Cobalt selenide with high electrochemical energy storage performance

oxide and sulfide, including high specific capacitance (1656 F g-1 at 1 A g-1) .... voltammogram (CV), galvanostatic charge/discharge (GCD), electroch...
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Binder-Free Hierarchical Urchin-like Manganese−Cobalt Selenide with High Electrochemical Energy Storage Performance Chenxu Miao,† Panpan Xu,† Jing Zhao,† Kai Zhu,† Kui Cheng,† Ke Ye,† Jun Yan,† Dianxue Cao,† Guiling Wang,*,† and Xianfa Zhang*,‡ †

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Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China ‡ Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China S Supporting Information *

ABSTRACT: Transition metal selenides are widely considered as good electron conductor materials, showing bright prospect in energy storage and conversion. However, binary metal selenides as supercapacitor electrode materials are rarely reported. Herein, a simple and binder-free hydrothermal method is employed to grow hierarchical urchin-like MnCo-selenide on nickel foam. The unique hierarchical microstructure, synergetic effect, and excellent conductivity enable the electrode exhibit outstanding supercapacitor performance compared with counterpart oxide and sulfide, including high specific capacitance (1656 F g−1 at 1 A g−1) and extraordinary cycle performance (8.2% capacity decline after 8000 cycles). Additionally, the asymmetric supercapacitor (ASC), employing MnCo-selenide and AC as anode and cathode, exhibits remarkable energy density of 55.1 Wh kg−1 at 880 W kg−1, confirming the as-prepared urchin-like MnCo-selenide is a satisfactory material for the energy storage system. KEYWORDS: supercapacitor, binary metal selenides, conductivity, urchin-like structure, hydrothermal multistep method is more complex.15,16 Therefore, direct growth of highly conductive materials on current collectors is an effective approach to obtain high electrochemical energy storage performance.17 Se lives in the same group with the O element, theoretically sharing a similar chemical property. Analogously with the onestep hydrothermal synthesis method of transition metal oxides, transition metal selenides (TMS) might also be prepared via such a simple way.18,19 More importantly, Se has been demonstrated to have a metallic feature; therefore, the corresponding transition metal composites will exhibit better electronic conductivity than TMO, which is very essential for electrochemical application.20 Additionally, microstructure is also a key factor affecting electrochemical performance of supercapacitor. The reasonable nanostructure contributed to the increase the contact area of electrode materials and electrolyte, exposing more active sites. Moreover, the hierarchical structure could unite a few beneficial morphologies together, which could make full use of electroactive materials, promoting the electrochemical performance.

1. INTRODUCTION In the past decades, many researchers are devoted to the progress in high-performance energy storage system to solve environmental pollution and energy shortage problems. Supercapacitors, as one of the most promising candidates, have been widely investigated because of ultrahigh power density, long-term stability, and rapid charge−discharge capabilities.1,2 It is well-known that carbon materials, conducting polymers, and transition metal oxides/chalcogenides have been broadly applied in supercapacitors.3−6 Among these materials, carbon materials have a low capacity7 and conducting polymers8 show poor cycle stability. Transition metal oxides (TMO) and chalcogenides possess high capacitance because of the redox reactions, appearing at the surface of electrode.9 However, the poor conductivity of transition metal oxides limits their application. Hence, it is important to develop a novel active material possessing excellent electrical conductivity and reasonable nanostructures. Generally, the combination of carbon materials and TMO is a universal strategy to improve the conductivity.10,11 However, the Schottky contact between carbon material and active materials will definitely increase the charge transfer resistance.12 In addition, this kind of composite is commonly present as powders, which is difficult to directly grow on current collectors by a simple method.13,14 Moreover, the © 2019 American Chemical Society

Received: February 17, 2019 Accepted: May 2, 2019 Published: May 2, 2019 3595

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

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ACS Applied Energy Materials

Figure 1. Schematic illustration for the fabrication of hierarchical urchin-like MnCo-oxide, sulfide, and selenide on nickel foam. were introduced into a 50 mL Teflon-lined autoclave. After reaction at 100 °C for 5 h, the as-obtained MnCo precursor was cleaned by distilled water and then transferred into an oven at 70 °C for 10 h. 2.2. Synthesis of MnCo-Selenide, MnCo-Oxide, and Sulfide. First, 3 mmol of Se powder and 6 mmol of NaBH4 were introduced into 40 mL mixed solvent (ethanol/distilled water ratio of 1:1). After stirring for 10 min under a N2 atmosphere, a clear solution was formed, and then a piece of preprepared MnCo precursor and the mixture were added into the Teflon-lined autoclave. After reaction at 180 °C for 12 h, the sample was cleaned by distilled water and placed in oven at 70 °C overnight. Finally, the active mass of as-obtained product is 2.1 mg cm−2, defined as MnCo-selenide. For comparison, the Mn−Co−Se compounds with different Mn/Co ratios were synthesized by adjusting the addition ratio of Mn2+ and Co2+, and the total molar of metal salts was kept at 3 mmol. The Mn/Co molar ratio was set as 0:1, 1:3, 1:1, and 1:0, which were defined as MnCo-1, MnCo-2, MnCo-3, and MnCo-4. In addition, MnCo-oxide and sulfide electrodes were also synthesized. The MnCo precursor was treated thermally at 350 °C for 2 h to obtain MnCo-oxide. MnCo-sulfide was synthesized through ion exchange method. Briefly, the MnCo precursor was immersed into 0.1 M Na2S solution and kept at 120 °C for 12 h to synthesize the MnCo-sulfide electrode, and the mass loadings of MnCo-oxide and sulfide were 1.0 and 1.4 mg cm−2, respectively. 2.3. Preparation Activated Carbon Electrode. First, active carbon, Super P, and PVDF solution (7 wt %) with a mass ratio of 8:1:1 were mixed together via strong magnetic stirring. Subsequently, the homogeneous slurry was formed by adding the NMP drop by drop. After stirring for 5 h, the slurry uniformly spread on the nickel foam (1 × 1 × 0.1 cm3) and then transferred into a vacuum oven at 80 °C for 10 h. Finally, the AC negative electrode was obtained. 2.4. Materials Characterizations. A scanning electron microscope (SEM, JEOL JSM-6480) and a transmission electron microscope (TEM, JEOL 2010) were used to analyze the microstructure and lattice distance, respectively. The crystallographic phase was confirmed through an X-ray diffractometer (XRD, Rigaku TTR III). X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was applied to explore the elementary composition and valence state of target element. The elementary composition of as-prepared electrodes was measured using energy disperse spectroscopy (EDS). 2.5. Electrochemical Testing. A three-electrode system was employed to study the supercapacitor performance of as-obtained electrodes, wherein the as-obtained electrode as working electrode, saturated calomel electrode (SCE) as reference electrode, and

So far, TMS with reasonable structure has been wildly explored as electrode materials because of the satisfactory electronic conductivity and excellent capacitor performance. Particularly, the binary metal selenides exhibit ultrahigh specific capacitance and outstanding rate capability resulting from the synergic effect of the different metal cation and multiple metal oxidation states. Very recently, most research focuses on Co and Ni elements and obtains high electrochemical performance.21−23 However, the Mn element, with a variety of oxidation states, low toxicity, and high abundance, is rarely reported. Herein, Mn, which is also representative for the extreme stability, is introduced to cooperate with Co, so as to simultaneously obtain high capacitive performance and cyclic stability. Meanwhile, Se could supply fast electron transfer,24 ensuring commendable rate capability. Moreover, MnCo oxides and sulfides have been demonstrated to own excellent electrochemical performance; it can be inferred that MnCo-selenide exhibits outstanding performance in supercapacitors. As far as we know, there is not any similar work based on current reports. A simple and binder-free hydrothermal method is employed to grow hierarchical urchin-like MnCo-selenide on nickel foam. The unique structure, offering abundant contact area between active materials and electrolyte, and the synergetic effect of Co, Mn, and Se enable the as-prepared electrode to possess extraordinary supercapacitor performance, including superior capacitive performance (1656 F g−1 at 1 A g−1), capacitance retention of 62% from 1 to 20 A g−1, and cyclic capability of 91.8% after 8000 cycles. Moreover, the assembled MnCoselenide//AC device shows ultrahigh energy density of 55.1 Wh kg−1 at 880 W kg−1, suggesting the promising application prospect as electrode materials.

2. EXPERIMENTAL SECTION 2.1. Preparation of MnCo Precursor. The MnCo precursor is prepared through the hydrothermal method, which is described as follows: 2.0 mmol of Co(CH3COO)2·4H2O, 1.0 mmol of Mn(CH3COO)2·4H2O, 5.0 mmol of NH4F, and 6.0 mmol of urea were added into 40 mL of distilled water under stirring. Then the homogeneous mixture and a pretreated nickel foam (2 × 3 × 0.1 cm3) 3596

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

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Figure 2. SEM (a−c) and TEM (d−f) of MnCo-selenide. TEM mapping of Co (g), Mn (h), Se (i), and the merge image (j). platinum slice as counter electrode. Cyclic voltammogram (CV), galvanostatic charge/discharge (GCD), electrochemical impedance spectroscopy (EIS), and cycle performance test were conducted in 6 M KOH by an electrochemical workstation (VMP3/Z Biologic). The ASC device was tested through two-electrode system, in which the as-prepared MnCo-selenide and AC as anode and cathode. All experiments were tested at 6 M KOH solution.

seen that after oxidation and sulfidation the microstructure does not change significantly. Especially, as shown in Figure 2a−c, even after selenylation under hydrothermal reaction condition, the urchin-like structure is still kept very well and uniformly covers on the surface of Ni foam, indicating good structural stability, which is essential for improving cycle stability of electrode. Figures 2d,e are the TEM and HRTEM images of the MnCo-selenide. As shown in Figure 2d, the diameter of the microsphere is ∼4.28 μm, which is assembled by nanowire with a width of 36 nm. The lattice fringes in the HRTEM image (Figure 2f) show lattice distances of 0.223 and 0.165 nm, which are in agreement with (210) and (221) planes of MnCo-selenide, respectively. The TEM mapping demonstrates the coexistence and even distribution of Mn, Co, and Se elements, indicating the binary transition metal precursor has been synthesized and the selenylation is complete and successful. Moreover, it should be noted that the unique nanowire assembled microsphere provides an open network, which could not only provide abundant contact area on the surface of the electrode/electrolyte, promoting the utilization of active materials, but also facilitate the penetration of electrolyte into the center part, shortening the ion diffusion path; thereby, the electrochemical performance could be enhanced efficiently. XRD was employed to confirm the crystallographic phase and purity of as-synthesized materials. The nickel foam was

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 shows the scheme of the preparation process. First, the MnCo-precursor is prepared via a simple hydrothermal method, which Mn2+ and Co2+ react with OH− released by urea to form MnCo-LDH. The MnCooxide is obtained by calcination of MnCo-precursor under an O2 atmosphere. S2− is used to ion exchange with precursor, and the MnCo-sulfide is achieved. Analogously, Se2− is applied to react with precursor to acquire MnCo-selenide. As presented in Figure 1, the three samples possess similar morphology, 3-dimensional hierarchical urchin-like structure, which is because the MnCo-oxide, sulfide, and selenide are evolved from the urchin-like precursor. SEM is employed to analyze the morphology of as-obtained samples, which are presented in Figure S1 and Figure 2a−c. Figure S1a,b displays the SEM images of the MnCo precursor, which presents obviously a 3-dimensional microsphere composed of a hierarchical nanowire growing perpendicularly outward from the center. Figures S1c,d and S1e,f are SEM images of MnCo-oxide and sulfide, respectively. It could be 3597

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

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Figure 3. (a) XRD of as-obtained electrodes. (b−e) XPS and EDS (f) results of MnCo-selenide electrode. (g) ICP results.

To further analyze the elementary composition and valence state of MnCo-selenide, the XPS test is performed, of which the results are listed in Figure 3b−e. The full survey spectra (Figure 3b) suggest the coexistence of Mn, Co, and Se elements. The chemical state could be analyzed by the special region narrow spectrum. As shown in Figure 3c, there are two strong peaks in the Mn 2p region. The peaks located at 641.5 and 653.3 eV suggest the presence of Mn2+.25 For the spectrum in the Co 2p region (Figure 3d), the peaks at 778.5 and 793.6 eV belong to Co2+; the binding energy at 780.2 and 799.3 eV can be ascribed to shakeup satellites.26,27 Figure 3e shows the XPS spectrum of Se 3d, the Se 3d5/2 peak at 54.7 eV, and the Se 3d3/2 peak at 55.5 eV are typical features of the metal− selenium bonds.28−30 In addition, the wide peak at 59.5 eV is associated with the oxidized Se.31 The XPS analyses imply that the binary MnCo-selenide is successfully prepared. The distribution of elements is examined by EDS, which is shown in Figure 3f. The insets are elemental mapping images,

removed to avoid the signal interference during the XRD test process. As depicted in Figure S2, the XRD pattern of MnCo precursor is in accordance with MnCo-LDH (JCPDS: 480083). From the XRD patterns of as-prepared samples (Figure 3a), the diffraction peaks of MnCo-oxide could be assigned to cubic MnCo2O4 (JCPDS: 84-0482) and the MnCo-sulfide could be indexed to cubic Co3S4 (JCPDS: 73-1703). The selenized product shows diffraction peaks located at 23.8°, 28.9°, 30.7°, 34.5°, 35.9°, 37.0°, 40.4°, 43.9°, 47.7°, 48.7°, 50.2°, 53.4°, 55.4°, 56.9°, 59.2°, 63.3°, 65.1°, and 66.2°, corresponding to (110), (011), (101), (111), (120), (200), (210), (121), (211), (220), (002), (031), (221), (131), (310), (122), (311), and (320) crystal planes of the orthorhombic CoSe2 (JCPDS: 53-0449), respectively. Because of the less replacement amount of Mn2+ with Co2+, it is difficult to detect the existence of Mn through XRD. But definitely, the substitution does not change the orthorhombic crystal structure. 3598

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Figure 4. CV (a) and GCD (b) curves of as-prepared electrodes. (c) Specific capacitance of as-prepared electrodes at 1 A g−1.

Figure 5. (a) CV curves of MnCo-selenide. (b) Plots of square root of scan rate and peak current. (c) GCD curves of MnCo-selenide. (d) Rate capability. (e) EIS test results. (f) Cycling performance.

distinct from linear behavior of carbon materials, which is consistent with CV curves. In addition, the discharge time of MnCo-selenide electrode is larger than others, suggesting better capacity performance. The specific capacity can be evaluated according to discharge time, and the results are listed in Figure S3c. The MnCo-selenide electrode show highest specific capacity of 161.0 mAh g−1 (1656 F g−1), confirming Mn−Co−Se electrode with the Mn/Co ratio of 1:2 possesses the superior electrochemical properties. Therefore, the MnCoselenide electrode is used for subsequent testing. Figure 4a presents the CV curves of MnCo-oxide, MnCosulfide, and MnCo-selenide electrodes at 30 mV s−1. All CV curves display obvious peaks, implying a reversible faradaic reaction occurs on the electrode. Generally, the integral area of CV curve can be applied to estimate capacitive performance of electrode. The CV curve of MnCo-selenide electrode possesses the maximum integral area, implying that MnCo-selenide electrode possesses better charge storage ability than MnCooxide and MnCo-sulfide electrodes. To further investigate the capacitance property of as-obtained electrodes, the GCD were tested with the potential window of 0−0.35 V. Figure 4b

implying the existence of Mn, Co, and Se elements, which is in accordance with XPS results. Besides, it could be seen that Mn, Co, and Se are uniformly distributed on nickel foam. To further confirm the chemical formula of MnCo-selenide, the ICP test was carried out, and the results are listed in Figure 3g. The molar ratio of Mn:Co:Se is about 0.32:0.68:2, so the MnCo-selenide can be defined as Mn0.32Co0.68Se2. In addition, the molar ratio of Mn and Co elements approached 1:2, implying the selenylation could not change the ratio of metal elements. 3.2. Electrochemical Investigations of As-Prepared Electrodes. The supercapacitor performance of the asobtained electrodes was tested through a three-electrode system with potential window range from 0 to 0.4 V in 6 M KOH. To confirm the optimal ratio of Mn/Co, the CV curves of Mn−Co−Se electrodes with various Mn/Co ratios are plotted in Figure S3a. The obvious oxidation/reduction peaks are found in all CV curves, indicating a faradaic reaction appearing at the surface of active material. Figure S3b exhibits the discharge curves of the as-prepared Mn−Co−Se electrodes at 1 A g−1. All discharge curves possess an obvious plateau 3599

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Table 1. Summary of Electrochemical Properties of Monometal Selenide, MnCo-Oxides, MnCo-Sulfides, and Binary Metal Selenides material

morphology

specific capacitance (F g−1)

current density (A g−1)

cycling performance

ref

Co0.85Se Co0.85Se Co0.85Se Co0.85Se MnCo2O4 MnCo2O4 MnCo2S4 MnCo2S4 Ni0.9Co1.92Se4 Ni0.34Co0.66Se2 MnCo-selenide

nanosheet nanosheet petal-like nanowire nanorod cube nanoneedle hollow tubular coral-like nanorod urchin-like

1528 1378 294 674 845.6 480.5 543.3 1203 1021.1 543.7 1656

1 1 0.5 1.48 1 1 1 2 0.3 0.8 1

89.1% after 5000 cycles 95% after 1000 cycles 93% after 5000 cycles 89% after 2000 cycles 90% after 2000 cycles 96% after 3000 cycles 91% after 5000 cycles 94% after 2000 cycles 88% after 5000 cycles 92% after 6000 cycles 91.8% after 8000 cycles

35 36 37 38 39 40 41 42 21 43 this work

wherein I and ∫ V dt are discharge current and integral area of discharge curve, respectively. V and m represent the voltage window and active mass of electrode, respectively. Based on the GCD curves (Figure 5c), the specific capacity is obtained as 161.0, 155.4, 149.4, 140.7, 131.8, 121.4, and 99.6 mAh g−1, corresponding to 1656, 1598, 1536, 1447, 1355, 1248, and 1024 F g−1 at 1, 2, 4, 6, 8, 10, and 20 A g−1, respectively. As a contrast, the values of MnCo-oxide and MnCo-sulfide electrodes with different current densities are also collected based on Figure S5, which is depicted in Figure 5d. The specific capacitance and rate capability are key factors to evaluate the energy storage application value of as-prepared materials. It could be apparently observed that, among the three electrodes, MnCo-selenide displays highest specific capacitance than MnCo-oxide and MnCo-sulfide. Furthermore, as the current density expands to 20-fold higher (1 to 20 A g−1), the MnCo-selenide electrode still retains 62%, which is much better than the MnCo-sulfide (51%) and MnCo-oxide (46%) electrodes. This is in accordance with the order of electrically conductive ability (MnCo-selenide > MnCo-sulfide > MnCo-oxide). Moreover, the capacitance performance of the MnCo-selenide electrode is higher than most reported about monometal selenide, MnCo-oxides, MnCo-sulfides, and binary metal selenides, as seen from Table 1. The reasons for outstanding capacitance performance of MnCo-selenide may be as follows: (1) The hierarchical urchin-like MnCo-selenide is composed of a nanowire growing perpendicularly outward from the center, which could shorten the length of the diffusion path and provide abundant contact area of electrode material and electrolyte. (2) The inherent excellent electrical conductivity of MnCo-selenide promotes the faster electrons transfer, thereby showing more satisfactory electrochemical performance. (3) The hierarchical urchin-like MnCo-selenide directly grown on Ni foam ensures the excellent conductive between electrode materials and current collector, avoiding dead volume resulting from binder and conductive agent, and therefore exhibits excellent capacitance performance. Moreover, the space between the nanowires could act as a buffer, adapting the volume change results from redox reaction, thus ensuring outstanding cycling stability. EIS tests were performed at the open-circuit potential in the frequency range from 0.1 to 100000 Hz to further study the reaction kinetics of the three electrodes. Figure 5e shows the Nyquist plots of as-prepared electrodes, and the inset is the equivalent circuit model. Generally speaking, EIS curves consist of a semicircle and a straight line at various frequencies. Ru represents the equivalent series resistance, which is

presents the GCD traces of as-prepared three electrodes at 1 A g−1. The voltage plateaus can be seen in all GCD curves, indicating the process of faradaic reactions, which is in accordance with the CV results. Distinctly, the MnCo-selenide electrode exhibits the longest discharge time than MnCo-oxide and MnCo-sulfide electrodes at the same current density, implying the highest value of specific capacitance (Figure 4c). The high electrical conductivity of MnCo-selenide may account for excellent electrochemical performance. Compared with MnCo-oxide and MnCo-sulfide, the MnCo-selenide possess higher electrical conductivity, which contributes to fast electrons transfer from current collector to active material, thereby increasing the electrochemical performance. In addition, the electrochemical active surface area (ECSA) is also a key factor to affect the electrochemical properties of asobtained electrodes. We obtained double-layer capacitance (Cdl) through the CV tests with various scan rates to estimate the ECSA,32 and the results are listed in Figure S4. It can be found that MnCo-selenide possesses highest Cdl of 21.8 mF cm−2, implying large electrochemical active surface area and more exposed activate sites,33 which contributes to the increase of electrochemical properties. The supercapacitor properties of the MnCo-selenide are systematically investigated via CV, GCD, and EIS. Figure 5a depicts the CV curves of the MnCo-selenide electrode with various scan rates. As the scan rate increases to 30 mV s−1, the shape of CV curves is still retained and without obvious distortion, hinting at the good rate capability. In addition, it could be observed that the potential of oxidation and reduction peaks slightly moves to the right and left, respectively, because of the existence of polarization. Figure 5b displays the function of peak current relative to the square root of scan rate, which well exhibits a linear relationship, indicating the faradaic reactions occurring on the MnCo-selenide electrode is a diffusion control process. Figure 5c shows the GCD curves of the MnCo-selenide electrode. The charge and discharge sections are symmetric, revealing the faradaic reaction process is primely reversible. In addition, there is no obvious voltage drop in the discharge curve, which is mainly ascribed to the outstanding conductivity of the MnCo-selenide electrode, resulting in excellent rate capability.34 The specific capacity (C, mAh g−1) of as-prepared electrodes could be calculated based on the following equation: C=

2I∫ V dt mV

(1) 3600

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

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Figure 6. (a) CV curves of AC and MnCo-selenide at 30 mV s−1. CV (b) and GCD (c) curves of MnCo-selenide//AC. (d) Ragone plot. (e) Cycle stability of MnCo-selenide//AC. (f) Schematic of the asymmetric supercapacitor. (g) Images of the red LED lighted by two ASC in series.

estimated by the horizontal intercept of the EIS curve. The charge transfer resistance (Rct) could be measured via the diameter of a semicircle. Moreover, the low-frequency line of the as-prepared electrode is related to Warburg impedance originating from ion diffusion. As shown in Figure 5e, the Rct value of MnCo-selenide is 0.99 Ω, which is lower than that of MnCo-oxide (3.44 Ω) and MnCo-sulfide (1.66 Ω), indicating the MnCo-selenide electrode possesses the best electrical conductivity. The result is identical with recent reports.44 The cycle performance is a crucial parameter to assess the potential for application, which was measured at 4 A g−1 for 8000 cycles (Figure 5f). After 8000 cycles, the specific capacitance of MnCo-selenide electrode still retains 91.8% of the initial value. However, the MnCo-oxide and MnCo-sulfide electrodes only maintain 77.8% and 85.1%. The results show that the cycle stability of the MnCo-selenide electrode is not only superior to other two electrodes but also quite better than other reports about similar electrode materials (Table 1), which may result from the ultrastable microstructure of the MnCo-selenide. The specific capacitance of MnCo-selenide electrode gradually increases at the first 400 cycles, which mainly because of the activation of the electrode material to

maximum contact of active material and electrolyte. Subsequently, the decrease of the specific capacitance is attributed to the shedding of active materials after repeated GCD test. The Coulombic efficiency of MnCo-selenide electrode is listed in Figure S6. The inset displays the first 5 GCD curves, 4000− 4004 GCD curves, and the last 5 GCD curves. A similar shape could be observed in all GCD curves and without distinct deviation after long-term cycling test, implying extraordinary cycling stability. Besides, all the charge−discharge curves have obvious bend instead of ideal straight lines, suggesting the faradaic reaction occurring at the surface of active material. Moreover, the Coulombic efficiency of all the GCD curves approaches 100%, indicating excellent electrochemical reversibility. Figure S7 is the SEM image of the MnCo-selenide electrode after 8000 cycles; the morphology is still not destroyed even if the electrode undergoes as many as 8000 charge and discharge processes, indicating remarkable cycling stability. The superior electrochemical properties of the MnCoselenide electrode are associated with the following reasons: First, the MnCo-selenide directly grown on current collectors without binder and conductive additive could not only avoid 3601

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ACS Applied Energy Materials the dead volume, improving the utilization of electrode material, but also ensure better electrons transfer from current collectors to active materials. Second, the microsphere assembled by nanowires grown from the center to outward supplies a 3D porous network, which could efficiently increase electrode/electrolyte contact area, enabling more active sites to provide higher capacitance. Besides, the open space could facilitate the penetration of electrolyte into the center, reducing concentration polarization to enhance rate capability. Lastly, the synergetic effect between Mn, Co, and Se elements, which supply excellent cycling stability, high specific capacitance, and better electron conductivity, respectively, endows the MnCoselenide electrode with outstanding supercapacitor properties. 3.3. Electrochemical Performance of MnCo-Selenide//AC. To further verify the possibility of MnCo-selenide electrode for practical application in supercapacitor, an ASC was assembled employing MnCo-selenide and AC as anode and cathode (Figure 6f). Figure S8 exhibits the electrochemical properties of AC. Based on the discharge time of AC electrode (Figure S8b), the values of specific capacitance are plotted in Figure S8c with the help of eq 1. The AC electrode exhibits an outstanding specific capacitance of 265 F g−1 at 1 A g−1. To attain the optimal energy storage properties of ASC devices, the mass ratio of MnCo-selenide and AC was adjusted using the equation m+ C −ΔV − = m+ + m− Cm ΔV

E=

P=

2I∫ V dt 3.6m E t

(3)

(4)

wherein I, m, ∫ V dt, and t are the discharge current, the active mass of the two electrodes, integral area of discharge curve, and discharge times, respectively. As presented in the Ragone diagram (Figure 6d), when the power density changes from 880 to 8205 W kg−1 (29.5 to 274.9 mW cm−3), the energy density decreases from 55.1 to 25.3 Wh kg−1 (1.85 to 0.85 mWh cm−3). These values are superior to most of previously reported ASC devices based on Mn−Co oxides, sulfides, monometallic selenides, and binary metal selenides, such as MnCo 2 O 4 //AC (35.4 Wh kg −1 at 225 W kg −1 ), 45 MnCo2O4.5//AC (40.5 Wh kg−1 at a 376 W kg−1),46 MnCo 2 S 4 //rGO (31.3 Wh kg −1 at 800 W kg −1 ), 47 Co0.85Se//AC (39.7 Wh kg−1 at 789.6 W kg−1),36 Co0.85Se// N-PCNs (21.1 Wh kg−1 at 400 W kg−1),37 Ni0.9Co1.92Se4//AC (26.3 Wh kg−1 at 265 W kg−1),21 and Ni0.8Co0.2Se//AC (32.1 Wh kg−1 at 37.9 W kg−1).22 The cycling stability was studied by the repeated GCD process at 4 A g−1 (Figure 6e). After 8000 cycles, a 9.8% capacity decline could be observed and the coulombic efficiency maintains ∼100%. The extraordinary cycling stability is very important for practical applications. To further confirm the possibility of MnCo-selenide//AC for electronic devices, we connected two charged ASC devices in series. As shown in Figure 6g, the new device could easily illume a red (1.8 V) light-emitting diode (LED). Moreover, the red LED still retained bright even after 5 min, implying great potential for energy storage.

(2)

where m represents the active mass of MnCo-selenide and AC. C and ΔV are the specific capacitance and potential range of the corresponding electrode. By calculation, the final mass ratio of MnCo-selenide and AC electrode is about 1:2.18. Figure 6a gives the CV curves of the MnCo-selenide and AC electrode at 30 mV s−1. As the voltage ranges of the two electrodes are 0−0.4 and −1−0 V, respectively, the ASC is expected to operate at 0−1.4 V. The CV curves of ASC were tested at various scan rates and depicted in Figure 6b. It could be seen that there is not obvious oxygen or hydrogen evolution at the cutoff voltages of the positive and negative direction, implying the potential range is proper for the assembled device. Additionally, because of the combined contribution from the electrical double-layer capacitance (EDLC) and pseudocapacitance mechanism of AC and MnCo-selenide electrodes, the CV curves display a pair of wide redox peaks. Moreover, the shape of CV traces still remains as the scan rate expands to 50 mV s−1, implying an outstanding rate property, which is essential for power output equipment. Figure 6c displays the GCD curves of ASC at various current densities. The GCD curves are nearly symmetry, confirming the high reversibility. The specific capacitance of the ASC device could be evaluated using eq 1, and m represents the total active mass of ASC device (sum of MnCo-selenide and AC), which are 78.7, 71.1, 59.8, 51.1, 43.0, and 36.1 mAh g−1 at 1, 2, 4, 6, 8, and 10 A g−1, respectively. According to the electrode density of 0.0335 g cm−3, the volumetric capacity (Cvc, mAh cm−3) of the ASC is 2.64, 2.38, 2.00, 1.71, 1.44, and 1.21 mAh cm−3 at corresponding current densities. Energy density is an imperative parameter to evaluate the ACS device. The energy density (E, Wh kg−1) and power density (P, W kg−1) of MnCo-selenide//AC were studied using eqs 3 and 4:

4. CONCLUSIONS In summary, the hierarchical urchin-like MnCo-selenide nanowires are directly grown on Ni foam through the twostep hydrothermal method. The MnCo-selenide electrode shows better conductivity and higher specific capacitance than MnCo-oxide and MnCo-sulfide. Meanwhile, the MnCoselenide presents an extraordinary cycling performance (91.8% after 8000 cycles). Furthermore, the MnCoselenide//AC device possesses large energy density and excellent cycle performance. Moreover, two as-prepared ASC devices could easily illume a red LED. These results indicate that MnCo-selenide is a satisfactory material for high performance supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00338. Additional characterization of as-prepared materials; electrochemical performance of Mn−Co−Se electrodes with different Mn/Co ratios; electrochemical active surface area and GCD curves of as-prepared electrodes; coulombic efficiency of MnCo-selenide electrode; SEM images of MnCo-selenide after 8000 charge−discharge cycles; electrochemical performance of AC electrode (PDF) 3602

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

Article

ACS Applied Energy Materials

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(14) Jia, Y.; Wan, H.; Chen, L.; Zhou, H.; Chen, J. Hierarchical nanosheet-based MoS2/graphene nanobelts with high electrochemical energy storage performance. J. Power Sources 2017, 354, 1−9. (15) Zhou, R.; Han, C.-j.; Wang, X.-m. Hierarchical MoS2-coated three-dimensional graphene network for enhanced supercapacitor performances. J. Power Sources 2017, 352, 99−110. (16) AbdelHamid, A. A.; Yang, X.; Yang, J.; Chen, X.; Ying, J. Y. Graphene-wrapped nickel sulfide nanoprisms with improved performance for Li-ion battery anodes and supercapacitors. Nano Energy 2016, 26, 425−437. (17) Chen, W.; Zhang, X.; Mi, L.; Liu, C.; Zhang, J.; Cui, S.; Feng, X.; Cao, Y.; Shen, C. High-Performance Flexible Freestanding Anode with Hierarchical 3D Carbon-Networks/Fe7S8/Graphene for Applicable Sodium-Ion Batteries. Adv. Mater. 2019, 31, 1806664. (18) Guo, K.; Yang, F.; Cui, S.; Chen, W.; Mi, L. Controlled synthesis of 3D hierarchical NiSe microspheres for high-performance supercapacitor design. RSC Adv. 2016, 6, 46523−46530. (19) Tang, C.; Pu, Z.; Liu, Q.; Asiri, A. M.; Sun, X.; Luo, Y.; He, Y. In Situ Growth of NiSe Nanowire Film on Nickel Foam as an Electrode for High-Performance Supercapacitors. ChemElectroChem. 2015, 2, 1903−1907. (20) Yu, N.; Zhu, M.-Q.; Chen, D. Flexible all-solid-state asymmetric supercapacitors with three-dimensional CoSe2/carbon cloth electrodes. J. Mater. Chem. A 2015, 3, 7910−7918. (21) Hou, L.; Shi, Y.; Wu, C.; Zhang, Y.; Ma, Y.; Sun, X.; Sun, J.; Zhang, X.; Yuan, C. Monodisperse metallic NiCoSe2 hollow submicrospheres: formation process, intrinsic charge-storage mechanism, and appealing pseudocapacitance as highly conductive electrode for electrochemical supercapacitors. Adv. Funct. Mater. 2018, 28, 1705921. (22) Guo, K.; Cui, S.; Hou, H.; Chen, W.; Mi, L. Hierarchical ternary Ni-Co-Se nanowires for high-performance supercapacitor device design. Dalton Trans. 2016, 45, 19458−19465. (23) Song, X.; Huang, C.; Qin, Y.; Li, H.; Chen, H. C. Hierarchical hollow, sea-urchin-like and porous Ni0.5Co0.5Se2 as advanced battery material for hybrid supercapacitors. J. Mater. Chem. A 2018, 6, 16205−16212. (24) Nagaraju, G.; Cha, S. M.; Sekhar, S. C.; Yu, J. S. Metallic layered polyester fabric enabled nickel selenide nanostructures as highly conductive and binderless electrode with superior energy storage performance. Adv. Energy Mater. 2017, 7, 1601362. (25) Li, N.; Zhang, Y.; Zhao, H.; Liu, Z.; Zhang, X.; Du, Y. Synthesis of High-Quality alpha-MnSe Nanostructures with Superior Lithium Storage Properties. Inorg. Chem. 2016, 55, 2765−70. (26) Chen, T.; Li, S.; Wen, J.; Gui, P.; Guo, Y.; Guan, C.; Liu, J.; Fang, G. Rational Construction of Hollow Core-Branch CoSe2 Nanoarrays for High-Performance Asymmetric Supercapacitor and Efficient Oxygen Evolution. Small 2018, 14, 1700979. (27) Fang, L.; Li, W.; Guan, Y.; Feng, Y.; Zhang, H.; Wang, S.; Wang, Y. J. A. F. M. Tuning Unique Peapod-Like Co(SxSe1−x)2 Nanoparticles for Efficient Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1701008. (28) Chen, T.; Li, S.; Wen, J.; Gui, P.; Fang, G. Metal-Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage. ACS Appl. Mater. Interfaces 2017, 9, 35927−35935. (29) Lee, C.-P.; Chen, W.-F.; Billo, T.; Lin, Y.-G; Fu, F.-Y.; Samireddi, S.; Lee, C.-H; Hwang, J.-S.; Chen, K.-H.; Chen, L.-C. J. J. o. M. C. A. Beaded stream-like CoSe2 nanoneedle array for efficient hydrogen evolution electrocatalysis. J. Mater. Chem. A 2016, 4, 4553− 4561. (30) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. J. A. C. NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting. Angew. Chem. 2015, 127, 9483−9487. (31) Li, H.; Gao, D.; Cheng, X. J. E. A. Simple microwave preparation of high activity Se-rich CoSe2/C for oxygen reduction reaction. Electrochim. Acta 2014, 138, 232−239.

AUTHOR INFORMATION

Corresponding Authors

*(G.W.) E-mail: [email protected]. *(X.Z.) E-mail: [email protected]. ORCID

Kui Cheng: 0000-0001-9396-1545 Jun Yan: 0000-0002-9967-3912 Dianxue Cao: 0000-0001-9138-7295 Guiling Wang: 0000-0003-2842-2355 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (51572052).

(1) Miller, J. R.; Simon, P. Materials science- Electrochemical capacitors for energy management. Science 2008, 321, 651−652. (2) Shao, Y.; El-Kady, M. F.; Sun, J.; Li, Y.; Zhang, Q.; Zhu, M.; Wang, H.; Dunn, B.; Kaner, R. B. Design and Mechanisms of Asymmetric Supercapacitors. Chem. Rev. 2018, 118, 9233−9280. (3) Zhang, D.; Han, M.; Li, Y.; He, J.; Wang, B.; Wang, K.; Feng, H. Ultra-facile fabrication of phosphorus doped egg-like hierarchic porous carbon with superior supercapacitance performance by microwave irradiation combining with self-activation strategy. J. Power Sources 2017, 372, 260−269. (4) Pattananuwat, P.; Aht-Ong, D. One-step method to fabricate the highly porous layer of poly (pyrrole/(3, 4-ethylenedioxythiophene)/) wrapped graphene hydrogel composite electrode for the flexibile supercapacitor. Mater. Lett. 2016, 184, 60−64. (5) Hui, K. N.; Hui, K. S.; Tang, Z.; Jadhav, V. V.; Xia, Q. X. Hierarchical chestnut-like MnCo2O4 nanoneedles grown on nickel foam as binder-free electrode for high energy density asymmetric supercapacitors. J. Power Sources 2016, 330, 195−203. (6) Wei, W.; Mi, L.; Gao, Y.; Zheng, Z.; Chen, W.; Guan, X. Partial ion-exchange of nickel-sulfide-derived electrodes for high performance supercapacitors. Chem. Mater. 2014, 26, 3418−3426. (7) Zhang, L. L.; Zhao, X. S. Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 2009, 38, 2520−2531. (8) Zhou, H.; Han, G. J. E. A. One-step fabrication of heterogeneous conducting polymers-coated graphene oxide/carbon nanotubes composite films for high-performance supercapacitors. Electrochim. Acta 2016, 192, 448−455. (9) Wang, T.; Zhang, S.; Yan, X.; Lyu, M.; Wang, L.; Bell, J.; Wang, H. J. A. a. m. interfaces, 2-Methylimidazole-derived Ni−Co layered double hydroxide nanosheets as high rate capability and high energy density storage material in hybrid supercapacitors. ACS Appl. Mater. Interfaces 2017, 9, 15510−15524. (10) Qu, C.; Zhang, L.; Meng, W.; Liang, Z.; Zhu, B.; Dang, D.; Dai, S.; Zhao, B.; Tabassum, H.; Gao, S.; Zhang, H.; Guo, W.; Zhao, R.; Huang, X.; Liu, M.; Zou, R. MOF-derived alpha-NiS nanorods on graphene as an electrode for high-energy-density supercapacitors. J. Mater. Chem. A 2018, 6, 4003−4012. (11) Geng, P.; Zheng, S.; Tang, H.; Zhu, R.; Zhang, L.; Cao, S.; Xue, H.; Pang, H. Transition Metal Sulfides Based on Graphene for Electrochemical Energy Storage. Adv. Energy Mater. 2018, 8, 1703259. (12) Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X. Contactsbetween two-and threedimensional materials: ohmic, Schottky, and p−n heterojunctions. ACS Nano 2016, 10, 4895−4919. (13) Xu, L.; Lu, Y. One-step synthesis of a cobalt sulfide/reduced graphene oxide composite used as an electrode material for supercapacitors. RSC Adv. 2015, 5, 67518−67523. 3603

DOI: 10.1021/acsaem.9b00338 ACS Appl. Energy Mater. 2019, 2, 3595−3604

Article

ACS Applied Energy Materials (32) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution. Adv. Mater. 2013, 25, 5807−5813. (33) Yu, J.; Li, Q.; Li, Y.; Xu, C. Y.; Zhen, L.; Dravid, V. P.; Wu, J. Ternary Metal Phosphide with Triple-Layered Structure as a LowCost and Efficient Electrocatalyst for Bifunctional Water Splitting. Adv. Funct. Mater. 2016, 26, 7644−7651. (34) Zhang, Y.; Pan, A.; Wang, Y.; Cao, X.; Zhou, Z.; Zhu, T.; Liang, S.; Cao, G. Self-templated synthesis of N-doped CoSe2/C doubleshelled dodecahedra for high-performance supercapacitors. Energy Storage Mater. 2017, 8, 28−34. (35) Yang, J.; Yuan, Y.; Wang, W.; Tang, H.; Ye, Z.; Lu, J. Interconnected Co0.85Se nanosheets as cathode materials for asymmetric supercapacitors. J. Power Sources 2017, 340, 6−13. (36) Gong, C.; Huang, M.; Zhou, P.; Sun, Z.; Fan, L.; Lin, J.; Wu, J. Mesoporous Co0.85Se nanosheets supported on Ni foam as a positive electrode material for asymmetric supercapacitor. Appl. Surf. Sci. 2016, 362, 469−476. (37) Peng, H.; Ma, G.; Sun, K.; Zhang, Z.; Li, J.; Zhou, X.; Lei, Z. A novel aqueous asymmetric supercapacitor based on petal-like cobalt selenide nanosheets and nitrogen-doped porous carbon networks electrodes. J. Power Sources 2015, 297, 351−358. (38) Banerjee, A.; Bhatnagar, S.; Upadhyay, K. K.; Yadav, P.; Ogale, S. Hollow Co0.85Se nanowire array on carbon fiber paper for high rate pseudocapacitor. ACS Appl. Mater. Interfaces 2014, 6, 18844−18852. (39) Xu, J.; Sun, Y.; Lu, M.; Wang, L.; Zhang, J.; Tao, E.; Qian, J.; Liu, X. Fabrication of the porous MnCo 2 O 4 nanorod arrays on Ni foam as an advanced electrode for asymmetric supercapacitors. Acta Mater. 2018, 152, 162−174. (40) Li, M.; Yang, W.; Li, J.; Feng, M.; Li, W.; Li, H.; Yu, Y. Porous layered stacked MnCo2O4 cubes with enhanced electrochemical capacitive performance. Nanoscale 2018, 10, 2218−2225. (41) Jia, H.; Song, Y.; Wu, J.; Fu, W.; Zhao, J.; Liu, X. A novel Pdoped MnCo2S4 nanoneedles assembled dandelion-like structure for high performance hybrid supercapacitors. Mater. Lett. 2018, 233, 55− 58. (42) Chen, Y. M.; Li, Z.; Lou, X. W. General Formation of MxCo3‑xS4 (M = Ni, Mn, Zn) Hollow Tubular Structures for Hybrid Supercapacitors. Angew. Chem., Int. Ed. 2015, 54, 10521−10524. (43) Xu, P.; Zeng, W.; Luo, S.; Ling, C.; Xiao, J.; Zhou, A.; Sun, Y.; Liao, K. 3D Ni-Co selenide nanorod array grown on carbon fiber paper: towards high-performance flexible supercapacitor electrode with new energy storage mechanism. Electrochim. Acta 2017, 241, 41− 49. (44) Zhai, Z.-B.; Huang, K.-J.; Wu, X. Superior mixed Co-Cd selenide nanorods for high performance alkaline battery-supercapacitor hybrid energy storage. Nano Energy 2018, 47, 89−95. (45) Li, L.; Zhang, Y. Q.; Liu, X. Y.; Shi, S. J.; Zhao, X. Y.; Zhang, H.; Ge, X.; Cai, G. F.; Gu, C. D.; Wang, X. L.; Tu, J. P. Onedimension MnCo2O4 nanowire arrays for electrochemical energy storage. Electrochim. Acta 2014, 116, 467−474. (46) Kuang, L.; Ji, F.; Pan, X.; Wang, D.; Chen, X.; Jiang, D.; Zhang, Y.; Ding, B. Mesoporous MnCo2O4.5 nanoneedle arrays electrode for high-performance asymmetric supercapacitor application. Chem. Eng. J. 2017, 315, 491−499. (47) Liu, S.; Jun, S. C. Hierarchical manganese cobalt sulfide core− shell nanostructures for high-performance asymmetric supercapacitors. J. Power Sources 2017, 342, 629−637.

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