Grass-like CuCo₂O₄ nanowire arrays supported on nickel foam with high capacitances and desirable cycling performance

Abstract

Grass-like CuCo₂O₄ nanowire arrays supported on nickel foam have been successfully synthesized via a hydrothermal method. Electrochemical investigations demonstrate that the material is an excellent electrode for supercapacitors with remarkable performance including a high area capacitance of 611 F g⁻¹ at a current density of 1.7 A g⁻¹, and a desirable cycling stability of 94.8% capacitance retention after 8000 cycles.

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Supercapacitors are a new kind of energy storage device.^1–4 As yet, a variety of materials such as carbonaceous materials, metal oxides/hydroxides and conducting polymers are used as supercapacitor electrode materials.^5,6 Among the many kinds of electrode materials that are candidates for supercapacitors, ternary cobalt-based spinel oxides have been actively pursued due to their better electroactivity and superior supercapacitive properties compared to that of single cobalt oxides.^7–9 During supercapacitor research, the theoretical capacitance of a certain active material is calculated by the equation: C = nF/(ΔVM), where n is the moles of charge transferred per mole of the test substance, F is Faraday’s constant (96 485.3383 C mol⁻¹), M is the molar mass of the test substance, and ΔV is the potential sweep range. By using this equation, we found that CuCo₂O₄ has a theoretical capacitance of 984 F g⁻¹. Moreover, other features, including low cost, abundant resources, and environmental friendliness, make CuCo₂O₄ a promising pseudocapacitive electrode. Up to now, there are few reports about CuCo₂O₄ being applied as supercapacitors. In 2014, Pendashteh et al. reported CuCo₂O₄ nanoparticles with a specific capacitance of 338 F g⁻¹ at current densities of 1 A g⁻¹.¹⁰ However, owing to their high surface energy, the nanoparticles always tend to self-aggregate, which reduces the effective area of the electrode/electrolyte interface. Also, these materials suffer from severe volume variation during charge–discharge cycling, which results in serious pulverisation of the electrodes and thus rapid capacity degradation. Therefore, it is crucial to retain a large contact area to fully maintain the advantages of active materials and to inhibit the volume change of these materials during cycling. Recently, constructing three-dimensional (3D) architectures based on two-dimensional (2D) substrates has been proven to be an effective approach to solve these problems.^11–14

In this communication, we developed a cost-effective and simple strategy for the direct growth of hierarchical CuCo₂O₄ (CCO) nanowire arrays (NWAs) on a nickel foam substrate as a binder-free electrode for high-performance supercapacitors. In such structures, the CCO NWAs grown on Ni foam could effectively avoid the aggregation of active materials and provide more active sites for electrochemical reaction. The existence of the spacing between each nanowire of about 50 nm and the ultra-thin nanowires can provide a short ion diffusion path and facilitate the fast transport of ions during the charge–discharge process. In addition, Ni foam acts as an excellent backbone for loading CCO nanowire arrays and as a highly conductive matrix for them. Since Ni foam can provide pathways for electronic transport, it can serve as the current collector without conductive additives. This binder-free electrode by directly growing active materials on 3D Ni foam simplifies the procedure for electrochemical measurements, which is helpful in practical applications. The combination of these constituents is expected to provide highly effective electrochemical properties. As a result, CCO NWAs exhibit high specific capacitance (611 F g⁻¹), and excellent cycle stability (94.8% capacitance retention after 8000 charge–discharge cycles) in a 2 M KOH solution.

In order to confirm the phase structure of the products, XRD measurements were carried out. As shown in Fig. 1 all of the diffraction peaks of the CCO NWAs can be indexed to CuCo₂O₄ according to the Joint Committee on Powder Diffraction Standards (JCPDS, card no. 76-1887). Besides, three peaks around 44.3°, 51.6° and 76.1° are also observed, coming from the Ni foam (JCPDS, card no. 87-0712). No other impurity peaks were observed, implying products with high purity.

Fig. 1 XRD pattern of CCO NWAs on Ni foam.

The morphology of bare nickel foam and the CCO NWA covered Ni foam examined by SEM is shown in Fig. 2a–d.¹⁵ The submillimeter pores and nickel foam skeletons can be clearly seen in Fig. 2a. Fig. 2b–d display the SEM images of the obtained CCO NWAs on Ni foam. From the distant view, it can be seen that the Ni foam surface is uniformly covered with the product as shown in Fig. 2b. The close views of the CCO nanowires in Fig. 2c–d indicate that the nanowires lie perpendicular to the Ni foam support, forming an ordered nanoarray structure. The higher magnification SEM image (Fig. 2d) discloses the space of approximately 50 nm between nanowires, thus, most of the nanowire surface is highly accessible to the electrolyte when used as an electrode for supercapacitors.¹⁶

Fig. 2 SEM images of the morphologies of the CCO NWAs on Ni foam: (a) bare Ni foam; (b) Ni foam after the hydrothermal reaction; (c) and (d) higher resolution images of CCO NWAs.

Fig. S1a and b† are the side views of the substance under SEM. From these figures we can clearly see the previously prepared CCO NWAs with a length of approximately 3 μm and a thickness of approximately 100 nm. More distinctly, from Fig. S1c,† we learned that the CCO NWAs supported on nickel foam are just like grass growing in the ground. Obviously, such a large area of one-dimensional nanowires can greatly enhance the specific surface area of the CCO NWAs, leading directly to the increased contact area between the electrolyte and the active material. By direct growth on foam nickel, the nanowires are well-separated, making them fully available to OH⁻ in solution, resulting in high utilization of the material. Additionally, the nanoscale space between the nanowires (about 50 nm) enables a favorable morphological stability, helping to alleviate structure damage caused by the volume expansion during cycling. What’s more, the inherent contact between the nanowires and nickel foam avoids the need for binders and conducting additives that add extra contact resistance or weight.¹⁷ Thus an effective and stable pathway for charge transfer could be formed. From the above, the CCO NWAs supported on nickel foam are expected to provide high performances.

To test the electrochemical capacitive performance of the CCO NWAs, a cyclic voltammogram (CV) was firstly recorded in a three-electrode system using SCE as the reference and platinum foil as the counter-electrode in aqueous 2 M KOH at room temperature. Fig. 3a reveals the typical CV curves of the as-prepared CCO NWAs with a potential window −0.2 to 0.8 V (vs. SCE) at various scan rates ranging from 5, 10, 20, 30, 40 to 50 mV s⁻¹, respectively. All the CV curves consist of a pair of strong redox peaks, indicating that the capacitance of the material is mainly associated with Faradaic pseudocapacitors, and are quite different from the curves of electric double layer capacitors. Moreover, the shape of the CV curves was not affected by the increase of the scan rate, indicating that this unique structure favors fast redox reactions. Nearly symmetrical shapes observed under different scan rates imply an ideal pseudocapacitive behavior for the CCO NWAs.¹⁸ The specific capacitance (C) was calculated according to the following equation:²⁵

where Δu is the potential (V), v is the potential scan rate (mV s⁻¹), i is the discharging current (A), and m represents the active mass (g). Based on the equation the surface capacitance values can be calculated to be 361.41, 331.27, 294.38, 253.67, 234.00 and 211.61 F g⁻¹ at scan rates of 5, 10, 20, 30, 40 and 50 mV s⁻¹, respectively. Taking the geometrical area of the electrode of CCO NWAs into account (1 × 1 cm²), the specific capacitance values are 1.08, 0.99, 0.88, 0.76, 0.70 and 0.63 F cm⁻², respectively (Fig. S5†). For comparison, bare Ni foam without hydrothermal reaction after the annealing procedure in pure N₂ gas was also investigated in the same system. It can be found that the capacitance contribution from the Ni foam substrate is negligible, as shown in Fig. S2.†

Fig. 3 Electrochemical characterization of CCO NWAs on Ni foam: (a) CV curves at different scan rates, (b) galvanostatic charge/discharge curves at different current densities, (c) the current density dependence of the specific capacitance calculated from the discharge curves and (d) cycling performance at a current density of 5 mA cm⁻².

Fig. 3b displays the galvanostatic charge–discharge behavior of the CCO NWAs as electrodes at different current densities in the potential range between 0 and 0.6 V. The symmetry of the charge and discharge curves exhibits an excellent reversible forward and backward reaction process. The specific capacitances of the CCO NWAs at different charge and discharge current densities were calculated as shown in Fig. 3c according to the equation: C = It/(mΔV). I is the constant discharge current, t is the discharge time, ΔV is the voltage drop upon discharging and m is the mass of the active materials. The mass of the CCO NWAs was 3 mg cm⁻² and it was taken into account to calculate the specific capacitance and current density. The CCO NWA electrode exhibits a superior pseudocapacitance of 611, 504, 426, 347, 211 and 140 F g⁻¹ at current densities of 1.7, 3.4, 6.7, 10, 16.7 and 23.3 A g⁻¹, respectively. It can be seen that increasing the specific current results in a decrease in the specific capacitance, in agreement with the trend of capacitance variation calculated from CV curves.

The CCO NWAs exhibit a specific capacitance as high as 611 F g⁻¹ at a charge and discharge current density of 1.7 A g⁻¹, higher than 338 F g⁻¹ at a current density of 1 A g⁻¹ as reported by Pendashteh et al. Even at a high charge and discharge density of 23.3 A g⁻¹, a specific capacitance of 140 F g⁻¹ can also be achieved, implying that the CCO NWAs have a relatively good rate capability at a large specific current (Fig. 3c). The cycling performance is a desirable parameter in order for supercapacitors to replace the lithium-ion battery.¹⁹ A large capacitance retention over a prolonged charge discharge cycle is essential for an outstanding electroactive material. Fig. 3d shows a small capacitance loss after 3000 cycles at a current density of 5 mA cm⁻². After 3000 cycles at 5 mA cm⁻², the specific capacitance remains at 98% of the initial value, shown in Fig. 3d. For a supercapacitor, long cycle life and high rate capability are important parameters for its practical application.²⁰ Following the above 3000 cycles, the electrode was subjected to a continuous discharge–charge process for 5000 cycles at different current densities. As shown in Fig. 4a, during the 4000 cycles with the current densities increasing from 10 to 20, 30, 50 mA cm⁻², the electrode exhibits a perfect capacitance stability and the capacitance retentions are more than 94.8%. When the current density returns to 10 mA cm⁻², a capacitance of 95.8% of the initial 1000 cycles under 10 mA cm⁻² (shown in Fig. 4b, blue curve) is observed, and has a capacitance retention of 96% during another 1000 cycles. The good stability of the as-prepared CCO NWAs with high specific capacitance makes them a promising candidate for potential application in supercapacitors. Besides, it is also noticed that the specific capacitance is about 60% of the theoretical capacitance (984 F g⁻¹). These results indicate that the 1D nanowires and their separate morphology from each other on the growth substrate with a 3D configuration can effectively prevent the volume expansion, contraction and aggregation of the electroactive materials during the charge–discharge process.

Fig. 4 (a) Rate performance of the CCO NWAs on Ni foam electrodes at progressively increased current densities; (b and c) corresponding charge/discharge curves of the first 10 and last 10 cycles of the CCO NWAs on Ni foam electrodes for 5000 cycles.

We also used electrochemical impedance spectroscopic (EIS) measurements to further investigate the electrochemical behavior of the CCO NWAs at an open circuit potential with an AC perturbation of 5 mV in the frequency ranging from 0.01 Hz to 100 KHz before and after cycling. Generally, Nyquist plots contain a semicircle and a straight line.²¹ In the low frequency area, the slope of the curve shows the Warburg impedance (W) which represents the electrolyte diffusion in the porous electrode and proton diffusion in the host material.²² Fig. 5a compares the Nyquist plots of the CCO NWA electrodes before and after 3000 cycles at a current density of 5 mA cm⁻² with a well-fitted equivalent circuit showing the components of the whole impedance.²³ Apparently, the slope of the curve after cycling barely changes compared with that before cycling. In the high frequency area, the intersection of the curve at the real part Z′ indicates the bulk resistance of the electrochemical system and the semicircle displays the charge-transfer process at the working electrode–electrolyte interface.²⁴ From this plot, the impedance of the as-prepared sample changes little after 3000 cycles, which further certifies that the CCO NWAs have a lower diffusion resistance and stable electrochemical properties. Fig. 5b is the schematic illustration of the electron path of the CCO NWAs on Ni foam, which provides a considerably shorter path for electron transport and a solid structure to improve their capacitive performance. In practice, the CCO NWAs produce more ion migration channels to promote the electrolyte ion transport in three-dimensional space, not just at the edge of the CuCo₂O₄ NWAs. For comparison, the CVs, CPs and EIS of pristine Ni foam are shown in the ESI, Fig. S2 and S4.† It can be found that the capacitance contribution from the Ni foam substrate is negligible.

Fig. 5 (a) Electrochemical impedance spectra (EIS) of the CCO NWAs on Ni foam electrodes. The blue curve refers to the EIS before cycling, and the red curve is the EIS after 3000 cycles. The inset is the equivalent circuit; (b) schematic representation of the rechargeable supercapacitor based on CCO NWAs on Ni foam.

Conclusions

In summary, we have illustrated the logical design and fabrication of CuCo₂O₄ (CCO) nanowire arrays on a Ni foam substrate using a facile hydrothermal method without any surfactant and template. The CCO NWA supercapacitor electrode exhibits extraordinary electrochemical properties in terms of its cyclic stability and rate performance, and particularly, the specific capacity is 60% of the theoretical capacitance of 984 F g⁻¹. The prodigious performance is attributed to the superiority of the high specific surface ratio of the 1-D nanowire and 3-D Ni foam for growth, which makes ion transport easy within and between the electroactive materials.^25,26 The freestanding and binder-free nature of the material means it has broader applications.²⁷ To sum up, this material made from abundant, low-cost precursors along with the facile synthesis technique makes it an ideal candidate for next-generation supercapacitor electrodes. The experimental results are inspiring considering that the electrodes have more space for the development of their applications.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09962e

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