Ting Zhua,
Zhen Yang Angb and
Ghim Wei Ho*abc
aDepartment of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583. E-mail: elehgw@nus.edu.sg
bEngineering Science Programme, National University of Singapore, 9 Engineering Drive 1, Singapore 117575
cInstitute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore
First published on 12th June 2015
Nickel cobalt oxides (NiCo2O4) superstructures with microsized flowers (MF), nanoflowers (NF), nanoneedles (NN), and nanospheres (NS) have been prepared by a facile self-assembly formation and subsequent conversion process. The various morphologies of the nickel cobalt precursors (NCP) are tuned by using different salts, solvents, surfactants and reaction temperatures. The as-obtained NCP can be readily converted to NiCo2O4 through an annealing process with their structures well retained. The electrochemical properties of the as-derived NiCo2O4 are assessed as the active electrode materials. The results indicate that the NiCo2O4 has exhibited promising pseudo-capacitive properties with high capacitance and good cycling stability. The NF has delivered the highest specific capacitance of 636 F g−1 (at a rate of 1 mV s−1) among all the four samples, showing its potential application for next generation electrochemical capacitors.
The surface charge storage does not require ionic diffusion within the electrode materials framework, and so generally supercapacitors exhibit fast redox reaction rates but relatively lower energy density than batteries. As a result, many efforts in the scientific fields have been devoted to improve their electrochemical performances.13,14 The capacitances and cycling performances can be enhanced by developing porous and/or smaller size particles accompanied with high specific surface area.15
Some active materials with fabulous specific capacitance and redox reaction rates have drawn great attentions from the scientists such as MnO2, NiO, Co3O4 and NiCo2O4. These transition metal oxides possess multiple oxidation states that exhibit excellent redox reaction rates and hence enhance the pseudocapacitive properties.16–24 As a binary metal oxide, NiCo2O4 is a low-cost and environmentally friendly metal combination that has a relatively higher electronic conductivity than single component metal oxides because of the redox contribution from both metals.25–28 The electrochemical performances of NiCo2O4 are reported to have a high dependence on the morphology.29–33 Feng et al. compiled various work on nickel based material and in their report.34 Lou et al. reported spinel mesoporous NiCo2O4 nanosheets with a specific capacitance of 2010 F g−1 at a scan rate of 2 mV s−1.35 Zou et al. synthesized 3D NiCo2O4 micro-sphere with specific capacitance of 1284 F g−1 at 2 A g−1 scan rate too.36 In a report by Lei et al., 3D hierarchical flower-shaped NiCo2O4 are fabricated with specific capacitance of 1006 F g−1 at a scan rate of 1 A g−1.37 However, the morphologies exploration based on facile hydrothermal self-assembly process that is tunable to obtain different structured materials have not been well-explored. In this work, the different morphologies of NiCo2O4 leading to different electrochemical properties have been investigated. The as-derived porous and unique materials are used as electrode materials in supercapacitors, where the sample NF exhibits the best performance among all the samples.
Ni(Ac)2·4H2O (g) | Co(Ac)2·4H2O (g) | Urea (g) | SDS (g) | Solvent (20 mL) | Temperature | |
---|---|---|---|---|---|---|
Micro-flowers | 0.125 | 0.25 | 0 | 0 | EtOH![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
180 °C for 6 h |
Nano-needles | 0.125 | 0.25 | 0.5 | 0 | EtOH![]() ![]() ![]() ![]() |
90 °C for 12 h |
Nano-flowers | 0.125 | 0.25 | 0 | 0.1 | EtOH![]() ![]() ![]() ![]() |
90 °C for 12 h |
Nano-spheres | 0.05 | 0.1 | 0 | 0 | IPA | 180 °C for 6 h |
Fig. 1 shows the FESEM images of all the NCP synthesized from different reaction conditions and surfactants. Fig. 1a was observed to present interconnected micro size sphere-like flower structures with diameters up to 10 μm. Fig. 1b presents needle-head nanorods with over 1 μm in length and 50 nm in width. Nanoflower-like structures are obtained by changing the volume ratio of ethanol and water in the presence of SDS (Fig. 1c). When the solvent was changed to sole IPA, we are able to produce the nanospheres with porous surfaces (Fig. 1d). It should be noted that the reaction during the hydrothermal synthesis is quite complicated due to the different solvents and surfactants used. The various morphologies appeared in the final products are the results by co-effect of solvents and surfactants. It is plausible that the solvents and surfactants have been served as the soft templates for the growth of NCP with different structures.
NiCo2O4 can be obtained by annealing the as-formed NCP samples at 300 °C for 2 hours in air to remove impurities and defects in lattice structures. Fig. 2 shows the FESEM images of all the four NiCo2O4 samples. The microsized flower-like structures (MF) have been well retained after calcination (Fig. 2a and b), which shows the robustness of the structures. It can be observed that the nanoflower (NF) particles are composed of ultrathin nanosheets with morphologies well preserved after heat treatment (Fig. 2c and d). Some broken nanoneedles (NN) have been seen in Fig. 2e and f, which may be due to the high aspect ratios of the structure. The porous NS clusters have no significant change after the calcination except some agglomerations after combustion (Fig. 2g and h). The as-obtained NiCo2O4 samples were then examined by XRD (Fig. 3) in order to obtain the crystallographic details. All the indexed peaks can be attributed to (111), (220), (311), (400), (422), (511) and (440) planes of the cubic NiCo2O4 (JCPDS 73-1702).38 No peaks of impurities have been found in the samples, which reveals the complete conversion of the NCP to the nickel cobalt oxides. The lowest crystallinity of MF sample can be observed among all the four samples, and this could be due to the largest particle sizes of MF, which makes it difficult for the heat treatment to achieve a high crystallinity at the same calcination temperature.
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Fig. 2 FESEM images of the NiCo2O4 with different morphologies: MF (a and b), NF (c and d), NN (e and f), and NS (g and h). |
In order to further study the morphology of the as-obtained NiCo2O4, the NF and NN samples were selected for the TEM characterization (Fig. 4). The NF sample is revealed to be composed of ultrathin nanosheets (Fig. 4a) with porous texture seen in Fig. 4b, which is consistent with the previous FESEM findings (Fig. 2d). The NN sample is observed to have a uniform rod-like body and a sharp tip (Fig. 4c). Its higher resolution TEM image (HR-TEM) presents the fringes with a spacing of 2.45 Å that can be attributed to the (311) plane of NiCo2O4.38,39 Both of the two samples are verified to be polymorph due to their circular structures from the SAED pattern (insets in Fig. 4b and d). Furthermore the EDX patterns shown in Fig. 4e and f indicate the presence of Ni and Co elements in the oxides, which has further strengthened the XRD results in Fig. 3. It should be noted that the presence of Cu and Pt peaks is attributed to that the EDX was performed on Cu substrate and the samples had been sputtered by Pt to further improve their electrical conductivity in order to obtain a clearer FESEM images.
The porous textural characteristics of the NiCo2O4 samples were examined by BET at 77 K. Fig. 5 shows the N2 adsorption–desorption isotherms of all the four samples and their corresponding pore size distributions obtained from respective desorption branches are displayed as insets. These type IV isotherms possess small hysteresis loops observed at a relative pressure of 0.4–1 for Sample MF (a), NF (b), and NN (c) and 0.8–1 for NS (d) with a similar pore size distribution around 3–5 nm (insets). The BET specific surface areas of MF, NN, NF and NS are determined to be 58, 73, 95 and 89 m2 g−1, respectively. It can be noted that NF shows the highest surface area than all the other samples, which could provide more channels for the access of hydrated electrolyte ions.40 These porous NiCo2O4 materials are then used as active materials for supercapacitor electrodes, where the electrochemical performances have been evaluated and compared.
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Fig. 5 BET results of NiCo2O4 samples MF (a), NF (b), NN (c) and NS (d). The insets show the corresponding pore size distributions, respectively. |
Fig. 6 illustrates the CV curves performed at different scan rates (1, 2, 5, 10, 20 mV s−1) with potential window ranging from 0 to 0.6 V (vs. SCE). Distinct redox peaks occurred at 0.3–0.45 V and 0.4–0.55 V can be observed clearly, which reveals the pseudocapacitive property of the electrodes. The different CV shapes of sample NF could be attributed to the different content of polarization of the electrode materials.41 The relevant redox reactions can be described as the following equation:42,43
NiCo2O4 + OH− + H2O ↔ NiOOH + 2CoOOH + e− |
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Fig. 6 CV curves of NiCo2O4 samples performed at different scan rates: MF (a), NN (b), NF (c) and NS (d). |
The specific capacitances can be calculated by the following equation:
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Fig. 7 Specific capacitances of NiCo2O4 samples calculated from CV curves with different scan rates. |
The charge–discharge is also conducted with various current densities using the same potential windows for all the four samples (Fig. 8). The charge–discharge curves displayed have conformed the pseudocapacitive characteristics, which is consistent with the previous CV results. Fig. 9 presents the graph of specific capacitances delivered at different current densities (2, 3, 5, 10, 20 A g−1). The highest specific capacitance among all the samples is delivered by the sample NF at current density of 2 A g−1 (566 F g−1). While a much lower value at 173.75 F g−1 is recorded at the same current density for sample MF, which demonstrates the sample trend with CV results. The highest values for the other two samples are calculated to be less than 200 F g−1, which shows poorer performance than the NF sample.
The highest capacitance of sample NF can also be revealed from the EIS result (Fig. 10). The internal resistance of the samples can be calculated from the range where high frequency semicircle intercepts with the x axis. It can be observed that the semicircle at high frequency range of NF is smaller, which indicates their lower charge-transfer impedance than all the other samples. Meanwhile, NF has a relatively straight vertical line in the low frequency range, indicating their better capacitive behavior and lower diffusion impedance.46
All the fabricated electrodes are also tested for prolonged cycles to evaluate the stability. The CV measurements were repeated for 1000 cycles (10 mV s−1), and the capacitances calculated from the cycles were displayed in Fig. 11. There is no distinct capacitance decay for all the four samples, which can be credited to their good cycling ability. The slightly increase of the capacitance in the first few hundreds cycles can be observed from all the samples. This increase can be attributed to the porous texture of the samples. The high porosity surface has allowed the electrolyte ions to diffuse into the inner area via different sizes of porous channels, which will lead to the electrode activation process and hence the capacitances are observed to be increased in the first few hundreds cycles.19 Sample NF has exhibited the highest specific capacitance due to its porous surface and high specific surface area by larger amount of electroactive sites for the redox reactions.47,48 It should be noted that the Ni foam template will only contribute ∼1.2% of the total capacitances of all the samples according to our control experiment, and this means majority of the capacitances are delivered from the electroactive materials.
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