This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2019, 4, 11863−11870
http://pubs.acs.org/journal/acsodf
CoNi2S4 Nanoplate Arrays Derived from Hydroxide Precursors for Flexible Fiber-Shaped Supercapacitors Jian Zhang, Xiaoxi Liu, Qing Yin, Yajun Zhao, Jianeng Luo, and Jingbin Han* State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
Downloaded via 91.204.14.130 on July 17, 2019 at 11:08:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: A high-quality porous CoNi2S4 nanoplates array was in situ synthesized on carbon fibers (CFs) by a hydrothermal method via a CoNilayered double hydroxide (LDH) precursor transformation process. The CoNi2S4@CFs electrode exhibits largely enhanced supercapacitor performance with a specific capacitance of 1724 F/g at 1 A/g, in comparison with that of the CoNi-LDH (1302 F/g) precursor. Furthermore, the CoNi2S4@CF electrode shows an extremely high rate capability with capacity retention of 79% under a charge density of 60 A/g, whereas the retention rate of CoNiLDH@CFs is only ∼34%. The abundant pore structure, improved electrical conductivity, and lower internal resistances of CoNi2S4@CFs (1.0 Ω) compared to those of CoNi-LDH@CFs (9.5 Ω) are responsible for the enhancement of energy storage performance. By using the CoNi2S4 nanoplate array as the positive electrode, an all-solid-state asymmetric fiber-shaped supercapacitor was further obtained, which exhibits outstanding flexible, foldable, and wearable capability. In view of the component tunability for LDH materials, the hydroxide precursor transformation method with merits of mild conditions and easy operation can be extended to the synthesis of a variety of metal sulfides for broad applications in electronic devices.
1. INTRODUCTION As one important type of energy storage device, supercapacitors (SCs) have attracted tremendous attention because of their fast charging/discharging, high power density, and long service life.1−4 Pseudocapacitive transition-metal oxides and hydroxides5−11 have been widely investigated as promising SC materials due to their high redox activity and ease of preparation. Nevertheless, they generally suffer from inadequate electrical conductivity, resulting in limited rate performance and low power density at a high discharging rate. To overcome these obstacles, anion doping or substitution by nonmetallic elements such as nitrogen, sulfur, or phosphorus has been proved to be an effective strategy to improve the electrical conductivity.12−15 Among these materials, CoNi2S4 is regarded as a promising alternative with the advantages of high capacitance, superior conductivity, and acceptable cost.16−19 In particular, CoNi2S4 exhibits an excellent electrical conductivity that is ∼100 times higher than that of CoNi2O420,21 as a result of its smaller band gap energy.22 In addition, CoNi2S4 possesses multiple oxidation states that are capable of enriching redox reactions compared to single-metal sulfides, in the same way as that in CoNi2O4 versus NiOx (or CoOx) electrodes. However, its practical application is still hindered by rigorous synthesis conditions, such as high operation temperature (above 400 °C) and use of toxic gases (H2S) or thiourea as the sulfur source.20,21,23 Therefore, it is highly desired to develop a mild and © 2019 American Chemical Society
environmentally friendly method for the facile synthesis of transition-metal sulfides. Flexible fiber-shaped SCs (FSSCs) as a new type of SCs emerged in response to the demand of portability, flexibility, wearability, and braided characteristics for next-generation electronic devices.24 In numerous studies, carbon fibers (CFs) have been judged as optimal alternative electrodes as a result of their high strength, electrical/thermal conductivities, and lowcost.25 However, CFs can only offer an unsatisfactory low energy density due to the electrical double-layer charge storage mechanism, which restricts their practical application as energy storage devices. To this end, it is still a big challenge to develop a new kind of fiber electrode material integrating high energy density, good rate capability, and mechanical flexibility. Herein, we designed and fabricated a hierarchical electrode for high-performance FSSCs, which is composed of highquality CoNi2S4 nanoplate arrays anchored on a CF backbone (denoted as CoNi2S4@CFs). A hydrothermal in situ vulcanization method was used to obtain such an electrode, which shows advantages of low synthesis temperature and simple operation. The resulting CoNi2S4@CF electrode exhibits high capacitance, good rate performance, and excellent cycling stability, as a result of a synergistic effect among the nanoarray Received: May 12, 2019 Accepted: June 18, 2019 Published: July 9, 2019 11863
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
not only facilitate the dispersion of CFs in aqueous solution but also serve as nucleation centers for the deposition of LDH. CoNi-LDH was grown vertically on the CF filament by a facile hydrothermal method in the presence of hexadecyl trimethyl ammonium bromide (CTAB). As shown in Figure 1D−F, LDH nanoplate arrays with a plate thickness of ∼15 nm can be observed to interlace with each other, forming an opened macroporous structure. Corresponding energy-dispersive X-ray spectrometry (EDS) mapping analysis shows that the ratio of Co/Ni in the LDH is close to 1:2 (Figure S2, Supporting Information). The mass ratio of CoNi-LDH/CFs is around 1:11 in the hybrid fibers, obtained by comparing the weight deference before and after LDH deposition. The X-ray diffraction (XRD) pattern of the as-prepared CoNi-LDH arrays (Figure 2A, bottom line) shows a series of reflections at 10.1°, 20.2°, 34.8°, 38.9°, 59.7°, and 60.9°, which, respectively, correspond to the [003], [006], [012], [015], [110], and [113] diffractions of the typical LDH phase,27−30 indicating their high crystallinity and purity. After a hydrothermal vulcanization treatment in sodium sulfide solution, the diffraction peaks of CoNi-LDH completely disappear; while new diffraction peaks appear at 2θ = 26.7°, 31.5°, 38.2°, 50.3°, and 55.0° (Figure 2A, top line), corresponding to [220], [311], [400], [511], and [440] of the CoNi2S4 spinel structure,20−22 respectively. The SEM image shows that the thickness of the nanoplates was reduced to ∼10 nm (Figure 2B). EDS mapping results (Figures 2C and S3) show a uniform distribution of the Co, Ni, and S elements, with Co/Ni of ∼1/2, indicating no obvious loss of metal elements after vulcanization. Transmission electron microscopy (TEM) observation shows that the CoNi2S4 bimetallic sulfide nanoplates were interlaced with each other (Figure 2D), in agreement with the SEM photograph (Figure 2B). A typical TEM image (Figure 2E) of one single nanoplate displays a large number of nanopores in the CoNi2S4 plates. A series of lattice fringes with spacing of 0.55, 0.17, and 0.24 nm were observed in the highresolution TEM (HR-TEM) image (Figure 2F), corresponding to the [111], [440], and [400] planes of CoNi 2 S 4 , respectively.31 These results further confirm the formation of spinel-structured CoNi2S4 after hydrothermal vulcanization of CoNi-LDH. To understand the detailed transformation process of the CoNi-LDH precursor to spinel CoNi2S4, the structural evolution with increasing vulcanization time (t) was studied. XRD patterns (Figure 3) show that the layered structure of the CoNi-LDH precursor was still preserved within the initial 45 min, accompanied by decreased diffraction intensity as t increases. When the vulcanization time reached 60 min, it was found that the LDH diffraction patterns became very weak and new peaks appeared at 2θ = 31.5° and 55.0°, indicating the formation of the CoNi2S4 spinel structure. When the vulcanization time was extended to 90 min, the diffraction peaks of LDH disappeared completely. Upon further increasing the vulcanization time from 90 to 240 min, the integral diffractions of CoNi2S4 at 31.5°, 38.2°, 50.3°, and 55.0° are increasingly obvious, implying an increased crystallinity of the bimetal sulfide. The conductivity of electrode materials is of crucial importance for their application in electrochemical energyrelated fields. Current−voltage (I−V) curves (Figure 4A) were used to measure the conductivity of CoNi2S4 at different vulcanization times. It was found that the slope of the I−V
architecture, good conductivity of CFs, and high pseudocapacitive activity of CoNi2S4. In addition, the porous structure resulting from the vulcanization process provides numerous active sites for the transport of electrolyte ions. By considering the elemental tunability of LDHs, the hydrothermal in situ vulcanization method presented in this work can be extended to the preparation of other double- or multimetal sulfides, which show broad applications in energy storage and conversion systems.
2. RESULTS AND DISCUSSION The CoNi2S4 nanoplate array was prepared by a two-step process, which involves the in situ growth of CoNi-layered double hydroxide (LDH) on CFs followed by a vulcanization treatment (as shown in Scheme 1). Figure 1A shows a typical Scheme 1. Schematic Illustration for the Synthesis of CoNi2S4@CFs
Figure 1. SEM images of the CFs (A,B) before and (C) after activation in acid. (D−F) SEM images of the CoNi-LDH@CFs with increasing magnification.
scanning electron microscopy (SEM) image of the primary CFs, which displays an electrically conductive network consisting of numerous individual uniform fibers with a diameter of ∼6 μm. From the enlarged SEM photograph (Figure 1B), a relatively smooth surface can be observed. After acid treatment, some irregular bulges appear on the surface of the CFs (Figure 1C), which are caused by the modification of functional groups (e.g., carbonyl, carboxyl, and hydroxyl), resulting in a greatly increased roughness. In addition, X-ray photoelectron spectroscopy (XPS) displays two strong peaks at 284.6 and 286.1 eV ascribed to C−C and −COH groups for pristine CFs (Figure S1, Supporting Information). New peaks appear at 286.8 and 288.7 eV after acid treatment, which is indicative of the surface modification by −CO (epoxide) and −CO (O−CO) groups.26 These functional groups 11864
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
Figure 2. (A) XRD patterns of CoNi-LDH and CoNi2S4. (B) SEM image and (C) EDS spectrum of the CoNi2S4 nanoplate. (D−F) TEM and HRTEM images of the CoNi2S4 nanoplate.
further proves that the electron transport property of the material with t = 120 min was the optimum. The pore structure of the electrode material determines the active site exposure and diffusion kinetic of electrolyte ions and thereby plays significant roles in the electrochemical energy storage. N2-adsorption/desorption measurement was carried out to investigate the porosity property of the CoNi2S4@CFs composite, with precursor CoNi-LDH@CFs as a reference sample (Figure 5). CoNi-LDH and CoNi2S4 show Brunauer− Emmett−Teller (BET) surface areas of 35.35 and 47.95 m2 g−1, respectively, with type-IV isotherms, indicating mesoporous structures for both materials (Figure 5A). Based on the pore-size distribution curves (Figure 5B), we can conclude that the pore sizes of CoNi-LDH and CoNi2S4 are both mainly distributed at 20 nm, ascribed to the interlaced stacking of the nanoplates. Besides, it is noteworthy that CoNi2S4 also displays a pore-size distribution at ∼30 nm, demonstrating that the hydrothermal vulcanization treatment leads to the formation of nanopores in the CoNi2S plates, which is consistent with the TEM observation (Figure 2E). The enriched pore structure could increase the contact between the electrode material and the electrolyte, which will greatly reduce the diffusion resistance of the electrolyte ions and provide more reactive sites to promote the Faraday redox reaction.
Figure 3. XRD patterns of CoNi2S4 obtained at different vulcanization times.
curves increased with extending t from 0 to 120 min, which indicates that the electric conductivity of the material increased continuously. However, if the vulcanization time was further increased to 240 min, the conductivity decreased, probably because of the collapse of the array structure after long-time vulcanization. In addition, the slope of the Tafel curve is also a common means to characterize the internal electronic fluidity of electrode materials. As shown in Figure 4B, the Tafel slope
Figure 4. (A) I−V and (B) Tafel curves of CoNi2S4 synthesized at different vulcanization times. 11865
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
Figure 5. (A) Nitrogen sorption isotherms and (B) pore-size distribution of the CoNi-LDH@CFs and CoNi2S4@CFs electrodes.
Figure 6. (A) Ni 2p and (B) Co 2p XPS spectra of CoNi-LDH and CoNi2S4. (C) S 2p XPS spectra of CoNi2S4.
Figure 7. (A) CV curves of the CoNi-LDH and CoNi2S4 electrodes (scan rate: 60 mV/s). (B) CV curves of the CoNi2S4 electrode collected at various scan rates. (C) Galvanostatic CD curves of CoNi-LDH and CoNi2S4 at 1 A/g. (D) Cycling stability of the CoNi2S4 electrode after 5000 cycles at 5 A/g. (E) Specific capacitance as a function of current density. (F) Electrochemical impedance spectra of the CoNi-LDH and CoNi2S4 electrodes. 11866
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
Figure 8. (A) Schematic diagram illustrating the architecture of the asymmetrical wire-shaped SC device. (B) CV curves of CoNi-LDH@CFs// AC@CF, CoNi2S4@CFs//AC@CF, and AC@CFs//AC@CF devices at a scan rate of 60 mV/s. (C) CV curves collected at scan rates between 20 and 100 mV/s for CoNi2S4@CFs//AC@CF SCs. (D) CV curve of the parallel device at scan rate of 50 mV/s. (E) CV curve of the series device at a scan rate of 50 mV/s (the inset image shows such a device driving a red light-emitting diode).
CoNi2S4 electrode remains unchanged upon increasing the scan rate, with slightly shifted peak position, indicating that the reaction is highly reversible (Figure 7B). The galvanostatic charge/discharge (CD) curves exhibit remarkable pseudocapacitance behavior for both CoNi-LDH and CoNi2S4 electrodes (Figure 7C). The discharge time of the CoNi2S4 electrode is much longer than that of the CoNi-LDH electrode at the same discharging rate, indicating a higher specific capacitance of CoNi2S4, in accordance with the CV results. When the current density is 1 A/g, the calculated specific capacitance of the CoNi2S4 electrode is 1742 F/g; whereas the CoNi-LDH electrodes only shows a capacitance of 1302 F/g. Besides, the CoNi2S4 electrode retains 89% of its original capacitance after 5000 cycles at 5 A/g, showing an excellent cycling stability of the bimetallic sulfide (Figure 7D). Rate capability is a critical parameter of electrochemical capacitors for high-power applications. Figure 7E displays the high current charge and discharge characteristics of the two electrodes as the current density increases from 1 to 60 A/g. The pristine CoNi-LDH electrode retains only ∼34% of the initial capacitance at 60 A/g. In comparison, the CoNi2S4@CF hybrid electrode exhibits a much higher retention of 79% at the same current density, demonstrating its superior high-rate capability. The largely improved rate capability is believed to relate with the increased conductivity and enriched porous structure upon vulcanization treatment.
The valence state of the obtained CoNi-LDH and CoNi2S4 materials are characterized by XPS, as presented in Figure 6A− C. For both CoNi-LDH and CoNi2S4, the spectrum of the Ni 2p3/2 main peak (Figure 6A) can be differentiated into two peaks, locating at 853.2 and 856.5 eV, which are ascribed to Ni2+ and Ni3+, respectively.32 The peaks appearing at 778.2 and 783.2 eV (Figure 6B) correspond to Co3+ and Co2+, respectively.33 It is noted that, the relative ratio of Co3+/ Co2+ decreases and that of Ni3+/Ni2+ increases after vulcanization treatment, indicating the reduction of cobalt and oxidation of nickel upon transformation of CoNi-LDH to CoNi2S4. The typical spectra of S in CoNi2S4 material can be divided into two main peaks (Figure 6C): the peak at 163.2 eV can be attributed to the metal-sulfur bonds and the peak below 162.0 eV is ascribed to the surface-sulfur bonds.34−36 Cyclic voltammogram (CV) curves of the CoNi-LDH and CoNi2S4 electrodes recorded at 60 mV s−1 in a potential window of −0.20 to 0.60 V are shown in Figure 7A. A pair of redox peaks at 0.10 and 0.45 V can be clearly observed in each curve as a result of the Faradaic capacitive behavior, corresponding to the redox reactions of Ni2+/Ni3+, Co2+/ Co3+, and Co3+/Co4+. It is worth noting that the integrated CV area of the CoNi2S4 electrode is significantly larger than that of the CoNi-LDH electrode, which demonstrates that the specific capacitance of the electrode is greatly enhanced after vulcanization. Additionally, the shape of CV curves for the 11867
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
the spinel-structured CoNi2S4 shows a more abundant pore structure compared with CoNi-LDH, which can greatly reduce the transport resistance of electrolyte ions in the faradaic reactions. As a result, the CoNi2S4@CF hybrid electrode exhibits high specific capacitance, desirable rate capability, as well as excellent cycling stability. The CoNi2S4@CF electrode was further assembled into a fiber-shaped micro-supercapacitor device, exhibiting good flexible and wearable abilities, which has broad application prospects in next-generation wearable electronics.
In addition, electrochemical impedance spectroscopy (EIS) measurements were carried out to understand the electron transport and ion transfer property of the hybrid electrode. The EIS data reveal much smaller internal resistances (1.0 Ω) in the Nyquist plots for the CoNi2S4@CF electrode as compared to that of the CoNi-LDH electrode (9.5 Ω). Moreover, the larger slope of the straight line for CoNi2S4 than CoNi-LDH means a facilitated ion transport in the CoNi2S4 electrode, which is ascribed to its more abundant porous structure. These results clearly demonstrate that the CoNi2S4 arrays display favorable charge-transfer kinetics and fast electron transport and thus exhibit dramatically enhanced pseudocapacitive performances. A micro all-solid-state asymmetric supercapacitor device was fabricated to demonstrate the potential application of the CoNi2S4@CFs electrode (schematically shown in Figure 8A). Different from those of a three-electrode system, the CV curves of the all-solid-state devices exhibit a rectangular shape in a potential window of 0−1.0 V (Figure 8B). Compared with those of AC@CFs//AC@CFs and CoNi-LDH@CFs//AC@ CFs devices, the shape of CV curves for the CoNi2S4@CFs// AC@CFs supercapacitor exhibits a more excellent rectangular characteristic. The CV curves at different scan rates display a trend of nearly linear increase in current with the increasing charge/discharge rate, indicating rapid electron-ion transmission within the scan range (Figure 8C). Galvanostatic charge/discharge curves of the three devices (Figure S4, Supporting Information) show that the specific capacitances of the three devices are 632, 493, and 240 F/g for CoNi2S4@ CFs//AC@CFs, AC@CFs//AC@CFs, and CoNi-LDH@ CFs//AC@CFs, respectively. The specific capacitance of the CoNi2S4@CFs//AC@CF supercapacitor is higher than or comparable with that of previously reported microdevices.37−39 However, the Coulombic efficiency of our devices is relatively low, which needs to be further improved. In view of the excellent mechanical properties and flexibility of CFs, this kind of supercapacitors can be easily operated in series or parallel. Three individual CoNi-LDH@CF//AC@CF devices were connected in series or in parallel to evaluate their electrochemical properties. As shown in Figure 8D,E, both parallel and series devices show CV curves with similar shapes as that of the single device, which exhibits its practicability and extensibility. For the parallel device, its current trebles while the voltage remains almost constant, in comparison with single supercapacitors. Howxever, the series device shows nearly the same current but three times the voltage as that of the single supercapacitors, which was used to drive a red light-emitting diode (inset of Figure 8E). In addition, the fiber-shaped CoNi2S4@CF//AC@CF supercapacitor can be further woven into fabrics in a variety of series and parallel connections. These results demonstrate that the CoNi2S4@CF//AC@CF supercapacitor has broad application prospects in the field of flexible and wearable devices.
4. EXPERIMENT SECTION 4.1. Activation of CFs. The CFs were activated with an acid modification method according to previous report.26 In brief, 0.5 g CFs were distributed uniformly into a mixed H2SO4/HNO3 (volume ratio 3:1) solution at 60 °C for 3 h. After the treatment, the CFs were taken out, washed using deionized water, and then dried at 60 °C for 6 h. 4.2. Preparation of CoNi-LDH Arrays Precursor. The CoNi-LDH nanoplate array on CFs was prepared using a simple hydrothermal growth approach. Typically, Ni(NO3)2· 6H2O, CoCl2·6H2O, and CTAB were dissolved in 144 mL methanol/water mixed solution with the volume ratio of 5:1 to get the concentrations of 70, 70, and 38 mM, respectively. Then, the solution was transferred into a Teflon-lined stainless steel autoclave, and the activated CFs were immersed into the mixed solution. Subsequently, the autoclave was sealed and maintained at 180 °C for 24 h. The resulting CoNi-LDH@CFs were washed alternately with deionized water and ethanol several times and dried at 80 °C overnight. 4.3. Preparation of CoNi2S4 Arrays. CoNi2S4 was prepared via an in situ vulcanization treatment of the CoNiLDH precursor. In a typical procedure, one wisp of the CoNiLDH@CFs nanoplatelet array was placed into a Teflon-lined stainless steel autoclave with 0.02 M solution of Na2S·9H2O (60 mL) and then heated at 160 °C for 2 h. After the hydrothermal reaction, the obtained CoNi2S4 array was rinsed thoroughly and dried in 60 °C. Vulcanization with different spans followed the same steps. Except for the discussion part on the vulcanization time, the CoNi2S4 samples for other characterizations were obtained by vulcanization for 2 h. 4.4. Assembly of the All-Solid-State Asymmetric Supercapacitor. CoNi2S4@CFs and active carbon (AC)@ CFs were used as the positive and negative electrodes, respectively. The PVA/KOH used for the solid electrolyte was prepared as follows: KOH was added into 60 mL of deionized water to get the concentration of 1 M and then 6 g of PVA powder was added into this solution. The whole mixture was heated to 90 °C under stirring until the solution became clear. The AC@CFs electrode was obtained by repeated dipping of the CFs into a mixture slurry of AC, polyvinylidene fluoride, and acetylene black with a mass ratio of 8:1:1 several times. After drying in an oven at 60 °C, the AC@CFs electrode was obtained. Then, several roots of CoNi2S4@CFs and AC@CFs were dipped separately into the PVA/KOH gel electrolyte for 3 min. Finally, the two individual electrodes were taken out, dried at 60 °C in vacuum, and then bonded to make the area fully contacted. Thus, the CoNi2S4@ CFs//AC@CFs asymmetric all-solid-state hybrid microcapacitor was obtained. 4.5. Characterization. XRD patterns were recorded by a Rigaku XRD-6000 diffractometer, using Cu Kα radiation (0.15418 nm) at 40 kV, 30 mA. A Zeiss SUPRA 55 SEM and a
3. CONCLUSIONS In summary, a facile and cost-effective route is developed to prepare CoNi2S4@CF electrode materials for one-dimensional FSSCs. In this integrated and hierarchical electrode, spinel CoNi2S4 porous nanoplate arrays were obtained by vulcanization on a CoNi-LDH precursor, which has the advantages of mild conditions and simple synthesis. The internal electron transport capability of the CoNi2S4@CF electrode is nearly 10 times higher than that of CoNi-LDH@CFs. Most significantly, 11868
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
(5) Qiu, D.; Ma, X.; Zhang, J.; Lin, Z.; Zhao, B. In situ synthesis of mesoporous NiO nanoplates embedded in a flexible graphene matrix for supercapacitor electrodes. Mater. Lett. 2018, 232, 163−166. (6) Liu, P.; Zhu, Y.; Gao, X.; Huang, Y.; Wang, Y.; Qin, S.; Zhang, Y. Rational construction of bowl-like MnO2 nanosheets with excellent electrochemical performance for supercapacitor electrodes. Chem. Eng. J. 2018, 350, 79−88. (7) Yan, A.-L.; Wang, X.-C.; Cheng, J.-P. Research progress of NiMn layered double hydroxides for supercapacitors: a review. Nanomaterials 2018, 8, 747−763. (8) Wei, M.; Huang, Q. S.; Zhou, Y. P.; Peng, Z.; Chu, W. Ultrathin nanosheets of cobalt-nickel hydroxides hetero-structure via electrodeposition and precursor adjustment with excellent performance for supercapacitor. J. Energy Chem. 2018, 27, 591−599. (9) Guo, Z.; Ye, W.; Fang, X.; Wan, J.; Ye, Y.; Dong, Y.; Cao, D.; Yan, D. Amorphous cobalt−iron hydroxides as high-efficiency oxygenevolution catalysts based on a facile electrospinning process. Inorg. Chem. Front. 2019, 6, 687−693. (10) Gao, R.; Yan, D. Fast formation of single-unit-cell-thick and defect-rich layered double hydroxide nanosheets with highly enhanced oxygen evolution reaction for water splitting. Nano Res. 2018, 11, 1883−1894. (11) Arif, M.; Yasin, G.; Shakeel, M.; Mushtaq, M. A.; Ye, W.; Fang, X.; Ji, S.; Yan, D. Hierarchical CoFe-layered double hydroxide and gC3N4 heterostructures with enhanced bifunctional photo/electrocatalytic activity towards overall water splitting. Mater. Chem. Front. 2019, 3, 520−531. (12) Li, T.; Zhang, Q. Advanced metal sulfide anode for potassium ion batteries. J. Energy Chem. 2018, 27, 373−374. (13) Liu, H.; Ho, K. H.; Hu, Y.; Ke, Q.; Mao, L.; Zhang, Y.; Wang, J. Doping cobalt hydroxide nanowires for better supercapacitor performance. Acta Mater. 2015, 84, 20−28. (14) Feng, K.; Huang, H.; Shi, D.; Diao, G.; Zhang, X. Ni-Co composite metal embedded porous N-doped carbon for an effective binder-free supercapacitor electrode. J. Mater. Res. 2018, 33, 1167− 1178. (15) Chen, H.; Yu, F.; Wang, G.; Chen, L.; Dai, B.; Peng, S. Nitrogen and sulfur self-doped activated carbon directly derived from elm flower for high-performance supercapacitors. ACS Omega 2018, 3, 4724−4732. (16) Wang, H.-T.; Liu, Y.-N.; Kang, X.-H.; Wang, Y.-F.; Yang, S.-Y.; Bian, S.-W.; Zhu, Q. Flexible hybrid yarn-shaped supercapacitors based on porous nickel cobalt sulfide nanosheet array layers on gold metalized cotton yarns. J. Colloid Interface Sci. 2018, 532, 527−535. (17) Liu, Y.; Lu, Q.; Huang, Z.; Sun, S.; Yu, B.; Evariste, U.; Jiang, G.; Yao, J. Electrodeposition of Ni-Co-S nanosheet arrays on N-doped porous carbon nanofibers for flexible asymmetric supercapacitors. J. Alloys Compd. 2018, 762, 301−311. (18) Chai, Z.; Wang, Z.; Wang, J.; Li, X.; Guo, H. Potentiostatic deposition of nickel cobalt sulfide nanosheet arrays as binder-free electrode for high-performance pseudocapacitor. Ceram. Int. 2018, 44, 15778−15784. (19) Geng, P.; Zheng, S.; Tang, H.; Zhu, R.; Zhang, L.; Cao, S.; Xue, H.; Pang, H. Transition metal sulfides based on graphene for electrochemical energy storage. Adv. Energy Mater. 2018, 8, 1703259− 1703284. (20) Wang, Y.-K.; Zhang, W.-B.; Zhao, Y.; Li, K.; Kong, L.-B. Coprecipitation reaction system synthesis and lithium-ion capacitor energy storage application of the porous structural bimetallic sulfide CoMoS4 nanoparticles. ACS Omega 2018, 3, 8803−8812. (21) Giagloglou, K.; Payne, J. L.; Crouch, C.; Gover, R. K. B.; Connor, P. A.; Irvine, J. T. S. Synthesis and electrochemical study of CoNi2S4 as a novel cathode material in a primary Li thermal battery. J. Electrochem. Soc. 2017, 164, A2159−A2163. (22) Chen, H.; Jiang, J.; Zhang, L.; Wan, H.; Qi, T.; Xia, D. Highly conductively NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors. Nanoscale 2013, 5, 8879−8883. (23) Li, Z.; Zhao, D.; Xu, C.; Ning, J.; Zhong, Y.; Zhang, Z.; Wang, Y.; Hu, Y. Reduced CoNi2S4 nanosheets with enhanced conductivity
JEOL JEM-3010 TEM were used for morphological observation. HR-TEM was collected on an FEI Tecnai G2 F20 S-Twin (200 kV). Al Kα radiation used in XPS measurements were conducted on an ESCALAB 250 instrument (Thermo Electron). Nitrogen adsorption/desorption isotherms were measured on a Quantachrome Autosorb-1CVP analyzer. The specific surface area was calculated using the BET method. All electrochemical measurements were carried on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China). The electrochemical tests on the CoNi2S4@CFs electrode were performed in a three-electrode system by using a saturated Hg/HgCl2 (SCE) electrode and a platinum plate as the reference and counter electrode, respectively, in 3 M KOH aqueous solution. At the open-circuit voltage, an alternating current voltage with 5 mV amplitude was employed in the EIS measurement, while the frequency ranged from 0.01 to 100 kHz in 1.0 M KOH solution. The electrochemical performance of CoNi2S4@CFs//AC@CF all-solid-state devices was measured in a two-electrode system.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01374.
■
XPS results of carbon fibers before and after acid treatment, SEM image and corresponding EDS elemental mapping of Co, Ni and EDS spectrum for CoNi-LDH and CoNi2S4 array, and galvanostatic charge−discharge curves of the three devices (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86-10-64412131. Fax: +86-10-64425385. ORCID
Jingbin Han: 0000-0003-1237-9508 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21671015, 21521005) and the Fundamental Research Funds for the Central Universities (buctylkxj01, XK1802-6 and BHYC1702B).
■
REFERENCES
(1) Wang, R.; Sui, Y.; Huang, S.; Pu, Y.; Cao, P. High-performance flexible all-solid-state asymmetric supercapacitors from nanostructured electrodes prepared by oxidation-assisted dealloying protocol. Chem. Eng. J. 2018, 331, 527−535. (2) Wang, F.; Wu, X.; Yuan, X.; Liu, Z.; Zhang, Y.; Fu, L.; Zhu, Y.; Zhou, Q.; Wu, Y.; Huang, W. Latest advances in supercapacitors: from new electrode materials to novel device designs. Chem. Soc. Rev. 2017, 46, 6816−6854. (3) Lin, X.-Q.; Lü, Q.-F.; Li, Q.; Wu, M.; Liu, R. Fabrication of lowcost and ecofriendly porous biocarbon using konjaku flour as the raw material for high-performance supercapacitor application. ACS Omega 2018, 3, 13283−13289. (4) Zhao, F.; Huang, W.; Zhang, H.; Zhou, D. Facile synthesis of CoNi2S4/Co9S8 composites as advanced electrode materials for supercapacitors. Appl. Surf. Sci. 2017, 426, 1206−1212. 11869
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
ACS Omega
Article
for high-performance supercapacitors. Electrochim. Acta 2018, 278, 33−41. (24) Xu, H.; Hu, X.; Sun, Y.; Yang, H.; Liu, X.; Huang, Y. Flexible fiber-shaped supercapacitors based on hierarchically nanostructured composite electrodes. Nano Res. 2015, 8, 1148−1158. (25) Yang, J.; Wang, Y.; Luo, J.; Chen, L. Highly nitrogen-doped graphitic carbon fibers from sustainable plant protein for supercapacitor. Ind. Crops Prod. 2018, 121, 226−235. (26) Yuan, X.; Zhu, B.; Cai, X.; Qiao, K.; Zhao, S.; Yu, J. Influence of different surface treatments on the interfacial adhesion of graphene oxide/carbon fiber/epoxy composites. Appl. Surf. Sci. 2018, 458, 996−1005. (27) Meng, X.; Feng, M.; Zhang, H.; Ma, Z.; Zhang, C. Solvothermal synthesis of cobalt/nickel layer double hydroxides for energy storage devices. J. Alloys Compd. 2017, 695, 3522−3529. (28) Laing, J.; Ma, R.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. Topochemical synthesis, anion exchange, and exfoliation of Co-Ni layered double hydroxides: a route to positively charged Co-Ni hydroxide nanosheets with tunable composition. Chem. Mater. 2010, 22, 371−378. (29) Xie, L.-j.; Sun, G.-h.; Xie, L.-f.; Su, F.-y.; Li, X.-m.; Liu, Z.; Kong, Q.-q.; Lu, C.-x.; Li, K.-x.; Chen, C.-m. A high energy density asymmetric supercapacitor based on a CoNi-layered double hydroxide and activated carbon. N. Carbon Mater. 2016, 31, 37−45. (30) He, X.; Li, R.; Liu, J.; Liu, Q.; Chen, R.; Song, D.; Wang, J. Hierarchical FeCo2O4@NiCo layered double hydroxide core/shell nanowires for high performance flexible all-solid-state asymmetric supercapacitors. Chem. Eng. J. 2018, 334, 1573−1583. (31) Yang, J.; Ma, M.; Sun, C.; Zhang, Y.; Huang, W.; Dong, X. C. Hybrid NiCo2S4@MnO2 heterostructures for high-performance supercapacitor electrodes. J. Mater. Chem. A 2015, 3, 1258−1264. (32) Shen, L.; Wang, J.; Xu, G.; Li, H.; Dou, H.; Zhang, X. NiCo2S4 nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors. Adv. Energy Mater. 2015, 5, 1400977− 1400983. (33) Saravanakumar, B.; Jayaseelan, S. S.; Seo, M.-K.; Kim, H.-Y.; Kim, B.-S. NiCo2S4 nanosheet-decorated 3D, porous Ni film@Ni wire electrode materials for all solid state asymmetric supercapacitor applications. Nanoscale 2017, 9, 18819−18834. (34) Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang, S. Design hierarchical electrodes with highly conductive NiCo2S4 nanotube arrays grown on carbon fiber paper for high-performance pseudocapacitors. Nano Lett. 2014, 14, 831−838. (35) Wang, Q.; Jiao, L.; Du, H.; Si, Y.; Wang, Y.; Yuan, H. Co3S4 hollow nanospheres grown on graphene as advanced electrode materials for supercapacitors. J. Mater. Chem. 2012, 22, 21387−21391. (36) Zhou, Y.-X.; Yao, H.-B.; Wang, Y.; Liu, H.-L.; Gao, M.-R.; Shen, P.-K.; Yu, S.-H. Hierarchical hollow Co9S8 microspheres: solvothermal synthesis, magnetic, electrochemical, and electrocatalytic properties. Chem.Eur. J. 2010, 16, 12000−12007. (37) Chang, Y.-z.; Han, G.-y.; Xiao, Y.-m.; Zhou, H.-h.; Li, M.-y.; Fu, D.-y.; Zhou, W. High-performance flexible wire-shaped electrochemical capacitors based on gold wire @ reduced graphene oxide. N. Carbon Mater. 2017, 32, 581−591. (38) Yu, J.; Wang, M.; Xu, P.; Cho, S.-H.; Suhr, J.; Gong, K.; Meng, L.; Huang, Y.; Byun, J.-H.; Oh, Y.; Yan, Y.; Chou, T.-W. Ultrahighrate wire-shaped supercapacitor based on graphene fiber. Carbon 2017, 119, 332−338. (39) Kang, Q.; Zhao, J.; Li, X.; Zhu, G.; Feng, X.; Ma, Y.; Huang, W.; Liu, J. A single wire as all-inclusive fully functional supercapacitor. Nano Energy 2017, 32, 201−208.
11870
DOI: 10.1021/acsomega.9b01374 ACS Omega 2019, 4, 11863−11870
