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Facile Synthesis of NiCo2-xFexO4 Nanotubes/Carbon Textiles Composites for High-performance Electrochemical Energy Storage Devices Zhaohui Liu, Longqiang Wang, Yufeng Cheng, Xiangyang Cheng, Bo Lin, Longfei Yue, and Shougang Chen ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Nano Materials

Facile Synthesis of NiCo2-xFexO4 Nanotubes/Carbon Textiles Composites for High-performance Electrochemical Energy Storage Devices Zhaohui Liu, Longqiang Wang, Yufeng Cheng, Xiangyang Cheng, Bo Lin, Longfei Yue, Shougang Chen* *Corresponding author (Email: [email protected]) School of Material Science and Engineering, Ocean University of China, Qingdao 266100, China

Abstract To use advanced pseudo-capacitive electrode for improved energy density of supercapacitors has paid much attention recently. Herein, we develop a promising electrode architecture that containing carbon textiles uniformly covered with NiCo2-xFexO4 nanotubes, which are directly used as electrode materials in energy storages devices, through a simple and feasible two-step method and then a short period of post annealing treatment was adopted. The prepared NiCo2-xFexO4 nanotubes/carbon textiles composite electrode possesses a high specific capacitance of 2057 F g-1 when the discharging current is 1 A g-1, and a superior long-life stability with 90.32% retention of specific capacitance after 3,000 cycles. The increased capacitive property is ascribed to the increased specific surface area, the vacancy formation and the unique nanotube structure. Key words: NiCo2-xFexO4 nanotubes; carbon textiles; energy storage, supercapacitor; capacity and cycle stability

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1 Introduction With the traditional fuels depleting and the resulting environmental concerns, renewable, clean and sustainable energies are urgently needed. Particularly, high efficient energy storage has received worldwide attentions recently [1-6]. Supercapacitors are increasing popular with the advantages of a high energy density, fast charging/discharging ability, long cycling lifetime, low maintenance cost and environment-friendly [7-11]. Generally, supercapacitors include

two types:

electrical

double

layer

capacitors

(EDLCs)

and

pesudocapacitors. The EDLCs have a relatively low capacitance, while the transition metal oxides in pseudocapacitors have intrinsically a low conductivity and are easily deformed [12]. Composite materials can integrate advantages of different materials and proved a promising alterative for energy storage and conversion [13-14]. Qiu et al. synthesized ZnCo2O4/MnO2 nanocore forests with a mesoporous, hierarchical core-shell structure used as supercapacitor electrodes, exhibiting an excellent cycling stability, high capacity, an efficient charge/discharge performance, and a long cycling lifetime [15]. Composited transition-metal oxides (CTMOs), especially those with a spinel structure CxT3-XO4 (C, T= Ni, Co, Mo, et al), have been used as a potential pseudocapacitive electrode material in the regard of energy storage. Particularly, NiCo2O4 is one of the most widely studied CTMOs due to its outstanding pseudocapacitive performance, low price, rich sources and environmental benignity [7, 16-18]. Wei et al. synthesized NiCo2O4


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nanoparticles, which present an excellent electrical capacity (1400 F g−1), good reversibility, and superior electrochemical cycling stability [19]. Nevertheless, because of the low electrical conductivity, poor electrochemical stability and restricted specific surface area, the NiCo2O4 shows a much smaller capacitance than the theoretical value [20, 21]. Numberous methods have been made to increase the properties of the electrode material, especially the specific capacitance. Li et al. fabricated core-shell sub-microspheres (carbon nanosphere/ NiCo2O4) by a simple hydrothermal method, and obtained a relatively high electrical capacity (1420 F g-1) [20]. Hu et al. prepared the Co3O4/NiCo2O4 double-shelled nanocages and when it used as electrodes for pseudocapacitors, it showed a capacitance (972 F g-1) when the discharging current achieved 5 A g-1 [22]. Wu et al. reported an electrode consists of NiCo2O4@ CNT (carbon nanotube) for supercapacitors and it achieved a higher capacitance of 1,590 F g-1 at 0.5 A g-1 [23]. Gao et al. synthesized nanosheets (NiCo2O4) on Ni nanofoam for supercapacitors and at 1 A g-1, its specific capacitance can achieve 899 F g-1 [24]. In general, an increased active surface areas can enable a good energy storage performance. Nanomaterials usually possess increased specific surface areas, meanwhile, the nanostructure can shorten the ion and electron transfer pathways [25]. Hollow micro-/nanostructures have been obtained by various processes, where the acid etching and Kirkendall effect are the most common methods for complex hollow metal oxides and chalcogenides. They have been



used in various application areas, for example, energy storage, biomedicine, catalysis, etc. Oh et al. produced Mn3O4/γ-Fe2O3 “nanoboxes” (they are hollow box-shaped nanocrystals) by galvanic replacement using Mn3O4 nanocrystals reacting with iron (II) perchlorate in aqueous media [26]. Their compositions are nonequilibrium and structures are unique (hollow structures), so the synthesized composites exhibited an excellent property, for example, a higher capacitance and a better electrochemical cycling stability. Recently, the complex hollow nanostructures, including metal oxides and chalcogenides, are synthesized successfully through the Kirkendall effect [11, 16, 17]. Whereas, fabrication of hollow nanotubes of multimetallic oxides (such as NiCo2-xFexO4) has rarely been reported. In this paper, we prepare NiCo2-xFexO4 nanotubes on carbon textiles through a simple and feasible method. Due to the surface areas are increased obviously, ion and electron transport pathways are shorten on a certain extent, the assembly exhibits a relatively high specific capacitance (than some literatures) and a better electrochemical cycling stability. As far as we know, the synthesized electrode (after electrochemical tests) displays a higher specific capacitance than the other similar electrode. The composite can be directly used as electrode materials (binder-free) for high performance pesudocapacitors. 2 Experimental Sections 2.1 Sample fabrication 2.1.1 Growth of NiCo2O4 Nanowires on Carbon Textiles


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The method of NiCo2O4 nanowires grown on carbon textiles is basically according to the team of Shen [25]. The operations are as follows: Carbon Textiles with a dimension of 3 cm ×3 cm ×0.3 mm were cleaned ultrasonically in anhydrous ethanol first, and then in distilled water about 10 min, and last dried at 60℃. 1.1897 g of CoCl2·6H2O, 0.5942 g of NiCl2·6H2O , 0.7289 g of hexadecyl trimethyl ammonium bromide (CTAB), 0.5405 g of urea and deionized water (50 mL ) were added into a 200 ml beaker, and then stirred (under a proper aped) until they were dissolved uniformly. The Teflon-lined stainless steel autoclave with a pretreated carbon cloth (3 cm×3 cm× 0.3 mm) was prepared, after the dissolved solution was put into, they were maintained at 120℃ for 24 h. After the first step, the carbon textiles were uniformly covered with NiCo-precursor nanowires, then they were rinsed carefully with absolute ethanol, and next distilled water, and last dried in the oven at 60℃. 2.1.2 Preparation of NiCo2-xFexO4 Nanocomposites The NiCo2-xFexO4 Nanocomposites was prepared using a water bath method followed by an annealing treatment. The preparation process included: (1) the carbon textiles covered with NiCo-precursor nanowires, as prepared above, were added into a beaker containing 50 mL distilled water. After heating to 90℃, 1 mL FeCl3 solution (containing 0.0054 g of FeCl3·6H2O) was added, and continued to keep at 95℃ for 1.5 h. During the cationic exchange reaction, the Fe3+ could replace Co2+ of the Ni-Co-carbonates hydroxide nanowire arrays to form a hollow structure. The precursor was rinsed carefully with absolute



ethanol first, and next distilled water and last dried at 50℃ in the oven. (2) The precursors were annealed at 300 ℃ for 3 h to obtain the NiCo2-xFexO4 nanocomposites. A diagram (Figure 1) of the experimental process for preparation and in situ growth of NiCo2-xFexO4 nanotube arrays on textiles carbon substrate using the hydrothermal method and water bath method is given as follows. Fig. 1 Diagram of the experimental process showing the preparation of NiCo2-xFexO4 nanotube arrays on carbon textiles. ①: hydrothermal method ②: water bath method

2.2 Characterization methods The X-ray diffraction (XRD, D8 ADVANCE Bruker, Germany) was used to measure the crystalline structure of the prepared samples. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific) data fitting was conducted on a Thermo Advantage. The topography and size of the fabricated samples were certified with scanning electron microscopy (SEM, Phenom ProX ， China) and transmission electron microscopy (TEM, JEM-1200EX, Japan ). 2.3 Electrochemical tests Cyclic voltammograms (CV) were measured in a certain concentration of aqueous solution (6 mol L-1 KOH) by a three-electrode. The working electrode, counter and reference electrode are respectively as follows: the carbon textiles– supported NiCo2-xFexO4 nanocomposites (the surface area of tested sample was


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1 cm2), a platinum and a saturated calomel electrode (SCE). The loading mass of active material on working electrode was 1.5 mg cm-2. The CV curves were recorded within the voltage window: -0.1 to 0.5 V (SCE) and the potential scan rates: 2 to 50 mV·s-1. The frequency range of the electrochemical impedance spectra (EIS) was 0.1-100,000 Hz, and the perturbation of AC voltage was 5 mV. The supercapactior testing was carried out with electrochemical workstation (CHI 660E) in aqueous KOH (6 mol L-1) electrolyte, and it used a three-electrode system same as the CV tests. The value of capacitance was estimated based on the formula: Cm=I ∆t/m ∆V

(1)

where Cm (F g-1) is the specific capacitance of the working electrode, I (A g-1) is discharging current, ∆t (s) is the discharging time period, m (g) is the mass loading of the active material and the ∆V (v) is the potential drop during discharging. 3 Results and Discussion 3.1 Morphology and Structural Analysis The prepared NiCo2-xFexO4 nanotube arrays on textile carbon substrate is characterized by XRD to characterize its crystalline structure, it is shown in Fig. 2. The crystalline planes (002) and (100) reflection at 22.71° and 43.25° (Fig. 2a) are belong to the carbon textiles. In addition to the reflections belonging to carbon textiles, all major diffraction peaks are attributed to the NiCo2O4 phase ACS Paragon Plus Environment


(JCPDS card No.02-1074). Peaks at 2θ values (31.31°, 36.82°, 44.73°, 55.51°, 59.30°, 65.15°) from the XRD spectra of NiCo2-xFexO4 can be indexed to the (200), (311), (400), (422), (511) and (440) crystalline planes, respectively. The Fe peaks in the NiCo2-xFexO4 nanotube are covered by the spinel NiCo2O4. The elemental distribution analysis of the NiCo2-xFexO4 nanotube is shown in Fig. S1. The presence of C, Fe, Co, Ni and O elements can be seen in Fig. 2b and 2c, the atomic percentage of every element is shown in Fig. 2d, respectively. Fig.2 XRD patterns of NiCo2-xFexO4 nanotube/ carbon textiles composite (a), EDS (b), elements (c) and atomic percentage (d).

Further elemental composition and oxidation state of the NiCo2-xFexO4 nanotube are characterized by XPS. The full survey spectrum (Fig.S2) of the NiCo2-xFexO4 nanotube displays the elements of C, Fe, Co, Ni, and O. Using a fitting method (Gaussian), the Co 2p spectrum in Fig. 3a is composed of Co2+and Co3+ spin-orbit doublets, and meanwhile two shakeup satellites. Peaks around 781.85 eV and 797.1eV are associated with Co2+, whereas those at 780.4 eV and 795.3 eV are belong to Co3+ [7, 28]. The Ni 2p is also consists of Ni2+ and Ni3+ spin-orbit doublets, and also two shakeup satellites (Fig. 3b). Peaks around 855.4 eV and 872.9 eV are belong to Ni2+, whereas the peaks at 856.6 eV and 874.6 eV are indexed to Ni3+ [29, 30]. The peaks around 712.03, 717.87 and 724.12 eV are associated with Fe3+ which also has a shakeup satellite (Fig. 3c). As shown in Fig. 3d, the O1s spectrum has a sharp peak at 529.7 eV. It gives the presence of metal-oxygen bonds, suggesting that the primary state of


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oxygen in NiCo2O4 is the lattice oxygen [31, 32]. Fig. 3 High-resolution XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Fe 2p and (d) O 1s for the NiCo2-xFexO4 nanotube/ carbon textiles composite

The SEM images of the synthesized specimens are characterized and shown in Fig.4. Fig. 4a shows the low and 4b the high-magnification images of the blank carbon fiber cloth. Higher-magnification SEM images shown in Fig. 4c and 4d display the needle-like NiCo-precursors uniformly growing on the carbon fiber. After annealing treatment (Fig. 4e and 4f) the NiCo2-xFexO4 possesses a nanotube structure on the carbon textiles. The unique feature can benefit the electron transfer, contributing to the optimal electrochemical performance. Typical NiCo2-xFexO4 nanotubes have diameters at about 100~300 nm, and the tube wall thickness about 20~100 nm. In comparison, NiCo2O4 nanowires are formed under similar preparation conditions without the addition of iron perchlorate, (as seen in Fig 4c and 4d). The TEM is further used to characterize the structure of NiCo2-xFexO4 nanotubes. As shown in Fig.5, the tubular NiCo2-xFexO4 nanotubes with an average diameter about 200 nm, and a length about several micrometers can be observed, which are consistent with the SEM images.

Fig. 4 The SEM images of the carbon textiles: (a) low magnification and (b) high. The SEM images of the crystalline NiCo2O4 nanowires/carbon textiles composite: (c) low magnifications and (e) high. The SEM images of the NiCo2-xFexO4 nanotube microsphere prepared in the absence of carbon textiles: (e) low



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magnifications and (f) high.

Fig. 5 TEM pattern of the NiCo2O4 nanowires (a) and the NiCo2-xFexO4 nanotube (b) scraped down from the carbon textiles

3.2 Supercapacitors performance The pseudocapacitive properties of the NiCo2-xFexO4 nanotube/carbon textiles

nanocomposites

are

measured

by

CV

and

galvanostatic

charging-discharging testing. The CV of the NiCo2-xFexO4 nanotube/carbon textiles composite electrode at a series of scan rates (2 to 50 mV s-1), in KOH (6 mol/L) solution can been seen in Fig. 6a. As is shown, that all curves are featured with a pair of well-defined redox peaks, and that indicate the pseudocapacitive features. When the potential scan rate increases, the peak current becomes larger, whereas, the oxidation and reduction peaks of the CV curves do not change obviously, showing that the electrode architecture makes the fast redox reactions possible for electrochemical energy storage. Galvanostatic charging-discharging measurements are carried out at a stable potential window (0~0.4 V, SCE) to further assess the potential applications of the NiCo2-xFexO4 nanotube/carbon textiles composite for pseudocapacitors. As shown in Fig. 6b, the presence of platform at about 0.25 V indicates the unique pseudocapacitive features, and it is consistent with the results in Fig. 6a. The specific capacitance that calculated by the formula (1) at


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the different charge/discharging current density (1, 2, 4, 8, 20 A g-1) is shown in Fig. 6c. Obviously, the specific capacitance values of NiCo2-xFexO4 nanotube/carbon textiles nanocomposite electrode are higher than the NiCo2O4 /carbon textiles electrode. Calculated can be drawn that the specific capacitance of the NiCo2-xFexO4 nanotube/carbon textiles composite are 2057, 1912, 1650, 1400, 1045 and 950 F g-1 at a series of current densities (1, 2, 4, 8, 10, 20 A g-1), respectively, as shown in Fig. 6b. The superior electrochemical performance of the NiCo2-xFexO4 nanotube/carbon textiles are mainly ascribed to the unique and advantageous structure of the prepared electrode. The specific capacitance of NiCo2-xFexO4 nanotube/carbon textiles is attributed to the increased surface area, the NiCo2-xFexO4 nanotubes/carbon textiles (67.36 m2 g-1) is much larger than the NiCo2O4 nanowires (31.42 m2 g-1), which is consistent with the BET testing (Fig.S3) and the unique nanostructure, which supplies more active sites (the carbon textiles are uniform covered with NiCo2-xFexO4 nanotubes) for redox reactions, and more efficient penetration of electrolyte (the increased surface area) into the electroactive materials (the unique structure). The NiCo2-xFexO4 nanotube composite electrode is more suitable for asymmetric supercapacitors for energy storage and power supply. Another crucial functionality affecting the application of supercapacitors is the long term cycle life. As shown in Fig. 6d, a cycle life test of the electrode made of NiCo2-xFexO4 nanotube/carbon textiles is performed. The carbon textiles-based NiCo2-xFexO4 nanotubes exhibit a relatively high specific



capacitance (2057 F g-1) and long cycling lifetime at 1 A g-1. After 3000 cycles, the capacitance still maintains at 90.32% and the SEM images can be seen in Fig. S4. In addition, the coulombic efficiency of the NiCo2-xFexO4 nanotube/carbon textiles during the whole cycle is in the range of 90-100%, as seen in Fig. 6d. The excellent long-term cycling stability of the NiCo2-xFexO4 nanotube/carbon textiles electrode due to the unique and advantageous structure of the prepared electrodes. Due to the NiCo2-xFexO4 nanotubes possess a large porosity, the electrons transfer through the nanochannels favors efficient electrochemical reactions in the processes of Faradic charge storage. The results indicate that the NiCo2-xFexO4 nanotube/carbon textiles electrode can be a potential alternative for high performance supercapacitors. Fig. 6 (a) CV and (b) constant-current charge-discharge voltage profiles of NiCo2-xFexO4 nanotubes/carbon textiles composite. (c) The specific capacitance at different current densities of 1, 2, 4, 8, 10, 20 A g-1, (d) cycling performance at current densities of 1 A g-1 of NiCo2-xFexO4 nanotubes/carbon textiles composite

Generally, the electrochemical performance, for example, internal resistance, electrons transfer resistance and capacity of supercapacitors also can be analyzed by EIS. The EIS data of the prepared electrode is investigated by using Nyquist plots, the results can show the impedances of the simulated component against the actual. As is shown, the equivalent circuit of the NiCo2-xFexO4 nanotubes/carbon textiles electrode is inset in Fig.7. The NiCo2-xFexO4 nanotubes/carbon textiles shows not obvious (at high frequency


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region), which indicates a tiny Rs (charge-transfer resistance) in the interface of the electrolyte and electrode. The impedance of the prepared electroactive substances is presented at low frequency and it may be because of the ion diffusion (ion of electroactive materials) [33]. Obviously, at a higher frequency range, a semicircle, which diameter corresponds to Rct (the charge transfer resistance) is observed and it is caused by Faradic reactions. And, from the vertical diffusion lines (a tiny slope) we can obtain that the good capacitive performance of the prepared electrode in charging/discharging experiments than that of the NiCo2O4/carbon textiles composite as well as carbon cloth [34-35]. The results reveal excellent stability of the NiCo2-xFexO4 nanotubes/carbon textiles composite.

Fig. 7 Nyquist plots of NiCo2-xFexO4 nanotubes/carbon textiles composite.

3.3 The mechanism for fabrication of the vacancy Oh, et al, has successfully prepared hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 (according to the Kirkendall effect) and meanwhile proved galvanic replacement reactions in metal oxide nanocrystals [26]. They demonstrated the mechanism of redox replacement through measuring the valence state with XAS (x-ray absorption spectroscopy) and XMCD (x-ray magnetic circular dichroism) [26]. And the XAS and XMCD spectra at the Fe L2,3-edges of the nanocages were similar to those of γ-Fe2O3 that contained only



Fe3+ [26]. According to this, we try to put forward the mechanism for fabrication of the vacancy and the process is shown in Fig. 8. When appropriate amount of aqueous iron (ш) is added, the core of the NiCo2O4 nanowire is partially dissolved, the “nanoboxes” with a certain thick walls are formed. With the concentration of iron (ш) perchlorate increasing, the pores expand further in size. During the intermediate stage of the replacement reaction, Co2+ is replaced with Fe3+ and the nanotubes exhibit continuous arrays. The mechanism of the redox replacement is supported by measurement of the valence state with XPS.

Fig.8 The formation mechanism of the vacancy ①: hole expansion; ②: inside deposition; ③: wall thickening

4 Conclusions Novel NiCo2-xFexO4 nanotubes are successfully prepared on carbon textiles substrates through a simple and feasible two-step method. The synthesized NiCo2-xFexO4 nanotubes/carbon textiles composite electrode possesses a high specific capacitance of 2057 F g-1 at 1 A g-1, and a superior cycling stability with 90.32% of specific capacitance retention after 3,000 cycles. The prepared nanocomposites possess an excellent capacitive property, for example, a high capacitance and long-life cycling stability, making the electrode ideal for supercapacitors. The improved capacitive behavior is ascribed to the increased specific surface area and the unique nanotube structure, which, by using the electronic transmission channel and large open space can shorten the electron


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transfer path and ensure more active substance participating in the fast Faradaic redox reactions. The superior supercapacitor properties enable the NiCo2-xFexO4 nanotube/carbon textiles composites to become an ideal electrode material for energy storage devices.

Acknowledgements: This work was supported by the National Natural Science Foundation (51572249) and the Fundamental Research Funds for the Central Universities (841562011).



The Supporting Information is Available: The elemental distribution analysis (EDS), The High-resolution XPS spectra (full survey spectrum) of NiCo2-xFexO4 nanotube/carbon textiles composites. The BET of NiCo2O4 nanowires and NiCo2-xFexO4 nanotube /carbon textiles. SEM images of the NiCo2-xFexO4 nanotube /carbon textiles after 3000 cycles.


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Fig. 1 Diagram of the experimental process showing the preparation of NiCo2-xFexO4 nanotube arrays on carbon textiles. ①: hydrothermal method ②: water bath method

Fig.2 XRD patterns of NiCo2-xFexO4 nanotube/ carbon textiles composite (a), EDS (b), elements (c) and atomic percentage (d).


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Fig. 3 High-resolution XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Fe 2p and (d) O 1s for the NiCo2-xFexO4 nanotube/ carbon textiles composite



Fig. 4 (a) Low and (b) high magnification of SEM images of the carbon textiles. (c) Low and (d) high magnifications of SEM images of the crystalline NiCo2O4 nanowires/carbon textiles composite. (e) Low and (f) high magnifications of SEM images of the NiCo2-xFexO4 nanotube microsphere prepared in the absence of carbon textiles.

Fig. 5 TEM pattern of the NiCo2O4 nanowires (a) and the NiCo2-xFexO4 nanotube (b) scratched down from the carbon textiles


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Fig. 6 (a) CV and (b) constant-current charge-discharge voltage profiles of NiCo2-xFexO4 nanotubes/carbon textiles composite. (c) The specific capacitance at different current densities of 1, 2, 4, 8, 10, 20 A g-1 (d) cycling performance at 1 A g-1 of NiCo2-xFexO4 nanotubes/carbon textiles composite



Fig. 7 Nyquist plots of NiCo2-xFexO4 nanotubes/carbon textiles composite.

Fig.8 The formation mechanism of the vacancy ①: hole expansion; ②: inside deposition; ③: wall thickening


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Electrode architecture containing carbon textiles uniformly covered with NiCo2-xFexO4 nanotubes and the composite electrode possesses a good electrochemical performance. 81x66mm (300 x 300 DPI)


Carbon Textiles

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