A Novel Hierarchical Porous 3D Structured Vanadium Nitride/Carbon Membranes for High-performance Supercapacitor Negative Electrodes
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KeywordsSupercapacitors Vanadium nitride/carbon 3D network Hierarchical porous structure
A novel and simple multi-phase polymeric strategy was used to fabricate hierarchical porous 3D structured vanadium nitride/carbon (VN/C) membranes.
The supercapacitor negative electrodes based on VN/C membranes exhibited a high specific capacitance of 392.0 F g−1 at 0.5 A g−1 and an excellent rate capability with capacitance retention of 50.5% at 30 A g−1.
The asymmetric device fabricated using Ni(OH)2/VN/C membranes has a high energy density of 43.0 Wh kg−1 at a power density of 800 W kg−1 and good cycling stability of 82.9% at 1.0 A g−1 after 8000 cycles.
With the rapid development of the global economy and growing population, energy, as a pillar of modern civilization, has received increasing attention. From the development of clean fuel such as wind power, solar energy, water energy, and tidal power, the tension between rising energy demand and environmental protection is easing [1, 2, 3, 4]. However, the existing energy output of clean-fuel technology is subject to discontinuity and variable environmental factors. For efficient use of renewable energy, it is important to develop high-efficient and stable energy storage devices. Supercapacitors, also called electrochemical capacitors, represent environment-friendly and irreplaceable energy storage devices compared to traditional capacitors and rechargeable batteries. Supercapacitors can achieve greater energy and power densities than conventional energy storage devices [5, 6, 7].
Supercapacitors can be classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors [8, 9, 10, 11, 12]. Considering their high-power densities, long cycle life, and fast charging/discharging rate, EDLCs have been widely used in commercial supercapacitor applications . Typical electrode materials for EDLCs are carbon-based and include activated carbon, carbon black, carbon onions, carbon nanotubes, and graphene [14, 15, 16, 17, 18]. However, a crucial limitation of EDLCs is their low energy densities of approximately 5–15 Wh kg−1, primarily due to the fast sorption and desorption of ions on the carbon-based electrode . Pseudocapacitors chemically store charge through fast and reversible redox reactions at the electrode interface. Electrode materials for pseudocapacitors should exhibit considerable capacity but are typically constrained by poor conductivity and stability. Most electrode materials consist of metal oxides and conducting polymers such as iron oxide , manganese oxide [20, 21], vanadium nitride [22, 23], tungsten nitride , and polyaniline .
To increase the energy storage and stability of the two-electrode materials, it is necessary to combine carbon-based materials with high-capacitance pseudocapacitive materials [6, 16, 20, 21, 22, 23]. Vanadium nitride (VN) has been shown to be a suitable candidate to improve the specific capacitance and energy density of negative electrode materials because of its excellent electrical conductivity as well as its wide and electrochemically stable potential window [26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. It has been reported that coating carbon on the VN surface largely improved its stability during electrochemical reaction . However, two major obstacles hinder the efficient fabrication of VN and carbon electrodes. The first is controlling the uniform distribution of VN nanoparticles in carbon matrix to prevent VN aggregation. The other problem is improving the infiltration of the carbon surface without affecting the VN state and its distribution within the internal holes of the material . In addition, the synthetic routes for nanocomposites of VN and carbon remain limited due to rather time-consuming, costly, and complex fabrication methods which include solution adsorption, chemical vapor deposition, laser atomic layer deposition, and electrospinning. Therefore, developing effective synthetic methods is particularly important for achieving novel supercapacitors with improved performance.
In this study, a novel and simple synthetic method was used to fabricate electrode membrane materials in which VN nanoparticles are uniformly incorporated into a 3D carbon matrix. Solvent exchange, PEG immigration, and self-assembly of the tri-block copolymer PAN-b-PMMA-b-PAN were applied to form an asymmetric 3D polymer membrane with hierarchical porous nanostructure. The electrochemical performances including specific capacitance, rate ability, and energy density based on the 3D VN/C membranes electrode and the supercapacitor device were investigated.
Vanadyl acetylacetonate was purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received without further treatment. Polyethylene glycol (PEG, Mw= 400) was obtained from Aladdin (Shanghai, China). PAN was prepared by solution polymerization as previously described [39, 40]. The tri-block copolymer (BCP) PAN-b-PMMA-b-PAN was synthesized by reversible addition fragmentation chain (RAFT) polymerization as previously described . All other chemicals (analytical grade) were obtained from Sinopharm Chemical Reagents Co. Ltd, China, and used without further purification.
3.2 Preparation of the V Source and Uniformly Distributed Polymer Membrane
Vanadyl acetylacetonate (0.5 g), PAN (1.5 g), PEG (0.5 g), and BCP (0.4 g) were dissolved in N,N-dimethylformamide (DMF, 11.2 g) with continuous stirring at 60 °C until a dark green homogeneous solution was obtained, which was subsequently used as casting solution. The material of vanadyl acetylacetonate/multi-phase polymeric membranes (I) was prepared by spin coating the casting solution on glass at 20 °C. Immediately, the membrane was submerged in deionized water as coagulation bath and subsequently the cured membrane was transferred to an air atmosphere at 40 °C to remove the residual water and solvent in the membrane. A casting solution without PEG and BCP was prepared to fabricate vanadyl acetylacetonate/polymeric membranes (II) using the same process as for vanadyl acetylacetonate/multi-phase polymeric membranes (I).
3.3 Synthesis of the Hierarchical Porous Vanadium Nitride/Carbon (VN/C) Membranes
The V/P-M membrane was first heated at 250 °C for 2 h under air flow and then sintered at 800 °C for 1.5 h under a mixed gas of NH3:N2 = 3:2. After cooling to room temperature, the hierarchical porous VN/C (I) and VN/C (II) were obtained from vanadyl acetylacetonate/multi-phase polymeric membranes (I) and vanadyl acetylacetonate/polymeric membranes (II), respectively.
3.4 Materials Characterization
The microscopic morphologies of the samples were characterized by field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Japan) and transmission electron microscopy (TEM, JEOL, JEM-2010, Japan). The crystal structure was identified by X-ray diffraction (XRD, D/MAX 2400, Japan) with Cu Kα radiation (k = 1.5418 Å) operating at 40 kV and 60 mA. The N2 adsorption–desorption isotherms of samples were measured at 77 K using an ASAP 2460 (Micromeritics, USA) instrument to measure the specific surface area. The specific surface area was calculated using the Brunauer–Emmett–Teller plot of the nitrogen adsorption isotherm. Non-local density functional theory (NLDFT) model was adopted to analyze the pore size distribution of samples (calculation model: slit/cylindrical pore, NLDFT equilibrium model). X-ray photoelectron spectroscopy (XPS) analysis was performed using a PerkinElmer PHI ESCA system with Al Kα (1486.6 eV) as the X-ray source. The electrical conductivity of the samples was determined using a four-point probe (RTS-9).
3.5 Electrochemical Measurements
4 Results and Discussion
In conclusion, the VN/C (I) design with interpenetrating carbon/VN networks, oxygen group-containing surfaces, and hierarchical porous structure was successfully fabricated for use as a supercapacitor electrode material. The advanced structure endowed VN/C (I) with a high specific surface area of approximately 523.5 m2 g−1 and excellent electrochemical behavior, including low resistance, good cyclic stability, and high specific capacitance. VN/C (I) presented a specific capacitance of 392.0 F g−1 at a current density of 0.5 A g−1 in 6.0 M KOH and a good rate capability with capacitance retention of 50.5% at 30 A g−1. Notably, the asymmetric device fabricated with Ni(OH)2//VN/C (I) exhibited a high energy density of 43.0 Wh kg−1 at a power density of 800 W kg−1, which only dropped to 32.3 Wh kg−1 at an increased power density of 4000 W kg−1. Moreover, excellent cycling stability (82.9%) was obtained at a current density of 1 A g−1 after 8000 cycles. This simple and novel strategy can be expanded to the synthesis of other hierarchical porous composite materials combining carbon-based and transition-metal oxide (nitride or sulfide) materials for numerous application in sensors, catalysts, gas separators, and other electrodes in hybrid supercapacitors.
This work was partly supported by the National Natural Science Foundation of China (51203071, 51363014, 51463012, and 51763014), China Postdoctoral Science Foundation (2014M552509 and 2015T81064), Natural Science Funds of the Gansu Province (1506RJZA098), and the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology (J201402), and Joint fund between Shenyang National Laboratory for Materials Science and State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals (18LHPY002).
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