Hua Fang*a,
Gaoyun Chenb,
Lixia Wanga,
Ji Yana,
Linsen Zhanga,
Kezheng Gaoa,
Yongxia Zhanga and
Lizhen Wang*a
aSchool of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450001, PR China. E-mail: fh@zzuli.edu.cn; wlz@zzuli.edu.cn
bInstitute of Chemical Defense, Beijing, 102205, China
First published on 15th November 2018
A hierarchical film composed of Co(OH)2@carbon nanotube (CNT) core/sheath nanocables (CCNF) was generated via a simple and rapid electrophoretic deposition method. It is found that the Co(OH)2 sheath was uniformly anchored on the surface of conductive CNT core. The Co(OH)2 sheath, with a thickness of ∼20 nm, was composed of numerous very tiny nanoparticles. Such a unique nanostructure endows the CCNF with a high surface area of 126 m2 g−1 and a hierarchical porosity, resulting in a large accessible surface area for redox activity. As expected, the CCNF exhibits high specific capacitance and excellent rate performance. Its specific capacitance reached 1215 F g−1 under a low current density of 1 A g−1 and was maintained at 832 F g−1 when the current density was increased 20 times to 20 A g−1. A high capacitance retention of 99.3% was achieved after 10000 cycles at 1 A g−1. Such intriguing capacitive behavior is attributed to the synergistic effect of the CNT core and the Co(OH)2 sheath.
In order to improve the capacitive performances of the Co(OH)2 based electrodes, various carbon materials, such as carbon nanotubes (CNTs)1,10–15 and graphene,16–21 are introduced to form carbon/Co(OH)2 hybrids. Due to their high surface area and excellent conductivity, the carbon materials can act as structure matrix to enhance conductivity and to enlarge specific surface area.1 Among the various carbon materials, CNTs are an important one-dimensional (1D) carbon nanomaterial with a huge aspect ratio and a unique combination of mechanical, electrical, and thermal properties.11 Thus, several CNTs/Co(OH)2 hybrids have been reported with improved performances. For example, Mondal et al. synthesized CNT wrapped Co(OH)2 flakes by using a hydrothermal approach, which showed high specific capacitance of 603 F g−1 at 1 mV s−1 scan rate.11 Li et al. fabricated a CNT/Ni–Co hydroxide nanoflake core/shell structure by chemical bath deposition method, which exhibited a high specific capacitance of 1151 F g−1 at 1 A g−1 and an excellent high rate capability.13 The above mentioned progresses point the way forward in the search for further improvement of capacitive performances. However, most of the reported CNTs/Co(OH)2 hybrids were prepared by hydrothermal, precipitation or other complicated methods, which hinders their application.
Herein, a hierarchical film composed of Co(OH)2@CNT core/sheath nanocables(CCNF) was successfully fabricated by an facile electrophoretic deposition (EPD) approach. The obtained CCNF showed high surface area and hierarchical three-dimensional (3D) interconnected porosity. As designed, excellent electrochemical properties were achieved, presenting a low-cost and high performance material for pseudocapacitor.
For preparing CCNF, 25 mg of pretreated CNTs were ultrasonically dispersed in 200 ml of 2 M cobalt nitrate ethanol solution. A nickle foil (3 cm × 4 cm) and a platinum foil (3 cm × 4 cm) were put into the hybrid suspension as the cathode and anode, respectively. The two electrodes were kept parallel and 1 cm apart. The deposition voltage was set as 50 V and the deposition time was 300 s. After deposition, the obtained film was washed with absolute ethanol and dried at 120 °C for 2 h in an air dry oven. The experimental setup in the EPD process and the obtained CCNF electrode are shown in Fig. S1.†
XRD measurement was performed to characterize the phases of the CCNF composite. As shown in Fig. 1a, the characteristic graphitic peak at 25.6° is clearly evident, indicating the presence of CNTs. The diffraction peaks at 11.5°, 34.5° and 61.2° are consistent with the (003), (102) and (110) plane of the α-Co(OH)2, which is in well agreement with the standard pattern (JCPDS 46-0605).18 The XRD peaks are broad and weak, indicating the poor crystallinity and polycrystalline nature of the Co(OH)2.
Fig. 1b shows the TG curves of the CCNF from 25 to 900 °C in air. The first weight loss stage below 150 °C is due to the removal of adsorbed water and intercalated water. The second weight loss stage in the temperature range of 150–300 °C is attributed to the dehydroxylation of the Co(OH)2 sheath. Thereafter, the third weight loss between 300 and 500 °C should be corresponding to the complete oxygenolysis of CNT core, which is consistent with the previous reported results.11,14 As a result, the weight ratio of Co(OH)2 sheath in CCNF is ∼61.7%. This TG result is consistent with the EDS measurement. As shown in Fig. S3,† the EDS result showed that the weight ratio of Co(OH)2 sheath in CCNF is ∼67.7%.
SEM and TEM were performed to investigate the morphology and microstructure of the CCNF samples. Fig. 2a reveals that one-dimensional (1D) fibrous CCNF (with a diameter of ∼100 nm) are intertwined with each other into a three-dimensional (3D) network morphology, with an open and porous structure. Hardly any bare CNT left out, indicating that the Co(OH)2 sheath was homogeneously distributed on the sidewall of CNT core. Fig. 2b depicts that the CCNF shows a nanoporous sidewall, proving that the Co(OH)2 sheath is composed of very thin nanoparticles.
The TEM images of the CCNF were presented to demonstrate their microstructure more clearly. As seen in Fig. 2c, entire Co(OH)2 sheath is anchored on the surface of CNT, indicating the robust adhesion between Co(OH)2 and CNT substrate. The core/sheath nanocable microstructure can be further demonstrated by Fig. 2d, where a bare CNT end can be clearly found. The thickness of the Co(OH)2 sheath is ∼20 nm. As shown by Fig. 2e, the Co(OH)2 sheath is composed of number of very tiny Co(OH)2 nanoparticles, which are uniformly anchored on CNT matrix. The particle size ranges from ∼3 to ∼6 nm in diameters.
As shown in Fig. 2f, the lattice fringes with d-spacings of 1.5 Å and 3.4 Å can be well indexed to (110) plane of α-Co(OH)2 (JCPDS card no. 46-0605) and (002) plane of CNT, respectively. The grain boundary can be clearly seen in Fig. 2f, further proving that the Co(OH)2 sheath is composed of very tiny nanoparticles. The CNTs, which are interconnected with each other into a highly conductive matrix, are utilized to disperse Co(OH)2 nanoparticles uniformly. The CNT matrix can offer a fast pathway for electron transport, favouring high power performance. The very tiny Co(OH)2 nanoparticles can provide high surface area for fast electrochemical redox reactions.
As shown in Fig. 3a, the N2 adsorption–desorption isotherm of the CCNF showed a combined I/IV type adsorption–desorption isotherms with a H3 hysteresis loop, indicating the presence of slit-like pores.24 The CCNF possessed a high specific surface area of 126 m2 g−1, an average pore size of 9.3 nm and a total pore volume of 0.29 m3 g−1. As shown in Fig. 3b, the CCNF showed a wide pore size distribution ranging from 1 nm to 100 nm, which indicates that there are micropores (<2 nm), mesopore (2–50 nm) and macropore (>50 nm). Combined with the SEM images shown in Fig. 2a, conclusion can be drawn that the CCNF possesses a 3D hierarchically interconnected network structure. Such hierarchical structure is beneficial for the efficient ion transport during the charge/discharge processes, resulting in a large accessible surface area for redox activity.4 Specifically speaking, the macropores could serve as ion-buffering reservoirs to provide short diffusion distance for the diffusion of electrolyte ions, while the mesopores can facilitate ion transport to the interior of the bulk materials, which will in turn improve the availability of the micropores. The micropores can provide high specific surface, thus providing high pseudocapacitance and good power performance.24 In addition, such hierarchical nanostructure could prevent the electrode from severe structural damage and capacity loss during long-time charge/discharge test.
Electrochemical measurements were conducted to evaluate the electrochemical properties of the CCNF. Fig. 4a shows the CV curve of CCNF electrodes in the voltage range of 0–0.4 V (vs. SCE) at a scan rate of 10 mV s−1. A pair of redox peaks can be observed in the voltammogram, which indicates that the capacitance mainly comes from faradaic redox reaction of Co(OH)2. The redox reactions can be expressed as follows:25
Co(OH)2 + OH− ↔ CoOOH + H2O + e− | (1) |
CoOOH + OH− ↔ CoO2 + H2O + e− | (2) |
It is known that both rechargeable batteries and pseudocapacitors store charges via electrochemical redox reactions of the electrode-active materials. The former is limited by cation diffusion within the crystalline framework of active material, while the latter is not controlled by the diffusion process.26 CV investigation is an efficient tool to clarify this kinetic difference.26 Due to that typical capacitive behaviour or pseudocapacitive behaviour is not diffusion-controlled, the current (i) should vary linearly with the sweep rate (v) according to the equation of i = CdAv, where Cd represents the capacitance and A is the surface area of the active materials.26,27 Recently, an oriented multiwalled organic–Co(OH)2 nanotubes were reported and proved to be pseudocapacitive by CV tests.4
Thus, CV tests were performed at different scan rates to investigate the electrochemical kinetic properties. As shown in Fig. 4b and c, the anodic/cathodic peak currents were linear to sweep rates, indicating that the current response is predominately capacitive in nature. As mentioned above, the hierarchical porous nanostructure facilitates fast ion diffusion and the high surface area provides higher accessible surface area for fast surface faradic reactions. Therefore, the CCNF electrode should not be controlled by the diffusion process, which can explain its pseudocapacitive behavior.
Fig. 4d shows the GCD curves of the CCNF at different current densities ranging from 1 to 20 A g−1. The nonlinear lines also confirm the pseudocapacitance behaviour of the CCNF electrode. The specific capacitance is calculated from the GCD curves, which is depicted in Fig. 4e. The CCNF shows high specific capacitances of 1215 and 832 F g−1 at constant current densities of 1 and 20 A g−1, respectively. The capacitance retention rate reaches as high as 68.5% when the current density increased by 20 times from 1 to 20 A g−1, proving the superior rate capability of the CCNF.
The cycle stability is critical for the practical application in supercapacitors. As shown in Fig. 4f, the CCNF electrode show a capacitance retention rate of 99.3% after 10000 GCD cycles, proving its superior electrochemical stability and reversibility.
The excellent capacitive performance of CCNF is also compared with other previously reported similar materials. As shown by Table 1, the CCNF shows comparatively high specific capacitance and outstanding long cycle life stability, proving to be a promising electrode material for high performance supercapacitors.
Sample | Specific capacitance/current density | Capacitance retention rate/current density/cycle number | Reference |
---|---|---|---|
Layered α-Co(OH)2 nanocones | 1055 F g−1/1 Ag−1 | 95%/5 Ag−1/2000 | 5 |
Heterogeneous Co3O4-nanocube/Co(OH)2-nanosheet hybrid | 1164 F g−1/1.2 Ag−1 | 97.4%/1.2 Ag−1/6000 | 6 |
β-Co(OH)2 nanosheets/RGO hybrid | 3355 F g−1/1 Ag−1 | No decay/10 Ag−1/4000 | 9 |
CNT wrapped Co(OH)2 flakes | 603 F g−1/1 mV s−1 | 96%/1.5 Ag−1/1000 | 11 |
Ni–Co hydroxides/CNTs composites | 1151 F g−1/1 Ag−1 | 77%/1 Ag−1/10000 | 13 |
Co(OH)2/graphene/Ni foam nano-electrodes | 694 F g−1/2 Ag−1 | 91.9%/40 Ag−1/3000 | 18 |
Delaminated α-Co(OH)2@graphene | 567 F g−1/1 Ag−1 | 82%/1 Ag−1/2000 | 19 |
Flower-like Co(OH)2/N-doped graphene composite | 2276 F g−1/1 Ag−1 | 93.5%/10 Ag−1/2000 | 21 |
Co(OH)xCO3 with hierarchical flowery architecture | 550 F g−1/2 Ag−1 | 99.5%/5 Ag−1/1500 | 22 |
Hierarchical film composed of Co(OH)2@CNT core/sheath nanocable | 1215 F g−1/1 Ag−1 | 99.3%/1 Ag−1/10000 | This work |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07031h |
This journal is © The Royal Society of Chemistry 2018 |