Formation of Micron-Sized Nickel Cobalt Sulfide ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9945-9954

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Formation of Micron-Sized Nickel Cobalt Sulfide Solid Spheres with High Tap Density for Enhancing Pseudocapacitive Properties Li Su,†,‡ Lijun Gao,‡ Qinghua Du,‡ Liyin Hou,‡ Xucai Yin,‡ Mengya Feng,‡ Wang Yang,‡ Zhipeng Ma,‡ and Guangjie Shao*,†,‡ †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

‡

S Supporting Information *

ABSTRACT: The synthesis of nickel cobalt sulfide is wellestablished because of its good electrical conductivity and great structural flexibility with low cost. It is extremely challenging to fabricate a unique structure with high tap density to improve its volumetric energy density and practical application. Here, we report a simple one-step hydrothermal method to synthesize micron-sized Ni−Co mixed sulfide solid spheres (1−2 μm in diameter). Because of the high tap density of more than 1.0 g cm−3 and the unique structure, the Ni−Co mixed sulfides demonstrate exceptional energy-storage performance. When acting as an electrode material for supercapacitors, these Ni−Co solid spheres can deliver a specific capacitance of 1492 F g−1 at a current density of 1.0 A g−1, and a retention of 76% at 10 A g−1 after 10 000 cycles. An asymmetric supercapacitor based on these solid spheres exhibits a high energy density of 48.4 W h kg−1 at a power density of 371.2 W kg−1 with excellent long-term cycling performance (91% retention of the initial specific capacitance at 5 A g−1 after 20 000 cycles). All the experiment results illustrate that the micron-sized solid-sphere Ni−Co mixed sulfides will be a promising electrode material for high-performance supercapacitors. KEYWORDS: NiCo2S4 solid sphere, Micron-sized, Tap density, Long-term cycling performance, Supercapacitors

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(e.g., CoS,18,19 MoS2,20 NiS,21 etc.) have been viewed as the promising electrode materials for supercapacitors because of their good electrochemical activity and conductivity. However, they possess some shortcomings such as low specific capacity at high current densities and poor cycling stabilities. To overcome these problems, it is necessary to prepare mixed metal sulfides with richer redox sites, higher capacitance, and improved electrical conductivity compared with single transition metal sulfides and transition metal oxides.22−24 Among the ternary metal sulfides, NiCo2S4 has drawn great attention owing to its excellent electrochemical performance.25,26 Recently, huge efforts have been devoted to developing the morphologies of NiCo2S4 electrodes which mainly focus on the nanosheet and nanotube structures.27,28 However, when used as electrodes, it is easy for these produced nanomaterials to reunite and reflect a poor tap density, which limits their practical application. On the other hand, the previous synthesis routes of NiCo2S4 are relatively complicated and require high-temperature thermal

INTRODUCTION Extensive research efforts have been made to explore renewable and sustainable energy-storage devices to adapt to the rapid ever-increasing demand for our daily life.1−3 Because of their prominent properties such as high specific capacity, long cycle life, and environmental friendliness, supercapacitors have attracted significant attention in recent years.4−7 On the basis of the energy-storage mechanism, supercapacitors can be divided into two types, i.e., electrostatic double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs are composed of carbon-based materials, which store energy from the electrostatic charge accumulation at the electrode/electrolyte interface.8,9 Electrode materials for pseudocapacitors including transition metal oxides (TMOs) and conducting polymers (CPs) make use of the fast and reversible Faraday reaction occurring at the electrode surface, resulting in a higher specific capacitance than that of EDLCs.10−12 However, pseudocapacitors suffer from poor stabilities owing to the structure collapse during the charge−discharge process.13,14 As we all know, the properties of the electrode materials are the critical factors to the performance of the supercapacitors.15−17 For the past few years, single transition metal sulfides © 2017 American Chemical Society

Received: June 13, 2017 Revised: September 28, 2017 Published: October 11, 2017 9945

DOI: 10.1021/acssuschemeng.7b01906 ACS Sustainable Chem. Eng. 2017, 5, 9945−9954

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Fabrication process of the Ni−Co mixed sulfide solid-sphere structure. ρ = m/V

conditions. For example, Lou and co-workers synthesized hollow nanofibers, ball-in-ball hollow spheres, and onionlike NiCo2S4 particles via ion exchange;29−31 also, many other researchers prepared NiCo2S4 materials by sulfurating the Ni− Co precursor with Na2S,32−34 which makes the synthesis of NiCo2S4 time-consuming and costly. Therefore, it is essential to build a unique morphology with high tap density and splendid electrochemical performance through a cost-efficient method as a potential electrode material. Herein, we construct a micron-sized Ni−Co mixed sulfide solid sphere with a high tap density of more than 1.0 g cm−3 which acts as an outstanding electrode material through a simple one-step hydrothermal procedure. In the process of the reaction, the addition of the thiourea is so crucial for the morphology. The Ni−Co mixed sulfides synthesized with appropriate thiourea delivered a good rate capacity and excellent long-term cycling stability, because of the micronsized solid-sphere structure with higher tap density than other morphologies including nanowires, nanosheets, and hollow structures. It is also noted that NiCo2S4 possesses nice electrical conductivity, richer redox sites, and high theoretical capacitance, and achieves remarkable specific capacitance and low impedance. Concerning this, Ni−Co mixed sulfides may be regarded as a promising electrode material in supercapacitor fields.

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(1) −3

where ρ is the tap density (g cm ), m is the mass of the material (g), and V is the volume after the material is vibrated (cm3). Electrochemical Study. The main electrochemical tests were as follows. Electrochemical tests were carried out in a three-electrode system. A nickel foam coated with Ni−Co mixed sulfides was used as the working electrode, active carbon (AC) as the counter electrode, and a Hg/HgO electrode as the reference electrode. Our working electrode was prepared with a mass ratio of 80:10:10 by mixing the Ni−Co mixed sulfide powder as the active material, acetylene black as the conductive additive, and polytetrafluoroethylene (PTFE) as the binder, respectively. First, PTFE was dropped into ethanol, after which 8 mg of Ni−Co mixed sulfide powder and 1 mg of acetylene black were added into the above solution. Then, ultrasonic vibration was performed for 15 min to disperse the mixed solution. Subsequently, the as-prepared mixed slurry was coated onto a piece of Ni foam (1 cm × 1 cm), followed by the electrode being dried at 80 °C for 12 h. Then, the electrode was pressed at a pressure of 10 MPa. The typical loading mass of the as-prepared electrode material is 2.0 mg cm−2. The same method for the preparation of the counter electrode was used, also mixing AC, acetylene black, and PTFE with the same weight ratio of 80:10:10 to smear onto a nickel foam (2.5 cm × 2.5 cm). All of the electrochemical measurements were performed in a 6 M KOH aqueous solution at room temperature. NEWARE (Shenzhen, China) analysis, as the galvanostatic charge−discharge testing system, was performed at different current densities (0−0.55 V versus Hg/HgO). The CHI 660E electrochemical workstation was operated for the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) tests. In addition, CV measurements were carried out from 0 to 0.55 V at different scanning rates, and the EIS test was within a frequency range 0.01−100 000 Hz. The specific capacitance of the electrode material was obtained according to the charge−discharge test, with the following equation:

EXPERIMENTAL SECTION

Synthesis of Micron-Sized Ni−Co Mixed Sulfides. All the analytical reagents were used without further purification. First, 1 mmol of Ni(NO3)2·6H2O, 2 mmol of Co(NO3)2·6H2O, and 4 mmol of thiourea were prepared carefully, and then dissolved into a 40 mL mixture of deionized water and ethylene glycol (volume ratio is 1:1) with vigorous stirring to form a homogeneous solution. Then, the mixed solution was put into a Teflon-lined autoclave (80 mL), and kept in an oven with the temperature of 220 °C for 12 h. After some time when the autoclave cooled off, it was taken out. Then, the sample was washed with deionized water and absolute ethanol several times to remove excess impurities, and finally dried at 80 °C overnight. The obtained sample was marked Ni−Co mixed sulfides-4. Moreover, the Ni−Co mixed sulfides-2 and Ni−Co mixed sulfides-8 were obtained by the same process with 2 and 8 mmol of thiourea, respectively. Material Characterization. X-ray powder diffraction (XRD; RigakuSmart Lab, X-ray diffractometer) was performed to study the crystalline structure of the products. X-ray photoelectron spectroscopy (XPS; Thermal ESCALAB 250) was used to analyze the chemical binding energy of samples. The morphology and structural properties of the materials were characterized by field-emission scanning electron microscopy (FE-SEM; Carl Zeiss Super55 operated at 5 kV) and fieldemission transmission electron microscopy (TEM; Hitachi HT7700 operated at 120 kV), respectively. The tap density of the materials was measured by vibrating the glass cylinder until the surfaces of the samples did not fall off; then, the scale of the glass cylinder was read, and finally, the tap density was calculated by the following formula:

Csp = I Δt /(mΔV )

(2) −1

where Csp is the specific capacitance (F g ), I is the charge−discharge current (A), Δt is the discharge time (s), ΔV is the charge−discharge potential window (V), and m is the mass of the active material (g). Asymmetric Supercapacitor (ASC) Devices. The ASC devices were fabricated by using the Ni−Co mixed sulfides-4 electrode as the cathode and an AC electrode as the anode in 6 M KOH aqueous electrolyte, with one piece of cellulose paper as the separator. The mass ratio of negative electrode and positive electrode was determined by the charge balance as follows: q+ = q− (3)

q = mC ΔV

(4)

m+ /m− = (C −ΔV −)/(C+ΔV+)

(5)

where q is the charge, m is the mass of the electrode material, C is the specific capacitance in the three-electrode system, and ΔV is the voltage window of positive and negative electrodes, separately. The specific capacitances of Ni−Co mixed sulfides-4 and the AC were calculated to be 1492 and 204 F g−1, respectively. Therefore, the 9946


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Figure 2. (a) XRD patterns. XPS spectra of (b) Ni 2p, (c) Co 2p, and (d) S 2p of the Ni−Co mixed sulfide solid-sphere structure. optimized mass loading of the AC was estimated to be 15 mg after balancing the charge capacity on 2 mg of Ni−Co mixed sulfides-4. The specific capacity (C), the energy density (E), and the power density (P) were calculated using the following equation:

C = I Δt /(M ΔV )

(6)

E = (1/2)CV 2

(7)

from Ni−Co mixed sulfides-2 can be successfully identified as tetragonal Ni3(NO3)2(OH)4 (JCPDS 22-0752) marked by the olive star, which is caused by the absence of thiourea and the combination of Ni(NO3)2·6H2O and ethylene glycol. As we know, Ni2+ precipitates more preferentially because of the different solubility of metal hydroxides (the solubility of Ni hydroxide is lower than that of Co hydroxide). The precipitation rates of Ni2+ and Co2+ under the same alkaline solution were also not equal.37 As a consequence, under the competitive coprecipitation, Ni3(NO3)2(OH)4 formed first. Meanwhile, the XRD pattern of Ni−Co mixed sulfides-4 with poor crystallinity shows a few extra peaks marked by a violet five-pointed star, which can be approximately indexed to the tetragonal Co1−xS phase (JCPDS 42-0826) although the peak intensity is relatively weak. In addition, the peak at about 2θ = 47° is higher than that at about 2θ = 51°, which is caused by the potentiation of Co1−xS and NiCo2S4. The corresponding XRD pattern of Ni−Co mixed sulfides-8 is readily indexed to the cubic NiCo2S4 phase (JCPDS 20-0782) with no residues or other phases detected. X-ray photoemission spectroscopy (XPS) results of the Ni−Co mixed sulfides-4 are also presented in Figure 2b−d. In addition, for a comparison with the pure NiCo2S4, the XPS analysis of Ni−Co mixed sulfides-8 is also performed as shown in Figure S1. In the high-resolution XPS spectrum of the Ni 2p region (Figure 2b), two spin−orbit doublets can be deconvoluted by using the Gaussian fitting method; the binding energy situated at 855.7 eV in Ni 2p3/2 and 872.7 eV in Ni 2p1/2 corresponds to the characteristics of Ni2+. However, peaks at 856.8 eV in Ni 2p3/2 and 875.3 eV in Ni 2p1/2 symbolize Ni3+,38 which is in agreement with pure NiCo2S4 (Figure S1a). Regarding the Co 2p region (Figure 2c), it shows two distinguished doublets located at a low energy band (Co 2p3/2) and a high energy band (Co 2p1/2). The binding energies at around 781.1 and 794.9 eV of the Co 2p peaks are assigned to Co2+, and those at 777.8 and 792.8 eV are

(8)

P = E /Δt −1

where C is the total cell discharge specific capacitance (F g ), and M is the total mass of active materials on both electrodes (g).

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RESULTS AND DISCUSSION Morphological and Structural Characterization. Figure 1 illustrates the schematic of our synthesis approach of the Ni− Co mixed sulfide solid sphere. In the process of thiourea hydrolyzation, ammonia and sulfur sources (S2−) were generated, and the alkaline environment was developed.35 Afterward, the Ni2+ and Co2+ are coprecipitated to form Ni−Co hydroxides, and then, S2− in solution reacted with Ni−Co hydroxides which were further converted to NiCo2S4.30,36 The synthetic strategy is so simple in that it simplified the previous experimental procedure by combining the two steps of chemical coprecipitation and sulfidation, which makes it timesaving and low-cost. Furthermore, the morphology of NiCo2S4 can be well-controlled by tuning the amount of thiourea. Figure 2a presents the XRD pattern of Ni−Co mixed sulfides-2, Ni−Co mixed sulfides-4, and Ni−Co mixed sulfides8, which indicates that diffraction peaks of the as-derived product can be approximately indexed to the cubic NiCo2S4 (JCPDS 20-0782, space group: Fd3̅m(227), a = 9.387, b = 9.387, c = 9.387), especially Ni−Co mixed sulfides-8. However, it can be seen that the peaks of Ni−Co mixed sulfides-2 and Ni−Co mixed sulfides-4 show some difference from the standard card. Obviously, all of these extra diffraction peaks 9947


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Figure 3. SEM and TEM images of Ni−Co mixed sulfide solid-sphere structures with different additions of thiourea: (a−c) Ni−Co mixed sulfides-2, (d−f) Ni−Co mixed sulfides-4, and (g−i) Ni−Co mixed sulfides-8. (j) Energy-dispersive X-ray spectroscopy (EDS) mapping image of Ni−Co mixed sulfides-4.

characteristics of Co3+.39 Compared with the Co 2p region of pure NiCo2S4 (Figure S1b), a slight deviation happens in its Co 2p1/2 signature, which may be due to the presence of Co1−xS. However, the Co 2p3/2 signature is close to that of NiCo2S4, suggesting similar valence states and ion distribution as in NiCo2S4.40 Figure 2d shows the S 2p region. In detail, the binding energy of 162.2 eV (S 2p1/2) is associated with sulfur− metal bonds (Ni−S and Co−S bonding). In addition, 161.0 eV (S 2p3/2) was also detected which implies the S2− at the surface of the material.36,41,42 Additionally, the high energy of 168.7 eV is attributed to the S4+ species at the surface and/or edges of Ni−Co mixed sulfides-4 with a highly oxidized state.43 Moreover, the spectrum of the S 2p region is also similar to that of Ni−Co mixed sulfides-8 (Figure S1c), which indicates that Co1−xS shows similar valence states as in pure NiCo2S4.44 According to the XPS analysis, we can estimate that the nearsurface constituent of the Ni−Co mixed sulfides-4 sample includes Ni2+/Ni3+, Co2+/Co3+, and S2−. Figure 3 presents the SEM images of samples prepared with different additions of thiourea by the hydrothermal process, and there are some differences among these morphologies. From Figure 3a,b, we can see that the like-spheres consist of massive particles, and the surface of the agglomerate is fairly rough. Simultaneously, few nanorods or nanotubes exist in the Ni−Co mixed sulfides2, which may be caused by the absence of thiourea. Figure 3d,e shows the morphology of Ni−Co mixed sulfides-4. There are uniform round spheres with an average size of about 2 μm and which are independent of each other, which contributes to diffusion and transmission of the active materials in the electrolyte. In addition, close observation shows that there are some mild textures composed of nanoparticles on the surface of

each round sphere, as the inset of Figure 3e shows, which is critical to the increase of efficient redox reactions with abundant electrochemical active sites, the enlargement of the material−electrolyte contact area, and the decrease of the electrolyte ion transportation path in the process of charging and discharging. Figure 3g,h demonstrates the morphology of Ni−Co mixed sulfides-8. As we have seen, the sphere structure remains, but the rough spheres occur with slight adhesion to each other, which does not contribute to the ion diffusion in the electrolyte. Moreover, as illustrated in Figure 3h, the surface becomes rough because of the increasing amount of thiourea. For further indication of the uniform distribution of elements for Ni−Co mixed sulfides-4, the energy-dispersive X-ray spectroscopy (EDS) mapping for the selected area of the sample is shown in Figure 3j, which presents a uniform distribution of elements Ni, Co, and S within the Ni−Co mixed sulfides-4. For further revelation of the detailed morphology of the samples, the TEM of Ni−Co mixed sulfides was analyzed (Figure 3c,f,i). We can see that all of these spheres are solid, in contrast; this enhances the tap density and volumetric energy density of the electrode. Three-Electrode Electrochemical Evaluation. For measurement of the electrochemical performance of these micronsized NiCo2S4 solid spheres as an active supercapacitor electrode, the galvanostatic charging−discharging, CV, and EIS tests were investigated in a standard three-electrode system with 6 M KOH aqueous electrolyte, which uses Hg/HgO and AC as the reference and counter electrodes, respectively. At the same scan rate of 5 mV s−1 (Figure 4a), all of the electrodes exhibit a similar Faradaic charge-storage mechanism, and the Ni−Co mixed sulfides-4 electrode exhibits a dramatically 9948


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Figure 4. CV curves of (a) Ni−Co mixed sulfides-2, Ni−Co mixed sulfides-4, and Ni−Co mixed sulfides-8 at 5 mV s−1, and (b) Ni−Co mixed sulfides-4 under different scan rates in 6 M KOH. Charge−discharge curves of (c) Ni−Co mixed sulfides-2, Ni−Co mixed sulfides-4, and Ni−Co mixed sulfides-8 at 1 A g−1, and (d) Ni−Co mixed sulfides-4 at different current densities. (e) Specific capacitance of Ni−Co mixed sulfides-2, Ni− Co mixed sulfides-4, and Ni−Co mixed sulfides-8 at different current densities. (f) EIS curves of the Ni−Co mixed sulfides-2, Ni−Co mixed sulfides4, and Ni−Co mixed sulfides-8.

densities from 1 to 20 A g−1 as shown in Figure 4d. It is suggested that the distinct plateau regions in the curves can further confirm the intriguing pseudocapacitive properties of the Ni−Co mixed sulfides-4 once again. As shown in Figure 4e, the specific capacitances of the Ni−Co mixed sulfides-2, Ni− Co mixed sulfides-4, and Ni−Co mixed sulfides-8 were calculated with different current densities. The specific capacitance of Ni−Co mixed sulfides-4 is 1492.36, 1407.27, 1240.36, 1091.13, and 909.16 F g−1 at current densities of 1, 2, 5, 10, and 20 A g−1, respectively. About 61% of the capacitance can be retained with the increased current density, suggesting good rate performance of these solid spheres. Compared with Ni−Co mixed sulfides-4, the Ni−Co mixed sulfides-2 and Ni− Co mixed sulfides-8 deliver a similar rate capability but lower specific capacitance. In general, the pseudocapacitive performance is mostly dependent on the surface chemistry and structure of the electrode materials, which results from the efficient redox reactions with abundant electrochemical active sites and high reactivity of the surface.45−47 The excellent capacitance of the Ni−Co mixed sulfides-4 as compared to those of the other two is due to the independent solid spheres with more active sites exposed, which enlarge the material−

expanded CV integrated area and higher current density with respect to those of the Ni−Co mixed sulfides-2 electrode and Ni−Co mixed sulfides-8 electrode, which suggests that the Ni− Co mixed sulfides-4 electrode facilitates electron transport and enhanced the electrochemical utilization of NiCo2S4. Figure 4b illustrates a series of CV curves of the Ni−Co mixed sulfides-4 electrode at various sweep rates of 5, 10, 50, 100, and 200 mV s−1 with potential window 0−0.55 V (versus Hg/HgO). Obviously, all of the CV curves show a set of redox peaks. These peaks point to typical redox reactions of Ni(II)/Ni(III) and Co(II)/Co(III) conversion in accordance with XPS, which stands for the faradaic behavior of the battery-type electrodes. In addition, with the current density of 1 A g−1, the charge− discharge curves of the three products are displayed in Figure 4c. Obviously, Ni−Co mixed sulfides-4 has the longest discharge time of 843 s when the current density is 1 A g−1 (within a voltage window 0−0.55 V), which represents the higher specific capacitance of 1492 F g−1 as compared to those of Ni−Co mixed sulfides-2 (1016 F g−1) and Ni−Co mixed sulfides-8 (884 F g−1). Galvanostatic charging−discharging tests were performed to investigate the excellent electrochemical behavior of Ni−Co mixed sulfides-4 at a series of current 9949


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Figure 5. Long-term cycle performance of Ni−Co mixed sulfides-4 at 10 A g−1. [Inset shows (a) SEM and (b) EIS results of Ni−Co mixed sulfides-4 after the stability test.]

migrate when v is infinitesimal, and the capacitance results from both pseudocapacitance and electrolyte ion insertion/extraction (Figure S2a). Thus, we can calculate from the inset of Figure S2a that the maximum specific capacity of Ni−Co mixed sulfides-2, Ni−Co mixed sulfides-4, and Ni−Co mixed sulfides8 is 1960.78, 2008.03, and 1587.30 F g−1, respectively. On the other hand, once ion diffusion is the control step when v is infinite, the capacitance mainly derives from the surface pseudocapacitance (Figure S2b), which is calculated from the inset of Figure S2b that the minimum specific capacity of Ni− Co mixed sulfides-2, Ni−Co mixed sulfides-4, and Ni−Co mixed sulfides-8 is 478.56, 1210.26, and 581.67 F g−1, respectively. As a consequence, the surface capacitance occupies 60.27% of the total capacitance for the Ni−Co mixed sulfides-4 electrode, which is higher than that of Ni−Co mixed sulfides-2 (24.41%) and Ni−Co mixed sulfides-8 (36.65%). It thus appears that the electrochemical reaction mostly occurs on the electrode surface, and the capacitance of Ni−Co mixed sulfides4 mainly ascribes to pseudocapacitance resulting from the redox reaction on the surface of the material. Hence, it displays outstanding specific capacitance and good rate performance. Additionally, Figure 4f shows the EIS measurement, in the low-frequency area; the three products display similar impedance spectra with a straight line and also with a single semicircle in the high frequency. As we all know, the chargetransfer resistance (the semicircle at the real axis) stands for the resistance of the electrochemical reaction at the electrode and electrolyte interface. On the other hand, the Warburg impedance (the slope of the curves) reveals the electrolyte diffusion in the electrode.52 It can be seen that the chargetransfer resistances of the Ni−Co mixed sulfides-2 and Ni−Co mixed sulfides-4 are similar and smaller than that of the Ni−Co mixed sulfides-8. Meanwhile, the Ni−Co mixed sulfides-4 shows a more vertical line than Ni−Co mixed sulfides-2 and Ni−Co mixed sulfides-8, implying the lower diffusion resistance. This is because the appropriate size of these independent solid spheres contributes to diffusion and transmission of the active materials in the electrolyte. Moreover, a repeated charging−discharging test at a current density of 10 A g−1 is conducted to study the long-term cycling stability of the Ni−Co mixed sulfides-4 electrode, as presented in Figure 5. The capacitance increases to 1073 F g−1 in the first 200 cycles, which is caused by the enlarged effective interfacial area at the electrode and electrolyte when the electrode is gradually activated.53 Impressively, the specific capacitance still reaches 800 F g−1 at 10 A g−1, and about 76% of the initial capacitance is preserved after 10 000 cycles, revealing that the

electrolyte contact area and provide efficient redox reactions in the process of charging and discharging. The higher pseudocapacitance contributed mostly from the use of surface active materials promotes more excellent electrochemical performance (Figure S2). In addition, we analyzed the pseudocapacitance contribution with the CV method from the perspective of electrochemistry.48−50 The capacitance of transition metal oxides mainly consists of electric double layer capacitance (EDLC), pseudocapacitance resulting from the redox reaction on the material surface, and capacitance of electrolyte ion insertion/extraction. In addition, the EDLC is so small that it can be ignored. These capacitive contributions were obtained by analyzing the CV tests at a set of sweep rates as follows:51 i = avb

(9)

where the i follows a power law relationship with the sweep rate v. Both a and b are adjustable parameters, with b-values determined from the slope of the plot of log i versus log v. For b = 0.5, the current is proportional to the square root of the scan rate v, according to the following equation:50 i = nFAC × D1/2v1/2(αnF /RT )1/2 π 1/2χ (bt )

(10)

The current response is diffusion controlled, which is indicative of a faradaic intercalation process. For b = 1.0, the capacitive current is proportional to the sweep rate, according to the following equation:51

i = vCdA

(11)

This is representative of a surface capacitive effect. A closer examination of the voltammetric sweep rate dependence is used for quantitatively distinguishing the capacitive contribution to the current response. Through the theories above, we can combine two separate mechanisms, surface capacitive effects and diffusion-controlled insertion processes, to express the current response at a fixed potential. Thus, the relationship of response current (i) and scanning speed (v) in the CV test is available from the following: i(v) = k1v1/2 + k 2v

(12) 1/2

In the formula, k1 and k2 are constants. k1v and k2v represent the current contributions from the diffusion process and the surface capacitive effects, respectively. The process of electrolyte ion insertion/extraction is related to v1/2, while the redox reaction on the material surface is related to v. Therefore, according to eq 12, the curve of v1/2 and the discharge specific capacitance shows that electrolyte ions have plenty of time to 9950


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Figure 6. (a) Comparative CV curves of the AC and the Ni−Co mixed sulfides-4 electrodes performed in a three-electrode cell in 6 M KOH aqueous solution at a scan rate of 10 mV s−1. (b) CV curves of the Ni−Co mixed sulfides-4//AC ASC at various scan rates. (c) Galvanostatic charge−discharge curves of the asymmetric supercapacitor at different current densities. (d) Ragone plot related to energy and power densities of the Ni−Co mixed sulfides-4//AC ASC. (e) Long-term cycle performance of the Ni−Co mixed sulfides-4//AC ASC at 5 A g−1.

Electrochemical Characterization of the ASC. For further evaluation of the practical application of these NiCo2S4 solid spheres, an ASC device is constructed by using the Ni−Co mixed sulfides-4 electrode as the cathode and an AC electrode as the anode in 6 M KOH aqueous electrolyte, with one piece of cellulose paper as the separator. Additionally, for estimation of the appropriate potential window of Ni−Co mixed sulfides-4 and AC electrodes, CV measurements were performed with a standard three-electrode system in an aqueous solution of 6 M KOH. The good rate capability of pure AC is confirmed by CV curves as a function of scan rate and galvanostatic charge−discharge curves at various current densities (Figure S3). The Ni−Co mixed sulfides-4 electrode was measured at a scan rate of 10 mV s−1 with a 0−0.55 V potential window (versus Hg/HgO), and within −1.0 and 0 V (versus Hg/HgO) for the AC electrode. Therefore, the voltage window of the ASC was chosen to be 1.55 V in 6 M KOH from electrochemical behaviors of both electrodes (Figure 6a). In addition, for assembly of the ASC device, the mass ratio of the two electrode materials needs to be obtained according to each specific capacitance. Figure 6b shows CV curves of the ASC at sweep rates from 5 to 100 mV s−1 between 0 and 1.55 V. A combination of both battery-like and electrical double-layer capacitive characteristics can be seen at all sweep rates. The

as-prepared Ni−Co mixed sulfides-4 electrode holds excellent long-term cyclability. At the same time, the Coulombic efficiency was maintained at nearly 100%, showing an excellent reversibility of the Ni−Co mixed sulfides-4 electrode. Furthermore, the morphology and EIS results of Ni−Co mixed sulfides-4 after the stability test are shown in the inset of Figure 5. From Figure 5a, we can seen that some changes have taken place in morphology, but it still remains as that of a sphere. As the original SEM shows that there are some mild textures on the surface of each round sphere (in the inset of Figure 3e), the electrochemical recrystallization process may occur because of the repeated charge and discharge processes, which cause the surface of the material to be composed of cross-linked nanosheets. In addition, the material reveals a large surface area and better electrochemical performance under long cycling. The EIS measurement of the Ni−Co mixed sulfides-4 was measured before and after stability testing, as shown in Figure 5b; the EIS curve was similar before and after stability testing, indicating that no morphological defects were created in the Ni−Co mixed sulfides-4. Therefore, the Ni−Co mixed sulfides-4 reveals outstanding cycling stability. The electrochemical performance of the Ni−Co mixed sulfides-4 electrode in our work is still much more superior to most Ni−Co-based mixed sulfide electrodes reported previously (Table S1). 9951


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Figure 7. Digital image of a green light-emitting diode (LED) lit by two ASC cells in series connection.

method with the core reaction of the hydrolyzation of thiourea. Outstanding electrochemical performances for the supercapacitor were demonstrated, especially an amazing discharge specific capacitance, nice rate performance, and splendid longterm cycling stability. Moreover, the ASC device made of Ni− Co mixed sulfides-4 and AC was constructed and showed a high energy density and power density as well as outstanding cycling performance. The superior energy-storage performance of these Ni−Co solid spheres may benefit from the following: (i) Micron-sized solid structures with high tap density of more than 1.0 g cm−3 can effectively avoid the reunion phenomenon of nanomaterials and improve the volumetric energy density of the electrode. (ii) Compositions of nanoparticles of each round sphere can provide abundant electrochemical active sites for redox reaction on the material surface, and also the high pseudocapacitance contribution is beneficial to the outstanding electrochemical performance. (iii) Independent of appropriate size, solid spheres contribute to diffusion and transmission of the active materials in the electrolyte. Thus, we believe that the construction of a micron-sized solid sphere with high tap density as an advanced Ni−Co-based mixed sulfide electrode material is a significant idea for high-performance energystorage devices.

galvanostatic charge−discharge curves of the Ni−Co mixed sulfides-4//AC ASC at different current densities of 1, 2, 5, 10, and 20 A g−1 are displayed in Figure 6c. We can see that the ASC delivers a specific capacitance of 304 F g−1 at a current density of 1 A g−1, and the capacitance retention is about 64% with a 10-fold increase in the current density (Figure S4). Furthermore, A Ragone plot, as shown in Figure 6d, was displayed to demonstrate the relationship between the energy and power density of the ASC. An energy density of 48.4 W h kg−1 can be achieved at a power density of 371.2 W kg−1, and an energy density of 26.3 W h kg−1 can still be retained at a high power density of 6309.9 W kg−1. The performance is also compared with the previous reports about Ni−Co-based ASC devices, such as HM-NCS//AC (28.9 W h kg−1 at 188 W kg−1),54 NiCo2S4/CFP//AC (17.3 W h kg−1 at 180 W kg−1),55 NiCo2S4//C (22.8 W h kg−1 at 160 W kg−1),56 and NiCo2S4// RGO (31.5 W h kg−1 at 156.6 W kg−1).57 Figure 6e shows the long-term cycling stability of the NiCo2S4//AC ASC at a current density of 5 A g−1. It still achieved 91% capacitance retention even enduring 20 000 long cycles, suggesting excellent stability of the ASC device. For further evaluation of the practical application of the Ni−Co mixed sulfides-4//AC ASC, a green light-emitting diode is lit for 15 min with two cells in a series connection, as presented in Figure 7. The ASC device assembled by the unique Ni−Co mixed sulfides-4 and AC electrode proves to have great value for practical applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01906. Additional XPS and electrochemical performance test results (PDF)

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CONCLUSION In summary, micron-sized Ni−Co mixed sulfide solid spheres have been synthesized via a simple one-step hydrothermal 9952


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0086-335-8061569. Fax: 0086-335-8059878. ORCID

Zhipeng Ma: 0000-0003-1394-9795 Guangjie Shao: 0000-0001-6957-4828 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the Natural Science Foundation of China (51674221), the China Postdoctoral Science Foundation Funded Project (2016M591405), Youth Scholars Research Fund of Yanshan University (16GA012), and Science and Technology Research and Development Program of Qinhuangdao (201602A004).

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Formation of Micron-Sized Nickel Cobalt Sulfide ... - ACS Publications

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