Advanced binder-free electrodes based on CoMn2O4@Co3O4 core/shell nanostructures for high-performance supercapacitors

Three-dimensional (3D) hierarchical CoMn2O4@Co3O4 core/shell nanoneedle/nanosheet arrays for high-performance supercapacitors were designed and synthesized on Ni foam by a two-step hydrothermal route. The hybrid nanostructure exhibits much more excellent capacitive behavior compared with either the pristine CoMn2O4 nanoneedle arrays alone or Co3O4 nanosheets alone. The formation of an interconnected pore hybrid system is quite beneficial for the facile electrolyte penetration and fast electron transport. The CoMn2O4@Co3O4 electrode can achieve a high specific capacitance of 1627 F g−1 at 1 A g−1 and 1376 F g−1 at 10 A g−1. In addition, an asymmetric supercapacitor (ASC) was assembled by using the CoMn2O4@Co3O4 core/shell hybrid nanostructure arrays on Ni foam as a positive electrode and activated carbon as a negative electrode in an aqueous 3 M KOH electrolyte. A specific capacitance of 125.8 F g−1 at 1 A g−1 (89.2% retention after 5000 charge/discharge cycles at a current density of 2 A g−1) and a high energy density of 44.8 W h kg−1 was obtained. The results indicate that the obtained unique integrated CoMn2O4@Co3O4 nanoarchitecture may show great promise as ASC electrodes for potential applications in energy storage.


Introduction
Supercapacitors have attracted a lot of attention due to their ultrafast charge-discharge capability, reversibility, safe operation, long cycle life, high power density and environmental friendliness. [1][2][3] Supercapacitors are usually divided into two categories according to their energy storage behavior: electrical double-layer capacitors (EDLCs) and pseudocapacitors. 4,5 Among them, pseudocapacitors exhibit higher specic capacitance due to the fast and fully reversible faradaic redox reactions at the interface between the electrode and electrolyte. As electrode materials for pseudocapacitors, transition metal oxides including Co 3 O 4 and MnO 2 and their related compounds have been widely studied. [6][7][8][9][10][11][12] Recently, manganese-based transitional metal oxides (such as CoMn 2 O 4 ) have emerged as a promising electrode material for supercapacitors due to their high reversible capacities and eco-benignity. 13 Jiang et al. synthesized hierarchical nanosheets of CoMn 2 O 4 on Ni foam using a hydrothermal method, which exhibited a high capacitance of 840 F g À1 at 10 A g À1 and retained 102% of its initial capacitance aer 7000 cycles. 14 Ren et al. fabricated owerlike microspheres of CoMn 2 O 4 , and it showed the specic capacitance of 188 F g À1 at 1 A g À1 with a capacitance retention of 93% aer 1000 cycles. 15 However, the experimental values of the capacitance are appreciably lower than the theoretical ones as has already been observed. There is a great need and a challenge to enhance the capacitive performance of CoMn 2 O 4 . Three-dimensional (3D) nanostructured electrodes have been intensively studied as attractive candidates for electrodes of high-performance supercapacitors due to their unique morphological architectures and super electrochemical properties, [16][17][18] and especially unique porous 3D core/ shell nanostructures usually exhibit novel physicochemical properties. [19][20][21] Therefore, intensive research efforts have been expended to design 3D core/shell nanostructured electrodes to shorten diffusion length of electrolytes to interior surfaces. Cai et al. fabricated 3D Co 3 O 4 @NiMoO 4 core/shell nanowire arrays via a facile two-step hydrothermal method. The material showed excellent electrochemical performance with a remarkable areal capacitance of 5.69 F cm À2 (1094 F g À1 ) at a current density of 30 mA cm À2 . 22 Liu et al. fabricated Co 3 O 4 @MnO 2 core/shell hierarchical nanowire arrays and showed remarkably improved electrochemical performance (about 4 to 10 times increase in areal capacitance with respect to single Co 3 O 4 array). 23 Therefore, taking into account the above-mentioned consideration, it is therefore of great possibility to the rational design and fabrication of CoMn 2 O 4 @Co 3 O 4 core/shell hybrid nanostructure arrays as electrode materials with combined properties of large areal capacitance and rate capability for high performance pseudocapacitor applications.
In this paper, CoMn 2 O 4 @Co 3 O 4 core/shell hybrid nanostructure arrays on Ni foam with unique hierarchical nanostructure for supercapacitor applications were successfully prepared via a facile hydrothermal method. The CoMn 2 O 4 @-Co 3 O 4 core/shell hybrid nanostructure arrays provides a large surface area and a number of electrochemical reactive sites, and faster ion diffusion and electron transport at electrode/ electrolytes interface compared with single CoMn 2 O 4 nanoneedles arrays or Co 3 O 4 nanosheets electrode. Therefore, such interconnected core/shell hybrid network conguration can effectively increase capacitance and cycling stability. The CoMn 2 O 4 @Co 3 O 4 hybrid electrodes presented herein exhibited remarkable electrochemical performance for SCs. The CoMn 2 -O 4 @Co 3 O 4 //AC ASC device also exhibits high specic energy and energy density. These results suggest that the CoMn 2 -O 4 @Co 3 O 4 core/shell hybrid nanostructure arrays can act as a high performance electrode material for SCs applications.

Synthesis of CoMn 2 O 4 nanoneedles arrays
In a typical process, rstly, commercially available Ni foam was pretreated with acetone, 3 M HCl solution, deionized water, and ethanol in sequence, and kept in vacuum oven at 60 C for 6 h. To obtain a homogeneous precursor solution, 2.10 g of Co(NO 3 ) 2 $6H 2 O, 3.48 g of Mn(CH 3 COO) 2 $4H 2 O, 5.07 g of urea and 1.63 g of NH 4 F were dissolved into the 70 mL ethanol under magnetic stirring. The total volume was then made up to 350 mL by adding distilled water and transferred to a Teonlined stainless steel autoclave. Then, the well-cleaned Ni foam (1 Â 1 cm 2 in sizes) was immersed in the autoclave. Subsequently, the autoclave was sealed and placed in an electrical oven at 160 C for 16 h. Aer reaction and cooled to room temperature, the precursor deposited Ni foam was taken out and cleaned with ethanol and deionized water, then dried in a vacuum furnace at 60 C for 6 h. The dried sample was then calcined in air at 400 C with the heating rate of 10 C min À1 and kept for 3 h (deposition weight ¼ 3.5 mg cm À2 ).

Synthesis of CoMn 2 O 4 @Co 3 O 4 core/shell hybrid nanostructure arrays
The CoMn 2 O 4 nanoneedles arrays were used as the skeleton for the growth of Co 3 O 4 nanosheets shell, which were synthesized by a simple hydrothermal method. Firstly, 0.8 g of Co(CH 3 COO) 2 $4H 2 O was were dissolved in 35 mL ethylene glycol/H 2 O (6 : 1) mixed solvent and stirring for 30 min at roomtemperature. Then, 0.6 g of sodium dodecyl sulfate as a surfactant was added under stirring. Aerwards, this solution was transferred into a Teon-lined stainless steel autoclave and heated in an oven at 180 C for 12 h and then cooled to room temperature. Finally, the solid product was washed with deionized water. Then, the precursor was dried at 60 C and calcined at 300 C for 3 h in air to obtain CoMn 2 O 4 @Co 3 O 4 . The mass loading of CoMn 2 O 4 @Co 3 O 4 was 4.6 mg, corresponding with the gain weight of Ni foam. For comparison, the sole Co 3 O 4 nanosheets electrode growing on Ni foam directly was prepared with the same method described above.

Characterization
The product scratched down from the Ni foam were characterized by an X-ray diffractometer (XRD, X'pert MPD Pro, Philips, Netherlands) using Cu Ka radiation (l ¼ 1.5406Å). The surface morphologies and micro-structures of the electrodes were characterized by eld emission scanning electron microscopy (FESEM, Zeiss Supra 35VP, USA) and high resolution transmission electron microscopy (HRTEM, JEM-2100, Japan). The specic surface area (BET method) and pore size distribution measurements were performed in a micromeritics ASAP 2020 sorptometer at 77 K.

Electrochemical measurements
The electrochemical performances were carried out on an electrochemical workstation (CHI660E, Shanghai Chen Hua Co. Ltd, China) using a three-electrode system, in which asprepared sample was used as the working electrode, Pt wire as the counter electrode, a mercury oxide mercury electrode (Hg/ HgO) as the reference electrode and freshly prepared 3 M KOH aqueous solution was used as the electrolyte, respectively. The cycling tests were conducted using a LAND battery program-control test system (CT2001A, Wuhan LANHE Co. Ltd, China).

Assembling of CoMn 2 O 4 @Co 3 O 4 //AC ASC
The prepared CoMn 2 O 4 @Co 3 O 4 core/shell nanoowers electrode (positive) and the charge balanced activated carbon (AC) electrode (negative) were pressed together and separated by a brin separator (140 mm thick). The electrodes and separator were immersed in an aqueous 3 M KOH electrolyte and assembled layer by layer with so polyethylene terephthalate (PET) membranes at room temperature.
The specic capacitance was determined from galvanostatic charge/discharge via eqn (1): where C m is the specic capacitance of the active material (F g À1 ), I (A), Dt (s), DV (V) and m (g) represent the applied current, total discharge time, potential window and the mass of the active materials, respectively. The energy and power density of the ASC devices were calculated as follows: where E (W h kg À1 ) is the average energy density; C m (F g À1 ) is the specic capacitance of the ASC device; DV (V) is the voltage window; P (W kg À1 ) is the average power density and Dt (s) is the discharge time. The structure and morphology of the as-prepared samples were characterized by FESEM and TEM. Fig. 2(a-c) present the typical FESEM images of the CoMn 2 O 4 nanoneedles arrays supported on Ni foam. On the observation of high-magnication image (Fig. 2(c)), we found that the obtained CoMn 2 O 4 nanoneedles arrays (with lengths of $1mm and diameters of $20-70 nm) are highly densely. Fig. 2 Fig. 2(j-l) also further reveal the hybrid nanostructure of an individual CoMn 2 O 4 @Co 3 O 4 core/shell nanowire, although hierarchical nanoower structure cannot be clearly observed, probably owing to the partial damage from TEM sample preparation procedure (sonication). The HRTEM image of CoMn 2 O 4 @Co 3 O 4 further veries the crystal structure. Fig. 2(l) (Fig. S3 †). The pore size distributions of these samples are shown in the insets, con-rming that the samples have mesoporous characteristics. Obviously, the CoMn 2 O 4 @Co 3 O 4 core/shell conguration can provide a higher surface area, which is mainly attributed to the interconnected Co 3 O 4 nanosheets and the aligned CoMn 2 O 4 nanoneedles scaffold that creating a 3D interconnected porous network and highly porous surface morphology. Such interconnected network conguration not only provides large surface area for charge storage but also facilitates electrolyte penetration through the mesopores and increase the utilization of the active materials. 26-29 Based on the above mentioned merits, the CoMn 2 O 4 @Co 3 O 4 core/shell nanoowers can be employed as excellent electrode material for SCs and the detailed electrochemical measurements are performed as follows. Fig. 3(a)  As shown, two pairs of redox peaks for the Co 3 O 4 electrode appear in the CVs, which is due to the Co 2+ /Co 3+ and Co 3+ /Co 4+ reactions, described by the following reaction: 30,34 (+)3Co(OH) 2 + OH À / Co 3 O 4 + 4H 2 O + e À (6)

Electrochemical performance of CoMn 2 O 4 @Co 3 O 4 core/ shell electrodes
CoOOH + OH À / CoO 2 + H 2 O + e À (8) (À)CoO 2 + H 2 O + e À / CoOOH + OH À 3CoOOH + e À / Co 3 O 4 + OH À + H 2 O (11) A pair of signicantly enhanced redox peaks can be observed at the voltage of 0.16 V and 0.41 V for the CoMn 2 O 4 @Co 3 O 4 electrode, which may originate from the fast faradaic redox reactions and the short ion diffusion path provided by CoMn 2 O 4 and Co 3 O 4 , as shown in reaction (4)-(12). Fig. 3(b) shows the CV curves of the CoMn 2 O 4 @Co 3 O 4 electrode at various scan rates. As the sweep rate increases, the cathodic peak position was shied from 0.18 to 0.07 V and the anode peak position was shied from 0.38 to 0.51 V, which is due to the polarization effect of the electrode.
The improved capacitive performance was also evaluated by galvanostatic charge-discharge (GCD) measurements over This journal is © The Royal Society of Chemistry 2018 a potential range from 0 to 0.5 V. Fig. 3(c) Fig. 3(d)   also conrmed by electrochemical impedance spectroscopy (EIS). The impedance spectra were obtained using an AC voltage of 5 mV in a frequency range from 0.01 Hz to 100 kHz. The electrochemical impedance data were analyzed with a Randles equivalent circuit that includes the charge transfer resistance (R ct ), and the straight line in the low-frequency region represents the diffusive resistance (W). In addition, the internal resistance (R e ) is obtained from the high-frequency intersection of the Nyquist plot in the real axis. 35 There are a single semicircle in the high-frequency region and a straight line in the lowfrequency region in the Nyquist plots of the three electrodes ( Fig. 3(f)). The semicircle at high frequency region corresponds to the charge transfer processes whereas the straight line at lowfrequency region relates to the ion diffusion processes. 36 The smaller semicircle diameter (lower R ct ) indicates faster electrontransfer kinetics of corresponding electrode. 37-39 CoMn 2 O 4 @-Co 3 O 4 electrode exhibits a smaller radius in the high-frequency region and a steeper line in the low-frequency region than those of other two electrodes, indicating superior charge transfer and ion diffusion kinetics behavior.
Cycling stability is another important parameter for highperformance supercapacitors. The long-term cycling stability of as-synthesized supercapacitors was tested through a repetitive galvanostatic charge/discharge process at a constant current density of 4 A g À1 for 3000 cycles (Fig. 4). It can be observed that the specic capacitance of CoMn 2 O 4 @Co 3 O 4 electrode rstly increases before 200 cycles, and then decreases with cycle number increasing, suggesting an activation process occurring during the beginning of a successive scan. 40 It is important to indicate that the electrode shows capacitance retention of 87.6% aer 3000 cycles. The decrease of the specic capacitance could be attributed to the dissolution of the outer Co 3 O 4 in alkaline electrolyte, leading to loss of active materials. The excellent stability may be mainly attributed to the unique hierarchical porous core/shell morphology of the CoMn 2 O 4 @-Co 3 O 4 hybrid. The CoMn 2 O 4 nanowire arrays on the Ni foam serve as a supporting framework for the ultrathin and interconnected Co 3 O 4 nanosheets to produce an hierarchical nanostructures array and therefore enhance the mechanical stability.
In conclusion, the outstanding electrochemical performance of the CoMn 2 O 4 @Co 3 O 4 hybrid electrode can be mainly ascribed to the unique core-shell hierarchical porous nanostructure, providing the advantages as follows: (i) both CoMn 2 O 4 and Co 3 O 4 are good pseudo-capacitive materials in KOH electrolyte, hence contributing to the enhanced pseudocapacitance activity. (ii) The unique hierarchical core/shell porous nanostructure is anticipated to enhance the amount of accessible active sites for the capacitive reactions, 41 and can supply a short and fast ion diffusion pathway, thus enhancing the faradic reaction.

Electrochemical characterization of CoMn 2 O 4 @Co 3 O 4 // AC ASC
As shown in Fig. 5(a), the CV curve of AC electrode from À1.0 to 0 V (vs. Hg/HgO) exhibits a nearly rectangular shape, typical of capacitive behavior, which indicates the characteristic of the electric double layer capacitance, while that of CoMn 2 O 4 @-Co 3 O 4 within a voltage the potential window of 0 to 0.6 V (vs. Hg/ HgO). As shown in Fig. 6(a), the CoMn 2 O 4 @Co 3 O 4 //AC device  Paper showed a nearly ideal capacitive behavior with a cell voltage up to 1.6 V at a scan rate of 50 mV s À1 . The achieved high operating voltage of the device benets from the advantage of the different stable potential windows of CoMn 2 O 4 @Co 3 O 4 and AC electrodes.
To obtain the maximum performance of the CoMn 2 O 4 @-Co 3 O 4 //AC ASC, it is crucial to keep the charges balanceable with the relationship q+ ¼ q À . In order to get the charge balanceable, the optimum loading mass of AC was decided by the following equation: where m is the mass of activated material, C m represents the specic capacitance and DV is the potential window in the three-electrode test system. The charge/discharge curve of the AC electrode at 1 A g À1 is depicted in Fig. 5(b) and the calculated C m value of the AC electrode is 254 F g À1 . The C m value of the CoMn 2 O 4 @Co 3 O 4 electrode is 1627 F g À1 at 1 A g À1 (Fig. 3(e)).
The mass loading of AC in CoMn 2 O 4 @Co 3 O 4 //AC was 17.7 mg. Fig. 6(a) shows the CV curves of the CoMn 2 O 4 @Co 3 O 4 //AC ASC at various scan rates. The CoMn 2 O 4 @Co 3 O 4 //AC ASC exhibits an irregular shape CV curve, which is distinct from the CV curve of the AC electrode with a rectangular shape. The distortion of CV curves from rectangular-like shapes may be attributed to the pseudo-capacitance from CoMn 2 O 4 @Co 3 O 4 // AC cathode. 42,43 The discharge curves of the ASC device at various current densities within the potential window of 0-1.6 V are shown in Fig. 6(b). The specic capacitances of CoMn 2 -O 4 @Co 3 O 4 //AC ASC were calculated according to the eqn (1) to be 125.8, 120.6, 114.7, 97.1 and 87.4 F g À1 at the current densities of 1, 2,4, 8, 10 A g À1 , respectively. It is worth noting that even at a high current density of 10 A g À1 , the specic capacitance still reaches up to 87.4 F g À1 (about 69.5% of the capacitance retained), indicating its good rate capability. Note that the specic capacitance is about six times larger than that of conventional AC-based symmetric capacitors ($20 F g À1 ), 44 which is enhanced by the ultra-high pseudo-capacitance of the CoMn 2 O 4 @Co 3 O 4 electrode. To examine the supercapacitor performance of the CoMn 2 O 4 @Co 3 O 4 //AC device, the energy density (E) and power density (P) of the ASC were calculated according to the eqn (2) and (3), and corresponding Ragone plot is in Fig. 6(c). The CoMn 2 O 4 @Co 3 O 4 //AC supercapacitor could achieve a high E value of 44.8 W h kg À1 (with a P value of 800.5 W kg À1 ) and still maintain 31.1 W h kg À1 at a high power density of 8010.3 W kg À1 . Note that the energy and power densities of the present ASC are superior to that of many ASCs using core-shell nanostructure materials as electrode positive electrode, such as MnMoO 4 $H 2 O@MnO 2 //AC, 45 Co 3 O 4 @-Ni(OH) 2 //AC, 46 NiCo 2 S 4 @Co(OH) 2 //AC 47 and NiCo 2 O 4 @MnO 2 // AC. 48 We believe that the high energy and power density of our ASC are mainly due to the wide working voltage window and the improved specic capacitance comes from the high synergistic effect of these two electrodes.
The long-term cycling stability of the CoMn 2 O 4 @Co 3 O 4 //AC ASC was examined by successive galvanostatic chargedischarge cycling at a current density of 2 A g À1 for 5000 cycles. As shown in Fig. 6(d), the ASC shows capacitance retention of 89.2% aer 5000 cycles. Note that the rate capability and cycling stability of the CoMn 2 O 4 @Co 3 O 4 //AC ASC is comparable or superior to many state-of-art ASC systems (Table S2 †), further indicating the advantage of the CoMn 2 O 4 @Co 3 O 4 //AC ASC. The high power density with energy density, excellent rate capability and cycling stability make the CoMn 2 O 4 @Co 3 O 4 //AC ASC as a promising candidate for practical energy storage.

Conclusions
We have successfully prepared hierarchical CoMn 2 O 4 @Co 3 O 4 core/shell nanowire arrays with attractive pseudocapacitance behaviors in situ grown on Ni foam by simple, cost-effective, and facile hydrothermal method. The resultant CoMn 2 O 4 @Co 3 O 4 core/shell nanowire arrays exhibited signicant capacitance, as compared with bare CoMn 2 O 4 and Co 3 O 4 . The CoMn 2 O 4 @-Co 3 O 4 hybrid with unique architecture is then used as binderfree electrode for supercapacitors. This electrode exhibits a high specic capacitance of 1627 F g À1 at 1 A g À1 and 1376 F g À1 at 10 A g À1 . An ASC device employing the CoMn 2 O 4 @Co 3 O 4 electrode and active carbon electrode delivers a high specic energy of 125.8 F g À1 at 1 A g À1 as well as high energy density of 44.8 W h kg À1 . Also, the ASC exhibits high cycling stability with capacitance retention of 89.2% aer 5000 charge/discharge cycles at a current density of 2 A g À1 . The superior electrochemical performance indicate that the present low-cost CoMn 2 O 4 @Co 3 O 4 core/shell hybrid can serve as a promising electrode material for high-performance supercapacitors.

Conflicts of interest
The authors declare no conict of interest.