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Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors
Journal Pre-proofs Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors Zhengwei Xiong, Jiang Yang, Zhipeng Gao, Qiang Yang, Deli Shi PII: DOI: Reference:
S2211-3797(19)32198-9 https://doi.org/10.1016/j.rinp.2020.102941 RINP 102941
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Results in Physics
Received Date: Revised Date: Accepted Date:
22 July 2019 9 January 2020 12 January 2020
Please cite this article as: Xiong, Z., Yang, J., Gao, Z., Yang, Q., Shi, D., Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors, Results in Physics (2020), doi: https:// doi.org/10.1016/j.rinp.2020.102941
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Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors Zhengwei Xiong,1, * Jiang Yang,1 Zhipeng Gao,2 Qiang Yang,3 Deli Shi,1, * 1Joint
Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang 621010, China
2Institute
of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China 3China
Academy of Engineering Physics, Mianyang621000, China
Corresponding authors: Email: [email protected] and [email protected] Abstract: As a typical supercapacitor electrode material, the reported Mo-N compounds mostly own one part of outstanding electrochemical properties (specific capacitance (Csp), voltage window and cycling stability), and the well integration with three aspects was rare. Here using a novel MoO3 precursor with ultra-long nanobelts (NBs) and stable orthorhombic crystal structure, we prepared unique Mo3N2 NBs, which inherited the morphology and structure of the precursor greatly. Compared with other Mo-N compounds, the Mo3N2 NBs both possessed the high Csp (220.3 F/g at 0.4 A/g), great cycling stability (98.3% Csp retention after 1500 cycles) and wide voltage window (0.95 V) due to the improved morphology, stable structure and excellent conductivity. Key words: Electron microscopy; Energy storage and conversion; Microstructure; Supercapacitor; Mo3N2 Introduction Electrochemical capacitors, also known as supercapacitors, provide higher power density than traditional batteries and capacitors [1]. The electrode materials that make up the
supercapacitors include: (1) carbon materials, which hold excellent conductivity but low Csp [2-4]; (2) conducting polymers, which have great conductivity but inferior cycling stability [5]; (3) transition metal oxides, which have high Csp but bad conductivity [10-12]; and (4) transition metal nitrides (TMNs), which possess fantastic electrical conductivity, superior stability and Csp concurrently [13]. Many TMNs were researched, such as WN, VN, MoN and so on [14]. Among the TMNs, molybdenum-nitride (Mo-N) was the first metal nitride to be reported as supercapacitor electrode material, because of exceptional chemical stability and good electrical conductivity [14]. In the past works, many scholars took MoO3 as precursor by a temperature-programmed reaction with NH3 in a closed chamber to prepare Mo-N [14-16]. Liu et al. synthesized Mo2N NBs with a low Csp of 160 F/g with a potential window ranging from -0.2 to 0.4 V and 91% Csp retention after 1000 cycles [14]. Lee et al. prepared Mo2N nanowires, which own 220 F/g with voltage window ranging from -1.1 to -0.3 V [15]. Yuan et al. fabricated Mo3N2 NBs composites, which display a high Csp of 282 F/g with a narrow potential window ranging from 0 to 0.45 V, leading to the low Energy densities and power densities [16]. Until now, there are no Mo-N compounds which could combine the excellent Csp, great cycling stability and wide voltage window. Lots of investigations revealed that the electrochemical properties of the Mo-N were determined by the crystal structure and morphology of MoO3 precursor [16-18]. Generally, two crystal structures of MoO3, orthorhombic and hexagonal phase, were applied in electrode material [17]. Compared with the hexagonal structure, orthorhombic MoO3 (α-MoO3) holds better stability but less Csp [17-21]. Recently, Yao et al. fabricated ultra-long α-MoO3 NBs which deliver an outstanding Csp of 1198 F/g and long-term stability
over 20000 cycles with 96.5% Csp retention [22]. The α-MoO3 NBs solved the less Csp problem, which could be attributed to the unique morphology and stable crystal structure. In this work, we took this outstanding α-MoO3 NBs as precursor to synthesize Mo-N via a temperature-programmed reaction. Based on this, as-prepared Mo3N2 NBs both possess preferable Csp, great cycling stability and a wide voltage window. Experimental section Synthesis of molybdenum nitride NBs
Scheme 1 Schematic diagram and chemical reaction equation of the formation process of Mo3N2 NBs.
As Scheme 1 shows, the formation process and chemical reaction equation for the as-prepared samples is displayed. For ultra-long α-MoO3 NBs as a precursor, the products were synthesized via a modified hydrothermal method. Typically, 2 g molybdenum powder (Aladdin, 99.5%) was added into 10 mL deionized water to form a uniform mixture. Next, 20 mL 30% (wt. %) H2O2 was slowly added into the mixture until the solution became light-yellow. Then, the solution was strongly stirred for 30 min to react thoroughly. After that, the solution was transferred to a Teflon-lined stainless steel autoclave and heated to 220 °C with a heating rate of 5 °C/min. The precipitate was rinsed by deionized water and
ethanol alternately for 3 times. The Mo3N2 NBs were prepared by a temperature-programmed reaction of α-MoO3 NBs with NH3 at the temperature of 750 °C. During the annealing process, the oxygen atom would escape from the Mo3N2 material, leading to the coarse surface of Mo3N2 NBs. Firstly, 200 mg MoO3 NBs samples were placed into a ceramic crucible. After that, the ceramic crucible was put into a quartz tube. The quartz tube was further put into constant temperature zone of the SiC electron furnace and the NH3 was slowly introduced with the rate at 0.06 L/h. Then, the furnace was heated from room temperature to 750 °C at the heating rate of 5 °C/min and kept this temperature for 4.5 h to ensure the reaction react thoroughly. When the reaction was finished, the quartz tube was cooled to the room temperature by natural cooling in the NH3 atmosphere. Finally, the product was took out for the next test. Characterizations of as-prepared molybdenum nitride NBs The crystalline structure of the samples was characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ=1.54 Å). Field emission scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEM-2100F, 200 kV) were used to observe the morphologies, elements and structures of the samples. The elemental surface chemical compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS). The Brunauer-Emmett-Teller (BET) method was used to measure the specific surface area. Electrochemical characterization of molybdenum nitride NBs The electrochemical measurements were taken by electrochemical workstation (CHI 660e). For a typical three-electrode test, the Mo-N NBs were used as the work electrode, Ag/AgCl
and platinum foil were used as the reference electrode and counter electrode, respectively. The electrochemical performance of Mo-N NBs work electrode was evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrum (EIS) in 1M H2SO4 electrolyte. Fabrication of working electrode in three-electrode system The electrode was fabricated by Mo-N NBs, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1. N-Methyl pyrrolidone (NMP) was used as a solvent to let Mo3N2, acetylene black and PVDF mix adequately. Then the mixture was evenly coated onto a stainless steel sheet (2 cm2), and the mass loading of active material for electrochemical characterization is 3~8 mg. After that, the stainless steel sheet was dried at 65 °C for 18 h in a vacuum drying chamber. Finally, the obtained working electrode was immersed in 1 M H2SO4 aqueous solution for 12 h before testing. Fabrication of symmetric supercapacitor The polyvinyl alcohol (PVA)/ H2SO4 gel electrolyte was prepared as follows: 5 g of PVA were added to 50 mL 1 M H2SO4 aqueous solution, and then the mixture was heated to 85 °C under vigorous stirring until the PVA dissolved completely. The working electrode was same as the electrode in the three-electrode system, then a PVA/H2SO4 transparent film was sandwiched in between to work as both an electrolyte and a separator. After that, the device was left in 60 °C for 24 h to remove excess water in the electrolyte. Results and discussion Structural and component properties
Fig. 1 (a) XRD pattern of Mo3N2 NBs. (b) XPS pattern of Mo3N2 NBs.
The XRD pattern of the original α-MoO3 and Mo3N2 NBs are shown in Fig. 1(a). The experimental results indicate that the XRD peaks from both material are in good agreement with the standard peaks of the orthorhombic MoO3 phase (ICSD#03-0258) and orthorhombic Mo3N2 (ICSD#04-4377) [17]. Furthermore, the intensity of the (200) diffraction peak of Mo3N2 NBs is stronger than other peaks, revealing a high growth in the (200) direction. As the difficulty to deconvolute the N signal, thus, the Mo spectra were analyzed. Although molybdenum shows variable oxidation states ranging from Mo6+ to Mo1+, of which Mo6+ and Mo4+ states are the most stable. Therefore, the peaks of Mo6+ and Mo4+ could be found obviously in Fig. 1(b). The deconvoluted XPS spectra of Mo binding energy region plotted in Fig. 1(b) shows two distinct peaks at 229.64 and 232.68 eV of Mo 3d 5/2 and Mo 3d 3/2 doublet with a splitting distance of 3.04 eV, corresponding to Mo-N [23, 24]. The satellite peaks at 228.81 eV and 231.93 eV are attributed to a mixed state of Mod+ where the value of d is in between 0 and 4 [24]. The Mod+ peaks could be assigned to the formation, agreeing well with the reported Mo3N2 in the past papers [24-26].
Fig. 2 (a) SEM image of α-MoO3 NBs. (b) SEM image of Mo3N2 NBs. (c) EDS pattern of Mo3N2 NBs. The inset in (a) and (b) shows an enlarged view of α-MoO3 and Mo3N2 NBs, respectively. The inset in (c) exhibits the weight (wt.) % and atom (at.) % of Mo and N.
In Fig. 2(a), the morphologies of ultra-long α-MoO3 NBs could be observed. Besides, the α-MoO3 NBs with a smooth surface shown in the inset possess a length of ~200 um and width of ~400 nm. As shown in Fig. 2(b), the Mo3N2 NBs were curled and have a coarse surface (see the inset). For Fig. 2(c), the EDS spectra reveals that the samples contain the Mo and N element. In addition, combining with the 61.36 at. % Mo and 38.64 at. % N concentration in the inset of Fig. 2(c), the existence of Mo3N2 was indicated, agreeing well with the XRD and XPS analysis.
Fig. 3 (a) TEM image of Mo3N2 NBs. (b) HR-TEM image of Mo3N2 NBs. The inset in (a) and (b) shows an enlarged view and FFT pattern of Mo3N2 NBs, respectively.
Fig. 4 BET image of Mo3N2 and MoO3 NBs.
As shown in Fig. 3(a), the width of Mo3N2 NBs is ~400 nm, and a coarse surface could be observed in the inset. After the BET test, the coarse Mo3N2 displayed a higher specific surface area (11.79 m2/g) than the smooth α-MoO3 NBs (7.08 m2/g) (see Fig. 4). Based on this, much more electroactive sites could be provided, benefiting to the contact of electrode and electrolyte [27]. The HR-TEM image of Mo3N2 in Fig. 3(b) exhibits the well-defined lattice fringe with inner planar spacing of 2.08 Å, which is attributed to the distance of the
(200) plane of Mo3N2, coinciding with the XRD results. Besides, as the inset of Fig. 3(b) shown, the FFT test indicates the orthorhombic crystal structure of Mo3N2. Electrochemical properties
Fig. 5 CV curves at various scan rates of (a) Mo3N2 and (b) MoO3 NBs, (c) comparative CV curves at 200 mV/s, GCD curves at different current density of (d) Mo3N2 and (e) MoO3, (f) comparative GCD curves at 0.4 A/g.
Fig. 6 Electrochemical performance of this work and other materials.
Fig. 5(a, b) display the CV curves of Mo3N2 and MoO3 NBs at different scanning rates with a
voltage ranging from -0.35 to 0.6 V. The oxidation and reduction peaks could be observed obviously, indicating that both Mo3N2 and MoO3 are pseudocapacitor material for the electrode [28-30]. Here, the voltage window of 0.95 V is larger than other investigations of the Mo3N2 (0.8 V) and Mo(O, N)x (0.6 V) [15, 31]. From Fig. 5(a), the Csp of 235, 217, 205, 192, 187, and 174 F/g at scan rate of 5, 10, 20, 50, 100, and 200 mV/s are achieved, respectively. Compared with the CV curves at 200 mV/s of Mo3N2 and MoO3, the lower Csp of MoO3 (181F/g) could be indicated in Fig. 5(c), suggesting a better electrochemical Csp of Mo3N2 than that of MoO3. From GCD cures of Mo3N2 NBs (Fig. 5d), the highest Csp of 220.3 F/g was obtained at 0.4 A/g, higher than that of precursor MoO3 (184 F/g at 0.4 A/g) in Fig. 5(e). Although the maximum Csp of Mo3N2 (220.3 F/g) for supercapacitor is lower than those of Mo(O, N)x (246 F/g) and MoN0.74C0.08H0.6 compounds (275 F/g) [23, 31], it is still higher than the pure Mo2N (148 F/g) and MoN (172 F/g) [32, 33], and comparable to other Mo3N2 (see Fig. 6) [15]. Furthermore, we gained a coulombic efficiency of 91.3% and little IR drop of Mo3N2 NBs in Fig. 5(f). These results are better than the 83.7% coulombic efficiency and IR drop of the MoO3, indicating a great electrochemical property of Mo3N2 [34, 35].
Fig. 7(a) Csp versus the inverse square root of scan rate of Mo3N2 NBs, (b) Csp at different scan rates, and the ratio of surface (black area) and diffusion Csp (red area) derived from Dunn's method of Mo3N2 NBs, (c) Cycling stability of Mo3N2 and MoO3. The inset in (c) displays the Csp at various current densities of Mo3N2 and MoO3 NBs, respectively.
To investigate the Csp in a more fundamental level, the Dunn’s method is employed to recognize the hybrid electrical energy storage of MoO3 NBs, which combines surface pseudocapacitance and double-layer diffusion Csp [36]. We can separate this two electrical storage mechanisms by the equation as follows: i(V)=k1v+k2v1/2
(1)
For more convenient analysis, we divide both sides of this equation with the scan rate, then: i(V)/v=k1+k2v-1/2
(2)
In the equation (2), i(V) is the current of given voltage, v is the scan rate, k1 and k2 are scan rate independent constant. The k1v and k2v1/2 in the equation (1) are corresponded to the double-layer diffusion Csp and surface pseudocapacitance, respectively. According to the
equation (2), we can plot the scan rate dependence of current (Fig. 7a). Thus, the k1 and k2 could be defined as the slope and the y-axis intercept point, respectively. By this procedure, we can distinguish quantitatively between the diffusion and surface Csp, exhibiting in the Fig. 7(b). Noticeably, with the increase of scan rate, both the Csp and ratio of surface and diffusion Csp of Mo3N2 are decreased, which are due to the more active material insufficient in the redox reaction as the scan rate increases [36]. As shown in Fig. 7(c), this material could still maintain 98.3% Csp after 1500 cycles, which is better than the pure MoO3 with 83.4% Csp retention after 1500 cycles, the Mo2N with the Csp retention of 91% after 1000 cycles [14], and Mo3N2 composites with the 93% after 1000 cycles [16]. The great cycling stability is attributed to the stable orthorhombic phase [17], as indication of XRD and FFT. In the low current density, both Mo3N2 and MoO3 NBs have a high Csp, which is due to the high utilization of active materials [34]. But they both deliver a low Csp in the high current density, indicating a poor rate capacity. This could be attributed to the fact that the high current makes more active material insufficient in the redox reaction [36]. The electronic performance of this work and others are exhibited in Fig. 6. Compared with other Mo-N compounds [14, 16, 17, 31-33], this Mo3N2 shows a great electronic property at all aspects (a high Csp, wide voltage window range and great cycling stability).
Fig. 8 (a) Nyquist plot of Mo3N2 and MoO3 NBs, (b) Warburg impedance spectra and Bode
phase angle plot of Mo3N2. The inset in (a) is the equivalent circuit model employed to simulate the Nyquist plot.
The typical Nyquist plot and equivalent circuit model of the Mo3N2 NBs are shown in Fig. 8(a). The Rs, Rct, Cp and CPE in the inset, represent the electrolyte resistance, the charge-transfer resistance, and the pseudocapacitance element and double-layer capacitance of active materials, respectively [37]. The calculated Rs and Rct values of the Mo3N2 are 0.9195 and 0.0546 Ω, respectively. Compared with the Rs (2.22 Ω) and Rct (2.83 Ω) of MoO3 [38], this Mo3N2 NBs own a lower resistance value, indicating the improved electrical conductivity [39]. Meanwhile, by calculating the frequency corresponded to the highest point of the semi-circle in the high-frequency region, we could get the time constant (the reciprocal of the frequency) of this material [40, 41]. For this Mo3N2 material, the time constant is 0.57 s, displaying its excellent electrochemical performance [40, 41]. As shown in Fig. 8(b), the Warburg impedance spectra of Mo3N2 NBs at high-frequency is closed to a constant (-0.001 ohm) which is equal to Log(Rs), further demonstrating the great electrochemical property of this Mo3N2 [41]. Combining with the Bode phase angle plot of, we could find that the phase angle at 0.1 Hz for the electrode material is nearly -90, representing an ideal capacitive behavior [40, 41]. To evaluate the voltage response of this material, relaxation time constant (t0) which is a quantitative measure of how fast a material can be charged and discharged, is measured from the Bode phase angle plot [40, 41]. In Fig. 8(b), the reciprocal of the frequency (2.53 Hz) corresponds to the t0. Therefore, the t0, 0.39s, of Mo3N2 is gotten, clearly revealing the quick potential response of this active material.
Overall, by the EIS approach, we proved the fast charging and discharging processes of Mo3N2 NBs, benefiting to understand the preferable electrochemical performance of our prepared material.
Fig. 9 (a) CV curves at various scan rates, (b) GCD curves at different current densities, (c) cycling stability, (d) Nyquist plot of Mo3N2 and MoO3 measured by two-electrode system. The inset in (c) and (d) displays the Csp at various current densities and an enlarged view of Nyquist plot of Mo3N2 NBs, respectively.
To cater the practical application, we used this Mo3N2 NBs to fabricate symmetric supercapacitor. The electrochemical test of this supercapacitor under a two-electrode system is shown in Fig. 9. From Fig. 9(a), the Csp of 236, 209, 192, 177, 165, and 151 F/g at scan rate of 10, 20, 50, 100, 200, and 400 mV/s are achieved, respectively. The CV curves indicate a rectangular shape without the cathode and anode peaks, differing from the shape of the CV
curves in the traditional three-electrode test. This could be attributed that the Mo3N2-based symmetric electrodes are charged and discharged at a constant rate over the entire voltammetric cycles, leading the absence of the redox peaks [36]. In Fig. 9(b), the symmetric supercapacitor delivers a high Csp of 251, 223, 212, 203, 180, and 153 F/g at discharge current densities of 0.5, 1, 1.5, 2.0, 3.5, and 5A/g, respectively. The decrease of Csp as the scan rate and current densities increases, for the same reason in the three-electrode system, is due to the fact that the high current makes more active material insufficient in the redox reaction. The cycling stability of this symmetric supercapacitor was tested through a cyclic GCD process at a current density of 3.5 A/g, showing in Fig. 9(c). The electrical device proved an excellent stability, remaining at 96.3% of its initial Csp after 5000 GCD cycles. For the symmetric device, its energy and power densities are calculated from the GCD curves by the equations as follows: 1
𝐸 = 2C𝑠𝑝 ∗ (𝛥𝑉)2 𝑃 = 𝐸/𝛥𝑡
(3) (4)
where the ΔV and Δt is the voltage window and time of discharge, respectively. After evaluating, the energy densities could be reached with various power densities: 25.45 Wh/kg at 916.04 W/kg, 26.57 Wh/kg at 683.32 kW/kg, and 28.21 Wh/kg at 406.13 W/kg. The maximum energy density of 31.72 Wh/kg is achieved at a power density of at 228.38 W/kg, and the highest power density is 2301.38 W/kg at the energy density of 19.19 Wh/kg. These values are superior to the previously reported Mo3N2 hybrid NBs symmetric systems (17.4 Wh/kg at 697 W/kg)，its low energy densities and power densities might be attributed to the
narrow potential window caused by its electrolyte [16]. Meanwhile, the energy density and power density are also higher than other material systems, such as MoO3/AC composites (16.75 Wh/kg at 325 W/kg), MoO3/MWCNTs//MoO3/MWCNTs (7.28 Wh/kg at 672 W/kg) and so on [42, 43]. Tested in a frequency range from 0.01 Hz to10 kHz, the charge-transfer process of this symmetric supercapacitor could be analyzed by the electrochemical impedance spectra. The Rs, and Rct of this Mo3N2 device is 1.22 and 0.47 Ω, respectively, which indicates a low internal resistance for the electrical device, suggesting a great conductivity of the electrode material. Conclusions In summary, the Mo3N2 NBs were obtained successfully via a temperature-programmed reaction with NH3 by using ultra-long α-MoO3 NBs as precursor. Noteworthy, the Mo3N2 NBs electrode exhibited superior electrochemical performance with a high specific Csp (220.3 F/g at 0.4 A/g), a wide voltage window at 0.95 V and excellent cycling performance (98.3% of original Csp after 1500 cycles). The great electrochemical performance can be contributed to the improved morphology and stable orthorhombic crystal structure. Conflicts of interest There are no conflicts to declare. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 11904299 and U1930124).
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for
high
energy
density
supercapacitors.
Electrochimica
Acta
2013;112:663-669. Zhengwei Xiong: Conceptualization, Methodology, Data curation, Writing- Reviewing and Editing, Supervision. Jiang Yang: Data curation. Zhipeng Gao: Investigation and Conceptualization. Qiang Yang: Methodology and Investigation. Deli Shi: Validation, Writing - original draft, Writing - review & editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
1. We prepared unique Mo3N2 NBs with ultra-long nanobelts and stable orthorhombic crystal structure. 2. Compared with other Mo-N compounds, the Mo3N2 NBs both possessed better electrochemical properties. 3. More excellent conductivity of Mo3N2 NBs was obtained.
S2211-3797(19)32198-9 https://doi.org/10.1016/j.rinp.2020.102941 RINP 102941
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
22 July 2019 9 January 2020 12 January 2020
Please cite this article as: Xiong, Z., Yang, J., Gao, Z., Yang, Q., Shi, D., Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors, Results in Physics (2020), doi: https:// doi.org/10.1016/j.rinp.2020.102941
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Orthorhombic Mo3N2 nanobelts with improved electrochemical properties as electrode material for supercapacitors Zhengwei Xiong,1, * Jiang Yang,1 Zhipeng Gao,2 Qiang Yang,3 Deli Shi,1, * 1Joint
Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang 621010, China
2Institute
of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China 3China
Academy of Engineering Physics, Mianyang621000, China
Corresponding authors: Email: [email protected] and [email protected] Abstract: As a typical supercapacitor electrode material, the reported Mo-N compounds mostly own one part of outstanding electrochemical properties (specific capacitance (Csp), voltage window and cycling stability), and the well integration with three aspects was rare. Here using a novel MoO3 precursor with ultra-long nanobelts (NBs) and stable orthorhombic crystal structure, we prepared unique Mo3N2 NBs, which inherited the morphology and structure of the precursor greatly. Compared with other Mo-N compounds, the Mo3N2 NBs both possessed the high Csp (220.3 F/g at 0.4 A/g), great cycling stability (98.3% Csp retention after 1500 cycles) and wide voltage window (0.95 V) due to the improved morphology, stable structure and excellent conductivity. Key words: Electron microscopy; Energy storage and conversion; Microstructure; Supercapacitor; Mo3N2 Introduction Electrochemical capacitors, also known as supercapacitors, provide higher power density than traditional batteries and capacitors [1]. The electrode materials that make up the
supercapacitors include: (1) carbon materials, which hold excellent conductivity but low Csp [2-4]; (2) conducting polymers, which have great conductivity but inferior cycling stability [5]; (3) transition metal oxides, which have high Csp but bad conductivity [10-12]; and (4) transition metal nitrides (TMNs), which possess fantastic electrical conductivity, superior stability and Csp concurrently [13]. Many TMNs were researched, such as WN, VN, MoN and so on [14]. Among the TMNs, molybdenum-nitride (Mo-N) was the first metal nitride to be reported as supercapacitor electrode material, because of exceptional chemical stability and good electrical conductivity [14]. In the past works, many scholars took MoO3 as precursor by a temperature-programmed reaction with NH3 in a closed chamber to prepare Mo-N [14-16]. Liu et al. synthesized Mo2N NBs with a low Csp of 160 F/g with a potential window ranging from -0.2 to 0.4 V and 91% Csp retention after 1000 cycles [14]. Lee et al. prepared Mo2N nanowires, which own 220 F/g with voltage window ranging from -1.1 to -0.3 V [15]. Yuan et al. fabricated Mo3N2 NBs composites, which display a high Csp of 282 F/g with a narrow potential window ranging from 0 to 0.45 V, leading to the low Energy densities and power densities [16]. Until now, there are no Mo-N compounds which could combine the excellent Csp, great cycling stability and wide voltage window. Lots of investigations revealed that the electrochemical properties of the Mo-N were determined by the crystal structure and morphology of MoO3 precursor [16-18]. Generally, two crystal structures of MoO3, orthorhombic and hexagonal phase, were applied in electrode material [17]. Compared with the hexagonal structure, orthorhombic MoO3 (α-MoO3) holds better stability but less Csp [17-21]. Recently, Yao et al. fabricated ultra-long α-MoO3 NBs which deliver an outstanding Csp of 1198 F/g and long-term stability
over 20000 cycles with 96.5% Csp retention [22]. The α-MoO3 NBs solved the less Csp problem, which could be attributed to the unique morphology and stable crystal structure. In this work, we took this outstanding α-MoO3 NBs as precursor to synthesize Mo-N via a temperature-programmed reaction. Based on this, as-prepared Mo3N2 NBs both possess preferable Csp, great cycling stability and a wide voltage window. Experimental section Synthesis of molybdenum nitride NBs
Scheme 1 Schematic diagram and chemical reaction equation of the formation process of Mo3N2 NBs.
As Scheme 1 shows, the formation process and chemical reaction equation for the as-prepared samples is displayed. For ultra-long α-MoO3 NBs as a precursor, the products were synthesized via a modified hydrothermal method. Typically, 2 g molybdenum powder (Aladdin, 99.5%) was added into 10 mL deionized water to form a uniform mixture. Next, 20 mL 30% (wt. %) H2O2 was slowly added into the mixture until the solution became light-yellow. Then, the solution was strongly stirred for 30 min to react thoroughly. After that, the solution was transferred to a Teflon-lined stainless steel autoclave and heated to 220 °C with a heating rate of 5 °C/min. The precipitate was rinsed by deionized water and
ethanol alternately for 3 times. The Mo3N2 NBs were prepared by a temperature-programmed reaction of α-MoO3 NBs with NH3 at the temperature of 750 °C. During the annealing process, the oxygen atom would escape from the Mo3N2 material, leading to the coarse surface of Mo3N2 NBs. Firstly, 200 mg MoO3 NBs samples were placed into a ceramic crucible. After that, the ceramic crucible was put into a quartz tube. The quartz tube was further put into constant temperature zone of the SiC electron furnace and the NH3 was slowly introduced with the rate at 0.06 L/h. Then, the furnace was heated from room temperature to 750 °C at the heating rate of 5 °C/min and kept this temperature for 4.5 h to ensure the reaction react thoroughly. When the reaction was finished, the quartz tube was cooled to the room temperature by natural cooling in the NH3 atmosphere. Finally, the product was took out for the next test. Characterizations of as-prepared molybdenum nitride NBs The crystalline structure of the samples was characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ=1.54 Å). Field emission scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEM-2100F, 200 kV) were used to observe the morphologies, elements and structures of the samples. The elemental surface chemical compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS). The Brunauer-Emmett-Teller (BET) method was used to measure the specific surface area. Electrochemical characterization of molybdenum nitride NBs The electrochemical measurements were taken by electrochemical workstation (CHI 660e). For a typical three-electrode test, the Mo-N NBs were used as the work electrode, Ag/AgCl
and platinum foil were used as the reference electrode and counter electrode, respectively. The electrochemical performance of Mo-N NBs work electrode was evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrum (EIS) in 1M H2SO4 electrolyte. Fabrication of working electrode in three-electrode system The electrode was fabricated by Mo-N NBs, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1. N-Methyl pyrrolidone (NMP) was used as a solvent to let Mo3N2, acetylene black and PVDF mix adequately. Then the mixture was evenly coated onto a stainless steel sheet (2 cm2), and the mass loading of active material for electrochemical characterization is 3~8 mg. After that, the stainless steel sheet was dried at 65 °C for 18 h in a vacuum drying chamber. Finally, the obtained working electrode was immersed in 1 M H2SO4 aqueous solution for 12 h before testing. Fabrication of symmetric supercapacitor The polyvinyl alcohol (PVA)/ H2SO4 gel electrolyte was prepared as follows: 5 g of PVA were added to 50 mL 1 M H2SO4 aqueous solution, and then the mixture was heated to 85 °C under vigorous stirring until the PVA dissolved completely. The working electrode was same as the electrode in the three-electrode system, then a PVA/H2SO4 transparent film was sandwiched in between to work as both an electrolyte and a separator. After that, the device was left in 60 °C for 24 h to remove excess water in the electrolyte. Results and discussion Structural and component properties
Fig. 1 (a) XRD pattern of Mo3N2 NBs. (b) XPS pattern of Mo3N2 NBs.
The XRD pattern of the original α-MoO3 and Mo3N2 NBs are shown in Fig. 1(a). The experimental results indicate that the XRD peaks from both material are in good agreement with the standard peaks of the orthorhombic MoO3 phase (ICSD#03-0258) and orthorhombic Mo3N2 (ICSD#04-4377) [17]. Furthermore, the intensity of the (200) diffraction peak of Mo3N2 NBs is stronger than other peaks, revealing a high growth in the (200) direction. As the difficulty to deconvolute the N signal, thus, the Mo spectra were analyzed. Although molybdenum shows variable oxidation states ranging from Mo6+ to Mo1+, of which Mo6+ and Mo4+ states are the most stable. Therefore, the peaks of Mo6+ and Mo4+ could be found obviously in Fig. 1(b). The deconvoluted XPS spectra of Mo binding energy region plotted in Fig. 1(b) shows two distinct peaks at 229.64 and 232.68 eV of Mo 3d 5/2 and Mo 3d 3/2 doublet with a splitting distance of 3.04 eV, corresponding to Mo-N [23, 24]. The satellite peaks at 228.81 eV and 231.93 eV are attributed to a mixed state of Mod+ where the value of d is in between 0 and 4 [24]. The Mod+ peaks could be assigned to the formation, agreeing well with the reported Mo3N2 in the past papers [24-26].
Fig. 2 (a) SEM image of α-MoO3 NBs. (b) SEM image of Mo3N2 NBs. (c) EDS pattern of Mo3N2 NBs. The inset in (a) and (b) shows an enlarged view of α-MoO3 and Mo3N2 NBs, respectively. The inset in (c) exhibits the weight (wt.) % and atom (at.) % of Mo and N.
In Fig. 2(a), the morphologies of ultra-long α-MoO3 NBs could be observed. Besides, the α-MoO3 NBs with a smooth surface shown in the inset possess a length of ~200 um and width of ~400 nm. As shown in Fig. 2(b), the Mo3N2 NBs were curled and have a coarse surface (see the inset). For Fig. 2(c), the EDS spectra reveals that the samples contain the Mo and N element. In addition, combining with the 61.36 at. % Mo and 38.64 at. % N concentration in the inset of Fig. 2(c), the existence of Mo3N2 was indicated, agreeing well with the XRD and XPS analysis.
Fig. 3 (a) TEM image of Mo3N2 NBs. (b) HR-TEM image of Mo3N2 NBs. The inset in (a) and (b) shows an enlarged view and FFT pattern of Mo3N2 NBs, respectively.
Fig. 4 BET image of Mo3N2 and MoO3 NBs.
As shown in Fig. 3(a), the width of Mo3N2 NBs is ~400 nm, and a coarse surface could be observed in the inset. After the BET test, the coarse Mo3N2 displayed a higher specific surface area (11.79 m2/g) than the smooth α-MoO3 NBs (7.08 m2/g) (see Fig. 4). Based on this, much more electroactive sites could be provided, benefiting to the contact of electrode and electrolyte [27]. The HR-TEM image of Mo3N2 in Fig. 3(b) exhibits the well-defined lattice fringe with inner planar spacing of 2.08 Å, which is attributed to the distance of the
(200) plane of Mo3N2, coinciding with the XRD results. Besides, as the inset of Fig. 3(b) shown, the FFT test indicates the orthorhombic crystal structure of Mo3N2. Electrochemical properties
Fig. 5 CV curves at various scan rates of (a) Mo3N2 and (b) MoO3 NBs, (c) comparative CV curves at 200 mV/s, GCD curves at different current density of (d) Mo3N2 and (e) MoO3, (f) comparative GCD curves at 0.4 A/g.
Fig. 6 Electrochemical performance of this work and other materials.
Fig. 5(a, b) display the CV curves of Mo3N2 and MoO3 NBs at different scanning rates with a
voltage ranging from -0.35 to 0.6 V. The oxidation and reduction peaks could be observed obviously, indicating that both Mo3N2 and MoO3 are pseudocapacitor material for the electrode [28-30]. Here, the voltage window of 0.95 V is larger than other investigations of the Mo3N2 (0.8 V) and Mo(O, N)x (0.6 V) [15, 31]. From Fig. 5(a), the Csp of 235, 217, 205, 192, 187, and 174 F/g at scan rate of 5, 10, 20, 50, 100, and 200 mV/s are achieved, respectively. Compared with the CV curves at 200 mV/s of Mo3N2 and MoO3, the lower Csp of MoO3 (181F/g) could be indicated in Fig. 5(c), suggesting a better electrochemical Csp of Mo3N2 than that of MoO3. From GCD cures of Mo3N2 NBs (Fig. 5d), the highest Csp of 220.3 F/g was obtained at 0.4 A/g, higher than that of precursor MoO3 (184 F/g at 0.4 A/g) in Fig. 5(e). Although the maximum Csp of Mo3N2 (220.3 F/g) for supercapacitor is lower than those of Mo(O, N)x (246 F/g) and MoN0.74C0.08H0.6 compounds (275 F/g) [23, 31], it is still higher than the pure Mo2N (148 F/g) and MoN (172 F/g) [32, 33], and comparable to other Mo3N2 (see Fig. 6) [15]. Furthermore, we gained a coulombic efficiency of 91.3% and little IR drop of Mo3N2 NBs in Fig. 5(f). These results are better than the 83.7% coulombic efficiency and IR drop of the MoO3, indicating a great electrochemical property of Mo3N2 [34, 35].
Fig. 7(a) Csp versus the inverse square root of scan rate of Mo3N2 NBs, (b) Csp at different scan rates, and the ratio of surface (black area) and diffusion Csp (red area) derived from Dunn's method of Mo3N2 NBs, (c) Cycling stability of Mo3N2 and MoO3. The inset in (c) displays the Csp at various current densities of Mo3N2 and MoO3 NBs, respectively.
To investigate the Csp in a more fundamental level, the Dunn’s method is employed to recognize the hybrid electrical energy storage of MoO3 NBs, which combines surface pseudocapacitance and double-layer diffusion Csp [36]. We can separate this two electrical storage mechanisms by the equation as follows: i(V)=k1v+k2v1/2
(1)
For more convenient analysis, we divide both sides of this equation with the scan rate, then: i(V)/v=k1+k2v-1/2
(2)
In the equation (2), i(V) is the current of given voltage, v is the scan rate, k1 and k2 are scan rate independent constant. The k1v and k2v1/2 in the equation (1) are corresponded to the double-layer diffusion Csp and surface pseudocapacitance, respectively. According to the
equation (2), we can plot the scan rate dependence of current (Fig. 7a). Thus, the k1 and k2 could be defined as the slope and the y-axis intercept point, respectively. By this procedure, we can distinguish quantitatively between the diffusion and surface Csp, exhibiting in the Fig. 7(b). Noticeably, with the increase of scan rate, both the Csp and ratio of surface and diffusion Csp of Mo3N2 are decreased, which are due to the more active material insufficient in the redox reaction as the scan rate increases [36]. As shown in Fig. 7(c), this material could still maintain 98.3% Csp after 1500 cycles, which is better than the pure MoO3 with 83.4% Csp retention after 1500 cycles, the Mo2N with the Csp retention of 91% after 1000 cycles [14], and Mo3N2 composites with the 93% after 1000 cycles [16]. The great cycling stability is attributed to the stable orthorhombic phase [17], as indication of XRD and FFT. In the low current density, both Mo3N2 and MoO3 NBs have a high Csp, which is due to the high utilization of active materials [34]. But they both deliver a low Csp in the high current density, indicating a poor rate capacity. This could be attributed to the fact that the high current makes more active material insufficient in the redox reaction [36]. The electronic performance of this work and others are exhibited in Fig. 6. Compared with other Mo-N compounds [14, 16, 17, 31-33], this Mo3N2 shows a great electronic property at all aspects (a high Csp, wide voltage window range and great cycling stability).
Fig. 8 (a) Nyquist plot of Mo3N2 and MoO3 NBs, (b) Warburg impedance spectra and Bode
phase angle plot of Mo3N2. The inset in (a) is the equivalent circuit model employed to simulate the Nyquist plot.
The typical Nyquist plot and equivalent circuit model of the Mo3N2 NBs are shown in Fig. 8(a). The Rs, Rct, Cp and CPE in the inset, represent the electrolyte resistance, the charge-transfer resistance, and the pseudocapacitance element and double-layer capacitance of active materials, respectively [37]. The calculated Rs and Rct values of the Mo3N2 are 0.9195 and 0.0546 Ω, respectively. Compared with the Rs (2.22 Ω) and Rct (2.83 Ω) of MoO3 [38], this Mo3N2 NBs own a lower resistance value, indicating the improved electrical conductivity [39]. Meanwhile, by calculating the frequency corresponded to the highest point of the semi-circle in the high-frequency region, we could get the time constant (the reciprocal of the frequency) of this material [40, 41]. For this Mo3N2 material, the time constant is 0.57 s, displaying its excellent electrochemical performance [40, 41]. As shown in Fig. 8(b), the Warburg impedance spectra of Mo3N2 NBs at high-frequency is closed to a constant (-0.001 ohm) which is equal to Log(Rs), further demonstrating the great electrochemical property of this Mo3N2 [41]. Combining with the Bode phase angle plot of, we could find that the phase angle at 0.1 Hz for the electrode material is nearly -90, representing an ideal capacitive behavior [40, 41]. To evaluate the voltage response of this material, relaxation time constant (t0) which is a quantitative measure of how fast a material can be charged and discharged, is measured from the Bode phase angle plot [40, 41]. In Fig. 8(b), the reciprocal of the frequency (2.53 Hz) corresponds to the t0. Therefore, the t0, 0.39s, of Mo3N2 is gotten, clearly revealing the quick potential response of this active material.
Overall, by the EIS approach, we proved the fast charging and discharging processes of Mo3N2 NBs, benefiting to understand the preferable electrochemical performance of our prepared material.
Fig. 9 (a) CV curves at various scan rates, (b) GCD curves at different current densities, (c) cycling stability, (d) Nyquist plot of Mo3N2 and MoO3 measured by two-electrode system. The inset in (c) and (d) displays the Csp at various current densities and an enlarged view of Nyquist plot of Mo3N2 NBs, respectively.
To cater the practical application, we used this Mo3N2 NBs to fabricate symmetric supercapacitor. The electrochemical test of this supercapacitor under a two-electrode system is shown in Fig. 9. From Fig. 9(a), the Csp of 236, 209, 192, 177, 165, and 151 F/g at scan rate of 10, 20, 50, 100, 200, and 400 mV/s are achieved, respectively. The CV curves indicate a rectangular shape without the cathode and anode peaks, differing from the shape of the CV
curves in the traditional three-electrode test. This could be attributed that the Mo3N2-based symmetric electrodes are charged and discharged at a constant rate over the entire voltammetric cycles, leading the absence of the redox peaks [36]. In Fig. 9(b), the symmetric supercapacitor delivers a high Csp of 251, 223, 212, 203, 180, and 153 F/g at discharge current densities of 0.5, 1, 1.5, 2.0, 3.5, and 5A/g, respectively. The decrease of Csp as the scan rate and current densities increases, for the same reason in the three-electrode system, is due to the fact that the high current makes more active material insufficient in the redox reaction. The cycling stability of this symmetric supercapacitor was tested through a cyclic GCD process at a current density of 3.5 A/g, showing in Fig. 9(c). The electrical device proved an excellent stability, remaining at 96.3% of its initial Csp after 5000 GCD cycles. For the symmetric device, its energy and power densities are calculated from the GCD curves by the equations as follows: 1
𝐸 = 2C𝑠𝑝 ∗ (𝛥𝑉)2 𝑃 = 𝐸/𝛥𝑡
(3) (4)
where the ΔV and Δt is the voltage window and time of discharge, respectively. After evaluating, the energy densities could be reached with various power densities: 25.45 Wh/kg at 916.04 W/kg, 26.57 Wh/kg at 683.32 kW/kg, and 28.21 Wh/kg at 406.13 W/kg. The maximum energy density of 31.72 Wh/kg is achieved at a power density of at 228.38 W/kg, and the highest power density is 2301.38 W/kg at the energy density of 19.19 Wh/kg. These values are superior to the previously reported Mo3N2 hybrid NBs symmetric systems (17.4 Wh/kg at 697 W/kg)，its low energy densities and power densities might be attributed to the
narrow potential window caused by its electrolyte [16]. Meanwhile, the energy density and power density are also higher than other material systems, such as MoO3/AC composites (16.75 Wh/kg at 325 W/kg), MoO3/MWCNTs//MoO3/MWCNTs (7.28 Wh/kg at 672 W/kg) and so on [42, 43]. Tested in a frequency range from 0.01 Hz to10 kHz, the charge-transfer process of this symmetric supercapacitor could be analyzed by the electrochemical impedance spectra. The Rs, and Rct of this Mo3N2 device is 1.22 and 0.47 Ω, respectively, which indicates a low internal resistance for the electrical device, suggesting a great conductivity of the electrode material. Conclusions In summary, the Mo3N2 NBs were obtained successfully via a temperature-programmed reaction with NH3 by using ultra-long α-MoO3 NBs as precursor. Noteworthy, the Mo3N2 NBs electrode exhibited superior electrochemical performance with a high specific Csp (220.3 F/g at 0.4 A/g), a wide voltage window at 0.95 V and excellent cycling performance (98.3% of original Csp after 1500 cycles). The great electrochemical performance can be contributed to the improved morphology and stable orthorhombic crystal structure. Conflicts of interest There are no conflicts to declare. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 11904299 and U1930124).
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for
high
energy
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supercapacitors.
Electrochimica
Acta
2013;112:663-669. Zhengwei Xiong: Conceptualization, Methodology, Data curation, Writing- Reviewing and Editing, Supervision. Jiang Yang: Data curation. Zhipeng Gao: Investigation and Conceptualization. Qiang Yang: Methodology and Investigation. Deli Shi: Validation, Writing - original draft, Writing - review & editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
1. We prepared unique Mo3N2 NBs with ultra-long nanobelts and stable orthorhombic crystal structure. 2. Compared with other Mo-N compounds, the Mo3N2 NBs both possessed better electrochemical properties. 3. More excellent conductivity of Mo3N2 NBs was obtained.