CoNi2S4 Nanoplate Arrays Derived from Hydroxide Precursors for Flexible Fiber-Shaped Supercapacitors.

A high-quality porous CoNi2S4 nanoplates array was in situ synthesized on carbon fibers (CFs) by a hydrothermal method via a CoNi-layered 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 CoNi-LDH@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.

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 hydroxides[5−11] 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 CoNi2O4[20,21] 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 NiO (or CoO) 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 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 low-cost.[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 high-quality 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 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.

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 ). Figure A shows a typical 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 B), a relatively smooth surface can be observed. After acid treatment, some irregular bulges appear on the surface of the CFs (Figure C), 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 −C=O (epoxide) and −C=O (O–C=O) groups.[26] These functional groups not only facilitate the dispersion of CFs in aqueous solution but also serve as nucleation centers for the deposition of LDH.

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.

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 D–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 A, 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 A, 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 B). EDS mapping results (Figures C 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.

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 HR-TEM images of the CoNi2S4 nanoplate.

Transmission electron microscopy (TEM) observation shows that the CoNi2S4 bimetallic sulfide nanoplates were interlaced with each other (Figure D), in agreement with the SEM photograph (Figure B). A typical TEM image (Figure E) 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 high-resolution TEM (HR-TEM) image (Figure F), corresponding to the [111], [440], and [400] planes of CoNi2S4, 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 ) 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.

Figure 3

XRD patterns of CoNi2S4 obtained at different vulcanization times.

The conductivity of electrode materials is of crucial importance for their application in electrochemical energy-related fields. Current–voltage (I–V) curves (Figure A) were used to measure the conductivity of CoNi2S4 at different vulcanization times. It was found that the slope of the I–V 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 B, the Tafel slope further proves that the electron transport property of the material with t = 120 min was the optimum.

Figure 4

(A) I–V and (B) Tafel curves of CoNi2S4 synthesized at different vulcanization times.

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 ). 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 A). Based on the pore-size distribution curves (Figure B), 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 E). 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 5

(A) Nitrogen sorption isotherms and (B) pore-size distribution of the CoNi-LDH@CFs and CoNi2S4@CFs electrodes.

The valence state of the obtained CoNi-LDH and CoNi2S4 materials are characterized by XPS, as presented in Figure A–C. For both CoNi-LDH and CoNi2S4, the spectrum of the Ni 2p3/2 main peak (Figure A) 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 B) 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 C): 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]

Figure 6

(A) Ni 2p and (B) Co 2p XPS spectra of CoNi-LDH and CoNi2S4. (C) S 2p XPS spectra of CoNi2S4.

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 A. 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 CoNi2S4 electrode remains unchanged upon increasing the scan rate, with slightly shifted peak position, indicating that the reaction is highly reversible (Figure B). The galvanostatic charge/discharge (CD) curves exhibit remarkable pseudocapacitance behavior for both CoNi-LDH and CoNi2S4 electrodes (Figure C). 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 D).

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.

Rate capability is a critical parameter of electrochemical capacitors for high-power applications. Figure E 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.

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 A). 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 B). 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 C). 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.

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).

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 D,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 E). 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.

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, 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.

Experiment Section

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.

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.

Preparation of CoNi2S4 Arrays

CoNi2S4 was prepared via an in situ vulcanization treatment of the CoNi-LDH precursor. In a typical procedure, one wisp of the CoNi-LDH@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.

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.

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 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.