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Article

Facile Synthesis of NiCo2S4/rGO Composites in a Micro-Impinging Stream Reactor for Energy Storage

by
Jiawei Zhang
,
Xiguan Chen
,
Chunyu Liu
and
Lixiong Wen
*
Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(6), 2882; https://0-doi-org.brum.beds.ac.uk/10.3390/app12062882
Submission received: 14 January 2022 / Revised: 5 March 2022 / Accepted: 8 March 2022 / Published: 11 March 2022
(This article belongs to the Special Issue Process Intensification via Rotating Packed Bed (Higee))
Browse Figures
Versions Notes

Abstract

:
Using a process-enhanced micro-impinging stream reactor (MISR) and a co-precipitation route, NiCo2S4 and NiCo2S4/rGO electrode materials were successfully prepared, respectively. Owing to its excellent micromixing performance, the MISR-prepared NiCo2S4/rGO composites had a smaller size and less agglomeration than the same composites prepared in a traditional stirred reactor (STR). The specific capacity of the MISR-prepared composites was as high as 198.0 mAh g−1 under the current density of 1 A g−1. The cycling stability of the composites also improved significantly after being modified with reduced graphene oxide (rGO), and they displayed a fine cycling stability, which maintained a retention rate of 83.6% after 1000 cycles of charging and discharging.

1. Introduction

Supercapacitors (SCs) or electrochemical capacitors (ECs) are novel energy storage systems that can complement traditional battery equipment owing to their high power density and excellent cycle stability. Due to different device structures and charging/discharging mechanisms, supercapacitors are divided into three categories: symmetrical supercapacitors, asymmetrical supercapacitors, and hybrid supercapacitors [1]. However, the energy density of supercapacitors cannot be compared with batteries. Therefore, how to solve the defect of low energy density of supercapacitors without reducing power density and cycle life becomes particularly important. Electrical double-layer capacitors (EDLCs) and pseudo-capacitors are the two basic forms of supercapacitors with different energy storage mechanisms [2,3,4]. With charges accumulating at the electrode/electrolyte interface, the electrical double-layer materials can ensure high power density but low energy density. The specific capacitance and energy density of pseudo-capacitive materials are relatively higher, which depends on the reversible and rapid redox reaction at the surface of electrode materials. Battery-type materials also demonstrate strong Faradaic behavior, which is different with pseudo-capacitive materials [5,6,7]. Hybrid supercapacitors combine battery-type materials with traditional capacitor materials, which provides a novel form of energy storage and can effectively solve some difficult electrochemical problems [8]. Therefore, researchers have done a lot of work to explore the application of high-performance electrode materials in electrochemical energy storage devices.
Electrochemical energy storage materials include traditional carbon materials, metal compounds, and emerging conductive polymer materials, among which metal oxides and sulfides are widely used [9,10]. In recent years, many reports have indicated that electrochemically active transition metal sulfides (TMSs), especially nickel sulfides (NiS and Ni2S3), cobalt sulfides (Co9S8 and Co3S4), and spinel structure nickel-cobalt sulfides (NiCo2S4) are effective electrode materials. Compared with their oxide counterparts, they have better conductivity and higher electrochemical activity. In addition, compared with single-component sulfides, the electrochemical action of nickel-cobalt bimetallic sulfides can give rise to more complex redox reactions, resulting in higher specific capacities [11].
However, NiCo2S4 is highly dependent on the surface Faraday redox reaction and phase transition process caused by ion intercalation, and the reaction kinetics are unfavorable during the rapid charge and discharge process, leading to poor rate performance and electrochemical stability, which will limit its practical application in hybrid supercapacitor technology [11]. Current studies have found that doping nano-structured electroactive materials on a carbon material substrate may be one of the effective and feasible strategies to solve the above problems [12], which guarantees the efficient utilization of active substances and reaction kinetics. For example, graphene has been reported as an exceptional substrate material for improving the electrochemical activity of ultrafine materials [13,14]. However, due to poor interface bonding with the graphene surface, nanoparticles generally tend to agglomerate and detach from the graphene surface, resulting in a decrease in energy storage performance. Therefore, the application of metal sulfides such as NiCo2S4 that can bind to the graphene surface stably without agglomerating will be a good solution to improve the performance of its hybrid supercapacitors [15].
NiCo2S4/rGO composites can be synthesized by different routes, such as hydrothermal [16], electrodeposition [17], co-precipitation [18], and self-template methods [19]. Compared with other methods, co-precipitation has the advantages of a simple preparation process, low cost, and easy control of preparation conditions for large-scale production. The co-precipitation method involves using the precipitant and two or more metal salt solutions to obtain a solid product after co-precipitation. It can precipitate several components at the same time, and the distribution of the components is relatively uniform [20]. It is generally believed that the degree of supersaturation has a great nonlinear effect on the nucleation rate and a high level of supersaturation is the thermodynamic driving force for the phase transition during crystallization processes [21,22]. Since the precipitation process is exceptionally fast, rapid micromixing is essential to create a homogeneous supersaturation, which will in turn ensure the product quality [23]. However, due to the poor micromixing performance and mass transfer efficiency in traditional stirred reactors (STRs), it is difficult to achieve uniform supersaturation, which will have a significant impact on the crystal size distribution and also lead to uneven element distribution in the prepared materials [24].
A proper measure to solve this problem is to develop a reactor with excellent micromixing performance in which a uniform supersaturated environment for nucleation will be provided [25,26]. In the last couple of decades, numerous studies have been invested in the development of microstructure equipment, such as microchannel reactors [27], microfluidic reactors [28], T-mixers [29], and confined impinging jet reactors [30], to enhance the micromixing process. All the reactors mentioned above have the ability to quickly and uniformly mix two miscible fluids, which are suitable for continuous precipitation. However, these devices have various application limitations: (1) the complex structure makes it difficult for manufacturing; (2) some reactors have extremely small inner diameters, which can cause severe blockage problems during precipitation [31,32]. In this regard, the development of new types of micromixing enhancement device, which should be easily constructed and scaled up with slight blockages, is urgently desired for the chemical co-precipitation process.
In this work, we constructed a micro-impinging stream reactor (MISR) based on a capillary reactor and impinging stream technology to enhance the micromixing performance between the reacting fluids for preparing NiCo2S4/rGO composite materials. The MISR consisted of two stainless steel capillary tubes connected to commercial T-joints with no downstream channel at the outlet, which would greatly reduce the clogging problem and pressure drop in the T-junction chamber. When two fluids converged and collided at the T-shaped intersection, uniform supersaturation could be obtained. The homogeneous environment required for uniform particle co-precipitation was thus guaranteed, which has potential to promote the synergistic effect of hybrid electrode materials, thereby obtaining composite materials with excellent electrochemical properties. In addition, loading of particles on graphene has also been investigated in MISR to further facilitate the superelectric properties of the NiCo2S4 composites.

2. Experimental

2.1. Materials

Graphite powder, hydrogen peroxide (H2O2, 30% aqueous solution), and hydrazine hydrate (N2H4·H2O, 80% aqueous solution) used to prepare reduced graphene oxide were purchased from Beijing Tongguang Fine Chemical Company. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), and sodium sulfide nonahydrate (Na2S·9H2O) used to synthesize metal sulfides were provided by Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Acetylene black, polyvinylidene fluoride (PVDF), and N-methyl-2-pyrrolidinone (NMP) used for electrode slurry coating were obtained from Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Potassium hydroxide (KOH) was supplied by Shanghai Meryer Chemical Technology Co., Ltd. (Shanghai, China). Activated carbon (AC) was purchased from Japan Kuraray Co., Ltd. (Tokyo, Japan). All chemicals involved in this work could be used directly without further treatment.

2.2. Construction of Micro-Impinging Stream Reactor

Figure 1a shows the MISR system, which consists of two parallel stainless steel capillaries and a commercial T-joint (Shanghai Emyte Hydraulic Equipment Co., Ltd., Shanghai, China, SS304), similar to that built in our previous work [33]. The two liquid feeds were introduced from the corresponding capillary tubes and collided in the T-joint cavity with a certain volume flow rate through two advection pumps (China Aerospace Science and Technology Corporation, Beijing Satellite Manufacturing Factory, Beijing, China, 2PB00C) to quickly reach a stable supersaturation state, thereby providing a uniform nucleation environment. Reaction products were collected at the outlet for further processing. Figure 1b is a schematic diagram of the MISR setup. Figure 1c demonstrates the internal structure of the T-joint with geometric details.

2.3. Synthesis of NiS, Co3S4, and NiCo2S4

NiCo2S4 composites were synthesized through a co-precipitation method with MISR as follows: precursor solution A was obtained by slowly dissolving 1.1693 g Ni(NO3)2 and 0.5915 g Co(NO3)2 in 100 mL of deionized water. Precursor solution B was prepared by dissolving 1.8133 g Na2S·9H2O in another 100 mL of deionized water. The two precursor solutions were pumped into the MISR through two advection pumps at the same flow rate, and flowed into the collector after impacting in the T-joint. Subsequently, the reaction precipitates in the collector were aged for 3 h. The product was filtered after aging, then needed to be washed repeatedly with deionized water and absolute ethanol until the pH of the filtrate reached neutral, and was then dried in an oven at 60 °C for 12 h to obtain the final NiCo2S4 composites. For comparison, the preparation of NiCo2S4 composites was also carried out under the same conditions in STR. In addition, Co3S4 and NiS were prepared in MISR following the same methods as those for NiCo2S4 composites, but without adding Ni(NO3)2·6H2O and Co(NO3)2·6H2O, respectively.

2.4. Synthesis of NiCo2S4/rGO Composites

The co-precipitation processes of NiCo2S4/rGO were carried out in MISR. GO was first prepared with the improved Hummers’ method, then reduced to rGO using hydrazine hydrate (N2H4·H2O) as the reducing agent. In a typical preparation, 0.05 g of GO was dispersed in 100 mL of deionized water and sonicated for 0.5 h. Subsequently, 1.1693 g of Ni(NO3)2·6H2O and 0.5915 g of Co(NO3)2·6H2O were dissolved in the homogeneous GO dispersion and stirred for 2 h as the precursor solution A. The precursor solution B was obtained by dissolving 1.8133 g Na2S·9H2O in another 100 mL of deionized water. These two precursor solutions collided at high speed inside the MISR and then flowed into the collector. After aging the precipitates for 3 h, an appropriate amount of N2H4·H2O was added to the solution for 1 h reduction treatment at 98 °C. Finally, using the same post-treatment method as the NiCo2S4 composites, the products were filtered, washed, and dried to obtain the NiCo2S4/rGO composites.

2.5. Material Characterization

In this work, an Ultima IV X-ray diffractometer (XRD, Rigaku, Tokyo, Japan) was used to test the crystal structure within 5° to 90° at a scan rate of 10° min−1, along with Cu Kα radiation (λ = 1.542 Å). The Raman spectrum was measured and collected by an inVia Reflex confocal Raman microscope (Renishaw, London, UK) with a shift range of 120–3250 cm−1. The three-dimensional morphology and size of the homogeneously dispersed particles were observed through a HITACHI S-4700 field emission scanning electron microscope (SEM, Hitachi, Tokyo, Japan) with an accelerating voltage of 10 kV. The same microscope was used to perform energy dispersive spectroscopy (EDS) analysis at an increased accelerating voltage of 20 kV to determine the element distribution of the particles. A JEOL JEM-3010 transmission electron microscope (TEM, JEOL, Tokyo, Japan) was used to further observe the particle morphology under 120 kV acceleration voltage. The surface chemical compositions were specified by ESCALAB 250 X-ray photoelectron spectroscopy (XPS, THERMO VG, Waltham, MA, USA) with the C 1s peak (284.8 eV) as the standard peak position for charging correction.

2.6. Preparation and Electrochemical Characterization of Electrodes

Powder materials prepared by MISR and traditional STR needed to be further processed for testing. The working electrode of the prepared composites was constructed first. In an agate mortar, the active material, acetylene black (conductive agent), and polyvinylidene fluoride (PVDF binder) were mixed in a mass ratio of 80:15:5, and the mixture was ground until the powder became uniformly black. An appropriate amount of NMP was added dropwise into the powder and stirred well to form a homogeneous slurry. An amount of 3~5 mg of the prepared mixture was pressed on 1 cm2 of square nickel foam under a pressure of 10 MPa through a tablet press, and then dried at 60 °C in a blast drying oven for 12 h to obtain preliminary electrodes.
The electrochemical performance of the prepared electrodes was characterized with a traditional three-electrode system, in which the electrode to be measured, the Hg/HgO electrode, and the carbon rod were considered as the working electrode, the reference electrode, and the counter electrode, respectively. The aqueous electrolyte was 6 M KOH. Cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) tests were performed on a CHI660C electrochemical workstation (Shanghai CH Instruments Co., Ltd., China), with voltage windows of 0.5 V and 0.45 V, respectively. Electrochemical impedance spectroscopy (EIS) was measured at a frequency of 0.01 to 100 kHz under open circuit voltage. The cycle performance was examined by a CT2001A Land Test System (Wuhan Land Electronics Co., Ltd., Wuhan, China). The specific capacity (Q, mAh g−1) was calculated by the following formula:
Q = I Δ t 3.6 m
where I (A) represents the discharge current, Δt (s) represents the discharge time, and m (g) represents the mass of the active material.
A coin-cell hybrid supercapacitor (HSC) was also assembled using NiCo2S4/rGO particles, activated carbon (AC), and 6 M KOH as positive electrode, negative electrode, and electrolyte, respectively, and the two electrodes were totally separated with a water-based diaphragm. The electrodes were constructed using the same method. The charge balance between the positive and negative electrodes could be achieved with the mass ratio of the positive and negative active materials according to the following formula:
m + m − = Q − Δ V − Q + Δ V +
where m represents the mass of the active material, Q represents the specific capacity, ∆V represents the potential window, and the subscripts “+” and “–” represent the positive and negative electrodes, respectively.
The energy density (E, Wh kg−1) and power density (P, W kg−1) were calculated using the following formulas:
E = C cell V 2 2 × 3.6
P = 3600 E Δ t
where V and Δt represent the potential window (V) and time of discharge (s), respectively, and Ccell represents the specific capacitance of the HSC device according to the overall amount of activated materials in both electrodes.

3. Results and Discussion

3.1. Morphological and Electrochemical Characterization of NiS, Co3S4, and NiCo2S4

XRD patterns of NiS, Co3S4, and NiCo2S4 prepared by MISR are shown in Figure 2a. There were four obvious cobalt sulfide diffraction peaks at 2θ = 19.0°, 31.3°, 36.8°, and 51.3° on the Co3S4 curve, which corresponded to the plane reflections of (111), (220), (311), and (400) for the spinel Co3S4 (JCPDS: 42-1448), respectively. No peaks of other phases appeared in the spectrum, demonstrating that the sample was of high purity. For the NiS curve, the presence of diffraction peaks at 2θ of 19.3°, 34.1°, and 60.3° indicated the successful preparation of NiS (JCPDS: 86-2281). In addition, the XRD curve of NiCo2S4 composites also showed characteristic peaks of NiS with noticeably lower intensity but shifted to higher positions than those of pure NiS, indicating that Co was doped into the NiS lattice.
Figure 2b presents the cycling performance of the prepared materials to explore the effects of Co doping on the electrochemical performance of NiS at different molar ratios. It shows that Co3S4 had the lowest specific capacity (SC) while NiCo2S4 had the highest SC, but the cycling performance of NiS was the poorest with only 26.2% of its initial capacity retained after 1000 cycles. Therefore, when NiS was doped with Co, not only its specific capacity but also its cycling performance were greatly improved, which can be attributed to the synergistic interaction between Co3S4 and NiS in the NiCo2S4 composites. The optimal mole ratio of Ni2+:Co2+ was found to be 2:1 in this work.
Under the designed voltage window of 0~0.5 V, the electrochemical performance of NiS, Co3S4, and NiCo2S4 as working electrodes was tested at a scan rate of 5 mV s−1. Their obvious difference from the nearly ideal rectangular CV curves of the electric double-layer capacitance was that unique redox peaks were displayed in each of the cyclic voltammetry curves in Figure 2c, indicating the typical Faraday behavior of battery-type electrodes [34]. The redox peak pairs of NiCo2S4, NiS, and Co3S4 electrodes appeared at 0.43/0.18 V, 0.47/0.25 V, and 0.31/0.17 V, respectively, which agreed well with the reversible Faraday reactions between Co2+/Co3+/Co4+ and Ni2+/Ni3+ combined with OH- anions, and the charge-discharge mechanism of NiCo2S4 is listed as follows in Equations (5) and (6) [35]. Obviously, the integrated area enclosed by the CV curve of the NiCo2S4 composite material was the largest, which indicates that the NiCo2S4 composite material has the largest specific capacity [36].
N i C o 2 S 4 + O H − + H 2 O ↔ N i S O H + 2 C o S O H + e −
C o S O H + O H − ↔ C o S O + H 2 O + e −
An EIS test was accomplished in the specified frequency range of 0.01–100 kHz to further explore the electron transfer characteristics of the metal sulfide electrode materials. Figure 2d presents the Nyquist fitting plots of NiCo2S4, NiS, and Co3S4. Due to the Faraday reactions of the electrochemical system, its bulk resistance Rs could be determined by the value of the intersection of the EIS curve and the X axis in the high-frequency region, and the charge transfer resistance Rct could be determined by the diameter of the semicircle in the Nyquist diagram [37,38]. The testing results illustrate that the NiCo2S4 electrode had a relatively lower bulk resistance and smaller charge transfer resistance as compared with the Co3S4 and NiS electrodes, demonstrating that the doping of Co into NiS can reduce the intrinsic resistance of the NiCo2S4 electrode and accelerate the electron transport capability. In the low-frequency region, the slope of the curve was related to the Warburg impedance, which was caused by ion diffusion [39]. The curve slope of the NiCo2S4 electrode was close to 90°, which indicates that the ion diffusion in the electrolyte was much faster than that of NiS and Co3S4.
To explore the effects of micromixing efficiency of the reactor on the co-precipitation process, NiCo2S4 composites were prepared using MISR and STR. By SEM observation of the materials in Figure 3a,b, it was found that both composites had an irregular shape. However, the NiCo2S4 composite prepared by STR was more agglomerated than the sample prepared by MISR, with a larger particle size and uneven distribution. The reason for this phenomenon was that the micromixing performance of MISR was better than that of traditional STR, which was similar to other composite materials prepared by MISR and STR as previously reported [40]. Compared with STR, the porous structure of the composite material obtained by MISR can lead to a larger surface area, which is a great advantage for electrode materials to achieve excellent electrochemical performance. In order to investigate the distribution of the Ni, Co, and S elements in the NiCo2S4 particles, EDS mapping analysis was performed as illustrated in Figure 3c, showing uniform distribution of all three elements and no other impurity elements observed, which suggests that the products were of high purity.
Porosity and specific surface area are important textural properties that affect the electrochemical performance of materials, and can be obtained by N2 adsorption and desorption isotherms. The N2 adsorption and desorption curves of the MISR-prepared NiCo2S4 shown in Figure 3d displayed typical IV adsorption behavior. The obvious hysteresis loop with relative pressure (P/P0) in the range 0.6–1.0 indicates that the material had an ordered mesoporous structure. The pore size distribution (PSD) deduced by the separate desorption curve shows that the NiCo2S4 composite had a wide size distribution between 10 and 100 nm. The sharp peak around 20 nm might be caused by the interspace between NiCo2S4 particles. The ordered mesoporous structure of NiCo2S4 resulted in a BET surface area (SBET) of 55.3 m2 g−1, which was much larger than the SBET values of NiS and Co3S4 of 41.5 and 32.7 m2 g−1, respectively. Previous studies reported that a Faraday redox reaction only occurred on a thin surface of the active electrode materials [40]. Increasing BET surface area could enhance the effective contact between electrolyte ions and active materials, thereby promoting the electrochemical performance of the prepared NiCo2S4 composite materials.
The Nyquist plots of the NiCo2S4 prepared by MISR and STR are presented in Figure 4a. These demonstrate that the Rs of NiCo2S4 prepared by MISR was smaller than that prepared by STR, indicating the better diffusion ability of the NiCo2S4 prepared by MISR. The semicircle in the plot was significantly reduced when changing the reactor from STR to MISR, suggesting the decrease of Rct and the enhanced electrical conductivity of the MISR-prepared NiCo2S4, which could be ascribed to the micromixing intensification in MISR.
The cycling performance of the NiCo2S4 composites prepared by MISR and STR was investigated as shown in Figure 4b. The results show that the initial specific capacities of the composites prepared by the two methods at a current density of 1 A g−1 were 198.0 and 149.9 mAh g−1, and their specific capacities remained 74.43% and 52.44%, respectively, after 1000 cycles of charge-discharge. All results show that the MISR-prepared NiCo2S4 composite not only had an excellent initial specific capacity, but also had superior cycling performance due to its faster and more uniform ion diffusion.

3.2. Structure and Morphology of rGO

Owing to the large specific surface area and excellent conductivity of reduced graphene oxide (rGO), NiCo2S4 was also combined with rGO to improve its conductivity and cycling performance.
Graphite was firstly used to synthesize GO by the modified Hummer method and the obtained GO was then reduced with hydrazine hydrate to produce rGO. The rGO Raman curve in Figure 5a shows two high-intensity peaks at 1340 cm−1 and 1618 cm−1, which are consistent with the graphene D band and G band (in-plane vibration of sp2 carbon atoms) [14]. The D and G characteristic peaks of rGO obtained from flake graphite redox were more remarkable than those of GO. The disorder of graphene crystal structure is usually determined by the ratio of D-band and G-band intensity (ID/IG), which also represents the defect density of graphene [13]. The value of ID/IG in Figure 5a is 1.08, indicating that the rGO prepared by the modified Hummer method was much more disordered than the untreated graphite. In addition, the ID/IG value of rGO was also greater than that of the untreated GO, revealing that the average size of the sp2 domain decreased after GO reduction, which is common in GO and other chemically reduced graphene.
XRD spectra of graphite, GO, and rGO are shown in Figure 5b. All curves display the (002) characteristic diffraction peak of graphene around 25°. Compared with GO, the (001) reflection of rGO almost disappeared, suggesting that GO has been effectively reduced [41].
Figure 5c,d present SEM and TEM images of the prepared rGO. Both images reflect successful synthesis of corrugated rGO. The almost transparent and layered flakes indicate an ultra-high specific surface area, which could allocate adequate attachment sites for anchoring NiCo2S4.

3.3. Electrochemical Properties and Characterization of NiCo2S4/rGO Composites

As discussed above, the Co doping in MISR has successfully improved the cycling performance of NiS. However, the improvement in conductivity and rate performance of NiCo2S4 is far from enough. Therefore, the corrugated rGO prepared above was used to modify NiCo2S4 in MISR. The XRD pattern of the obtained NiCo2S4/rGO is displayed in Figure 6a. It demonstrates that the rGO modification did not change the original crystal structure of NiCo2S4. The characteristic peaks of rGO were masked because of its low adding amount, resulting in low diffraction intensity [42]. The above results show that the introduction of rGO would not change the chemical composition of NiCo2S4.
Figure 6b displays the Raman spectrum of NiCo2S4/rGO. Compared with pure rGO, the composites also had two prominent peaks D and G at 1340 and 1618 cm−1, but the intensity was slightly lower. However, the peak-intensity ratio ID/IG still maintained a high value of 1.02, which means that the complex reaction did not change the highly disordered structure of rGO, probably because the deposition of NiCo2S4 particles on the rGO surface could effectively inhibit the stacking of graphene. In addition, there were peaks at 509 cm−1 and 661 cm−1 belonging to metal sulfides [12], confirming that MISR has successfully loaded NiCo2S4 onto rGO.
Elemental composition and corresponding chemical states on the surface of the NiCo2S4/rGO sample were evaluated by XPS. Figure 7a shows an XPS survey spectrum of the NiCo2S4/rGO sample in the range 100–1100 eV. The peaks at 169.7, 284.1, 781.5, and 855.7 eV correspond to S 2p, C 1s, Co 2p, and Ni 2p, respectively [43], indicating the existence of S, C, Co, and Ni elements in the as-prepared sample. The deconvolution fitting of the Ni 2p high-resolution XPS spectrum (Figure 7b) confirmed the existence of the two spin orbitals of Ni2+ and Ni3+ corresponding to Ni 2p3/2 and Ni 2p1/2. Specifically, the Ni 2p3/2 spectrum had a peak at 852.8 eV, corresponding to Ni2+ ions at the octahedral position, and a peak at 855.9 eV, corresponding to Ni3+ ions at the tetrahedral position. The Co 2p XPS spectrum exhibited similar behavior. The convex peaks of Co 2p3/2 at 778.9 eV and Co 2p1/2 at 796.3 eV are shown in Figure 8c, indicating that Co3+ and Co2+ coexisted. The XPS results for nickel and cobalt elements indicate that the prepared NiCo2S4/rGO hybrids contained Ni (Ni2+, Ni3+) and Co (Co2+, Co3+) with different valence states, which is consistent with the CV analysis. This Faraday behavior guaranteed the excellent electrochemical performance of the as-fabricated electrode materials [44,45]. Figure 7d shows the S 2p spectrum at high resolution, where the two main peaks at approximately 162.5 and 161.5 eV correspond to the metal-sulfur bond and the S2- in low coordination on the surface, respectively. The C 1s XPS curve was fitted by peak splitting and four peaks at 284.6 eV, 285.1 eV, 286.1 eV, and 288.9 eV were obtained, corresponding to the four bonding modes of carbon elements in the NiCo2S4/rGO composites (carbon-carbon double bond C=C, carbon-oxygen single bond CO, carbonyl carbon C=O, and carboxylic acid carbon OC=O), respectively. This meant that some oxygen-containing functional groups still existed on the graphene surface [42,46], which might help anchor the NiCo2S4 species on graphene more tightly and strengthen the structure of the hybrid electrodes.
Figure 8 presents TEM and SEM images of the NiCo2S4/rGO composites. It shows that the composites had a much rougher lamellar structure as compared with the smooth wrinkles of rGO (Figure 5d) due to the anchoring of the bimetallic sulfides on the rGO surface. The NiCo2S4 anchoring on rGO appeared uniform, which would take full advantage of the excellent conductivity and large specific surface area of rGO. In addition, the tight anchoring of NiCo2S4 composites on rGO built a barrier-free high-speed transmission channel, which would allow the charge to diffuse quickly and effectively and improve the overall conductivity. From the low-magnification SEM image (Figure 8b), the overall morphology and size distribution of the particles are clearly demonstrated. The particles in the high-magnification SEM image (Figure 8c) exhibited characteristic graphene wrinkles with a grainy surface as a consequence of uniform attachment of NiCo2S4 nanoparticles. Such attachment could act as a spacer between graphene layers [41], which significantly reduces the agglomeration behavior of rGO. Therefore, NiCo2S4 and rGO complemented each other and significantly enhanced the superelectric properties of the electrode materials.
Similar three-electrode systems of the NiCo2S4 composites were built to test the electrochemical properties of NiCo2S4/rGO, including cyclic voltammetry (CV, Figure 9a), galvanostatic charge and discharge (GCD, Figure 9b), electrochemical impedance (EIS, Figure 9c), and cycling performance (Figure 9d). Using 6 M KOH aqueous solution as the electrolyte, CV curves were obtained at the scan rate range 2 to 40 mV−1. The CV curves were similar to that of the NiCo2S4 composites, illustrating remarkable Faraday redox peaks caused by the interactions between Co2+/Co3+ or Ni2+/Ni3+ cations and the OH− anions from the KOH electrolyte. The CV curve of the material at a scan rate of 5 mV s−1 was remarkably symmetrical and the potential difference of the redox peaks was only 209 mV, which was much smaller than that of NiCo2S4 (259 mV), indicating that the electrochemical reversibility was greatly improved. In addition, as the scan rate increased, a shift in the redox peak could be observed, which was due to the increase in the internal diffusion resistance of the electrode [47].
In order to further explore the potential of NiCo2S4/rGO composites as electrode materials, the constant current charge-discharge performance at different current densities (1 to 20 A g−1) was tested in 6 M KOH electrolyte with the voltage window being set to 0 to 0.5 V (vs. Hg/HgO). Figure 9b illustrates that the initial specific capacity values of the NiCo2S4/rGO composite were 198.0, 194.1, 188.3, 176.0, 171.4, and 161.1 mAh g−1, respectively, under different current densities of 1, 2, 5, 7, 10, and 20 A g−1. Based on the initial specific capacity at the fixed current density of 1 A g−1, which corresponded to the best performance, the retention rates at another five current densities of 2, 5, 7, 10, and 20 A g−1 were 98.0%, 95.1%, 88.8%, 86.5%, and 81.4%, respectively. This shows that an excellent rate performance was achieved by the NiCo2S4/rGO composite material to ensure fast charge-discharge ability, due to the special 3D conductive network of the hybrid material for rapid transmission of ions; therefore, it suggests that the rGO sheets are ideal substrates for the uniform nucleation and growth of NiCo2S4 particles.
Figure 9c reveals the fitting Nyquist plots for the NiCo2S4 and NiCo2S4/rGO composites. The semicircle diameter of NiCo2S4 modified with rGO became smaller, suggesting that the Rct was decreased and the electrical conductivity of NiCo2S4 was enhanced due to the doping of rGO and the even loading process in MISR.
The cycling performance of NiCo2S4/rGO and NiCo2S4 composites at a current density of 1 A g−1 was recorded as illustrated in Figure 9d. It shows that the initial specific capacities of the two composite materials prepared at the same optimal conditions were 198.0 and 191.8 mAh g−1, respectively. After 1000 charge-discharge cycles, their specific capacity retention rates were 83.6% and 74.4%, respectively. Therefore, the NiCo2S4 composites modified by rGO had not only higher initial specific capacity, but also more superior cycling performance, because the unique structure of NiCo2S4/rGO can offer more electroactive sites for rapid diffusion of ions.
Figure 9e,f show the SEM images of NCo2S4/rGO before and after the 1000 cycle test. The particles on the electrode sheet consisted of the active material, acetylene black, and the PVDF binder. Their shape and size both remained nearly the same, suggesting that the electrode had good stability during consecutive charge and discharge processes.
Table 1 compares the electrochemical performance of the NiCo2S4/rGO composites prepared in this study with other similar nickel-cobalt-based sulfide materials by different synthesis methods, all of which have been converted into capacity with the same unit [48]. The electrode materials in this work and the references had the same charge-discharge mechanism with a 3-electron process, implying the same theoretical capacity of 263.8 mAh g−1. This shows that our NiCo2S4/rGO composite had the highest initial specific capacity, but its cycling stability was not very satisfactory. However, compared with other methods, MISR technology has several significant advantages. Firstly, the MISR equipment is convenient to construct and easy to operate. Then, it can operate continuously to reduce the reaction time dramatically and to assure the stability of product quality. Moreover, the numbering-up technology of MISR can readily increase its production capacity. Hence, the MISR method might be a prospective technology for preparing high-performance composites by co-precipitation.
In addition, to further explore the electrochemical performance of NiCo2S4/rGO in specific devices, a simple hybrid supercapacitor (HSC) was constructed as shown in Figure 10a. Figure 10b shows the CV curve of activated carbon as a negative electrode with a voltage window of –1 to 0 V, which maintained the characteristic rectangle of an electric double-layer capacitor. Therefore, the cyclic voltammetry (CV) voltage window of the HSC device shown in Figure 10b should be set to 1.5 V. CV curves of the ASC device at various scan rates (10 to 100 mV s−1) are presented in Figure 10c. The broad redox peak indicates that the HSC device composed of NiCo2S4/rGO had the battery-type capacity. The charging and discharging platform of the HSC device is not obvious in Figure 10d, but it shows an almost axisymmetric curve, suggesting an excellent capacitive characteristic. At different current densities of 1, 2, 5, 7, and 10 A g−1, the initial specific capacitances of the HSC device were obtained as 121.2, 107.8, 92.0, 86.6, and 77.5 F g−1, respectively. Moreover, the HSC device demonstrated a fine cycling performance, with a capacitance retention rate of 83.8% after 1000 cycles at a current density of 1 A g−1 (Figure 10e). The coulombic efficiency of this hybrid device is also shown in Figure 10e, which has remained above 98.6% during the entire cycle test process. Figure 10f shows the corresponding IR drop of the device at different current densities. It had the smallest IR drop of 0.092 V at a current density of 1 A g−1, and the IR drops at 2, 5, and 7A g−1 were 0.119 V, 0.126 V, and 0.180 V, respectively, demonstrating a small increase of the IR drop within such a current density range. When the current density reached 10 A g−1, the IR drop increased to 0.658 V sharply. Therefore, the HSC device had a good electrochemical performance at low current densities.
There are two other important electrochemical properties of electrode materials that have often been ignored by researchers, i.e., the self-discharge and leakage current of hybrid devices. In this work, the device was firstly charged to 1.5 V under a very small current density, and then kept at this voltage for 1 h. The current change over such time was recorded to obtain the leakage current curve, as shown in Figure 11a. It can be observed that the leakage current dropped rapidly from the initial 1.9 mA to 0.33 mA within the first 10 min. After that, the leakage current decreased slowly and was finally stabilized at 0.25 mA. For water-based hybrid supercapacitors studied in the past, a leakage current of 0.0255 mA per mg was regarded as an extremely low value, indicating that our device had excellent cycle stability. In addition, in order to further evaluate the charge retention performance of the hybrid device, a self-discharge test was performed by charging the device to 1.5 V with the same method used in the leakage current test, and then removing the power supply and letting it stand for 30 min, as shown in Figure 11b. Unlike the leakage current, self-discharge maintained a mild trend throughout the test. The initial voltage was 1.460 V and remained at 1.216 V after 1 h with a potential difference of 0.244 V, which indicates that this device has an excellent self-discharge performance and hence will have an appealing prospect in the field of energy storage.

4. Conclusions

This work constructed a new type of micro-impinging stream reactor (MISR), and explored the syntheses of NiS, Co3S4, NiCo2S4, and NiCo2S4/rGO composites by the co-precipitation process using this equipment and their applications in hybrid supercapacitor electrode materials. As compared to NiS and Co3S4, the NiCo2S4 composites were found to possess several advantages such as smaller particle size, more uniform mesoporous structure, larger specific surface area and, in turn, more superior electrochemical performance. In addition, NiCo2S4 could be quickly anchored on the surface of rGO through MISR to enhance its performance. The NiCo2S4/rGO composites had the highest specific capacity of 198.0 F g−1 and the capacity retention after 1000 electrochemical cycles was 83.6% under a current density of 1 A g−1. Changing the current density to 20 A g−1, the specific capacity of NiCo2S4/rGO could reach 161.1 mAh g−1, and the retention rate was 81.4%, suggesting an outstanding rate capability. Furthermore, the NiCo2S4/rGO//AC asymmetric supercapacitor exhibited a fine cycling performance with 83.8% of the maximum capacitance retention after 1000 electrochemical cycles at a current density of 1 A g−1. This work shows that the MISR method may have great prospects in preparing high-performance ultrafine composites.

Author Contributions

Conceptualization, J.Z. and L.W.; methodology, J.Z. and L.W.; investigation, J.Z., X.C. and C.L.; resources, L.W. and X.C.; data curation, J.Z.; writing—original draft preparation, J.Z. and X.C.; writing—review and editing, J.Z. and L.W.; supervision, L.W.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Major Project of National Science and Technology from China (No. 2018ZX09721002-002), the National Key R&D Program of China (No. 2017YFB0307202), and the National Natural Science Foundation of China (Nos. 21576012).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) MISR experimental system; (b) operation flow chart of MISR (A, B, and C are precursor containers and product collector, respectively); (c) internal structure of the T-joint (geometric details: inner diameter of the feed pipes d = 0.6 mm, length of the T-joint L = 2 mm, inner diameter of the T-joint D = 1.8 mm).
Figure 1. (a) MISR experimental system; (b) operation flow chart of MISR (A, B, and C are precursor containers and product collector, respectively); (c) internal structure of the T-joint (geometric details: inner diameter of the feed pipes d = 0.6 mm, length of the T-joint L = 2 mm, inner diameter of the T-joint D = 1.8 mm).
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Figure 2. (a) XRD analyses; (b) cycling stability under a current density of 1 A g−1; (c) CV measurements at a scan rate of 5 mV s−1; (d) Nyquist spectra (EIS) of the NiS, Co3S4, and NiCo2S4 composites prepared in MISR.
Figure 2. (a) XRD analyses; (b) cycling stability under a current density of 1 A g−1; (c) CV measurements at a scan rate of 5 mV s−1; (d) Nyquist spectra (EIS) of the NiS, Co3S4, and NiCo2S4 composites prepared in MISR.
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Figure 3. SEM images of (a) the MISR-prepared and (b) the STR-prepared NiCo2S4. (c) Element distribution (EDS) of the MISR-prepared NiCo2S4; (d) N2 adsorption/desorption isotherms and PSDs (the insertion) of the MISR-prepared NiCo2S4.
Figure 3. SEM images of (a) the MISR-prepared and (b) the STR-prepared NiCo2S4. (c) Element distribution (EDS) of the MISR-prepared NiCo2S4; (d) N2 adsorption/desorption isotherms and PSDs (the insertion) of the MISR-prepared NiCo2S4.
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Figure 4. (a) EIS Nyquist plots and (b) cycling performance of the MISR-prepared and the STR-prepared NiCo2S4 composites.
Figure 4. (a) EIS Nyquist plots and (b) cycling performance of the MISR-prepared and the STR-prepared NiCo2S4 composites.
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Figure 5. (a) Raman spectra and (b) XRD spectra of graphite, GO, and rGO; (c) SEM and (d) TEM image of rGO.
Figure 5. (a) Raman spectra and (b) XRD spectra of graphite, GO, and rGO; (c) SEM and (d) TEM image of rGO.
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Figure 6. (a) XRD pattern and (b) Raman spectrum of NiCo2S4/rGO (D, G: Characteristic peaks of rGO).
Figure 6. (a) XRD pattern and (b) Raman spectrum of NiCo2S4/rGO (D, G: Characteristic peaks of rGO).
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Figure 7. (a) XPS spectrum; (b) Ni 2p XPS; (c) Co 2p XPS; (d) S 2p XPS; (e) C 1s XPS of NiCo2S4/rGO composites.
Figure 7. (a) XPS spectrum; (b) Ni 2p XPS; (c) Co 2p XPS; (d) S 2p XPS; (e) C 1s XPS of NiCo2S4/rGO composites.
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Figure 8. (a) TEM; (b) low-magnification and (c) high-magnification SEM images of the NiCo2S4/rGO composites.
Figure 8. (a) TEM; (b) low-magnification and (c) high-magnification SEM images of the NiCo2S4/rGO composites.
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Figure 9. (a) CV curves at various scan rates, (b) galvanostatic charge and discharge diagram at different current densities, (c) EIS spectra, and (d) cycling performance of the NiCo2S4/rGO composites; SEM images of NiCo2S4/rGO composites (e) before and (f) after 1000 charge-discharge cycles.
Figure 9. (a) CV curves at various scan rates, (b) galvanostatic charge and discharge diagram at different current densities, (c) EIS spectra, and (d) cycling performance of the NiCo2S4/rGO composites; SEM images of NiCo2S4/rGO composites (e) before and (f) after 1000 charge-discharge cycles.
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Figure 10. (a) Scheme of the hybrid supercapacitor (NiCo2S4/rGO//activated carbon); (b) CV measurements of NiCo2S4/rGO and activated carbon electrodes at a scan rate of 5 mV s−1 in a three-electrode system; (c) CV curves of NiCo2S4/rGO//activated carbon HSC device at different scan rates; (d) initial GCD curves of NiCo2S4/rGO//activated carbon HSC device at different current densities; (e) the cycle stability and coulombic efficiency of NiCo2S4/rGO//activated carbon HSC device at a current density of 1 A g−1; (f) IR drop of HSC device under different current densities.
Figure 10. (a) Scheme of the hybrid supercapacitor (NiCo2S4/rGO//activated carbon); (b) CV measurements of NiCo2S4/rGO and activated carbon electrodes at a scan rate of 5 mV s−1 in a three-electrode system; (c) CV curves of NiCo2S4/rGO//activated carbon HSC device at different scan rates; (d) initial GCD curves of NiCo2S4/rGO//activated carbon HSC device at different current densities; (e) the cycle stability and coulombic efficiency of NiCo2S4/rGO//activated carbon HSC device at a current density of 1 A g−1; (f) IR drop of HSC device under different current densities.
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Figure 11. (a) Leakage current and (b) self-discharge of supercapacitor device.
Figure 11. (a) Leakage current and (b) self-discharge of supercapacitor device.
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Table
Table 1. Electrochemical comparison of NiCo-based sulfides synthesized by different methods.
Table 1. Electrochemical comparison of NiCo-based sulfides synthesized by different methods.
MaterialsMethodsCapacityCapacity Retention Rate
NiCo2S4/rGO
(This work)		MISR198 mAh g−1
(1 A g−1)		83.6%
(1 A g−1, 1000 cycles)
Tube-like NiCo2S4 [49]Solvothermal method116.4 mAh g−1
(3 A g−1)		75.9%
(10 A g−1, 5000 cycles)
Ni1.5Co1.5S4 [12]Polyol method167.0 mAh g−1
(1 A g−1)		108%
(10 A g−1, 2000 cycles)
NiCo2S4 arrays on nickel foam [50]Hydrothermal method102.2 mAh g−1
(0.5 A g−1)		85%
(5 A g−1, 5000 cycles)
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Zhang, J.; Chen, X.; Liu, C.; Wen, L. Facile Synthesis of NiCo2S4/rGO Composites in a Micro-Impinging Stream Reactor for Energy Storage. Appl. Sci. 2022, 12, 2882. https://0-doi-org.brum.beds.ac.uk/10.3390/app12062882

AMA Style

Zhang J, Chen X, Liu C, Wen L. Facile Synthesis of NiCo2S4/rGO Composites in a Micro-Impinging Stream Reactor for Energy Storage. Applied Sciences. 2022; 12(6):2882. https://0-doi-org.brum.beds.ac.uk/10.3390/app12062882

Chicago/Turabian Style

Zhang, Jiawei, Xiguan Chen, Chunyu Liu, and Lixiong Wen. 2022. "Facile Synthesis of NiCo2S4/rGO Composites in a Micro-Impinging Stream Reactor for Energy Storage" Applied Sciences 12, no. 6: 2882. https://0-doi-org.brum.beds.ac.uk/10.3390/app12062882

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