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Sulfur and nitrogen Co-doped activated CoFe2O4@C nanotubes as an efficient material for supercapacitor applications
Carbon 162 (2020) 124e135
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Carbon journal homepage: www.elsevier.com/locate/carbon
Sulfur and nitrogen Co-doped activated CoFe2O4@C nanotubes as an efficient material for supercapacitor applications Yujin Li a, b, d, Cuimeng Song a, b, d, Jinchao Chen a, b, d, Xueni Shang a, b, Jinping Chen c, Yun Li d, Min Huang d, Fanbin Meng a, b, * a
School of Material Science and Engineering, Hebei University of Technology, Tianjin, China Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Hebei University of Technology, Tianjin, China Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials, School of Materials, Tianjin University of Technology, Tianjin, China d CAS Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences (CAS), Ningbo, 315201, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2019 Received in revised form 11 February 2020 Accepted 17 February 2020 Available online 18 February 2020
Developing low-cost and highly efficient electrochemical materials toward supercapacitor applications is crucial for energy conversion systems. In the present work, dual S, N-doped activated CoFe2O4@CNTs was successfully synthesized using carbamide and sodium thiosulfate as N and S precursors and CNTs as a substrate via a convenient two-step hydrothermal activation procedure, which presents good performance for supercapacitor. The characterization results indicate that activity S and N atoms can be successfully doped into the framework of CoFe2O4@CNTs with little impact on the inner morphology and structure. However, the electrode material of N2S1eCoFe2O4@CNTs exhibits a superior electrochemical performance with 1053.60 F g1 at 1 A g1 in KOH electrolyte due to synergistic effects between spineltype metal oxides, heteroatoms and sp2 lattice of graphitic carbon. In addition to high energy and power densities, the capacitance retention of charging-discharging reaches 93.15% at a current density of 30 A g1 after 5000 cycles, exhibiting an outstanding cycle stability and potential lifetime in an alkaline electrolyte. © 2020 Elsevier Ltd. All rights reserved.
Keywords: CoFe2O4/CNTs S N-doped Binder-free Electrode material Supercapacitor
1. Introduction In modern society, pursuing more powerful electric power and green sustainable energy resources have become global challenges with the increasing depletion of fossil fuel and the serious environmental pollution [1e4]. In the circumstance, electrochemical supercapacitors (SCs) are highly important energy storage systems with long cycle life, rapid charge-discharge rate, large power density and high rate capacity compared with batteries, thus has again attracted significant interest in the past a few years in energy storage devices, such as emergency power supplies, pulse energy for laser weapons, vehicle starting power and short-term power boosters,etc [5e8]. Carbon, one of the most promising energy storage materials,
* Corresponding author. 29 Guangrong dao, hongqiao District, Tianjin, 300130, China. E-mail address: [email protected] (F. Meng). https://doi.org/10.1016/j.carbon.2020.02.050 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
has provoked great interest in recent years for use as electrodes in SCs, due to its high specific surface area (SSA), low cost, high electrical conductivity, excellent cyclic stability, wide operating temperature and superior mechanical [9e11]. So far, shaped carbon nanomaterials (SCNMs), such as graphene, carbon fibres (CF), carbon sphere (CS) and carbon nanotubes (CNTs), have been widely explored as alternative supports due to unavailability of active site on some supports coupled with the poor interactions between reactants and active sites [12]. In the case of CNTs, onedimensional allotrope of carbon, has notably shown short ion diffusion distance, larger surface area and high interface areas, which is helpful for fast electron transport and tuneable when employed as support. Recently, the spinel-type metal oxides nanoparticles (NPs) have been introduced as the supported pseudocapacitive materials, showing substantial enhancement in capacitance storage performance due to their high energy density at a high chargedischarge rate, robustness at the higher potential and reversible and rapid Faradaic surface reactions [5,13]. Further, carbons
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doped by heteroatoms (e.g., N, S, and so on) are considered as one of the promising candidates for electrode materials, since C atom in the sp2 lattice of graphitic carbon substituted by heteroatoms can change the electronic arrangement and tailor their electron donor property effectively [14e19]. In addition, dual heteroatom-doped with different electronegativity can further improve the electrochemical performance as synergistic effects are able to create unique electronic coupling structure on the sp2 hybridized carbon. Especially, sulfur and nitrogen with strong synergistic effects have successfully attracted extensive attention due to their superior ability to replace carbon atoms [20e23]. Inspired by these factors, herein, we have developed the N and S co-doped into the framework of CoFe2O4@CNTs as an efficient and durable electrode material for supercapacitor with power densities and high capacitance retention due to the cooperative effect between spinel-type metal oxides, heteroatoms and sp2 lattice of graphitic carbon. The framework of binder-free CoFe2O4@CNTs is fabricated by the hydrothermal activation procedure, followed by annealing in argon atmospheres. It has been found that the spinel-type CoFe2O4 were deposited on the surface of CNTs uniform and extensive without surfactant, while a negligible and loose CoFe2O4 forms detachment the substrate. Subsequently, the N and S doping ratio, level and form in the CoFe2O4@CNTs as secondary template can be readily tuned by a simple control of molar ratio of reactants. As prepared NxSy-CoFe2O4@CNTs exhibits the superior enhanced stability and bifunctional activity than the CoFe2O4@CNTs, CoFe2O4, CNTs and some other materials [24e26]. As a result, the NxSy-CoFe2O4@CNTs nanocomposites are one of the most suitable electrode materials with desirable specific capacitance, outstanding stability and excellent redox supercapacitive performances, indicating it a promising electrode for supercapacitor. 2. Experimental 2.1. Materials Cobalt acetate (C4H6CoO4$4H2O, purity: 99.5%), ferric acetate (C4H7FeO5, purity: 98.5%), sodium thiosulfate (Na2S2O3$5H2O, purity: 99.0%), carbamide (CH4N2O, purity: 99.95%), sodium borohydride (NaBH4, purity: 98%) and sodium dodecylbenzenesulphonate (SDBS, C18H29NaO3S, purity: 99.5%) were supplied by Aladdin (China). All reagents materials were analytical-grade and used as received without further purification. The CNTs (carbon content: 99.8%) were purchased from Cyberelectrochemical materials Co. Ltd (Tianjin, China). 2.2. Synthesis of activated CNTs and binder-free CoFe2O4@CNTs The preparation procedures of CoFe2O4@CNTs and NxSyCoFe2O4@CNTs are schematically illustrated in Fig. 1. In brief, 0.3g of CNTs were mixed in the SDBS solution (30 wt%; 50 mL), stirred for 2 h, sonicated for 40 min, allowed to stand for 24 h, filtered, washed and dried in a mild condition. Next, 0.382g of C4H7FeO5 and 0.249g C4H6CoO4$4H2O were dissolved in 40 ml of ultrapure water. After thorough mixing, 0.12 g pretreated active CNTs were added to the solution by continuously stirring for 1h under a magnetic stirrer, followed by being added NaBH4 solution (20 wt%; 5 mL) and stirred for 2h. After that, the mixture was transferred to a 100 ml Teflonlined stainless steel autoclave, sealed tightly, and heated at 150 C for 12 h. Then, the black precipitate was collected after washed and vacuum dried. Subsequently, the CoFe2O4@CNTs nanocomposites were obtained by calcining the collected powders in shielding gas (Ar) atmosphere at 700 C for 2 h in a muffle furnace.
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2.3. Synthesis of NxSy-CoFe2O4@CNTs In order to obtain the N1S1eCoFe2O4@CNTs, 3.723g of Na2S2O3$5H2O and 0.3003g CH4N2O with different quality were dissolved in 40 ml of ultrapure water, followed by being added 0.12g prepared CoFe2O4@CNTs powders and stirred for 2h. Afterwards, the mixture was transferred to a 100 ml Teflon-lined stainless steel autoclave, sealed tightly, and heated at 180 C for 16 h. Finally, the obtained black hydrogel was subsequently washed with distilled water for several times, filtered and dried to obtain N1S1eCoFe2O4@CNTs nanocomposites. Similarly, the samples were named based on the molar ratio of nitrogen and sulfur, such as N2S1eCoFe2O4@CNTs, N3S1eCoFe2O4 @CNTs, N1S2eCoFe2O4@CNTs and so on.
2.4. Characterization The X-ray diffraction (XRD, CuKa, l ¼ 0.15406 Å, Bruker D8 Advance) patterns were characterized at a scanning speed of 6 / min in the 2 q range from 10 to 90 . Raman spectra of the samples were acquired by a DXR Microscope Raman spectrometer. The chemical vibration mode was determined by Fourier transform infrared spectroscopy (FTIR) (Bruker, Vector 22, Germany) in the range of 400 cm1 to 4000 cm1 by means of KBr disc technique. Surface functional groups and bonding characterization were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB-250Xi, America) system using monochromatized AI-Ka X-ray source. The morphology observation and crystalline structure of the as-obtained products were investigated by field emission scanning electron microscopy (FESEM, NovaNanoSEM450, American FEI Company) and transmission electron microscopy (TEM, JEM-2010FEF, Japan JEOL), respectively. The correlative elemental analysis was performed with an energy dispersive X-ray spectroscope (EDS) attached to the FESEM. The magnetic properties were investigated by vibrating sample magnetometer (VSM, 7407, America LakeShore) running under 20,000 Oe field, respectively. The thermal gravimetric analysis (TGA) curves were obtained using instrument of LLCSDTQ-600 (America TA Instruments-water) at a heating rate of 5 C/min from room temperature to 1000 C in flowing air.
2.5. Electrochemical tests All the electrochemical studies were evaluated employing a three-electrode system by an electrochemical workstation (CHI660E, Chenhua, shanghai). The working electrodes were fabricated as follows: a slurry containing the carbon black, polyfluortetraethylene (PTFE) and active materials with a weight ratio of 1:1:8 was well mixed, and then was pressed on nickel foams (1.0 cm 1.0 cm) and dried at 50 C for 48 h in vacuum. The mass loading of active materials on each current collector was 3 mg. The electrochemical measurements were carried out in 1 M KOH aqueous solution with a three-electrode system where the modified nickel foam, platinum foil electrode and a standard Hg/HgO electrode acted as working, counter and reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurements and electrochemical impedance spectroscopy (EIS, frequency ranging from 0.01 KHz to 100 kHz at 5 mV amplitude) were investigated by the potential range of negative 1-0.1V. The corresponding specific capacitance was calculated based on the following equation:
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Fig. 1. Schematic illustrations of the preparation of CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs nanocomposites. (a) CNTs were mixed in the SDBS solution and dried to obtain active CNTs. (bec) fabrication of the CoFe2O4@CNTs nanocomposites by hydrothermal activation and high-temperature sintering procedure. (ced) the preparation process and structure of NxSy-CoFe2O4@CNTs nanocomposites. . (A colour version of this figure can be viewed online.)
Cm ¼
0:8I Dt m DV
where Cm (F g1) is the specific capacitance, I (A) is the discharge current, Dt (s) is the full discharge time (s), m (g) is the electrode active material mass and DV (V) is the potential voltage, respectively. The energy and power densities were calculated as follows:
1 Cm DV 2 E¼ : 2 3:6 P¼
3:6E Dt
where E (Wh kg1) is the energy density and P (Kw kg1) is the power density, respectively.
3. Results and discussions The crystal structure and phase composition were studied by XRD. As shown in Fig. 2(a), well-resolved diffraction lines in the patterns of CNTs and CoFe2O4@CNTs have a distinct characteristic diffraction peaks at 2q ¼ 26.08 , corresponding to the lattice planes
(002) of CNTs (PDF#41e1487). Moreover, the curves of CoFe2O4 and CoFe2O4@CNTs also exhibit a series of lattice plane peaks at 18.29 , 30.08 , 34.04 , 36.26 , 43.06 , 53.45 , 56.97 and 62.59 , which are assigned to (111), (220), (311), (400), (422), (511) and (440) reflections of CoFe2O4 (PDF#22e1086), respectively. In addition, the XRD patterns of CoFe2O4@CNTs show no other diffraction peaks, indicating their high purity and crystallinity. After N and S modified on the surface of CoFe2O4@CNTs, as shown in Fig. 2(b), all NxSyCoFe2O4@CNTs synthesized products have two distinct characteristic diffraction peaks at 26.18 and 33.95 , which are ascribed to (002) and (311) crystal planes of pure CNTs and CoFe2O4. Furthermore, it cannot effectively differentiate the pyrite-phase products due to the isostructural with one another and very similar lattice constants, particularly in the case of FeS2 and CoS2. XRD reaches its resolution limit and therefore the positions of the peaks are not corresponding to the standard card of CoS2 or FeS2. Hence, the five samples show other diffraction peaks indexed as crystallographic planes of CoS2 and FeS2 (PDF#41e1471 and PDF#42e1340), demonstrating that polymetallic sulphide was formed during the reaction, which is consistent with the report of Matthew S. Faber.et al. [27]. In order to analyze the disordered degrees and crystal defects of the samples, the Raman spectra of CNTs, CoFe2O4, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs are shown in Fig. 2(c). The typical peaks of pure CNTs were observed at 1346 cm1 and 1574 cm1, which
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Fig. 2. XRD diffraction patterns of (a) CNTs, CoFe2O4, CoFe2O4@CNTs and (b) NxSy-CoFe2O4@CNTs samples. (c) Raman spectra of the as-prepared samples (200 cm1e2000 cm1). (d) FTIR patterns of the as-prepared samples (400 cm1e4000 cm1). (A colour version of this figure can be viewed online.)
corresponded to the well-documented D and G bands of graphitic carbon, respectively. The prominent D-band is related to the vibration of sp3 carbon atoms stemming from the degree of lattice disorder and defects, while the G peaks is ascribed to the in-plane stretching mode of sp2 carbon domains in a graphitic 2D hexagonal lattice. Generally, the integral intensity ratios of D-band to G-band (ID/IG) are usually used to evaluate the disorder degree of defect density [28,29]. Furthermore, both NxSy-CoFe2O4@CNTs and CoFe2O4 exhibit a series of small peaks at 288 cm1, 458 cm1 and 665 cm1, corresponding to the characteristic peaks of metal oxides. In addition, there is also a distinct peak at 364 cm1 in all NxSyCoFe2O4@CNTs samples, which is caused by the metal polysulfides [27,30]. Compared with the CNTs, the ID/IG ratio of the CoFe2O4@CNTs varies from 0.391 to 0.804, indicating the great disordering of the crystal structure, which may be caused by the introduction of Co and Fe inserted into the inside cavity with it deposited on the surface of the CNTs simultaneously. In addition, the values of intensities ratios of ID/IG are increased from 0.899, 0.959 and 0.977 for N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs after the doping of the different molar
ratio of nitrogen and sulfur, representing the enhanced disorder and defects with increasing ratios, which illustrates C atom in graphitic carbon substituted by heteroatoms [31]. The constitutions of spinel structure and chemical properties of as-obtained CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs were investigated by FTIR spectra. As shown in Fig. 2(d), the strong peak at about 3440 cm1 in all as-obtained samples is assigned to the typical eOH stretching vibration, which can result from surface absorption of water molecules. In addition, a characteristic peak of CNTs, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs at 1627 cm1 corresponds to the typical bending vibrations of C]C, indicating the graphitization of the as-prepared samples [32]. Furthermore, there is a strong absorption peak at about 585 cm1 of CoFe2O4, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs, corresponding to the typical metal-oxygen bond in spinel type crystal structure. Notably, the FTIR spectra of NxSy-CoFe2O4@CNTs also exhibit distinct absorption peaks at 425 cm1, 1012 cm1, 1139 cm1 and 1383 cm1, which are attributed to the CoS2 & FeS2, CeN stretching vibrations, CeO deformation vibrations and CeS bending vibrations, respectively. The main unconspicuous band corresponds to the CeO
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Fig. 3. XPS spectra of N1S2eCoFe2O4@CNTs nanocomposites. (a) survey scan, (b,c,d,e,f) C 1s scan, Fe 2p scan, Co 2p scan, N 1s scan and S 2p scan. (A colour version of this figure can be viewed online.)
deformation vibrations due to the residues of the incompletely removed oxygen-containing functional groups of CNTs and the incorporation of physicochemically adsorbed oxygen. Moreover, the characterization data of CeN and CeS have demonstrated C
atom in graphitic carbon bond with heteroatoms, indicating successful coating surface of N and S on the CoFe2O4@CNTs. The bonding characteristics and functional groups of the N1S2eCoFe2O4@CNTs were evaluated by XPS, which are shown in
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Fig. 4. FESEM and HRTEM images of samples. SEM images of (a) pristine CNTs, (b,c), (d) N1S1eCoFe2O4@CNTs at low magnification and high magnification. (e,f) HRTEM images of N1S1eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
Table S1 and Fig. 3. In brief, the spectra distinctly show the peaks of C 1s, Fe 2p, Co 2p, N 1s and S 2p, which confirms the successful formation of CoFe2O4 nanoparticles on surface of CNTs and indicates the successful N and S doping with the hydrothermal method. The spectrum for a C 1s can be deconvoluted into three peaks located at 284.7 eV, 285.8 eV and 287.4 eV, representing for the sp2 CeC, CeS & CeO and CeN &C]O [33]. For the Fe 2p spectra, it can be divided into several peaks at 706.4 eV and 707.2 eV within Fe 2p3/2 region, and 718.3 eV, 720.5 eV and 711.3 eV within Fe 2p1/2 and satellite peaks regions, which reveals the formation of CoFe2O4 [34e36]. Moreover, the deconvolution of Co 2p spectra can be deconvoluted into binding energies of 779.6 eV, 780.2 eV, 782.3 eV and 785.6 eV within Co 2p3/2 and satellite peaks regions, and 794.6 eV, 795.5 eV, 798.3 eV and 803.6 eV within Co 2p1/2 and satellite peaks regions [37,38]. In the N 1s spectra, the four fitted peaks are assigned at 398.5 eV, 400.4 eV, 401.3 eV and 404.8 eV, indicating pyridinic-N, pyrrolic-N, graphitic-N and oxidized
pyridinic-N doped in the CoFe2O4@CNTs, respectively [39,40]. Additionally, the high-resolution S 2p can be assigned to five peaks centered at 161.9 eV, 162.8 eV, 163.4 eV, 164.8 eV and 168.8 eV, associated with SeCo, SeFe, 2p3/2 CeSeC, 2p1/2 CeSeC and C-SOXC, respectively, and further convinced the successful doping of N and S atoms into the CoFe2O4@CNTs framework [41,42]. The morphology and microstructure features of the CNTs, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs were characterized by FESEM and FETEM in Fig. 4 and Fig. S1, respectively. As can be seen from Fig. 4(a), the as-spun CNTs show 1D fibrous morphologies with an average diameter of 150 nm and smooth surface. As shown in Fig. 4(b), the N1S1eCoFe2O4@CNTs inherits the continuously fibrous morphology of the as-spun CNTs with smooth surface and shows successful coating surface of N and S on the CoFe2O4@CNTs with slight detached CoFe2O4. In Fig. 4(c) and (d), the images of N1S1eCoFe2O4@CNTs demonstrate a relatively muddledness and roughness appearance with several ultra-quantity small-sized
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Fig. 5. Elemental mapping images of N1S1eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
Fig. 6. The hysteresis loop of CoFe2O4@CNTs, N2S1eCoFe2O4@CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs nanocomposites. (A colour version of this figure can be viewed online.)
CoFe2O4 NPs and activated heteroatoms of N and S doping on the surface of CNTs whose interlamellar spacings are clearly discernible. As can be seen from Fig. S1(A) and Fig. S1(a), we can see that the CoFe2O4 is distributed on the main body of CNTs uniform and extensive. Fig. S2 displays the TGA curves of CNTs and CoFe2O4@CNTs. The CoFe2O4 content (29.3 wt%) in CoFe2O4@CNTs could be calculated through the curves. Importantly, the muddledness and roughness structures are clearly visible on the exposed CNTs surface from Fig. S1(B) to Fig. S1(e), implying the presence of substantial CoFe2O4 NPs and N and S heteroatoms doping. In addition, the randomly three-dimensional interconnected network built of CNTs, CoFe2O4 and heteroatoms with uniformly diameter ranging from 150 nm to 200 nm facilitates electron transport, which helps to improve the rate performance of
electrode. Moreover, the substrate CNTs in the NxSy-CoFe2O4@CNTs nanocomposites with a large surface area can provide new electroactive sites due to the strong synergistic interactions between the individual CoFe2O4 NPs and CNTs surface, and can also prevent CoFe2O4 NPs agglomeration [43e45]. Fig. 4(e) and (f) clearly reveal the typical HRTEM images of N1S1eCoFe2O4@CNTs, in which the CoFe2O4 nanoparticles and N, S coating layer are uniformly embedded in the 1D carbon NTs matrix. The interplanar spacing of the adjacent fringes is measured approximate 0.209 nm and 0.255 nm, corresponding to the (400) and (311) crystal planes of CoFe2O4, respectively. In addition, the adjacent lattice spacing in the CNTs substrate is calculated to be 0.3348 nm, which corresponds to adjacent (002) plane of the sp2 carbon. Apparently, a thin N, S coating layer with thickness approximate ranging from 3 nm to 10 nm coats on the surface of CNTs substrate and active carrier CoFe2O4 NPs, which is conducive to further strengthening the bonding effect between exterior CoFe2O4 NPs, N, S heteroatoms and CNTs [46]. The element distribution of N1S1eCoFe2O4@CNTs was also determined by EDX mappings in Fig. 5. Obviously, we can see more intuitively from element mapping that uniform constituent elements of O, Fe, Co, N, and S are distributed on the main body of CNTs, demonstrating that CoFe2O4@CNTs and CNTs substrates are well coated by N, S heteroatoms in hybridization process. The hysteresis loops of the CoFe2O4@CNTs, N2S1eCoFe2O4 @CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs were investigated by VSM at room temperature. As can be seen from Fig. 6, all of the magnetization curves show a normal S-shape type, which is a ferrimagnetism characteristic. Furthermore, the CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs show saturation magnetization Ms 80.2 emu/ g, 71.9 emu/g, 58.8 emu/g and 54.2emu/g, respectively. In general, the saturation magnetization are mainly dependent on the magnetic grain size, grain composition, particle content and crystal orientation. Notably, the saturation magnetization decreases along with the increase of N, S heteroatoms content, indicating that the tetrahedral sites and octahedral sites in spinel structure of CoFe2O4 NPs could be occupied by N, S heteroatoms, which is in accordance with that of XRD patterns of NxSy-CoFe2O4@CNTs. Ultimately, the
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Fig. 7. Electrochemical performance of the CoFe2O4@CNTs nanocomposites electrodes for supercapacitors. (a) CV curves at different scan rates and (b) GCD curves at different current densities. (c,d) EIS spectra of as-prepared samples. (e) Corresponding specific capacitance as a function of current density. (f) cyclic stability of CoFe2O4@CNTs at a current density of 20 A g1 for 5000 cycles. (A colour version of this figure can be viewed online.)
decrease of saturation magnetization for higher concentration of N, S heteroatoms may be attributed to the formation of second phase, which decreases the resultant magnetic moment and thereby the magnetzation [47].
The electrochemical performances of CoFe2O4@CNTs were performed by cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) measurements at different scan rates (10 mV s1-200 mV s1) using 1 M KOH aqueous solution as
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Fig. 8. Electrochemical performance of the NxSy-CoFe2O4@CNTs nanocomposites electrodes for supercapacitors. (a) CV curves of N2S1eCoFe2O4@CNTs at different scan rates. (b) CV curves at 100 mV s1 for CNTs, CoFe2O4, CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs, respectively. (c) GCD curves of N2S1eCoFe2O4@CNTs
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electrolyte in a three-electrode system. As shown in Fig. 7(a), the CV curve of CoFe2O4@CNTs exhibits nonrectangular-like shape, which may be result from the cooperation of pseudo-capacitance nature and double-layer capacitance behavior. In addition, all the curves exhibit a dominant pair of redox peaks, indicating an obviously faradaic redox reaction behavior occurring in the working process, which are the characteristics of the extraction and insertion of protons, respectively [48]. The corresponding mechanism can be interpreted based on the following equation:
CoFe2 O4 þ OH4CoOOH þ 2FeOOH þ e CoOOH þ OH4CoO2 þ H2 O þ e Moreover, the area surrounded magnifies and remains a homologous shape with voltage sweep rate increasing, which means the enhancement of the capacitance value and an excellent rate capability with reversible and fast capacitive characteristics at high sweep rate. The galvanostatic charge-discharge curves of CoFe2O4@CNTs at different current densities are shown in Fig. 7(b). Apparently, every discharge time is approximate to the charging time of GCD curves even at different current densities among 1 A g1 to 8 A g1, exhibiting no obvious ohmic drop for all discharging curves [49]. Electrochemical impedance spectroscopy (EIS) is an effective way to get more information about electrochemical performances and capacitive behavior. As shown in the Nyquist plots of Fig. 7(c), the charge transfer resistance (Rct) of CoFe2O4@CNTs is 7.78 U, indicating a favorable ion diffusion process and excellent charge-transfer kinetic between residual oxygen-containing functional groups from CNTs and the redox reactions of CoFe2O4 NPs [7,50]. Furthermore, the intrinsic ohmic resistance (Rs) is 2.11 U, demonstrating efficient electron transportation with loading of CoFe2O4 NPs. As shown in Fig. 7(e), the specific capacitance of CoFe2O4@CNTs is 615.5 F g1, 537.6 F g1, 488.2 F g1, 421.3 F g1, 417.8 F g1 and 412 F g1 with an increase of current density from 1 A g1 to 8 A g1. The charging and diffusion of ions get longer time at the lower current densities, which enables the most of the active surface area to get charged respective to the discharging time, hence the capacitance gets increased with the current density decreased [51]. Especially, the capacitance retention of charging-discharging retains 94.96% after 5000 cycles of 20 A g1, exhibiting an excellent cyclic stability and potential lifetime in an alkaline electrolyte [52]. Similarly, the electrochemical properties of the NxSyCoFe2O4@CNTs were evaluated in 1 M KOH under the same conditions. As shown in Fig. 8(a) and Figs. S3(a and c), the CV curves of N2S1eCoFe2O4@CNTs, N1eCoFe2O4@CNTs and S1eCoFe2O4@CNTs display nonrectangular-like shape and a dominant pair of redox peaks at scan rates of 10 mV s1-200 mV s1, demonstrating the cooperation of pseudo-capacitance and double-layer capacitance nature of the charge-discharge process. The as-obtained CV curves at scan rates of 100 mV s1 of CNTs, CoFe2O4, CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4 @CNTs are shown in Fig. 8(b). These results suggest that the asprepared N1S2eCoFe2O4@CNTs nanocomposites electrodes could deliver high capacity and reversibility, showing that the content of molar ratio of nitrogen and sulfur in the composite is most appropriate for the full utilization of CoFe2O4 and N, S heteroatoms [34,53]. Fig. 8(c) and Figs. S3(b and d) display the selected GCD curves for N2S1eCoFe2O4@CNTs, N1eCoFe2O4@CNTs and S1eCoFe2O4@CNTs hybrid electrodes at different current densities. The approximately
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symmetric charge-discharge curves were observed at all current densities, exhibiting excellent electrochemical capacitive property and outstanding electrochemical reversibility [54]. The GCD curves of all the samples are compared at a current density of 1 A g1 in Fig. 8(d), indicating that the N1S2eCoFe2O4@CNTs electrode displays improved capacitive performance, which is in accordance with that of CV curves. EIS tests were conducted to get the intrinsic mechanism of the asprepared samples, as shown in Fig. 8(e) and Fig. S3(e). In addition, Table S2 summarizes the specific capacitance (Cm), energy density (E), intrinsic ohmic resistance (Rs) and charge transfer resistance (Rct) of all the electrode materials. The charge transfer resistance (Rct) of the nanohybrid electrodes decrease slightly from 14.84 U to 2.46 U for CNTs and N1S2eCoFe2O4@CNTs, demonstrating the enhanced electrochemical performance with the CoFe2O4 and N, S co-doped into the framework of CNTs, which is benefit from the combination of fast ion diffusion as well as low electron-transfer resistance [54]. Moreover, the slope of the straight line at low frequency is significantly steeper for the N1S2eCoFe2O4@CNTs electrode than for the other electrodes, clearly indicating a high electrolyte diffusion rate [55]. As shown in the specific capacitances in Fig. 8(f), Fig. S3(f) and Table S3, the N1S2eCoFe2O4@CNTs exhibits a high capacitance of 1213 F g1, 1119 F g1, 1030 F g1, 978 F g1, 936 F g1, 913 F g1 and 900 F g1 at 0.5 A g1, 1 A g1, 2 A g1, 3 A g1, 4 A g1, 6 A g1 and 12 A g1, respectively, which is mainly due to more electroactive site for diffusion efficient provided by the high surface area and optimal porous textures with the increase of N, S heteroatoms content. Encouragingly, the N1S2eCoFe2O4@CNTs electrode retains 80.4% of its initial capacitance even at a very high current density of 12 A g1, implying excellent rate capability and high reversible redox reaction. Additionally, the cyclic stability of different electrode materials at the same current density (20 A g1) are compared in Figs. S3(g and h) and Fig. S4. The N, S doping level of N1S3eCoFe2O4@CNTs and N3S1eCoFe2O4@CNTs are relatively high with a N/S ratio, which are the higher and closer to the value of 96.11% and 95.47%, implying that a superior electrochemical performance should possess a relatively high ratio of doped N and S. Furthermore, as shown in Fig. 8(g), the capacitance retention remains 93.92%, 93.15% and 92.7% of N1S2eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S1eCoFe2O4 @CNTs asymmetric supercapacitor at a current density of 30 A g1 for 5000 cycles, respectively, revealing the excellent cycling stability and potential lifetime in an alkaline electrolyte [52,56].
4. Conclusion In summary, various molar ratio of nitrogen and sulfur Codoped activated CoFe2O4@CNTs nanocomposites have been directly synthesized by substrate-assisted hydrothermal activation procedure and high-temperature sintering technique. The saturation magnetization decreases along with the increase of N, S heteroatoms content, indicating that the tetrahedral sites and octahedral sites in spinel structure of CoFe2O4 NPs could be occupied by N, S heteroatoms, which decreases the resultant magnetic moment and thereby the magnetzation. However, the NxSyCoFe2O4@CNTs electrode materials exhibit a high capacity, good rate capability and excellent cyclic stability with the increase of N, S molar ratios, demonstrating that dual heteroatoms synergistic effects are able to create unique electronic coupling structure on the sp2 hybridized carbon and spinel-type metal oxides nanoparticles, which can further improve the electrochemical performance.
at different current densities. (d) GCD curves of as-prepared samples at a current density of 1 A g1 and (e) corresponding EIS spectra of as-prepared samples. (f) specific capacitance at various current densities and (g) corresponding cyclic stability in 30 A g1 for 5000 cycles of N2S1eCoFe2O4@CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
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Hence, the distinctive structure and favorable electrochemical properties make the NxSy-CoFe2O4@CNTs materials constructed in this work a promising candidate towards the next generation highperformance supercapacitor. Declaration of competing interest There are no conflicts of interest to declare. CRediT authorship contribution statement Yujin Li: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. Cuimeng Song: Conceptualization, Formal analysis, Data curation. Jinchao Chen: Investigation. Xueni Shang: Data curation, Formal analysis. Jinping Chen: Investigation, Data curation. Yun Li: Investigation. Min Huang: Investigation. Fanbin Meng: Resources, Writing - original draft, Writing - review & editing, Project administration, Funding acquisition. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51371075) and scientific research project of Hebei Education Department (Grant No. E2019202143). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2020.02.050. References [1] X. Zhang, C.Y. Wang, Y.N. Chen, Z. Zhou, Binder-free NiFe2O4/C nanofibers as air cathodes for Li-O2 batteries, J. Power Sources 377 (2018) 136e141. [2] Y.H. Xiao, D.C. Su, X.Z. Wang, L.M. Zhou, S.M. Fang, In suit growth of ultradispersed NiCo2S4 nanoparticles on graphene for asymmetric supercapacitors, Electrochim. Acta 176 (2015) 44e50. [3] Y.L. Xu, B. Ren, S.S. Wang, Z.F. Liu, Carbon aerogel-based supercapacitors modified by hummers oxidation method, J. Colloid Interface Sci. 27 (2018) 25e32. [4] A.M. Wang, H.L. Wang, S.Y. Zhang, C.J. Mao, Controlled synthesis of nickel sulfide/graphene oxide nanocomposite for high-performance supercapacitor, Appl. Surf. Sci. 282 (2013) 704e708. [5] Y.F. Zhang, S.J. Park, Incorporation of RuO2 into charcoal-derived carbon with controllable microporosity by CO2 activation for high-performance supercapacitor, Carbon 122 (2017) 287e297. [6] S. Saha, P. Samanta, N.C. Murmu, T. Kuila, J.H. Lee, Electrochemical functionalization and in-situ deposition of the SAA@rGO/h-BN@Ni electrode for supercapacitor applications, J. Ind. Eng. Chem. 52 (2017), 321e708. [7] X.Y. Xie, X.J. He, H.F. Zhang, F. Wei, Interconnected sheet-like porous carbons from coal tar by a confined soft-template strategy for supercapacitors, Chem. Eng. J. 350 (2018) 49e56. [8] V. Veeramani, R. Madhu, S.M. Chen, M. Sivakumar, C.T. Hung, N. Miyamoto, NiCo2O4-decorated porous carbon nanosheets for high-performance supercapacitors, Electrochim. Acta 247 (2017) 288e295. [9] F. Li, Y.Y. Li, X. Yang, X.X. Han, X.P. Xu, G.G. Nie, Highly fluorescent chiral N-Sdoped carbon dots from cysteine: affecting cellular energy metabolism, Angew. Chem. Int. Ed. 57 (2018) 2377e2382. [10] M.A. Rafi, T.N. Narayanan, D.P. Hashim, N. Sakhavand, R. Shahsavari, R. Vajtai, Hexagonal boron nitride and graphite oxide reinforced multifunctional porous cement composites, Adv. Funct. Mater. 10 (2013) 1e7. [11] S.N. Liu, J. Wu, J. Zhou, S.Q. Liang, Mesoporous NiCo2O4 nanoneedles grown on three dimensional graphene networks as binder-free electrode for highperformance lithium-ion batteries and supercapacitors, Electrochim. Acta 176 (2015) 1e9. [12] A.H. Labulo, B. Omond, V.O. Nyamori, SuzukieMiyaura reaction and solventfree oxidation of benzyl alcohol by Pd/nitrogen-doped CNTs catalyst, J. Mater. Sci. 23 (2018) 15817e15836. [13] A. Sivanantham, P. Ganesan, S. Shanmugam, A synergistic effect of Co and CeO2 in nitrogen-doped carbon nanostructure for the enhanced oxygen electrode activity and stability, Appl. Catal. B Environ. 237 (2018) 1148e1159. [14] H.H. Zhang, X.Q. Liu, G.L. He, X.X. Zhang, W.H. Hu, Bioinspired synthesis of nitrogen/sulfur co-doped graphene as an efficient electrocatalyst for oxygen reduction reaction, J. Power Sources 279 (2015) 252e258.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Sulfur and nitrogen Co-doped activated CoFe2O4@C nanotubes as an efficient material for supercapacitor applications Yujin Li a, b, d, Cuimeng Song a, b, d, Jinchao Chen a, b, d, Xueni Shang a, b, Jinping Chen c, Yun Li d, Min Huang d, Fanbin Meng a, b, * a
School of Material Science and Engineering, Hebei University of Technology, Tianjin, China Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Hebei University of Technology, Tianjin, China Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials, School of Materials, Tianjin University of Technology, Tianjin, China d CAS Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences (CAS), Ningbo, 315201, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2019 Received in revised form 11 February 2020 Accepted 17 February 2020 Available online 18 February 2020
Developing low-cost and highly efficient electrochemical materials toward supercapacitor applications is crucial for energy conversion systems. In the present work, dual S, N-doped activated CoFe2O4@CNTs was successfully synthesized using carbamide and sodium thiosulfate as N and S precursors and CNTs as a substrate via a convenient two-step hydrothermal activation procedure, which presents good performance for supercapacitor. The characterization results indicate that activity S and N atoms can be successfully doped into the framework of CoFe2O4@CNTs with little impact on the inner morphology and structure. However, the electrode material of N2S1eCoFe2O4@CNTs exhibits a superior electrochemical performance with 1053.60 F g1 at 1 A g1 in KOH electrolyte due to synergistic effects between spineltype metal oxides, heteroatoms and sp2 lattice of graphitic carbon. In addition to high energy and power densities, the capacitance retention of charging-discharging reaches 93.15% at a current density of 30 A g1 after 5000 cycles, exhibiting an outstanding cycle stability and potential lifetime in an alkaline electrolyte. © 2020 Elsevier Ltd. All rights reserved.
Keywords: CoFe2O4/CNTs S N-doped Binder-free Electrode material Supercapacitor
1. Introduction In modern society, pursuing more powerful electric power and green sustainable energy resources have become global challenges with the increasing depletion of fossil fuel and the serious environmental pollution [1e4]. In the circumstance, electrochemical supercapacitors (SCs) are highly important energy storage systems with long cycle life, rapid charge-discharge rate, large power density and high rate capacity compared with batteries, thus has again attracted significant interest in the past a few years in energy storage devices, such as emergency power supplies, pulse energy for laser weapons, vehicle starting power and short-term power boosters,etc [5e8]. Carbon, one of the most promising energy storage materials,
* Corresponding author. 29 Guangrong dao, hongqiao District, Tianjin, 300130, China. E-mail address: [email protected] (F. Meng). https://doi.org/10.1016/j.carbon.2020.02.050 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
has provoked great interest in recent years for use as electrodes in SCs, due to its high specific surface area (SSA), low cost, high electrical conductivity, excellent cyclic stability, wide operating temperature and superior mechanical [9e11]. So far, shaped carbon nanomaterials (SCNMs), such as graphene, carbon fibres (CF), carbon sphere (CS) and carbon nanotubes (CNTs), have been widely explored as alternative supports due to unavailability of active site on some supports coupled with the poor interactions between reactants and active sites [12]. In the case of CNTs, onedimensional allotrope of carbon, has notably shown short ion diffusion distance, larger surface area and high interface areas, which is helpful for fast electron transport and tuneable when employed as support. Recently, the spinel-type metal oxides nanoparticles (NPs) have been introduced as the supported pseudocapacitive materials, showing substantial enhancement in capacitance storage performance due to their high energy density at a high chargedischarge rate, robustness at the higher potential and reversible and rapid Faradaic surface reactions [5,13]. Further, carbons
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doped by heteroatoms (e.g., N, S, and so on) are considered as one of the promising candidates for electrode materials, since C atom in the sp2 lattice of graphitic carbon substituted by heteroatoms can change the electronic arrangement and tailor their electron donor property effectively [14e19]. In addition, dual heteroatom-doped with different electronegativity can further improve the electrochemical performance as synergistic effects are able to create unique electronic coupling structure on the sp2 hybridized carbon. Especially, sulfur and nitrogen with strong synergistic effects have successfully attracted extensive attention due to their superior ability to replace carbon atoms [20e23]. Inspired by these factors, herein, we have developed the N and S co-doped into the framework of CoFe2O4@CNTs as an efficient and durable electrode material for supercapacitor with power densities and high capacitance retention due to the cooperative effect between spinel-type metal oxides, heteroatoms and sp2 lattice of graphitic carbon. The framework of binder-free CoFe2O4@CNTs is fabricated by the hydrothermal activation procedure, followed by annealing in argon atmospheres. It has been found that the spinel-type CoFe2O4 were deposited on the surface of CNTs uniform and extensive without surfactant, while a negligible and loose CoFe2O4 forms detachment the substrate. Subsequently, the N and S doping ratio, level and form in the CoFe2O4@CNTs as secondary template can be readily tuned by a simple control of molar ratio of reactants. As prepared NxSy-CoFe2O4@CNTs exhibits the superior enhanced stability and bifunctional activity than the CoFe2O4@CNTs, CoFe2O4, CNTs and some other materials [24e26]. As a result, the NxSy-CoFe2O4@CNTs nanocomposites are one of the most suitable electrode materials with desirable specific capacitance, outstanding stability and excellent redox supercapacitive performances, indicating it a promising electrode for supercapacitor. 2. Experimental 2.1. Materials Cobalt acetate (C4H6CoO4$4H2O, purity: 99.5%), ferric acetate (C4H7FeO5, purity: 98.5%), sodium thiosulfate (Na2S2O3$5H2O, purity: 99.0%), carbamide (CH4N2O, purity: 99.95%), sodium borohydride (NaBH4, purity: 98%) and sodium dodecylbenzenesulphonate (SDBS, C18H29NaO3S, purity: 99.5%) were supplied by Aladdin (China). All reagents materials were analytical-grade and used as received without further purification. The CNTs (carbon content: 99.8%) were purchased from Cyberelectrochemical materials Co. Ltd (Tianjin, China). 2.2. Synthesis of activated CNTs and binder-free CoFe2O4@CNTs The preparation procedures of CoFe2O4@CNTs and NxSyCoFe2O4@CNTs are schematically illustrated in Fig. 1. In brief, 0.3g of CNTs were mixed in the SDBS solution (30 wt%; 50 mL), stirred for 2 h, sonicated for 40 min, allowed to stand for 24 h, filtered, washed and dried in a mild condition. Next, 0.382g of C4H7FeO5 and 0.249g C4H6CoO4$4H2O were dissolved in 40 ml of ultrapure water. After thorough mixing, 0.12 g pretreated active CNTs were added to the solution by continuously stirring for 1h under a magnetic stirrer, followed by being added NaBH4 solution (20 wt%; 5 mL) and stirred for 2h. After that, the mixture was transferred to a 100 ml Teflonlined stainless steel autoclave, sealed tightly, and heated at 150 C for 12 h. Then, the black precipitate was collected after washed and vacuum dried. Subsequently, the CoFe2O4@CNTs nanocomposites were obtained by calcining the collected powders in shielding gas (Ar) atmosphere at 700 C for 2 h in a muffle furnace.
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2.3. Synthesis of NxSy-CoFe2O4@CNTs In order to obtain the N1S1eCoFe2O4@CNTs, 3.723g of Na2S2O3$5H2O and 0.3003g CH4N2O with different quality were dissolved in 40 ml of ultrapure water, followed by being added 0.12g prepared CoFe2O4@CNTs powders and stirred for 2h. Afterwards, the mixture was transferred to a 100 ml Teflon-lined stainless steel autoclave, sealed tightly, and heated at 180 C for 16 h. Finally, the obtained black hydrogel was subsequently washed with distilled water for several times, filtered and dried to obtain N1S1eCoFe2O4@CNTs nanocomposites. Similarly, the samples were named based on the molar ratio of nitrogen and sulfur, such as N2S1eCoFe2O4@CNTs, N3S1eCoFe2O4 @CNTs, N1S2eCoFe2O4@CNTs and so on.
2.4. Characterization The X-ray diffraction (XRD, CuKa, l ¼ 0.15406 Å, Bruker D8 Advance) patterns were characterized at a scanning speed of 6 / min in the 2 q range from 10 to 90 . Raman spectra of the samples were acquired by a DXR Microscope Raman spectrometer. The chemical vibration mode was determined by Fourier transform infrared spectroscopy (FTIR) (Bruker, Vector 22, Germany) in the range of 400 cm1 to 4000 cm1 by means of KBr disc technique. Surface functional groups and bonding characterization were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB-250Xi, America) system using monochromatized AI-Ka X-ray source. The morphology observation and crystalline structure of the as-obtained products were investigated by field emission scanning electron microscopy (FESEM, NovaNanoSEM450, American FEI Company) and transmission electron microscopy (TEM, JEM-2010FEF, Japan JEOL), respectively. The correlative elemental analysis was performed with an energy dispersive X-ray spectroscope (EDS) attached to the FESEM. The magnetic properties were investigated by vibrating sample magnetometer (VSM, 7407, America LakeShore) running under 20,000 Oe field, respectively. The thermal gravimetric analysis (TGA) curves were obtained using instrument of LLCSDTQ-600 (America TA Instruments-water) at a heating rate of 5 C/min from room temperature to 1000 C in flowing air.
2.5. Electrochemical tests All the electrochemical studies were evaluated employing a three-electrode system by an electrochemical workstation (CHI660E, Chenhua, shanghai). The working electrodes were fabricated as follows: a slurry containing the carbon black, polyfluortetraethylene (PTFE) and active materials with a weight ratio of 1:1:8 was well mixed, and then was pressed on nickel foams (1.0 cm 1.0 cm) and dried at 50 C for 48 h in vacuum. The mass loading of active materials on each current collector was 3 mg. The electrochemical measurements were carried out in 1 M KOH aqueous solution with a three-electrode system where the modified nickel foam, platinum foil electrode and a standard Hg/HgO electrode acted as working, counter and reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurements and electrochemical impedance spectroscopy (EIS, frequency ranging from 0.01 KHz to 100 kHz at 5 mV amplitude) were investigated by the potential range of negative 1-0.1V. The corresponding specific capacitance was calculated based on the following equation:
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Fig. 1. Schematic illustrations of the preparation of CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs nanocomposites. (a) CNTs were mixed in the SDBS solution and dried to obtain active CNTs. (bec) fabrication of the CoFe2O4@CNTs nanocomposites by hydrothermal activation and high-temperature sintering procedure. (ced) the preparation process and structure of NxSy-CoFe2O4@CNTs nanocomposites. . (A colour version of this figure can be viewed online.)
Cm ¼
0:8I Dt m DV
where Cm (F g1) is the specific capacitance, I (A) is the discharge current, Dt (s) is the full discharge time (s), m (g) is the electrode active material mass and DV (V) is the potential voltage, respectively. The energy and power densities were calculated as follows:
1 Cm DV 2 E¼ : 2 3:6 P¼
3:6E Dt
where E (Wh kg1) is the energy density and P (Kw kg1) is the power density, respectively.
3. Results and discussions The crystal structure and phase composition were studied by XRD. As shown in Fig. 2(a), well-resolved diffraction lines in the patterns of CNTs and CoFe2O4@CNTs have a distinct characteristic diffraction peaks at 2q ¼ 26.08 , corresponding to the lattice planes
(002) of CNTs (PDF#41e1487). Moreover, the curves of CoFe2O4 and CoFe2O4@CNTs also exhibit a series of lattice plane peaks at 18.29 , 30.08 , 34.04 , 36.26 , 43.06 , 53.45 , 56.97 and 62.59 , which are assigned to (111), (220), (311), (400), (422), (511) and (440) reflections of CoFe2O4 (PDF#22e1086), respectively. In addition, the XRD patterns of CoFe2O4@CNTs show no other diffraction peaks, indicating their high purity and crystallinity. After N and S modified on the surface of CoFe2O4@CNTs, as shown in Fig. 2(b), all NxSyCoFe2O4@CNTs synthesized products have two distinct characteristic diffraction peaks at 26.18 and 33.95 , which are ascribed to (002) and (311) crystal planes of pure CNTs and CoFe2O4. Furthermore, it cannot effectively differentiate the pyrite-phase products due to the isostructural with one another and very similar lattice constants, particularly in the case of FeS2 and CoS2. XRD reaches its resolution limit and therefore the positions of the peaks are not corresponding to the standard card of CoS2 or FeS2. Hence, the five samples show other diffraction peaks indexed as crystallographic planes of CoS2 and FeS2 (PDF#41e1471 and PDF#42e1340), demonstrating that polymetallic sulphide was formed during the reaction, which is consistent with the report of Matthew S. Faber.et al. [27]. In order to analyze the disordered degrees and crystal defects of the samples, the Raman spectra of CNTs, CoFe2O4, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs are shown in Fig. 2(c). The typical peaks of pure CNTs were observed at 1346 cm1 and 1574 cm1, which
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Fig. 2. XRD diffraction patterns of (a) CNTs, CoFe2O4, CoFe2O4@CNTs and (b) NxSy-CoFe2O4@CNTs samples. (c) Raman spectra of the as-prepared samples (200 cm1e2000 cm1). (d) FTIR patterns of the as-prepared samples (400 cm1e4000 cm1). (A colour version of this figure can be viewed online.)
corresponded to the well-documented D and G bands of graphitic carbon, respectively. The prominent D-band is related to the vibration of sp3 carbon atoms stemming from the degree of lattice disorder and defects, while the G peaks is ascribed to the in-plane stretching mode of sp2 carbon domains in a graphitic 2D hexagonal lattice. Generally, the integral intensity ratios of D-band to G-band (ID/IG) are usually used to evaluate the disorder degree of defect density [28,29]. Furthermore, both NxSy-CoFe2O4@CNTs and CoFe2O4 exhibit a series of small peaks at 288 cm1, 458 cm1 and 665 cm1, corresponding to the characteristic peaks of metal oxides. In addition, there is also a distinct peak at 364 cm1 in all NxSyCoFe2O4@CNTs samples, which is caused by the metal polysulfides [27,30]. Compared with the CNTs, the ID/IG ratio of the CoFe2O4@CNTs varies from 0.391 to 0.804, indicating the great disordering of the crystal structure, which may be caused by the introduction of Co and Fe inserted into the inside cavity with it deposited on the surface of the CNTs simultaneously. In addition, the values of intensities ratios of ID/IG are increased from 0.899, 0.959 and 0.977 for N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs after the doping of the different molar
ratio of nitrogen and sulfur, representing the enhanced disorder and defects with increasing ratios, which illustrates C atom in graphitic carbon substituted by heteroatoms [31]. The constitutions of spinel structure and chemical properties of as-obtained CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs were investigated by FTIR spectra. As shown in Fig. 2(d), the strong peak at about 3440 cm1 in all as-obtained samples is assigned to the typical eOH stretching vibration, which can result from surface absorption of water molecules. In addition, a characteristic peak of CNTs, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs at 1627 cm1 corresponds to the typical bending vibrations of C]C, indicating the graphitization of the as-prepared samples [32]. Furthermore, there is a strong absorption peak at about 585 cm1 of CoFe2O4, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs, corresponding to the typical metal-oxygen bond in spinel type crystal structure. Notably, the FTIR spectra of NxSy-CoFe2O4@CNTs also exhibit distinct absorption peaks at 425 cm1, 1012 cm1, 1139 cm1 and 1383 cm1, which are attributed to the CoS2 & FeS2, CeN stretching vibrations, CeO deformation vibrations and CeS bending vibrations, respectively. The main unconspicuous band corresponds to the CeO
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Fig. 3. XPS spectra of N1S2eCoFe2O4@CNTs nanocomposites. (a) survey scan, (b,c,d,e,f) C 1s scan, Fe 2p scan, Co 2p scan, N 1s scan and S 2p scan. (A colour version of this figure can be viewed online.)
deformation vibrations due to the residues of the incompletely removed oxygen-containing functional groups of CNTs and the incorporation of physicochemically adsorbed oxygen. Moreover, the characterization data of CeN and CeS have demonstrated C
atom in graphitic carbon bond with heteroatoms, indicating successful coating surface of N and S on the CoFe2O4@CNTs. The bonding characteristics and functional groups of the N1S2eCoFe2O4@CNTs were evaluated by XPS, which are shown in
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Fig. 4. FESEM and HRTEM images of samples. SEM images of (a) pristine CNTs, (b,c), (d) N1S1eCoFe2O4@CNTs at low magnification and high magnification. (e,f) HRTEM images of N1S1eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
Table S1 and Fig. 3. In brief, the spectra distinctly show the peaks of C 1s, Fe 2p, Co 2p, N 1s and S 2p, which confirms the successful formation of CoFe2O4 nanoparticles on surface of CNTs and indicates the successful N and S doping with the hydrothermal method. The spectrum for a C 1s can be deconvoluted into three peaks located at 284.7 eV, 285.8 eV and 287.4 eV, representing for the sp2 CeC, CeS & CeO and CeN &C]O [33]. For the Fe 2p spectra, it can be divided into several peaks at 706.4 eV and 707.2 eV within Fe 2p3/2 region, and 718.3 eV, 720.5 eV and 711.3 eV within Fe 2p1/2 and satellite peaks regions, which reveals the formation of CoFe2O4 [34e36]. Moreover, the deconvolution of Co 2p spectra can be deconvoluted into binding energies of 779.6 eV, 780.2 eV, 782.3 eV and 785.6 eV within Co 2p3/2 and satellite peaks regions, and 794.6 eV, 795.5 eV, 798.3 eV and 803.6 eV within Co 2p1/2 and satellite peaks regions [37,38]. In the N 1s spectra, the four fitted peaks are assigned at 398.5 eV, 400.4 eV, 401.3 eV and 404.8 eV, indicating pyridinic-N, pyrrolic-N, graphitic-N and oxidized
pyridinic-N doped in the CoFe2O4@CNTs, respectively [39,40]. Additionally, the high-resolution S 2p can be assigned to five peaks centered at 161.9 eV, 162.8 eV, 163.4 eV, 164.8 eV and 168.8 eV, associated with SeCo, SeFe, 2p3/2 CeSeC, 2p1/2 CeSeC and C-SOXC, respectively, and further convinced the successful doping of N and S atoms into the CoFe2O4@CNTs framework [41,42]. The morphology and microstructure features of the CNTs, CoFe2O4@CNTs and NxSy-CoFe2O4@CNTs were characterized by FESEM and FETEM in Fig. 4 and Fig. S1, respectively. As can be seen from Fig. 4(a), the as-spun CNTs show 1D fibrous morphologies with an average diameter of 150 nm and smooth surface. As shown in Fig. 4(b), the N1S1eCoFe2O4@CNTs inherits the continuously fibrous morphology of the as-spun CNTs with smooth surface and shows successful coating surface of N and S on the CoFe2O4@CNTs with slight detached CoFe2O4. In Fig. 4(c) and (d), the images of N1S1eCoFe2O4@CNTs demonstrate a relatively muddledness and roughness appearance with several ultra-quantity small-sized
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Fig. 5. Elemental mapping images of N1S1eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
Fig. 6. The hysteresis loop of CoFe2O4@CNTs, N2S1eCoFe2O4@CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs nanocomposites. (A colour version of this figure can be viewed online.)
CoFe2O4 NPs and activated heteroatoms of N and S doping on the surface of CNTs whose interlamellar spacings are clearly discernible. As can be seen from Fig. S1(A) and Fig. S1(a), we can see that the CoFe2O4 is distributed on the main body of CNTs uniform and extensive. Fig. S2 displays the TGA curves of CNTs and CoFe2O4@CNTs. The CoFe2O4 content (29.3 wt%) in CoFe2O4@CNTs could be calculated through the curves. Importantly, the muddledness and roughness structures are clearly visible on the exposed CNTs surface from Fig. S1(B) to Fig. S1(e), implying the presence of substantial CoFe2O4 NPs and N and S heteroatoms doping. In addition, the randomly three-dimensional interconnected network built of CNTs, CoFe2O4 and heteroatoms with uniformly diameter ranging from 150 nm to 200 nm facilitates electron transport, which helps to improve the rate performance of
electrode. Moreover, the substrate CNTs in the NxSy-CoFe2O4@CNTs nanocomposites with a large surface area can provide new electroactive sites due to the strong synergistic interactions between the individual CoFe2O4 NPs and CNTs surface, and can also prevent CoFe2O4 NPs agglomeration [43e45]. Fig. 4(e) and (f) clearly reveal the typical HRTEM images of N1S1eCoFe2O4@CNTs, in which the CoFe2O4 nanoparticles and N, S coating layer are uniformly embedded in the 1D carbon NTs matrix. The interplanar spacing of the adjacent fringes is measured approximate 0.209 nm and 0.255 nm, corresponding to the (400) and (311) crystal planes of CoFe2O4, respectively. In addition, the adjacent lattice spacing in the CNTs substrate is calculated to be 0.3348 nm, which corresponds to adjacent (002) plane of the sp2 carbon. Apparently, a thin N, S coating layer with thickness approximate ranging from 3 nm to 10 nm coats on the surface of CNTs substrate and active carrier CoFe2O4 NPs, which is conducive to further strengthening the bonding effect between exterior CoFe2O4 NPs, N, S heteroatoms and CNTs [46]. The element distribution of N1S1eCoFe2O4@CNTs was also determined by EDX mappings in Fig. 5. Obviously, we can see more intuitively from element mapping that uniform constituent elements of O, Fe, Co, N, and S are distributed on the main body of CNTs, demonstrating that CoFe2O4@CNTs and CNTs substrates are well coated by N, S heteroatoms in hybridization process. The hysteresis loops of the CoFe2O4@CNTs, N2S1eCoFe2O4 @CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs were investigated by VSM at room temperature. As can be seen from Fig. 6, all of the magnetization curves show a normal S-shape type, which is a ferrimagnetism characteristic. Furthermore, the CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs show saturation magnetization Ms 80.2 emu/ g, 71.9 emu/g, 58.8 emu/g and 54.2emu/g, respectively. In general, the saturation magnetization are mainly dependent on the magnetic grain size, grain composition, particle content and crystal orientation. Notably, the saturation magnetization decreases along with the increase of N, S heteroatoms content, indicating that the tetrahedral sites and octahedral sites in spinel structure of CoFe2O4 NPs could be occupied by N, S heteroatoms, which is in accordance with that of XRD patterns of NxSy-CoFe2O4@CNTs. Ultimately, the
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Fig. 7. Electrochemical performance of the CoFe2O4@CNTs nanocomposites electrodes for supercapacitors. (a) CV curves at different scan rates and (b) GCD curves at different current densities. (c,d) EIS spectra of as-prepared samples. (e) Corresponding specific capacitance as a function of current density. (f) cyclic stability of CoFe2O4@CNTs at a current density of 20 A g1 for 5000 cycles. (A colour version of this figure can be viewed online.)
decrease of saturation magnetization for higher concentration of N, S heteroatoms may be attributed to the formation of second phase, which decreases the resultant magnetic moment and thereby the magnetzation [47].
The electrochemical performances of CoFe2O4@CNTs were performed by cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) measurements at different scan rates (10 mV s1-200 mV s1) using 1 M KOH aqueous solution as
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Fig. 8. Electrochemical performance of the NxSy-CoFe2O4@CNTs nanocomposites electrodes for supercapacitors. (a) CV curves of N2S1eCoFe2O4@CNTs at different scan rates. (b) CV curves at 100 mV s1 for CNTs, CoFe2O4, CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs, respectively. (c) GCD curves of N2S1eCoFe2O4@CNTs
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electrolyte in a three-electrode system. As shown in Fig. 7(a), the CV curve of CoFe2O4@CNTs exhibits nonrectangular-like shape, which may be result from the cooperation of pseudo-capacitance nature and double-layer capacitance behavior. In addition, all the curves exhibit a dominant pair of redox peaks, indicating an obviously faradaic redox reaction behavior occurring in the working process, which are the characteristics of the extraction and insertion of protons, respectively [48]. The corresponding mechanism can be interpreted based on the following equation:
CoFe2 O4 þ OH4CoOOH þ 2FeOOH þ e CoOOH þ OH4CoO2 þ H2 O þ e Moreover, the area surrounded magnifies and remains a homologous shape with voltage sweep rate increasing, which means the enhancement of the capacitance value and an excellent rate capability with reversible and fast capacitive characteristics at high sweep rate. The galvanostatic charge-discharge curves of CoFe2O4@CNTs at different current densities are shown in Fig. 7(b). Apparently, every discharge time is approximate to the charging time of GCD curves even at different current densities among 1 A g1 to 8 A g1, exhibiting no obvious ohmic drop for all discharging curves [49]. Electrochemical impedance spectroscopy (EIS) is an effective way to get more information about electrochemical performances and capacitive behavior. As shown in the Nyquist plots of Fig. 7(c), the charge transfer resistance (Rct) of CoFe2O4@CNTs is 7.78 U, indicating a favorable ion diffusion process and excellent charge-transfer kinetic between residual oxygen-containing functional groups from CNTs and the redox reactions of CoFe2O4 NPs [7,50]. Furthermore, the intrinsic ohmic resistance (Rs) is 2.11 U, demonstrating efficient electron transportation with loading of CoFe2O4 NPs. As shown in Fig. 7(e), the specific capacitance of CoFe2O4@CNTs is 615.5 F g1, 537.6 F g1, 488.2 F g1, 421.3 F g1, 417.8 F g1 and 412 F g1 with an increase of current density from 1 A g1 to 8 A g1. The charging and diffusion of ions get longer time at the lower current densities, which enables the most of the active surface area to get charged respective to the discharging time, hence the capacitance gets increased with the current density decreased [51]. Especially, the capacitance retention of charging-discharging retains 94.96% after 5000 cycles of 20 A g1, exhibiting an excellent cyclic stability and potential lifetime in an alkaline electrolyte [52]. Similarly, the electrochemical properties of the NxSyCoFe2O4@CNTs were evaluated in 1 M KOH under the same conditions. As shown in Fig. 8(a) and Figs. S3(a and c), the CV curves of N2S1eCoFe2O4@CNTs, N1eCoFe2O4@CNTs and S1eCoFe2O4@CNTs display nonrectangular-like shape and a dominant pair of redox peaks at scan rates of 10 mV s1-200 mV s1, demonstrating the cooperation of pseudo-capacitance and double-layer capacitance nature of the charge-discharge process. The as-obtained CV curves at scan rates of 100 mV s1 of CNTs, CoFe2O4, CoFe2O4@CNTs, N1S1eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S2eCoFe2O4 @CNTs are shown in Fig. 8(b). These results suggest that the asprepared N1S2eCoFe2O4@CNTs nanocomposites electrodes could deliver high capacity and reversibility, showing that the content of molar ratio of nitrogen and sulfur in the composite is most appropriate for the full utilization of CoFe2O4 and N, S heteroatoms [34,53]. Fig. 8(c) and Figs. S3(b and d) display the selected GCD curves for N2S1eCoFe2O4@CNTs, N1eCoFe2O4@CNTs and S1eCoFe2O4@CNTs hybrid electrodes at different current densities. The approximately
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symmetric charge-discharge curves were observed at all current densities, exhibiting excellent electrochemical capacitive property and outstanding electrochemical reversibility [54]. The GCD curves of all the samples are compared at a current density of 1 A g1 in Fig. 8(d), indicating that the N1S2eCoFe2O4@CNTs electrode displays improved capacitive performance, which is in accordance with that of CV curves. EIS tests were conducted to get the intrinsic mechanism of the asprepared samples, as shown in Fig. 8(e) and Fig. S3(e). In addition, Table S2 summarizes the specific capacitance (Cm), energy density (E), intrinsic ohmic resistance (Rs) and charge transfer resistance (Rct) of all the electrode materials. The charge transfer resistance (Rct) of the nanohybrid electrodes decrease slightly from 14.84 U to 2.46 U for CNTs and N1S2eCoFe2O4@CNTs, demonstrating the enhanced electrochemical performance with the CoFe2O4 and N, S co-doped into the framework of CNTs, which is benefit from the combination of fast ion diffusion as well as low electron-transfer resistance [54]. Moreover, the slope of the straight line at low frequency is significantly steeper for the N1S2eCoFe2O4@CNTs electrode than for the other electrodes, clearly indicating a high electrolyte diffusion rate [55]. As shown in the specific capacitances in Fig. 8(f), Fig. S3(f) and Table S3, the N1S2eCoFe2O4@CNTs exhibits a high capacitance of 1213 F g1, 1119 F g1, 1030 F g1, 978 F g1, 936 F g1, 913 F g1 and 900 F g1 at 0.5 A g1, 1 A g1, 2 A g1, 3 A g1, 4 A g1, 6 A g1 and 12 A g1, respectively, which is mainly due to more electroactive site for diffusion efficient provided by the high surface area and optimal porous textures with the increase of N, S heteroatoms content. Encouragingly, the N1S2eCoFe2O4@CNTs electrode retains 80.4% of its initial capacitance even at a very high current density of 12 A g1, implying excellent rate capability and high reversible redox reaction. Additionally, the cyclic stability of different electrode materials at the same current density (20 A g1) are compared in Figs. S3(g and h) and Fig. S4. The N, S doping level of N1S3eCoFe2O4@CNTs and N3S1eCoFe2O4@CNTs are relatively high with a N/S ratio, which are the higher and closer to the value of 96.11% and 95.47%, implying that a superior electrochemical performance should possess a relatively high ratio of doped N and S. Furthermore, as shown in Fig. 8(g), the capacitance retention remains 93.92%, 93.15% and 92.7% of N1S2eCoFe2O4@CNTs, N2S1eCoFe2O4@CNTs and N1S1eCoFe2O4 @CNTs asymmetric supercapacitor at a current density of 30 A g1 for 5000 cycles, respectively, revealing the excellent cycling stability and potential lifetime in an alkaline electrolyte [52,56].
4. Conclusion In summary, various molar ratio of nitrogen and sulfur Codoped activated CoFe2O4@CNTs nanocomposites have been directly synthesized by substrate-assisted hydrothermal activation procedure and high-temperature sintering technique. The saturation magnetization decreases along with the increase of N, S heteroatoms content, indicating that the tetrahedral sites and octahedral sites in spinel structure of CoFe2O4 NPs could be occupied by N, S heteroatoms, which decreases the resultant magnetic moment and thereby the magnetzation. However, the NxSyCoFe2O4@CNTs electrode materials exhibit a high capacity, good rate capability and excellent cyclic stability with the increase of N, S molar ratios, demonstrating that dual heteroatoms synergistic effects are able to create unique electronic coupling structure on the sp2 hybridized carbon and spinel-type metal oxides nanoparticles, which can further improve the electrochemical performance.
at different current densities. (d) GCD curves of as-prepared samples at a current density of 1 A g1 and (e) corresponding EIS spectra of as-prepared samples. (f) specific capacitance at various current densities and (g) corresponding cyclic stability in 30 A g1 for 5000 cycles of N2S1eCoFe2O4@CNTs, N1S1eCoFe2O4@CNTs and N1S2eCoFe2O4@CNTs. (A colour version of this figure can be viewed online.)
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Hence, the distinctive structure and favorable electrochemical properties make the NxSy-CoFe2O4@CNTs materials constructed in this work a promising candidate towards the next generation highperformance supercapacitor. Declaration of competing interest There are no conflicts of interest to declare. CRediT authorship contribution statement Yujin Li: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. Cuimeng Song: Conceptualization, Formal analysis, Data curation. Jinchao Chen: Investigation. Xueni Shang: Data curation, Formal analysis. Jinping Chen: Investigation, Data curation. Yun Li: Investigation. Min Huang: Investigation. Fanbin Meng: Resources, Writing - original draft, Writing - review & editing, Project administration, Funding acquisition. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51371075) and scientific research project of Hebei Education Department (Grant No. E2019202143). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2020.02.050. References [1] X. Zhang, C.Y. Wang, Y.N. Chen, Z. Zhou, Binder-free NiFe2O4/C nanofibers as air cathodes for Li-O2 batteries, J. Power Sources 377 (2018) 136e141. [2] Y.H. Xiao, D.C. Su, X.Z. Wang, L.M. Zhou, S.M. Fang, In suit growth of ultradispersed NiCo2S4 nanoparticles on graphene for asymmetric supercapacitors, Electrochim. Acta 176 (2015) 44e50. [3] Y.L. Xu, B. Ren, S.S. Wang, Z.F. Liu, Carbon aerogel-based supercapacitors modified by hummers oxidation method, J. Colloid Interface Sci. 27 (2018) 25e32. [4] A.M. Wang, H.L. Wang, S.Y. Zhang, C.J. Mao, Controlled synthesis of nickel sulfide/graphene oxide nanocomposite for high-performance supercapacitor, Appl. Surf. Sci. 282 (2013) 704e708. [5] Y.F. Zhang, S.J. Park, Incorporation of RuO2 into charcoal-derived carbon with controllable microporosity by CO2 activation for high-performance supercapacitor, Carbon 122 (2017) 287e297. [6] S. Saha, P. Samanta, N.C. Murmu, T. Kuila, J.H. Lee, Electrochemical functionalization and in-situ deposition of the SAA@rGO/h-BN@Ni electrode for supercapacitor applications, J. Ind. Eng. Chem. 52 (2017), 321e708. [7] X.Y. Xie, X.J. He, H.F. Zhang, F. Wei, Interconnected sheet-like porous carbons from coal tar by a confined soft-template strategy for supercapacitors, Chem. Eng. J. 350 (2018) 49e56. [8] V. Veeramani, R. Madhu, S.M. Chen, M. Sivakumar, C.T. Hung, N. Miyamoto, NiCo2O4-decorated porous carbon nanosheets for high-performance supercapacitors, Electrochim. Acta 247 (2017) 288e295. [9] F. Li, Y.Y. Li, X. Yang, X.X. Han, X.P. Xu, G.G. Nie, Highly fluorescent chiral N-Sdoped carbon dots from cysteine: affecting cellular energy metabolism, Angew. Chem. Int. Ed. 57 (2018) 2377e2382. [10] M.A. Rafi, T.N. Narayanan, D.P. Hashim, N. Sakhavand, R. Shahsavari, R. Vajtai, Hexagonal boron nitride and graphite oxide reinforced multifunctional porous cement composites, Adv. Funct. Mater. 10 (2013) 1e7. [11] S.N. Liu, J. Wu, J. Zhou, S.Q. Liang, Mesoporous NiCo2O4 nanoneedles grown on three dimensional graphene networks as binder-free electrode for highperformance lithium-ion batteries and supercapacitors, Electrochim. Acta 176 (2015) 1e9. [12] A.H. Labulo, B. Omond, V.O. Nyamori, SuzukieMiyaura reaction and solventfree oxidation of benzyl alcohol by Pd/nitrogen-doped CNTs catalyst, J. Mater. Sci. 23 (2018) 15817e15836. [13] A. Sivanantham, P. Ganesan, S. Shanmugam, A synergistic effect of Co and CeO2 in nitrogen-doped carbon nanostructure for the enhanced oxygen electrode activity and stability, Appl. Catal. B Environ. 237 (2018) 1148e1159. [14] H.H. Zhang, X.Q. Liu, G.L. He, X.X. Zhang, W.H. Hu, Bioinspired synthesis of nitrogen/sulfur co-doped graphene as an efficient electrocatalyst for oxygen reduction reaction, J. Power Sources 279 (2015) 252e258.
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