One-Step In Situ Self-Assembly of Cypress Leaf-Like Cu(OH)2 Nanostructure/Graphene Nanosheets Composite with Excellent Cycling Stability for Supercapacitors
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Transition metal hydroxides and graphene composite holds great promise to be the next generation of high performance electrode material for energy storage applications. Here we fabricate the cypress leaf-like Cu(OH)2 nanostructure/graphene nanosheets composite through one-step in situ synthesis process, employed as a new type of electrode material for high efficiency electrochemical energy storage in supercapacitors. A solution-based two-electrode system is applied to synthesize Cu(OH)2/graphene hybrid nanostructure, where anodic graphene nanosheets firmly anchor cathodic Cu(OH)2 nanostructure due to the electrostatic interaction. The in situ self-assembly of Cu(OH)2/graphene ensures good structural robustness and the cypress leaf-like Cu(OH)2 nanostructure prompt to form the open and porous morphology. The hybrid structure would facilitate charge transport and effectively mitigate the volume changes during long-term charging/discharging cycles. As a consequence, the Cu(OH)2/graphene composite exhibits the highest capacitance of 317 mF/cm2 at the current density of 1 mA/cm2 and superior cyclic stability with no capacitance decay over 20,000 cycles and remarkable rate capability at increased current densities.
KeywordsCypress leaf-like Cu(OH)2 nanostructure graphene nanosheets outstanding cycling performance
Electrical double layer capacitors
Electrochemical impedance spectroscopy
Field emission scanning electron microscopy
Galvanostatic charge-discharge measurements
High- resolution transmission electron microscopy
Selected-area electron diffraction
Scanning electron microscopy
Transmission electron microscopy
X-ray photoelectron spectroscopy
The ever depletion of fossil fuels and aggravation of environmental pollutions call for urgently exploring sustainable energy sources and developing energy storage technologies to meet application requirements of many electronic devices and hybrid vehicles in our modern society [1, 2]. As a promising energy storage device, supercapacitors (SCs) have attracted much attention in view of their small size, high power density, fast recharge ability, long lifespan and desirable operational safety [3, 4, 5, 6, 7, 8] There are two classes of SCs, pseudocapacitors and electrical double layer capacitors (EDLCs), on the basis of energy storage mechanism . Carbon material with many advantages of abundance, non-toxic, large surface area, good conductivity, excellent chemical durability, is a typical electrode material for double-layer capacitors (EDLCs), storing charge in the electric double-layer near electrolyte/electrode surface by electrostatic adsorption [10, 11, 12, 13, 14, 15, 16]. However, carbon material generally exhibits a relatively low specific capacitance. By comparison, many inexpensive transition metal hydroxides, such as Ni(OH)2 [17, 18], NiO , MnO2 , Co3O4  store energy partially relied on fast reversible Faradic redox reactions occurring on the electrode surface, offering much higher pseudo-capacitance [22, 23]. Unfortunately, most of them suffer from the intrinsic poor electric conductivity and undergo huge volume change during electrochemical processes, which results in the poor reversibility and short cycle life . Obviously, to synthesize the high-performance electrode material at a low cost, it is of great significance to combine easily available transition metal hydroxides with carbon material by a cost-effective and facile fabrication strategy.
Among various transition hydroxides, Cu(OH)2 is one of the most promising electrode material because of its natural abundance, environmentally friendly and fast redox couple [25, 26, 27]. Besides the above -mentioned characteristics of most carbon material, graphene has an exceptionally large specific surface area, whose major surfaces are exposed to the electrolyte, exhibiting a high specific capacitance (550 F/g) . To improve the electric conductivity and enhance capacity of electrode, Cu(OH)2 and graphene composite have been designed as electrode, efficiently inhibiting the volume changes of Cu(OH)2 and preventing serious agglomeration and re-stacking of graphene because the typical flexible and robust nature of graphene enable electrode materials to effectively maintain the structural integration [26, 29, 30, 31]. Mahanty et al. presented that the reduced graphene oxide/Cu(OH)2 composite, which exhibited a high capacitance of 602 F g−1 and good capacitance retention of 88.8% over 5000 cycles. Both specific capacitance and cyclic stability were dramatically enhanced, compared with pristine Cu(OH)2 . Ghasemi et al. prepared Cu2O-Cu(OH)2-graphene nanocomposite by multiple steps, including electrophoretic deposition and electrodeposition techniques, exhibited specific capacitance of 425 F g−1 and maintained about 85% of initial capacitance with a current density of 10 A g−1 after 2500 cycles . Although supercapacitive properties have been enhanced in the report, most of these approaches are complicated and expensive. Furthermore, the cycling stability of reported Cu(OH)2/graphene composite for supercapacitance needs to be further improved.
In this work, we report the one-step in situ self-assembly of cypress leaf-like Cu(OH)2 nanostructure/graphene nanosheets composite realizes in a two-electrode system, where graphene nanosheets generate from electrochemical exfoliation of graphite at anode and simultaneously Cu(OH)2 nanostructure forms on Cu foam at cathode. The morphology and structure, together with the interaction between different components of nanocomposite would influence their electrochemical energy storage properties. The transparent few-layer graphene nanosheets firmly anchor on cypress leaf-like Cu(OH)2 surface, forming a porous, open and interconnected structure. This unique hybrid structure is expected to endow this composite fast charge transfer velocity, high electrochemical activity, and excellent stability. As a result, the Cu(OH)2/graphene composite presents excellent electrochemical energy storage performance with high specific capacitance and wonderful cyclic stability over 20,000 cycles, making it an ideal electrode material for high-performance SCs.
Driven by the electric field, the exfoliated graphene nanosheets with residual negative charges at its edge were electrically attracted to the surface of cathodic Cu(OH)2, assembling into this unique porous nanostructure. The resulting cypress leaf-like Cu(OH)2 nanostructure/graphene nanosheets composite was air dried.
The X-ray diffraction (XRD) was carried out on a Rigaku Ultima IV X-ray Diffractometer by Cu Kα radiation with the scan rate of 2°min−1 over a 2θ range from 10° to 80°. Raman spectroscopy was acquired on Renishaw in a Via-reflex system, with the excitation source of a laser wavelength (532 nm). We obtain the details of morphology, structure, crystal size, and other parameters by field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus), transmission electron microscope (TEM), and selected-area electron diffraction (SAED) (JEOL JEM-2100F operating at 200 kV). The surface chemical components and valance states of the sample were researched by X-ray photoelectron spectroscopy (XPS).
, in which C (mF cm−2) represents the area capacitance, J (mA cm−2) is the current density, t (s) is the discharging time, ΔV (V) is the voltage window for cycling tests.
Results and Discussions
With increasing scan rate, CV curves maintain a similar profile and the current response increased, indicating the good rate capability and good reversibility of Faradic reactions [17, 27]. Meanwhile, the oxidation and reduction peak respectively shift to more positive and more negative potentials, due to the limited ion diffusion time or high-electron hopping resistance .
Figure 6b displays the area capacitance and galvanostatic charge-discharge curves at different current densities of 1, 2, 4, 8, and 10 mA cm−2. The galvanostatic charge-discharge curves of the composite electrode exhibit the typical pseudo-capacitive nature, which finely agrees with its CV curves. The Cu(OH)2/graphene composite achieves the highest area-specific capacitance of 317 mF cm-2 at a current density of 1 mA cm-2. The specific capacitance can maintain 303, 293, 280, 273 mF cm−2 at different current densities. The Cu(OH)2/graphene nanocomposite electrode shows a good rate capability with only 14% capacitance loss at a high current density of 10 mA cm−2, which can be ascribed to the unique nanostructure in favor of fast and efficient electrolyte ion diffusion and charge transfer .
The cycling stability of the Cu(OH)2/graphene nanocomposite electrode was studied by charging-discharging cycling measurements at the constant current density of 2 mA cm−2 (Fig. 6c). The specific capacitance till 20,000 cycles keep the initial value of 303 mF cm−2 with 100% retention, exhibiting the outstanding cycling performance. Moreover, the Coulombic efficiency can maintain 100% which further demonstrates that the electrode possesses good electrochemical stability. Form Fig. 6d, the intercept value about 2.35 on real axis represents the internal resistance (RS) in high-frequency area. The slightly high internal resistance is mainly attributed to the inherent resistance of active material, due to the natural defect in electric conductivity of Cu(OH)2. The slope of the Nyquist plot reflects the Warburg impedance, which demonstrates a low electrolyte diffusion resistance. The open porous Cu(OH)2/graphene nanocomposite nanostructure with large surface area endows the electrode with abundant reactive sites and shorten ion diffusion path.
The excellent electrochemical energy storage properties of the Cu(OH)2/graphene nanocomposite are ascribed to the following reasons: (i) the 3D Cu foam substrate analogous to the reported Ni foam also has many advantages of high-electric conductivity, large surface area, microscale pores and many flow channels, providing the active material with high mass loading, and large effective surface area [35, 36]; (ii) due to the cypress leaf-like Cu(OH)2 synthesized by in situ oxidation of Cu foam, this binder-free electrode not only reduce the dead volume effect and the internal resistance but also prompt the effective charge transfer and fast redox reactions [37, 38]; (iii) the electric conductivity of the Cu(OH)2 can be improved by assembling with graphene, facilitating the electrolyte ions diffusion and electron transport ; (iv) to some extent, the volume changes of Cu(OH)2 and especially the agglomeration of graphene all can be alleviated, increasing the stability of both nanostructure and electrochemical performance during continuous charge-discharge processes ; (v) the unique open, porous, and interconnected nanostructure can reserve electrolyte ions to ensure the sufficient redox reactions particularly at high current densities .
We have adopted a simple electrochemical method based on solution to in situ synthesize cypress leaf-like Cu(OH)2 nanostructure/graphene nanosheets on Cu foam serving as a promising electrode for supercapacitors. This novel hybrid nanostructure endows the Cu(OH)2/graphene nanocomposite with abundant redox reactions, good charge transfer, and short electrolyte ion diffusion pathway. When evaluated as the electrode material for supercapacitors, the Cu(OH)2/graphene nanocomposite demonstrates high reversible capacitance of 317 mF cm−2 and excellent stability with 100 % retention over 20,000 cycles at current densities of 2 mA cm−2 and remarkable rate capability at increased current densities. This synthesis method will open a new door for the facile fabrication of other hydroxides and provides an effective strategy for remarkable electrochemical energy storage devices.
We gratefully acknowledge the support of this work by the National Natural Science Foundation of China (grant nos. 11174197 and 11574203).
Availability of Data and Materials
The datasets used during the current study are available from the corresponding author on reasonable request.
ZZH performed the experiments. ZZH, YYX, and MLG designed the experiment. ZZH, YYX, JDK, LFG, and YH analyzed the data. ZZH wrote the paper. ZMJ, SWZ checked the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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