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titanium nitride nanowire array for flexible supercapacitor
Journal of Power Sources 286 (2015) 561e570
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced electrochemical performance of polyaniline/carbon/ titanium nitride nanowire array for flexible supercapacitor Yibing Xie a, b, *, Chi Xia a, b, Hongxiu Du a, b, Wei Wang a, b a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Suzhou Research Institute of Southeast University, Suzhou 215123, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PANI/C/TiN NWA ternary composite with shell/shell/core configuration is fabricated. PANI/C/TiN NWA exhibits capacitance of 1093 F g1 and capacitance retention of 98%. PANI/C shell enables to promote capacitance and cycling stability of PANI/C/TiN NWA. PANI/C/TiN NWA exhibits higher capacitance than C/PANI/TiN and PANI/C/TiO2 NWA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 30 March 2015 Accepted 5 April 2015 Available online 6 April 2015
The ternary nanocomposite of polyaniline/carbon/titanium nitride (PANI/C/TiN) nanowire array (NWA) is fabricated as electroactive electrode material for flexible supercapacitor application. Firstly, TiN NWA is formed through ammonia nitridation treatment of TiO2 NWA, which is synthesized via seed-assisted hydrothermal reaction. PANI/C/TiN NWA is then formed through sequentially coating carbon and PANI on the surface of TiN NWA. PANI/C/TiN NWA has unique shell/shell/core architecture, including a core layer of TiN NWA with a diameter of 40e160 nm and a length of 1.5 mm, a middle shell layer of carbon with a thickness of about 6.0 nm and an external surface layer of PANI with a thickness of 20e50 nm. PANI/C/TiN NWA provides ion diffusion channel at interspaces between the neighboring nanowires and electron transfer route along independent nanowires. The carbon shell layer is able to protect TiN NWA from electrochemical corrosion during charge/discharge process. PANI/C/TiN NWA displays high specific capacitance of 1093 F g1 at 1.0 Ag1, and good cycling stability with a capacity retention of 98% after 2000 cycles, presenting better supercapacitive performance than other integrated nanocomposites of C/ PANI/TiN, PANI/TiN and PANI/C/TiO2 NWA. Such a ternary nanocomposite of PANI/C/TiN NWA can be used as an electrode material of flexible supercapacitors. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Titanium nitride Carbon Nanowire array Supercapacitor
1. Introduction
* Corresponding author. School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China. E-mail address: [email protected] (Y. Xie). http://dx.doi.org/10.1016/j.jpowsour.2015.04.025 0378-7753/© 2015 Elsevier B.V. All rights reserved.
There is currently a strong demand for the development of inexpensive, flexible, light-weight and environmentally friendly energy storage devices [1,2]. As an intermediate system between dielectric capacitors and batteries, supercapacitors are emerging as an efficient energy storage devices due to their fast charge/
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discharge capability, high power density and long-term stability [3,4]. Recently, much effort has been devoted to developing the nanostructured and good conductive electrode materials that has high electrochemical capacitance [5e9]. However, it is still a critical challenge to enhance the energy density and cycling stability while retain the high-power density of electrode materials for flexible supercapacitors [10]. Conducting polymers are commonly used as flexible pseudocapacitive materials which exhibit superior specific energy to various carbon materials used in electrical double-layer capacitors [11,12]. Moreover, conducting polymers generally show higher conductivity than the metal oxides and consequently have larger power capability [13,14]. Polyaniline (PANI) is considered as one of most promising and versatile conducting polymers, which is ascribed to its high theoretical capacitance, easy synthesis, low cost and environmental stability [15,16]. However, PANI also suffers from the volumetric swelling and shrinking during charge/ discharge process as a result of ion doping and dedoping [17,18]. This volumetric change destroys the backbone of PANI, which seriously weakens its cycling life. To overcome this shortcoming, various composites of PANI modified with carbon materials, metal oxides or others have been explored to enhance the mechanical stability of the PANI [19e21]. Recently, both titanium dioxide (TiO2) and titanium nitride (TiN) nanoarray have become new kinds of promising materials for electrochemical and photochemical applications because of its high surface area and good physicochemical stability [22e25]. Intensively, TiN with good electrical conductivity is widely used in different fields such as dye-sensitized solar cells, fuel cells and supercapacitors [26]. TiN compact film usually serves as a current collector of electrode materials. Alternatively, TiN nanoarray is able to act as good electrode materials of supercapacitors [27,28]. On the other hand, the previous studies proved that TiN was somewhat instability in aqueous electrolyte solution due to irreversible electrochemical oxidation reaction [29]. Carbon coating treatment is regarded as a simple and effective strategy to substantially enhance the cycling stability of electrode materials in energy storage devices [30]. Many carbon-modified electrode materials, such as LiFePO4/C, NiO/C and TiO2/C et al., have been developed for energy storage applications [31e34]. In this study, a novel composite material with a shell/shell/core nanostructure was designed as flexible supercapacitor electrode to overcome the limitations associated with individual components. The ternary nanocomposite PANI/C/TiN NWA NWA with a shell/ shell/core nanoarray structure was prepared through sequentially coating carbon and PANI on TiN NWA. It is expected that the PANI/ C/TiN NWA nanocomposite can provide the improved performance of electrochemical capacitance and cycling stability. 2. Experimental The PANI/C/TiN NWA with unique shell/shell/core structure was synthesized to support on CC substrate via a stepwise deposition and coating process. Fig. 1 shows the schematic illustration of the fabrication process for PANI/C/TiN NWA supported on CC substrate. Firstly, TiN NWA was grown on CC substrate by seed-assisted hydrothermal synthesis and ammonia nitridation process. Secondly, carbon layer was coated on TiN NWA via a glucose-assisted hydrothermal treatment, and subsequent heating treatment. Finally, a thin film of PANI layer was coated onto C/TiN NWA through an electropolymerization deposition process. 2.1. Preparation of PANI/C/TiN NWA TiN NWA grown directly on a carbon cloth using a previously reported method [10]. Briefly, the clean carbon cloth (CC,
6.0 cm 1.0 cm) was immersed into 0.2 M tetrabutyl titanate ethanol solution for 10 min and then further heated in air atmosphere at 350 C for 10 min TiO2 nanoparticles accordingly were coated on the surface of CC to form TiO2/CC. The reaction solution was prepared by mixing 90 ml 12 mol L1 hydrochloric acid solution with 90 ml deionized water and 2.7 ml tetrabutyl titanate. As-prepared mixture solution and above TiO2/CC were transferred into a Teflon-lined stainless steel autoclave. The sealed autoclave was heated at 180 C for 7 h, and then was cooled down slowly to room temperature. A white TiO2 NWA film was uniformly coated on the surface of CC. The TiO2 NWA conducted the heating treatment at 550 C for 1 h under nitrogen atmosphere. Finally, the TiO2 NWA was converted to TiN NWA by annealing treatment at 900 C for 1 h in ammonia atmosphere with a flow rate of 40 mL min1 in a tubular furnace. The C/TiN NWA was synthesized by a glucose-assisted hydrothermal method. The as-prepared TiN NWA was heated at 180 C for 3 h in a Teflon-lined stainless steel autoclave with 25 mL 0.1 M glucose solution. This sample was further annealed at 800 C for 1 h in N2 atmosphere, forming C/TiN NWA. PANI/C/TiN NWA was prepared by electrochemical deposition method in a standard three-electrode setup, including a saturated calomel electrode (SCE) as the reference electrode, Pt plate as the counter electrode and C/TiN NWA as the working electrode. PANI was coated on C/TiN NWA through cyclic voltammetry (CV) electrodeposition process at a sweep rate of 25 mV s1 and a potential range of 0.2e0.9 V in an electrolyte solution containing 0.1 M aniline monomer and 1.0 M sulfuric acid. The loading amount could be obtained by measuring the weight difference before and after the deposition of active materials utilizing an electronic balance with an accuracy of 0.1 mg. Herein, the loading amount of PANI was 2.5 mg. For a comparison, C/PANI/TiN NWA was also prepared by changing the coating sequence of PANI and carbon. PANI/TiN NWA was firstly prepared according to our previously reported method [15]. The carbon layer was then coated on PANI/TiN NWA to form C/ PANI/TiN NWA through a glucose-assisted hydrothermal synthesis method. This C/PANI/TiN NWA did not conduct a further annealing treatment process. 2.2. Structure characterization The surface morphology and microstructure of PANI/C/TiN NWA was investigated by means of scan electron microscopy (SEM, Zeiss Ultra Plus) and transmission electron microscopy (TEM, JEM-2100) and energy dispersive X-ray (EDX) spectroscopy. Raman spectroscopy was performed on a Raman spectrometer (Renishaw microRaman spectroscopy) using a HeeNe laser that emitted the sample at 785 nm excitation with wave between 0 and 2000 cm1. X-ray diffraction patterns were recorded with a Bruker-AXS Micro-diffractometer (XRD, D8 ADVANCE, Germany) with the use of Cu-Ka radiation source. 2.3. Electrochemical measurement The electrochemical performances of the electrode materials were measured in a three-electrode system by cyclic voltammetry and galvanostatic charge/discharge measurement, using CHI760C electrochemical workstation. Pt plate and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Electrochemical impedance spectra were recorded using an electrochemical workstation (EIS, ZAHNERim6ex, Germany) from 10 mHz to 100 kHz with an potential amplitude of 5 mV. EIS measurements were carried out at an open potential of 0.5 V (vs. SCE). All electrolyte used above was 1 M H2SO4 aqueous solution. The specific capacitance (C) is calculated using the following equations.
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Fig. 1. Schematic illustration of the fabrication process for PANI/C/TiN NWA supported on CC substrate.
Q It ¼ C¼ DV m DV m
(1)
where C is the specific capacitance, I is the charge/discharge current, t is the time of discharge, △V is the voltage difference between the upper and lower potential limits, and m is the mass of active materials. 3. Results and discussion 3.1. Morphological characterization Fig. 2 shows photographs of PANI/C/TiN NWA, C/TiN NWA, C/
Fig. 2. Photographs of the CC coated with active materials of TiO2 NWA, C/TiO2 NWA, C/TiN NWA and PANI/C/TiN NWA.
TiO2 NWA and TiO2 NWA supported on the CC. The layer-by-layer formation of different materials is associated with the changing color of nanowire film, which ranged from white TiO2 NWA to brown C/TiO2 NWA and black C/TiN NWA. The dark green film (in web version) indicated the formation of PANI/C/TiN NWA. Fig. 3 shows SEM images of TiO2 NWA, C/TiO2 NWA, TiN NWA, C/ TiN NWA, PANI/C/TiO2 NWA and PANI/C/TiN NWA. The entire CC fibers were uniformly covered by TiO2 NWA with a diameter of 40e160 nm and a length of 1.5 mm (Fig. 3A and B). The C/TiO2 NWA remained similar morphology to pure TiO2 NWA, maintaining a smooth surface (Fig. 3C and D). This suggested that the thickness of carbon shell was very thin. The TiN NWA had a relatively rougher surface, accordingly increasing the surface area of the materials. The C/TiN NWA exhibited a similar shape and size compared with TiN NWA (Fig. 3E and F). The SEM images of PANI/C/TiO2 NWA and PANI/C/TiN NWA were shown in Fig. 3G and H. The diameter of PANI/C/TiO2 NWA and PANI/C/TiN NWA were obvious larger than that of C/TiO2 NWA and C/TiN NWA. It indicated that the film of PANI layer was uniformly coated on the entire C/TiO2 or C/TiN NWA. Fig. 4 shows TEM images of C/TiN NWA and PANI/C/TiN NWA. The TEM image of C/TiN NWA revealed that the carbon shell had an average thickness of 6.0 nm (Fig. 4A). The carbon coating layer in C/ TiN NWA composite provided dual functionalities. Firstly, it promoted electron transport along independent nanowires. Secondly, it protected the TiN NWA from electrochemical corrosion and also facilitated the deposition of PANI layer. Previous research work had proven that the PANI was easier to be adsorbed and grafted on the surface of carbon materials [35]. The TEM image of PANI/C/TiN NWA revealed that the PANI effectively coated on the surface of C/ TiN NWA, forming a ternary nanocomposite with heterogeneous coaxial structure (Fig. 4B). The external surface layer shell of PANI had the thickness of 20e50 nm. This kind of shell/shell/core nanostructure possessed large electrochemical surface and an optimal ion diffusion path, which could act well as supercapacitor electrode material.
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Fig. 3. SEM images of (A, B) TiO2 NWA, (C, D) C/TiO2 NWA, (E) TiN NWA, (F) C/TiN NWA, (G) PANI/C/TiO2 NWA and (H) PANI/C/TiN NWA.
3.2. Structural analysis Fig. 5A shows the XRD patterns of TiO2 NWA, C/TiO2 NWA, TiN NW and C/TiN NWA. The XRD patterns of all samples revealed the weak characteristic diffraction peak at 25.5 , which was ascribed to the (002) crystal diffraction plane peak of the graphite carbon of the CC substrate. Curve a shows that the diffraction peaks at 27.5 , 36.2 , 41.5 and 54.5 could be indexed as the (110), (101), (111) and
(211) crystal planes of rutile TiO2, respectively [33]. Comparatively, curve b shows that the diffraction peaks of C/TiO2 NWA were similar to that of TiO2 NWA, confirming rutile phase of C/TiO2 NWA and amorphous phase of carbon shell. Curve c shows that the characteristic diffraction peaks at 37.0 , 43.1 and 62.5 could match well with the (111), (200) and (220) crystal plane of cubic TiN, respectively [26]. In addition, the absence of characteristic TiO2 peaks at 27.5 , 41.5 and 54.5 in TiN indicated the full conversion of
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Fig. 4. TEM images of (A) C/TiN NWA and (B) PANI/C/TiN NWA.
rutile TiO2 to cubic TiN through annealing treatment at 900 C in NH3 atmosphere. Comparatively, the diffraction peaks of C/TiN NWA in curve d were similar to that of TiN NWA in curve c, confirming that the cubic phase crystal structure of TiN NWA was preserved after coating carbon shell. Fig. 5B shows EDX spectrum of C/TiN NWA. The EDX analysis result revealed that the formation of TiN NWA proved the complete conversion of TiO2 NWA into TiN NWA, which was consistent with the XRD analysis result. Fig. 5C shows Raman spectra of the PANI/C/TiO2 NWA, PANI/C/ TiN NWA, C/TiO2 NWA, TiO2 NWA, C/TiN NWA and TiN NWA. The curve d shows that TiO2 NWA had the characteristic Raman peaks at 449 and 609 cm1 [29]. Comparatively, the curve c shows that C/ TiO2 NWA had additional two new peaks at 1317 and 1601 cm1, which were corresponding to the D and G bands of carbon [31]. The curve e shows that C/TiN NWA revealed the significantly enhanced intensity of the D and G bands when compared with TiN NWA, suggesting that the carbon layer was successfully covered on the surface of TiN NWA. The Raman spectra of TiO2 NWA and TiN NWA grown on CC substrate did not present the obvious characteristic Raman peaks of CC substrate. It was believed that the characteristic Raman peaks of the D and G bands of graphite carbon were so weak to be observed when CC substrate was completely covered by TiO2 or TiN NWA. The Raman spectra of PANI/C/TiO2 NWA, PANI/C/TiN NWA, C/TiO2 NWA, and C/TiN NWA grown on CC substrate revealed the characteristic Raman peaks of the D and G bands of carbon, which were mostly ascribed to the carbon coating layer on TiO2 NWA or TiN NWA rather than the graphite carbon of CC substrate. The curve f shows that TiN NWA had the characteristic peak at 576 cm1. In comparison to TiO2 NWA, TiN NWA did not show the characteristic peaks at 449 and 609 cm1 of TiO2 NWA, indicating that rutile TiO2 NWA could be completely converted to cubic TiN NWA through ammonia nitridation process. In view of curve a and b, the prominent peaks at 1163 cm1, 1338 cm1, 1480 cm1 and 1591 cm1 were corresponding to CeH bending of quinoid ring, CeNþ stretching, CeC stretching of the benzene ring and C]C stretching [36]. The peaks at 420 cm1 and 517 cm1 should be assigned to amine deformation and ring deformation [37]. The similar characteristic peaks of PANI were observed in curve a and b, indicating that PANI was successfully deposited on both C/TiO2 NWA and C/TiN NWA substrates. 3.3. Electrochemical properties The electrochemical performances of these shell/shell/core structured NWA electrode materials mostly depend on their porosity, conductivity and inherent electroactivity. The CV and galvanostatic charge/discharge experiments were carried out in 1.0 M H2SO4 aqueous electrolyte. Fig. 6A shows the CV curves of
TiO2 NWA, TiN NWA, C/TiO2 NWA and C/TiN NWA at a scan rate of 10 mV s1. Usually, the shape of CV curves of TiO2 and TiN was related to inherent electroactivity, reactive electrolyte solution, scan rate and potential range. In a strict sense, the CV curve of TiO2 NWA did not present the ideal rectangular shape due to its semiconductor property. Comparatively, the CV curve shape of TiN NWA was improved due to its good conductivity. Both TiO2 NWA and TiN NWA were identified as the nearly rectangle-like shape of CV curves, indicating electric double layer capacitive behavior. In comparison with TiO2 NWA, the C/TiO2 NWA exhibited a higher current response. Generally, as-prepared TiO2 NWA has a quite low capacitance due to its poor conductivity. The C/TiO2 NWA exhibited the increased capacitance because the carbon layer as an electroactive material could improve the electron conductivity of TiO2 NWA. In addition, the integral areas of TiN NWA and C/TiN NWA in CV curves were obviously larger than that of TiO2 NWA and C/TiO2 NWA, respectively. This result indicated that the nitridation process of TiO2 NWA could help to enhance its specific capacitance since TiN NWA possessed the better electrical conductivity than TiO2 NWA. Interestingly, the C/TiN NWA exhibited a smaller specific capacitance when compared with TiN NWA. The carbon shell layer in C/TiN NWA could protect the TiN core layer from the electrochemical corrosion reaction in H2SO4 electrolyte solution. On the other hand, this carbon shell layer also had an inhibiting effect on the diffusion of electrolyte ions into the core layer of TiN NWA. Eventually, the electroactive TiN NWA would serve only as conductive supporting role and unlikely form an electric double layer, resulting in lower capacitance of C/TiN NWA. Fig. 6B shows the galvanostatic charge/discharge curves of TiO2 NWA, TiN NWA, C/TiO2 NWA and C/TiN NWA at a current density of 1.0 mA cm2. The shapes of the charge curves were almost linear and somewhat symmetrically mirrored their discharge counterparts, indicating the good electrochemical capacitance performance of these electrode materials. TiN NWA and C/TiN NWA obviously exhibited their relatively higher capacitance than TiO2 NWA and C/TiO2 NWA. The charge storage capacity in galvanostatic charge/discharge tests was consistent with the result in those previous CV tests. Notably, The IR drop of C/TiN NWA was even smaller than that of TiN NWA and C/ TiO2 NWA, which could be ascribed to the synergistic effect of superior electrical conductivity of TiN NWA core and carbon shell. To determine the synergistic effect and microstructure influence of the PANI/C shell on the capacitance performance, different integrated electrode materials were prepared for a comparison. Fig. 6C shows the CV curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA. Apparently, the corresponding CV curves did not show ideal rectangular shape since all these electrodes involved electroactive PANI. In general, PANI could conduct reversible proton (Hþ) doping/dedoping reaction in H2SO4 electrolyte solution during the charge/discharge
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Fig. 5. (A) XRD patterns of TiO2 NWA, C/TiO2 NWA, TiN NW and C/TiN NWA; (B) EDX spectrum of C/TiN NWA; (C) Raman spectra of PANI/C/TiO2 NWA, PANI/C/TiN NWA, C/ TiO2 NWA, TiO2 NWA, C/TiN NWA and TiN NWA.
process, which contributed to the pseudo-capacitance performance. The curve a and b indicate that the anodic peaks occurring at 0.30 V for PANI/TiO2 NWA, PANI/C/TiO2 NWA could be attributed to the transition of PANI from the semiconducting state to the conducting state [38]. Comparatively, the curve c, d and e indicate that the corresponding anodic peaks at 0.3 V became very unobvious for PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA. Such a difference was ascribed to the much higher conductivity of TiN NWA in comparison to TiO2 NWA. This relatively high anodic current response caused by TiN substrate could surpass the current response caused by PANI in the CV curves of PANI/TiN NWA, PANI/ C/TiN NWA and C/PANI/TiN NWA. Compared with PANI/TiO2 NWA and PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/ PANI/TiN NWA showed the increased CV integral areas, implying their relatively higher specific capacitance. The enhanced capacitance performance was due to the improved conductivity of the nanowire framework changed from TiO2 to TiN. PANI/TiO2, PANI/C/ TiO2, PANI/TiN and PANI/C/TiN with an outer layer of PANI mostly involved the pseudo-capacitance of PANI. The C/PANI/TiN with outer layer of carbon involved the pseudo-capacitance of PANI and the electrical double layer capacitance of carbon. Conductive carbon shell and TiN NWA could decrease the internal resistance of the PANI and ensure the high utilization of electroactive PANI. The galvanostatic charge/discharge curves of PANI/TiO2 NWA, PANI/TiN NWA, PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a current density of 1.0 mA cm2 were shown in Fig. 6D. The almost linear and symmetric curves indicated good electrochemical capacitance performance of all these samples. Compared with PANI/C/TiO2 NWA, the longer discharge time of the PANI/C/TiN NWA indicated its relatively higher capacitance. Additionally, the IR drop of PANI/C/TiN NWA was smaller than that of PANI/TiO2 NWA, suggesting the lower internal resistance of PANI/C/TiN NWA. This result confirmed that the good conductivity of electrode materials could facilitate high capacitance performance. Table 1 lists the specific capacitance of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA obtained by galvanostatic charge/discharge measurements at the same current density of 1.0 mA cm2. Just as expected, PANI based on the core of TiN NWA exhibited a much higher capacitance performance than that based on the core of TiO2 NWA. It was also noteworthy that PANI/C/TiN NWA had comparable pseudo-capacitive behavior and specific capacitance compared to PANI/TiN NWA. However, when the carbon layer was covered on PANI/TiN NWA to form C/PANI/TiN NWA, its specific capacitance was lower a bit than that of PANI/C/ TiN NWA. This could be attributed to the decline of the pseudocapacitance of PANI because the carbon shell could prevent electrolyte ions from freely penetrating into PANI layer. The area specific capacitances of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA were determined to be 4.7, 13.8, 65.2 and 85.9 mF cm2. Comparatively, the area specific capacitances of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA and PANI/C/TiN NWA were determined to be 250, 303, 491 and 480 mF cm2. It means that the pseudocapacitance of PANI was much larger than EDLC capacitance of TiO2 and TiN. Concerning PANI/C/TiN NWA, the overall capacitance performance was ascribed to the major contribution of PANI and the minor contribution of C/TiN. Accordingly, it was reasonable to just regard the PANI as the predominant electroactive material and the C/TiO2/CC and C/TiN/CC as the active substrates. EIS measurements were carried out to evaluate the charge transfer resistance and ion diffusion resistance of PANI/C/TiN NWA. Fig. 7 shows that the Nyquist plots exhibited a semicircle in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The experimental EIS results were fitted using the equivalent circuit depicted in the inset of Fig. 7. The equivalent circuit model included the main elements of the resistors (R), the
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Fig. 6. (A) CV curves of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA at a scan rate of 10 mV s1; (B) Galvanostatic charge/discharge curves of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA at a current density of 1.0 mA cm2; (C) CV curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a scan rate of 10 mV s1; (D) Galvanostatic charge/discharge curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a current density of 1.0 mA cm2.
Table 1 Specific capacitance of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA obtained by galvanostatic charge/discharge measurements at the same current density of 1.0 mA cm2. Electrode
Discharge time (s)
Specific capacitance (mF cm2)
Specific capacitance (F g1)
PANI/TiO2 NWA PANI/C/TiO2 NWA PANI/TiN NWA PANI/C/TiN NWA C/PANI/TiN NWA
150 182 295 288 248
250 303 491 480 413
500 606 982 960 826
constant phase element (CPE) and Warburg element (W). The resistance values included the solution resistance (Rs) and the charge transfer resistance (Rct) within the electrode materials [39]. Table 2 lists the fitting values of the equivalent circuit elements of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid. Apparently, the PANI/C/TiN NWA exhibited the smallest value of Rct (0.214 U), while PANI/C/TiO2 NWA showed the highest Rct (0.982 U). This result was due to the superior conductivity of TiN NWA. The C/ PANI/TiN NWA also showed bigger value of Rct (0.502 U) than PANI/ C/TiN NWA (0.214 U), owing to the presence of additional contact resistance between carbon shell and polymer layer of PANI. Herein, the carbon shell was generated from the hydrothermal reaction of glucose without further annealing treatment. However, considering PANI/C/TiN NWA, the further heating treatment could
decrease oxygen content and accordingly improve the conductivity of carbon shell. At low frequencies, the Nyquist plot of a simple electrode/electrolyte system usually shows a straight line with a slope of 45 if the system is a diffusion-controlled process, or a slope of 90 if the system is purely capacitive in nature [39]. The enlarged Nyquist plot in high frequency range was shown in the inset of Fig. 7. The C/PANI/TiN NWA started with a nearly 45 impedance line and extended to an almost vertical line, suggesting the presence of ions diffusion-controlled intercalation/deintercalation (or doping/dedoping) process occurred in the electrodes [40]. This could be due to that the carbon shell suppressed ions from penetrating into the PANI layer. The C/PANI/TiN NWA showed the highest Warburg resistance (0.94 U), while the PANI/C/TiN NWA exhibited a smallest value of WR (0.19 U). This indicated the
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Fig. 7. Nyquist plots of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid in 1 M H2SO4 electrolyte. The insets showed the corresponding equivalent circuit and the enlarged Nyquist plots in high frequency region.
Table 2 Fitting values of the equivalent circuit elements of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid. Parameters
PANI/C/TiO2 NWA
PANI/C/TiN NWA
C/PANI/TiN NWA
Rs (U) Rct (U) CPE (mF) nCPE WR (U)
4.06 0.982 148 0.980 0.220
3.47 0.214 307 1.07 0.192
3.58 0.502 257 0.997 0.940
PANI/C/TiN NWA had a better charge transport behavior. In view of PANI/C/TiN NWA, the electroactive PANI shell would conduct fast and reversible doping/depoding reaction to contribute pseudocapacitance. The carbon shell could protect TiN from the electrochemical corrosion reaction, improving the cycling stability. TiN core NWA could provide effective electron transfer route. So the PANI/C/TiN NWA exhibited good supercapacitance performance. More electrochemical tests were performed to evaluate capacitive performances of PANI/C/TiN NWA. Fig. 8A shows the CV curves of PANI/C/TiN NWA at different scan rates. At a low sweep rate of less than 10 mV s1, the CV curves revealed the sloping rectanglelike shape and current response was enhanced when increasing scan rate. This result indicated the PANI/C/TiN NWA had good electrochemical reversibility. In addition, at a higher sweep rate from 20 to 60 mV s1, the CV curves obviously deviated from regular rectangle shape and the current response had an unobvious enhancement when increasing scan rate. The proton would only approach the outer surface of the electrode materials at a higher sweep rate. The inner pores of electrode materials would have little capacitance contribution. The pseudo-capacitance decreased and therefore resulted in the capacitance loss at a higher current density. Fig. 8B shows the galvanostatic chargeedischarge curves of PANI/C/TiN NWA at different current densities. All the curves were highly linear and symmetrical at various current densities, which implied that this electrode had excellent electrochemical reversibility. The specific capacitance could be calculated from discharge curve according to Eq. (1). Encouragingly, the PANI/C/TiN NWA exhibited high specific capacitance and rate capability. Fig. 8C shows the specific capacitances of the PANI/C/TiN NWA, PANI/C/ TiO2 NWA and C/PANI/TiN NWA electrodes as a function of current density. Strikingly, PANI/C/TiN NWA displayed a specific capacitance of 1093 F g1 at 1.0 A g1, which was obvious higher than that
of PANI/C/TiO2 NWA (741 F g1) and even C/PANI/TiN NWA (988 F g1). This result was also higher than previously reported PANI/CC hybrid [41]. The difference of the capacitance performance of PANI/C/TiN NWA and C/PANI/TiN NWA may be also associated with the difference of their specific surface area and the different crystallinity of carbon. Generally, the specific capacitance gradually decreased with the increase of current density due to the incremental voltage drop and insufficient electroactive material involved in a redox reaction at higher current densities. While at a high current density of 5.0 A g1 (or 2.5 mA cm2), the PANI/C/TiN NWA remained the specific capacitance of 708 F g1, which was equal to 65% of that measured at 1.0 A g1 (or 0.5 mA cm2). This indicated a good rate capability of PANI/C/TiN NWA. The cycling stability of electrode materials is another critical requirement for practical supercapacitor applications. In order to investigate the effect of carbon shell on the stability of electrode materials, the C/PANI/TiN NWA, PANI/C/TiN NWA and PANI/TiN NWA conducted continuous charge/discharge measurements at a current density of 5.0 A g1 and the results were shown in Fig. 8D. As expected, PANI/TiN NWA suffered from a 10% capacitance loss after 2000 cycles. This capacitance loss could be attributed to the falling electrical conductivity of TiN NWA, which suffered from irreversible electrochemical oxidation in the corrosive electrolyte of H2SO4 solution. In contrast, PANI/C/TiN NWA exhibited remarkable cycling stability, keeping the capacitance retention of 98% after 2000 cycles. This result proved that the deposition of the uniform carbon shell could effectively protect TiN NWA from the electrochemical corrosion reaction and improve stability of whole electrode material. In addition, we also had prepared C/PANI/TiN NWA in order to protect TiN NWA and PANI synchronously using carbon layer. However, it is noteworthy that the C/PANI/TiN NWA retained 96% of its initial capacitance after 2000 cycles, showing a little lower stability than the PANI/C/TiN NWA. Interestingly, the C/PANI/ TiN NWA showed an increased capacity during the initial cycling process, which may be attributed to the improvement of ion accessibility in PANI [42]. This result indicated the TiN NWA still suffered from slight oxidation reaction, which could be attributed to the influence of proton acid doped polyaniline during the charge/ discharge processes. The PANI/C/TiN NWA showed the better cycling stability. The interconnecting carbon layer inserted between TiN and PANI not only protected TiN NWA core, but also guaranteed fast reversible charging/discharging process of PANI shell. In summary, the PANI/C/TiN NWA with shell/shell/core
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Fig. 8. (A) CV curves collected at different scan rate for the PANI/C/TiN NWA. (B) Galvanostatic charge/discharge curves collected at different current density for the PANI/C/TiN NWA. (C) Specific capacitances of the PANI/C/TiO2 NWA, C/PANI/TiN NWA and PANI/C/TiN NWA at different current density. (D) Cycle performance of the PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA measured at a current density of 2.5 mA cm2.
configuration had the reasonable combination of cycling stability and pseudo-capacitance performance, presenting the potential application as flexible supercapacitor electrode material. 4. Conclusions Heterogeneous coaxial PANI/C/TiN NWA ternary nanocomposite with a shell/shell/core structure was synthesized by sequentially coating carbon and PANI on TiN NWA for supercapacitor application. The PANI/C/TiN NWA achieved a high specific capacitance of 1093 F g1 at 1.0 A g1 and a remarkable capacity retention of 98% after 2000 cycles, which was higher than that of the C/PANI/TiN NWA and PANI/TiN NWA. The electroactive PANI shell would conduct fast and reversible doping/depoding reaction to contribute pseudo-capacitance. The carbon shell could protect TiN from the electrochemical corrosion reaction, improving the cycling stability. TiN core NWA could provide effective electron transfer route. The well-designed PANI/C/TiN NWA provided the ion diffusion channel at the interspace between the neighboring nanowires and the electron transfer route along independent nanowires, enabling a significant improvement of the electrochemical capacitance and cycling stability. The finding suggests that the promising application of PANI/C/TiN NWA ternary nanocomposite as flexible supercapacitor for electrochemical energy storage. Acknowledgments The work was supported by National Natural Science
Foundation of China (No. 21373047 and 20871029), Program for New Century Excellent Talents in University of the State Ministry of Education (No. NCET-08-0119) and Science & Technology Program of Suzhou City, China (No. ZXG2012026, SYN201208 and SYG201017). References [1] G.M. Zhou, F. Li, H.M. Cheng, Energy Environ. Sci. 7 (2014) 1307e1338. [2] L. Nyholm, G. Nystrom, A. Mihranyan, M. Stromme, Adv. Mater. 23 (2011) 3751e3769. [3] J.R. Miller, P. Simon, Science 321 (2008) 651e652. [4] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845e854. [5] Y. Xie, H. Du, C. Xia, Microporous Mesoporous Mater. 204 (2015) 163e172. [6] Y. Xie, D. Wang, Y. Zhou, H. Du, C. Xia, Synth. Met. 198 (2014) 59e66. [7] R. Zhang, H.A. Andersson, M. Andersson, B. Andres, H. Edlund, P. Edstrom, S. Edvardsson, S. Forsberg, M. Hummelgard, N. Johansson, K. Karlsson, H.E. Nilsson, M. Norgren, M. Olsen, T. Uesaka, T. Ohlund, H. Olin, Sci. Rep. 3 (2013) 1477. [8] X. Zhang, W. Lu, J. Dai, L. Bourgeois, J. Yao, H. Wang, J.R. Friend, D. Zhao, D.R. MacFarlane, Sci. Rep. 4 (2014) 7420. [9] Y. Xie, Y. Zhan, J. Porous Mater. 22 (2015) 403e412. [10] X.H. Lu, M.H. Yu, G.M. Wang, T. Zhai, S.L. Xie, Y.C. Ling, Y.X. Tong, Y. Li, Adv. Mater. 25 (2013) 267e272. [11] H. Du, Y. Xie, C. Xia, W. Wang, F. Tian, Y. Zhou, Mater. Lett. 132 (2014) 417e420. [12] H. Du, Y. Xie, C. Xia, W. Wang, F. Tian, New. J. Chem. 38 (2014) 1284e1293. [13] X.H. Lu, T.Y. Liu, T. Zhai, G.M. Wang, M.H. Yu, S.L. Xie, Y.C. Ling, C.L. Liang, Y. Tong, Y. Li, Adv. Energy Mater. 4 (2014) 1300994. [14] G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1e12. [15] C. Xia, Y. Xie, W. Wang, H. Du, Synth. Met. 192 (2014) 93e100. [16] C. Xia, Y. Xie, Y. Wang, W. Wang, H. Du, F. Tian, J. Appl. Electrochem. 43 (2013) 1225e1233. [17] T.Y. Liu, L. Finn, M.H. Yu, H.Y. Wang, T. Zhai, X.H. Lu, Y.X. Tong, Y. Li, Nano Lett.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced electrochemical performance of polyaniline/carbon/ titanium nitride nanowire array for flexible supercapacitor Yibing Xie a, b, *, Chi Xia a, b, Hongxiu Du a, b, Wei Wang a, b a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Suzhou Research Institute of Southeast University, Suzhou 215123, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PANI/C/TiN NWA ternary composite with shell/shell/core configuration is fabricated. PANI/C/TiN NWA exhibits capacitance of 1093 F g1 and capacitance retention of 98%. PANI/C shell enables to promote capacitance and cycling stability of PANI/C/TiN NWA. PANI/C/TiN NWA exhibits higher capacitance than C/PANI/TiN and PANI/C/TiO2 NWA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 30 March 2015 Accepted 5 April 2015 Available online 6 April 2015
The ternary nanocomposite of polyaniline/carbon/titanium nitride (PANI/C/TiN) nanowire array (NWA) is fabricated as electroactive electrode material for flexible supercapacitor application. Firstly, TiN NWA is formed through ammonia nitridation treatment of TiO2 NWA, which is synthesized via seed-assisted hydrothermal reaction. PANI/C/TiN NWA is then formed through sequentially coating carbon and PANI on the surface of TiN NWA. PANI/C/TiN NWA has unique shell/shell/core architecture, including a core layer of TiN NWA with a diameter of 40e160 nm and a length of 1.5 mm, a middle shell layer of carbon with a thickness of about 6.0 nm and an external surface layer of PANI with a thickness of 20e50 nm. PANI/C/TiN NWA provides ion diffusion channel at interspaces between the neighboring nanowires and electron transfer route along independent nanowires. The carbon shell layer is able to protect TiN NWA from electrochemical corrosion during charge/discharge process. PANI/C/TiN NWA displays high specific capacitance of 1093 F g1 at 1.0 Ag1, and good cycling stability with a capacity retention of 98% after 2000 cycles, presenting better supercapacitive performance than other integrated nanocomposites of C/ PANI/TiN, PANI/TiN and PANI/C/TiO2 NWA. Such a ternary nanocomposite of PANI/C/TiN NWA can be used as an electrode material of flexible supercapacitors. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Titanium nitride Carbon Nanowire array Supercapacitor
1. Introduction
* Corresponding author. School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China. E-mail address: [email protected] (Y. Xie). http://dx.doi.org/10.1016/j.jpowsour.2015.04.025 0378-7753/© 2015 Elsevier B.V. All rights reserved.
There is currently a strong demand for the development of inexpensive, flexible, light-weight and environmentally friendly energy storage devices [1,2]. As an intermediate system between dielectric capacitors and batteries, supercapacitors are emerging as an efficient energy storage devices due to their fast charge/
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discharge capability, high power density and long-term stability [3,4]. Recently, much effort has been devoted to developing the nanostructured and good conductive electrode materials that has high electrochemical capacitance [5e9]. However, it is still a critical challenge to enhance the energy density and cycling stability while retain the high-power density of electrode materials for flexible supercapacitors [10]. Conducting polymers are commonly used as flexible pseudocapacitive materials which exhibit superior specific energy to various carbon materials used in electrical double-layer capacitors [11,12]. Moreover, conducting polymers generally show higher conductivity than the metal oxides and consequently have larger power capability [13,14]. Polyaniline (PANI) is considered as one of most promising and versatile conducting polymers, which is ascribed to its high theoretical capacitance, easy synthesis, low cost and environmental stability [15,16]. However, PANI also suffers from the volumetric swelling and shrinking during charge/ discharge process as a result of ion doping and dedoping [17,18]. This volumetric change destroys the backbone of PANI, which seriously weakens its cycling life. To overcome this shortcoming, various composites of PANI modified with carbon materials, metal oxides or others have been explored to enhance the mechanical stability of the PANI [19e21]. Recently, both titanium dioxide (TiO2) and titanium nitride (TiN) nanoarray have become new kinds of promising materials for electrochemical and photochemical applications because of its high surface area and good physicochemical stability [22e25]. Intensively, TiN with good electrical conductivity is widely used in different fields such as dye-sensitized solar cells, fuel cells and supercapacitors [26]. TiN compact film usually serves as a current collector of electrode materials. Alternatively, TiN nanoarray is able to act as good electrode materials of supercapacitors [27,28]. On the other hand, the previous studies proved that TiN was somewhat instability in aqueous electrolyte solution due to irreversible electrochemical oxidation reaction [29]. Carbon coating treatment is regarded as a simple and effective strategy to substantially enhance the cycling stability of electrode materials in energy storage devices [30]. Many carbon-modified electrode materials, such as LiFePO4/C, NiO/C and TiO2/C et al., have been developed for energy storage applications [31e34]. In this study, a novel composite material with a shell/shell/core nanostructure was designed as flexible supercapacitor electrode to overcome the limitations associated with individual components. The ternary nanocomposite PANI/C/TiN NWA NWA with a shell/ shell/core nanoarray structure was prepared through sequentially coating carbon and PANI on TiN NWA. It is expected that the PANI/ C/TiN NWA nanocomposite can provide the improved performance of electrochemical capacitance and cycling stability. 2. Experimental The PANI/C/TiN NWA with unique shell/shell/core structure was synthesized to support on CC substrate via a stepwise deposition and coating process. Fig. 1 shows the schematic illustration of the fabrication process for PANI/C/TiN NWA supported on CC substrate. Firstly, TiN NWA was grown on CC substrate by seed-assisted hydrothermal synthesis and ammonia nitridation process. Secondly, carbon layer was coated on TiN NWA via a glucose-assisted hydrothermal treatment, and subsequent heating treatment. Finally, a thin film of PANI layer was coated onto C/TiN NWA through an electropolymerization deposition process. 2.1. Preparation of PANI/C/TiN NWA TiN NWA grown directly on a carbon cloth using a previously reported method [10]. Briefly, the clean carbon cloth (CC,
6.0 cm 1.0 cm) was immersed into 0.2 M tetrabutyl titanate ethanol solution for 10 min and then further heated in air atmosphere at 350 C for 10 min TiO2 nanoparticles accordingly were coated on the surface of CC to form TiO2/CC. The reaction solution was prepared by mixing 90 ml 12 mol L1 hydrochloric acid solution with 90 ml deionized water and 2.7 ml tetrabutyl titanate. As-prepared mixture solution and above TiO2/CC were transferred into a Teflon-lined stainless steel autoclave. The sealed autoclave was heated at 180 C for 7 h, and then was cooled down slowly to room temperature. A white TiO2 NWA film was uniformly coated on the surface of CC. The TiO2 NWA conducted the heating treatment at 550 C for 1 h under nitrogen atmosphere. Finally, the TiO2 NWA was converted to TiN NWA by annealing treatment at 900 C for 1 h in ammonia atmosphere with a flow rate of 40 mL min1 in a tubular furnace. The C/TiN NWA was synthesized by a glucose-assisted hydrothermal method. The as-prepared TiN NWA was heated at 180 C for 3 h in a Teflon-lined stainless steel autoclave with 25 mL 0.1 M glucose solution. This sample was further annealed at 800 C for 1 h in N2 atmosphere, forming C/TiN NWA. PANI/C/TiN NWA was prepared by electrochemical deposition method in a standard three-electrode setup, including a saturated calomel electrode (SCE) as the reference electrode, Pt plate as the counter electrode and C/TiN NWA as the working electrode. PANI was coated on C/TiN NWA through cyclic voltammetry (CV) electrodeposition process at a sweep rate of 25 mV s1 and a potential range of 0.2e0.9 V in an electrolyte solution containing 0.1 M aniline monomer and 1.0 M sulfuric acid. The loading amount could be obtained by measuring the weight difference before and after the deposition of active materials utilizing an electronic balance with an accuracy of 0.1 mg. Herein, the loading amount of PANI was 2.5 mg. For a comparison, C/PANI/TiN NWA was also prepared by changing the coating sequence of PANI and carbon. PANI/TiN NWA was firstly prepared according to our previously reported method [15]. The carbon layer was then coated on PANI/TiN NWA to form C/ PANI/TiN NWA through a glucose-assisted hydrothermal synthesis method. This C/PANI/TiN NWA did not conduct a further annealing treatment process. 2.2. Structure characterization The surface morphology and microstructure of PANI/C/TiN NWA was investigated by means of scan electron microscopy (SEM, Zeiss Ultra Plus) and transmission electron microscopy (TEM, JEM-2100) and energy dispersive X-ray (EDX) spectroscopy. Raman spectroscopy was performed on a Raman spectrometer (Renishaw microRaman spectroscopy) using a HeeNe laser that emitted the sample at 785 nm excitation with wave between 0 and 2000 cm1. X-ray diffraction patterns were recorded with a Bruker-AXS Micro-diffractometer (XRD, D8 ADVANCE, Germany) with the use of Cu-Ka radiation source. 2.3. Electrochemical measurement The electrochemical performances of the electrode materials were measured in a three-electrode system by cyclic voltammetry and galvanostatic charge/discharge measurement, using CHI760C electrochemical workstation. Pt plate and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Electrochemical impedance spectra were recorded using an electrochemical workstation (EIS, ZAHNERim6ex, Germany) from 10 mHz to 100 kHz with an potential amplitude of 5 mV. EIS measurements were carried out at an open potential of 0.5 V (vs. SCE). All electrolyte used above was 1 M H2SO4 aqueous solution. The specific capacitance (C) is calculated using the following equations.
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Fig. 1. Schematic illustration of the fabrication process for PANI/C/TiN NWA supported on CC substrate.
Q It ¼ C¼ DV m DV m
(1)
where C is the specific capacitance, I is the charge/discharge current, t is the time of discharge, △V is the voltage difference between the upper and lower potential limits, and m is the mass of active materials. 3. Results and discussion 3.1. Morphological characterization Fig. 2 shows photographs of PANI/C/TiN NWA, C/TiN NWA, C/
Fig. 2. Photographs of the CC coated with active materials of TiO2 NWA, C/TiO2 NWA, C/TiN NWA and PANI/C/TiN NWA.
TiO2 NWA and TiO2 NWA supported on the CC. The layer-by-layer formation of different materials is associated with the changing color of nanowire film, which ranged from white TiO2 NWA to brown C/TiO2 NWA and black C/TiN NWA. The dark green film (in web version) indicated the formation of PANI/C/TiN NWA. Fig. 3 shows SEM images of TiO2 NWA, C/TiO2 NWA, TiN NWA, C/ TiN NWA, PANI/C/TiO2 NWA and PANI/C/TiN NWA. The entire CC fibers were uniformly covered by TiO2 NWA with a diameter of 40e160 nm and a length of 1.5 mm (Fig. 3A and B). The C/TiO2 NWA remained similar morphology to pure TiO2 NWA, maintaining a smooth surface (Fig. 3C and D). This suggested that the thickness of carbon shell was very thin. The TiN NWA had a relatively rougher surface, accordingly increasing the surface area of the materials. The C/TiN NWA exhibited a similar shape and size compared with TiN NWA (Fig. 3E and F). The SEM images of PANI/C/TiO2 NWA and PANI/C/TiN NWA were shown in Fig. 3G and H. The diameter of PANI/C/TiO2 NWA and PANI/C/TiN NWA were obvious larger than that of C/TiO2 NWA and C/TiN NWA. It indicated that the film of PANI layer was uniformly coated on the entire C/TiO2 or C/TiN NWA. Fig. 4 shows TEM images of C/TiN NWA and PANI/C/TiN NWA. The TEM image of C/TiN NWA revealed that the carbon shell had an average thickness of 6.0 nm (Fig. 4A). The carbon coating layer in C/ TiN NWA composite provided dual functionalities. Firstly, it promoted electron transport along independent nanowires. Secondly, it protected the TiN NWA from electrochemical corrosion and also facilitated the deposition of PANI layer. Previous research work had proven that the PANI was easier to be adsorbed and grafted on the surface of carbon materials [35]. The TEM image of PANI/C/TiN NWA revealed that the PANI effectively coated on the surface of C/ TiN NWA, forming a ternary nanocomposite with heterogeneous coaxial structure (Fig. 4B). The external surface layer shell of PANI had the thickness of 20e50 nm. This kind of shell/shell/core nanostructure possessed large electrochemical surface and an optimal ion diffusion path, which could act well as supercapacitor electrode material.
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Fig. 3. SEM images of (A, B) TiO2 NWA, (C, D) C/TiO2 NWA, (E) TiN NWA, (F) C/TiN NWA, (G) PANI/C/TiO2 NWA and (H) PANI/C/TiN NWA.
3.2. Structural analysis Fig. 5A shows the XRD patterns of TiO2 NWA, C/TiO2 NWA, TiN NW and C/TiN NWA. The XRD patterns of all samples revealed the weak characteristic diffraction peak at 25.5 , which was ascribed to the (002) crystal diffraction plane peak of the graphite carbon of the CC substrate. Curve a shows that the diffraction peaks at 27.5 , 36.2 , 41.5 and 54.5 could be indexed as the (110), (101), (111) and
(211) crystal planes of rutile TiO2, respectively [33]. Comparatively, curve b shows that the diffraction peaks of C/TiO2 NWA were similar to that of TiO2 NWA, confirming rutile phase of C/TiO2 NWA and amorphous phase of carbon shell. Curve c shows that the characteristic diffraction peaks at 37.0 , 43.1 and 62.5 could match well with the (111), (200) and (220) crystal plane of cubic TiN, respectively [26]. In addition, the absence of characteristic TiO2 peaks at 27.5 , 41.5 and 54.5 in TiN indicated the full conversion of
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Fig. 4. TEM images of (A) C/TiN NWA and (B) PANI/C/TiN NWA.
rutile TiO2 to cubic TiN through annealing treatment at 900 C in NH3 atmosphere. Comparatively, the diffraction peaks of C/TiN NWA in curve d were similar to that of TiN NWA in curve c, confirming that the cubic phase crystal structure of TiN NWA was preserved after coating carbon shell. Fig. 5B shows EDX spectrum of C/TiN NWA. The EDX analysis result revealed that the formation of TiN NWA proved the complete conversion of TiO2 NWA into TiN NWA, which was consistent with the XRD analysis result. Fig. 5C shows Raman spectra of the PANI/C/TiO2 NWA, PANI/C/ TiN NWA, C/TiO2 NWA, TiO2 NWA, C/TiN NWA and TiN NWA. The curve d shows that TiO2 NWA had the characteristic Raman peaks at 449 and 609 cm1 [29]. Comparatively, the curve c shows that C/ TiO2 NWA had additional two new peaks at 1317 and 1601 cm1, which were corresponding to the D and G bands of carbon [31]. The curve e shows that C/TiN NWA revealed the significantly enhanced intensity of the D and G bands when compared with TiN NWA, suggesting that the carbon layer was successfully covered on the surface of TiN NWA. The Raman spectra of TiO2 NWA and TiN NWA grown on CC substrate did not present the obvious characteristic Raman peaks of CC substrate. It was believed that the characteristic Raman peaks of the D and G bands of graphite carbon were so weak to be observed when CC substrate was completely covered by TiO2 or TiN NWA. The Raman spectra of PANI/C/TiO2 NWA, PANI/C/TiN NWA, C/TiO2 NWA, and C/TiN NWA grown on CC substrate revealed the characteristic Raman peaks of the D and G bands of carbon, which were mostly ascribed to the carbon coating layer on TiO2 NWA or TiN NWA rather than the graphite carbon of CC substrate. The curve f shows that TiN NWA had the characteristic peak at 576 cm1. In comparison to TiO2 NWA, TiN NWA did not show the characteristic peaks at 449 and 609 cm1 of TiO2 NWA, indicating that rutile TiO2 NWA could be completely converted to cubic TiN NWA through ammonia nitridation process. In view of curve a and b, the prominent peaks at 1163 cm1, 1338 cm1, 1480 cm1 and 1591 cm1 were corresponding to CeH bending of quinoid ring, CeNþ stretching, CeC stretching of the benzene ring and C]C stretching [36]. The peaks at 420 cm1 and 517 cm1 should be assigned to amine deformation and ring deformation [37]. The similar characteristic peaks of PANI were observed in curve a and b, indicating that PANI was successfully deposited on both C/TiO2 NWA and C/TiN NWA substrates. 3.3. Electrochemical properties The electrochemical performances of these shell/shell/core structured NWA electrode materials mostly depend on their porosity, conductivity and inherent electroactivity. The CV and galvanostatic charge/discharge experiments were carried out in 1.0 M H2SO4 aqueous electrolyte. Fig. 6A shows the CV curves of
TiO2 NWA, TiN NWA, C/TiO2 NWA and C/TiN NWA at a scan rate of 10 mV s1. Usually, the shape of CV curves of TiO2 and TiN was related to inherent electroactivity, reactive electrolyte solution, scan rate and potential range. In a strict sense, the CV curve of TiO2 NWA did not present the ideal rectangular shape due to its semiconductor property. Comparatively, the CV curve shape of TiN NWA was improved due to its good conductivity. Both TiO2 NWA and TiN NWA were identified as the nearly rectangle-like shape of CV curves, indicating electric double layer capacitive behavior. In comparison with TiO2 NWA, the C/TiO2 NWA exhibited a higher current response. Generally, as-prepared TiO2 NWA has a quite low capacitance due to its poor conductivity. The C/TiO2 NWA exhibited the increased capacitance because the carbon layer as an electroactive material could improve the electron conductivity of TiO2 NWA. In addition, the integral areas of TiN NWA and C/TiN NWA in CV curves were obviously larger than that of TiO2 NWA and C/TiO2 NWA, respectively. This result indicated that the nitridation process of TiO2 NWA could help to enhance its specific capacitance since TiN NWA possessed the better electrical conductivity than TiO2 NWA. Interestingly, the C/TiN NWA exhibited a smaller specific capacitance when compared with TiN NWA. The carbon shell layer in C/TiN NWA could protect the TiN core layer from the electrochemical corrosion reaction in H2SO4 electrolyte solution. On the other hand, this carbon shell layer also had an inhibiting effect on the diffusion of electrolyte ions into the core layer of TiN NWA. Eventually, the electroactive TiN NWA would serve only as conductive supporting role and unlikely form an electric double layer, resulting in lower capacitance of C/TiN NWA. Fig. 6B shows the galvanostatic charge/discharge curves of TiO2 NWA, TiN NWA, C/TiO2 NWA and C/TiN NWA at a current density of 1.0 mA cm2. The shapes of the charge curves were almost linear and somewhat symmetrically mirrored their discharge counterparts, indicating the good electrochemical capacitance performance of these electrode materials. TiN NWA and C/TiN NWA obviously exhibited their relatively higher capacitance than TiO2 NWA and C/TiO2 NWA. The charge storage capacity in galvanostatic charge/discharge tests was consistent with the result in those previous CV tests. Notably, The IR drop of C/TiN NWA was even smaller than that of TiN NWA and C/ TiO2 NWA, which could be ascribed to the synergistic effect of superior electrical conductivity of TiN NWA core and carbon shell. To determine the synergistic effect and microstructure influence of the PANI/C shell on the capacitance performance, different integrated electrode materials were prepared for a comparison. Fig. 6C shows the CV curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA. Apparently, the corresponding CV curves did not show ideal rectangular shape since all these electrodes involved electroactive PANI. In general, PANI could conduct reversible proton (Hþ) doping/dedoping reaction in H2SO4 electrolyte solution during the charge/discharge
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Fig. 5. (A) XRD patterns of TiO2 NWA, C/TiO2 NWA, TiN NW and C/TiN NWA; (B) EDX spectrum of C/TiN NWA; (C) Raman spectra of PANI/C/TiO2 NWA, PANI/C/TiN NWA, C/ TiO2 NWA, TiO2 NWA, C/TiN NWA and TiN NWA.
process, which contributed to the pseudo-capacitance performance. The curve a and b indicate that the anodic peaks occurring at 0.30 V for PANI/TiO2 NWA, PANI/C/TiO2 NWA could be attributed to the transition of PANI from the semiconducting state to the conducting state [38]. Comparatively, the curve c, d and e indicate that the corresponding anodic peaks at 0.3 V became very unobvious for PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA. Such a difference was ascribed to the much higher conductivity of TiN NWA in comparison to TiO2 NWA. This relatively high anodic current response caused by TiN substrate could surpass the current response caused by PANI in the CV curves of PANI/TiN NWA, PANI/ C/TiN NWA and C/PANI/TiN NWA. Compared with PANI/TiO2 NWA and PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/ PANI/TiN NWA showed the increased CV integral areas, implying their relatively higher specific capacitance. The enhanced capacitance performance was due to the improved conductivity of the nanowire framework changed from TiO2 to TiN. PANI/TiO2, PANI/C/ TiO2, PANI/TiN and PANI/C/TiN with an outer layer of PANI mostly involved the pseudo-capacitance of PANI. The C/PANI/TiN with outer layer of carbon involved the pseudo-capacitance of PANI and the electrical double layer capacitance of carbon. Conductive carbon shell and TiN NWA could decrease the internal resistance of the PANI and ensure the high utilization of electroactive PANI. The galvanostatic charge/discharge curves of PANI/TiO2 NWA, PANI/TiN NWA, PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a current density of 1.0 mA cm2 were shown in Fig. 6D. The almost linear and symmetric curves indicated good electrochemical capacitance performance of all these samples. Compared with PANI/C/TiO2 NWA, the longer discharge time of the PANI/C/TiN NWA indicated its relatively higher capacitance. Additionally, the IR drop of PANI/C/TiN NWA was smaller than that of PANI/TiO2 NWA, suggesting the lower internal resistance of PANI/C/TiN NWA. This result confirmed that the good conductivity of electrode materials could facilitate high capacitance performance. Table 1 lists the specific capacitance of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA obtained by galvanostatic charge/discharge measurements at the same current density of 1.0 mA cm2. Just as expected, PANI based on the core of TiN NWA exhibited a much higher capacitance performance than that based on the core of TiO2 NWA. It was also noteworthy that PANI/C/TiN NWA had comparable pseudo-capacitive behavior and specific capacitance compared to PANI/TiN NWA. However, when the carbon layer was covered on PANI/TiN NWA to form C/PANI/TiN NWA, its specific capacitance was lower a bit than that of PANI/C/ TiN NWA. This could be attributed to the decline of the pseudocapacitance of PANI because the carbon shell could prevent electrolyte ions from freely penetrating into PANI layer. The area specific capacitances of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA were determined to be 4.7, 13.8, 65.2 and 85.9 mF cm2. Comparatively, the area specific capacitances of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA and PANI/C/TiN NWA were determined to be 250, 303, 491 and 480 mF cm2. It means that the pseudocapacitance of PANI was much larger than EDLC capacitance of TiO2 and TiN. Concerning PANI/C/TiN NWA, the overall capacitance performance was ascribed to the major contribution of PANI and the minor contribution of C/TiN. Accordingly, it was reasonable to just regard the PANI as the predominant electroactive material and the C/TiO2/CC and C/TiN/CC as the active substrates. EIS measurements were carried out to evaluate the charge transfer resistance and ion diffusion resistance of PANI/C/TiN NWA. Fig. 7 shows that the Nyquist plots exhibited a semicircle in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The experimental EIS results were fitted using the equivalent circuit depicted in the inset of Fig. 7. The equivalent circuit model included the main elements of the resistors (R), the
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Fig. 6. (A) CV curves of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA at a scan rate of 10 mV s1; (B) Galvanostatic charge/discharge curves of TiO2 NWA, C/TiO2 NWA, TiN NWA and C/TiN NWA at a current density of 1.0 mA cm2; (C) CV curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a scan rate of 10 mV s1; (D) Galvanostatic charge/discharge curves of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA at a current density of 1.0 mA cm2.
Table 1 Specific capacitance of PANI/TiO2 NWA, PANI/C/TiO2 NWA, PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA obtained by galvanostatic charge/discharge measurements at the same current density of 1.0 mA cm2. Electrode
Discharge time (s)
Specific capacitance (mF cm2)
Specific capacitance (F g1)
PANI/TiO2 NWA PANI/C/TiO2 NWA PANI/TiN NWA PANI/C/TiN NWA C/PANI/TiN NWA
150 182 295 288 248
250 303 491 480 413
500 606 982 960 826
constant phase element (CPE) and Warburg element (W). The resistance values included the solution resistance (Rs) and the charge transfer resistance (Rct) within the electrode materials [39]. Table 2 lists the fitting values of the equivalent circuit elements of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid. Apparently, the PANI/C/TiN NWA exhibited the smallest value of Rct (0.214 U), while PANI/C/TiO2 NWA showed the highest Rct (0.982 U). This result was due to the superior conductivity of TiN NWA. The C/ PANI/TiN NWA also showed bigger value of Rct (0.502 U) than PANI/ C/TiN NWA (0.214 U), owing to the presence of additional contact resistance between carbon shell and polymer layer of PANI. Herein, the carbon shell was generated from the hydrothermal reaction of glucose without further annealing treatment. However, considering PANI/C/TiN NWA, the further heating treatment could
decrease oxygen content and accordingly improve the conductivity of carbon shell. At low frequencies, the Nyquist plot of a simple electrode/electrolyte system usually shows a straight line with a slope of 45 if the system is a diffusion-controlled process, or a slope of 90 if the system is purely capacitive in nature [39]. The enlarged Nyquist plot in high frequency range was shown in the inset of Fig. 7. The C/PANI/TiN NWA started with a nearly 45 impedance line and extended to an almost vertical line, suggesting the presence of ions diffusion-controlled intercalation/deintercalation (or doping/dedoping) process occurred in the electrodes [40]. This could be due to that the carbon shell suppressed ions from penetrating into the PANI layer. The C/PANI/TiN NWA showed the highest Warburg resistance (0.94 U), while the PANI/C/TiN NWA exhibited a smallest value of WR (0.19 U). This indicated the
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Fig. 7. Nyquist plots of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid in 1 M H2SO4 electrolyte. The insets showed the corresponding equivalent circuit and the enlarged Nyquist plots in high frequency region.
Table 2 Fitting values of the equivalent circuit elements of PANI/C/TiO2 NWA, PANI/C/TiN NWA and C/PANI/TiN NWA hybrid. Parameters
PANI/C/TiO2 NWA
PANI/C/TiN NWA
C/PANI/TiN NWA
Rs (U) Rct (U) CPE (mF) nCPE WR (U)
4.06 0.982 148 0.980 0.220
3.47 0.214 307 1.07 0.192
3.58 0.502 257 0.997 0.940
PANI/C/TiN NWA had a better charge transport behavior. In view of PANI/C/TiN NWA, the electroactive PANI shell would conduct fast and reversible doping/depoding reaction to contribute pseudocapacitance. The carbon shell could protect TiN from the electrochemical corrosion reaction, improving the cycling stability. TiN core NWA could provide effective electron transfer route. So the PANI/C/TiN NWA exhibited good supercapacitance performance. More electrochemical tests were performed to evaluate capacitive performances of PANI/C/TiN NWA. Fig. 8A shows the CV curves of PANI/C/TiN NWA at different scan rates. At a low sweep rate of less than 10 mV s1, the CV curves revealed the sloping rectanglelike shape and current response was enhanced when increasing scan rate. This result indicated the PANI/C/TiN NWA had good electrochemical reversibility. In addition, at a higher sweep rate from 20 to 60 mV s1, the CV curves obviously deviated from regular rectangle shape and the current response had an unobvious enhancement when increasing scan rate. The proton would only approach the outer surface of the electrode materials at a higher sweep rate. The inner pores of electrode materials would have little capacitance contribution. The pseudo-capacitance decreased and therefore resulted in the capacitance loss at a higher current density. Fig. 8B shows the galvanostatic chargeedischarge curves of PANI/C/TiN NWA at different current densities. All the curves were highly linear and symmetrical at various current densities, which implied that this electrode had excellent electrochemical reversibility. The specific capacitance could be calculated from discharge curve according to Eq. (1). Encouragingly, the PANI/C/TiN NWA exhibited high specific capacitance and rate capability. Fig. 8C shows the specific capacitances of the PANI/C/TiN NWA, PANI/C/ TiO2 NWA and C/PANI/TiN NWA electrodes as a function of current density. Strikingly, PANI/C/TiN NWA displayed a specific capacitance of 1093 F g1 at 1.0 A g1, which was obvious higher than that
of PANI/C/TiO2 NWA (741 F g1) and even C/PANI/TiN NWA (988 F g1). This result was also higher than previously reported PANI/CC hybrid [41]. The difference of the capacitance performance of PANI/C/TiN NWA and C/PANI/TiN NWA may be also associated with the difference of their specific surface area and the different crystallinity of carbon. Generally, the specific capacitance gradually decreased with the increase of current density due to the incremental voltage drop and insufficient electroactive material involved in a redox reaction at higher current densities. While at a high current density of 5.0 A g1 (or 2.5 mA cm2), the PANI/C/TiN NWA remained the specific capacitance of 708 F g1, which was equal to 65% of that measured at 1.0 A g1 (or 0.5 mA cm2). This indicated a good rate capability of PANI/C/TiN NWA. The cycling stability of electrode materials is another critical requirement for practical supercapacitor applications. In order to investigate the effect of carbon shell on the stability of electrode materials, the C/PANI/TiN NWA, PANI/C/TiN NWA and PANI/TiN NWA conducted continuous charge/discharge measurements at a current density of 5.0 A g1 and the results were shown in Fig. 8D. As expected, PANI/TiN NWA suffered from a 10% capacitance loss after 2000 cycles. This capacitance loss could be attributed to the falling electrical conductivity of TiN NWA, which suffered from irreversible electrochemical oxidation in the corrosive electrolyte of H2SO4 solution. In contrast, PANI/C/TiN NWA exhibited remarkable cycling stability, keeping the capacitance retention of 98% after 2000 cycles. This result proved that the deposition of the uniform carbon shell could effectively protect TiN NWA from the electrochemical corrosion reaction and improve stability of whole electrode material. In addition, we also had prepared C/PANI/TiN NWA in order to protect TiN NWA and PANI synchronously using carbon layer. However, it is noteworthy that the C/PANI/TiN NWA retained 96% of its initial capacitance after 2000 cycles, showing a little lower stability than the PANI/C/TiN NWA. Interestingly, the C/PANI/ TiN NWA showed an increased capacity during the initial cycling process, which may be attributed to the improvement of ion accessibility in PANI [42]. This result indicated the TiN NWA still suffered from slight oxidation reaction, which could be attributed to the influence of proton acid doped polyaniline during the charge/ discharge processes. The PANI/C/TiN NWA showed the better cycling stability. The interconnecting carbon layer inserted between TiN and PANI not only protected TiN NWA core, but also guaranteed fast reversible charging/discharging process of PANI shell. In summary, the PANI/C/TiN NWA with shell/shell/core
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Fig. 8. (A) CV curves collected at different scan rate for the PANI/C/TiN NWA. (B) Galvanostatic charge/discharge curves collected at different current density for the PANI/C/TiN NWA. (C) Specific capacitances of the PANI/C/TiO2 NWA, C/PANI/TiN NWA and PANI/C/TiN NWA at different current density. (D) Cycle performance of the PANI/TiN NWA, PANI/C/TiN NWA and C/PANI/TiN NWA measured at a current density of 2.5 mA cm2.
configuration had the reasonable combination of cycling stability and pseudo-capacitance performance, presenting the potential application as flexible supercapacitor electrode material. 4. Conclusions Heterogeneous coaxial PANI/C/TiN NWA ternary nanocomposite with a shell/shell/core structure was synthesized by sequentially coating carbon and PANI on TiN NWA for supercapacitor application. The PANI/C/TiN NWA achieved a high specific capacitance of 1093 F g1 at 1.0 A g1 and a remarkable capacity retention of 98% after 2000 cycles, which was higher than that of the C/PANI/TiN NWA and PANI/TiN NWA. The electroactive PANI shell would conduct fast and reversible doping/depoding reaction to contribute pseudo-capacitance. The carbon shell could protect TiN from the electrochemical corrosion reaction, improving the cycling stability. TiN core NWA could provide effective electron transfer route. The well-designed PANI/C/TiN NWA provided the ion diffusion channel at the interspace between the neighboring nanowires and the electron transfer route along independent nanowires, enabling a significant improvement of the electrochemical capacitance and cycling stability. The finding suggests that the promising application of PANI/C/TiN NWA ternary nanocomposite as flexible supercapacitor for electrochemical energy storage. Acknowledgments The work was supported by National Natural Science
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