2.1. Morphology and Structural Characteristics
Figures S1 and S2 present the XRD patterns of NiAl LDH and CoAl LDH at different concentrations of KOH. NiAl LDH presents a remarkable alkali stability. Alkali etching leads to the slight decrease of crystallinity of NiAl LDH without any change of crystalline. However, CoAl LDH presents a poor alkali stability. The crystalline phase of CoAl LDH is firstly converted to CoOOH and then to Co(OH)
2. As is well-known, the alkaline environment is necessary for LDH-based electrode materials to produce electrochemical performance. Hence, alkali stability may be an important factor affecting the electrochemical stability of electrode materials. According to the
Figure S3, the CoAl LDH-E obtained by alkali etching at 3 M KOH presents a terrible specific capacitance of only 85.35 F g
−1 at 1 A g
−1. For comparison, CoAl LDH presents an outstanding specific capacitance of 838.03 F g
−1 at 1 A g
−1. Based on the outstanding alkali stability of NiAl LDH, prepared NiCoAl LDH may be a preferable method to improve the alkali stability of CoAl LDH. Hence, the Ni
1Co
2Al LDH and Ni
2Co
1Al LDH are prepared. Considering the positive influence of electrical conductivity, the derived Ni
1Co
2S
4 and Ni
2Co
1S
4 from Ni
1Co
2Al LDH and Ni
2Co1Al LDH are also prepared. The schematic illumination of the synthesis and alkali etching of NiCo sulfides is shown in
Scheme 1.
The XRD patterns of the as-prepared samples were shown in
Figure 1a–c. As shown in
Figure 1a, all the XRD patterns exhibit a standard LDH characteristic peaks of (003) and (006) planes at around 11° and 23°, respectively. From
Figure 1b, the interlayer spacing of the as-prepared samples are found in the order as Ni
1Co
2Al LDH < Ni
2Co
1Al LDH = Ni
1Co
2Al LDH-E < Ni
2Co
1Al LDH-E due to the content variation of a trivalent ion (Co
3+ and Al
3+). For example, the interlayer spacing of Ni
1Co
2Al LDH is 7.654 Å. The interlayer spacing of the obtained Ni
2Co
1Al LDH is increasing to 7.694 Å with the increase of the content of Co
3+ due to the receding charge density of host layer caused by the decrease of the Co
3+. The interlayer spacing of Ni
2Co
1Al LDH-E and Ni
1Co
2Al LDH-E are increased to 7.748 and 7.694 Å, respectively, due to the receding charge density of host layer caused by the decrease of the leaching of Al
3+ after the alkali etching process. From red and green lines of
Figure 1c, the peaks at around 31.58°, 38.32°, and 55.31° are assigned to the (311), (400), and (440) planes of cubic Ni
1Co
2S4 (JCPDS No. 73–1704), suggesting the success of preparation process of Ni
1Co
2S
4 and Ni
2Co
1S
4. From the yellow line, the peaks at 20.24°, 34.41°, 38.89°, 59.98°, 61.25°, and 65.34° are assigned to the (003), (012), and (110) planes of hexagonal CoOOH (JCPDS No. 07-0169) and the (012), (110), and (113) planes of Ni(OH)
2·0.75H
2O (JCPDS No. 38-0715). The result suggests that Ni
2Co
1S
4 have been decomposed into CoOOH and Ni(OH)
2 due to its alkali instability. For comparison, the XRD pattern of Ni
1Co
2S
4 has almost no change due to the superior alkali stability. As shown in
Figure 1d, obvious peaks at 1352 and 576 cm
−1 in spectra of Ni
2Co
1Al LDH, Ni
2Co
1Al LDH-E, and Ni
2Co
1S
4-E are assigned to the characteristic adsorption of CO
32− and M-O (Ni-O and Co-O) components, respectively, indicating that the Ni
2Co
1S
4-E has been decomposed into hydroxide. The broad peak at around 536 cm
−1 is assigned to the characteristic adsorption of M-S (Ni-S and Co-S).
From
Figure 2a, Ni, Co, and S can be found in XPS spectra of Ni
2Co
1S
4 and Ni
2Co
1S
4-E, and the Ni, Co, Al O, and C can be found in Ni
2Co
1Al LDH and Ni
2Co
1Al LDH-E, the chemical composition of these as-obtained samples is in line with forecast, indicating that success of the preparation process. From
Figure 2b,d, the peaks of Ni 2p
1/2 and 2p
3/2 of Ni
2Co
1S
4 appeared at 873.3 and 855.7 eV, and the peaks of Co 2p
1/2 and 2p
3/2 of Ni
2Co
1S
4 appeared at 796.6 and 780.7 eV. For comparison, the peaks of Ni 2p
1/2 and 2p
3/2 of Ni
2Co
1S
4-E appeared at 872.9 and 855.3 eV, and the peaks of Co 2p
1/2 and 2p
3/2 of Ni
2Co
1S
4-E appeared at 796.2 and 779.6 eV. The binding energies of Ni and Co after alkali etching decreased obviously, indicating the change of chemical environment of the elements of Ni and Co. Combined with XRD results, the decrease of binding energy of Ni and Co may be attributed to the decomposition of the Ni
2Co
1S
4 after the alkali etching. From
Figure 2f, the signal of S in Ni
2Co
1S
4-E disappeared, suggesting that the decomposition of the Ni
2Co
1S
4 once again. From
Figure 2c,e, the peaks of Ni 2p
1/2 and 2p
3/2 of Ni
2Co
1Al LDH appeared at 872.8 and 855.3 eV, and the peaks of Co 2p
1/2 and 2p
3/2 of Ni
2Co
1Al LDH appeared at 796.1 and 780.1 eV. For comparison, the peaks of Ni 2p
1/2 and 2p
3/2 of Ni
2Co
1Al LDH-E appeared at 872.9 and 855.3 eV, and the peaks of Co 2p
1/2 and 2p
3/2 of Ni
2Co
1Al LDH-E appeared at 795.6 and 779.8 eV. The binding energy of Ni is almost unchanged, while the binding energy of Co shifts significantly, indicating that Co is more easily affected by alkali etching. From
Figure 2g, the signals of Al decreased obviously, indicating the leaching of Al during the alkali etching process. From
Figure 2h, the fitted peak at 529.24 eV in Ni
2Co
1S
4-E is assigned to the M-O (Ni-O or Co-O) bond, which should be ascribed to the Ni(OH)
2 and CoOOH produced by the decomposition of Ni
2Co
1S
4. From
Figure 2i, the sharp declining of adsorbed oxygen peak intensity of Ni
2Co
1Al LDH-E at 532.53 eV should be ascribed to the desorption of adsorbed oxygen after alkali etching.
From
Figures S4 and S5, the Ni
2Co
1Al LDH and Ni
2Co
1Al LDH-E exhibit a typical morphology of hexagonal nanosheet, which are consistent with the morphology of LDH in previous reports [
23]. Clear and bright diffraction spots can be found in
Figure S4b. These diffraction spots form two diffraction circles which should be ascribed to the (113) and (012) planes of Ni
2Co
1Al LDH. As shown in
Figure S4d, the elements of Ni, Co, Al, and O are equably dispersed on the surface of Ni
2Co
1Al LDH, indicating that the preparation process is successful. As shown in
Figure S5b, the diffraction spots become blurred, suggesting that the crystallinity of Ni
2Co
1Al LDH is decreased slightly after alkali etching. Comparing the EDS map**s of Ni
2Co
1Al LDH and Ni
2Co
1Al LDH-E, similar element contents indicate that alkali etching has little effect on the components of Ni
2Co
1Al LDH. As shown in
Figure 3a, the Ni
2Co
1S
4 obtained by sulfidation exhibits a morphology of an amorphous nanosheet, indicating that the sulfidation process destroys the stable hexagonal structure. The 0.2315 nm lattice spacing is assigned to the (400) plane of Ni
2Co
1S
4 (JCPDS No.73-1704). As shown in
Figure 3c, the Ni
2Co
1S
4-E displays a morphology of nanosheets, which is different from the morphology before alkali etching, indicating that the sample of Ni
2Co
1S
4-E have been decomposed completely. Combined with the XRD result, the 0.2429 and 0.2660 nm lattice spacings are assigned to the (101) plane of CoOOH (JCPDS No. 07-0169) and (101) plane of Ni(OH)
2·0.75H
2O (JCPDS No. 38-0715). From
Figure 3b, the feeble signal of Al indicates that the Al has not been completely removed from the lattice of the Ni
2Co
1Al LDH, the feeble signal of S, and the intense signal of O indicate that the element of S is replaced by OH
− gradually during alkali etching.
To obtain the microstructure information of the as-obtained materials, the BET measurement of Ni
2Co
1S
4, Ni
2Co
1S
4-E, Ni
2Co
1Al LDH, and Ni
2Co
1Al LDH-E is carried out, the corresponding N
2 adsorption/desorption isotherms and pore-size distribution (inset) curves are shown in
Figure 4 and the relevant data for specific surface area, pore volume, and average diameter of these samples are shown in
Table S1. The specific surface areas of Ni
2Co
1S
4, Ni
2Co
1S
4-E, Ni
2Co
1Al LDH, and Ni
2Co
1Al LDH-E are 7.1648, 31.0860, 58.8592, and 63.2865 m
2 g
−1, respectively. The pore volumes of Ni
2Co
1S
4, Ni
2Co
1S
4-E, Ni
2Co
1Al LDH, and Ni
2Co
1Al LDH-E are 0.040893, 0.151196, 0.376361, and 0.376361 cm
3 g
−1, respectively. The average pore diameters of Ni
2Co
1S
4, Ni
2Co
1S
4-E, Ni
2Co
1Al LDH, and Ni
2Co
1Al LDH-E are 18.4100, 14.9734, 13.6747, and 13.0023 m
2 g
−1, respectively. The Ni
2Co
1Al LDH-E exhibits a maximal specific surface area due to the unique layer structure of LDH and pore-forming by alkali etching. The slight decrease in pore diameter and average pore volume of Ni
2Co
1Al LDH-E when compared with Ni
2Co
1Al LDH should be ascribed to the small size of the newly formed pores by alkali etching which dragged down the average data. The specific surface area of Ni
2Co
1S
4 is lower than Ni
2Co
1Al LDH obviously due to the destruction of layered structure by sulfidation. After the alkali etching of Ni
2Co
1S
4 is carried out, the Ni
2Co
1S
4-E is obtained and exhibits a higher specific surface area than initial Ni
2Co
1S
4 due to the decomposition from sulfide to hydroxide. These results are consistent with those of XRD, XPS, FTIR, and TEM, indicating that the Ni
2Co
1S
4 possesses dissatisfied alkali stability.
2.2. Electrochemical Properties
To evaluate the electrochemical performance of the as-prepared Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, Ni
2Co
1S
4, and Ni
1Co
2S
4, the CV curves at a scan rate of 30 mV s
−1 and GCD curves at a current density of 1 A g
−1 are carried out, as shown in
Figure 5a,b. The integral areas of Ni
2Co
1S
4 and Ni
1Co
2S
4 from CV curves are higher than those of Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, and the discharge times of Ni
2Co
1S
4 and Ni
1Co
2S
4 from GCD curves are longer than those of Ni
2Co
1Al LDH and Ni
1Co
2Al LDH, indicating that sulfidation possesses a superior enhancing effect on the electrochemical properties of NiCoAl LDH. The specific capacitances of Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, Ni
2Co
1S
4, and Ni
1Co
2S
4 at 1 A g
−1 are 271.72, 343.37, 718.51, and 1312.87 F g
−1, respectively, and the specific capacities of Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, Ni
2Co
1S
4, and Ni
1Co
2S
4 are 37.06, 47.17, 98.46, and 181.10 mAh g
−1, respectively. From
Figure 5c, the rate capabilities of Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, Ni
2Co
1S
4, and Ni
1Co
2S
4 from 1 to 7 A g
−1 are 70.41, 32.15, 80.28, and 52.96%, respectively. The charge transfer resistances (
Rct) of Ni
2Co
1Al LDH, Ni
1Co
2Al LDH, Ni
2Co
1S
4, and Ni
1Co
2S
4 fitted by the equivalent circuit diagram (
Figure 5i) are 2.13, 17.00, 3.81, and 5.93 Ω, respectively (
Figure 5d). Ni
2Co
1S
4 possesses superior electrical conductivity, outstanding specific capacitance and satisfied rate capability. Although Ni
2Co
1Al LDH exhibits a superior alkali stability, the specific capacitance of Ni
2Co
1Al LDH is lower than that of two sulfides (Ni
2Co
1S
4 and Ni
1Co
2S
4) obviously. Hence, Ni
2Co
1Al LDH does not meet the standard of advanced electrode materials. Based on the above, the Ni
2Co
1S
4 is selected as the target electrode material for subsequent tests. The CV curves of Ni
2Co
1S
4 at various scan rates are shown in
Figure 5e. The strong redox peaks should be ascribed to the Ni
2+/N
i3+ and Co
2+/Co
3+ peaks. The possible electrochemical reaction is shown as follows:
Additionally, the oxidation and reduction peaks of CV curves shift to the direction of high potential and low potential, respectively, without any shape change, indicating a considerable cycling stability. The capacitance retention of Ni
2Co
1S
4 from 1 to 30 A g
−1 is 20.74%, exhibiting glorious rate capability (
Figure 5f). From
Figure 5g, Ni
2Co
1S
4 exhibits superior cycling stability of 85.48% capacitance retention after 5000 cycles. In addition, the shapes of GCD curves of Ni
2Co
1S
4 before and after cycling exhibit a stable shape. In addition, the discharge time of the Ni
2Co
1S
4 at 5000th cycle decreases slightly when compared with the discharge time at the 1st cycle, suggesting an excellent cycling stability. According to
Figure 5h, the
Rct values before and after cycling are 3.815 and 18.53 Ω, respectively, suggesting that the decrease of specific capacitance of Ni
2Co
1S
4 is mainly due to the decrease of electrical conductivity.
To evaluate the influence of alkali etching on the electrochemical performance of electrode material, the electrochemical tests of as-etched samples are carried out, as shown in
Figure S6. The specific capacitances of Ni
2Co
1S
4-E, Ni
1Co
2S
4-E, Ni
2Co
1Al LDH-E, and Ni
1Co
2Al LDH-E at 1 A g
−1 are 612.13, 482.97, 118.47, and 109.35 F g
−1, respectively, all of which are much lower than that of electrode materials before etching, indicating that the obvious negative effect of alkali etching on the electrochemical performance. Ni
2Co
1S
4 possesses poor alkali stability. However, the cycling stability of Ni
2Co
1S
4 is excellent and is not affected by prolonged alkali immersion. This result may be ascribed that the alkali concentration on the surface of the electrode material is insufficient during the process of the electrochemical test.
To analyze the electrochemical behavior of Ni
2Co
1S
4, the CV curves with small scan rates are carried out, as shown in
Figure 6a. The peak current (
Ip, A) and scan rate (
v, mv s
−1) obey the relationship as follows [
24]:
where
a and
b are the adjustable parameters. The
b value is concerned with the charge storage mechanism: (1)
b = 0.5, represents the diffusion-controlled mechanism, (2)
b ≥ 1.0, represents the surface capacitance-controlled mechanism, (3) 0.5 <
b < 1.0, represents a mixing capacitance mechanism. According to
Figure 6b, the
b-value of Ni
2Co
1S
4 from anodic and cathodic peaks are 0.66937 and 0.77223, respectively, suggesting that both surface capacitance effect and diffusion-controlled capacitance dominate the electrochemical reaction. The detailed capacitance contribution ratio can be calculated from the following formula [
25]:
where
I (A) represents the current at the fixed potential, and
k1 and
k2 are the constants.
k1v and
k2v0.5 represent the charge stored mechanism by the surface capacitance effect and diffusion-controlled capacitance effect. The CV curves and fitted integral area at 1.1, 1.4, and 1.7 mV s
−1 are shown in
Figure 6c–e and display 87.02, 83.09, and 80.18% diffusion-controlled capacitance effect, respectively. From
Figure 6f, the contribution proportion of diffusion-controlled capacitance effect increases with the increase of scan rates. The result should be ascribed that high current density leads to the shrinkable diffusion time and diffusion distance.
To evaluate the practical application potential, the commercial active carbon (AC) is used as the negative material. The specific capacitances of AC at 1 A g
−1 are 150 F g
−1 (
Figure S7).
Figure 7a and b exhibit the CV and GCD curves of the Ni
2Co
1S
4//AC ASC device at various potential windows. The CV curves exhibit a quasi-quadrilateral shape within the potential range of 1.0–1.6 V. However, when the potential window reached 1.8 V, obvious polarizations at the high potential region appeared. Similarly, a distinct charge platform appeared in the GCD curves with the potential window of 1.6 V. Hence, the above results indicate that the optimal potential window of ASC device is 1.6 V. The CV curves of ASC device at various scan rates exhibit a stable and homomorphic shape, indicating that the ASC device possesses satisfied stability (
Figure 7c). The specific capacitances of ASC device at 1, 2, 3, 4, 5, and 6 A g
−1 are 54.625, 31.5, 23.81, 19.625, 16.25, and 13.39 F g
−1, respectively (
Figure 7d), the corresponding rate capability from 1 to 6 A g
−1 is 20.54 % (
Figure 7e). The ASC device exhibits a satisfied cycling stability of 61.29% capacitance retention after 2000 cycles (
Figure 7f), and the corresponding
Rct value increases from 29.52 to 135 Ω (
Figure 7g), indicating that the decrease of specific capacitance of ASC device is mainly due to the decrease of electrical conductivity. Compared with the three-electrode system, the cycling stability decreases obviously. The results should be ascribed to the negative influence of the negative material of commercial AC. The ASC device displays an appreciable energy density of 18.7 Wh kg
−1 at 800 W kg
−1 (
Figure 7h), which is higher than that of previously reported Ni/Co-based ASC devices [
26,
27,
28,
29,
30,
31]. In addition, as shown in
Figure 7i, the LED indicator can be lit up by two ASC device in series, exhibiting a desirable application potential.