Nousheen Iqbalac,
Xianfeng Wang*abc,
Jianlong Gebc,
Jianyong Yubc,
Hak-Yong Kimd,
Salem S. Al-Deyab*e,
Mohamed El-Newehyf and
Bin Ding*abc
aState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
bKey Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China. E-mail: wxf@dhu.edu.cn; binding@dhu.edu.cn
cNanofibers Research Centre, Modern Textile Institute, Donghua University, Shanghai 200051, China
dDepartment of BIN Fusion Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea
ePetrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. E-mail: ssdeyab@ksu.edu.sa
fDepartment of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt
First published on 17th May 2016
Introducing flexibility and high porosity into carbon nanofibers (CNFs) is one of the critical challenges for the next generation of multifunctional energy storage and CO2 adsorption materials. Herein, we developed an efficient strategy for the controllable fabrication of a flexible and mechanically robust Co3O4 nanoparticles (NPs) doped CNFs (CNFs-Co) hybrid membrane via electrospinning and subsequent carbonization treatment. The quantitative pore size distribution and fractal analysis revealed that the CNFs-Co possessed a tunable porous structure with high surface area of 483 m2 g−1. Therefore, it exhibited exceptional performance in CO2 capture, i.e. a high CO2 adsorption capacity of 5.4 mmol g−1 at 1 bar and room temperature. Electrochemical measurements performed on CNFs-Co for supercapacitor applications demonstrated very high capacitance of up to ∼911 F g−1 at 5 mV s−1 (76% capacitance retention after 1000 cycles) in 1 M H2SO4 solution. The successful synthesis of this hybrid membrane may also provide new insights towards the development of materials for various multifunctional applications.
Carbon materials such as graphene, carbon nanotubes,6 mesoporous carbon7 and CNFs, are among the most common materials for energy storage and adsorption. Excellent conductivity, high chemical stability, tunable structural flexibility, and cost-effectiveness were the key reasons for the selection of CNFs for the current work focusing on the synthesis of a flexible membrane. Meanwhile, a versatile and industrially feasible method, i.e. electrospinning, has achieved remarkable interest for the fabrication of CNFs in both academia and in industry.8 Significantly, the high mechanical strength and flexibility, good electrical conductivity, excellent porosity and surface area to volume ratio of cobalt oxide doped CNFs could be further tuned and enhanced when produced by electrospinning.9,10
The reported literature shows that RuO2 is considered as an ideal material for electrochemical capacitors, owing to its exceptional electron/proton conductivity with high rate capability, and huge theoretical specific capacitance.11,12 Nevertheless, its toxicity and high cost hinder its large-scale application in supercapacitors therefore, substitute electrode materials with outstanding electrochemical performance are being targeted for advanced supercapacitors.13,14 Recently, NiO, CoO, MnO2,15,16 etc. possessing attractive supercapacitive properties have been reported as substitute electrode materials for RuO2.17 Moreover, Co3O4 membranes have been found to exhibit substantial capacitance up to 3560 F g−1, having high redox reactivity.18 However, similar to other metal oxides, Co3O4 still suffers from poor electrical conductivity and limited electrochemical stability during cycles. To address these issues, nanostructured Co3O419 has been assembled with a carbon matrix to form composites in various forms for supercapacitors.
CNFs have been turned into flexible membranes to extend their range of applications, such as in numerous portable electronic facilities and adsorption materials.20–22 However, it is hard to manufacture them at a large scale due to the complicated fabrication process, and these integrated membranes are generally brittle.2,23 Although many previous approaches have been proposed to predict the flexible properties of CNFs with highly-idealized structures,24 a predictive understanding of mechanical properties and rupture behavior, based on the internal structure of CNFs have proven challenging.
Additionally, an extensive reliance on fossil fuels for energy production has ignited concerns over global warming, the most severe environmental issue of this time. Porous solid sorbents are prospective candidates for the purpose of CO2 capture including porous carbon,5 metal–organic frameworks (MOFs),25 porous polymer networks (PPNs),26 covalent organic polymers (COPs), zeolites,27,28 and amine functionalized materials.29–31 Among these sorbents, carbon based components such as CNFs have many advantages i.e., their high thermal, mechanical, and chemical stabilities, low production cost, hydrophobic properties, and ease of modification with other NPs. However, CNFs are generally brittle in nature, offering poor mechanical characteristics, high flow rate and lower efficiency for CO2 adsorption. Recently, three-dimensional (3D) metal oxide based flexible CNFs composites with interrelated paths have attracted growing attention,5 since they can display lower diffusive resistance to mass and ion transport from micropores, good flexibility and a high surface area for active site dispersion.
CNFs have been used for the controlled analysis established for a superficial mechanism of elasticity with the help of metal oxides. Different hybrid materials have thus been made from Co3O4,32 with different carbon materials. Co3O4/graphene and Co3O4/carbon nanotubes (CNTs) have been synthesized with improved conductivities of electrodes, and a large surface area between the active materials and electrolytes for efficient chemical reactions and CO2 adsorption. Apart from graphene and CNTs, electrospun CNFs have been widely applied as an electrode material for various types of energy storage applications.
In this contribution, we developed a facile in situ technique for the synthesis of flexible CNFs membranes derived from hierarchical porous CNFs embedded with Co3O4 NPs. CNFs-Co hybrid membranes were prepared by altering the relative quantity of cobalt acetylacetonate (Co(acac)2) in the precursor solution. The PAN nanofibers were carbonized at a high temperature in order to increase the electrical conductivity of the final CNFs matrix. The stepwise process of CNFs formation and FE-SEM (field emission electron microscopy) images of the pure, oxidized and doped nanofiber membranes are represented in Fig. 1. The resulting CNFs-Co exhibited a large surface area and high flexibility with an exceptional CO2 capturing capacity of 5.4–6.9 mmol g−1 at 25 °C and 0 °C (1 bar), also offering a reasonable specific capacitance of 911 F g−1 (in 1 M H2SO4), showing good enough potential to be used in supercapacitors as well.
Cs = Q/mΔU, | (1) |
(2) |
Cs = S/2VmΔU, | (3) |
It can be seen that for electrodes undergoing similar testing parameters (the same mass, potential window, and scan rate), where m is the mass of the two electrodes. For cyclic voltammetry, the specific capacitance was taken as,36,37
(4) |
In this equation Vh is the high voltage cut-off, VI is the low voltage cut-off, and v is the scan rate.
Fig. 2 FE-SEM images of CNFs with different salt contents of (a) 0, (b) 2, (c) 3, and (d) 4 wt%. (e) Digital images presenting the robust flexibility of the CNFs-Co-4 membranes. |
Although previous reports have proposed to predict the flexible properties of CNFs with highly-idealized structures, the poor mechanical properties and rupture behavior of CNFs are still big challenges to overcome. Here, we use organized analysis to establish an apparent mechanism of elasticity for the nanostructures of CNFs membranes, that has been revealed in Fig. 2e. The Co3O4 nanoparticles and the carbon layers are uniformly distributed in the carbon matrix, forming an organized composite nanostructure as explained previously (see movie 1 in ESI†). The external stress on the CNFs membranes leads to bending distortion of the fibers.34 For pure CNFs, the pressure is concentrated on the bending area, resulting in rapid extended cracks across the fiber matrix, and finally causing the fracture of fibers (for pure CNFs, see movie 2 in ESI†).
Nitrogen adsorption–desorption measurements at 77 K were conducted to investigate the porous textures of the as-prepared CNFs. Fig. 3a shows that the isotherms of all samples demonstrate typical adsorption/desorption behavior, including micropore filling, monolayer adsorption, multilayer adsorption and capillary condensation.35,36 For pure CNFs, the majority of nitrogen adsorption occurred at a low relative pressure (P/P0 < 0.1), and a plateau appeared at a middle relative pressure, demonstrating a typical microporous structure with few mesopores. A slight decrease of the adsorption plateau could be attributed to the distortion of micropores. To further investigate the formation of the porous structure of the samples in detail, the specific surface area, total pore volume, average pore diameter and pore size distribution were determined using the BET surface area method (Fig. 3). Table 1 shows the specific surface areas of conventional CNFs samples for CNFs-Co-2, CNFs-Co-3, and CNFs-Co-4, i.e. ∼131, 416, 457 and 483 m2 g−1, respectively, indicating the major contributing role of Co NPs on determining the specific surface area.
Fig. 3 (a) N2 adsorption–desorption isotherms, and (b) 2D-NLDFT pore size distribution curves of relevant CNFs-Co materials. |
Samples | SBETa (m2 g−1) | Smesob (m2 g−1) | Vtotalc (cm3 g−1) | Vmesod (cm3 g−1) | PVFmesoe (%) | Davf (nm) |
---|---|---|---|---|---|---|
a Total surface area was calculated by the BET method.b Mesopore surface area was calculated by the Barrett, Joyner, and Halenda (BJH) method.c Total pore volume was estimated was calculated at P/P0 = 0.99.d Vmeso was calculated by the BJH method.e PVFmeso indicates the pore volume fraction of mesopores.f Average pore width was estimated by the BET method (4V/A by BET). | ||||||
Pure CNF | 131 | 43 | 0.7 | 0.1 | 14 | 2.81 |
CNFs-Co-2 | 416 | 307 | 0.30 | 0.11 | 33 | 2.91 |
CNFs-Co-3 | 457 | 340 | 0.29 | 0.12 | 34 | 2.94 |
CNFs-Co-4 | 483 | 388 | 0.3 | 0.13 | 36 | 2.95 |
On the other hand, the pore size distribution and pore volume in active materials are very significant characteristics for supercapacitors and CO2 adsorption. The enhanced mesopore volume provides a low resistance pathway and short diffusion route for ions because of the admission of a larger amount of electrolyte into the pores. Therefore, the adjustment of salt content directly effects the performance of supercapacitors. In order to comprehensively investigate the hierarchical porous structures of CNFs/CNFs-Co, pore size distribution analysis was performed using a two dimensional nonlocal density functional theory (2D-NLDFT) method.39 Fig. 3b depicts the 2D-NLDFT pore size distribution curves. As expected, CNFs-Co exhibited both microporous (pore width < 2 nm) and mesoporous (pore volume 2–50 nm) structures, more polydisperse peaks appeared in the range of 2–15 nm, and the volume of pores increased greatly indicating an efficient enhancement of the pore size. The detailed parameters of surface area and porous structures are listed in Table 1.
ln(V/Vmono) = A[ln(ln(p0/p))] + constant | (5) |
The FHH plots shown in Fig. 4 determine a diverse linear trend with different slopes in the high coverage region. The resultant D values of CNFs, CNFs-Co-2, CNFs-Co-3, and, CNFs-Co-4, were about 2.81, 2.91, 2.94, and 2.95 respectively, indicating a typical surface fractal feature. It can be recognized that the D values exhibit a change with the increment of salt content, confirming the enhancement of mesoporous structures. This phenomenon may be contributed to by three aspects: (i) the decomposition of the salts with gas release increased the porosity of the carbon matrix, (ii) a catalytic reaction between the salts and the carbon matrix occurred, resulting in the consumption of carbon,39 and (iii) the extrusion of Co nanoparticles in the carbon matrix created defects in the carbon matrix.
Fig. 4 Plots of ln(V/Vmono) against ln(ln(p0/p)) reconstructed from the N2 adsorption isotherms of the relevant CNFs-Co materials. |
To further confirm of the existence of Co3O4 in CNFs-Co and their distribution in the carbon matrix, TEM analysis with SAED was carried out. Fig. 5 shows the TEM images of CNFs-Co-2 after calcination at 850 °C, demonstrating the intact structure of individual CNFs and also a uniform distribution of Co3O4 NPs in the carbon matrix (Fig. 5a). The naturally turbostratic stacking structure revealed graphitic carbon layers parallel to each other along the axis of the fiber as well as apparent bends and branches. The ambiguous lattice fringes of the nano shell and clear diffraction rings in Fig. 5b support the presence of ordered carbon layers and Co3O4. The region below the turbostratic stacking structure displays amorphous carbon phase. CNFs-Co nanofibers are undoubtedly recognizable from the HR-TEM images, showing a lattice distance of 0.2 nm (Fig. 5c). Fig. 5d shows the carbon matrix entailed regions with different levels of carbon layers. The FE-SEM and HR-TEM images show no significant change to the morphology and internal structure after measuring the properties of optimized samples after supercapacitance and CO2 adsorption performance (see S2 in ESI†).
Fig. 5 (a) HR-TEM images of CNFs with a salt content of 2%. (b) SAED pattern, (c) showing lattice distance of CNFs-Co-2, (d) HR-TEM image showing amorphous carbon layers. |
The microstructures of the CNFs derived from various concentrations of salts were investigated using Raman spectroscopy. For all samples, pure CNFs, CNFs-Co-2, CNFs-Co-3 and CNFs-Co-4, main peaks centered at 1344, 1339, 1332, 1327 cm−1 (D bands) and 1582, 1598, 1599, 1604 cm−1 (G bands) can be observed respectively, indicating that organic substances underwent fruitful decomposition following carbonization (Fig. 6). The G band originates from the in-plane stretching motion of sp2 bonded carbon atoms, and the highly ordered graphitic structure, while the D band corresponds to the amorphous sp3 carbon amorphous carbon. The high intensity ratio means a high degree of graphitization and the relative intensities of the G and D bands (IG/ID) are used to evaluate the degree of graphitization. The values of IG/ID for pure CNFs, CNFs-Co-2, CNFs-Co-3 and CNFs-Co-4 are ∼0.86, 0.86, 0.92 and 0.97 respectively, based on the fitting curves, indicating that the carbon exists mainly in an amorphous form in the samples. The degree of crystallinity of carbon (R) calculated from the ratios of IG/ID for CNFs confirms the enhancement of the graphitization degree with the addition of salts.
Fig. 6 (a) Raman spectra of CNF fibers with various salt concentrations of (a) 0, (b) 2, (c) 3, and (d) 4 wt% and (b) representative XRD graphs of CNFs with salt contents of (a) 0, and (b) 4 wt%. |
Besides the graphitic nanofibers, embedded Co3O4 nanoparticles were also obtained from thermal decomposition, which could explain the burst-nucleation and crystal growth mechanism. Normally, this conversion would contain three related processes: firstly, the pyrolysis of Co(acac)2 generates a large amount of cobalt via a series of complicated decomposition reaction and redox reaction. Secondly, the salt concentration exceeds the critical concentration and aggregates to form clusters. Thirdly, in the burst nucleation process, the monomer concentration decreases, and nuclei gradually grow into Co3O4 nanoparticles by the consumption of monomers.27
XPS analysis also validated the growth of Co3O4 (Fig. S4†). High resolution XPS spectra of Co 2p and O 1s and C 1s for Co3O4 are shown in Fig. S4a.† Peaks at 782.4 eV for 2p3/2 and 797.2 eV for 2p1/2 show the contribution of cobalt in the doped CNFs (Fig. S4b). The O 1s peak at 540 eV is attributed to the typical metal oxygen bonds and (Fig. S4c†) the carbon 1s spectrum is used for calibration (295.5 eV), shown in Fig. S4d.†The presence of Co3O4 is further confirmed with XRD examination. Fig. 6b shows the XRD patterns of the pure CNFs and CNFs-Co-4 (see S3 in ESI† for CNFs-Co-2 and CNFs-Co-3).
As expected, it can be observed that the locations and relative intensities of the diffraction peaks matched well with carbon and standard Co3O4 patterns, indicating a successful synthesis of Co3O4 and carbon composites. The broad diffraction peak at a 2θ value of 43° can be allocated to the specific reflection (100) diffraction planes of ordered graphitic carbon. The additional broad diffraction peak at 2θ = 26° is indexed to the characteristic (002) reflection of amorphous carbon, which has further been verified by Raman characterization. This proves that the Co NPs are decomposed and carbothermally reduced to CNFs-Co. After annealing in a N2 atmosphere, three diffraction peaks at 2θ angles of about 36.2°, 51.5° and 75.8° are observed, which can be assigned to the (111), (200), and (220) reflections of the Co3O4 NPs. In addition, the size of Co3O4 NPs is calculated to be about ∼100 nm.
The electrochemical properties of the CNFs-Co flexible composite membranes are characterized by CV analysis. The CV curves of CNFs-Co-4 in the voltage range of 0–0.5 V are shown in Fig. 7. The typical CV response at various scan rates (5–15 m s−1) was measured in 1 M H2SO4 electrolyte (Fig. 7a). The rectangular shape of the CV curves suggested potential capacitive behavior. Increasing the scan rate from 5 to 15 mV s−1 resulted in decreased specific capacitances (SC) of 911, 698 and 510 F g−1, and 380, 298, and 236 F g−1 for CNFs-Co-4 (Fig. 7a) and CNFs-Co-2 respectively (see S6 in ESI†). A higher scanning rate inhibited the electrolyte ions in accessing the full surface of CNFs-Co by decreasing the diffusion time, leading to reduced capacitance. The gradual activation process of the electroactive Co3O4 NPs linearly enhanced the SC of the hybrid porous electrode during the first 200 cycles, which was even maintained after 1000 cycles. Comparing to CNFs, the improved hierarchical structure of CNFs-Co-4 facilitated electrolyte migration to the electrode, resulting in considerable rate capability, whereas the higher surface area and total pore volume of the mesoporous structure could be credited for the better capacitance (see S7 in ESI† for comparison). Fig. 7b shows the specific capacitance and scan rates of pure CNFs and CNFs-Co-4 that give the capacitance behavior at different scan rates. By increasing the scan rate, the specific capacitance decreases due to the diffusion of electrolyte.
The frequency range of EIS analysis was carried out at 0.1–10000 Hz. In the Nyquist plots, the intercept at the real part is a combination of the ionic resistance of the electrolyte, the intrinsic resistance of the substrate and active material, and the contact resistance of the active material/current collector interface. An ideal polarizable capacitance gives rise to a straight line along the imaginary axis. In our system, polarizable capacitance gives rise to a straight line along the imaginary axis that shows less resistance (Fig. 7c). The inset in Fig. 7c shows the high values of the Nyquist plot. It is intriguing to note that the equivalent series resistance (ESR) is not altered much, confirming ideal capacitive behavior with the change in salt content in CNFs-Co-4. The near 90° phase angles at high frequency show inductive behavior, while the near ∼90° phase angles at low frequency indicate capacitive behavior. It is commonly recognized that the electrochemical capacitance is dependent upon both the surface area and the electrical conductivity of the electrode material. Fig. 7d shows SC vs. cycle number for the CNFs-Co-4 membrane electrode. It is shown that the SC was measured at 1000 cycles, demonstrating cyclic stability and high specific capacitance. To further demonstrate the advantage of the synergistic effect in this electrode design, the cycling performance of the Co-CNFs at different current densities was recorded. Even though suffering from the sudden change of the current delivery, the electrode exhibits stable capacitance at each current density value. After 1000 continuous cycles at varying current densities, the capacitance was stable in Fig. 7d. This result highlights the capability of the pseudo-capacitive material-based CNFs-Co electrode to meet the requirements of both long cycle lifetime and good rate capability, which are important merits for practical energy storage devices.
The normalized capacitance retention tested at a current density of 0.5 A g−1 over 1000 cycles, is demonstrated in Fig. 8. Comparing to pure CNFs, the loading of Co3O4 resulted in higher SC retention for both CNFs-Co-4 (76%) and CNFs-Co-2 (78%). The inset is the corresponding charge–discharge curves of 30 cycles. Consequently, the good stability of the doped CNFs mainly derived from the three dimensional network structures, leads to good ion mobility. The contact between the PVA/H2SO4 electrolyte and electrode increases due to the high specific surface area.
Fig. 8 Cyclic performance of CNFs-Co-4 at the current density of 0.5 A g−1. The inset is the corresponding charge–discharge curves of 30 cycles. |
The charge curves are closely symmetric to their corresponding discharge curves in the potential range, which showed a high reversibility between charge and discharge processes. It can be seen that all the charge and discharge curves have a symmetric nature indicating a rapid current–voltage response and good coulombic efficiency. It is significant to point out that the electrospun doped CNFs show electrochemical activity (galvanic charge/discharge) in 1 M H2SO4 solution in the voltage range at 0.5 V. The superior compatibility and robust adhesion between these well-dispersed and stabilized fine Co3O4 nanoparticles and the conductive CNFs may be responsible for the SC and excellent electrochemical stability. It is challenging to significantly increase the specific surface area, electrical conductivity, stability and SC, which are achieved in this study by the incorporation of different concentrations of Co(acac)2 compared to pure CNFs, making CNFs-Co promising candidates for supercapacitors and lithium ion batteries.
Fig. 9 (a) CO2 adsorption isotherms for CNFs-Co measured at 25 °C, and (b) CO2 adsorption isotherms for CNFs-Co-4 measured at 25 °C and 0 °C. |
The CNFs doped with Co3O4 NPs possess similar volumes of micropores, and one can reason that these three samples might have similar CO2 capacities. It is clearly seen that there was a small CO2 uptake rise for CNFs-Co-4 as compared to CNFs-Co-3 and CNFs-Co-2, indicating the effect of Co3O4 NPs in the CNFs on CO2 adsorption. These values are advanced than those of many reported carbons and are comparable with the salt containing carbons. Therefore, it can be concluded that the overall CO2 adsorption of the CNFs-Co studied is a combination of two contributions: a major contribution related to adsorption in the fine micropores and a minor contribution due to the adsorption of Co3O4 NPs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06077c |
This journal is © The Royal Society of Chemistry 2016 |