Magnetron sputtered TiN thin films toward enhanced performance supercapacitor electrodes
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Supercapacitors as a new type of energy storage devices bridging the gap between conventional capacitors and batteries have aroused widespread concern. Herein, binder-free titanium nitride (TiN) thin film electrodes for supercapacitors prepared by reactive magnetron sputtering technology are reported. The effect of N2 content on the supercapacitor performance is evaluated. A highest specific capacitance of 27.3 mF cm−2 at a current density of 1.0 mA cm−2, together with excellent cycling performance (98.2% capacitance retention after 20,000 cycles at 2.0 mA cm−2) is achieved in a 0.5 M H2SO4 aqueous electrolyte. More importantly, a symmetric supercapacitor device assembled on the basis of TiN thin films can deliver a maximum energy density of 17.6 mWh cm−3 at a current density of 0.2 mA cm−2 and a maximum power density of 10.8 W cm−3 at a current density of 2 mA cm−2 with remarkable cycling stability. As a consequence, TiN thin films demonstrate great potential as promising supercapacitor electrode materials.
KeywordsSupercapacitor Energy storage Titanium nitride Magnetron sputtering
Energy security has become a key factor that restricts the sustainable development of human civilization, triggering tremendous efforts to develop renewable and non-polluting new energy [1, 2]. Among energy storage systems, supercapacitors have been considered to have potential applications in portable electronics and hybrid electric vehicles due to fast charge/discharge rate, long-term cycle stability, excellent rate capability, high-power density, and low cost [3, 4, 5]. Significantly, supercapacitors can provide higher power densities than conventional capacitors, and they also possess energy densities much higher than those of batteries [5, 6]. As previously reported, carbon-based materials and transition metal oxides widely used in supercapacitors possess their own advantages and shortcomings. Although electrical double-layer capacitors (EDLCs) based on carbon-based materials can achieve high-power density and cycling stability, the relatively low energy density due to low-specific capacitance restricts widespread application [7, 8]. In contrast, pseudocapacitors using transition metal oxides can deliver higher energy density than that of EDLCs, however, suffer from poor cyclic stability and low power density . Recently, transition metal nitrides have become a research hotspot on account of excellent chemical and thermal stability, superior electrical conductivity and outstanding electrochemical property [10, 11]. Especially, titanium nitride (TiN) as one of important metal nitrides has aroused particular attention because of its electric conductivity close to that of metals (4000–55,500 S cm−1) . Sun et al. designed the TiN@C nanotube-based fiber electrodes by one-step nitridation and complete carbon coating process, and a specific capacitance of 19.4 mF cm−2 at a scan rate of 10 mV s−1 was achieved . Tian et al. prepared titanium nitride nanotube array by a simple adsorption-reduction process and it showed a specific capacitance of 25.2 mF cm−2 at a scan rate of 100 mV s−1 . However, they have the following drawbacks: (1) involvement of toxic gas (NH3) and high temperature, (2) difficulty to control the oxygen amount and (3) usage of binder and conductive additives. To avoid these problems, directly depositing the active materials onto the substrate has been proposed. Achour et al. demonstrated the TiN thin film electrodes sputtered with a highest specific capacitance of 8.8 mF cm−2 at a scan rate of 100 mV s−1 . Currently, a major challenge of using TiN thin films as supercapacitor electrodes is to further improve their specific capacitance. To the end, we design the TiN thin films with varying degrees of porosity by adjusting the nitrogen gas flow rate. Among them, the TiN thin film electrode with 9% N2 content is found to exhibit a significantly enhanced specific capacitance of 27.3 mF cm−2 at a current density of 1.0 mA cm−2 (42.6 mF cm−2 at 100 mV s−1) along with outstanding cycling performance. Significantly, we have successfully demonstrated a symmetric supercapacitor device using two identical TiN thin films that reaches an impressive energy density of 17.6 mWh cm−3 at a power density of 1.1 W cm−3.
Growth of TiN thin films
Summary of sample identification and deposition conditions for TiN thin films
Deposition pressure (Pa)
N2 flow rate (sccm)
Ar flow rate (sccm)
X-ray diffraction (XRD, Philips X’pert PRO) was taken to analyse the crystal structure and phase formation using Cu Kα radiation. Raman spectroscopy was acquired by an XploRA Raman at 532 nm excitation wavelength. X-ray photoelectron spectroscopy (XPS, PHI-Quantum 2000) was carried out with Al Kα radiation to study the chemical states and compositions of TiN thin films. The morphologies and microstructures of TiN thin films were observed by scanning electron microscopy (SEM, ZEISS Sigma) and transmission electron microscopy (TEM, Tecnai F30), respectively. The surface roughnesses of TiN thin films were determined using tapping-mode atomic force microscopy (AFM, Agilent 5500).
Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed on a CHI 660E electrochemical workstation. Owing to excellent electronic conductivity, the deposited TiN thin films can directly be used as working electrode and as current collector without further process steps. For single electrode, the electrochemical performance was investigated in a 0.5 M H2SO4 aqueous electrolyte within a three-electrode cell system where TiN thin films, Ag/AgCl and Pt wire were served as the working electrode, reference electrode and counter electrode, respectively. Whereas in the symmetric supercapacitor device, two identical pieces of TiN thin films were sandwiched by Celgard 3501 in a 0.5 M H2SO4 aqueous electrolyte.
Results and discussion
EIS analysis is an effective tool to examine the interfacial property of thin film electrodes. As shown in Fig. 5e, the Nyquist plot consists of a semicircle and a straight line at the high and low frequency region, respectively. The small diameter semicircle indicates low charge-transfer resistance (see inset of Fig. 5e), in concordance with GCD result. A good capacitor behavior can be corroborated due to the almost vertical line and nearly 90 ° phase angle observed in the Bode plot (Fig. 5f). In addition, the characteristic frequency (f0) at − 45 ° is around 0.87 Hz, and thereby the time constant calculated by 1/f0 equals 1.1 s, depicting a fast energy release capability to ensure ultrahigh power and energy .
In summary, we demonstrate the growth of TiN thin films on the silicon wafers using magnetron sputtering method under different nitrogen gas flow rates and investigate their electrochemical performance as supercapacitor electrodes. TiN thin film electrodes show long-term cycling performance with an optimized specific capacitance of 27.3 mF cm−2 at 1.0 mA cm−2, which excels most of the previous transition metal nitrides. Furthermore, a symmetric supercapacitor device based on TiN thin films is successfully produced and achieves a maximum energy density of 17.6 mWh cm−3 at a power density of 1.1 W cm−3, along with outstanding cycling stability. These findings suggest that TiN thin films possess good application prospects in the field of supercapacitor.
This research is financially supported by the National Nature Science Foundation of China (Nos. 51372212, 51601163).
Compliance with ethical standards
Conflict of interest
There are no conflicts of interest.
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