Facile synthesis of NiCoMnO₄ nanoparticles as novel electrode materials for high-performance asymmetric energy storage devices

Abstract

In attempt to reduce the amount of toxic Co in cobalt oxides, NiCoMnO₄ nanoparticles were synthesized via a facile hydrothermal route and their electrochemical properties have been investigated as a novel electrode material for supercapacitors. Samples have been characterized using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS). N₂ adsorption/desorption measurements demonstrated a high specific surface area of 175 m² g⁻¹. The morphology of the samples has been probed with scanning (SEM) and transmission (TEM) electron microscopy. The electrochemical properties of the NiCoMnO₄ nanoparticles have been evaluated as electrode materials for energy storage application by means of various electrochemical techniques including cyclic voltammetry, galvanostatic charge–discharge measurements, and electrochemical impedance spectroscopy. According to the obtained results, the NiCoMnO₄ electrodes showed significant improved capacitive performance in comparison with pure oxides (e.g. NiO, Co₃O₄, and Mn₃O₄). The NiCoMnO₄ electrodes with a high mass loading of ∼10 mg cm⁻² exhibited a high specific capacitance of 510 F g⁻¹ at 1 A g⁻¹, and a desirable rate capability (retaining 285 F g⁻¹ at 10 A g⁻¹). Finally, asymmetric devices based on NiCoMnO₄ nanoparticles as positive electrodes and reduced graphene oxide nanosheets as negative electrodes were assembled, showing a remarkable performance including high energy (20 W h kg⁻¹), and stunning power density (37.5 kW kg⁻¹).

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1. Introduction

Rapidly growing energy demands in our modern life have inspired a tremendous amount of scientific and industrial efforts to introduce and develop new electrode materials and technologies capable of fulfilling both high energy and high power requirements. Standing at the opposite ends of the electrochemical energy storage spectrum, batteries store charge based on faradaic redox reactions providing high energies while supercapacitors work by charge accumulation in the electrochemical double layer (EDLC) which yields inherently high power.^1,2 However, differences between batteries and supercapacitors are becoming blurred by the appearance of hybrid/asymmetric devices, merging both energy storage mechanisms.³ As opposed to simple transition metal oxides (TMOs), mixed transition metal oxides (MTMOs) are placed at the summit of high-energy materials due to improved electrical and electrochemical behavior.^4–6 Different varieties of MTMOs have been investigated and have shown impressive capacitances.^7,8 Among the many transition metal oxides, cobalt oxides have attracted great attention, suggesting this family as very promising electrode materials in Li-ion batteries⁹ and excellent active materials for supercapacitors.^10,11 Hence, considerable efforts have been made to partially substitute Co with less expensive and more benign elements to form the spinel structures with improved or comparable performance.^12,13 For instance, NiCo₂O₄ and MnCo₂O₄ have been extensively investigated, demonstrating extremely high capacitances due to synergistic effect of different cations present in lattice structure.^4,7,14–18 Very recently, we showed that substituting Co with common, abundant cations like Fe or Cu improves the electrochemical performance in comparison with pure Co₃O₄ synthesized in similar conditions.^19,20 This is while that substituting Co with different cations not only reduces Co content of the final spinel, but also may enhance electrical and/or electrochemical properties. However, although cobaltites with more than one transition metal cation have been synthesised and applied in other fields, those which were tried so far in supercapacitor applications were only substituted with one transition metal cation. As an interesting example, spinel NiCoMnO₄ includes three cations where all three are considered high capacitance materials in their simple oxide analogues.^21,22 This particular spinel is known as a precursor for the synthesis of LiNi_1/3Co_1/3Mn_1/3O₂ as a cathode in Li-ion batteries.²³ Furthermore, it has been already shown to be an efficient bifunctional catalyst for ORR/OER reactions and for CO oxidation as well.²⁴ However, to the best of our knowledge, the electrochemical properties of this material have never been examined in alkaline asymmetric energy storage devices. Moreover, most of the previously studied systems were usually employing very small mass loadings (<1 mg cm⁻²), resulting in huge gravimetrical capacitances. This is while the performance of the electrode material is highly dependent on the mass loading and significantly decreases in highly loaded electrodes.^25–27 Therefore, it is greatly important to introduce and develop high performance materials which can retain their properties under reasonable mass loading conditions (e.g. commercial scale of 8–10 mg cm⁻²).^1,25–30

Hydrothermal method, as a high-scale and very facile synthesis, has been extensively utilized to synthesize simple and binary metal oxides.^31–33 Herein, this facile method was employed for the first time to synthesize the ternary transition metal oxide, NiCoMnO₄ nanoparticles (NCMO NPs), without using any surfactants or templates. Electrochemical properties of NCMO NPs were studied as positive electrode materials in 3 M KOH solution using different electrochemical techniques. In order to assess the viability of NCMO NPs, electrodes with high mass-loading of NCMO NPs were prepared and used in asymmetric devices by their integration with highly conducting reduced graphene oxide (RGO) negative electrodes. Electrochemical performance of the asymmetric devices was evaluated and was compared to symmetric RGO-based supercapacitors, revealing excellent competence of the hybrid devices.

2. Experimental

2.1. Synthesis of NiCoMnO₄ nanoparticles

NiCoMnO₄ nanoparticles were synthesized via a facile hydrothermal route followed by thermal treatment (Scheme 1). Typically, equimolar amounts of NiCl₂·6H₂O, Mn(NO₃)₂·4H₂O, and Co(NO₃)₂·6H₂O were dissolved in a mixture of 20 ml of Milli-Q water and 20 ml of dimethylformamide (DMF) and stirred for 60 min to obtain a transparent solution. Afterwards, a 5% ammonium solution (NH₄OH) was added to the mixture up to pH ≈ 9. The resultant dark green solution was kept stirring for a further 30 min. Next, the solution was transferred into a 200 ml Teflon-lined stainless steel autoclave reactor where the hydrothermal reaction took place at 120 °C for 2 h. When the reactor cooled to room temperature (RT), the resulting precipitate was vacuum filtered, rinsed thoroughly by using a water/ethanol mixture, and dried at 60 °C. Finally, the precipitate was calcined in an electrical furnace at 300 °C (rate of 2° min⁻¹) for 2 h. For comparison, pure oxides (e.g. NiO, Co₃O₄, and Mn₃O₄) were also synthesized via the same route while only one of the metal salts was used as the precursor.

Scheme 1 Schematic illustration of the synthesis steps of NiCoMnO₄ nanoparticles.

2.2. Synthesis of reduced graphene oxide nanosheets (RGO NSs)

Graphene oxide (GO) was synthesized via modified Hummers method.³⁴ In a typical synthesis, 8 g K₂S₂O₈ and 8 g P₂O₅ were added to 24 ml of concentrated H₂SO₄ solution. Using a silicon oil bath, the solution was heated to 80 °C and 4 g graphite powder was gradually added followed by stirring for 6 h. After cooling to RT, the mixture was diluted by 300 ml Milli-Q water and then filtered and dried overnight at 60 °C. A 2 g quantity of the as-prepared powder (preoxidized graphite) was added to 92 ml concentrated H₂SO₄ solution, cooled in an ice bath. 12 g KMnO₄ was gradually added under vigorous stirring. After 15 min, 2 g NaNO₃ was added and the mixture was stirred at 35 °C for 2 h. Then 560 ml Milli-Q water and 10 ml H₂O₂ 30% solution were introduced turning the solution to a bright yellow. The sample was filtered, rinsed several times, and dried at 100 °C. Next, it was dispersed again in Milli-Q water and the brown dispersion was dialyzed for at least 1 day to remove residual metal ions and acids. After vacuum filtration, the GO gel was dried at 60 °C.

In order to reduce the GO sample, 100 mg of as-prepared sample was re-dispersed in 150 ml Milli-Q water and 70 μl of a hydrazine hydrate solution (50–60%) was added. The solution was then transferred to an oil bath where it was stirred for 6 h at 80 °C. The final sample was rinsed thoroughly with water and dried overnight at 60 °C.

2.3. Microstructural analysis

The prepared samples were structurally characterized using X-ray diffraction (XRD, PANalytical Empyream diffractometer utilizing Cu Kα radiation) at a generator voltage of 45 kV and an emission current of 40 mA. The crystalline size of the particles was calculated by Debye–Scherer equation:

d = Kλ/(β cos θ) (1)

where d is the particle dimension, K is the spherical shape factor (0.89), and β is the full width at half-maximum height (fwhm) of the respective peaks. The average crystallite size was obtained from (113), (004), and (044) peaks.

Furthermore, energy dispersive spectroscopy (EDAX) was used to confirm the elemental composition of the sample. Nova Nano SEM230 FEG scanning electron microscopes (SEM) and FEI Tecnai 20 transmission electron microscope (TEM) with a BaF6 filament at an accelerating voltage of 200 kV were used to investigate morphological properties of the samples. X-ray photoelectron spectroscopy (XPS) was conducted to analyze the surface of the NiCoMnO₄ samples by means of a monochromatic Al Kα radiation with a voltage of 12 kV, a current of 6 mA, and energy resolution of 0.1 eV for high resolution spectra. N₂ adsorption–desorption measurements were conducted using a Quadrasorb instrument preceded by a thermal pretreatment of samples at 250 °C for 10 h.

2.4. Electrochemical characterizations

The electrochemical properties of the samples were investigated in a three-electrode configuration employing a 3 M KOH solution as the electrolyte. Working electrodes were fabricated by applying a paste containing the active material, carbon black, and PTFE with a weight ratio of 7 : 2 : 1, on cleaned pieces of nickel foam as the current collector. The mass loading of the samples ranged from 4.75 to 10.6 mg cm⁻². A platinum mesh and a Hg/HgO (1 M NaOH, ALS Co., Ltd Japan) electrode were used as the counter and reference electrodes, respectively. The electrochemical behavior of the samples was evaluated on a Bio-Logic Science Instrument (VMP3) through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were conducted by applying an AC signal with amplitude of 10 mV in a frequency range from 200 kHz to 0.1 Hz.

Specific capacitance of the samples was estimated form CV curves based on the charge stored during anodic and cathodic scans using the following equation:

in which ∫idV is the integral under the curve and ν is the scan rate. Furthermore, discharge profiles were employed to calculate the specific capacitance of the samples at various current densities, according to:where i is the discharge current in A, Δt is the discharge duration in s, m is the mass of the active material in g, and ΔV is the potential range in V.

2.5. Assembly of hybrid energy storage devices

Asymmetric hybrid devices were assembled by integrating a battery-like NiCoMnO₄ electrode and a supercapacitor-like RGO electrode as the positive and negative electrodes respectively in 2-electrode Swagelok cells. A cellulosic filter paper was used as separator between two electrodes and was thoroughly soaked in 3 M KOH solution before measurements. Based on charge balance theory (Q⁺ = Q⁻), the mass ratio between positive and negative electrodes can be obtained as following:³⁵where C is the specific capacitance of either positive or negative electrodes in three-electrode configuration in F g⁻¹, m is the mass of the electrodes in g, and ΔV is the corresponding potential window in V obtained by CV in 3 electrode configuration. Accordingly, in order to reach charge balance in the full device and a maximum working potential, the mass of the positive and negative electrodes was adjusted, resulting in a m⁺/m⁻ = 0.56. Almost the same mass and areas as in the 3-electrode experiments were used in full devices in order to insure that the capacitance values used in the eqn (4) were appropriate. Asymmetric hybrid devices were then subjected to CV, GCD, and long-term cycling measurements. Specific real energy (calculated directly from the integration of the discharge curves) and specific power of the devices were also calculated according to the following equations:³⁶where E_sp is specific real energy in W h kg⁻¹, I is the discharge current in A, V is the working potential window in V, m_t is total mass of the active material in the expressed negative and positive electrodes in kg, and P_sp is the specific real power in W kg⁻¹.

3. Results and discussion

3.1. Microstructural and compositional analysis

3.1.1. Positive electrodes (metal oxides). Structural and compositional properties of the prepared samples were examined through Powder X-ray diffraction (XRD) and energy dispersive electron diffraction (EDX). Fig. 1a shows XRD patterns of different samples. As can be seen, all diffracted peaks (marked with hkl planes) could be well indexed to the corresponding oxides (e.g. NiO, JCPDS 98-006-2825; Co₃O₄, 98-010-3093; Mn₃O₄, 98-001-7918) without any evidence of impurities. In the case of NCMO NPs, diffraction peaks can be ascribed to the cubic spinel phase with the space group Fd3̄m and lattice parameters of a = b = c = 0.8145 nm (JCPDS 98-009-8472), in agreement with previous reports.^23,37 Moreover, broad diffracted peaks in this sample suggest nanoscale crystalline domains according to Scherrer theory (c.a. 14 nm obtained for particle sizes). EDX results (Fig. S1†) further demonstrate the purity of the mixed metal oxide. N₂ adsorption–desorption experiments were conducted for exploring textural properties of the NCMO NPs. As shown in Fig. 1b, the adsorption branch ascends continuously without leveling off, revealing interparticle adsorption. A small hysteresis (H3 type) appeared at high P/P₀, typically seen in type-IV isotherms.³⁸ The presence of such hysteresis suggests capillary condensation in mesopores (pore diameter of 2–50 nm).³⁹ Accordingly, a high specific surface area of S_BJH = 175 m² g⁻¹ and a total pore volume (V_tot) of 0.69 cm³ g⁻¹ was achieved. Moreover, pore size distribution was estimated by BJH analysis (Fig. S2†), confirming the presence of mesopores mostly centred at ∼15 nm.

Fig. 1 Microstructural analysis of the samples: (a) XRD patterns of NiO, Co₃O₄, Mn₃O₄, and NiCoMnO₄; (b) N₂ ad-/de-sorption isotherms; (c, d) SEM, and (e, f) TEM images of NCMO NPs.

Morphology of the NCMO sample was explored by Scanning Electron Microscopy (SEM) and results were shown in Fig. 1c and d. As can be seen, the sample is almost uniform, comprising of particles with irregular shape (Fig. 1c). Amplified to higher magnification, in Fig. 1d, one can see that the large particles are themselves comprised of smaller nanoparticles agglomerated forming larger particles having different sizes in a cauliflower-like shape. Transmission Electron Microscopy (TEM) was employed to shed more light on morphological properties of the samples. Transmitting electron beam through NCMO specimen (Fig. 1e) shows very fine NPs in the range of 2–5 nm (shown by red arrows). These particles are connected to each other, forming a mesoporous structure in agreement with already observed textural properties. A high resolution TEM image of the NCMO NPs (Fig. 1f) reveals that the nanoparticles are composed of nanocrystalline domains of the spinel cobaltite. As it is seen, lattice fringes with d-spacing of 0.47 and 0.24 nm could be observed, corresponding to the (111) and (311) crystallographic planes, respectively.

The surface elemental composition and chemical state of atoms in the mixed transition metal oxide were probed by X-ray Photoelectron Spectroscopy (XPS). High-resolution Ni 2p spectrum (Fig. 2a) includes two main peaks centred at binding energies (B.E.) of 855.98 and 873.58 eV, assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively. Two satellites (S1 and S2) also appear at 861.48 and 880.49 eV, indicating the presence of Ni²⁺.⁴⁰ By scrutinizing the core region for Co 2p, one might note the presence of Co²⁺ and Co³⁺ species based on appearance of Co 2p_3/2 at 780.18 eV and Co 2p_1/2 at 795.58 eV (Fig. 2b).¹⁹ Fig. 2c shows the high-resolution spectra for Mn 2p. Mn 2p_3/2 and Mn 2p_1/2 emerged in 642.58 and 653.48 eV, unrevealing the existence of Mn³⁺ and Mn⁴⁺ in the spinel structure.³⁷ O 1s spectrum (Fig. 2d) represents two peaks centred at 530.16 eV corresponding to M–O (M = Ni, Co, and Mn) and at 531.8 eV for –OH groups.³⁷ Hence, these XPS results disclose the mixed-valence nature of the NCMO sample which could be beneficial in energy storage applications.

Fig. 2 Surface elemental composition of the NCMO NPs: (a) high-resolution XPS spectra of Ni 2p, (b) Co 2p, (c) Mn 2p, and (d) O 1s.

3.1.2. Negative electrodes (RGO NSs). In order to serve as negative electrodes, RGO NSs has been synthesized via chemically reduced graphene oxide. XRD patterns of the prepared samples can be seen in Fig. S3a.† Disappearance of the sharp intense peak at 11.4° in the case of RGO sample confirms significant removal of oxygen-containing functional groups during the chemical reduction of GO, resulting in the revival of conjugated bonds turning insulating GO into highly conducting RGO NSs.⁴¹ For further tracing the structure and defects in graphene layers, Raman spectroscopy was employed before and after chemical reduction in GO and RGO NSs. The I_D/I_G ratio has been increased in the RGO sample (Fig. S3b†) which can be attributed to the smaller average size and larger amount of new graphitic domains and/or the increased fraction of graphene edges.⁴²

TEM analysis has been also used to explore the morphological properties of graphene layers. As can be seen, TEM image of RGO NSs (Fig. S3c†) depicts expected planar flaky forms of thin graphene layers wrinkled in some parts.

3.2. Electrochemical evaluation of the samples

The electrochemical behavior of samples was first evaluated in a three-electrode configuration using 3 M KOH as the electrolyte. The working electrodes were fabricated by active materials applied on Ni foam with high mass loadings of 4.75 to 10.6 mg cm⁻². Cyclic voltammograms of NiO, Co₃O₄, Mn₃O₄, and NiCoMnO₄ samples at a scan rate of 5 mV s⁻¹ are shown in Fig. 3. In extending the potential range to a positive 600 mV, it can be clearly seen that all samples display a battery-like signature with strong redox peaks arising from faradaic reactions.^43–45 Specific capacitances (C_sp) of electrode materials were calculated based on the charge stored during forward and backward scans. Accordingly, by integrating the area under the curves, one obtains a high C_sp of 425 F g⁻¹ at 5 mV s⁻¹ for NCMO NPs. This is significantly higher than that of obtained for pure oxides (e.g. 214, 173, and 105 F g⁻¹ for NiO, Co₃O₄, and Mn₃O₄ samples, respectively). This improvement clearly shows the importance of the ternary mixed oxides in comparison with those of their single oxide analogues, possibly due to the synergistic effect of the combined structure of multi-cations in the lattice.

Fig. 3 Electrochemical properties of the samples: cyclic voltammograms of different samples at a scan rate of 5 mV s⁻¹ in 3 M KOH solution.

The shape of the CVs for pure oxides are in agreement with previously reported studies, suggesting that Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ species are involved in charge storage in Co₃O₄, while Ni²⁺/Ni³⁺ and Mn³⁺/Mn⁴⁺ redox couples are possibly transformed to each other during charge and discharge in NiO and Mn₃O₄ samples, respectively.^21,46,47 Clarifying the exact redox mechanism in MTMOs is difficult due to complex structure and different possible phases in alkaline solutions. However, based on the obtained results from XPS analysis and previous reports on similar binary MOs,^43,45 following mechanism can be proposed for faradaic reactions, responsible for charge storage in NCMO NPs:

Ni²⁺(Co²⁺,Co³⁺)(Mn³⁺,Mn⁴⁺)O₄ + H₂O + OH⁻ ↔ NiOOH + CoOOH + MnOOH + e⁻

MnOOH + OH⁻ ↔ MnO₂ + H₂O + e⁻

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻

Cyclic voltammograms of NCMO NPs and pure oxides under various scan rates are shown in Fig. 4a and Fig. S4a–c,† respectively. Accordingly, specific capacitance values of the samples were plotted against scan rates, shown in Fig. S4d.† As seen, capacitance values decrease by increasing the scan rate in all samples. Capacitance fading can be attributed to kinetic polarization at higher scan rates, commonly seen in materials having a faradaic charge storage mechanism.⁴⁸ Accordingly, C_sp of NCMO NPs drops from 425 to 175 F g⁻¹ (41% retention) as the scan rate undergoes a 20-fold increase. It is clear that the capacitance of NCMO NPs is much higher than those of single oxides at all measured scan rates.

Fig. 4 Electrochemical properties of NCMO NPs and RGO NSs in 3-electrode configuration in 3 M KOH: cyclic voltammograms of (a) NCMO NPs and (b) RGO NSs at various scan rates; (d, e) GCD profiles of the samples; and rate capabilities (c) at various scan rates and (f) at various specific currents.

Unlike the oxides, CVs of RGO NSs (Fig. 4b) are quite close to the rectangular shape which is characteristic of ideal electric double layer capacitors (EDLCs). More importantly, this semi-rectangular shape of the CV is well preserved by increasing the potential scan rate, demonstrating high electrical conductivity of the sample.⁴⁹ This is in agreement with already obtained results in XRD and Raman which have suggested the significant removal of oxygen-containing functional groups.

According to calculations based on CV, it is been found that RGO NSs can deliver a capacitance of 145 F g⁻¹ at 5 mV s⁻¹ in negative potentials, which is a high capacitance for carbonaceous materials. Fig. 4c displays specific capacitance values of RGO NSs in comparison with NCMO NPs at various scan rates. The evolution of C_sp corresponding to different scan rates in RGO sample reveals excellent capacitance retention even after increasing the scan rate by a factor of 20 (70% retention). This can be understood from EDLC-based charge storage in RGO NSs and their high electric conductivity as well. Galvanostatic charge–discharge (GCD) measurements were performed at various specific currents (Fig. 4d and e) to monitor the energy storage behavior of NCMO NPs as positive and RGO NSs as negative electrodes. As seen in Fig. 4d, NCMO NPs have a non-triangular profile, indicating the faradaic nature of reaction as already understood from cyclic voltammetry experiments. Moreover, the voltage plateaus are exactly in accordance with the redox peaks in CV curves (Fig. 4a). High columbic efficiency of the sample can be also realized by almost symmetrical counterparts in charge–discharge profiles. Using discharge profiles, NCMO NPs showed a high specific capacitance of 510 F g⁻¹ at a current density of 1 A g⁻¹. Rate capability of the samples is also shown in Fig. 4f, revealing around 56% capacitance retention at 10 A g⁻¹ for NCMO NPs. In contrary, the nearly linear GCD profile of RGO NSs (Fig. 4e) portrays an explicit difference in charge storing mechanism, typical for EDLC-based electrodes. A specific capacitance value of 145 F g⁻¹ at 1 A g⁻¹ was obtained. This decreases to 80 F g⁻¹ at 10 A g⁻¹ (Fig. 4f).

To further illuminate the reasons of excellent electrochemical behavior of the NCMO NPs, Electrochemical Impedance Spectroscopy (EIS) measurements were conducted. Nyquist plot of the sample is depicted in Fig. S5.† In the high frequencies, the intercept of the curve with the real axis is considered as equivalent series resistance (ESR), a combination of the electrolyte resistance, intrinsic resistance of the active material, and the interfacial contact resistances. Accordingly, ESR value for NCMO electrode is as small as 0.944 Ω, revealing excellent electrical conductivity of the sample. The semicircle at high/medium frequency ranges can be attributed to charge transfer resistance (R_ct) of the faradaic reactions and its small diameter demonstrates rapid nature of the reaction kinetics in NCMO NPs. Moreover, in low frequency region, linear part corresponds to diffusion-controlled movement of ions which is close to ideal vertical line and reveals excellent electrochemical properties of NCMO NPs for energy storage applications.²⁰ All these indicate the viability of NCMO NPs as a promising electrode material with small impedance.

We also compared the obtained results for NCMO NPs with some corresponding previous reports, listed in Table 1, demonstrating the promising properties of the NCMO NPs as electrode materials for energy storage. As can be seen from the table and to our best knowledge, the energy storage performance of the NiCoMnO₄ as a ternary metal oxide has been investigated in alkaline solutions for the first time in the present work. Moreover, NCMO NPs showed superior or comparable specific capacitance value in comparison with previously reported simple or mixed binary cobalt oxides. All of these results suggest that the synthesized NCMO NPs can promisingly serve as positive electrode materials while RGO NSs can be employed as an ideal negative electrode in hybrid energy storage applications.

Table 1 Comparison of the electrochemical properties of the NiCoMnO₄ NPs with some previous corresponding reports of some mixed cobaltites and simple metal oxides

Sample	Synthesis route	Electrolyte	Csp (F g^−1)	Potential window	Discharge regime	Ref.
MnO2@CCNs	Pyrolysis and hydrothermal	1 M Na2SO4	262	0.9	0.2 A g^−1	51
Co3O4/RGO	Hydrothermal	1 M KOH	445	0.55	0.5 A g^−1	52
NiCo2O4 NS	Solvothermal	2 M KOH	578	0.5	1 A g^−1	53
MnCo2O4 NS	Electrodeposition	1 M NaOH	400	1.0	1 A g^−1	12
MnCo2O4 NS	Hydrothermal	6 M KOH	410	0.5	1 A g^−1	54
MnCo2O4 nanostructures	Sol–gel	2 M KOH	405	0.4	0.62 A g^−1	55
Ordered porous CuCo2O4	SBA-15 template	6 M KOH	1210	0.5	2 A g^−1	6
Ultrathin mesoporous NiCo2O4	Electrodeposition	3 M KOH	2010	0.4	2 A g^−1	4
Co0.72Ni0.28LDH	Electrodeposition	1 M KOH	2104	0.6	1 A g^−1	26
NiCo-LDH@CNT/NF	CVD and electrodeposition	1 M KOH	2046	0.5	1 A g^−1	28
MnO2/CFP	Hydrothermal	1 M Na2SO4	251	1.0	1 A g^−1	56
Co3O4 nanoplate arrays	Electrodeposition	1 M KOH	146	0.5	1 A g^−1	57
NiCoMnO4 NPs	Hydrothermal	3 M KOH	510	0.55	1 A g^−1	This study
			425		5 mV s^−1

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3.3. Performance of NCMO NPs//RGO NSs asymmetric devices

To take advantage of both high-energy faradaic reaction and high-power EDLC phenomenon, two different charge storage fashions were integrated in hybrid NCMO NPs//RGO NSs devices. These positive and negative electrodes were gravimetrically balanced (m⁺/m⁻ = 0.56) according to charge balance theory to reassure that we have achieved the maximum potential window. According to the CV signature of these electrodes in our three-electrode configuration (Fig. S6†), by integrating the two electrodes with different working potential windows, one can expand the operating voltage window to 1.5 V (above the thermodynamic decomposition voltage of water, i.e. >1.2 V).⁵⁰ This is much higher than the available operating voltage in a symmetric RGO NSs//RGO NSs (e.g. 1 V, Fig. S7†). Fig. 5a shows CV curves of hybrid NCMO NPs//RGO NSs device at different voltage windows at 50 mV s⁻¹ in 3 M KOH solution. As can be seen, EDLC plays a dominant role in storing the electric charge up to 800 mV. Beyond this, faradaic reactions take charge of the process and appearance of redox peaks lead to a significant deviation from the ideal rectangular shape typically seen in EDLC symmetric devices (e.g. RGO//RGO in S7a). Conducting CV measurements at various scan rates on the hybrid cell (Fig. 5b) reveals insignificant change in shape of the curve by increasing the sweep rate, demonstrating excellent rate capability and reversibility of this energy storage device.

Fig. 5 CV of hybrid NCMO NPs//RGO NSs in (a) various voltage windows and (b) different scan rates; (c) rate capability of hybrid device and symmetric RGO//RGO supercapacitor; and (d) GCD profile of hybrid device at 0.5 A g⁻¹ and individual profile for each of electrodes.

Rate capability of the asymmetric NCMO NPs//RGO NSs and symmetric RGO NSs//RGO NSs is shown in Fig. 5c. A high specific capacitance value (based on the mass of both electrodes, c.a. 14.9 mg cm⁻²) of 45 F g⁻¹ was achieved at a scan rate of 5 mV s⁻¹. This value is 1.7 times greater than the value obtained for a symmetric RGO NSs//RGO NSs cell (26 F g⁻¹). This is due to embedding NCMO NPs as the positive electrode in the cell which provides high-capacitance as well as fast faradaic reactions. As can be seen, the asymmetric cell could retain 75% of its initial capacitance at a high scan rate of 100 mV s⁻¹, and the capacitance was still much higher than the symmetric cell (34 F g⁻¹ against 17 F g⁻¹). Fig. 5d displays GCD profile of the asymmetric NCMO NPs//RGO NSs cell at a current density of 0.5 A g⁻¹. Nearly identical wings in charge–discharge counterparts demonstrate the excellent reversibility and columbic efficiency of the cell. The charge–discharge wings showed a slightly deviated linear shape where the slope-changing points are in accordance with the observed peaks in CV curves (Fig. 5a). Furthermore, the behavior of individual electrodes in the full cell was monitored via a Hg/HgO reference electrode placed in a T-shape Swagelock® cell to insure that the charge of electrodes are well balanced by adjusting the mass of electrodes; the signals for positive and negative electrodes are shown in Fig. 5d. As can be seen, each electrode behaved in almost the same way as it performed in the three-electrode system. This confirms that mass of electrodes are well balanced with each electrode performing in its corresponding potential region. Clearly, the overall profile is a combination of the two charge storage mechanisms.

A Ragone plot is considered as a common tool for comparing the performance of energy storage devices. The corresponding Ragone chart of our hybrid NCMO NPs//RGO NSs device is displayed in Fig. 6a. (according to GCD profiles at Fig. S8†). As can be seen, the hybrid cell could attain a high specific energy of c.a. 20 W h kg⁻¹ at a specific power of 0.377 kW kg⁻¹ at a current density of 0.5 A g⁻¹. This is more than 6 times greater than the maximum energy of the RGO//RGO cell, originating from significantly increased capacitance and operating voltage by embedding the novel NCMO electrodes in the positive terminal. By increasing the current load to 50 A g⁻¹ (∼385 mA cm⁻²), an impressively high specific power of 37.5 kW kg⁻¹ was achieved at 3.68 W h kg⁻¹; this is much higher (15 times greater) than the RGO//RGO cell for the same energy density. These results demonstrate that by enclosing the NCMO electrode in the cell one not only increases available energy but also enhances significantly the attainable power.

Fig. 6 (a) Ragone plot for the hybrid device and symmetric RGO//RGO supercapacitor; (b) cycling performance of hybrid NCMO NPs//RGO NSs cells over 2000 cycles; and (c) photographs of blue and red LEDs powered on by 3 cells in series.

The capability of providing a prolonged charge is an important factor in energy storage devices. On that account, we inspected capacitance retention of our devices for 2000 cycles at a current density of 2 A g⁻¹ and the outcome is shown in Fig. 6b. Accordingly, a capacitance retention of around 87% was obtained after 2000 cycles, showing the competence of the hybrid device. Moreover, no significant change was realized in the shape of profiles as shown in first three and last cycles (inset of Fig. 6b), revealing good reversibility of the system during consecutive long cycles. Table 2 summarizes a comparison of the performance of NCMO NPs as an electrode material in an asymmetric NCMO NPs//RGO NSs device with some recent corresponding reports for similar systems, showing superior or comparable characteristics of the assembled devices. Moreover, Fig. 6c provides an indication regarding the viability of NCMO NPs//RGO NSs for practical applications. Here, blue and red LEDs (20 mA) were powered by connecting 3 cells in series for around 10 min (see video file in ESI†).

Table 2 Comparison of the performance of NCMO NPs//RGO NSs asymmetric cell with some recent reports on metal-based asymmetric energy storage devices

Sample	Electrolyte	Csp (F g^−1)	Potential window (V)	Max. specific energy (W h kg^−1)	Max. specific power (kW kg^−1)	Ref.
MnO2//graphene	1 M Na2SO4	37 (5 mA cm^−2)	2	25.2	2.5	58
Ordered porous CuO//AC	6 M KOH	72.4 (1 A g^−1)	1.4	19.7	7	50
MnO2//3D graphene	0.5 M Na2SO4	30 (1.5 mA cm^−2)	1	6.8	2.5	59
NiCo2S4//AC	2 M KOH	∼80 (2 A g^−1)	1.5	24.7	17.2	60
CoO/Co3O4//AC	3 M KOH	38.6 (0.2 A g^−1)	1.4	10.5	4.1	61
NiCoDH//AC	2 M KOH	N/A	1.5	17.5	∼10	62
NiCoMnO4 NPs//RGO NSs	3 M KOH	66 (0.5 A g^−1)	1.5	20	37.5	This study
		45 (5 mV s^−1)

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All of these results suggest NiCoMnO₄ NPs as an impressive positive electrode material for aqueous asymmetric devices. Accordingly, combination of NCMO NPs as a novel high capacitance electrode material with a highly conductive RGO NSs which assure high-power, leads to a high-performance promising hybrid device. More importantly, facile synthesis routes and high mass loadings employed during this study ensure suitable applicability of the electrode materials in commercial devices. Despite the competence of NCMO NPs as a novel promising positive electrode material, it should be mentioned that there is obviously room to improve its performance; e.g. by utilization of ordered porous structures to ensure utilization of full capacitance of material or by taking advantage of synergistic effects in nanocomposite materials. Attempts for improving the merits of this material are currently being pursued.

4. Conclusion

In summary, NiCoMnO₄ nanoparticles were synthesized via facile hydrothermal route and their electrochemical properties as electrode materials for energy storage application have been investigated in alkaline medium. The NiCoMnO₄ nanoparticles showed excellent electrochemical properties with a high capacitance of 510 F g⁻¹ at 1 A g⁻¹, much higher than the pure oxide analogues (e.g. NiO, Co₃O₄, and Mn₃O₄) synthesized via the same route. NiCoMnO₄ electrodes were integrated with RGO nanosheets in asymmetric devices, resulting a high energy density of 20 W h kg⁻¹ and a maximum power density of 37.5 kW kg⁻¹. Moreover, NCMO NPs//RGO NSs hybrid devices showed excellent rate capability at various current loads and cycling stability over 2000 cycles.

Acknowledgements

Authors gratefully acknowledge financial support from the European Commission through the Marie-Curie AMAROUT II Fellowship program (A. P.), MINECO (former MICINN) through the Ramon y Cajal Program (RYC-2011-08093), ENE2012-31516, European Union structural funds and the Comunidad de Madrid MAD2D-CM Program (S2013/MIT-3007).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00960c

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