High-performance binder-free supercapacitor electrode by direct growth of cobalt-manganese composite oxide nansostructures on nickel foam
© Jiang et al.; licensee Springer. 2014
Received: 10 June 2014
Accepted: 9 September 2014
Published: 13 September 2014
A facile approach composed of hydrothermal process and annealing treatment is proposed to directly grow cobalt-manganese composite oxide ((Co,Mn)3O4) nanostructures on three-dimensional (3D) conductive nickel (Ni) foam for a supercapacitor electrode. The as-fabricated porous electrode exhibits excellent rate capability and high specific capacitance of 840.2 F g-1 at the current density of 10 A g-1, and the electrode also shows excellent cycling performance, which retains 102% of its initial discharge capacitance after 7,000 cycles. The fabricated binder-free hierarchical composite electrode with superior electrochemical performance is a promising candidate for high-performance supercapacitors.
Due to the depletion of fossil fuels and increasingly serious environmental pollution, there has been an urgent demand for advanced and high-performance energy storage devices to satisfy the needs of modern society and emerging ecological concerns[1, 2]. Supercapacitors, also called electrochemical capacitors, have attracted a great deal of attention due to their excellent performance like high power density, long cycle life, high reliability, etc.[3–5]. They store energy through ion adsorption at the electrode/electrolyte interface or based on faradaic redox reactions by using high-energy electrode materials such as metal oxides, metal-doped carbons, or conductive polymers. Growing interest has concentrated on the metal-oxide nanostructures with excellent electrochemical performance in recent years. Simple binary metal oxide materials such as manganese dioxide (MnO2)[7–9], cobaltosic oxide (Co3O4)[10, 11], nickel oxide (NiO)[12, 13], and ruthenium oxide (RuO2)[14, 15] have been widely studied as supercapacitor electrodes and show good electrochemical performance. Meanwhile, ternary metal oxides with two different metal cations like NiCo2O4[16–18], ZnCo2O4[19, 20], and MnCo2O4[21, 22] have also garnered attentions in recent years due to their promising applications in energy storage fields. The coupling of two metal species could make oxidation-state-rich redox reactions which are essential for pseudocapacitor and various combinations of the cations during the charging-discharging process, and the tunable stoichiometric/non-stoichiometric compositions of the mixed transition metal oxides could provide great opportunities to manipulate the physical/chemical properties[17, 22–24].
Ternary Co-Mn oxides have been widely studied as lithium-ion battery electrodes in recent reports[22, 23, 25, 26], which demonstrates excellent electrochemical performance. For example, Li et al. prepared MnCo2O4 quasi-hollow microspheres which maintained remarkable reversible capacities of 755 mA h g-1 at a current density of 200 mA g-1 after 25 cycles when used as an anode material for lithium ion batteries. Recently, efforts have been devoted to the study of Co-Mn oxide structures as supercapacitor electrode materials[21, 24, 27–29]. Pure MnCo2O4[21, 24] and MnCo2O4.5 nanostructures have been synthesized through the hydrothermal method or solvothermal technique and tested as supercapacitor electrodes, showing potential applications in supercapacitors. Co-Mn composite oxide structures have also been fabricated through the thermally decomposing method or electroless electrolytic technique, showing improved electrochemical performance compared with the pure MnCo2O4 and MnCo2O4.5 nanostructures.
In this work, a facile approach was proposed to fabricate binder-free Co-Mn composite electrodes ((Co,Mn)3O4 nanostructures/Ni foam) for supercapacitors. The method mainly consisted of two steps. Firstly, Co-Mn composite oxide hierarchical nanosheets were directly grown on three-dimensional (3D) Ni foam substrates through a hydrothermal process. Then a post-annealing treatment was conducted at 400°C in air. Directly growing nanostructured arrays on conductive substrates without the use of a binder or an additive can avoid the ‘dead surface’ in conventional slurry-derived electrodes and allow for more efficient charge and mass exchange. The Co-Mn composite oxide ((Co,Mn)3O4) nanostructures/Ni foam electrode was evaluated as a supercapacitor electrode and showed superior electrochemical performance.
All the reagents used in the experiment were of analytical grade. The facile approach was proposed to grow Co-Mn composite nanostructures on Ni foam through a hydrothermal method followed by annealing treatment. In a typical process, commercially available Ni foam was used as current collector and treated with acetone, hydrochloric acid, deionized water, and ethanol in sequence. To obtain a homogeneous precursor solution, 0.338 g manganese sulfate (MnSO4.H2O), 0.291 g cobaltous nitrate (Co(NO3)2.6H2O), 0.721 g urea, and 0.444 g ammonium fluoride (NH4F) were dissolved into the mixed solvent of 40 ml deionized water and 40 ml ethanol under magnetic stirring. The role of NH4F in the formation of different morphologies has been investigated by changing the weight of NH4F such as 0.222 and 0.888 g. After drying, the well-cleaned Ni foam was immersed in the precursor solution. Then the solution was transferred into a Teflon-lined stainless steel autoclave and heated at 120°C for 8 h. After reaction and cooled to room temperature, the substrate was taken out and cleaned with ethanol and deionized water before being dried in air. The dried sample was then annealed in air at 400°C with the heating rate of 1°C min-1 and kept for 4 h to obtain the Co-Mn composite oxide hierarchical structures. Both the Co-Mn composite oxide hierarchical structures/Ni foam and bare Ni foam were weighed in a high-precision analytical balance (Sartorius, Bradford, MA, USA), respectively. The loading density of the Co-Mn nanostructures in the sample is calculated as around 0.80 mg cm-2.
The morphologies were observed with scanning electron microscopy (SEM, SIRION200, Hillsboro, OR, USA) coupled with an energy-dispersive X-ray (EDX, Oxford Instrument, Abingdon, UK). Transmission electron microscopy (TEM) observation was carried out on a JEOL 2100 F microscope (JEOL Ltd., Tokyo, Japan). The nanostructures scratched down from the nickel foam were characterized by X-ray diffraction (XRD) analysis using Bruke D8-Advance (Bruker AXS, Inc., Madison, WI, USA). The specific surface area of the Co-Mn oxide nanostructures was determined by the Brunauer-Emmett-Teller (BET) equation and the pore size distribution was obtained through the BJH method. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG MultiLab 2000 system with a monochromatic Al Ka X-ray source (ThermoVG Scientific, East Grinstead, West Sussex, UK). The Co-Mn composite oxide nanostructures/Ni foam electrode was evaluated for a high-performance supercapacitor by the three-electrode system in 1 M KOH aqueous solution. The three-electrode assembly was constructed using the sample as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt foil as the counter electrode. The close contact of Co-Mn nanostructures on the current collector Ni foam allows for efficient charge transport, and waives the need for adding ancillary conducting material or binders. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted on an Autolab work station (PGSTAT-302 N, Eco Echemie B.V. Company, Utrecht, Netherlands). The electrochemical performance of pure Ni foam after being annealed at the same conditions as those for Co-Mn composite oxide nanostructures/Ni foam was tested by the CV technique. Galvanostatic charging/discharging and cycling tests were conducted using a battery measurement system (LAND CT2001A, Wuhan LAND Electronics, Wuhan, China).
Results and discussion
where I (A), Δt (s), m (g), and ΔV (V) are the discharge current, discharge time, mass of the active materials, and the potential windows, respectively. The small capacitance of nickel foam could be neglected in the calculation of specific capacitance. As shown in Figure 6b, the Co-Mn composite oxide structures/Ni foam electrode exhibits high specific capacitances of about 910.1, 898.6, and 889.2 F g-1 at relatively low current densities of 1, 2, and 3 A g-1, respectively. When the current density is increased to 10 A g-1, the specific capacitance of the electrode is around 840.2 F g-1, which keeps 92.3% of the specific capacitance at the current density of 1 A g-1. In comparison with literature reports, MnCo2O4.5 hierarchical architectures with a specific capacitance of 151.2 F g-1 at the scan rate of 5 mV s-1, one-dimensional MnCo2O4 nanowire arrays with a specific capacitance of 349.8 F g-1 at the current density of 1 A g-1, and Co-Mn oxide/carbon-nanofiber composite electrodes with a specific capacitance of 630 F g-1 at the scan rate of 5 mV s-1, this study demonstrates much higher specific capacitance of Co-Mn composites. The reported (Co,Mn)3O4 nanowires/Ni foam electrode exhibits a specific capacitance of 611 F g-1 at the current density of 2.38 A g-1 in 6.0 mol dm-3 KOH electrolyte. The excellent rate capability and high specific capacitance of the as-fabricated Co-Mn composite oxide nanosheets/Ni foam mainly attribute to the unique 3D hierarchical porous structures of the Co-Mn composite oxide electrode. The binder-free nanosheet structures can ensure good electric contact with the 3D Ni foam, and the open spaces between neighboring nanosheets allow for easy diffusion of the electrolyte, which is helpful for charging or discharging at a high current density. Moreover, the porous nature of the nanosheet structures will enhance the electrolyte/electrode contact area, shorten the ion diffusion distance, and accommodate the strain induced by the volume change during the redox reaction.
EIS measurement was also employed to characterize the composite electrodes over the frequency range from 0.1 to 106 Hz as shown in Figure 6c. The intersection of the Nyquist plot on the real axis is related to the equivalent series resistance. The resistance of the hierarchical Co-Mn oxide nanosheets/Ni foam electrode is around 1.15 Ω, which is lower than that of the reported Co-Mn oxide hierarchical structure electrode (1.3 Ω), indicating the improved charge transport properties of the electrode. The excellent rate capability and high specific capacitance of the electrode are also due to the relatively small ESR, which could improve the fast redox reaction. The cycling performance of the electrode was tested at the current density of 10 A g-1 and the result is shown in Figure 6d. When the electrode was cycled up to 7,000 cycles, over 102% of its initial discharge capacitance is retained. In addition, during the first 1,000 cycles, the specific capacitance was increasing gradually, indicating an electroactivation process of the electrode under the given testing conditions (in a voltage range of 0 to 0.5 V, at the current density of 10 A g-1). Figure 6e,f shows the morphologies of the Co-Mn composite oxide hierarchical nanostructures before and after cycling tests, respectively. There are no obvious cracks and collapses on the hierarchical structures after being tested for 7,000 cycles, indicating good mechanical stability of the fabricated Co-Mn composite oxide hierarchical structures. The electrode with both long-term stability and superior electrochemical performance shows promising applications for high-performance supercapacitors.
In summary, we have developed a facile approach to grow Co-Mn composite oxide ((Co,Mn)3O4) hierarchical nanosheets on 3D conductive Ni foam through a hydrothermal method together with a post-annealing treatment. The porous nanosheets/Ni foam electrode shows a high specific capacitance of 910.1 and 840.2 F g-1 at the current density of 1 and 10 A g-1, respectively. Over 102% of its initial discharge capacitance is retained after 7,000 cycling cycles at the current density of 10 A g-1. The outstanding electrochemical performance will undoubtedly make the Co-Mn composite oxide hierarchical nanostructures a promising candidate for high performance supercapacitor.
This work is supported by the National Science Foundation of China (Grant Nos. 51275195 and 91323106), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13017), and the National Instrument Development Specific Project of China (Grant No. 2011YQ16000204).
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