Efficient energy storage capabilities promoted by hierarchical MnCo₂O₄ nanowire-based architectures

Abstract

A facile, two-step hydrothermal/calcination approach to grow uniform MnCo₂O₄ nanowires (NWs) on a binder- and conductive-agent-free nickel foam substrate has been demonstrated. The hierarchical MnCo₂O₄ NW-based architectures on a conductive substrate allow for enhanced electrolyte transport and charge transfer towards/from the MnCo₂O₄ NWs' surfaces with large numbers of electroactive sites. In addition, the direct growth and attachment of the MnCo₂O₄ NWs on the supporting conductive substrates provide much reduced contact resistances and efficient charge transfer. These excellent features allow the use of MnCo₂O₄ NWs as a lithium-ion battery electrode, with high capacity at a current density of 200 mA h g⁻¹, as well as a very good cycling stability. Moreover, these MnCo₂O₄ NWs have also demonstrated a high capacitive behavior for symmetric supercapacitor application, indicating the potential application of MnCo₂O₄ NW high-performance energy-storage devices.

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Developing new and renewable energy systems based on multifunctional energy storage sources is necessary because of growing concerns over fossil fuel usage, global warming, and resource consumption.¹ Among these electrical energy storage systems are batteries and supercapacitors (SCs) which utilize different mechanisms for energy storage. Batteries utilize the bulk of an electrode material for charge storage via reduction–oxidation (redox) reactions, whereas SCs ideally store electric energy in the electric field of an electrochemical double layer.^2,3 Batteries can store significantly more energy per unit mass than SCs because the former utilizes electrochemical reactions called faradaic processes. These processes involve charge transfer across the interfaces between battery electrodes and an electrolyte solution, leading to redox reactions of species at the interfaces. The charge-storing processes employed by SCs are significantly faster than the faradaic processes in batteries; thus, while SCs have lower energy densities than batteries, the capacitors have higher power densities.³ Lithium ion batteries (LIBs) have the highest energy densities among rechargeable batteries. LIBs are used in portable electronic devices, power tools, stationary power supplies, and medical instruments in military, automotive, and aerospace applications.⁴ Based on their charge storage mechanism, SCs are classified into (1) electric double-layer capacitors (EDLCs), in which the capacitance arises from simple charge separation at the electrode/electrolyte interface and (2) pseudo-capacitors, in which the pseudocapacitance arises from faradaic charge-transfer chemical reactions during charge–discharge process at the electrode interface.^5,6 These mechanisms can work separately or together, depending on the active electrode materials used in the SC.⁷

Metal oxides have long been studied as potential electrode materials for LIBs and SCs because of the ease associated with their large-scale fabrication and their rich redox reactions involving different ions, which contribute to high specific capacities/capacitances.⁸ Transition-metal complex oxides have recently attracted research interest because of their superior physicochemical properties and significant potential for many technological applications, ranging from catalysts to electrode materials. Partial substitution of M (M = Fe, Mn, Co, or Ni) in spinel M₃O₄ with other 3d-transition metals could improve the catalytic activity, selectivity, stability, and resistance of the oxides to poisoning in a number of catalytic or electrocatalytic processes.⁹ Complex oxides formed by the combination of two transition-metal oxides or a transition-metal oxide and a post-transition metal oxide have also been studied because of their good cycling capacity resulting from complementary and synergistic Li⁺ charge–discharge processes and superior capacitive performance.^1,10 Among the transition-metal oxides currently available, cobalt-containing spinel oxides (MCo₂O₄, where M = Cu,^11,12 Mn,^13,14 Ni,^15–18 Zn,^19–22 or Mg^23,24) have drawn considerable attention because of their excellent physicochemical properties, which are suitable for many technological applications, ranging from catalysts and sensors to electrode materials and electrochemical devices.²⁵ For instance, NiCo₂O₄ has been investigated as high-performance electrode material for LIBs and SCs because of its relatively better electrical conductivity and higher electrochemical activity compared with binary NiO and Co₃O₄.^26–29 While spinel MnCo₂O₄ has been studied as a magnetic^30,31 and catalytic^32–34 material, MnCo₂O₄ has received little attention as an anode material for LIB and SC electrodes. Lavela et al.³⁵ studied the electrochemical properties of MnCo₂O₄ particles prepared using the sol–gel method with an initial discharge capacity of 1200 mA h g⁻¹ at 1 C rate. Liu and Wang³⁶ prepared nanosized MnCo₂O₄ using the hydrothermal method and conducted charge–discharge testing of MnCo₂O₄ as an anode for LIBs at 0.2 mA cm⁻² with an initial discharge capacity of 1448 mA h g⁻¹. These previous studies indicate that MnCo₂O₄ is a promising electrode material for LIBs. Li et al.¹³ recently prepared multiporous MnCo₂O₄ hollow structures and studied their performance as an anode material for LIBs; results indicated that MnCo₂O₄ has a first discharge capacity of 1473 mA h g⁻¹ at 200 mA g⁻¹ and possesses a reversible capacity of 755 after 25 cycles. Studying the use of MnCo₂O₄ as electrodes for SCs, Gomez and Kalu³⁷ fabricated a thin film binder-free Co–Mn composite oxide electrode using a two-step process involving both electroless and electrolytic oxidation of plated film; here, the molar ratio of Mn and Co were controlled in the electroless bath. A specific capacitance of 832 F g⁻¹ at a scan rate of 20 mV s⁻¹ was obtained using cyclic voltammetry (CV) in a 0.5 M Na₂SO₄ aqueous electrolyte. Various non-aqueous electrolytes have also been developed to achieve cell operating voltages >2.5 V. Given that the specific energy of SCs is proportional to the square of the operating voltage, non-aqueous electrolyte mixtures, such as propylene carbonate (PC) or acetonitrile, have been used in many commercial SCs, particularly those for high-energy applications.³⁸ However, the low capacitance of the non-aqueous electrolyte mixtures, as well as being unfriendly electrolyte, undermine the overall effectiveness of such electrolyte for SC applications.³⁹

In the current paper, we report the direct growth of MnCo₂O₄ nanowires (NWs) onto Ni foam (binder- and conductive-agent-free) using simple hydrothermal synthesis followed by heat treatment at 350 °C for 3 h (for the detailed experimental procedure, see the ESI†). To the best of our knowledge, this study is the first to report the performance of MnCo₂O₄ NWs as anode materials for LIBs and SCs. We fabricated non-aqueous electrolyte-based symmetric SCs with capacitances consistent with the capacitance values reported in previous studies that used aqueous electrolytes.

The X-ray diffraction (XRD) pattern of MnCo₂O₄ NWs grown on Ni foam after calcination at 350 °C for 3 h is shown in Fig. 1a. Except for two typical peaks derived from Ni foam (at approximately 2θ of 44.5° and 51.9°, JCPDS File no. 04-0850), all of the diffraction peaks can be clearly indexed and assigned to the spinel MnCo₂O₄ phase (JCPDS File no. 23-1237, space group Fd3̄m). No other peak is detected, thereby indicating the absence of any impurities (e.g., NiO, MnO₂, or CoO) in the product and uniform growth of MnCo₂O₄ on the Ni foam surface. The morphology of MnCo₂O₄ NWs grown on Ni foam was characterized using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Fig. 1b, inset, (higher magnification) and S2 (ESI†) show SEM images of the MnCo₂O₄ NWs grown on Ni foam after heat treatment at 350 °C for 3 h. Uniform growth of numerous NWs on the entire foam can be observed. Typical NWs have diameters of ∼100 nm (Fig. S1d†). Fig. 1c and d show the corresponding TEM and high-resolution TEM (HRTEM) images of a single MnCo₂O₄ NW with a length of ∼1 μm and diameter of ∼100 nm (Fig. 1c and its inset). A higher magnification TEM image in Fig. 1c (inset) reveals that a typical MnCo₂O₄ NW has a porous architecture. The resolved lattice fringes (Fig. 1d) are about ∼0.28 nm and 24 nm, corresponding well to the (220) and (311) planes, respectively, of spinel MnCo₂O₄. The selected-area electron diffraction (SAED) pattern shown in Fig. 1d (inset); reflection planes confirm the successful synthesis of single-crystalline MnCo₂O₄ NWs similar to the ZnCo₂O₄ found in our previous work¹⁹ via the facile hydrothermal reaction. Energy-dispersive X-ray spectrometry (EDS) microanalysis of the NWs (Fig. S2, ESI†) shows that the nanostructure contains only Mn, Co, and O elements, which indicates the formation of pure MnCo₂O₄. EDS mapping (Fig. S2, ESI†) provides clearer information about the element distribution within the NWs and confirms the formation of pure MnCo₂O₄ products. EDS mapping (Fig. S2, ESI†) provides clearer information about the element distribution within the NWs and confirms the formation of pure MnCo₂O₄ products.

Fig. 1 (a) XRD pattern, (b) FE-SEM, (c) TEM images, and (d) HRTEM images of MnCo₂O₄ NWs. Inset in (b) shows a higher-magnification image of the MnCo₂O₄ NWs. Inset in (c) shows a higher-TEM magnification image of a single MnCo₂O₄ NW. Inset in (d) shows the corresponding SAED pattern. The scale bars of (c) and (d) are 200 nm (20 nm in inset) and 5 nm, respectively.

The electrochemical properties of MnCo₂O₄ NWs grown on Ni foam were initially analyzed using cyclic voltammetry (CV). Fig. 2a shows the first three CV curves of a MnCo₂O₄ NWs on Ni foam electrode obtained at a scan rate of 0.5 mV s⁻¹, room temperature, and potentials ranging from 0.01 V to 3.0 V. The CV curve for the first cycle obviously differs from those of the following cycles, and no significant alteration is found between the second and third cycles. In the first cycle, a sharp reduction peak is found at 0.58 V and a small reduction peak is observed at 0.41 V in the cathodic process. The peak at 0.58 V can be assigned to Li intercalation into the lattice followed by MnCo₂O₄ crystal structure destruction and formation of the respective nanosize metal particles⁴⁰ (reduction of Co³⁺ to Co²⁺, Co²⁺ and Mn²⁺ to metallic Co and Mn, respectively, in MnCo₂O₄ crystals).^13,36 The small sharp peak at 0.41 V may be assigned to Li₂O formation and decomposition of the organic electrolyte to form a solid electrolyte interphase (SEI) layer at the electrode/electrolyte interphase. These cathodic irreversible peaks can be elucidated by the following equation: MnCo₂O₄ + 8Li⁺ + 8e⁻ → Mn + 2Co + 4Li₂O. In the anodic scan, two broad oxidation peaks are observed at 1.68 and 2.21 V, corresponding to the oxidation of Mn to Mn²⁺ and of Co to Co²⁺, respectively.^13,41 The reduction peak in the second and third cycles gradually moves to ∼0.88 V, differing significantly from the irreversible electrochemical reaction observed in the first discharge cycle. In the anodic polarization process, two peaks are recorded at 1.7 and 2.26 V, which can be attributed to the oxidation of Mn⁰ to Mn²⁺ and of Co⁰ to Co²⁺, respectively. MnO and CoO redox reactions can be elucidated by the following equation; Mn + 2Co + 3Li₂O → MnO + 2CoO + 6Li⁺ + 6e⁻.¹³

Fig. 2 Electrochemical performance of MnCo₂O₄ NWs electrode for LIBs. (a) First three cycles of CVs at a scan rate of 0.5 mV s⁻¹ and voltage ranging from 0.01 V to 3.0 V; (b) voltage versus capacity profiles of the first, second, third, seventh, and thirtieth discharge–charge cycles (at 200 mA g⁻¹); (c) cycle performance at 200 mA g⁻¹; and (d) rate capability at various current densities.

The MnCo₂O₄ NWs were then tested as anodes for LIBs. Representative charge–discharge profiles of the first, second, third, seventh, and thirtieth cycles at a current density of 200 mA g⁻¹ are shown in Fig. 2b. The initial discharge and charge capacities are 2264 and 1634 mA h g⁻¹, respectively, corresponding to 72% of the first coulombic efficiency. In addition to the contribution of the partially reversible formation/decomposition of the SEI and organic polymeric/gel-like layers by electrolyte decomposition,⁴¹ the high discharge capacity (theoretical value = 906 mA h g⁻¹) of the first cycle may also be related to the shortening of the Li⁺ diffusion distance and increases in the number of diffusion ions caused by the NW porous structure.¹⁹ The first discharge–charge capacities are higher than those recorded in previous studies on MnCo₂O₄ in LIBs.^13,35,36 The first cycle discharge plateau appears at ∼0.9 V, which corresponds to Li intercalation into the MnCo₂O₄ crystal lattice followed by crystal structure destruction and formation metallic particles. These results are consistent with the CV behavior. The plateau at ∼0.9 V shifts to ∼1.2–1.3 V in later cycles, which indicates that irreversible reactions occur during the first cycle; the findings further confirm that different electrochemical reactions govern the successive cycles. The discharge capacities of the electrode in the second, third, and seventh cycles range from 1708 mA g⁻¹ to 1740 mA g⁻¹, indicating high stability of the electrode material. The thirtieth discharge capacity shows a high capacity value of ∼1273 mA h g⁻¹. The irreversible capacity loss in the first cycle (∼23%) can be attributed to the SEI formation and metal oxide reduction to metal with Li₂O formation, which is commonly observed in various electrode materials.⁴² Fig. 2c shows the charge–discharge capacity of the MnCo₂O₄ NWs electrode as a function of the number of cycles at a current density of 200 mA g⁻¹. The coulombic efficiency is significantly improved. Within 45 cycles, capacity loss is observed only in the first several cycles. The specific discharge capacity of the electrode is 1710 mA h g⁻¹ after seven cycles. However, the reversible capacities gradually decrease in subsequent cycles. The obtained coulombic efficiency (Fig. 2c) indicates that the charge–discharge process gradually stabilizes. From the 2^nd cycle to the 45^th cycle, the charge–discharge capacities in the measured range tend to stabilize, and their corresponding values change from ∼1708 mA h g⁻¹ (2^nd cycle) to ∼1038 mA h g⁻¹ (45^th cycle); these values are greater than the theoretical capacity of MnCo₂O₄. To investigate the performance rate of the MnCo₂O₄ NWs, the rate capability was evaluated using multiple-step charging–discharging at different current densities ranging from 200 mA g⁻¹ to 1600 mA g⁻¹ (Fig. 2d). Fig. 2d shows that the discharge capacity slightly decreases from ∼1600 mA h g⁻¹ at 200 mA g⁻¹ (after seven cycles) to 1180, 708, and 388 mA h g⁻¹ at current densities of 400, 800, and 1600 mA g⁻¹, respectively. The current density returns to the initial values in reverse order. The capacity also recovers to 515, 811, and 1174 mA h g⁻¹ at current densities of 800, 400, and 200 mA g⁻¹, respectively, after seven cycles. These results show that MnCo₂O₄ NWs anode for LIB has significantly high capability for capacity retention and good rate performance.

The high capacity and good rate capability can be attributed to the unique morphology and structure of the MnCo₂O₄ NWs/Ni foam electrodes brought about by the following characteristics: (1) MnCo₂O₄ NWs directly grown on Ni foam have high electronic conductivity because MnCo₂O₄ NWs are firmly bound to the Ni foam substrate, yielding significantly good adhesion and electrical contact; and (2) the 3D configuration of the MnCo₂O₄ NWs/Ni foam confirms the loose textures and open spaces between neighboring NWs, which significantly enhances the electrolyte/MnCo₂O₄ contact area; therefore, ideal conditions for the facile diffusion of the electrolyte are provided and accommodation of the strain is induced by the volume change during electrochemical reactions. These conditions lead to higher lithiation and delithiation efficiencies under electrolyte penetration.⁴²

In addition to LIB anodes, the potential use of the MnCo₂O₄ NWs electrode as electrochemical capacitors was evaluated. A symmetrical SC was assembled from MnCo₂O₄ NWs/Ni foam electrodes using glass fiber as a separator in 1 M LiClO₄/PC nonaqueous electrolyte at voltage between 0 and 2.5 V. Fig. 3a shows the CV curves obtained at various scan rates between 1 and 100 mV s⁻¹. The CV curves are nearly rectangular-shape and show almost-ideal capacitive behaviors for scan rates up to 50 mV s⁻¹ (Fig. S3, ESI†). This observation indicates the low contact resistance and close-to-ideal pseudocapacitive nature of the electrodes.⁴³

Fig. 3 Electrochemical performance of MnCo₂O₄ NWs electrode for symmetric SCs. (a) CVs at different scan rates of 1–100 mV s⁻¹; (b) capacity retention of 3000 cycles at 25 mV s⁻¹; (c) galvanostatic charge–discharge curves of the first five cycles of the SCs at 0.1 A g⁻¹, and (d) first cycle of the SCs at different current densities in 1 M LiClO₄/PC.

At higher scan rates (100 to 1000 mV s⁻¹, Fig. S3, ESI†), the CV curves deviate from a rectangular shape and instead show an oval shape, which can be ascribed to the inherent resistivity of the electrode. Such resistivity can be attributed to intensified polarization, rapid charge transfer, and cationic diffusion. High scan rates of 0.5 and 1 V s⁻¹ suggest the possible application of the electrode in high-power SCs. The MnCo₂O₄ NWs/Ni foam electrode exhibits high specific capacitances of 783.1, 346.9, 275.2, 217.8, 179.2, 145.4, 76.7, and 53.0 F g⁻¹ at scan rates of 1, 5, 10, 25, 50, 100, 500, and 1000 mV s⁻¹, respectively. Even under higher scan rate of 5 V s⁻¹, capacitive behavior can still be observed (Fig. S3d, ESI†) with a specific capacitance of 19.3 F g⁻¹. Our results are comparable with or better than those previously reported for MnO₂,^44–46 and Co₃O₄ (ref. 47) and consistent with the values obtained recently for Mn–Co composites,³⁷ even though aqueous electrolytes were used in these studies. The cycling stability of the electrodes was evaluated at 25 mV s⁻¹ (Fig. 3b). The obtained capacitance values, which vary from 217 F g⁻¹ for cycle 1 to 210 F g⁻¹ for cycle 3000, are almost constant up to 3000 cycles and feature 96.8% retention of the initial capacitance. The electrochemical performance of the MnCo₂O₄ NWs/Ni foam electrode was further evaluated by galvanostatic charge–discharge measurements performed at different current densities. Fig. 3c shows the constant current charge–discharge curves of symmetrical SCs at 0.1 A g⁻¹ (first five cycles; see also Fig. S4, ESI†). During the charging and discharging steps, the charge curve is nearly symmetrical with its corresponding discharge counterpart and shows a small decrease in internal resistance (IR drop), which indicates pseudocapacitive contributions. The anodic charging segment is symmetrical with its corresponding cathodic discharging counterpart (triangular shape) in the galvanostatic charge–discharge curve, which is indicative of a highly reversible capacitive material or a strongly reversible charge–discharge reaction. The results show the linear variation of the cell potential with time, which suggests the pseudocapacitive nature of the electrode resulting from charge storage via adsorption/desorption at the electrode/electrolyte interface as well as double layer contributions. This is consistent with that reported by Ramchandra et al.,⁴⁸ who found that Li⁺ insertion/extraction is the only mechanism in organic electrolytes. The charge and discharge curves of several representative current rates (0.1, 0.5, 1, 2, 5, and 10 A g⁻¹) are shown in Fig. 3d and its inset. All of the curves present symmetrical features between the charging and discharging branches, which suggests the ideal pseudocapacitive nature of fast charge–discharge processes.⁴³

Impedance measurements were performed to investigate the electrochemical characteristics of the SC electrode/electrolyte interface quantitatively. Fig. 4a shows the Nyquist plots of symmetrical SC cells assembled based on MnCo₂O₄ NWs/Ni foam before and after charge–discharge cycling under different current densities. All of the Nyquist plots show a semicircle at the high-to-middle frequency range, followed by a straight line in the low frequency region. The lines at lower frequencies are almost straight in all of the plots, which indicates pronounced capacitive behaviors with very small diffusion resistance.^49,50 The intercept of the semicircle at high frequency on the real impedance axis (Z′) represents a combined resistance (R_s), which includes the intrinsic resistance of electrode materials, the resistance of bulk electrolyte solution, and the contact resistance. The diameter of the semicircle corresponds to the charge transfer resistance (R_ct) caused by faradaic reactions and EDLC at the electrode/electrolyte interface.⁴⁹

Fig. 4 (a) Nyquist plot recorded for symmetrical SC cells containing MnCo₂O₄ NWs/Ni foam before and after cycling under different current densities. (b) Equivalent circuit of the MnCo₂O₄ NWs/Ni foam-based SC.

The linear part in the low frequency region of the Nyquist plot is related to the Warburg resistance (W_o, diffusive resistance) of the electrolyte into the interior of the electrode surface and ion diffusion/transport into the electrode surface. A line that is almost vertical to the real axis in the imaginary part of the impedance at the low-frequency region represents swift ion diffusion in the electrolyte and adsorption onto the electrode surface, which, in turn, suggests the ideal capacitive behavior of the electrodes.⁵⁰ The symmetrical SC cell is illustrated in Fig. 4b. In the equivalent circuit, R_s is connected in series to a double-layer capacitance (C_dl), which is connected in parallel to R_ct and W_o. By fitting the experimental data using Zview™ software (Scribner Associates, Inc.), R_s values of 5.98 and 6.5 Ω and R_ct values of 3.7 and 4.2 Ω can be respectively obtained from the electrode before and after charging–discharging tests. The R_s values are nearly the same, while the R_ct values vary insignificantly. The vertical diffusion lines show minimal slope differences, which indicate the good capacitive behavior of the electrode before and after cycling. Such capacitive behavior may be attributed to the NWs structure, which facilitates fast electron transfer between the active materials and the charge collector. NWs can function as transport channels for storing and transferring more electrical charges to the electrodes. NWs have large specific surface areas that can increase the effective liquid–solid interfacial area and result in efficient utilization of the active material.⁵¹ In general; excellent electrochemical performance could be derived from the dense and porous features of MnCo₂O₄ NWs, which significantly increase the number of electroactive sites. Furthermore, the direct growth of interconnected 1D NWs on conductive substrate ensures good mechanical adhesion as well as enhanced electrical contact with the conductive substrate; the substrate also serves as a current collector in highly integrated electrodes.

In summary, hierarchical MnCo₂O₄ NWs were successfully grown on conductive substrates using a facile hydrothermal method combined with calcination at 350 °C in air. The MnCo₂O₄ NWs supported on Ni foam directly served as binder- and conductive-agent-free electrodes for LIBs and SCs. The LIB anode fabricated from the MnCo₂O₄ NWs on Ni foam shows a high first discharge capacity of 2264 mA h g⁻¹ at a current density of 200 mA g⁻¹. This LIB anode also exhibits significant rate capabilities and long-term cycling performance. The SC electrode fabricated from MnCo₂O₄ NWs on Ni foam shows an electrochemical capacitance of 783.1 and 346.9 F g⁻¹ at scan rates of 1 and 5 mV s⁻¹, respectively. This SC electrode also exhibits capacitance behaviors under high scan rates of 1 and 5 V s⁻¹. Therefore, MnCo₂O₄ NWs on Ni foam can be used for the fabrication of high-performance energy-storage devices.

Acknowledgements

The financial support from the Ministry of Science and Technology of Taiwan (MOST 100-2112-M-003-009-MY3 and MOST 101-2113-M-002-014-MY3) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supporting results. See DOI: 10.1039/c4ra01677g

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