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Magnetron sputtered NbN thin film electrodes for supercapacitors
Accepted Manuscript Magnetron sputtered NbN thin film electrodes for supercapacitors Hao Shen, Binbin Wei, Dongfang Zhang, Zhengbing Qi, Zhoucheng Wang PII: DOI: Reference:
S0167-577X(18)30957-1 https://doi.org/10.1016/j.matlet.2018.06.052 MLBLUE 24492
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
29 March 2018 13 June 2018 14 June 2018
Please cite this article as: H. Shen, B. Wei, D. Zhang, Z. Qi, Z. Wang, Magnetron sputtered NbN thin film electrodes for supercapacitors, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.06.052
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetron sputtered NbN thin film electrodes for supercapacitors Hao Shena, Binbin Weia, Dongfang Zhanga, Zhengbing Qib,*, Zhoucheng Wanga,* a
b
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China *Corresponding author: Tel./fax: +86-592-2180738, E-mail: [email protected] (Z.C. Wang) Tel./fax: +86-592-6291328, E-mail: [email protected] (Z.B. Qi)
Abstract: Selecting suitable electrode materials is the key to develop high-performance supercapacitors. Herein, we report the niobium nitride (NbN) thin films prepared by reactive magnetron sputtering and investigate their electrochemical performances when used as supercapacitor electrodes. NbN thin film electrodes can achieve the optimal electrochemical performance with a volumetric capacitance of 707.1 F cm-3 and an outstanding cycling stability (remaining 92.2% after 20000 cycles). Such impressive properties render the NbN thin films qualified as the promising candidates for supercapacitors or other energy storage systems. Keywords: NbN thin film; magnetron sputtering; physical vapor deposition; supercapacitor; energy storage and conversion.
1. Introduction Nowadays, for handling the severe energy and environment crisis, regenerative energy resources have been developed to optimize the traditional energy structures, but still constrained by inherent intermittence and disequilibrium distribution [1]. To cope with these knotty issues, high-efficient energy storage systems are available and in growing demands. The most wildly utilized energy storage systems 1
are batteries and electrochemical capacitors (namely supercapacitors). Compared with batteries, supercapacitors attract increasing attention on account of some outstanding merits, such as long cycle life and high power density, which suffice to meet the requirements of energy storage systems. Recently, transition metal nitrides (TMNs) are being spotlighted as electrode materials for supercapacitors by virtue of high specific capacitance, outstanding chemical resistance, good conductivity and excellent cycling stability [2]. Some TMNs have been already applied in the supercapacitor field, such as VN [3], TiN [4] and CrN [5, 6]. As a member of TMNs, NbN is particularly noticeable due to its higher specific capacitance and nevertheless, has been reported only a few times [7-9]. To the best of our knowledge, these NbN electrode materials reported up to date were obtained by annealing corresponding precursors under high temperature in the NH3 atmosphere, inducing high energy-consumption and environmental pollution. Differing from conventional approaches, magnetron sputtering technologies are environment-friendly and simply tunable, by which the active materials are directly deposited onto substrates and the electrodes are prepared without additional binders. Herein, we successfully fabricated NbN thin films via reactive DC magnetron sputtering and further investigated their electrochemical performances. The optimal NbN thin film electrodes can achieve a volumetric capacitance of 707.1 F cm-3 and exhibit an exceptional cycling stability with 92.2% capacitance retention after 20000 cycles. This study furthers the investigation of NbN thin films as promising electrodes for supercapacitors.
2. Experimental 2.1. Fabrication of NbN thin film electrodes 2
NbN thin films were deposited on the smooth Si (100) substrates by reactive DC magnetron sputtering from a pure niobium target (purity 99.95%). Before being placed into the sputtering chamber, the Si substrates were stepwise cleaned with acetone and ethanol for minimizing adhesive pollutants. To eliminate surface contaminants and accelerate the deposition rate, the niobium target was pre-sputtered for 5 min. The detailed deposition parameters are presented in Table S1. 2.2. Characterization X-ray diffraction (XRD, Rigaku Ultima IV) was employed to identify the crystallographic structure. The surface and cross-sectional morphologies of NbN thin films were observed by scanning electron microscopy (SEM, ZEISS Sigma). 2.3. Electrochemical testing All the electrochemical measurements were carried out on an electrochemical workstation (CHI660E, CHI Instruments). The electrochemical performances of NbN thin film electrodes were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 0.5 M H2SO4 aqueous electrolyte using a conventional three-electrode cell configuration in which the thin film electrode, an Ag/AgCl and a Pt foil were used as the working electrode, reference electrode and counter electrode, respectively.
3. Results and discussion 3.1. Composition and microstructure As shown in Fig. 1a, all diffraction peaks can be indexed to the fcc NbN (JCPDS: 65-9399) and the slight deviations are attributed to the internal stress [10]. The predominant peak located at ~35.5° reveals 3
Fig. 1. (a) XRD patterns and (b-i) SEM images of T1-T4 thin films. the preferential orientation with (111) plane. Meanwhile, the other two weak peaks positioned at ~41.2° and ~59.7° correspond to the (200) and (220) planes, respectively. The (111) diffraction peak of the T2 thin film is sharper than those of other thin films, denoting its best crystallinity [8]. The surface and cross-sectional morphologies of NbN thin films (T1-T4) are shown in Fig. 1b-i. Obviously, dense grains are dispersed throughout the surface and the cross-sectional views show the columnar texture, which is consistent with typical island structure characteristic. With increasing nitrogen content, the grain sizes increase at first and then gradually decrease, reaching the maximum value at 10%. During the deposition, growing films are under ion bombardment, and energies of incident ions are inversely proportional to deposition rates [11]. If nitrogen content increases, deposition rates generally decline due to target poison, leading to higher energies of incident ions. Although the energies increase as nitrogen content increases from 5% to 10%, the incident ions do not have enough energies to penetrate beyond the first atomic layer. Therefore, these energies are essentially transferred to adatoms and increase the surface mobility, which could make grain sizes larger [12]. As nitrogen content increases from 10% to 20%, the incident ions with further increased energies can penetrate into and collide with near surface atoms by re-sputtering effect, causing more nucleation sites and consequently smaller grain sizes [13]. The T2 thin film possesses the largest grain size, which implies larger voids between the grains and is beneficial for 4
electrochemical performance. 3.2. Electrochemical performance
Fig. 2. (a) CV curves at the scan rate of 100 mV s-1, (b) GCD curves and (c) Comparison of volumetric capacitances at the current density of 1 mA cm-2 of NbN thin films (T1-T4). Electrochemical performances of all thin films are evaluated by various electrochemical techniques and the results are presented in Fig. 2. Fig. 2a depicts the CV curves of all thin films at the scan rate of 100 mV s-1. It is well known that the capacitance is proportional to the integral area of CV curves [14]. By comparison, the capacitance of the T2 thin film electrode is larger than those of other thin film electrodes. This is also corroborated by the GCD curves in Fig. 2b, where the discharge duration of the T2 thin film electrode is obviously longer. As shown in Fig. 2c, the calculated volumetric capacitances of the T1, T2, T3 and T4 thin film electrodes are 462.5, 707.1, 549.3 and 292.4 F cm-3 at 1 mA cm-2, respectively. The difference in capacitances can be ascribed to the different crystallinity [15]. Besides, the larger voids for the T2 thin film electrode may promote the accessibility of electrolyte ions to the electrode surface, and thus contribute to improving the capacitance. We further investigate the electrochemical behavior of the T2 thin film electrode. Apparently, CV curves in Fig. 3a presents a deviation from the rectangular shape, implying that the capacitance contributions mainly originate from the pseudocapacitance. There is no distinct redox peak observed from the CV curves, probably because the charge/discharge process is conducted at a pseudo-constant rate [8]. As displayed in Fig. 3b, the potential-time curves are approximately symmetric with slight 5
distortions, further confirming the supercapacitive behavior arising from the chemisorption of the electrolyte ions, i.e. H+. Fig. 3c shows the volumetric capacitance and coulombic efficiency at different
Fig. 3. Electrochemical performance of the T2 thin film electrode: (a) CV curves at varying scan rates; (b) GCD curves at different current densities; (c) Volumetric capacitance and coulombic efficiency as a function of current density; (d) Cycling performance at the scan rate of 200 mV s-1 (inset shows the CV curves of 1st, 10,000th and 20,000th cycle); (e) Nyquist plot (inset shows the magnified high-frequency region); (f) Corresponding equivalent circuit model. current densities. The T2 thin film electrode can achieve a remarkable volumetric capacitance of 707.1 F cm-3 at 1 mA cm-2 and still remain 51.3% when the current density increases to 10 mA cm-2. Moreover, the high coulombic efficiency demonstrates good reversibility of the charge/discharge process. This outstanding performance is comparable with previously reported works (shown in Table S2). Fig. 3d indicates the superior cycling stability that remains 92.2% of the initial capacitance after 20000 cycles, which is also demonstrated by the CV curves in the inset. The EIS analysis is widely employed to investigate the kinetic performance of the electrodes. The Nyquist plot and the corresponding equivalent circuit models are shown in Fig. 3e and 3f, respectively. At the low-frequency region, a beeline with a slope approaching 90° presents the typical capacitive behavior. At the high-frequency region (inset), the circular arc originates from the charge transfer process at the electrode/electrolyte interface and its intersection with the real axle corresponds to the internal resistance. The relative low internal resistance
6
(Rs, 1.8 Ω) and charge transfer resistance (Rct, 8.7 Ω) are helpful for the good electrochemical performance.
4. Conclusions In summary, NbN thin films were prepared as binder-free electrodes for supercapacitors by reactive magnetron sputtering. By optimizing the nitrogen content, NbN thin film electrodes deliver a volumetric capacitance of 707.1 F cm-3 and an outstanding cycling stability with 92.2% capacitance remaining after 20000 cycles. As a result, our works can forward the application of NbN thin films in supercapacitors or other energy storage systems.
Acknowledgments This research is financially supported by the National Nature Science Foundation of China (No. 51372212, 51601163).
References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19-29. [2] Y. Zhong, X.H. Xia, F. Shi, J.Y. Zhan, J.P. Tu, H.J. Fan, Transition metal carbides and nitrides in energy storage and conversion, Adv. Sci. 3 (2016) 1500286. [3] Y.G. Wu, F. Ran, Vanadium nitride quantum dot/nitrogen-doped microporous carbon nanofibers electrode for high-performance supercapacitors, J. Power Sources 344 (2017) 1-10. [4] A. Achour, R.L. Porto, M.A. Soussou, M. Islam, M. Boujtita, K.A. Aissa, L. L. Brizoual, A. Djouadi, T. Brousse, Titanium nitride films for micro-supercapacitors: effect of surface chemistry and film morphology on the capacitance, J. Power Sources 300 (2015) 525-532. 7
[5] B.B. Wei, H.F. Liang, D.F. Zhang, Z.T. Wu, Z.B. Qi, Z.C. Wang, CrN thin films prepared by reactive DC magnetron sputtering for symmetric supercapacitors, J. Mater. Chem. A 5 (2017) 2844-2851. [6] M. Arif, A. Sanger, A. Singh, Sputter deposited chromium nitride thin electrodes for supercapacitor applications, Mater. Lett. 220 (2018) 213-217. [7] D. Choi, P.N. Kumta, Synthesis and characterization of nanostructured niobium and molybdenum nitrides by a two-step transition metal halide approach, J. Am. Ceram. Soc. 94 (2011) 2371-2378. [8] H.L. Cui, G.L. Zhu, X.Y. Liu, F.X. Liu, Y. Xie, C.Y. Yang, T.Q. Lin, H. Gu, F.Q. Huang, Niobium nitride Nb4N5 as a new high-performance electrode material for supercapacitors, Adv. Sci. 2 (2015) 1500126. [9] B. Gao, X. Xiao, J.J. Su, X.M. Zhang, X. Peng, J.J. Fu, P.K. Chu, Synthesis of mesoporous niobium nitride nanobelt arrays and their capacitive properties, Appl. Surf. Sci. 383 (2016) 57-63. [10] M. Benkahoul, E. Martinez, A. Karimi, R. Sanjinés, F. Lévy, Structural and mechanical properties of sputtered cubic and hexagonal NbNx thin films, Surf. Coat. Tech. 180 (2004) 178-183. [11] J. Musil, Recent advances in magnetron sputtering technology, Surf. Coat. Technol. 100 (1998) 280-286. [12] G. Abadias, Y.Y. Tse, Ph. Guérin, Interdependence between stress, preferred orientation, and surface morphology of nanocrystalline TiN thin films deposited by dual ion beam sputtering, J. Appl. Phys. 99 (2006) 113519. [13] J.H. Huang, K.W. Lau, G.P. Yu, Effect of nitrogen flow rate on structure and properties of nanocrystalline TiN thin films produced by unbalanced magnetron sputtering, Surf. Coat. Technol. 191 (2005) 17-24. [14] S.L. Zhang, N. Pan, Supercapacitors performance evaluation, Adv. Energy Mater. 5 (2015) 1401401. [15] L. Mao, K. Zhang, H.S.O. Chan, J.S. Wu, Nanostructured MnO2/graphene composites for supercapacitor electrodes: the effect of morphology, crystallinity and composition, J. Mater. Chem. 22 (2012) 1845-1851.
8
Figure captions:
Fig. 1. (a) XRD patterns and (b-i) SEM images of T1-T4 thin films. Fig. 2. (a) CV curves at the scan rate of 100 mV s-1, (b) GCD curves and (c) Comparison of volumetric capacitances at the current density of 1 mA cm-2 of NbN thin films (T1-T4). Fig. 3. Electrochemical performance of the T2 thin film electrode: (a) CV curves at varying scan rates; (b) GCD curves at different current densities; (c) volumetric capacitance and coulombic efficiency as a function of current density; (d) cycling performance at the scan rate of 200 mV s-1 (inset shows the CV curves of 1st, 10,000th and 20,000th cycle); (e) Nyquist plot (inset shows the magnified high-frequency region); (f) the corresponding equivalent circuit model.
9
Highlights
1. The NbN thin films are fabricated by reactive magnetron sputtering. 2. The optimal NbN thin film exhibits high volumetric capacitance and outstanding cycling stability remaining. 3. These NbN thin films are directly deposited onto substrates without additional binders.
13
S0167-577X(18)30957-1 https://doi.org/10.1016/j.matlet.2018.06.052 MLBLUE 24492
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
29 March 2018 13 June 2018 14 June 2018
Please cite this article as: H. Shen, B. Wei, D. Zhang, Z. Qi, Z. Wang, Magnetron sputtered NbN thin film electrodes for supercapacitors, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.06.052
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetron sputtered NbN thin film electrodes for supercapacitors Hao Shena, Binbin Weia, Dongfang Zhanga, Zhengbing Qib,*, Zhoucheng Wanga,* a
b
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China *Corresponding author: Tel./fax: +86-592-2180738, E-mail: [email protected] (Z.C. Wang) Tel./fax: +86-592-6291328, E-mail: [email protected] (Z.B. Qi)
Abstract: Selecting suitable electrode materials is the key to develop high-performance supercapacitors. Herein, we report the niobium nitride (NbN) thin films prepared by reactive magnetron sputtering and investigate their electrochemical performances when used as supercapacitor electrodes. NbN thin film electrodes can achieve the optimal electrochemical performance with a volumetric capacitance of 707.1 F cm-3 and an outstanding cycling stability (remaining 92.2% after 20000 cycles). Such impressive properties render the NbN thin films qualified as the promising candidates for supercapacitors or other energy storage systems. Keywords: NbN thin film; magnetron sputtering; physical vapor deposition; supercapacitor; energy storage and conversion.
1. Introduction Nowadays, for handling the severe energy and environment crisis, regenerative energy resources have been developed to optimize the traditional energy structures, but still constrained by inherent intermittence and disequilibrium distribution [1]. To cope with these knotty issues, high-efficient energy storage systems are available and in growing demands. The most wildly utilized energy storage systems 1
are batteries and electrochemical capacitors (namely supercapacitors). Compared with batteries, supercapacitors attract increasing attention on account of some outstanding merits, such as long cycle life and high power density, which suffice to meet the requirements of energy storage systems. Recently, transition metal nitrides (TMNs) are being spotlighted as electrode materials for supercapacitors by virtue of high specific capacitance, outstanding chemical resistance, good conductivity and excellent cycling stability [2]. Some TMNs have been already applied in the supercapacitor field, such as VN [3], TiN [4] and CrN [5, 6]. As a member of TMNs, NbN is particularly noticeable due to its higher specific capacitance and nevertheless, has been reported only a few times [7-9]. To the best of our knowledge, these NbN electrode materials reported up to date were obtained by annealing corresponding precursors under high temperature in the NH3 atmosphere, inducing high energy-consumption and environmental pollution. Differing from conventional approaches, magnetron sputtering technologies are environment-friendly and simply tunable, by which the active materials are directly deposited onto substrates and the electrodes are prepared without additional binders. Herein, we successfully fabricated NbN thin films via reactive DC magnetron sputtering and further investigated their electrochemical performances. The optimal NbN thin film electrodes can achieve a volumetric capacitance of 707.1 F cm-3 and exhibit an exceptional cycling stability with 92.2% capacitance retention after 20000 cycles. This study furthers the investigation of NbN thin films as promising electrodes for supercapacitors.
2. Experimental 2.1. Fabrication of NbN thin film electrodes 2
NbN thin films were deposited on the smooth Si (100) substrates by reactive DC magnetron sputtering from a pure niobium target (purity 99.95%). Before being placed into the sputtering chamber, the Si substrates were stepwise cleaned with acetone and ethanol for minimizing adhesive pollutants. To eliminate surface contaminants and accelerate the deposition rate, the niobium target was pre-sputtered for 5 min. The detailed deposition parameters are presented in Table S1. 2.2. Characterization X-ray diffraction (XRD, Rigaku Ultima IV) was employed to identify the crystallographic structure. The surface and cross-sectional morphologies of NbN thin films were observed by scanning electron microscopy (SEM, ZEISS Sigma). 2.3. Electrochemical testing All the electrochemical measurements were carried out on an electrochemical workstation (CHI660E, CHI Instruments). The electrochemical performances of NbN thin film electrodes were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 0.5 M H2SO4 aqueous electrolyte using a conventional three-electrode cell configuration in which the thin film electrode, an Ag/AgCl and a Pt foil were used as the working electrode, reference electrode and counter electrode, respectively.
3. Results and discussion 3.1. Composition and microstructure As shown in Fig. 1a, all diffraction peaks can be indexed to the fcc NbN (JCPDS: 65-9399) and the slight deviations are attributed to the internal stress [10]. The predominant peak located at ~35.5° reveals 3
Fig. 1. (a) XRD patterns and (b-i) SEM images of T1-T4 thin films. the preferential orientation with (111) plane. Meanwhile, the other two weak peaks positioned at ~41.2° and ~59.7° correspond to the (200) and (220) planes, respectively. The (111) diffraction peak of the T2 thin film is sharper than those of other thin films, denoting its best crystallinity [8]. The surface and cross-sectional morphologies of NbN thin films (T1-T4) are shown in Fig. 1b-i. Obviously, dense grains are dispersed throughout the surface and the cross-sectional views show the columnar texture, which is consistent with typical island structure characteristic. With increasing nitrogen content, the grain sizes increase at first and then gradually decrease, reaching the maximum value at 10%. During the deposition, growing films are under ion bombardment, and energies of incident ions are inversely proportional to deposition rates [11]. If nitrogen content increases, deposition rates generally decline due to target poison, leading to higher energies of incident ions. Although the energies increase as nitrogen content increases from 5% to 10%, the incident ions do not have enough energies to penetrate beyond the first atomic layer. Therefore, these energies are essentially transferred to adatoms and increase the surface mobility, which could make grain sizes larger [12]. As nitrogen content increases from 10% to 20%, the incident ions with further increased energies can penetrate into and collide with near surface atoms by re-sputtering effect, causing more nucleation sites and consequently smaller grain sizes [13]. The T2 thin film possesses the largest grain size, which implies larger voids between the grains and is beneficial for 4
electrochemical performance. 3.2. Electrochemical performance
Fig. 2. (a) CV curves at the scan rate of 100 mV s-1, (b) GCD curves and (c) Comparison of volumetric capacitances at the current density of 1 mA cm-2 of NbN thin films (T1-T4). Electrochemical performances of all thin films are evaluated by various electrochemical techniques and the results are presented in Fig. 2. Fig. 2a depicts the CV curves of all thin films at the scan rate of 100 mV s-1. It is well known that the capacitance is proportional to the integral area of CV curves [14]. By comparison, the capacitance of the T2 thin film electrode is larger than those of other thin film electrodes. This is also corroborated by the GCD curves in Fig. 2b, where the discharge duration of the T2 thin film electrode is obviously longer. As shown in Fig. 2c, the calculated volumetric capacitances of the T1, T2, T3 and T4 thin film electrodes are 462.5, 707.1, 549.3 and 292.4 F cm-3 at 1 mA cm-2, respectively. The difference in capacitances can be ascribed to the different crystallinity [15]. Besides, the larger voids for the T2 thin film electrode may promote the accessibility of electrolyte ions to the electrode surface, and thus contribute to improving the capacitance. We further investigate the electrochemical behavior of the T2 thin film electrode. Apparently, CV curves in Fig. 3a presents a deviation from the rectangular shape, implying that the capacitance contributions mainly originate from the pseudocapacitance. There is no distinct redox peak observed from the CV curves, probably because the charge/discharge process is conducted at a pseudo-constant rate [8]. As displayed in Fig. 3b, the potential-time curves are approximately symmetric with slight 5
distortions, further confirming the supercapacitive behavior arising from the chemisorption of the electrolyte ions, i.e. H+. Fig. 3c shows the volumetric capacitance and coulombic efficiency at different
Fig. 3. Electrochemical performance of the T2 thin film electrode: (a) CV curves at varying scan rates; (b) GCD curves at different current densities; (c) Volumetric capacitance and coulombic efficiency as a function of current density; (d) Cycling performance at the scan rate of 200 mV s-1 (inset shows the CV curves of 1st, 10,000th and 20,000th cycle); (e) Nyquist plot (inset shows the magnified high-frequency region); (f) Corresponding equivalent circuit model. current densities. The T2 thin film electrode can achieve a remarkable volumetric capacitance of 707.1 F cm-3 at 1 mA cm-2 and still remain 51.3% when the current density increases to 10 mA cm-2. Moreover, the high coulombic efficiency demonstrates good reversibility of the charge/discharge process. This outstanding performance is comparable with previously reported works (shown in Table S2). Fig. 3d indicates the superior cycling stability that remains 92.2% of the initial capacitance after 20000 cycles, which is also demonstrated by the CV curves in the inset. The EIS analysis is widely employed to investigate the kinetic performance of the electrodes. The Nyquist plot and the corresponding equivalent circuit models are shown in Fig. 3e and 3f, respectively. At the low-frequency region, a beeline with a slope approaching 90° presents the typical capacitive behavior. At the high-frequency region (inset), the circular arc originates from the charge transfer process at the electrode/electrolyte interface and its intersection with the real axle corresponds to the internal resistance. The relative low internal resistance
6
(Rs, 1.8 Ω) and charge transfer resistance (Rct, 8.7 Ω) are helpful for the good electrochemical performance.
4. Conclusions In summary, NbN thin films were prepared as binder-free electrodes for supercapacitors by reactive magnetron sputtering. By optimizing the nitrogen content, NbN thin film electrodes deliver a volumetric capacitance of 707.1 F cm-3 and an outstanding cycling stability with 92.2% capacitance remaining after 20000 cycles. As a result, our works can forward the application of NbN thin films in supercapacitors or other energy storage systems.
Acknowledgments This research is financially supported by the National Nature Science Foundation of China (No. 51372212, 51601163).
References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19-29. [2] Y. Zhong, X.H. Xia, F. Shi, J.Y. Zhan, J.P. Tu, H.J. Fan, Transition metal carbides and nitrides in energy storage and conversion, Adv. Sci. 3 (2016) 1500286. [3] Y.G. Wu, F. Ran, Vanadium nitride quantum dot/nitrogen-doped microporous carbon nanofibers electrode for high-performance supercapacitors, J. Power Sources 344 (2017) 1-10. [4] A. Achour, R.L. Porto, M.A. Soussou, M. Islam, M. Boujtita, K.A. Aissa, L. L. Brizoual, A. Djouadi, T. Brousse, Titanium nitride films for micro-supercapacitors: effect of surface chemistry and film morphology on the capacitance, J. Power Sources 300 (2015) 525-532. 7
[5] B.B. Wei, H.F. Liang, D.F. Zhang, Z.T. Wu, Z.B. Qi, Z.C. Wang, CrN thin films prepared by reactive DC magnetron sputtering for symmetric supercapacitors, J. Mater. Chem. A 5 (2017) 2844-2851. [6] M. Arif, A. Sanger, A. Singh, Sputter deposited chromium nitride thin electrodes for supercapacitor applications, Mater. Lett. 220 (2018) 213-217. [7] D. Choi, P.N. Kumta, Synthesis and characterization of nanostructured niobium and molybdenum nitrides by a two-step transition metal halide approach, J. Am. Ceram. Soc. 94 (2011) 2371-2378. [8] H.L. Cui, G.L. Zhu, X.Y. Liu, F.X. Liu, Y. Xie, C.Y. Yang, T.Q. Lin, H. Gu, F.Q. Huang, Niobium nitride Nb4N5 as a new high-performance electrode material for supercapacitors, Adv. Sci. 2 (2015) 1500126. [9] B. Gao, X. Xiao, J.J. Su, X.M. Zhang, X. Peng, J.J. Fu, P.K. Chu, Synthesis of mesoporous niobium nitride nanobelt arrays and their capacitive properties, Appl. Surf. Sci. 383 (2016) 57-63. [10] M. Benkahoul, E. Martinez, A. Karimi, R. Sanjinés, F. Lévy, Structural and mechanical properties of sputtered cubic and hexagonal NbNx thin films, Surf. Coat. Tech. 180 (2004) 178-183. [11] J. Musil, Recent advances in magnetron sputtering technology, Surf. Coat. Technol. 100 (1998) 280-286. [12] G. Abadias, Y.Y. Tse, Ph. Guérin, Interdependence between stress, preferred orientation, and surface morphology of nanocrystalline TiN thin films deposited by dual ion beam sputtering, J. Appl. Phys. 99 (2006) 113519. [13] J.H. Huang, K.W. Lau, G.P. Yu, Effect of nitrogen flow rate on structure and properties of nanocrystalline TiN thin films produced by unbalanced magnetron sputtering, Surf. Coat. Technol. 191 (2005) 17-24. [14] S.L. Zhang, N. Pan, Supercapacitors performance evaluation, Adv. Energy Mater. 5 (2015) 1401401. [15] L. Mao, K. Zhang, H.S.O. Chan, J.S. Wu, Nanostructured MnO2/graphene composites for supercapacitor electrodes: the effect of morphology, crystallinity and composition, J. Mater. Chem. 22 (2012) 1845-1851.
8
Figure captions:
Fig. 1. (a) XRD patterns and (b-i) SEM images of T1-T4 thin films. Fig. 2. (a) CV curves at the scan rate of 100 mV s-1, (b) GCD curves and (c) Comparison of volumetric capacitances at the current density of 1 mA cm-2 of NbN thin films (T1-T4). Fig. 3. Electrochemical performance of the T2 thin film electrode: (a) CV curves at varying scan rates; (b) GCD curves at different current densities; (c) volumetric capacitance and coulombic efficiency as a function of current density; (d) cycling performance at the scan rate of 200 mV s-1 (inset shows the CV curves of 1st, 10,000th and 20,000th cycle); (e) Nyquist plot (inset shows the magnified high-frequency region); (f) the corresponding equivalent circuit model.
9
Highlights
1. The NbN thin films are fabricated by reactive magnetron sputtering. 2. The optimal NbN thin film exhibits high volumetric capacitance and outstanding cycling stability remaining. 3. These NbN thin films are directly deposited onto substrates without additional binders.
13