet the power demands for the PNGV (Partnership for a New Generation of Vehicles). These results reveal that our Mn0.75Ni0.25CO3//GNS solid-state ASC device possess a great potential for practical applications. The high energy and power density of solid-state ASC applications electrode materials are due to the uniform nano/sub-microspheres morphology as cathode and more surface area of GNS as an anode material that lead to more accessible to smaller electrolyte ions and favourable for the fast diffusion of the electrolyte ions at the surface of electrode materials. Meanwhile, the solid-state ASC electrode materials prevent the loss of capacity retention during the repetitive incorporation/extraction process. The highlighted solid-state ASC device (Mn0.75Ni0.25CO3//GNS) is novel, cost effective, easy to fabricate and possesses an excellent potential for energy and power density devices.

Figure 7
figure 7

(a) EIS curves of the Mn0.75Ni0.25CO3 nano/sub-microspheres//GNS solid-state ASC initial cycles and after 7500 cycles, (b) Ragone plots of the Mn0.75Ni0.25CO3 nano/sub-microspheres//GNS solid-state ASC, lighted LED (insert).

Table 2 Compression of energy density, power density, potential window and specific capacitance previous literatures work on Mn based asymmetric supercapacitors.

Conclusions

In conclusion, we have reported the facile template-free synthesis of Mn1-xNixCO3 (x-0, 0.20,0.25 and 0.30) nano/sub-microspheres via facile co-precipitation method using sodium bicarbonate as the precipitating agent for the first time. The microscopic and X-ray diffraction results reveal that the synthesized rhombohedral Mn1-xNixCO3 particles are homogeneously dispersed crystalline nano/sub-microspheres morphologies with 430–470 nm size. Galvanostatic charge-discharge result showed high specific capacity of 364 F g−1 offered by Mn0.75Ni0.25CO3 nano/sub-microspheres electrode which is 23.07% higher capacity than the capacity offered by MnCO3 in 1 M of Na2SO4 electrolyte. The Mn0.75Ni0.25CO3 nano/sub-microspheres electrode is to be one of the promising electrodes for long-term cycle stability as it exhibited stabilized performance at 5 A g−1 for 7500 cycles. Further, the as-fabricated Mn0.75Ni0.25CO3//GNS device displayed a power density of 499 W kg−1 at high specific energy density of 25 Wh kg−1, as well as the solid-state ASC capacitance retention of 87.7% was delivered even after 7500 cycles, which is an additional benefit, and suitable for the commercial applications. The Mn0.75Ni0.25CO3 nano/sub-microspheres materials further provides a new pathway for the new Li-ion anode, sensor and photocatalyst applications.

Experimental

Materials

Nickel sulphate hexahydrate (NiSO4.6H2O), Manganese sulphate tetrahydrate (MnSO4.4H2O), sodium bicarbonate (NaHCO3), Sodium sulfate (Na2SO4), Acetylene black, Polyvinylidene fluoride and N-Methyl-2-Pyrrolidone were procured from Sigma-Aldrich, India. Ethanol was purchased from SRL Pvt. Ltd, India. All the purchased chemicals and regents were in analytical grade and used as received without any further purifications.

Synthesis of Mn1-xNixCO3 (x = 0.0, 0.20, 0.25 and 0.30) nano/sub-microspheres

Mn1-xNixCO3 nano/sub-microspheres were synthesized by a facile co-precipitation method. Typically, manganese sulphate (0.75 mmol) and nickel sulphate (0.25 mmol) were dissolved in double distiller water (70 mL) and sodium bicarbonate (10 mmol) was dissolved separately in double distiller water (70 mL). A 7 mL of ethanol was then added to the MnSO4 and NiSO4 solutions with constant stirring. After its complete dispersion, the NaHCO3 solution was added to the above mixture at room temperature. After 5 min, the reaction solution was turned to green colour. This indicated that the initial formation of Mn0.75Ni0.25CO3 nano/sub-microspheres and mixture was to continuously stirred for 3 hours at room temperature to form Mn0.75Ni0.25CO3 nano/sub-microspheres. The Mn0.75Ni0.25CO3 nano/sub-microspheres formed were separated by filtration and washed several times with ultrapure water and ethanol to remove impurities. Finally, Mn0.75Ni0.25CO3 nano/sub-microspheres were dried at 120 °C for 12 h to remove the adsorbed water molecules on the surface of the nano/sub-microspheres. The same procedure was followed for the synthesis of Mn1-xNixCO3 (x = 0.0, 0.20, 0.25 and 0.30) nano/sub-microspheres.

Materials characterization

The structural, functional and morphologies of Mn1-xNixCO3 nano/sub-microspheres were examined using X-ray diffraction (XRD) measurements using a PAN Analytical X’ per PRO Model X-ray diffractometer with Cu Kα radiation (α = 1.5418 Å) from 10–80 °, Fourier transform infrared (FT-IR) spectral analyses were performed using a Nicolet Avater 370 with KBr pellet technique from 450 to 4000 cm−1. The morphology and surface nature of Mn1-xNixCO3 nano/sub-microspheres were characterized using scanning electron microscopy using JEOL-JSM and transmission electron microscopy (TEM) taken on a JEOL/JEM 2100. X-ray photoelectron spectroscopy (Carl Zeiss) was carried out in ultra-high vacuum with Al kα line (1486.6 eV) as an exciting source to analyse the surface chemistry and valence state of Mn0.75Ni0.25CO3 sample20.

Electrochemical measurements

To evaluate the electrochemical properties of the synthesized Mn1-xNixCO3 (x = 0.0, 0.20, 0.25 and 0.30) nano/sub-microspheres electrodes the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy studies with three electrode system used. The working electrode was prepared by mixing the synthesized Mn1-xNixCO3 nano/sub-microspheres materials (active materials-70%), conductive materials (acetylene black-20%), binder (polyvinylidene fluoride (PVDF-10%)) and a few drops of n-methyl-2-pyrrolidone (NMP) was used as the solvent. The active material (1.5 mg) was coated on a nickel foam substrate (1 cm ×1 cm) and was dried at 90 °C for 10 h. This served as the working electrode, saturated calomel (SCE) and a platinum wire was used as the reference and counter electrode, respectively. The electrode evaluated in 1 M of Na2SO4 aqueous solutions electrolyte at different scan rate and current densities.

The constructed solid-state ASC device was measured with a two-electrode system, including two slices of nickel foam (2 ×1 cm) as current collectors and a cellulosic paper as a separator. In the two-electrode system, the Mn0.75Ni0.25CO3 nano/sub-microspheres were used as the positive electrode (1.2 mg) and the graphene nanosheet (GNS) (3.87 mg) was used as the negative electrode, which was prepared by pasting of 20% acetylene black and 10% of PVDF in NMP slurry on a nickel foam. The solid-state electrolyte was prepared by the addition of 2 g PVA powder into 20 ml of deionized water under vigorous stirring at 95 °C, a clear solution was obtained. Later, 1 g of Na2SO4 was added to the above clear solution under stirring for 30 min to form PVA- Na2SO4 solid -state gel. The positive (Mn0.75Ni0.25CO3 nano/sub-microspheres) and negative (GNS) electrode and separator were dipped into the PVA- Na2SO4 solid-state solution for 5 min, taken out, assembled and kept under vacuum desicater to form solid-state ASC. For the optimal mass ratio between the positive and negative electrodes (m+/m) was obtained from the maximum capacitance and operating potential window, which was calculated by using the following charge balance equation42,

$${q}^{+}={q}^{-}$$
(5)
$$q=m\times c\times v$$
(6)
$${m}^{+}/{m}^{-}={c}^{-}{v}^{-}/{c}^{+}{v}^{+}$$
(7)

Where, m is the mass, c is the specific capacitance and v is the operating voltage range. The electrochemical performances in both configurations such as in three electrode and in the two electrode systems were carried out using the CHI 660D electrochemical work station.