Bimetallic NiCo2S4 Nanoneedles Anchored on Mesocarbon Microbeads as Advanced Electrodes for Asymmetric Supercapacitors
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A facile method was used to anchor pseudocapacitive bimetallic NiCo2S4 (NCS) nanoneedles on mesocarbon microbeads (MCMBs), forming a novel urchin-like structure.
The NCS@MCMB electrode and an assembled asymmetric supercapacitor displayed good electrochemical performance.
KeywordsBimetallic sulfides NiCo2S4 Nanoneedles Mesocarbon microbeads Asymmetric supercapacitor
In the last few decades, the growing demand for high-performance mobile devices and renewable energy has promoted the continuous development of energy storage devices [1, 2, 3, 4, 5]. Supercapacitors have shown potential applications in energy storage, owing to their high power density, short charge–discharge time, long cycle life, and environmental friendliness [6, 7, 8, 9, 10, 11, 12, 13, 14]. The electrochemical performance of supercapacitors is strongly dependent on the electrode materials, which is also the key element for overcoming the limited energy density issues affecting supercapacitors. From the viewpoint of electrochemical performance, an ideal electrode material is expected to provide high specific capacitance and outstanding electrochemical stability. Despite their high electrical conductivity and long cycle life, traditional carbon materials exhibit capacitances based on the electrochemical double-layer capacitor (EDLC) mechanism; therefore, they cannot provide the large energy densities required by advanced energy devices [15, 16, 17]. In comparison, the specific capacitance of pseudocapacitive materials such as conducting polymer and transition metal compounds generally originates from reversible redox reactions; therefore, these materials can provide larger specific capacitances and higher energy densities [18, 19, 20, 21, 22, 23, 24].
Transition metal sulfides, especially bimetallic Ni–Co sulfides, have recently attracted considerable scientific and technological interest due to their excellent electrochemical performance [25, 26, 27, 28, 29]. On the one hand, these materials possess higher electrical conductivity, due to their lower band gap than that of ternary Ni–Co oxides [30, 31, 32]. On the other hand, the Ni–Co sulfides show richer Faradaic redox reactions than monometallic sulfides [33, 34], due to the contribution from the bimetallic elements, which results in higher specific capacitances [35, 36, 37, 38]. However, similar to most other pseudocapacitive materials, the intrinsically low electronic conductivity, irreversible redox reactions, and structural instability of Ni–Co sulfides often limit their rate capability and cycle life after repeated Faradaic redox reactions [39, 40]. Although various bimetallic Ni–Co sulfides with different morphologies and components have been explored, their electronic conductivity and structural stability during long-term cycling are still unsatisfactory. To overcome these drawbacks, various carbonaceous materials were considered as hosts to load active materials and prepare composites. Several advantageous features have been identified for these carbon–NCS composites, including a highly open structure based on their carbon framework, an improved electrical conductivity of the electrode, an enhanced utilization of the active material, and a reduced dissolution of active materials into electrolytes. Compared to the widely used carbon nanotubes [41, 42, 43, 44] and graphene [45, 46, 47, 48], mesocarbon microbead (MCMB) materials are regarded as promising carbon hosts due to their low cost and easy synthesis, which are favorable for large-scale applications. The high conductivity of MCMB is expected to enhance the overall electrical conductivity of the composite, thus resulting in an improved electron transfer during electrochemical reactions. The spherical and open structure of MCMB can also provide a large number of adsorption sites to anchor other materials, which would enable a high loading of active materials. In addition, the robust solid structure of MCMB can produce a highly stable composite without massive agglomeration during electrochemical reactions, leading to a high structural stability of the electrode.
In this work, we adopted a facile two-step hydrothermal method to prepare a novel NiCo2S4@MCMB composite with urchin-like core–shell structure, by directly growing NiCo2S4 (NCS) nanoneedles on MCMBs. When evaluated as an electrode material for supercapacitors, the as-prepared NCS@MCMB composite displayed satisfactory electrochemical performance, which was attributed to its stable and open urchin-like structure. Moreover, the NCS@MCMB composite was also tested as an electrode material of asymmetric supercapacitors (ASCs), further confirming its potential in practical applications such as energy storage devices.
2 Experimental Section
2.1 Material Preparation
2.1.1 Synthesis of NCS@MCMB Precursors
The MCMB reactant was an industrial product prepared from coal tar pitch. The composites were synthesized by a facile two-step hydrothermal process: 101.6 mg of MCMB was first dispersed in 40 mL of deionized water by ultrasonication for 30 min, to obtain a suspension. Then, 1 mmol of Ni(NO3)2·6H2O, 2 mmol of Co(NO3)2·6H2O, and 12 mmol of urea were dissolved into the well-dispersed MCMB suspension, and the mixture was stirred for 20 min to obtain a clear dark pink solution. The suspension was placed into a 50-mL Teflon-lined stainless steel autoclave and maintained at 120 °C for 8 h. After cooling down to room temperature, the precipitate was collected and washed several times with deionized water and ethanol, followed by drying at 60 °C for 24 h in vacuum prior to the subsequent hydrothermal process.
2.1.2 Synthesis of NCS@MCMB
Na2S·9H2O (1.17 g) was dissolved in 40 mL of deionized water, followed by the addition of the obtained NCS@MCMB precursor. The suspension was stirred for 20 min, transferred into a 50 mL Teflon-lined stainless steel autoclave, and maintained at 160 °C for 12 h. After cooling down to room temperature, the precipitate was collected and washed several times with deionized water and ethanol, and then dried at 60 °C for 24 h in vacuum to yield a sample containing 75% NCS, labeled NCS@MCMB-75%.
For comparison, two additional samples, labeled NCS@MCMB-65% and NCS@MCMB-85%, were synthesized under the same reaction conditions but with NCS percentages of 65% and 85%, respectively. Pure NCS, CoS@MCMB, and NiS@MCMB were also synthesized under the same conditions.
2.2 Material Characterization
The crystalline structure of the samples was identified by X-ray diffraction (XRD) using a Rigaku D/max 2500 V diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å), operating voltage and current of 40 kV and 40 mA, respectively, and 2θ range from 20° to 70°. The morphology was characterized by Sirion 200 scanning electron microscopy (SEM, FEI, USA) and JEM 2100 F transmission electron microscopy (TEM, JEOL, Japan) with operating voltage 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken using a Kratos Axis spectrometer (Kratos, Britain) with monochromatic Al Kα radiation (1486.71 eV), operating voltage and current of 15 kV and 10 mA, respectively, and a hemispherical electron energy analyzer. Raman spectra were recorder on an inVia spectrometer (Renishaw, Britain) with a 514 nm excitation source. Nitrogen adsorption/desorption isotherms at 77 K were measured using a Micromeritics Autosorb-iQ2-C instrument (Quantachrome, USA); specific surface areas were estimated by the Brunauer–Emmett–Teller (BET) method. Thermogravimetric analysis (TGA) was performed using Pyris 1 analyzer (PerkinElmer, USA) by heating under air flow from 30 to 900 °C at a rate of 10 °C min−1.
2.3 Electrochemical Measurements
Electrochemical tests were performed on an electrochemical workstation (CHI 760e, Shanghai Chenhua, China), using a 3 M KOH aqueous solution at room temperature. The tests were performed in a three-electrode configuration with Pt foil, Ag/AgCl, and sample-modified nickel foam as counter, reference, and working electrodes, respectively. To prepare the working electrodes, the samples, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a 80:10:10 weight ratio in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was pasted uniformly onto Ni foam (1 cm × 4 cm) with a thickness of 2 mm and dried in a vacuum oven at 120 °C for 12 h to remove the solvent. The mass loading of each electrode was about 2.5 mg (pasting surface area: 1 cm × 1 cm), corresponding to an areal mass loading capacity of 2.5 mg cm−2. The specific capacitance (F g−1) and current rate (A g−1) values were calculated based on the total mass of active material. Cyclic voltammetry (CV) measurements were taken from 0 to 0.5 V, at scanning rates from 2 to 100 mV s−1. Galvanostatic charge–discharge (GCD) measurements were taken from 0 to 0.5 V at current densities of 1–10 A g−1. Electrochemical impedance spectroscopy (EIS) measurements were taken by applying an alternate current voltage amplitude of 5 mV in the frequency range from 100 kHz to 0.01 Hz. All electrochemical measurements were taken at room temperature, and the specific capacitance (C) was calculated as C = (I × Δt)/(ΔV × m), where m is the mass (g) of active material, I is the constant discharge current (A), ΔV is the applied potential window (V), and t denotes the discharge time (s).
The ASC device was fabricated using active carbon (AC) and the NCS@MCMB composite as the negative and positive electrodes, respectively. The device was fabricated using a CR2016-type coin cell with 3 M KOH solution as the electrolyte and one piece of cellulose paper as the separator. The energy density (E, Wh kg−1) and power density (P, W kg−1) of the NCS@MCMB//AC ASC device were calculated from the equations: E = CV2/2 and P = E/Δt, respectively, where V is the applied potential and Δt is the discharge time.
3 Results and Discussion
The long-term cycling performance of the NCS and NCS@MCMB-75% electrodes were investigated at 5 A g−1 for 3000 cycles, and the corresponding results are shown in Fig. 6d. The capacitance retention of NCS@MCMB over 3000 cycles was 94%, larger than the 82% value obtained for pure NCS, which highlights an obvious improvement in cycling stability. The cycling stability of NCS@MCMB was also improved with respect to that of previously reported NCS electrodes [59, 60, 61, 62], demonstrating its good structural stability during electrochemical reactions. Apart from the high electrical conductivity revealed by the EIS results, the sustained cycling stability of the NCS@MCMB composite also benefitted from the tight integration, better interfacial interaction, and improved electrical contact between NCS nanoneedles and MCMB. In addition, MCMB served as an ideal substrate material to reduce the agglomeration of NCS nanoneedles during the long-term cycling tests, resulting in a highly stable composite structure.
As shown in Fig. 6e, the CV curves of the NCS@MCMB-75% electrode were further evaluated at different scan rates ranging from 2 to 100 mV s−1, within the potential window of 0–0.5 V. The 50-fold increase in scan rate resulted in the anode peak shifting slightly from 0.29 to 0.23 V, demonstrating the excellent high-rate reversibility of the composite electrode [63, 64]. The obvious redox peaks in the CV curves confirmed the pseudocapacitive behavior of NCS, which was responsible for its large capacitance [62, 65]. Similar Faradaic redox peaks were observed in both NCS@MCMB-65% and NCS@MCMB-85% composites (Fig. S6b, d), which also exhibited more obvious redox peaks compared with those of pure NCS or MCMB (Fig. S7b, d), further confirming the enhanced pseudocapacitive characteristics of NCS@MCMB. The nonlinear trend of the GCD curves at high current densities ranging from 1 to 10 A g−1 (Fig. 6f) further confirmed the Faradaic characteristic of NCS@MCMB. The GCD curves presented voltage plateaus at around 0.35 V consistent with the CV curves, implying the high reversibility of the composite. The specific capacitance values at different current densities and the capacitance retention values (Fig. 6g, h) were calculated from the GCD measurements of various electrodes (Figs. 6f, S6a, c, and S7a, c). Although pure NCS possessed a high capacitance of 632 F g−1 at 1 A g−1, this value sharply decreased with the gradual increase in current density (1.6% capacitance retention at 10 A g−1). In contrast, MCMB exhibited a lower specific capacitance (32 F g−1 at 1 A g−1) but also an excellent rate performance (62.5% capacitance retention). Therefore, the NCS@MCMB composite exhibited a synergistic effect between the two components, which led to greatly improved capacitance and rate performance. Among the composites with different NCS contents, the NCS@MCMB-75% electrode displayed the highest specific capacitances, with values of 936, 888, 794, and 670 F g−1 at current densities of 1, 2, 5, and 10 A g−1, respectively. This indicated that about 71.58% of the specific capacitance was retained after a tenfold increase in current density, indicating excellent rate capability. In contrast, the lower capacitance of NCS@MCMB-85% (890 F g−1 at 1 A g−1) might be due to the underutilization of the NCS grains that were not loaded on MCMB. Despite a slightly higher capacitance retention of 80.96% than that of NCS@MCMB-75%, the NCS@MCMB-65% composite displayed a much lower capacitance (788 F g−1 at 1 A g−1), due to its lower NCS content. Therefore, we inferred that the NCS@MCMB-75% composite possessed an optimal NCS:MCMB ratio, which achieved a favorable balance between high capacitance and high-rate performance. The superior electrochemical performances of the NCS@MCMB composites originated from their unique urchin-like structure (Fig. 6i). On the one hand, the NCS nanoneedles were sufficiently unfolded to gain full contact with the OH− groups in the electrolyte, thereby promoting the Faradaic redox processes on the NCS nanoneedles. On the other hand, the highly conductive MCMB served as a substrate to support individual NCS nanoneedles, which greatly enhanced the electron transfer in the composite. The favorable charge transfer in the NCS@MCMB composite resulted in high specific capacitances, promising high-rate performance, and excellent long-term cycling stability. We also prepared monometallic CoS@MCMB and NiS@MCMB composites, and their CV and GCD curves are shown in Fig. S8. Both materials showed smaller capacitances than NCS@MCMB (391 F g−1 for CoS@MCMB and 474 F g−1 for NiS@MCMB), demonstrating the beneficial effect of bimetallic Ni–Co sulfides.
The outstanding electrochemical performances of the MCMB@NCS composite could be ascribed to its unique microstructure and the synergistic effect of MCMB and NCS. First, the bimetallic sulfides possessed abundant active sites for the Faradaic redox reactions, resulting in a high specific capacitance of the composite. Second, the MCMB component imparted excellent electrical conductivity to the composite, leading to rapid electron transfer and high-rate performance. Third, the composite presented a highly porous structure, which enhanced cationic diffusion in the electrolyte, thereby leading to improved ion transport efficiency. Fourth, the NCS nanoneedles were uniformly anchored on the MCMB surface, which allowed them to be fully unfolded and effectively contribute to the Faradaic redox reactions, thereby leading to a greatly increased utilization ratio of NCS and superior pseudocapacitive characteristics. Moreover, the stable composite structure with a strong NCS-MCMB interaction allowed maintaining a high structural stability during the redox reactions, thereby leading to highly reversible electrochemical reactions and significantly enhanced cycling performance.
In summary, a simple two-step hydrothermal strategy was developed to synthesize a NCS@MCMB composite with unique urchin-like core–shell structure, in which bimetallic NCS nanoneedles were uniformly anchored on MCMB. When evaluated as an electrode material for supercapacitors, the unique structure and synergistic effects endowed the composite electrode with a high specific capacitance of 936 F g−1 at 1 A g−1, high cyclic stability (96.2% capacitance retention after 3000 cycles), and promising rate capability (670 F g−1 at 10 A g−1). An asymmetric supercapacitor was also fabricated, which showed great promise for practical applications in the energy storage field. Moreover, the present fabrication procedure and device architecture could be extended to other active bimetallic and carbonaceous materials and promote future applications in high-performance electrochemical energy storage/conversion devices.
This work was jointly supported by the National Natural Science Foundations of China (No. 51572246) and the Fundamental Research Funds for the Central Universities (Nos. 2652017401 and 2652015425).
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