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Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance
Author’s Accepted Manuscript Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance Peng Sun, Rui Lin, Zilong Wang, Meijia Qiu, Zhisheng Chai, Bodong Zhang, Hui Meng, Shaozao Tan, Chuanxi Zhao, Wenjie Mai
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S2211-2855(16)30550-X http://dx.doi.org/10.1016/j.nanoen.2016.11.052 NANOEN1644
To appear in: Nano Energy Received date: 31 October 2016 Revised date: 24 November 2016 Accepted date: 25 November 2016 Cite this article as: Peng Sun, Rui Lin, Zilong Wang, Meijia Qiu, Zhisheng Chai, Bodong Zhang, Hui Meng, Shaozao Tan, Chuanxi Zhao and Wenjie Mai, Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.11.052 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 galley proof before it is published in its final citable 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.
Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance
Peng Suna, Rui Linb, Zilong Wanga, Meijia Qiua, Zhisheng Chaia, Bodong Zhanga, Hui Menga, Shaozao Tanb, Chuanxi Zhaoa*, Wenjie Maia,c*
a
Siyuan laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies
and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong 510632, China, b
Department of Chemistry, Jinan University, Guangzhou, Guangdong 510632, P. R.
China. c
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and
Communications, Jinan University, Guangzhou, Guangdong 510632, China.
[email protected] [email protected]
*
Corresponding authors.
Abstract As an emerging energy storage device for wearable electronics, fiber supercapacitors 1
(FSCs) have distinguished themselves from two dimensional SCs. Herein, TiN@C nanotube-based fiber electrodes are designed through a one-step nitridation and complete carbon coating process. They exhibit 260 % higher capacitance than the samples obtained by traditional two-step nitridation and carbonization method. The carbon shell and the fabrication method are critical for the performance of the electrodes. The carbon shell remarkably protects the polycrystalline TiN core from mechanical cracking and chemical oxidation. FSCs based on these fiber electrodes demonstrate excellent electrochemical performance and mechanical stability. High capacitance of 2.4 mF∙cm-1 (19.4∙mF∙cm-2 and 1.9 F∙cm-3) at a scan rate of 10 mV s-1 with an energy density of 2.69 μWh∙cm-2 and a power density of 809 μW∙cm-2 can be achieved. Moreover, nearly 80% capacitance retention after 10,000 cycle tests can be realized and the novel FSC can be bent for 2,000 times with capacitance decay of 7.2 %. Interestingly, our FSCs show almost no decay when cut into two segments due to the adoption of solid state electrolyte, demonstrating excellent tailorability. Finally, they can be also used as electric cables while store energy, demonstrating their bi-functionality.
Graphical abstract
Fiber supercapacitors based on TiN@C nanotubes have been firstly fabricated by a new one-step nitridation and complete carbon coating method. It demonstrates a mechanical tailorability, excellent bending stability and convenience to be woven into textile, which promotes the exploration for proper energy storage garments. 2
Keywords: TiN@C nanotubes, carbon shell, fiber supercapacitor, mechanical stability
1. Introduction With the demand for flexible and portable electronic products continues to rise, wearable electronics gains increasing popularity among customers [1]. Hence, how to develop novel energy storage devices for powering wearable electronic systems becomes a key issue. Supercapacitors (SCs) with flexible substrates are potential candidates that can be employed in energy storage textile [2-4]. However, clothes fabricated from conventional 2 dimensional (2D) planar devices are usually not breathable, which means they are not convenient for the volatilization of sweat or air from human body and will result in discomfort. Flexible SCs with 1 dimensional fiber shape are lightweight, reconfigurable and can be easily woven into different shapes [5, 6]. Because of the weaving property, they are more breathable than the conventional 2D planar devices, thus more suitable for being employed in wearable SCs. Fiber shape supercapacitor (FSC) has become fascinating since Wang’s group first put forward the model of FSCs [7-10]. And there have been a number of publications about their application in wearable devices. For example, Qu and his co-workers designed series of graphene FSC with good bending cycle of 1000 times (nearly 90 % capacitance retention) [11]. Yu et al. fabricated coil-type FSC that can preserve 93 % initial capacitance after experiencing 1000 electrochemical cycle in 3
different bending state [12]. However, better anti-bending ability is needed when these FSCs are applied in clothes. In addition, it’s important to retain mechanical and electrochemical performance when FSC suffers some damage such as snap or being cut off, which is seldom studied [13, 14]. According to the classification of substrate, there are two kinds of FSC, carbon based FSC and metal based FSC [15-19]. Metal based FSC demonstrates better mechanical strength and electric conductivity, confirming the possibility for both transmitting and storing charge concurrently [12, 20]. Thomas’s group constructed a coil type FSC by employing its inner substrate Cu as conductive component while outer asymmetric SC can work steady [20]. Wang et al. assembled flexible FSC into commercial electric cables to achieve the two fuctions of energy storage and transmission [21]. To combine fashionable designs with advanced technology in smart garments, it’s necessary to integrate these two functions into single cable electric FSC. Many researches have focused on pseudocapacitors because of their higher energy density and capacitance [22-28]. However, the low conductivity of traditional pseudocapacitive materials like metal oxide or hydroxide limits their performance. Due to the excellent electric conductivity (4000-55500 S∙cm-1), stable chemical properties and rather high capacitance, metal nitrides, especially TiN, have gradually become a hot research topic in SCs [29-32]. But its poor electrochemical stability hinders its further development in practical applications. Lei’s group synthesized carbon coated TiN nanotube arrays through two step atomic layer deposition process and achieved much better electrochemical performance and stability than bare TiN 4
NTs [33]. Lu et al. indicated that the electrochemical oxide reaction leads to the instability and pointed that the facile method, which coats a thin layer of carbon on TiN, can effectively prevent its electrochemical oxidation [34, 35]. Nevertheless, the two-step method, which first nitrogenizes the TiO2 into TiN and then coats carbon shell, includes some redundant procedure. It’s necessary to simplify the synthesizing process via other cost-effective methods. Herein, a TiN@C nanotubes (NTs)-based FSC with both energy transmission and storing functions was designed. TiO2 NTs were first synthesized on Ti wire via an anode oxidation method. After that, a carbonaceous shell was prepared on the TiO2 NTs by hydrothermal method. Finally, the TiO2@C NTs experienced further nitridation and complete carbonization concurrently, transforming into TiN@C NTs. Two parallel electrodes can be made into a solid-state FSC with good electrochemical performance. The device demonstrates a capacitance of 2.4 mF∙cm-1 (19.4 mF∙cm-2 and 1.9 F∙cm-3) at a scan rate of 10 mV s-1 and an energy density of 2.69 μWh∙cm-2 with a power density of 809 μW cm-2. After 10,000 cycles, the capacitance can still retain 78 %. Excitingly, it can bear a rather long bending cycle of 2000 times with 92.8 % capacitance preservation. The most special feature is its tailorability. There is almost no performance decay when it was cut into halves. Its excellent bending resistance and tailorability prove the possibility of its application in wearable electronics. Because of the outstanding conductive contact between active materials and the substrate, it can also be used as a cable for current transfer while storing charge at the same time, which broadens the applications of this novel FSC. 5
2. Experimental 2.1 Chemicals and materials Ammonium fluoride (AR, Chemical Guangzhou China), ethylene glycol (AR, YongHua Jiangsu China), glucose (AR, BoAo Shanghai China), potassium hydroxide (AR, Chemical Guangzhou China), polyvinyl alcohol (AR, 1799, Aladdin), ethanol (AR, Chemical Guangzhou China), acetone (AR, HongChuan Dongguan China) and Ti wire (diameter of 200 μm) were purchased from vendors. 2.2 Synthesis of TiN@C NTs electrode Firstly, Ti wires (average diameter of 200 μm) were washed by acetone, deionized (DI) water and ethanol one after another each for 10 min. Then, the clean substrates were soaked into an ethylene glycol solution with 0.3 wt % NH4F and 2 vol% deionized water under a constant voltage of 60 V for 2 h, as reported previously [36]. After that, two different operations were conducted for gathering two types of TiN@C NTs samples. (1). The as prepared TiO2 NTs were transferred into a tube furnace in NH3 atmosphere at 800 ℃ for 1 h, converting into TiN NTs. Then, a hydrothermal carbon coating and further anneal were proceeded for covering a thin layer of carbon out of the NTs. We name this kind of samples from traditional two-step process as TiN@C NTs II. (2). The as prepared TiO2 NTs were firstly coated a thin layer of carbonaceous shell through a hydrothermal reaction, and then went through a further nitridation process to get this TiN@C NTs I directly. 2.3 Synthesis of two kinds of carbon shell material 30 mL 0.05 M glucose solution was first poured into a 50 mL Teflon-lined stainless 6
autoclave. Then the sealed autoclave was kept in an electric oven at 180 ℃ for 3 h (kept the identical condition during the hydrothermal carbon coating process). After that, the residual solution was dried in hot air to get the deposition powder. Next, the carbonaceous powder was divided into two parts, which were transferred to the tube furnace. For better comparing the converting process of two kinds of carbon shell, we kept the annealing temperature (800 ℃) and duration time (1 h) the same, only changed the annealing atmosphere of N2 and NH3. After the annealing process, two kinds of carbon shell material can be obtained. 2.4 Fabrication of solid-state FSC The KOH/Polyvinyl Alcohol (PVA) sol gel electrolyte was firstly prepared by adding 6 g PVA powder into 60 mL 1 M KOH solution and stirred under 85 ℃ for 2 h. Two TiN@C NTs electrodes were soaked into KOH/PVA sol gel electrolyte for 1 min, and then were put into a silica gel tube filled with KOH/PVA electrolyte for 1 day to extract redundant water. 2.5 Characterization The morphology and nanostructure of the samples were characterized by field emission scanning electron microscope (FE-SEM, ZEISS ULTRA 55), transmission electron microscope (TEM, JEOL 2100F, 200 kV) equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD, Rigaku, MiniFlex600, Cu Kα) analyzer and X-ray Photoelectron Spectroscope (XPS, Thermo K-alpha). The electrochemical properties of the FSCs were on the CHI 660E (ChenHua Shanghai) electrochemical workstation, and the electrochemical impedance spectra (EIS) were 7
measured using VerasSTAT 3-400 (Princeton Applied Research) at a frequency ranging from 100 mHz to 10 kHz with a potential amplitude of 10 mV. The single electrode test was performed in 1 M KOH solution. Saturated calomel electrode (SCE) reference electrode and Pt counter electrode were used in the measurement.
3. Results and Discussion Two kinds of TiN@C NTs electrode materials were synthesized via two different methods, as shown in scheme of Figure 1. Firstly, TiO2 NTs were grown on Ti wire through anodic oxidation process. Different procedures were employed in the two methods. In two-step method (lower flow diagram), the first step was conducted to obtain TiN NTs. Then, a carbon shell was attached to the surface of the TiN NTs through a hydrothermal glucose-assisted method and further annealing process, resulting in TiN@C NTs II. For this method, the nitridation and carbonization processes were two separate steps, requiring two different annealing processes, which is uneconomical and inefficient. Moreover, the carbon shell does not adhere well to the surface of TiN, which reduces the surface area of the electrode. While in the one-step method (upper flow diagram), TiO2 NTs were first coated a layer of carbonaceous shell (hydrothermal procedure) and then annealed in NH3 atmosphere. The annealing process in NH3 can achieve the nitridation and complete carbonization at the same time and produce TiN@C NTs I. The details of synthesis are shown in experiment part. By achieving nitridation and complete carbonization in one step, this new method can greatly shorten the synthetic time and consume less energy. 8
3.1 Characterization and electrochemical performance of the TiN@C NTs electrode Scanning electron microscope (SEM) images of TiO2 NTs, TiO2@C NTs, TiN NTs and TiN@C NTs (I and II) were presented in Figure 2 and S1. From the side and top views of TiO2 NTs on the Ti wire in Figures 2a, 2b and S1a, we can find after anodic oxidation Ti wire is uniformly covered by a layer of TiO2 NTs whose average length is about 10 μm (cross-sectional image in Figure 2a). Top view image of the TiO2 layer in Figure 2b demonstrates a perfect NTs array with average outer diameter of 100 nm. After coating a carbonaceous shell, there are numbers of nanoparticles covering the TiO2 NTs as shown in Figure S1b. If TiO2 NTs are transformed into TiN directly, the produced TiN can keep the morphology of nanotube (Figure S1c). SEM image of TiN@C NTs II synthesized by two-step method is shown in Figure 2c. It can be found that the two-step method brings aggregation of carbon particles and breaks the tube structures. This is probably because the bare TiN NTs are rather fragile, which are easily broken under high temperature (800 oC) during the carbonization process. While for TiN@C NTs I synthesized by one-step method shown in Figure 2d, uniform and compact carbon nanoparticle layer formed on their surface. This result reveals that in the one-step method the carbonaceous shell covering TiO2 NTs can protect the nanotube structure at high temperature (~ 800 oC). Top view images of TiN@C NTs I and II are demonstrated in Figure S1d and S1e, which clearly manifesting their tube structures. Furthermore, the element composition of two kinds of electrodes was investigated. Different kinds of carbon can be obtained from the carbonaceous 9
solution in the previous hydrothermal process. The dried carbonaceous powder from the solution experienced high-temperature reaction in different atmosphere (N2 and NH3) and then was converted to two different kinds of carbon, as detailed in Experimental Section. Next,
energy dispersive X-ray spectrometer (EDS)
measurements were conducted to characterize these two kinds of carbon shell. As shown in Figure S1f, the carbon shell synthesized from two-step method contains only C and O elements, where O comes from the organics absorbed in air environment. But for carbon shell obtained from one-step method it contains not only C and O elements but also a small quantity of N atoms (Figure S1g). This result proves the N atoms have been successfully doped into the carbon shell. X-ray diffraction (XRD) analysis is used to determine the composition and crystal structure of the samples. Figure 3a shows the XRD pattern of TiO2@C NTs and TiN@C NTs I. Peaks of TiO2@C NTs in the bottom diffraction curve can be assigned to the anatase TiO2 phase (JCPDS# 02-0387). It is notable that the peaks of top diffraction curve correspond to the (111), (200), (220), (311) and (222) faces of cubic TiN (JCPDS# 65-5774). Transmission electron microscope (TEM) images in Figure 3b-f presented a clearer verification of the detailed morphology and substance components. It is obvious that the synthesized TiN is of NT structure, as shown in Figure 3b. The inset demonstrates the corresponding high resolution TEM image. The interplanar distances of 0.236 nm and 0.209 nm match well with the (111) and (200) facets, respectively. Finally, scanning transmission electron microscope (STEM) image of the single TiN@C NT I and the corresponding EDS element mapping 10
images are shown in Figure 3c-f. It is apparent that the TiN NT is covered by a uniform carbon shell. To further ascertain the element composition and chemical bonding state of TiN@C NT I, X-ray photoelectron spectroscopy (XPS) data were collected. High resolution Ti-2p and N-1s XPS spectra are shown in Figure S2a and b, respectively. The core level Ti-2p spectrum (Figure S2a) can be divided into 6 peaks (three pairs of spin-orbit split doublets) after deconvolution, which are Ti−N (2p3/2 = 455.7 eV and 2p1/2 = 461.8 eV), Ti−N−O (2p3/2 = 456.9 eV and 2p1/2 = 463.2 eV), and Ti−O (2p3/2 = 458.5 eV and 2p1/2 = 464.4 eV). The N-1s detailed spectrum is shown in Figure S2b, presenting a broad peak that can be split into two peaks at 395.8 eV (Ti–N) and 396.7 eV (Ti–N–O); a small peak at 398.7 eV may belong to the chemisorbed nitrogen [37, 38]. For a fiber electrode to be used in SC, its electrochemical performance is important. A series of electrochemical tests were carried to evaluate the capacitive property of TiN NTs electrode. The tests were carried out in a three electrode system with a piece of Pt sheet as counter electrode and saturated calomel electrode (SCE) as the reference electrode. Capacitive properties of TiN@C NTs I and II electrodes were compared
through
cyclic
voltammetry
(CV)
curves
and
galvanostatic
charging/discharging (GCD) curves in Figure 4a and 4b. Calculated from CV curves, the length specific capacitance of the TiN@C NTs I is 9.22 mF cm-1 at a scan rate of 100 mV s-1, which is much higher than the TiN@C NTs II (2.56 mF cm-1). The result manifests the one-step method can bring a great enhancement in capacitive property. What’s more, compared with the TiN@C NTs II, the TiN@C NTs I shows a much 11
smaller IR drop, confirming its better charge transport ability (Figure 4b). The smaller IR drop of TiN@C NTs I can be explained by the electrochemical impedance spectroscopy (EIS) result shown in Figure S3. The Rs (equivalent internal resistance) of the TiN@C NTs I is about 3.1 Ω which is smaller than Rs of the TiN@C NTs II (3.5 Ω). All of these results can be explained by following two aspects: 1. TiN@C NTs I remain the nanotube structure and keep impacted and intact, but in contrast, TiN@C NTs II are loose and easily broken; The broken structure of TiN@C NTs II impair the charge transport and storage capability. 2. The N-doped carbon layer in TiN@C NTs I can contribute to a higher electrochemical performance than the pure carbon layer of the TiN@C NTs II [39-41]. Since the TiN@C NTs I demonstrates a better electrochemical performance than II, it was chosen as the electrodes materials in the following studies. The electrochemical performance of the TiN@C NTs I was studied using rate dependent CV curves and GCD curves at different current. As shown in Figure 4c, all of the CV curves show typical capacitive behaviors. Specific capacitance of TiN@C NTs I can reach a high value of 11.15 mF cm-2 at the scan rate of 10 mV s-1. And the GCD curves (Figure 4d) are nearly symmetric triangles with a minimum IR drop of 0.053 V at 0.5 mA. All these results suggest that the TiN@C NTs I electrode is a perfect candidate for SC material. 3.2 Electrochemical and mechanical performance of the TiN@C NTs FSC Two parallel TiN@C NTs I electrodes were fabricated into a solid-state FSC using KOH/PVA gel as electrolyte. CV curves at scan rates from 10-100 mV s-1 in Figure 5a display near rectangular shapes, manifesting the perfect capacitive performance of the 12
solid-state FSC. As shown in Table S1, the longitudinal specific capacitance of the FSC calculated from CV curves reaches 2.4 mF cm-1, which is much higher than previous reports such as MnO2/ZnO nanowires/polymer FSC (0.02 mF cm-1) [42], ZnO nanowire/graphene fiber (0.025 mF cm-1) [10], MnO2/CNT fiber (0.221 mF cm-1) [43], PEDOT/CNT fiber (0.47 mF cm-1) [8], and Pen ink/carbon metal fiber (1.008 mF cm-1) [19]. Its areal specific capacitance is as high as 19.4 mF cm-2, which is also superior than the all graphene core–sheath fiber (1.7 mF cm-2) [44], MnO2/ZnO nanowires/polymer fiber (2.4 mF cm-2) [42] and comparable to the MnO2/CNT fiber (23.74 mF cm-2) [43], buckled MnO2/CNT fiber (27.98 mF cm-2) [43]. Figure S4a presents the correlation between length specific capacitance and scan rate. It can be seen that less than 20 % decay in capacitance when the scan rate increases from 10 to 100 m∙V s-1, indicating its perfect rate capacitance performance. Pure TiO2 NTs based FSC with ultrahigh rate performance has been reported previously [45]. It can reach a high scan rate of 200 V s-1 with a perfect capacitive performance. Figure S4b shows CV curves of the TiN@C NTs I FSC at higher scan rates. Even at a high scan rate of 5 V∙s-1, the CV curve can still obtain a capacitance of 0.83 m∙Fcm-1. Furthermore, GCD measurements were performed to confirm its good capacitive properties. As shown in Figure 5b, all GCD curves are approximate mirror-image quasi triangular shapes and coulombic efficiencies are calculated to be higher than 90 % at all currents. Minimum IR drop reaches a low value of 0.0029 V at 0.1 mA, indicating the small charge and ion transfer resistance which is brought by the one-step method. The result was further confirmed by EIS result in Figure 5c. The Rs showed a rather small value of 13
3.4 Ω. These results may come from the following reasons: 1. the TiN material has extraordinary electronic conductivity; 2. the NT structure provides enough transferring channels for electron and ions. For comparison, two parallel TiN@C NTs II electrodes were also fabricated into a FSC. Electrochemical stabilities of TiN@C NTs I and II FSCs were tested via GCD cycle tests. As shown in Figure 5d, after experiencing 10,000 electrochemical cycles, TiN@C NTs II FSC retains 64 % capacitance while the TiN@C NTs I FSC keeps 78 % capacitance, demonstrating the better stability of TiN@C NTs I FSC. To confirm the stability result, GCD tests were also conducted on three extra TiN@C NTs I FSCs for 10,000 cycles, the results of which all show nearly 80 % capacitance retention (as shown in Figure S5). The integration of high mechanical flexibility and stability in FSCs has demonstrated promising application in energy storage garment electronics. Hence, we studied the mechanical performance of the fabricated TiN@C NTs FSCs. Firstly, bending test was performed on single TiN@C NTs I FSC (Figure 6a, from linear shape to a circle is one cycle, as shown in the inset pictures). After 2000 cycles, all of them maintain over 90 % of their original capacitance. For comparison, the same test was also conducted on four single TiN@C NTs II FSCs. They show only 75% retention of the capacitance, suggesting its inferior mechanical stability as compared with the TiN@C NTs I FSCs. This can be attributed to the protective carbon shell of the TiN@C NTs I FSC, which is confirmed by the above SEM pictures (Figure 1c and 1d). In Figure S6, CV curves of the TiN@C NTs I FSC with a bent angle of 0°, 180° and 360° are almost the same, further verifying its perfect flexibility. SCs that can be 14
tailored as desired at the device level are significant for wearable electronics. To test this ability, as shown in Figure 6b, a single TiN@C NTs I FSC was cut into halves. 49 % and 51 % capacitance retentions were calculated from the CV curves in Figure 6c, which were proportionate to the sizes of the two segments. In addition, the two separated FSCs were reconnected in series or parallel to explore their mechanical stability after being cut. Amazingly, the capacitance of the paralleled devices shows only 3.3 % decay compared with the original FSC, revealing that the FSC we prepared can be repaired via a simple paralleling connection if it experienced damage. Weaving these FSCs into textile was also demonstrated in Figure 6c, where the textile was woven with a plain weave. Four FSCs were inserted among the colorful yarns, as indicated by those white arrows. The magnified picture shows that our devices could be well weaved into the fabric, which demonstrates the possibility of their application in wearable electronics. With excellent flexibility, tailorability and stitchability, this TiN@C NTs FSC presents great application potential in energy storage clothes. Energy density and power density are two important parameters for practical application of energy storage devices. Herein, areal energy density and power density of our TiN@C NTs I FSC were calculated to compare with recent researches (since most studies using areal data), and the results are shown in Figure 7a [10, 43, 44, 46-49]. Notably, our FSC reaches high average energy densities of 2.25-2.69 μWh cm-2 with power densities of 97-809 μW cm-2 (scan rate ranging from 10-100 mV s-1), which is much superior than many previous reports such as ZnO nanowire-based fiber (0.027 μWh cm-2 and 14 μW cm-2) [10], GF@3D-G fiber (0.17 μWh cm-2 and 6 μW 15
cm-2) [44], PANI/CNT FSC by wet-spinning (≈0.56 μWh cm-2 and 30 μW cm-2) [49], PANI/stainlless steel FSC (0.95 μWh cm-2 and 100 μW cm-2) [46], MnO2 coated CNT FSC (1.14 μWh cm-2 and 210 μW cm-2) [43], Ti@MnO2 FSC (1.4 μWh cm-2 and 580 μW cm-2) [47] and CNT/ordered mesoporous carbon fiber (1.77 μWh cm-2 and 43 μW cm-2) [48]. Its energy density is even comparable to the wet-spinning rGO/CNT FSC (3.84 μWh cm-2) [50]. To further study its potential as a power source, three devices were connected in series to demonstrate the practical application of our FSC. CV curves and GCD curves of single FSC, two and three FSCs in series were shown in Figure 7b and Figure S7a, manifesting that voltage can be extended to a high value via simply connecting several FSCs in series. Next, three tandem FSCs can be used to power a green light-emitting diode (LED). The sketch-map was shown in Figure 7d. This result proves that the FSC we prepared can form a high voltage device by series connection. Besides, paralleled connection can be used to enhance the capacitance of the whole device group, as shown in Figure S7b and S7c. Incorporating both energy transport and storage features at the same time is an attractive but challenging attempt. Besides its bent stability, tailorablity and high electrochemical performance, the TiN@C NTs FSC can also be used as a conducting cable because of the Ti substrate of the electrode. To study this property, a longer FSC (over 10 cm as shown in Figure S8) was fabricated for revealing the bi-functionality. Firstly, three long FSCs in series were connected to the power source (a pair of dry batteries with a switch that can control on and off) and a red LED. When batteries were working (switch turned on), the red LED shone brilliantly, as presented in Figure 16
8a. Once the switch of batteries was turned off, the LED did not go out at once, but could stay on for several seconds (Figure 8b), which proves that the FSCs can store energy while the Ti substrate serves as electric cable simultaneously. The mechanism of the bi-functional FSC in working state can be illustrated in Figure 8c. The blue Ti wire in the center of the TiN@C NTs FSC is in charge of transferring electrons in the outer circle while the K+ and OH- in the gel electrolyte were stored in negative and positive TiN@C NTs electrodes, respectively.
4. Conclusion
In summary, we developed a novel one-step method for obtaining high performance TiN@C NTs on Ti wire. It simplifies the carbon coating process via a one-step nitridation and complete carbonization procedure, resulting in higher electrochemical performance than the samples prepared by traditional two-step method or other materials in previous studies. The TiN@C NTs I electrode obtained from the new one-step method can achieve a high length specific capacitance of 9.22 mF cm-1, which is triples higher than the TiN@C NTs II electrode (2.56 mF cm-1). FSCs using two paralleled TiN@C NTs I electrodes presented rather good electrochemical and mechanical performance. The capacitance of the TiN@C NTs I FSCs reached 2.4 mF cm-1 and 19.4 mF cm-2. Moreover, they can withstand 10,000 cycles GCD test with nearly 80 % capacitance retention. Significantly, these FSCs showed excellent mechanical flexibility and stability, all of which retains over 90 % capacitance after bending test of 2,000 cycles was performed. Tailorability is another special feature of the device we assembled. It can be cut into two parts with almost no performance decay and easily repaired via a paralleled connection. The energy density and power 17
density of single FSC could be as high as 2.69 μWh cm-2 and 809 μW cm-2, respectively. By integrating three devices into one group, a green LED can be successfully lightened. Interestingly, these FSCs can also be weaved into a piece of cloth, which firmly proved the possible application of our TiN@C NTs FSC in wearable energy storage device. Finally, this FSC with Ti substrate can be also used as electric cable while storing charges in the TiN@C NTs array. All in all, we have developed a novel method to prepare the TiN@C NTs based FSC which has showed excellent electrochemistry performance and can be a promising candidate for wearable energy supplying devices of future electronics.
Acknowledgement
We thank Kun Wang from Sun Yat-sen University for assistance with the XPS test. We are grateful for the financial supports from the National Natural Science Foundation of China (Grants 21376104 and 61604061) and the Natural Science Foundation of Guangdong Province, China (Grants 2014A030306010 and 2014A030310302).
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20
Fig 1 Schematic of the one-step (new method) and two-step (traditional method) synthesis process of two kinds of TiN@C NTs fiber electrode.
Fig 2 SEM images of TiO2 NTs and TiN@C NTs (I and II). (a) Cross-sectional image of the TiO2 NTs. (b) Top view image of the NTs array. (c) Cross-sectional image of the TiN@C NTs II array, whose tube structures are broken. Inset shows the corresponding magnified image. (d) Cross-sectional image of the TiN@C NTs I array, whose tube structures are intact. Inset shows the corresponding magnified image. 21
Fig 3 (a) XRD spectra of the TiO2@C NTs and TiN@C NTs I. (b) TEM image of the single TiN@C NT I. Inset shows the corresponding HRTEM image. (c) STEM image of a single TiN@C NT I and (d-f) the corresponding EDS element mapping images.
22
Fig 4 CV curves and GCD curves of two kinds of TiN@C NTs electrodes. (a) CV curves of TiN@C NTs I and II. (b) GCD curves of TiN@C NTs I and II. (c) CV curves of the TiN@C NTs I at different scan rates. (d) GCD curves of the TiN@C NTs I at different currents.
23
Fig 5 Electrochemical performance of the TiN@C NTs FSCs. (a) and (b) CV curves of the TiN@C NTs I FSC at different scan rate. (c) EIS test of the TiN@C NTs I FSC. (d) GCD Cycle time comparasion of the TiN@C NTs I and II FSC.
Fig 6 Bending resistance and tailorability of the TiN@C NTs FSC. (a) Bent cycle stability of four different TiN@C NTs I and II FSCs, which demonstrates that the TiN@C NTs I FSCs are highly bending resistant due to significantly improved mechanical stability. Inset photo shows the bending cycle operation from 0° to 360°. 24
(b) Photos showing one single FSC is cut into two parts, which are still functional. The white scale bar is 1 cm. (c) CV curves of the original FSC, two parts and their series and parallel connection. (d) A practical fabric weaved by wool wires and the FSCs.
Fig 7 Devices’ performance and application demonstration. (a) Ragone plots of the TiN@C NTs FSC and other previous works. (b) CV curves of several FSCs in series. (c) Three tandem FSCs power a green LED. (d) Sketch map of the structure in (c).
25
Fig 8 TiN@C NTs FSC can be used both as cable and energy storage device. (a) Tandem cable FSCs were used as connection for powering a red LED. (b) The cable FSCs were employed to be a power source to light the LED. (c) Schematic demonstrating the multi-function of transferring and storing charge at the same time. Highlights A facile one-step nitridation and carbon coating process is introduced for TiN@C nanotube.
The TiN@C based SC shows remarkably enhanced electrochemical and mechanical performance.
The TiN@C based SC can serve as electric cables and store energy devices at the same time.
26
PII: DOI: Reference:
www.elsevier.com/locate/nanoenergy
S2211-2855(16)30550-X http://dx.doi.org/10.1016/j.nanoen.2016.11.052 NANOEN1644
To appear in: Nano Energy Received date: 31 October 2016 Revised date: 24 November 2016 Accepted date: 25 November 2016 Cite this article as: Peng Sun, Rui Lin, Zilong Wang, Meijia Qiu, Zhisheng Chai, Bodong Zhang, Hui Meng, Shaozao Tan, Chuanxi Zhao and Wenjie Mai, Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.11.052 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 galley proof before it is published in its final citable 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.
Rational design of carbon shell endows TiN@C nanotube based fiber supercapacitors with significantly enhanced mechanical stability and electrochemical performance
Peng Suna, Rui Linb, Zilong Wanga, Meijia Qiua, Zhisheng Chaia, Bodong Zhanga, Hui Menga, Shaozao Tanb, Chuanxi Zhaoa*, Wenjie Maia,c*
a
Siyuan laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies
and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong 510632, China, b
Department of Chemistry, Jinan University, Guangzhou, Guangdong 510632, P. R.
China. c
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and
Communications, Jinan University, Guangzhou, Guangdong 510632, China.
[email protected] [email protected]
*
Corresponding authors.
Abstract As an emerging energy storage device for wearable electronics, fiber supercapacitors 1
(FSCs) have distinguished themselves from two dimensional SCs. Herein, TiN@C nanotube-based fiber electrodes are designed through a one-step nitridation and complete carbon coating process. They exhibit 260 % higher capacitance than the samples obtained by traditional two-step nitridation and carbonization method. The carbon shell and the fabrication method are critical for the performance of the electrodes. The carbon shell remarkably protects the polycrystalline TiN core from mechanical cracking and chemical oxidation. FSCs based on these fiber electrodes demonstrate excellent electrochemical performance and mechanical stability. High capacitance of 2.4 mF∙cm-1 (19.4∙mF∙cm-2 and 1.9 F∙cm-3) at a scan rate of 10 mV s-1 with an energy density of 2.69 μWh∙cm-2 and a power density of 809 μW∙cm-2 can be achieved. Moreover, nearly 80% capacitance retention after 10,000 cycle tests can be realized and the novel FSC can be bent for 2,000 times with capacitance decay of 7.2 %. Interestingly, our FSCs show almost no decay when cut into two segments due to the adoption of solid state electrolyte, demonstrating excellent tailorability. Finally, they can be also used as electric cables while store energy, demonstrating their bi-functionality.
Graphical abstract
Fiber supercapacitors based on TiN@C nanotubes have been firstly fabricated by a new one-step nitridation and complete carbon coating method. It demonstrates a mechanical tailorability, excellent bending stability and convenience to be woven into textile, which promotes the exploration for proper energy storage garments. 2
Keywords: TiN@C nanotubes, carbon shell, fiber supercapacitor, mechanical stability
1. Introduction With the demand for flexible and portable electronic products continues to rise, wearable electronics gains increasing popularity among customers [1]. Hence, how to develop novel energy storage devices for powering wearable electronic systems becomes a key issue. Supercapacitors (SCs) with flexible substrates are potential candidates that can be employed in energy storage textile [2-4]. However, clothes fabricated from conventional 2 dimensional (2D) planar devices are usually not breathable, which means they are not convenient for the volatilization of sweat or air from human body and will result in discomfort. Flexible SCs with 1 dimensional fiber shape are lightweight, reconfigurable and can be easily woven into different shapes [5, 6]. Because of the weaving property, they are more breathable than the conventional 2D planar devices, thus more suitable for being employed in wearable SCs. Fiber shape supercapacitor (FSC) has become fascinating since Wang’s group first put forward the model of FSCs [7-10]. And there have been a number of publications about their application in wearable devices. For example, Qu and his co-workers designed series of graphene FSC with good bending cycle of 1000 times (nearly 90 % capacitance retention) [11]. Yu et al. fabricated coil-type FSC that can preserve 93 % initial capacitance after experiencing 1000 electrochemical cycle in 3
different bending state [12]. However, better anti-bending ability is needed when these FSCs are applied in clothes. In addition, it’s important to retain mechanical and electrochemical performance when FSC suffers some damage such as snap or being cut off, which is seldom studied [13, 14]. According to the classification of substrate, there are two kinds of FSC, carbon based FSC and metal based FSC [15-19]. Metal based FSC demonstrates better mechanical strength and electric conductivity, confirming the possibility for both transmitting and storing charge concurrently [12, 20]. Thomas’s group constructed a coil type FSC by employing its inner substrate Cu as conductive component while outer asymmetric SC can work steady [20]. Wang et al. assembled flexible FSC into commercial electric cables to achieve the two fuctions of energy storage and transmission [21]. To combine fashionable designs with advanced technology in smart garments, it’s necessary to integrate these two functions into single cable electric FSC. Many researches have focused on pseudocapacitors because of their higher energy density and capacitance [22-28]. However, the low conductivity of traditional pseudocapacitive materials like metal oxide or hydroxide limits their performance. Due to the excellent electric conductivity (4000-55500 S∙cm-1), stable chemical properties and rather high capacitance, metal nitrides, especially TiN, have gradually become a hot research topic in SCs [29-32]. But its poor electrochemical stability hinders its further development in practical applications. Lei’s group synthesized carbon coated TiN nanotube arrays through two step atomic layer deposition process and achieved much better electrochemical performance and stability than bare TiN 4
NTs [33]. Lu et al. indicated that the electrochemical oxide reaction leads to the instability and pointed that the facile method, which coats a thin layer of carbon on TiN, can effectively prevent its electrochemical oxidation [34, 35]. Nevertheless, the two-step method, which first nitrogenizes the TiO2 into TiN and then coats carbon shell, includes some redundant procedure. It’s necessary to simplify the synthesizing process via other cost-effective methods. Herein, a TiN@C nanotubes (NTs)-based FSC with both energy transmission and storing functions was designed. TiO2 NTs were first synthesized on Ti wire via an anode oxidation method. After that, a carbonaceous shell was prepared on the TiO2 NTs by hydrothermal method. Finally, the TiO2@C NTs experienced further nitridation and complete carbonization concurrently, transforming into TiN@C NTs. Two parallel electrodes can be made into a solid-state FSC with good electrochemical performance. The device demonstrates a capacitance of 2.4 mF∙cm-1 (19.4 mF∙cm-2 and 1.9 F∙cm-3) at a scan rate of 10 mV s-1 and an energy density of 2.69 μWh∙cm-2 with a power density of 809 μW cm-2. After 10,000 cycles, the capacitance can still retain 78 %. Excitingly, it can bear a rather long bending cycle of 2000 times with 92.8 % capacitance preservation. The most special feature is its tailorability. There is almost no performance decay when it was cut into halves. Its excellent bending resistance and tailorability prove the possibility of its application in wearable electronics. Because of the outstanding conductive contact between active materials and the substrate, it can also be used as a cable for current transfer while storing charge at the same time, which broadens the applications of this novel FSC. 5
2. Experimental 2.1 Chemicals and materials Ammonium fluoride (AR, Chemical Guangzhou China), ethylene glycol (AR, YongHua Jiangsu China), glucose (AR, BoAo Shanghai China), potassium hydroxide (AR, Chemical Guangzhou China), polyvinyl alcohol (AR, 1799, Aladdin), ethanol (AR, Chemical Guangzhou China), acetone (AR, HongChuan Dongguan China) and Ti wire (diameter of 200 μm) were purchased from vendors. 2.2 Synthesis of TiN@C NTs electrode Firstly, Ti wires (average diameter of 200 μm) were washed by acetone, deionized (DI) water and ethanol one after another each for 10 min. Then, the clean substrates were soaked into an ethylene glycol solution with 0.3 wt % NH4F and 2 vol% deionized water under a constant voltage of 60 V for 2 h, as reported previously [36]. After that, two different operations were conducted for gathering two types of TiN@C NTs samples. (1). The as prepared TiO2 NTs were transferred into a tube furnace in NH3 atmosphere at 800 ℃ for 1 h, converting into TiN NTs. Then, a hydrothermal carbon coating and further anneal were proceeded for covering a thin layer of carbon out of the NTs. We name this kind of samples from traditional two-step process as TiN@C NTs II. (2). The as prepared TiO2 NTs were firstly coated a thin layer of carbonaceous shell through a hydrothermal reaction, and then went through a further nitridation process to get this TiN@C NTs I directly. 2.3 Synthesis of two kinds of carbon shell material 30 mL 0.05 M glucose solution was first poured into a 50 mL Teflon-lined stainless 6
autoclave. Then the sealed autoclave was kept in an electric oven at 180 ℃ for 3 h (kept the identical condition during the hydrothermal carbon coating process). After that, the residual solution was dried in hot air to get the deposition powder. Next, the carbonaceous powder was divided into two parts, which were transferred to the tube furnace. For better comparing the converting process of two kinds of carbon shell, we kept the annealing temperature (800 ℃) and duration time (1 h) the same, only changed the annealing atmosphere of N2 and NH3. After the annealing process, two kinds of carbon shell material can be obtained. 2.4 Fabrication of solid-state FSC The KOH/Polyvinyl Alcohol (PVA) sol gel electrolyte was firstly prepared by adding 6 g PVA powder into 60 mL 1 M KOH solution and stirred under 85 ℃ for 2 h. Two TiN@C NTs electrodes were soaked into KOH/PVA sol gel electrolyte for 1 min, and then were put into a silica gel tube filled with KOH/PVA electrolyte for 1 day to extract redundant water. 2.5 Characterization The morphology and nanostructure of the samples were characterized by field emission scanning electron microscope (FE-SEM, ZEISS ULTRA 55), transmission electron microscope (TEM, JEOL 2100F, 200 kV) equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD, Rigaku, MiniFlex600, Cu Kα) analyzer and X-ray Photoelectron Spectroscope (XPS, Thermo K-alpha). The electrochemical properties of the FSCs were on the CHI 660E (ChenHua Shanghai) electrochemical workstation, and the electrochemical impedance spectra (EIS) were 7
measured using VerasSTAT 3-400 (Princeton Applied Research) at a frequency ranging from 100 mHz to 10 kHz with a potential amplitude of 10 mV. The single electrode test was performed in 1 M KOH solution. Saturated calomel electrode (SCE) reference electrode and Pt counter electrode were used in the measurement.
3. Results and Discussion Two kinds of TiN@C NTs electrode materials were synthesized via two different methods, as shown in scheme of Figure 1. Firstly, TiO2 NTs were grown on Ti wire through anodic oxidation process. Different procedures were employed in the two methods. In two-step method (lower flow diagram), the first step was conducted to obtain TiN NTs. Then, a carbon shell was attached to the surface of the TiN NTs through a hydrothermal glucose-assisted method and further annealing process, resulting in TiN@C NTs II. For this method, the nitridation and carbonization processes were two separate steps, requiring two different annealing processes, which is uneconomical and inefficient. Moreover, the carbon shell does not adhere well to the surface of TiN, which reduces the surface area of the electrode. While in the one-step method (upper flow diagram), TiO2 NTs were first coated a layer of carbonaceous shell (hydrothermal procedure) and then annealed in NH3 atmosphere. The annealing process in NH3 can achieve the nitridation and complete carbonization at the same time and produce TiN@C NTs I. The details of synthesis are shown in experiment part. By achieving nitridation and complete carbonization in one step, this new method can greatly shorten the synthetic time and consume less energy. 8
3.1 Characterization and electrochemical performance of the TiN@C NTs electrode Scanning electron microscope (SEM) images of TiO2 NTs, TiO2@C NTs, TiN NTs and TiN@C NTs (I and II) were presented in Figure 2 and S1. From the side and top views of TiO2 NTs on the Ti wire in Figures 2a, 2b and S1a, we can find after anodic oxidation Ti wire is uniformly covered by a layer of TiO2 NTs whose average length is about 10 μm (cross-sectional image in Figure 2a). Top view image of the TiO2 layer in Figure 2b demonstrates a perfect NTs array with average outer diameter of 100 nm. After coating a carbonaceous shell, there are numbers of nanoparticles covering the TiO2 NTs as shown in Figure S1b. If TiO2 NTs are transformed into TiN directly, the produced TiN can keep the morphology of nanotube (Figure S1c). SEM image of TiN@C NTs II synthesized by two-step method is shown in Figure 2c. It can be found that the two-step method brings aggregation of carbon particles and breaks the tube structures. This is probably because the bare TiN NTs are rather fragile, which are easily broken under high temperature (800 oC) during the carbonization process. While for TiN@C NTs I synthesized by one-step method shown in Figure 2d, uniform and compact carbon nanoparticle layer formed on their surface. This result reveals that in the one-step method the carbonaceous shell covering TiO2 NTs can protect the nanotube structure at high temperature (~ 800 oC). Top view images of TiN@C NTs I and II are demonstrated in Figure S1d and S1e, which clearly manifesting their tube structures. Furthermore, the element composition of two kinds of electrodes was investigated. Different kinds of carbon can be obtained from the carbonaceous 9
solution in the previous hydrothermal process. The dried carbonaceous powder from the solution experienced high-temperature reaction in different atmosphere (N2 and NH3) and then was converted to two different kinds of carbon, as detailed in Experimental Section. Next,
energy dispersive X-ray spectrometer (EDS)
measurements were conducted to characterize these two kinds of carbon shell. As shown in Figure S1f, the carbon shell synthesized from two-step method contains only C and O elements, where O comes from the organics absorbed in air environment. But for carbon shell obtained from one-step method it contains not only C and O elements but also a small quantity of N atoms (Figure S1g). This result proves the N atoms have been successfully doped into the carbon shell. X-ray diffraction (XRD) analysis is used to determine the composition and crystal structure of the samples. Figure 3a shows the XRD pattern of TiO2@C NTs and TiN@C NTs I. Peaks of TiO2@C NTs in the bottom diffraction curve can be assigned to the anatase TiO2 phase (JCPDS# 02-0387). It is notable that the peaks of top diffraction curve correspond to the (111), (200), (220), (311) and (222) faces of cubic TiN (JCPDS# 65-5774). Transmission electron microscope (TEM) images in Figure 3b-f presented a clearer verification of the detailed morphology and substance components. It is obvious that the synthesized TiN is of NT structure, as shown in Figure 3b. The inset demonstrates the corresponding high resolution TEM image. The interplanar distances of 0.236 nm and 0.209 nm match well with the (111) and (200) facets, respectively. Finally, scanning transmission electron microscope (STEM) image of the single TiN@C NT I and the corresponding EDS element mapping 10
images are shown in Figure 3c-f. It is apparent that the TiN NT is covered by a uniform carbon shell. To further ascertain the element composition and chemical bonding state of TiN@C NT I, X-ray photoelectron spectroscopy (XPS) data were collected. High resolution Ti-2p and N-1s XPS spectra are shown in Figure S2a and b, respectively. The core level Ti-2p spectrum (Figure S2a) can be divided into 6 peaks (three pairs of spin-orbit split doublets) after deconvolution, which are Ti−N (2p3/2 = 455.7 eV and 2p1/2 = 461.8 eV), Ti−N−O (2p3/2 = 456.9 eV and 2p1/2 = 463.2 eV), and Ti−O (2p3/2 = 458.5 eV and 2p1/2 = 464.4 eV). The N-1s detailed spectrum is shown in Figure S2b, presenting a broad peak that can be split into two peaks at 395.8 eV (Ti–N) and 396.7 eV (Ti–N–O); a small peak at 398.7 eV may belong to the chemisorbed nitrogen [37, 38]. For a fiber electrode to be used in SC, its electrochemical performance is important. A series of electrochemical tests were carried to evaluate the capacitive property of TiN NTs electrode. The tests were carried out in a three electrode system with a piece of Pt sheet as counter electrode and saturated calomel electrode (SCE) as the reference electrode. Capacitive properties of TiN@C NTs I and II electrodes were compared
through
cyclic
voltammetry
(CV)
curves
and
galvanostatic
charging/discharging (GCD) curves in Figure 4a and 4b. Calculated from CV curves, the length specific capacitance of the TiN@C NTs I is 9.22 mF cm-1 at a scan rate of 100 mV s-1, which is much higher than the TiN@C NTs II (2.56 mF cm-1). The result manifests the one-step method can bring a great enhancement in capacitive property. What’s more, compared with the TiN@C NTs II, the TiN@C NTs I shows a much 11
smaller IR drop, confirming its better charge transport ability (Figure 4b). The smaller IR drop of TiN@C NTs I can be explained by the electrochemical impedance spectroscopy (EIS) result shown in Figure S3. The Rs (equivalent internal resistance) of the TiN@C NTs I is about 3.1 Ω which is smaller than Rs of the TiN@C NTs II (3.5 Ω). All of these results can be explained by following two aspects: 1. TiN@C NTs I remain the nanotube structure and keep impacted and intact, but in contrast, TiN@C NTs II are loose and easily broken; The broken structure of TiN@C NTs II impair the charge transport and storage capability. 2. The N-doped carbon layer in TiN@C NTs I can contribute to a higher electrochemical performance than the pure carbon layer of the TiN@C NTs II [39-41]. Since the TiN@C NTs I demonstrates a better electrochemical performance than II, it was chosen as the electrodes materials in the following studies. The electrochemical performance of the TiN@C NTs I was studied using rate dependent CV curves and GCD curves at different current. As shown in Figure 4c, all of the CV curves show typical capacitive behaviors. Specific capacitance of TiN@C NTs I can reach a high value of 11.15 mF cm-2 at the scan rate of 10 mV s-1. And the GCD curves (Figure 4d) are nearly symmetric triangles with a minimum IR drop of 0.053 V at 0.5 mA. All these results suggest that the TiN@C NTs I electrode is a perfect candidate for SC material. 3.2 Electrochemical and mechanical performance of the TiN@C NTs FSC Two parallel TiN@C NTs I electrodes were fabricated into a solid-state FSC using KOH/PVA gel as electrolyte. CV curves at scan rates from 10-100 mV s-1 in Figure 5a display near rectangular shapes, manifesting the perfect capacitive performance of the 12
solid-state FSC. As shown in Table S1, the longitudinal specific capacitance of the FSC calculated from CV curves reaches 2.4 mF cm-1, which is much higher than previous reports such as MnO2/ZnO nanowires/polymer FSC (0.02 mF cm-1) [42], ZnO nanowire/graphene fiber (0.025 mF cm-1) [10], MnO2/CNT fiber (0.221 mF cm-1) [43], PEDOT/CNT fiber (0.47 mF cm-1) [8], and Pen ink/carbon metal fiber (1.008 mF cm-1) [19]. Its areal specific capacitance is as high as 19.4 mF cm-2, which is also superior than the all graphene core–sheath fiber (1.7 mF cm-2) [44], MnO2/ZnO nanowires/polymer fiber (2.4 mF cm-2) [42] and comparable to the MnO2/CNT fiber (23.74 mF cm-2) [43], buckled MnO2/CNT fiber (27.98 mF cm-2) [43]. Figure S4a presents the correlation between length specific capacitance and scan rate. It can be seen that less than 20 % decay in capacitance when the scan rate increases from 10 to 100 m∙V s-1, indicating its perfect rate capacitance performance. Pure TiO2 NTs based FSC with ultrahigh rate performance has been reported previously [45]. It can reach a high scan rate of 200 V s-1 with a perfect capacitive performance. Figure S4b shows CV curves of the TiN@C NTs I FSC at higher scan rates. Even at a high scan rate of 5 V∙s-1, the CV curve can still obtain a capacitance of 0.83 m∙Fcm-1. Furthermore, GCD measurements were performed to confirm its good capacitive properties. As shown in Figure 5b, all GCD curves are approximate mirror-image quasi triangular shapes and coulombic efficiencies are calculated to be higher than 90 % at all currents. Minimum IR drop reaches a low value of 0.0029 V at 0.1 mA, indicating the small charge and ion transfer resistance which is brought by the one-step method. The result was further confirmed by EIS result in Figure 5c. The Rs showed a rather small value of 13
3.4 Ω. These results may come from the following reasons: 1. the TiN material has extraordinary electronic conductivity; 2. the NT structure provides enough transferring channels for electron and ions. For comparison, two parallel TiN@C NTs II electrodes were also fabricated into a FSC. Electrochemical stabilities of TiN@C NTs I and II FSCs were tested via GCD cycle tests. As shown in Figure 5d, after experiencing 10,000 electrochemical cycles, TiN@C NTs II FSC retains 64 % capacitance while the TiN@C NTs I FSC keeps 78 % capacitance, demonstrating the better stability of TiN@C NTs I FSC. To confirm the stability result, GCD tests were also conducted on three extra TiN@C NTs I FSCs for 10,000 cycles, the results of which all show nearly 80 % capacitance retention (as shown in Figure S5). The integration of high mechanical flexibility and stability in FSCs has demonstrated promising application in energy storage garment electronics. Hence, we studied the mechanical performance of the fabricated TiN@C NTs FSCs. Firstly, bending test was performed on single TiN@C NTs I FSC (Figure 6a, from linear shape to a circle is one cycle, as shown in the inset pictures). After 2000 cycles, all of them maintain over 90 % of their original capacitance. For comparison, the same test was also conducted on four single TiN@C NTs II FSCs. They show only 75% retention of the capacitance, suggesting its inferior mechanical stability as compared with the TiN@C NTs I FSCs. This can be attributed to the protective carbon shell of the TiN@C NTs I FSC, which is confirmed by the above SEM pictures (Figure 1c and 1d). In Figure S6, CV curves of the TiN@C NTs I FSC with a bent angle of 0°, 180° and 360° are almost the same, further verifying its perfect flexibility. SCs that can be 14
tailored as desired at the device level are significant for wearable electronics. To test this ability, as shown in Figure 6b, a single TiN@C NTs I FSC was cut into halves. 49 % and 51 % capacitance retentions were calculated from the CV curves in Figure 6c, which were proportionate to the sizes of the two segments. In addition, the two separated FSCs were reconnected in series or parallel to explore their mechanical stability after being cut. Amazingly, the capacitance of the paralleled devices shows only 3.3 % decay compared with the original FSC, revealing that the FSC we prepared can be repaired via a simple paralleling connection if it experienced damage. Weaving these FSCs into textile was also demonstrated in Figure 6c, where the textile was woven with a plain weave. Four FSCs were inserted among the colorful yarns, as indicated by those white arrows. The magnified picture shows that our devices could be well weaved into the fabric, which demonstrates the possibility of their application in wearable electronics. With excellent flexibility, tailorability and stitchability, this TiN@C NTs FSC presents great application potential in energy storage clothes. Energy density and power density are two important parameters for practical application of energy storage devices. Herein, areal energy density and power density of our TiN@C NTs I FSC were calculated to compare with recent researches (since most studies using areal data), and the results are shown in Figure 7a [10, 43, 44, 46-49]. Notably, our FSC reaches high average energy densities of 2.25-2.69 μWh cm-2 with power densities of 97-809 μW cm-2 (scan rate ranging from 10-100 mV s-1), which is much superior than many previous reports such as ZnO nanowire-based fiber (0.027 μWh cm-2 and 14 μW cm-2) [10], GF@3D-G fiber (0.17 μWh cm-2 and 6 μW 15
cm-2) [44], PANI/CNT FSC by wet-spinning (≈0.56 μWh cm-2 and 30 μW cm-2) [49], PANI/stainlless steel FSC (0.95 μWh cm-2 and 100 μW cm-2) [46], MnO2 coated CNT FSC (1.14 μWh cm-2 and 210 μW cm-2) [43], Ti@MnO2 FSC (1.4 μWh cm-2 and 580 μW cm-2) [47] and CNT/ordered mesoporous carbon fiber (1.77 μWh cm-2 and 43 μW cm-2) [48]. Its energy density is even comparable to the wet-spinning rGO/CNT FSC (3.84 μWh cm-2) [50]. To further study its potential as a power source, three devices were connected in series to demonstrate the practical application of our FSC. CV curves and GCD curves of single FSC, two and three FSCs in series were shown in Figure 7b and Figure S7a, manifesting that voltage can be extended to a high value via simply connecting several FSCs in series. Next, three tandem FSCs can be used to power a green light-emitting diode (LED). The sketch-map was shown in Figure 7d. This result proves that the FSC we prepared can form a high voltage device by series connection. Besides, paralleled connection can be used to enhance the capacitance of the whole device group, as shown in Figure S7b and S7c. Incorporating both energy transport and storage features at the same time is an attractive but challenging attempt. Besides its bent stability, tailorablity and high electrochemical performance, the TiN@C NTs FSC can also be used as a conducting cable because of the Ti substrate of the electrode. To study this property, a longer FSC (over 10 cm as shown in Figure S8) was fabricated for revealing the bi-functionality. Firstly, three long FSCs in series were connected to the power source (a pair of dry batteries with a switch that can control on and off) and a red LED. When batteries were working (switch turned on), the red LED shone brilliantly, as presented in Figure 16
8a. Once the switch of batteries was turned off, the LED did not go out at once, but could stay on for several seconds (Figure 8b), which proves that the FSCs can store energy while the Ti substrate serves as electric cable simultaneously. The mechanism of the bi-functional FSC in working state can be illustrated in Figure 8c. The blue Ti wire in the center of the TiN@C NTs FSC is in charge of transferring electrons in the outer circle while the K+ and OH- in the gel electrolyte were stored in negative and positive TiN@C NTs electrodes, respectively.
4. Conclusion
In summary, we developed a novel one-step method for obtaining high performance TiN@C NTs on Ti wire. It simplifies the carbon coating process via a one-step nitridation and complete carbonization procedure, resulting in higher electrochemical performance than the samples prepared by traditional two-step method or other materials in previous studies. The TiN@C NTs I electrode obtained from the new one-step method can achieve a high length specific capacitance of 9.22 mF cm-1, which is triples higher than the TiN@C NTs II electrode (2.56 mF cm-1). FSCs using two paralleled TiN@C NTs I electrodes presented rather good electrochemical and mechanical performance. The capacitance of the TiN@C NTs I FSCs reached 2.4 mF cm-1 and 19.4 mF cm-2. Moreover, they can withstand 10,000 cycles GCD test with nearly 80 % capacitance retention. Significantly, these FSCs showed excellent mechanical flexibility and stability, all of which retains over 90 % capacitance after bending test of 2,000 cycles was performed. Tailorability is another special feature of the device we assembled. It can be cut into two parts with almost no performance decay and easily repaired via a paralleled connection. The energy density and power 17
density of single FSC could be as high as 2.69 μWh cm-2 and 809 μW cm-2, respectively. By integrating three devices into one group, a green LED can be successfully lightened. Interestingly, these FSCs can also be weaved into a piece of cloth, which firmly proved the possible application of our TiN@C NTs FSC in wearable energy storage device. Finally, this FSC with Ti substrate can be also used as electric cable while storing charges in the TiN@C NTs array. All in all, we have developed a novel method to prepare the TiN@C NTs based FSC which has showed excellent electrochemistry performance and can be a promising candidate for wearable energy supplying devices of future electronics.
Acknowledgement
We thank Kun Wang from Sun Yat-sen University for assistance with the XPS test. We are grateful for the financial supports from the National Natural Science Foundation of China (Grants 21376104 and 61604061) and the Natural Science Foundation of Guangdong Province, China (Grants 2014A030306010 and 2014A030310302).
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Fig 1 Schematic of the one-step (new method) and two-step (traditional method) synthesis process of two kinds of TiN@C NTs fiber electrode.
Fig 2 SEM images of TiO2 NTs and TiN@C NTs (I and II). (a) Cross-sectional image of the TiO2 NTs. (b) Top view image of the NTs array. (c) Cross-sectional image of the TiN@C NTs II array, whose tube structures are broken. Inset shows the corresponding magnified image. (d) Cross-sectional image of the TiN@C NTs I array, whose tube structures are intact. Inset shows the corresponding magnified image. 21
Fig 3 (a) XRD spectra of the TiO2@C NTs and TiN@C NTs I. (b) TEM image of the single TiN@C NT I. Inset shows the corresponding HRTEM image. (c) STEM image of a single TiN@C NT I and (d-f) the corresponding EDS element mapping images.
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Fig 4 CV curves and GCD curves of two kinds of TiN@C NTs electrodes. (a) CV curves of TiN@C NTs I and II. (b) GCD curves of TiN@C NTs I and II. (c) CV curves of the TiN@C NTs I at different scan rates. (d) GCD curves of the TiN@C NTs I at different currents.
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Fig 5 Electrochemical performance of the TiN@C NTs FSCs. (a) and (b) CV curves of the TiN@C NTs I FSC at different scan rate. (c) EIS test of the TiN@C NTs I FSC. (d) GCD Cycle time comparasion of the TiN@C NTs I and II FSC.
Fig 6 Bending resistance and tailorability of the TiN@C NTs FSC. (a) Bent cycle stability of four different TiN@C NTs I and II FSCs, which demonstrates that the TiN@C NTs I FSCs are highly bending resistant due to significantly improved mechanical stability. Inset photo shows the bending cycle operation from 0° to 360°. 24
(b) Photos showing one single FSC is cut into two parts, which are still functional. The white scale bar is 1 cm. (c) CV curves of the original FSC, two parts and their series and parallel connection. (d) A practical fabric weaved by wool wires and the FSCs.
Fig 7 Devices’ performance and application demonstration. (a) Ragone plots of the TiN@C NTs FSC and other previous works. (b) CV curves of several FSCs in series. (c) Three tandem FSCs power a green LED. (d) Sketch map of the structure in (c).
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Fig 8 TiN@C NTs FSC can be used both as cable and energy storage device. (a) Tandem cable FSCs were used as connection for powering a red LED. (b) The cable FSCs were employed to be a power source to light the LED. (c) Schematic demonstrating the multi-function of transferring and storing charge at the same time. Highlights A facile one-step nitridation and carbon coating process is introduced for TiN@C nanotube.
The TiN@C based SC shows remarkably enhanced electrochemical and mechanical performance.
The TiN@C based SC can serve as electric cables and store energy devices at the same time.
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