Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors

Journal of Alloys and Compounds xxx (xxxx) xxx

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Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors Ya Wang a, Xingyi Ma b, Shanshan Li a, Jiale Sun a, Yanfei Zhang a, Huiyu Chen a, *, Chunju Xu a, ** a b

School of Materials Science and Engineering, North University of China, Taiyuan, 030051, China Division of Intelligent Bio-systems, Korea Blockchain Institute, Seoul, 06651, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 June 2019 Received in revised form 1 November 2019 Accepted 3 November 2019 Available online xxx

In this paper, MgCo2O4 twinned-hemispheres (MgCo2O4 THSs) composed of a large number of nanoparticles were obtained via a solvothermal reaction accompanied with subsequent annealing process. Such MgCo2O4 THSs possessed a specific surface area of 63.8 m2 g
1 with a dominate pore size of 6 nm, and the mean pore size was calculated to be 13.5 nm. The electrochemical performance of these MgCo2O4 THSs was evaluated in a typical three-electrode system. The results revealed that such MgCo2O4 electrode material delivered a specific capacitance of 626.5 F/g at 1 A g
1 in 2 M of KOH aqueous electrolyte, and possessed a rate capability of 71.8% at 16 A g
1. The capacitance could preserve 68.9% of the original value after 5000 cycles at the current density of 5 A g
1. The assembled MgCo2O4 THSs//AC asymmetric supercapacitor attained an energy density of 30.6 W h kg
1 at a power density of 861 W kg
1, and a long cycling life with 99.06% specific capacitance retention after 5000 cycles at 5 A g
1. The outstanding electrochemical properties of MgCo2O4 THSs can be attributed to their unique structure, which make them suitable and promising electrode material candidate for supercapacitors. In addition, the synthesis strategy is easy to be operated, which is able to be adopted to fabricate other binary transition metal oxides. © 2019 Elsevier B.V. All rights reserved.

Keywords: MgCo2O4 Solvothermal synthesis Electrochemical performance Supercapacitors

1. Introduction In recent years, to develop eco-benign energy storage system with high performance has been nourished by the excessive consumption of fossil fuels and environmental deterioration issues [1e3]. Among many kinds of modern energy storage devices, supercapacitor is considered to be significantly promising one for its merits of high power density, superior cycling characteristic, and rapid charge-discharge capability, and so it has occupied a critical position in the market [4e6]. To date, supercapacitors with excellent properties are able to meet the needs of many applications requiring high power density and long cycling life, such as in the fields of hybrid vehicles, emergency lighting, and military devices. Additionally, in order to further improve the application potential of supercapacitor, extensive efforts have been paid to enhancing its relatively low energy density [7e9]. Electrode material is one of the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Chen), [email protected] (C. Xu).

critical components of supercapacitor, and its type of material, morphology and porosity have significant impact on the electrochemical properties of supercapacitor. Hence, selecting suitable electrode material plays crucial role for the development of supercapacitor with favorable performance, which requires a deep fundamental understanding about the energy storage mechanism of supercapacitors [10e12]. So far, researchers have recognized that promising electrode materials not only should provide as many electrons and ions transport pathways and electrochemically active sites as possible, but also should possess advantages of large specific surface area, excellent conductivity, and good cycling stability. On the basis of different energy storage mechanisms, the supercapacitors are divided into two categories with one of electric double layer capacitors (EDLCs) and the other one of pseudocapacitors (PCs). Ordinarily, the EDLCs rely on charge accumulation at the surface of electrode material to store energy, which is a physical process without involving any redox reactions. Therefore, the porosity of electrode material is one of vital factors in determining the specific capacitance. In this regard, conductive carbonaceous materials represented by carbon nanotubes, graphene, and

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Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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activated carbon are ideal electrode materials for EDLCs, in that they often have good conductivity as well as large specific surface areas [13]. Different from EDLCs, performing Faradaic reactions at or close to the electrode material is the mechanism by which the PCs store energy. Common electrode materials for PCs such as metal oxides and conductive polymers have considerable pseudocapacitive characteristics, which can provide larger specific capacitance than EDLCs using carbon-based materials with inherently low capacitance characteristics [14,15]. To sum up, carbonaceous materials, conducting polymers as well as metal oxides are the basic electrode materials used in supercapacitors. Unfortunately, each of them has certain disadvantages, such as low capacitance of conventional carbon-based materials, poor conductivity of metal oxides, and mechanical collapse of conductive polymers, which severely hinder the further practical application of supercapacitors [16,17]. Among various electrode materials, metal oxides have attracted many interests attributing to the low cost, environmental friendliness as well as high theoretical specific capacitance [18]. It has been found that binary transition metal oxides (BTMOs), which are formed by introducing another metal element to metal oxides, can provide higher electric conductivity attributing to the synergy effect between different metals. In view of the metallic cations emerged in the same molecular structure, the improved electrochemical activity of BTMOs provides favorable conditions for the rapid progress of redox reaction [19,20]. Particularly, spinel type cobalt-based BTMOs, which can be represented by a general formula MCo2O4 (M ¼ Ni, Mn, Zn, Cu, Mg, and etc), have exhibited great potential in the field of electrode materials, for their advantages of high theoretical capacitance, good electrochemical activity and remarkable cycling stability [21,22]. In terms of various MCo2O4 with spinel structure, it is necessary to choose a proper material and design its structure for optimizing electrochemical performance. MgCo2O4 stands out for its extremely high theoretical capacitance of ~3122 F/g. For this electrode material, the elemental Mg does not take part in the redox reaction due to none of the valence variation, and all the redox reactions come from the elemental cobalt. However, Mg has a better conductivity than that of cobalt, and therefore in the practical applications, the capacitance of Co3O4 may be enhanced by the replacement of Co atom in Co3O4 crystal structure with elemental Mg [23]. It has urged tremendous efforts devoted to its preparation and morphology design as supercapacitor electrode [24,25]. For example, the MgCo2O4 synthesized by molten salt method exhibited spheroidal particle morphology, and the specific capacitance tested at 0.5 A/g was 321 F/g [26]. Using facile hydrothermal method, MgCo2O4 cuboidal microcrystals were successfully prepared and showed a specific capacity of ~345 C/g at 1 A/g [23]. Xu et al. reported that porous MgCo2O4 with double-urchin-like shape could be synthesized by a hydrothermal method, and it delivered a specific capacitance of 508 F/g [27]. Since powdered MgCo2O4 is affected small specific surface area as well as relatively poor conductivity, it is difficult to deliver satisfactory specific capacitance in practical applications, which has prompted researchers to invest numerous efforts to solve these problems [28]. A binder-free electrode formed by directly growing MgCo2O4 nanocone arrays on nickel foam was successfully prepared by Cui et al. Such electrode delivered a high specific capacitance of 750 F/g [29]. Xu et al. synthesized porous MgCo2O4 nanoneedle arrays on Ni foam through a two-step method, and the MgCo2O4 electrode material exhibited a specific capacitance of 804 F/g [30]. Furthermore, MgCo2O4@ppy composite could be directly grown on nickel foam, and the specific capacitance of such electrode could reach 1079.6 F/g. When it was tested at 1 A/g for 1000 cycles of charge-discharge process, the specific capacitance maintained 97.4% of the original value [31]. According to the existing literatures, it should be pointed out that

these approaches (such as fabricating binder-free structure or composite electrode) are indeed effective ways to improve the specific capacitance of MgCo2O4-modified electrode. While enhancing the electrochemical performance, the preparation methods are inevitably complicated, and the production cost is increased, which are disadvantageous for large-scale production. In this work, MgCo2O4 twinned-hemispheres (MgCo2O4 THSs) were successfully obtained via a solvothermal reaction in ethylene glycol solvent with an annealing treatment of the precursors in air. To our knowledge, MgCo2O4 with similar morphology have never been reported via the same synthesis process. The MgCo2O4 THSs exhibited a specific capacitance of 626.5 F/g at a current density of 1 A/g, and the capacitance still remained 68.9% after a test of 5000 cycles at 5 A/g, suggesting its outstanding electrochemical performance and long-life cycling durability. In addition, the MgCo2O4 THSs//AC asymmetric supercapacitor (ASC) was fabricated, and it exhibited excellent electrochemical performance in terms of energy densities and cycling stability. Such results demonstrate that the MgCo2O4 twinned-hemispheres have great potential as electrode material for supercapacitors. 2. Experimental 2.1. Materials synthesis Typically, 1 mmol of Mg(NO3)2$6H2O and 2 mmol of Co(NO3)2$6H2O were initially introduced into 40 mL of ethylene glycol. When the nitrates of magnesium and cobalt were completely dissolved by magnetic stirring, urea with amount of 20 mmol was added. Later on, the uniform rosy solution obtained after another 2 h of magnetic stirring was loaded into an autoclave with 50 mL of capacity. After the solvothermal reaction proceeded for 15 h at 150  C, the resulting precipitate was collected by centrifugation, and washed several times with water and absolute ethanol, respectively. The dried sample was calcined at 500  C for 2 h with temperature increase rate of 2  C/min. 2.2. Materials characterization Thermogravimetric analysis (TGA) was conducted by a TG 209 F3 thermal analyzer (Netzsch, Germany) at a ramping rate of 10  C min
1 from room temperature to 800  C under air atmosphere. The X-ray diffraction (XRD) pattern of the sample was acquired from a Bruker D8 Advance powder X-ray diffractometer, in which it used Cu-ka (l ¼ 0.1548 nm) as X-ray source and the 2q value ranged from 10 to 80 . The shapes of the samples were observed by a fieldemission scanning electron microscope (FESEM) on JEOL JSM7100F equipment. In order to perform transmission electron microscopy (TEM) measurement, the sample needs to be dispersed in absolute ethanol and dropped on carbon-coated copper mesh. The JEOL JEM2100F transmission electron microscope was used at 200 kV to obtain TEM and high-resolution TEM (HRTEM) images. Xray photoelectron spectroscopy (XPS) was performed on an ESCA 2000 spectrometer to further investigate the composition as well as elemental valence of product. Based on the nitrogen adsorptiondesorption test at 77 K on a Quantachrome Autosorb 1-C adsorption analyzer, the porosity of the sample including specific surface area and pore size distribution could be obtained according to the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods, respectively. 2.3. Electrochemical measurements The as-obtained powder was fabricated into a working electrode according to the following procedures to evaluate its

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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electrochemical properties. The MgCo2O4 THSs, acetylene black and polyvinylidene fluoride (PVDF) were dispersed in N-methyl-2pyrrolidone with a weight ratio of 80:15:5. After gentle stirring for a few minutes, the homogeneous slurry was obtained and then was dropped onto clean nickel foam with 1  1 cm2 in size. The Ni foam with loaded MgCo2O4 THSs was put into a vacuum oven at 85  C for drying over night. Finally, it was pressed under a 10 MPa pressure supplied by a hydraulic press to obtain the working electrode. Using CHI 660E electrochemical workstation and KOH electrolyte with a concentration of 2 M, various electrochemical tests were conducted to measure the electrochemical performance of MgCo2O4 THSs. Cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), cycling, and electrochemical impedance spectroscopy (EIS) tests were all performed at room temperature. A typical three-electrode configuration was employed, and saturated calomel electrode served as reference electrode. The CV measurements were performed with potential between 0 and 0.58 V, and the scan rate ranged from 2 to 40 mV/s. As for the GCD tests, the potential was set between 0 and 0.45 V, and the current density varied from 1 to 16 A/g. EIS measurement was conducted under an open circuit potential. In addition, AC amplitude of 5 mV was used, and the frequency varied from 105 Hz to 10
2 Hz. For the twoelectrode tests, an ASC was fabricated by using MgCo2O4 THSs as the positive electrode and AC as the negative electrode, respectively. A 2 M of KOH solution was freshly prepared and used as electrolyte. According to the GCD curve, the specific capacitance, energy density, and power density of MgCo2O4 THSs can be obtained by the following equations [32,33]: Cs ¼ I△t /(m△V)

(1)

E ¼ Cs△V2 / 7.2

(2)

P ¼ 3600E / △t

(3)

In the above equations, Cs (F$g
1) indicates the specific capacitance, I (A) is the current of discharge process, △t (s) means the discharge time, m (g) is the weight of MgCo2O4 material and △V (V) is the applied potential window. E (W h kg
1) is the energy density and P (W kg
1) is the power density, respectively. In the two-electrode system, the m (g) is the total mass of active materials on both the positive and negative electrodes. 3. Results and discussion The preparation of final product was realized via two steps including the initial formation of precursor and the subsequent calcination process. In order to determine a proper temperature for the calcinations of the precursor, TGA and DTG analysis of the asprepared precursor were employed in this work and the corresponding curves were shown in Fig. S1 in the Supporting Information. The precursor has three-step weight loss process. It was found that about 9.82% weight was lost below 255  C, ascribing to the removal of water molecules physically adsorbed on the surface or the crystalline water. The second step of weight loss (28.38%) below 385  C was associated with the conversion of the precursor to oxide. The third slight weight loss of around 3.67% between 385 and 500  C was attributed to the continuous decomposition of the organic residues. After that, the precursor almost decomposed completely, and no obvious loss of weight was observed at higher temperature, suggesting the formation of stable oxides. Therefore, the calcination temperature could be chosen to be 500  C. The phase purity of MgCo2O4 was investigated by XRD measurement. Fig. 1a illustrated several typical XRD patterns of

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MgCo2O4 samples prepared at different calcination temperatures. It was obvious that the calcination temperature greatly influenced the crystallinity of MgCo2O4 product, because all the diffraction peaks in the XRD pattern became stronger and sharper with the calcination temperature increasing. Better crystallinity was achieved at higher calcination temperature. Eight well-defined diffraction peaks located at 18.82 , 31.21, 36.85 , 38.56 , 44.47, 55.42 , 59.65 , and 65.08 were the characteristic peaks of spinel cubic phase of MgCo2O4 (JCPDS No. 81e0667, a ¼ b ¼ c ¼ 8.10 Å) [34]. Furthermore, there were no additional peaks indexing to impurities at 500  C, demonstrating that the synthesized MgCo2O4 was in highly pure phase. The morphology and structure of MgCo2O4 was investigated by FESEM. It could be seen from the SEM image in Fig. 1b that a large number of uniform particles with twinned-hemisphere-like shape were produced. Fig. 1c presented a further enlarged SEM image, from which a joint boundary was clearly observed between the twinned hemispheres. The high-magnification SEM image in Fig. 1d further demonstrated that the structure was composed of two hemispheres. In addition, the twinned-hemispheres with rough surface were approximately 2 mm in size. What’s more, the element composition and spatial distribution of the sample were further evaluated by energy dispersive spectroscopy (EDS) measurement. As the EDS spectrum is shown in Fig. S2, the atomic ratio of Mg/Co/ O is 1/2.06/3.94, close to the stoichiometric ratio of 1/2/4. The SEM image and its corresponding elemental mappings for Mg, Co, as well as O are presented in Fig. 1eeh, respectively. The similar shape of the three elemental mappings not only proves the presence of Mg, Co, and O elements, but also suggests that they are uniformly distributed in the MgCo2O4 THSs. TEM analysis was used for investigating the detailed microstructures and morphological characteristics of the as-prepared MgCo2O4 THSs, and the images were presented in Fig. 2. The typical TEM image (Fig. 2a) revealed the twinned-hemisphere-like structure, which matched well with the SEM observations. The magnified TEM image (Fig. 2b) demonstrated that a large number of nanoparticles (NPs) aggregated together to form such twinned hemispheres structure, and the mesopores among adjacent particles were clearly visible. Fig. 2c illustrated a representative HRTEM image that contained several NPs and pores among them. The enlarged HRTEM image (Fig. 2d) was taken in the circled area. In this image, the lattice spacings were measured to be 0.28 and 0.45 nm, which corresponded well to the spinel MgCo2O4 with (220) and (111) planes, and were in accordance with XRD data. XPS is a useful tool to analyze the elemental characteristics, from which the chemical composition and elemental valence of MgCo2O4 THSs are obtained. In this XPS analysis, the spectra for magnesium, cobalt, and oxygen were all calibrated by using the 284.6 eV of C 1s as a reference. The survey spectrum of MgCo2O4 THSs in Fig. 3a proved that the sample was composed of elemental Mg, Co, O, and C. In addition, there were no other characteristic peaks indexing to impure elements, indicating high purity of the MgCo2O4 THSs. Fig. 3b presented Mg 1s XPS spectrum with high resolution, and the peak located at the energy of 1304.9 eV corresponded to the presence of magnesium oxide [23]. The Co 2p spectrum consisted of two dominant peaks, as shown in Fig. 3c, one of them located at binding energy of 781.3 eV, attributing to Co 2p3/ 2, and the other one at binding energy of 796.4 eV was ascribed to Co 2p1/2 [35]. The energy separation between the two peaks was about 15.1 eV, which suggested the presence of Co3þ in the MgCo2O4 THSs [36]. Interestingly, the two main peaks were separated into two spin-orbit doublets after Gaussian fitting. Two peaks with positions of 781.0 and 796.1 eV could be ascribed to Co3þ, while another two peaks at 782.5 and 797.9 eV of binding energies were related to Co2þ state [37,38]. Furthermore, the two satellite

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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Fig. 1. (a) XRD pattern and (bed) SEM images of MgCo2O4 THSs in various magnifications synthesized in ethylene glycol solvent with subsequent calcination at 500  C for 2 h, (e) a typical SEM image of MgCo2O4 THSs and corresponding EDS elemental mappings for (f) Mg, (g) Co, as well as (h) O.

Fig. 2. (aeb) TEM images of MgCo2O4 THSs in different magnifications, (c) HRTEM image of the assembled porous NPs for MgCo2O4 THSs, and (d) enlarged HRTEM image taken from the circled position in (c).

Fig. 3. (a) XPS survey spectrum of the obtained MgCo2O4 THSs, and XPS core-level spectra for (b) Mg 1s, (c) Co 2p, and (d) O 1s.

peaks (marked as “sat”) centered at binding energies of 804.9 and 789 eV demonstrated that the Co cations held a valence of þ2 [10]. It was observed in Fig. 3d that the O 1s spectrum was fitted into four curves with positions at 529.6, 530.4, 531.7 and 532.9 eV, indicating four different oxygen components (O1, O2, O3, and O4) [39]. The O1 component with the lowest binding energy of 529.6 eV is ascribed to the typical MgeO and CoeO bonds. Lattice oxygen is represented by the O2 component at 530.4 eV [40,41]. The O3 component at 531.7 eV is due to the presence of hydroxyl group. The peak at the highest binding energy (532.9 eV, O4) corresponds to water molecules that are chemically and physically adsorbed on the surface of MgCo2O4 THSs [42]. N2 adsorption-desorption measurements were conducted at 77 K to investigate the porosity of the as-synthesized MgCo2O4 THSs. As shown in Fig. 4a, the isotherms can be classified as typical type IV with type H3 hysteresis, and it reveals that the MgCo2O4 THSs possess mesoporous structure [43]. The BET specific surface area of MgCo2O4 THSs was calculated to be 63.8 m2 g
1, which was larger than that of double-urchin like MgCo2O4 (26.45 m2 g
1) [27] as well as MgCo2O4 flaky flowery structure (55 m2 g
1) [44]. Electrode materials with larger specific surface area can be expected to improve the electrochemical performance, for it may lead to a large number of active sites participating in the redox reaction, and the electrode material can achieve efficient utilization. As the plot of pore size distribution presented in Fig. 4b, the pore size is dominantly distributed at 6 nm, and the average pore diameter is 13.5 nm. The mesoporous structure can provide shorter transport paths for both electrolyte penetration and ion diffusion to facilitate the rapid Faradaic process. More active positions can be provided by increasing the specific surface area, and these active sites are beneficial for the occurrence of redox reactions, which are favorable

Fig. 4. (a) Nitrogen adsorption-desorption isotherms of the obtained MgCo2O4 twinned-hemispheres, and (b) the plot of corresponding pore size distribution.

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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factors in improving electrochemical performance of electrode materials. Herein, we considered that the concentration of urea played an essential effect in the formation of such MgCo2O4 THSs, in order to verify this point more convincingly, a series of experiments with various dosages of urea were conducted. Urea is a commonly used homogeneous precipitant for the formation of micro/nanostructures in aqueous solution, which hydrolyzes and releases CO2 and NH3 at high temperature, resulting in an alkaline solution system. In addition, since the reactions proceed within closed autoclave, the released CO2 is further converted into CO2
3 and reacts with metal ions to form a precipitate of MgCo-complex precursor [45]. Fig. 5 displays the SEM images of the MgCo2O4 products obtained with different amounts of urea. It was worth mentioning that no precipitates were available if the solvothermal treatment was conducted at 150  C for 15 h without using any urea, indicating that no reaction was taken place under the above condition. When 5 mmol of urea was added to the system, spherical MgCo2O4 particles with rough surface were formed (Fig. 5a). If the urea content was increased to 10 mmol, the particle was formed by two hemispheres jointed face to face, namely, the twinned-hemisphere was generated (Fig. 5b). Urea can influence the size and morphology of the product by controlling the nucleation and crystal growth processes, which are two key factors in affecting the performance of electrode materials. When the urea concentration is high, the number of nuclei in the solution increases, resulting in the gradual decrease of final grain size. Fig. 5c displayed the SEM image of MgCo2O4 sample obtained with 30 mmol of urea, a larger number of twinned-hemisphere-like structure was formed, which possessed smaller size than that of 10 mmol urea adopted. When the molar weight of urea was further increased to 40 mmol, as shown in Fig. 5d, a serious aggregation phenomenon emerged and the irregular large block structure replaced the twinnedhemisphere-like morphology. The shape of MgCo2O4 products influenced by urea content can be easily proved by the above experimental results. A series of time-dependent experiments were conducted to produce different MgCo2O4 samples in order that we could comprehend the formation mechanism of the twinnedhemispheres. Fig. 6 presented the SEM images of MgCo2O4 products obtained with various solvothermal durations. In the initial 6 h of the reaction, a large number of NPs with 30 nm in size were

Fig. 5. SEM images of MgCo2O4 products prepared with various amounts of urea: (a) 5, (b) 10, (c) 30, and (d) 40 mmol, respectively. All the samples were calcined at 500  C for 2 h.

5

Fig. 6. SEM images of MgCo2O4 products prepared with various solvothermal durations: (a) 6, (b) 9, (c) 12, and (d) 24 h, respectively. All the samples were calcined at 500  C for 2 h.

formed, as shown in Fig. 6a. Once the solvothermal treatment was prolonged to 9 h, many twinned hemispheres appeared, and some MgCo2O4 NPs coexisted at the same time (Fig. 6b). If we further extended the reaction time to 12 h, as the SEM image illustrated in Fig. 6c, the product was consisted of a large quantity of twinnedhemispheres. It could be seen from Fig. 6d that there was no significant change in the size and morphology of the MgCo2O4 sample prepared by reacting for 24 h. Based on the above observations, the formation process of MgCo2O4 THSs could be interpreted by Ostwald ripening process. At the initial stage of reaction, nuclei were generated and the crystals grew larger at the expense of smaller ones. The subsequently produced nuclei or smaller NPs were attached to minimize of the total surface energy. Especially, the viscosity of the ethylene glycol solvent is very high, and the free movement of the nuclei or smaller NPs is greatly limited, leading to the formation of microspheres. Surely the detailed mechanism needs further investigation, and more related work is currently under way. The MgCo2O4 is one of the excellent electrode materials and is expected to be applied in energy savings. The MgCo2O4 THSs in this work are further considered as electrode material for supercapacitors. So some related electrochemical tests including CV, GCD, cycling as well as EIS were processed in 2 M of KOH aqueous electrolyte to evaluate its electrochemical performance. All the measurements were conducted in a typical three-electrode system at room temperature The CV test was applied over a potential window ranging from 0 to 0.58 V, and the scan rate differed from 2 to 40 mV/s, as the result was shown in Fig. 7a. A couple of redox peaks are clearly seen on all CV curves obtained at different scan rate, which is the typical characteristic of pseudocapacitive behavior. This is a significant difference from the EDLCs in which the CV curve exhibits typical rectangular shape [46]. The reversible redox reactions performing at or near electrode surface generate such redox peaks, which can be expressed by the following formulas [10,31]: MgCo2O4 þ 2H2O 4 2CoOOH þ Mg2þ þ 2OH


(4)

CoOOH þ H2O þ e
4 Co(OH)2 þ OH


(5)

Interestingly, with the scan rate increasing from 2 to 40 mV/s, the positions of the anodic peaks shift from 0.35 to 0.42 V, and the

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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Fig. 7. (a) CV curves of MgCo2O4 THSs measured with scan rate ranging from 2 to 40 mV/s, (b) GCD curves of MgCo2O4 THSs measured with current density ranging from 1 to 16 A/g, (c) GCD curves at 1 A g
1 for MgCo2O4 THSs, Sample R1, R2, R3, and R4, respectively, and (d) the plots of specific capacitance vs current density for MgCo2O4 THSs, Sample R1, R2, R3, and R4, respectively. All the tests were performed in 2 M of KOH aqueous electrolyte.

cathode ones move from 0.2 to 0.17 V. It is attributed to the ohmic resistance and polarization of MgCo2O4 THSs-modified electrode [47]. Lower scan rate provides favorable condition for electrolyte penetration, and both the outer and inner active sites can participate in the Faradaic process. However, the active materials cannot be fully utilized at higher scan rate, given the fact that the diffusion of ions and electrons is limited in this situation. Basically, all shapes of the CV curves keep similar at different scan rate except that the positions of anodic and cathode peaks shift slightly, suggesting outstanding reversibility of the Faradaic process. Galvanostatic charge-discharge (GCD) test was performed under the potential of 0e0.45 V to further evaluate the electrochemical performance of MgCo2O4 THSs, and Fig. 7b presented the GCD curves at various current densities. The presence of plateaus on each discharge curve demonstrated the pseudocapacitive properties, and it was consistent with the conclusion obtained from CV measurement. Besides, the shape of all GCD curves at different current densities was approximately symmetrical, indicating that the electrode material possessed excellent Coulombic efficiency. The specific capacitances of MgCo2O4 THSs at different current density were calculated on the basis of Equation (1), and the values were 626.5, 570.2, 538.6, 502.8, 479.8, and 449.8 F/g at 1, 3, 5, 8, 12, and 16 A g
1, respectively. For comparison, four samples with shapes quite different from MgCo2O4 THSs obtained with urea of 5 mmol (MgCo2O4 spherical particles with rough surface, named Sample R1) and 40 mmol (MgCo2O4 irregular large block structure, named Sample R2), and solvothermal duration of 6 h (MgCo2O4 NPs, named Sample R3) and 9 h (MgCo2O4 THSs coexisted with some NPs, named Sample R4), respectively, were selected to measure their CV and GCD curves. The related CV curves at different scan rates, GCD curves at various current densities, and plots of current density vs specific capacitance were provided in Figs. S3 and S4. All the four samples exhibited pseudo-capacitive behaviors. The GCD curves tested at 1 A g
1 for all the five samples were presented in Fig. 7c, from which the specific capacitances of the

Sample R1, R2, R3, and R4 were calculated to be 469.2, 354.2, 345.7, and 533.8 F/g, lower than that of MgCo2O4 THSs with 626.5 F/g. At higher current density, the addition of PVDF binder leads to more severe potential drop, and the continuous redox reaction causes a certain degree of structural collapse of the material, resulting in sacrifice of specific capacitance. The value of capacitance decreases gradually with the increase of current density, and such relation is intuitively demonstrated in Fig. 7d. The diffusion and transport of electrolyte ions are suppressed at high current density, and the active sites inside MgCo2O4 cannot participate in all Faradaic processes. So a poor specific capacitance emerges under this condition [48]. The rate capability of MgCo2O4 THSs can reach 71.8% as the current density increases from 1 to 16 A/g, indicating its favorable electrochemical performances. The specific capacitances of MgCo2O4 THSs are also superior to some of single MgCo2O4 nanostructures, such as MgCo2O4 powders (321 F/g @ 0.5 A/g) [26] and double urchin-like MgCo2O4 (508 F/g @ 2 A/g) [27]. However, the capacitance value is still lower than some binder-free MgCo2O4 structures or MgCo2O4-based composites, such as MgCo2O4 cones (750 F/g @ 1 A/g) or MgCo2O4 needle arrays (804 F/g @ 1 A/g) directly grown on nickel foam [29,30], urchin-like MgCo2O4@ppy core-shell composite on nickel foam (1079.6 F/g @ 1 A/g) [31], and so on. The detailed specific capacitances of some MgCo2O4-based electrode materials were summarized in Table 1. In the case of taking into consideration factors such as preparation operations and costs, MgCo2O4 THSs is still a potential electrode material with excellent advantages. Furthermore, the long-term cycling stability of MgCo2O4 THSs was evaluated by a 5000 consecutive GCD measurement at 5 A/g, which was a chief factor in determining the application potential in supercapacitors. From Fig. 8a it can be found that the specific capacitance decreases from 538.6 to 391 F/g in the initial 1000 cycles, and then almost keeps stable in the next 4000 cycles. The phenomenon may be attributed to the following two reasons. The first one is probably due to the partially destruction of the MgCo2O4

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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7

Table 1 The specific capacitances of MgCo2O4-based electrode materials. Morphology

BET specific surface area (m2/g)

Specific capacitance

Ref.

MgCo2O4 prisms MgCo2O4 needles@b-polytype silicon carbide MgCo2O4 NWs on graphene-coated Ni foam MgCo2O4 NWs@MnO2 flakes on graphene supported on Ni foam MgCo2O4 powders MgCo2O4 cuboidal crystals double urchin-like MgCo2O4 MgCo2O4 brushes@carbon fibers MgCo2O4 cones on Ni foam MgCo2O4 needle arrays@Ni foam urchin-like MgCo2O4 on Ni foam@ppy core-shell composite MgCo2O4 flowers@rGO MgCo2O4 sheets on Ni foam MgCo2O4 sheets on Ni foam MgCo2O4 twinned-hemispheres

/ 1069 / / 0.45 18 26.45 / 335.8 25.6 /

613.5 C/g @ 2 mA/cm2 310.02 C/g @ 5 mV/s 658 F/g @ 1 A/g 887.3 F/g @ 1 A/g 321 F/g @ 0.5 A/g 345 C/g @ 1 A/g 508 F/g @ 2 A/g 854 mF/cm2 @ 2.5 mA/cm2 750 F/g @ 1 A/g 804 F/g @ 1 A/g 1079.6 F/g @ 1 A/g

[4] [22] [24] [25] [26] [23] [27] [28] [29] [30] [31]

55 / 146.6 63.8

570 F/g @ 2.5 mA/cm2 947 C/g @ 2 A/g 853.06 C/g @ 1 mA/cm2 626.5 F/g @ 1 A/g

[44] [47] [56] This work

“/” denotes no related data.

Fig. 8. (a) A consecutive 5000 GCD curves at 5 A/g and the related Coulombic efficiency, and (b) the last 10 GCD curves of MgCo2O4 THSs.

twinned-hemispheres during the working electrode fabrication process. The nickel foam with loaded MgCo2O4 active material is pressed under 10 MPa pressure, and the nickel foam becomes a nickel sheet. The active material is also pressed into the sheet tightly, under so high pressure, the structure of MgCo2O4 twinnedhemispheres with size of about 2 mm may be destroyed to some extent. The electrode after 5000 cycles is expected to be examined with SEM technique, but such characterization is very difficult to carry out. Another reason is probably due to the fact that the structure of twinned-hemispheres may be partially collapsed in the initial 1000 cycles, especially the average pore size is 13.5 nm and it is relatively large, and the structure is easily collapsed. The similar phenomenon was also reported previously [49,50]. So 72.6% specific capacitance was retained at the 1000th cycle. During the subsequent cycling measurement, the capacitance showed a slightly decay tendency, at the end of 5000th cycle, 68.9% of the original value was maintained. Fig. 8b presented the last 10 chargedischarge curves with stable and almost similar shape, indicating that such electrode exhibited outstanding cycling stability. Coulombic efficiency is also an important parameter for electrode materials, and the value of Coulombic efficiency can be obtained by the following equation:

h¼

td  100% tc

redox reaction. In order to have a further understanding about electrolyte diffusion mechanism and ion transport kinetics, electrochemical impedance spectroscopy (EIS) test was performed within the frequency scope of 105e10
2 Hz, and the related Nyquist plot was presented in Fig. 9. Inset of this figure demonstrated the enlarged Nyquist plot in the high frequency region and equivalent circuit fitted from the EIS data. The curve was consisted of a semicircle and a straight line. The series resistance (Rs) is associated with the sum of intrinsic resistance of MgCo2O4 THSs, ionic resistance of potassium hydroxide electrolyte, and the contact resistance between the MgCo2O4 THSs and nickel foam, which is represented by the

(6)

In the above equation, h represents Coulombic efficiency, td and tc mean discharge and charge time, respectively. It can be seen in Fig. 8a that the h of MgCo2O4 THSs almost remains 100% throughout the total 5000 GCD test, illustrating the favorable reversibility of the

Fig. 9. Nyquist plots of MgCo2O4 twinned-hemispheres before and after 5000 cycles, and the fitting equivalent circuit and the enlarged Nyquist plots at high frequency region were shown in the inset.

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

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Fig. 10. (a) CV curves of the MgCo2O4 THSs//AC ASC measured at different potential windows at a scan rate of 10 mV/s, and (b) GCD curves of the MgCo2O4 THSs//AC ASC measured at different potential windows at a current density of 1 A g
1.

intercept of the plot on the real axis (Z’) [51]. Charge transfer resistance (Rct) is determined by the semicircle diameter at high frequency. The slope of the line indicates the Warburg impedance (Zw), and the diffusion resistance of hydroxyl ions is the main reason of this impedance [52]. After calculation, the Rs values of MgCo2O4 THSs had almost no change before and after the cycling test, and the Rct value after 5000 cycles was 0.57 U, which was slightly increased compared with the value (0.49 U) before cycling. EIS measurement demonstrates that the fast ion diffusion and low charge transfer resistance can apply favorable conditions for the improvement of electrochemical performance. The unique structure of MgCo2O4 THSs not only provides abundant active sites to participate in the Faradaic process, but also shortens the transfer paths for both electrons and ions. Even in the case of long-term cycling, the resistance of MgCo2O4 THSs-modified electrode has no significant change, further suggesting that it may serve as a promising electrode material for energy savings. In order to evaluate the MgCo2O4 THSs electrode in the practical application of supercapacitors, an ASC was fabricated using MgCo2O4 THSs as the positive electrode and AC as the negative electrode (MgCo2O4 THSs//AC). The electrochemical behavior of the AC electrode was tested in a 2 KOH aqueous solution (Fig. S5). The

specific capacitance of the AC electrode was calculated to be 178.5 F/g at a current density of 1 A g
1 and 155.3 F/g at 16 A g
1 with a rate capability of about 87%. It suggests that the AC is a good negative electrode material for supercapacitors. To achieve the optimal electrochemical performance of the ASC, the two electrodes should satisfy the charge balance with qþ ¼ q
. Fig. 10a presented a series of CV curves of the MgCo2O4 THSs//AC ASC with different potential windows at a scan rate of 10 mV/s. It was clear that the operation potential of the ASC could be enlarged up to 1.7 V without obvious polarization curves. The GCD curves of the ASC were obtained over a potential window of 0.8e1.7 V at a current density of 1 A g
1 (Fig. 10b), further proving that no obvious polarization could be observed at 1.7 V. After calculation from Fig. 10b, the specific capacitance of the ASC increased from 24.6 to 76.3 F g
1 as the potential window changed from 0.8 to 1.7 V, and accordingly, the energy density increased from 2.2 to 30.6 W h kg
1. A potential window of 1.7 V is hence appropriate to investigate the electrochemical performance of the MgCo2O4 THSs//AC ASC. Fig. 11a displayed the CV curves of the MgCo2O4 THSs//AC ASC at scan rates ranging from 5 to 50 mV/s over a potential window of 0e1.7 V in the 2 M of KOH electrolyte. All the CV curves exhibited distorted and rectangular shapes, suggesting that both the pseudocapacitance from MgCo2O4 THSs and the electric double-layer capacitance from AC contributed to the total capacitance of the ASC. The GCD curves of the ASC tested with current density ranging from 1 to 16 A g
1 were presented in Fig. 11b. The specific capacitance of the ASC was calculated to be 76.3, 66.6, 63.6, 60.5, 56.6 and 54.0 F/g at 1, 3, 5, 8, 12, and 16 A g
1, respectively (Fig. 11c). About 70.8% capacitance was maintained as the current density increased from 1 to 16 A g
1, which indicated the excellent rate capability of the MgCo2O4 THSs//AC ASC. Fig. 11d showed the cycling performance of MgCo2O4 THSs//AC ASC through a continuous 5000 GCD process at a constant current density of 5 A g
1. About 99.06% specific capacitance was preserved after 5000 cycles. The coulombic efficiency of the ASC almost remained at 100% until the cycling process was over. The last 10 GCD curves were shown in the

Fig. 11. (a) CV curves of the MgCo2O4 THSs//AC ASC at different scan rates in the potential window of 0e1.7 V, (b) GCD curves of the MgCo2O4 THSs//AC ASC at different current densities, (c) the specific capacitance of the MgCo2O4 THSs//AC ASC at various current densities, and (d) continuous 5000 GCD curves and Coulombic efficiency of the MgCo2O4 THSs//AC ASC at a constant current density of 5 A g
1 with last 10 GCD curves in the inset.

Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905

Y. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

9

favorable long-term cycling stability at 5 A/g with 68.9% capacitance retention at 5000th charge-discharge test. It can be reasonably considered that the unique structure of the MgCo2O4 THSs has greatly contributed to the excellent electrochemical performance. In addition, the assembled MgCo2O4 THSs//AC asymmetric supercapacitor exhibited an energy density of 30.6 W h kg
1 at a power density of 861 W kg
1, and possessed a long cycling life with 99.06% specific capacitance retention after 5000 cycles at 5 A g
1. These results prove the great potential of the MgCo2O4 THSs as alternative electrode material in electrochemical energy storage fields. Declaration of competing interest The authors declare that they have no competing interests. Fig. 12. Ragone plots of the MgCo2O4 THSs//AC ASC and other ASCs reported previously.

inset, and they were consistent with the initial GCD curves at the beginning of cycling. All these results revealed the superior cycling durability of the MgCo2O4 THSs//AC ASC. Furthermore, the energy and power densities of the MgCo2O4 THSs//AC ASC were calculated, and the Ragone plots based on the GCD curves were presented in Fig. 12. The ASC achieved a high energy density of 30.6 W h kg
1 at a power density of 861 W kg
1, and still attained 24.3 W h kg
1 at a higher power density of 7522 W kg
1. As shown in Fig. 12, it is worth mentioning that the energy density of the MgCo2O4 THSs//AC ASC outperforms or is comparable with some of the previously reported ASCs from MgCo2O4 or other cobalt-based electrode materials, such as Co3O4/ NF//carbon aerogel microspheres (17.9 W h kg
1 at 750 W kg
1) [53], NiCo2S4 NPs//AC (28.3 W h kg
1 at 245 W kg
1) [43], NiCo2O4 nanosheets/CuCo2O4 nanocones/NF//AC (15 W h kg
1 at 814 W kg
1) [19], leaf-like CuCo2O4/NF//leaf-like CuCo2O4/NF (26.5 W h kg
1 at 763.7 W kg
1) [54], and MgCo2O4 nanosheets/ NF//AC (12.99 W h kg
1 at 448.7 W kg
1) [47]. However, this energy density is still lower than some of MgCo2O4 nanostructures, especially for the MgCo2O4 directly growing on the nickel foam or other conductive substrates such as MgCo2O4 submicron prisms//AC (39.7 W h kg
1 at 396 W kg
1) [4], SiCF/MgCo2O4 nanoneedles// SiCF (41.3 W h kg
1 at 464.7 W kg
1) [55], urchin-like MgCo2O4@ppy/NF//AC (33.4 W h kg
1 at 320 W kg
1) [31], and MgCo2O4 nanoneedles/NF//rGO (81 W h kg
1 at 1350 W kg
1) [30]. In brief, the high energy storage of the MgCo2O4 THSs//AC ASC benefited from the large cell potential of 1.7 V and the high specific capacitance. All the results above have proven the prepared MgCo2O4 THSs to be a promising candidate for supercapacitor applications. 4. Conclusions In summary, the twinned-hemispheres-like MgCo2O4 microstructure with size of about 2 mm was successfully synthesized through solvothermal reaction and post annealing at 500  C in air. Such MgCo2O4 structure formed by two hemispheres jointing together, which was composed of a large number of NPs, and the mesopores formed among these NPs gave rise to the large BET specific surface area. The surface area as well as average pore size of MgCo2O4 THSs was 63.8 m2/g and 13.5 nm, respectively. The electrochemical performance of MgCo2O4 THSs was evaluated in potassium hydroxide solution using three-electrode system. When tested at 1 A/g, the MgCo2O4 THSs-modified electrode exhibited 626.5 F/g, and the capacitance could still remain 71.8% even the current density reached 16 A/g. Besides, this electrode exhibited

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Please cite this article as: Y. Wang et al., Facile solvothermal synthesis of novel MgCo2O4 twinned-hemispheres for high performance asymmetric supercapacitors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152905