Assembly of porous NiO nanowires on carbon cloth as a flexible electrode for high-performance supercapacitors

Abstract

Porous NiO nanowire layers were deposited on conductive carbon cloth and are used as flexible electrodes for high performance supercapacitors. The unique electrode structure facilitates the accommodation of the volume change, increases the active surface area, improves the conductivity and achieves rapid electrolyte diffusion, resulting in high electrochemical performance.

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In recent years, increasing power and energy demands for next-generation portable and flexible electronic devices have raised critical requirements for the energy storage devices.^1–6 Among all the energy storage devices, supercapacitors have gained considerable interest because of their high power density, long cycle life, low cost, environmental friendliness and safety.^7,8 However, the traditional supercapacitors are unable to fully meet the mentioned requirements due to their large volume, low energy density, heavy weight and non-deformability.^9–12 Therefore, it is important to fabricate and design flexible supercapacitors with high electrochemical performance for the development of portable and flexible electronic devices.^11,13–18

Carbon cloth with high conductivity and hierarchically porous structure shows great potential in the flexible supercapacitors.^19–22 However, its low surface area results in undesirable electrochemical performance due to the limitation of electrical double-layer mechanism. Compared to carbon materials, pseudocapacitor materials possess lots of attractive properties such as high theoretical capacitance and energy density.^8,23–26 Although some reported work have indicated that composite electrodes constructed by pseudocapacitor materials and carbon cloth can greatly enhance the electrochemical performance.^27–29 However, the improvement achieved by nanostructure engineering, such as changing the particle size, morphology and porous structure, is still unsatisfactory in terms of specific capacitance, energy density and cycling stability.²⁷ Therefore, further rationally tuning the structure and morphology of pseudocapacitor material/carbon cloth composite electrode materials is still one of the most challenge research work to advance the development of flexible supercapacitors and their corresponding electrodes.

In the present study, porous NiO nanowire layers were successfully deposited on carbon cloth, which are used as flexible electrodes for high performance supercapacitors. The conductive carbon cloth greatly enhances the conductivity of the composite electrode and facilitates the electron transportation by bridging the adjacent NiO nanowires. The unique one-dimensional morphology and porous structure of NiO nanowires facilitate to achieve high active surface area and repaid electrolyte diffuse. Moreover, the porous structure of NiO effectively accommodated the volume change during the charge–discharge processes, resulting in high cycling stability.

The schematic illustration of the preparation of NiO/carbon cloth (NiO/CC) electrode was shown in Scheme 1. In the first step, NiCl₂ reacting with urea produces Ni₂(OH)₂CO₃·4H₂O nanowire arrays on the carbon cloth during the hydrothermal process (Scheme 1). The formation mechanism can be described by the following formulaes:³⁰

CO(NH)₂ + 3H₂O → 2NH₃·H₂O + CO₂ (1)

NH₃·H₂O → NH₄⁺ + OH⁻ (2)

CO₂ + 2OH⁻ → CO₃²⁻ + H₂O (3)

2Ni²⁺ + 2OH⁻ + CO₃²⁻ + 4H₂O → Ni₂(OH)₂CO₃·4H₂O (4)

Scheme 1 Schematic illustration of the preparation process of NiO/CC electrode.

In the second step, the Ni₂(OH)₂CO₃·4H₂O nanowires are in situ transformed into porous NiO nanowires on carbon cloth after calcination at 300 °C in N₂.

Fig. 1a shows the XRD pattern of the NiO precursor on carbon cloth. Several strong diffraction peaks were clearly observed at 17.2°, 25.9°, 28.9°, 33°, 35.5° and 62.9°, which are well indexed to Ni₂(OH)₂CO₃·4H₂O (JCPDS no. 38-0714).³⁰ As shown in Fig. S1b,† the XRD pattern of the calcined product shows some new diffraction peaks at 37.2°, 43.2°, 62.8°, 75.3° and 79.3°, which can be well indexed to the (111), (200), (220), (311) and (222) crystal planes of NiO (JCPDS no. 73-1523).³¹ The crystal size of NiO was calculated to be 4.6 nm according to the Scherrer formula.

Fig. 1 XRD patterns of the (a) Ni₂(OH)₂CO₃·4H₂O/carbon cloth and (b) NiO/CC electrode.

Fig. 2a shows that the carbon cloth's hierarchically porous structure was constructed by many adjacent micron-sized carbon fibers. Fig. 2b and c show the smooth surface of carbon fibers in carbon cloth. After the hydrothermal growth of Ni₂(OH)₂CO₃·4H₂O nanowires on carbon cloth, the structure of carbon cloth remained unchanged, indicating it has excellent chemical stability (Fig. 2d). The Ni₂(OH)₂CO₃·4H₂O nanowire layers firmly and fully coated the carbon fibers when close observing the surface of carbon fibers (Fig. 2e and f). As shown in Fig. 2g, the morphology and structure of NiO/CC electrode is similar to that of Ni₂(OH)₂CO₃·4H₂O/carbon cloth. The NiO nanowires kept their precursor's initial morphologies during the calcination process (Fig. 2h and i). Fig. 2j shows that the one-dimensional NiO nanowires with a diameter of around 100 nm. The high magnification TEM image in Fig. 2k clearly shows many small pores inside NiO nanowires due to the aggregation of small primary NiO nanoparticles with interparticle connections. The particle size of primary NiO nanoparticles was around 5 nm, which is consistent with the result of XRD analysis (Fig. 1). Some reported work indicated that the pores inside NiO nanowires resulted from the volatile H₂O and CO₂, which are generated during the thermal decomposition process of Ni₂(OH)₂CO₃·4H₂O.³² In order to gain further insight into the NiO nanowires, a high-resolution TEM image is shown in Fig. 2l. The lattice fringe of an interplane distance was measured to be 0.21 nm, corresponding to the d-spacing between adjacent (200) crystallographic planes of NiO.³³

Fig. 2 SEM images of (a–c) carbon cloth, (d–f) Ni₂(OH)₂CO₃·4H₂O/carbon cloth and (g–i) NiO/CC. (j and k) TEM and (l) high-resolution TEM images of porous NiO nanowire arrays on carbon cloth.

The NiO/CC electrode could be used as the binder- and conductive-agent-free electrode for flexible supercapacitors due to its unique morphology, structure and deformability. In order to investigate the effect of electrode structure on the electrochemical performance, Fig. 3a compares the CV curves of NiO/CC, NiO nanowire and carbon cloth electrodes at the same scan rate of 5 mV s⁻¹ within a potential window of 0.0–0.5 V. The CV curve area of NiO/CC electrode is almost 1000 and 2 times larger than that of bare carbon cloth and NiO electrodes, respectively, indicating that the unique electrode structure of NiO/CC electrode created a synergistic effect, which greatly enhanced the electrochemical performance. The NiO/CC electrode showed a pair of redox peaks at ∼0.25 V and ∼0.35 V, which may originate from the following reaction in alkaline electrolyte:^31,34–36

NiO + zOH⁻ ↔ zNiOOH + (1 − z)NiO + ze⁻

where z (z = 0–1) represents the fraction of nickel sites involved in the electrochemical process. The value of z reflects the material utilization of the electrode active material.³⁶ Fig. 3b shows the CV curves of NiO/CC electrode at different scan rates from 2 to 50 mV s⁻¹. The shape of CV curves was almost unchanged, indicating that the NiO/CC electrode has an ideal capacitive behavior. Fig. 3c shows that the specific capacitances of NiO/CC and NiO electrodes at different scan rates. The specific capacitances of NiO/CC electrode slightly changed with increasing scan rates and higher than that of bare NiO electrode. The maximum specific capacitance was determined to be 652 F g⁻¹ at a scan rate of 2 mV s⁻¹. As shown in Fig. 3d, the NiO/CC electrode exhibited the GCD curves with a nearly triangular shape at different current densities. No significant potential drop appeared at different current rates, indicating a rapid I–V response and an excellent electrochemical reversibility of the NiO/CC electrode during the charge–discharge processes.

Fig. 3 (a) CV curves of NiO/CC, NiO and carbon cloth electrodes. (b) CV curves of NiO/CC electrode at different scan rates. (c) Specific capacitances of NiO/CC and NiO electrodes at various scan rates. (d) GCD curves NiO/CC electrode at different current densities.

The NiO/CC electrode repeated the charge/discharge cycles at a current density of 13.7 A g⁻¹ to evaluate its cycling stability. Fig. 4 shows that the capacitance retention slightly decreased and then tended to be stable. No significant change was observed in the typical GCD curves during repeated cycles in the GCD curves, indicating the high cycling stability (Fig. S1†).³⁴ The capacitance retention reached around 92% of its initial capacitance after 10 000 cycles. The capacitance retention of NiO/CC is significantly higher than some previously reported NiO electrode materials: for example, 86% for NiO nanowires after 1000 cycles, 74% for NiO nanoparticles after 1000 cycles, and 75% for NiO nanowalls after 4000 cycles.^37–39 Some reported work indicated that metal oxides with porous structure could effectively accommodate the large volume charge during the charge–discharge processes.⁴⁰ Therefore, the excellent stability of NiO/CC may be due to the porous structure inside the one-dimensional NiO nanowires and its unique electrode structure.

Fig. 4 Capacitance retentions of NiO/CC electrode during repeated cycles.

Since flexible electrodes usually work under the bending and stretching conditions, the stable electrochemical performance is important for their practical application. Fig. 5a compares the CV curves of NiO/CC electrode with different elongation percents from 0 to 15%. The nearly unchanged CV shape indicates that the NiO/CC electrode has stable performances under the stretching conditions. The specific capacitance of NiO/CC electrode slightly varied when the elongation percent was less than 15% (Fig. 5b). However, further increasing the elongation percents above 15% resulted in the electrode breaking up. The effect of bending on the electrochemical performance was also investigated. Fig. 5c shows that the CV curves of NiO/CC electrode with different bending times. The CV curves remained nearly unchanged during 100 times of bending. As shown in Fig. 5d, the capacitance retention was over 85% after 100 times of bending. These results indicate that the NiO/CC electrode possesses good flexibility and stable electrochemical performance.

Fig. 5 (a) CV curves and (b) capacitance retentions of NiO/CC at different elongation percents. (c) CV curves and (d) capacitance retentions of NiO/CC at different bending times. The insets in (b and d) are the digital photographs of NiO/CC electrode under stretching and bending conditions, respectively.

In summary, we demonstrated a facile method to prepare flexible NiO/CC electrode by a hydrothermal deposition method and subsequently a thermal decomposition method. The conductive and hierarchically porous electrode structure facilitates to enhance the conductivity of NiO, the diffusion of electrolyte ion, the active surface area and relieve the large volume change during the charge–discharge processes, resulting in high electrochemical capacitance and cycling stability. Moreover, the electrochemical performance of the electrode was stable under the bending and stretching conditions. It is believed that the NiO/CC electrode has great potential in the flexible energy storage devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51402048), the Fundamental Research Funds for the Central Universities, DHU Distinguished Young Professor Program and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, XRD patterns of the Ni₂(OH)₂CO₃·4H₂O/carbon cloth and NiO/CC electrode, and typical GCD curves of NiO/CC electrode during repeated cycles. See DOI: 10.1039/c6ra17123k

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