Zichen Xu,
Zhiqiang Zhang,
Huiling Yin,
Shengxian Hou,
Hongtao Lin
*,
Jin Zhou
and
Shuping Zhuo*
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255000, P. R. China. E-mail: linht@sdut.edu.cn
First published on 17th January 2020
Conductive polymers, such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) and poly 3,4-ethylenedioxythiophene (PEDOT), play an important role in the application of pseudocapacitors. It is necessary to explore the effects of different conductive polymers in electrode composites. Herein, we prepare zinc sulfide/reduced graphene oxide (ZnS/RGO) by the hydrothermal method, and conductive polymers (PANI, PPy, PTh and PEDOT) doped with the same mass ratio (polymer to 70 wt%) via in situ polymerization on the surface of ZnS/RGO composite. For the supercapacitor application, the ZnS/RGO/PANI ternary electrode composite possesses the best capacitance performance and cycle stability out of all of the polymer-coated ZnS/RGO composites. In the three-electrode system, the discharge specific capacitance and cycle stability of ZnS/RGO/PANI are 1045.3 F g−1 and 160% at 1 A g−1 after 1000 loops. In a two-electrode symmetric system, the discharge specific capacitance and cycle stability of ZnS/RGO/PANI are 722.0 F g−1 and 76.1% at 1 A g−1 after 1000 loops, and the greatest energy and power density of the ZnS/RGO/PANI electrode are 349.7 W h kg−1 and 18.0 kW kg−1. In addition, conductive polymers can effectively improve the voltage range of the electrode composites in 6 M KOH electrolyte for the two-electrode system. The discharge voltage ∼1.6 V makes them promising electrode materials for supercapacitors.
To obtain a promising electrode material for pseudocapacitors, metal sulfides such as MoS2,7 SnS2,8 CoS9 and ZnS8 have been widely concerned with their excellent electrical conductivity, multiple active sites and low-cost. Compared to the limitations of electrode materials with single substance in the application of actual conditions, composite electrode materials possess better environmental stability and low-cost in the application of SCs. Composite electrode materials can also achieve better electrochemical performance by controlling their morphology. For example, Ramachandran et al.10 synthesized zinc sulfide decorated graphene (ZnS/G) nanocomposites via a facile solvothermal approach and its specific capacitance is up to 197.1 F g−1 at 5 mV s−1 in 6 M KOH electrolyte, which is better than the specific capacitance of ZnS as 32.8 F g−1 and that of graphene as 115 F g−1. Graphene as a typical two-dimensional material can improve the surface area of composites and prevent metal sulfide accumulation because of the effect of its conjugated π bonds.11,12 Although graphene/metal sulfides composites have excellent electro-conductibility and stable capacity performance, but poor capacitance and energy performance limit its application in pseudocapacitors.13 Therefore, doping a high capacitance material for pseudocapacitors, such as conductive polymers, may be an effective method t for improving the electrochemical performance of pseudocapacitors.
Conductive polymers (CPs) such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) and poly 3,4-ethylenedioxythiophene (PEDOT) are regarded as promising pseudo-capacitive materials because of their large specific capacitance and the characterization of many active sites.14 However, pure CPs are easier to accumulate into blocks and the contact areas between polymers and electrolyte are decreased, which reduces the electrochemical properties. Therefore, this composite with a lone pair of electrons and two-dimensional planar structure, such as graphene/metal sulfides, can effectively prevent the accumulation of CPs and improve the cycle stability of CPs.15–17 For example, Li et al.18 successfully synthesized MoS2/RGO@PANI electrode composite through a two-stage synthetic method, and the specific capacitance and cycle stability after 3000 loops of MoS2/RGO@PANI at 1 A g−1 were 1224 F g−1 and 82.5%, which were better than those of pure PANI as 774 F g−1 and 70.2%. Tang et al.19 has developed a scalable solution-based approach to controllable growth of PPy ultrathin films on two-dimensional MoS2 monolayers and the specific capacitance and cycle stability of MoS2/PPy (695 F g−1, 85% at 1 A g−1 after 4000 cycles) are far more than those of PPy (200 F g−1, 60% at 1 A g−1 after 4000 cycles). The results indicate that the synergistic effect among polymer, metal sulfide and graphene can effectively improve electrochemical performance of electrode materials.18,19 But various CPs have different effect on the electrode composites, which may be ascribed to their discrepant conductive and growth mechanism. So, it is necessary to study how various CPs improve electrochemical performance of electrode composites.
Herein, we chose to prepare ZnS/reduced graphene oxide (ZnS/RGO) by hydrothermal method, and CPs (PANI, PPy, PTh and PEDOT) doped with the same mass ratio (polymer to 70 wt%) via in situ polymerization on the surface of ZnS/RGO composite. CPs can effectively improve capacitance performance of electrode composites, and it also extended the voltage range in two-electrode system. In 6 M KOH electrolyte solution, the voltage range of various polymer-coated ZnS/RGO electrode with two-electrode system are up to 0–1.6 V, which is better than that (−0.2–0.6 V) in three-electrode system and improve energy and power density of electrode composites. For the application of SCs, ZnS/RGO/PANI possesses the best electrochemical properties respectively in two and three electrode system, and it is a promising electrode material of pseudocapacitors. These findings may be useful for developing effective and stable electrode materials for energy storage device.
All of the obtained precipitates were washed by deionized water and ethanol, and freeze-drying for 24 h. Finally, we successfully synthesized four kinds of ternary composite, ZnS/RGO/PANI, ZnS/RGO/PPy, ZnS/RGO/PTh and ZnS/RGO/PEDOT. The synthesis steps was shown in Scheme 1.
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Fig. 1 The XRD patterns (a) and FT-IR spectra (b) of ZnS/RGO/PANI, ZnS/RGO/PPy, ZnS/RGO/PTh and ZnS/RGO/PEDOT. |
Polymer-coated ZnS/RGO ternary composites are explored as an electrode material in 6 M KOH electrolyte solution for three-electrode system, where nickel foam as current collector. The electrochemical characterization of polymer-coated ZnS/RGO ternary electrode materials is targeted in Fig. 2. Besides a single CV curve of nickel foam, the redox peaks intensity of polymer-coated ZnS/RGO electrode materials are enhanced as shown in Fig. 2a. The values of oxidation and reduction peak of ZnS/RGO/PANI are 0.36 and 0.26 V respectively, which is lower than those of the ZnS/RGO/PPy and ZnS/RGO/PTh (0.38 and 0.28 V). ZnS/RGO/PEDOT has a similar value of redox peak to nickel foam (0.39 and 0.29 V). ZnS/RGO/PANI has the largest integrate area in CV curves and GCD profiles as shown in Fig. 2a and b, which indicates that it has excellent specific capacitance property. The specific capacitance of ZnS/RGO/PANI at 1 A g−1 is up to 1045 F g−1, which is larger than that of the ZnS/RGO/PTh (854 F g−1), ZnS/RGO/PPy (787 F g−1) and ZnS/RGO/PEDOT (680 F g−1). Meanwhile, the specific capacitance of ZnS/RGO/PANI (1045 F g−1 at 1 A g−1) is also better than other composites reported, such as GO/PANI (425 F g−1 at 0.2 A g−1),33 ZnS/G-60 (191.7 F g−1 at 5 mV s−1),10 PANI/Zn2+ film (738 F g−1 at 5 mA cm−2),34 rGO/Fe3O4/PANI (283.4 F g−1 at 1 A g−1)35 and SnS2/NRGO/PANI (1021.7 F g−1 at 1 A g−1).31 The specific capacitance (CS) of faradaic pseudocapacitance for three-electrode system can be calculated based on the following equation:36
![]() | (1) |
We speculate that the difference in the electrochemical properties for the polymer-coated ZnS/RGO electrode materials are attributed to the different conductive mechanism for the polymers. PANI depolarizes a positive charge on a nitrogen atom into a large conjugated π bond via a redox reaction, thereby obtaining high conductivity.37,38 However, PPy and PTh have similar electrical conduction mechanisms, both are transferred by conjugated π electrons.39,40 In PEDOT, the epoxy ring provides the electron-donating group with improved conductivity but greatly increases its steric hindrance.41,42 To confirm this speculation, we test the EIS plots of the polymer-coated ZnS/RGO electrode materials as shown in Fig. 2c. In the high and intermediated frequencies region, the semicircular portion reflects charge-transfer resistance (RCT) arranged in order as ZnS/RGO/PTh (0.17 Ω) < ZnS/RGO/PPy (0.18 Ω) < ZnS/RGO/PANI (0.21 Ω) < ZnS/RGO/PEDOT (0.27 Ω). The process of redox reaction and the effect of steric hindrance result in the larger RCT of ZnS/RGO/PANI and ZnS/RGO/PEDOT, respectively.42,43 In the low frequency region, the line with a low slope predicts that it has significant faradaic pseudocapacitance performance. ZnS/RGO/PANI also possesses the best conductivity as shown in Table S1† and the values of ionic conductivity (σ) are putted in order as ZnS/RGO (0.61 × 10−3 S m−1) < ZnS/RGO/PEDOT (0.74 × 10−3 S m−1) < ZnS/RGO/PPy (0.81 × 10−3 S m−1) < ZnS/RGO/PTh (1.46 × 10−3 S m−1) < ZnS/RGO/PANI (1.77 × 10−3 S m−1). Not only that, ZnS/RGO/PANI also has excellent capacitance retention as 63.9% at different current density from 0.3 to 20 A g−1 in Fig. 2d, which is higher than that of ZnS/RGO/PPy (63%), ZnS/RGO/PTh (54.6%) and ZnS/RGO/PEDOT (53.3%). Simultaneously, ZnS/RGO/PANI possesses the largest discharge specific capacitance at various current densities and it still remains 672.6 F g−1 when the current density reaches to 20 A g−1. As the cycle numbers increase, the discharge specific capacitance of all polymer-coated ZnS/RGO electrode materials are increased as shown in Fig. 2e. ZnS/RGO/PANI has the largest growth rate as 160% and its discharge specific capacitance is up to 1662 F g−1 at 1 A g−1 after 1000 cycles, which is better than those of composites reported, such as GO/PANI (83% after 500 cycles),33 ZnS/G-60 (94.1% after 1000 cycles),10 PANI/Zn2+ film (65.4% after 500 cycles),34 rGO/Fe3O4/PANI (78% after 1000 cycles)35 and SnS2/NRGO/PANI (82% after 1000 cycles).31 In addition, the electrochemical characterizations of ZnS/RGO electrode material are targeted in Fig. S1c–g,† which display the low capacitance retention as 44%, poor cycle stability as 88% and low specific capacitance as 425.4 F g−1. And the electrochemical performance of four pure CPs is also displayed in Fig. S2a–d,† which introduces that PEDOT has the best capacitance performance (the highest value is 718.2 F g−1 at 0.3 A g−1) than the others (PANI, 642.6 F g−1 at 0.3 A g−1; PPy, 586.2 F g−1 at 0.3 A g−1; PTh, 697.1 F g−1 at 0.3 A g−1) and PTh has the same trend as PEDOT in the change of specific capacitance with different current density. Meanwhile, PANI and PPy have the smaller RCT than the both PTh and PEDOT, it indirectly proves that PANI and PPy have better electronic conductivity than those of PTh and PEDOT, which is mainly reflected in the capacitance retention rate (PANI (64.0%) > PPy (56.4%) > PEDOT (54.3%) > PTh (51.6%)). However, the electrochemical properties of the above polymer-coated ZnS/RGO electrode materials are better than those of ZnS/RGO and four pure CPs, which indicates that the interaction between CPs and ZnS/RGO in composites can effectively improve the electrochemical performance of ZnS/RGO and CPs electrode.
According to above studied results, we have found that ZnS/RGO/PANI composite has the best capacitance performance and cycle stability in polymer-coated ZnS/RGO composites. We further explore what the effects of various ingredients on the electrochemical of ZnS/RGO/PANI composite. So we tested the electrochemical properties of ZnS/RGO/PANI, ZnS/RGO, PANI and RGO/PANI (the mass ratio of PANI is 70% as same as that of ZnS/RGO/PANI) with 6 M KOH electrolyte in three-electrode system as shown in Fig. S3.† As shown in Fig. S3a andb,† ZnS/RGO/PANI possesses the best capacitance performance and smart pseudocapacitor behavior, and the specific capacitance at 1 A g−1 can be calculated by eqn (1) as 1045.3 F g−1 (ZnS/RGO/PANI) > 703.9 F g−1 (RGO/PANI) > 568.7 F g−1 (PANI) > 425.4 F g−1 (ZnS/RGO). Meanwhile the RCT of different composites is stored as ZnS/RGO/PANI < ZnS/RGO < RGO/PANI < PANI, and the capacitance retention at current density from 0.3 to 20 A g−1 ordered as 63.9% (ZnS/RGO/PANI) > 63.8% (PANI) > 54.8% (RGO/PANI) > 44.0% (ZnS/RGO) as shown in Fig. S3c and d.† To sum up, we speculate that ZnS plays an important role in promoting the dispersion and polymerization of PANI in ZnS/RGO/PANI composite system. Because, PANI is easy to stack, which causes its poor capacitance performance, but when it compounded with graphene can effectively improve its dispersion and capacitance performance. However, when PANI grows on the surface of ZnS/RGO, its specific capacitance and conductivity will be further improved. It is indicated that ZnS particles can strengthen the connection between PANI and graphene, which is beneficial to the transfer of electrons in ZnS/RGO/PANI ternary composite.
As we all know, the electrochemical performances of the electrode materials are inextricably related to their morphologies. By SEM and TEM images, we can clearly observe the successful deposition of PANI, PPy, PTh and PEDOT on the surface of layered ZnS/RGO. As shown in the SEM and TEM images of ZnS/RGO composite (Fig. S1a and b†), ZnS spherical particles are distributed on the tissue-like RGO layer and its lattice spacing is 0.32 nm. Compared to the morphology of ZnS/RGO, all the polymer-coated ZnS/RGO materials remain the spherical shape of ZnS and the layered structure of graphene as shown in Fig. 3a–d. Meanwhile, the surfaces become more fluffy and multi-layered for all the polymer-coated ZnS/RGO composites. This fluffy multi-layered morphology causes the electrolyte gradually penetrate from the outside to the interior of electrode materials and activate more active sites as cycle numbers increases, which further promotes their capacitance performance. As targeted in Fig. 3a–d, the EDS images of various polymer-coated ZnS/RGO composites display distribution and percentage of the elements: ZnS/RGO/PANI (C: 46.14%, N: 42.9%, S: 5.07%, Zn: 5.62%), ZnS/RGO/PPy (C: 68.1%, N: 21.9%, S: 5.05%, Zn: 4.95%), ZnS/RGO/PTh (C: 47.54%, O: 17.61%, S: 18.2%, Zn: 16.65%) and ZnS/RGO/PEDOT (C: 14.8%, O: 44.1%, S: 28.87%, Zn: 12.24%). N and S elements doped in the composites provide more active sites, which makes them possess better capacitance performance.44,45 ZnS/RGO/PANI possesses the largest percentage of N and S elements, which explains the reason for the largest specific capacitance of ZnS/RGO/PANI (Fig. 2d). As shown in Fig. 3e–h, the TEM images further exhibit the morphologies of polymer-coated ZnS/RGO composites. The ZnS lattice spacing for all the composites is 0.32 nm, which is consistent with the plane (111) calculated by XRD in Fig. 1a. Various polymers have different morphologies, such as PANI in the shape of raspberry, PPy in the form of nanofibrous, PTh and PEDOT in the form of membrane structure, respectively. Meanwhile, the average thickness of different polymers on the surface of ZnS spherical particles are almost the same for the polymer-coated ZnS/RGO composites as seen in the HRTEM images (Fig. 3e–h). The specific surface areas are achieved from nitrogen sorption isotherms on the basis of Brunauer–Emmett–Teller (BET) theory (Fig. S4a–d†), and the order as follows: ZnS/RGO/PEDOT (18.18 m2 g−1) < ZnS/RGO/PTh (32.53 m2 g−1) < ZnS/RGO/PPy (40.60 m2 g−1) < ZnS/RGO/PANI (61.90 m2 g−1). Their pore distributions are mainly dominated by micropores and macropores, and the average pore size distributions are 10.63 nm (ZnS/RGO/PANI), 15.60 nm (ZnS/RGO/PPy), 16.65 nm (ZnS/RGO/PTh) and 13.63 nm (ZnS/RGO/PEDOT).
Further structural properties and composition of polymer-coated ZnS/RGO composites are obtained by XPS analysis and the spectra are displayed in Fig. 4. According to the C 1s spectrum as shown in Fig. 4a, the C–N characteristic peaks of PANI and PPy at 285.0 (ZnS/RGO/PANI) and 285.2 eV (ZnS/RGO/PPy), which are corresponding to 1298.9 cm−1 and 1190 cm−1 respectively in FT-IR spectra (Fig. 1b). Meanwhile, the C–S characteristic peaks of PTh and PEDOT at 288.7 (ZnS/RGO/PTh) and 288.9 eV (ZnS/RGO/PEDOT), which are corresponding to 917 cm−1 in FT-IR spectra. As targeted in Fig. 4b, the N 1s spectrums of ZnS/RGO/PANI and ZnS/RGO/PPy can be categorized into four peaks as pyridinic N, C–N, pyrrolic N and Zn–N. The binding energy of N 1s for ZnS/RGO/PPy is greater than that of ZnS/RGO/PANI, which indicates that the nitrogen atoms on ZnS/RGO/PANI are more reactive. As shown in Fig. 4c, the O 1s spectrums of ZnS/RGO/PANI, ZnS/RGO/PPy and ZnS/RGO/PTh are categorized into three peaks as O–C, OC and ROSR, which are corresponding to the bonding form of GO. But ZnS/RGO/PEDOT composite possesses the C–O–R peak at 534.0 eV, which is corresponding to the characteristic peak of PEDOT at 1210 cm−1 in FT-IR. In addition, the atomic percentage of various elements of these ternary composites are listed according to the XPS test in Table S2.† ZnS/RGO/PANI composite possesses the largest total percentage of N and S as 18%, which is consistent with the result in EDS graphic (Fig. 2a–d).
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Fig. 4 Deconvoluted XPS spectra: the high-resolution spectra (a) C 1s, (b) N 1s, (c) O 1s and (d) S 2p of ZnS/RGO/PANI, ZnS/RGO/PPy, ZnS/RGO/PTh and ZnS/RGO/PEDOT. |
To further explore the application of these polymer-coated ZnS/RGO composites, we studied their electrochemical performance in a symmetric two-electrode system with 6 M KOH as electrolyte as shown in Fig. 5. As shown in Fig. 5a and b, the redox peaks in CV curves at 0.80 and 1.56 V are consistent with the voltage platform in GCD profiles and D-ZnS/RGO/PANI (double ZnS/RGO/PANI electrode) possesses the largest integrate area. The potential window of these electrode materials is extended to 1.6 V in two-electrode system, because the inherent high voltage range of the polymer improves the energy of these electrode materials. Meanwhile, electrode composites in symmetrical two-electrode system are more easily activated to reach higher potentials than those in three-electrode system, because in three-electrode system the reference electrode can precisely adjust the potential of work electrode.46 In addition, extended potential window is also related to the electrolyte especially pH and concentration, which affects the diffusion of ions in the electrolyte solution.47 In this work, we use 30 mL of 6 M KOH as electrolyte, and its pH is around 13. In the GCD profiles, the discharge specific capacitance (CS) of these ternary composites in two-electrode system can be calculated by follow equation based on eqn (1):
![]() | (2) |
According to eqn (2), the discharge specific capacitance at the current density of 1A g−1 are 722.0 (D-ZnS/RGO/PANI), 613.8 (D-ZnS/RGO/PPy), 564.3 (D-ZnS/RGO/PTh) and 653.4 F g−1 (D-ZnS/RGO/PEDOT), respectively. D-ZnS/RGO/PANI has the largest discharge specific capacitance, which is ascribed to the maximum percentage of N and S elements on the surface of ZnS/RGO/PANI (Fig. 3a and Table S2†). As shown in Fig. 5c, D-ZnS/RGO/PANI also has the smallest slope of a line in the range of low frequency because of its significant redox reaction on the surface. In the range of high frequency, RCT are arrayed as D-ZnS/RGO/PPy (0.30 Ω) < D-ZnS/RGO/PTh (0.37 Ω) < D-ZnS/RGO/PEDOT (0.56 Ω) < D-ZnS/RGO/PANI (0.93 Ω), which are around twice than the RCT in three-electrode system as shown in Fig. 2c. The discharge specific capacitance of various composites at different current density are calculated by eqn (2) as shown in Fig. 5d. The initial values of these composites at the current density of 0.3 A g−1 are 777.2 (D-ZnS/RGO/PANI), 613.8 (D-ZnS/RGO/PPy), 569.3 (D-ZnS/RGO/PTh) and 673.2 F g−1 (D-ZnS/RGO/PEDOT), and their capacitance retention are 50.0% (D-ZnS/RGO/PANI), 10.13% (D-ZnS/RGO/PPy), 16.34% (D-ZnS/RGO/PTh) and 21.27% (D-ZnS/RGO/PEDOT) respectively when the current density is up to 20 A g−1. D-ZnS/RGO/PANI possesses the best capacitance performance because of its unique conductive mechanism from the constant conversion of oxidation and reduction states. This result also displays the difference between symmetric two-electrode system and three-electrode system. D-ZnS/RGO/PANI shows the best capacitance performance, which is consistent with the tests of three-electrode system in Fig. 2d. Meanwhile, D-ZnS/RGO/PANI also possesses the highest cycle stability as 76.1% in the above composites at the current density of 1 A g−1 as shown in Fig. 5e and its specific capacitance is slightly improved after 300 cycles, which is because of the raspberry-like PANI structure (Fig. 3a) and microporous structure (Fig. S4a†) favorable for electrolyte penetration. And the specific capacitance of D-ZnS/RGO/PEDOT is also slightly improved after 300 cycles. Because the surface of both ZnS/RGO/PANI and ZnS/RGO/PEDOT has dense polymer coatings, which leads their specific capacitance to decreasing in the first 300 cycles. As the cycles increasing, the surface of ZnS/RGO/PANI and ZnS/RGO/PEDOT has been fully activated to accommodate more ions, thereby increasing the value of their specific capacitance, and finally their specific capacitance reaches stability. In addition, D-ZnS/RGO/PEDOT (65.2%) has better cycle stability than D-ZnS/RGO/PTh (55.3%) and D-ZnS/RGO/PPy (50.0%), because membrane PEDOT structure (Fig. 3d and h) has better structural stability.
Energy density (E) and power density (P) as important parameters to measure the performance of symmetric supercapacitors can be calculated by follow equations:
E = (1/7.2) × ΔV2 × CS (W h kg−1) | (3) |
P = 3600E/Δt (W kg−1) | (4) |
As shown in Fig. 5f, D-ZnS/RGO/PANI exhibits the excellent values of energy and power density in the range of 0.3–20 A g−1, and the greatest energy and power density are calculated using eqn (3) and (4) to 349.7 W h kg−1 and 18.0 kW kg−1, respectively. These results are better than the reported symmetric supercapacitors based on PANI, such as MoS2/RGO/PANI (22.3 W h kg−1 and 5.1 kW kg−1),14 rGO/Fe3O4/PANI (47.7 W h kg−1 and 550 W kg−1),35 SnS2/NRGO/PANI (69.5 W h kg−1 and 575.5 W kg−1)31 and ternary nitrogen-doped graphene/nickel ferrite/polyaniline (92.7 W h kg−1 and 110.8 W kg−1).48
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra07842h |
This journal is © The Royal Society of Chemistry 2020 |