Zheyin Yu,
Zhenxiang Cheng*,
Zhixin Tai,
Xiaolin Wang,
Chandrasekar Mayandi Subramaniyam,
Chunsheng Fang,
Shaymaa Al-Rubaye,
Xiaotian Wang and
Shixue Dou
Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2500, Australia. E-mail: cheng@uow.edu.au; Fax: +61 2 4221 5731; Tel: +61 2 4298 1406
First published on 4th May 2016
In this work, NH4F was used as a vital additive to control the morphology of Co3O4 precursors on Ni foam in a conventional hydrothermal reaction, and then, via thermal decomposition, to obtain Co3O4 material. The amount of NH4F plays a pivotal role in the formed morphology of the Co3O4 precursors, and four morphologies of Co3O4 were obtained through close control of the amount of additive: nanowires, thin nanowire-clusters, thick nanowire-clusters, and fan-like bulks. The morphological evolution process of the Co3O4 precursors has been investigated according to their intermediates at different reaction stages, and some novel growth mechanisms are proposed: (1) the amount of NH4F in the solution system affects the chemical composition of the precursors; (2) with an increasing amount of NH4F in the solution system, the morphology will tend to form more ordered states and more distinct hierarchical structures; (3) with an increasing amount of NH4F in the solution system, the growth of products will tend to form denser structures; (4) the amount of NH4F in the solution system will affect the mass loading of products. The four different morphologies of Co3O4 were tested as free-standing electrode materials for supercapacitor application. Co3O4 with the thin-nanowire-cluster morphology exhibits the best electrochemical performance: the specific area capacitance is 1.92 F cm−2 at the current density of 5 mA cm−2 and goes up to 2.88 F cm−2 after 3000 charge–discharge cycles, while the rate capability is 72.91% at the current density of 30 mA cm−2.
Nowadays, energy storage and conversion materials have become an important research subject because of the increasing global demand for energy. Compared with Li-ion batteries, supercapacitors have the unique merits of good pulsed charge–discharge characteristics, high power density, and long lifetime, making them applicable and indispensable in many portable systems and hybrid electric vehicles.19,20 Co3O4 has been considered as a promising electrode material for supercapacitors owing to its high theoretical capacitance, well-defined redox activity, great reaction reversibility, low fabrication cost, natural abundance, and environmental friendliness.21
A free-standing high specific capacitance electrode should be the most promising type of electrode for potential practical applications in the future, because such electrodes allow highly efficient industrial production, since the active materials do not need to be mixed with carbon black (conductive agent) or polyvinylidene difluoride (PVDF) binder. Among the many synthesis methods for Co3O4, hydrothermal reaction could satisfy the above requirements, and it also could be suitable for large-scale production.21 The mass loading of Co3O4 on Ni foam through hydrothermal reaction is limited, however, which renders relatively low area capacitance. NH4F could be a potential promoter of mass loading on the current collector during the hydrothermal reaction, while which could affect the morphology.
In this report, four morphologies of Co3O4 on Ni foam were obtained through conventional hydrothermal reaction to fabricate the precursor, and then, via thermal decomposition reactions, the final product. During the hydrothermal reaction, different amounts of NH4F were applied as an additive, and it was found that the NH4F plays a pivotal role in forming the morphology of the Co3O4 precursors, so that four different morphologies were obtained: nanowires, thin nanowire-clusters, thick nanowire-clusters, and fan-like bulks. Furthermore, the morphological evolution process of the Co3O4 precursors was investigated according to their intermediates in different reaction stages, and some novel growth mechanisms are proposed: firstly, the amount of NH4F applied in the solution system affects the chemical composition of the precursor. With a low amount of NH4F, the precursor is Co(CO3)0.5(OH)·0.11H2O, while with medium and high amounts of NH4F, the precursor is CoF1.3(OH)0.7. Secondly, with increasing amounts of NH4F in the solution system, the morphology will tend to form more ordered states and more distinct hierarchical structures, from nanowires to thin nanowire-clusters, to thick nanowire-clusters, and then to fan-like bulks. Thirdly, with increasing amounts of NH4F in the solution system, the final products tend to form denser structures. Nanowires tend to stack together with the medium amount of NH4F, and the gaps among nanowires are reduced significantly compared with the sparsely distributed nanowires that are obtained with a small amount of NH4F. With a high amount of NH4F, the nanowires are replaced by nanosheets, which further promote a dense structure. Fourthly, the amount of NH4F in the solution system affects the mass loading of products on the Ni foam substrate. On introducing a medium amount of NH4F into the solution system, the mass loading on Ni foam will increase notably compared with that with a low amount of NH4F, although the mass loading on Ni foam does not increase linearly as the amount of NH4F increases, and a high amount of NH4F suppresses further increases in the mass loading on Ni foam.
The electrochemical performances of the four different morphologies of Co3O4 as free-standing electrode materials were tested for supercapacitor application. Co3O4 with the thin-nanowire-cluster morphology exhibits the best electrochemical performance among the four electrodes: the specific area capacitance is 1.92 F cm−2 at the current density of 5 mA cm−2, rising to 2.88 F cm−2 after 3000 charge–discharge cycles, and the rate capability is 72.91% at the current density of 30 mA cm−2. Its high specific area capacitance makes it an ideal potential supercapacitor electrode for practical application.
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Fig. 1 SEM images of the samples: Co3O4-1 (a and b), Co3O4-2 (c and d), Co3O4-3 (e and f), and Co3O4-4 (g and h). |
In Fig. 2, the XRD patterns reveal the crystal structure and phase purity of as-obtained Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4. The two highest intensity peaks at 44.5° and 51.8° are ascribed to the (111) and (200) planes of metallic nickel (JCPDS no. 04-0850), respectively. The peaks at 31.4°, 36.9°, 59.3°, and 65.2° in the four patterns belong to the (220), (311), (511), and (440) planes of Co3O4 (JCPDS no. 42-1467).
Further morphology and structure characterizations were conducted by TEM, as shown in Fig. 3. In Fig. 3(a, c, and e), the TEM images reveal the basic construction units to be nanowires in Co3O4-1, Co3O4-2, and Co3O4-3, respectively. It can be seen that these nanowires are highly porous, being composed of interconnected nanoparticles approximately 10–25 nm in size. The diameters of these nanowires are in the range of 100–150 nm, which is in accordance with those observed in the SEM images. From Fig. 3d, it can be seen that the basic construction unit of Co3O4-4 is the nanosheet, with a size in the range of 300–500 nm. The nanosheets display some wrinkling. From the HRTEM images in Fig. 3(b, d, f, and h), the (111), (220), and (400) interplane spacings of 0.47, 0.29, and 0.21 nm respectively correspond to the different crystallographic planes of Co3O4, as indicated, which also matches well with the XRD diffraction results.
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Fig. 3 TEM images of Co3O4-1 (a), Co3O4-2 (c), Co3O4-3 (e), and Co3O4-4 (g); HRTEM images of Co3O4-1 (b), Co3O4-2 (d), Co3O4-3 (f), and Co3O4-4 (h). |
Based on the above descriptions, it could be concluded that the amount of NH4F affects the morphology of the Co3O4 precursor during the hydrothermal reaction process. The most direct reason is that the compositions of the precursors for the different samples are different. From the XRD patterns of these precursors after the 6 h hydrothermal reaction (Fig. S1 and S2 in ESI†), the precursor with the lowest amount of NH4F (1 mmol) is Co(CO3)0.5(OH)·0.11H2O, while the precursors with medium amounts of NH4F (5 and 10 mmol) and the highest amount of NH4F (20 mmol) are CoF1.3(OH)0.7, which could explain our observations that introducing medium amounts of NH4F (5 and 10 mmol) or a high amount of NH4F (20 mmol) makes the morphology evolution process more complicated. The composition of Co3O4-1 precursor was further investigated by measuring a FTIR spectrum from 400 to 4000 cm−1 (Fig. S3 in ESI†). The peaks at 3501 and 1518 cm−1 are ascribed to O–H stretching vibration and bending vibration, respectively, the peaks at 1383 and 1070 cm−1 arise from CO32− asymmetrical and symmetrical stretching vibration, respectively, and the peaks at 834 and 693 cm−1 are assigned to out-of-plane and in-plane bending vibration of CO32−, respectively.22 From some previous reports, it is generally considered that F− anions could slow down the nucleation rate and activate the substrate, because F− ions can coordinate with Co2+ ions at the initial stage and then release the Co2+ ions slowly,23 but from this work, when there are medium amounts of NH4F (5 and 10 mmol) or a high amount of NH4F (20 mmol) in the solution system, the precursor product is CoF1.3(OH)0.7, which could explain the greater complication of the morphology evolution process. With a medium (5 or 10 mmol) or high (20 mmol) amount of NH4F in the solution system, the F− not only takes part in the reaction, but also forms the stable precursor product CoF1.3(OH)0.7. This also suggests that the formation of CoF1.3(OH)0.7 helps to reduce the nucleation rate to some extent, making the morphologies with 5 and 10 mmol of NH4F experience an evolution from bulks to nanowire clusters. It should be noted that 20 mmol of NH4F hinders the formation of nanowires, and the bulks directly evolve into nanosheets, thus reducing the interspaces among the nanowires, which further reveals that F− could slow down the nucleation rate and make the structure denser. The corresponding mechanisms for the effects of different amount of NH4F on the morphology are proposed, as shown in Fig. 5. The hydrothermal chemical reaction could be suggested as follows:
Co2+ + xF− → [CoFx](x−2)− | (1) |
H2NCONH2 + H2O → 2NH3 + CO2 | (2) |
NH3·H2O → NH4+ + OH− | (3) |
2[CoFx](x−2)− + CO32− + 2OH− + nH2O → Co2(OH)2CO3·nH2O + 2xF− | (4) |
[CoFx](x−2)− + OH− → Co(OH)F + (x−1)F− | (5) |
Subsequently, the precursors are transformed to stable Co3O4 phase in air.
3Co2(OH)2CO3·nH2O + O2 → 2Co3O4 + (3n + 3)H2O + 3CO2 | (6) |
6Co(OH)F + O2 → 2Co3O4 + 6HF | (7) |
In addition, the amount of NH4F in the reaction system plays a crucial role in the mass loading of products on Ni foam. After thermal treatment, the mass loading of Co3O4 is approximately 0.95, 6.8, 6.3, and 3.96 mg cm−2 for 1 mmol, 5 mmol, 10 mmol, and 20 mmol of NH4F, respectively. Although CoF1.3(OH)0.7 could reduce the nucleation rate and enhance the mass loading of products, a high amount of NH4F (20 mmol) still suppresses further increase of the mass loading. Optical images of these samples are shown in Fig. S4 in ESI.† In Fig. S4-a,† the differences in the colours of the precursors are due to their different morphologies and chemical compositions.
From the above analysis, this work further reveals that F− could slow down the nucleation rate during such a hydrothermal reaction process, which is in accordance with previous reports.23 Most importantly, it also elucidates a new growth mechanism: firstly, the amount of NH4F in the solution system affects the chemical composition of the precursor. Secondly, with increasing amounts of NH4F in the solution system, the growth of products will tend towards the formation of more ordered states and more distinct hierarchical structures. Thirdly, with increasing amounts of NH4F in the solution, the morphology of products will tend towards denser structures. Fourthly, the amount of NH4F in the solution system will affect the mass loading of products.
Fig. 6b shows typical charge and discharge curves of the four samples at the current density of 5 mA cm−2 within the potential range of 0–0.45 V. It can be seen that all the curves have apparent voltage plateaus in the discharging process, indicating their pseudocapacitive behaviour, although the lengths of the discharging times and discharging plateaus are different. The differences in discharging plateaus for the four electrodes also could be ascribed to their different available sites for different morphologies. Among these discharging curves, the Co3O4-2 discharging time is the longest. From the discharging curves, the specific area capacitance can be calculated according to the following equation:
CA = I × Δt/(ΔV × S) | (8) |
The capacitance per unit area of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 was 0.5, 1.92, 1.53, and 0.87 F cm−2, respectively. The Co3O4-2 electrode displays the highest capacitance among them, which could be ascribed to its high mass loading and unique morphology.
Fig. 6c presents the specific area capacitance of the Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 electrodes at various current densities, from 5 mA cm−2 to 30 mA cm−2, respectively. At the current density of 30 mA cm−2, the specific area capacitance retention is 60.00%, 72.91%, 64.70%, and 60.91% for the Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 electrodes, respectively. Among all these electrodes, the Co3O4-2 electrode displays the best specific area capacitance retention, which is also superior to previous reported similar freestanding Co3O4 material.29–31 The charge and discharge curves of the different electrodes at various current densities are shown in Fig. S9.†
In Fig. 6d, the cycling stability of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 was tested at current density of 5 mA cm−2 over 3000 cycles. After 3000 cycles, the capacitance retention was 100.00% (0.5 F cm−2), 150% (2.88 F cm−2), 107% (1.64 F cm−2), and 111% (0.97 F cm−2) for Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4, respectively, which reveals that Co3O4-2, Co3O4-3, and Co3O4-4 undergo a capacitance activation process during charge–discharge cycling. Compared with Co3O4-3 and Co3O4-4, there is an apparent trend towards increasing capacitance for Co3O4-2, which could be observed in the first 1150 cycles (from 1.92 F cm−2 to 3.16 F cm−2), with the capacitive activation process during charging–discharging similar to that in most previous reports.32–37 The capacitance retention of Co3O4-2, based on its maximum value of 3.16 F cm−2, still could amount to 91% after 3000 cycles.
According to many previous reports (Table S1†), free-standing Co3O4, NiCo2O4, or MnO2 electrodes synthesized by hydrothermal reaction methods for supercapacitors usually exhibit low specific area capacitance (approximately 0.1–0.8 F cm−2), which severely obstructs their practical application. In order to further enhance their specific area capacitance, a second step reaction was adopted to make them into “core/shell” structures, and the specific area capacitance could then reach 0.5–2.5 F cm−2.
In this work, we successfully demonstrate that NH4F could effectively control the active material mass loading on Ni foam and its product morphology through a one-step hydrothermal reaction, thereby obtaining the optimized high specific area capacitance of the Co3O4-2 electrode (1.92 F cm−2), which is an ideal potential electrode with high specific area capacitance for practical application.
Fig. S10(a and b)† show the electrochemical impedance spectroscopy plots collected before cycling and after 3000 cycles for the Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 electrodes, respectively. Fig. S10c† is the equivalent circuit for determining the electrochemical impedance. The semicircle in the high frequency region reveals the properties of the electrode surface, indicating the charge transfer resistance (Rct). After 3000 cycles, the Rct of the Co3O4-2, Co3O4-3, and Co3O4-4 electrodes all decreased dramatically, which could be the reason for their capacitance activation during cycling.38 In addition, reaction of the Ni foam with the electrolyte during cycling may be another factor behind the capacitance increase.28 Fig. S11(a–d)† shows the Bode plots of the Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 electrodes, respectively, and their phase angles reached nearly −80 degree at 0.01 Hz, which represents their ideal capacitor nature.28
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03400d |
This journal is © The Royal Society of Chemistry 2016 |