Enhanced cycle stability of a NiCo2S4 nanostructured electrode for supercapacitors fabricated by the alternate-dip-coating method.

Nanostructured nickel cobalt sulfide (NiCo2S4) electrodes are successfully fabricated using a simple alternate-dip-coating method. The process involves dipping a TiO2 nanoparticles-covered substrate in a nickel/cobalt precursor solution and sulfur precursor solution alternately at room temperature. The fabricated bimetallic sulfide electrode exhibits a synergetic improvement compensating for the disadvantages of the two single metal sulfide electrodes, i.e. the poor cycle stability of the nickel sulfide electrode and the low specific capacitance (Csp) of the cobalt sulfide electrode. The two capacitive properties are optimized by adjusting the ratio of nickel and cobalt concentrations in the metal precursor solution, reaching a Csp of 516 F g-1 at a current density of 1 mA cm-2, with its retention being 99.9% even after 2000 galvanostatic charge-discharge cycles.

Introduction

Supercapacitors have attracted increasing attention as a next-generation energy storage device because of their excellent capacitive characteristics such as a rapid charge–discharge process, large specific capacitance (Csp) and high power density [1-3]. Supercapacitors are generally categorized into two types according to their energy storing mechanism, electrical double layer capacitors (EDLCs) and pseudocapacitors [2-4]. Recently, the pseudocapacitors using fast Faradaic charge-transfer reactions have been studied extensively due to their superior capacitive performances compared to EDLCs. The electrode materials generally used for the pseudocapacitors include transition metal oxides (TMOs) and conductive polymers [5]. Transition metal sulfides (TMSs) have also been studied recently because of their superior capacitive properties to corresponding TMOs such as high mechanical and thermal stability, high electrical conductivity and rich redox reactions [6]. Among the various TMSs, nickel sulfide (NiSx) has been the most widely studied due to its large theoretical capacitance, high electrical conductivity, eco-friendly properties and affordable prices [7-11]. However, easy agglomeration and pulverization while repeating the charge–discharge process hamper the practical application of NiSx electrodes [7,12]. The nanostructuring of the electrodes is therefore essential; however, conventional nanostructuring techniques such as hydrothermal or solvothermal processes require harsh reaction conditions in an autoclave at high temperature for a long time [8,13-16]. To solve these problems, we recently introduced a significantly simpler nanostructuring technique, the so-called alternate-dip-coating method for fabricating nanostructured NiS electrodes [17]. The fabricated NiS nanostructured electrode exhibited a significantly improved specific capacitance and voltammetric response compared to the NiS planar film electrode. However, the Csp retention of the NiS nanostructured electrode after 1000 charge–discharge cycles was still only approximately 60%. This low cycle stability is the intrinsic problem of NiS electrodes that needs to be overcome for their practical use [18-20]. On the other hand, a cobalt sulfide (CoSx) electrode was reported to have considerably larger cycle stability although its specific capacitance was quite lower than that of the NiS electrode. We therefore expected that the combination of these two TMSs would lead to a synergetic enhancement in capacitive properties, especially in cycle stability. The improved capacitive properties of the NiCo2S4 electrodes compared to the single metal sulfide, i.e. the NiS and Co3S4, electrodes have recently been reported [21]. However, the NiCo2S4 nanostructured electrodes were also fabricated under harsh fabrication conditions, mostly using a hydrothermal or solvothermal technique [21-23].

In this work, the nanostructured bimetallic nickel cobalt sulfide (NiCo2S4) electrode was more simply and successfully fabricated by an alternate-dip-coating method and its electrochemical properties were investigated. While the specific capacitance of the NiCo2S4 electrode was positioned in between the Csp values of the NiS and Co3S4 electrodes, the cycle stability was dramatically improved, exhibiting over 99% Csp retention even after 2000 charge–discharge cycles.

Material and methods

Fabrication of nanostructured metal sulfide electrodes

Nickel acetate (Ni(Ac)2, 99%), cobalt acetate (Co(Ac)2, 99%) and sodium sulfide (Na2S, 98%) were purchased from Aldrich. The process for the alternate-dip-coating method and fabrication of the nanostructured NiS electrodes are described elsewhere [17]. The nanostructured Co3S4 electrode was fabricated using the same process. Briefly, a fluorine-doped tin oxide (FTO)-coated glass substrate was subjected to UV-O3 cleaning for 5 min. A 200 nm thick porous TiO2 (p-TiO2) layer was formed on the cleaned substrate by spin-coating of commercial TiO2 nanoparticle (NP) paste (90-T, Dyesol) diluted in ethanol (1 : 6 weight ratio) at 2500 r.p.m., followed by annealing at 300°C for 1 h. The TiO2-deposited FTO substrate was placed in a 0.15 M Co(Ac)2 methanol/water solution and a 0.15 M Na2S methanol/water solution for 3 min each, washed with distilled water and blown with N2 gas. This alternate-dip-coating cycle was repeated seven times. For the fabrication of nanostructured NiCo2S4 electrodes, a 0.0375 M Ni(Ac)2 and 0.075 M Co(Ac)2 mixed solution was used as a metal precursor solution, and the other process was carried out in the same manner. Finally, the nanostructured metal sulfide electrodes were annealed at 200–350°C for 1 h. The deposit weights of the metal sulfides were determined by a quartz crystal microbalance (QCM, Stanford Research System QCM 2000).

Characterization

The crystalline structure and surface morphology of the pristine and TMS-coated p-TiO2 layers were characterized by X-ray diffraction (XRD, Philips PW1827) and a field emission scanning electron microscope (FE-SEM, JEOL JSM-7410F, JEOL Ltd), respectively. The electrochemical properties of the electrodes were evaluated by cyclic voltammetry (CV) and the galvanostatic charge–discharge (GCD) technique in a 2.0 M aqueous KOH solution at room temperature using a cyclic voltammeter (ZIVE SP2, WonATech). The measurements were performed in a three-electrode electrochemical cell in which the metal sulfide electrodes were used as a working electrode, a platinum plate was used as a counter electrode and Ag/AgCl (in 3.0 M KCl) was used as a reference electrode.

Results and discussion

First, the p-TiO2 layer was prepared by the spin-coating of TiO2 nanoparticles with an average diameter of 20 nm, followed by sintering at 300°C for 1 h. The crystalline structure of the fabricated p-TiO2 layer was confirmed as a rutile structure as shown in figure 1a. The metal sulfide thin films were formed on the p-TiO2 layer by the alternate-dip-coating method in a corresponding metal precursor solution and sulfur precursor solution for 3 min each. For the deposition of the single metal sulfide thin films, i.e. NiS and Co3S4 thin films, Ni(Ac)2 and Co(Ac)2 solution were used as a metal precursor solution, respectively. The bimetallic sulfide, i.e. NiCo2S4, thin films were also prepared aiming for the synergetic improvement of the two single metal sulfide electrodes. For this preparation, a solution containing both Ni(Ac)2 and Co(Ac)2 was used as a metal precursor solution. The concentration of each precursor was adjusted stoichiometrically. After finishing the alternate-dip-coating cycles, the electrode was annealed to enhance the crystallinity of the active materials. From XRD patterns, it was observed that the nickel sulfide (figure 1b) and cobalt sulfide (figure 1c) film were composed of α-phase NiS (JCPDS card no. 02-1280) and Co3S4 (JCPDS card no. 75-1561) crystallites, respectively. The bimetallic sulfide film is composed of mainly NiCo2S4 (JCPDS card no. 20-0782) and a small amount of Co3O4 (JCPDS card no. 42-1467) as shown in figure 1d. The annealing temperature of the NiCo2S4 electrodes was fixed at 350°C in this work because the annealing at a lower temperature led to less crystallinity of the film as shown in figure 1e,f. Surface FE-SEM images of the electrodes are shown in figure 2. Before the deposition of the metal sulfides, the TiO2 nanoparticles with an average diameter of 20 nm were clearly observed in figure 2a. The number of alternate-dip-coating cycles for the three metal sulfide electrodes was fixed at seven. Deposit weights estimated by QCM measurements for the NiS, Co3S4 and NiCo2S4 were approximately 33, 90 and 53 µg cm−2, respectively. It was observed that the surface and interspace of the p-TiO2 nanoparticles were almost covered with metal sulfides after seven cycles of the deposition for all three electrodes. The elemental SEM-mapping results for Ni and Co atoms are also shown in the insets of figure 2. The green and red colours represent the Ni and Co atoms, respectively. As shown in the inset of figure 2b, the nickel atoms were well spread over the p-TiO2 layer for the NiS electrode. Likewise, Co atoms were also well spread over the p-TiO2 layer for the Co3S4 electrode (figure 2c). In the case of the NiCo2S4 electrode, Ni and Co atoms were observed to be uniformly spread over the p-TiO2 layer, as shown in the inset of figure 2d.

Figure 1.

Powder XRD patterns of (a) pristine, (b) NiS-coated, (c) Co3S4-coated and (d) NiCo2S4-coated p-TiO2 nanoparticles layer. The annealing temperatures for the NiS, Co3S4 and NiCo2S4 layer were 300°C, 200°C and 350°C, respectively. The XRD patterns of NiCo2S4-coated p-TiO2 nanoparticles layers annealed at (e) 250°C and (f) 300°C are also shown.

Figure 2.

Surface FE-SEM images of (a) pristine, (b) NiS-, (c) Co3S4- and (d) NiCo2S4-coated p-TiO2 layer. The surface SEM-mapping images for the Ni (green) and Co (red) atoms are also shown in the insets of (b–d).

The electrochemical performance of the three metal sulfide electrodes was estimated by CV measurements in a 2.0 M KOH aqueous solution. The potential windows were set differently for each active material, i.e. 0–0.5 V for the NiS, 0–0.6 V for the Co3S4 and 0–0.55 V for the NiCo2S4 electrodes. Figure 3a–c shows CV curves of the three electrodes at various scan rates from 10 to 100 mV s−1. The areal capacitance (Careal) values were calculated with the following equation: where J (mA cm−2) is the current density, ΔV (V) is the voltage range and dV/dt (mV s−1) is the scan rate. As the scan rate increased, the oxidation and reduction peak shifted to a more positive and more negative potential, respectively, which is probably caused by increased polarization at the elevated scan rates [24,25]. Figure 3d shows the plots of the Careal values as a function of the scan rate for the three metal sulfide electrodes. At a scan rate of 10 mV s−1, the NiS-coated p-TiO2 electrode showed the highest Careal value of 59.7 mF cm−2. The Co3S4-coated p-TiO2 electrode had the lowest Careal value of 36.6 mF cm−2. In the case of the NiCo2S4-coated p-TiO2 electrode, the Careal value was 47.2 mF cm−2, which is approximately an average value of the two single metal sulfide electrodes. However, at a scan rate of 100 mV s−1, the NiCo2S4-coated p-TiO2 electrode showed a higher Careal value than those of the single metal sulfide electrodes. The Careal retention with respect to the value at the scan rate of 10 mV s−1 was 75.9%. In contrast, the NiS-coated p-TiO2 electrode showed the lowest Careal retention of 53.1%. The improved voltammetric response of the NiCo2S4 electrode is probably due to the higher electrical conductivity of bimetallic sulfides compared to corresponding single metal sulfides [26].

Figure 3.

CV curves measured at various scan rates for the (a) NiS, (b) Co3S4 and (c) NiCo2S4 electrodes. The Careal values calculated from the CV measurements are plotted as a function of the scan rate in (d).

The same tendency was also observed in the GCD measurement. The discharge curves for the three metal sulfide electrodes measured at various current densities are shown in figure 4a–c. The Csp values are calculated with the following equation: where I (A) is the discharge current, m (g) is the deposited weight of the metal sulfides, Δt (s) is the total discharge time and ΔV (V) is the voltage drop during the discharge [27]. The calculated Csp values of the NiS, Co3S4 and NiCo2S4 electrodes at a current density of 1 mA cm−2 are 896.9, 187.0 and 515.7 F g−1, respectively. As expected, the NiS electrode showed the highest specific capacitance, and the Csp value of the NiCo2S4 electrode was placed between the values of the NiS and Co3S4 electrodes. The plots of the calculated Csp values of the three metal sulfide electrodes at various current densities are shown in figure 4d. Similar to the rate capabilities obtained from the CV curves, the NiS-coated p-TiO2 electrode had the lowest Csp retention of 50.4% at a current density of 5 mA cm−2. At the same measurement condition, the Csp retention of the Co3S4 electrode was 79.2%. The Csp retention of the NiCo2S4 electrode, 65.0%, was positioned between the values of the two single metal sulfide electrodes. The NiCo2S4 electrode exhibited an energy density of 21.7 W h kg−1 at a power density of 5200 W kg−1, as calculated using the following formulae: and where Csp (F g−1) is the specific capacitance obtained from the GCD measurements at a current density of 1 mA cm−2, ΔV (V) is the applied potential window and Δt (s) is the discharge time [28,29].

Figure 4.

Galvanostatic discharge curves measured at various current densities for the (a) NiS, (b) Co3S4 and (c) NiCo2S4 electrodes. The Csp values calculated from the discharge curves are plotted as a function of the current density in (d).

The cycle stability of the metal sulfide electrodes was estimated by repeating continuous galvanostatic charge–discharge cycles at a constant current density of 3 mA cm−2. Figure 5 represents the Csp retentions of the three electrodes as a function of the number of GCD cycles. The NiS electrode had the lowest retention of approximately 66% after 2000 charge–discharge cycles. In contrast, the Co3S4 and NiCo2S4 electrodes had Csp values that barely changed during the 2000 cycles. This indicates that after 2000 charge–discharge cycles, the Csp value of the NiCo2S4 electrode exceeds the Csp value of the NiS electrode, although at the initial stage, the Csp value of the NiS electrode is approximately 1.5 times larger than that of the NiCo2S4 electrode. The superior electrochemical properties such as the electrical conductivity, specific capacitance and cycle stability of bimetallic sulfides to those of the corresponding single metal sulfides have also been reported previously [26,30,31]. Overall, the simple fabrication and synergetic improvements of the nanostructured NiCo2S4 electrodes demonstrated in this study can be applied to manufacturing efficient metal sulfide electrodes for supercapacitors.

Figure 5.

Plots of the Csp retention for the three metal sulfide electrodes as a function of the number of galvanostatic charge–discharge cycles. The current density was fixed to 3.0 mA cm−2 during the measurements.

Conclusion

The nanostructured NiS, Co3S4 and NiCo2S4 electrodes for supercapacitors were simply fabricated by alternately dipping a TiO2 nanoparticles-covered FTO substrate into a metal and a sulfur precursor solution. When the Ni(Ac)2 and Co(Ac)2 solutions and their 1 : 2 (mole/mole) mixed solution were used as a metal precursor solution in the process, the α-NiS, Co3S4 and NiCo2S4 electrodes were fabricated, respectively. A maximum Csp value of 897 F g−1 at a current density of 1 mA cm−2 was obtained for the nanostructured NiS electrode, although it had the lowest capacitance retention of 66% after 2000 GCD cycles. In contrast, the Co3S4 electrode had a significantly higher voltammetric response and cycle stability although its Csp value was quite low at 187 F g−1. The bimetallic sulfide electrode, i.e. NiCo2S4 electrode, was then fabricated for synergetic improvement from the properties of the two single metal sulfide electrodes. As expected, the NiCo2S4 electrode showed a dramatic increase in the voltammetric response and cycle stability with a slightly reduced Csp value of 516 F g−1, compared to the NiS electrode. The Csp retention of the nanostructured NiCo2S4 electrode was approximately 100% with respect to its initial value even after 2000 GCD cycles.