| Energy crisis and environmental pollution are the two major challenges of the 21 st century. In this context, supercapacitors, the next generation of energy storage device, have attracted intense attention due to their long cycle life, fast recharge ability, high power density and environmental protection. As the main decision of the proverty of supercapacitors, the electrode materials have also become a major research topic. In the past few decades, transition metal-based oxides/hydroxides/ sulfides, conducting polymers and carbon materials have been reported for supercapacitve materials. Recently, layered double hydroxides(LDH) have aroused widespread attention in the area of catalysis, adsorption, molecular sieves and supercapacitors due to their unique layered configuration and properties. In particular, LDH have drawn great interest for the application of supercapacitors because of their layered structure with abundant channels between layers and the existence of transition metals, which can simultaneously take advantage of electrical double layered capacitance and Faradaic pseudocapacitance to get a relatively high capacitance. Nevertheless, the practical use of LDH, as a single electrode material, still could not meet the requirements of supercapacitors with high energy density. Thus, more researches have been focused on its composites, including integrating pseudocapacitive material with a highly conductive material and constructing a core/shell structure with different pseudocapacitive material. Based on this, we prepared the LDH-based composites and investigated its electrochemical performance.The CoAl-layered double hydroxide nanosheets/reduced graphene oxide(CAN-LDH-NS/rGO) composite was prepareded by an exfoliation-restacking process. First, we fabricated the nitrate-intercalated CoAl-LDH(CAN-LDH) via one-step co-precipitation method under the protection of N2 gas. Then, the obtained CAN-LDH was exfoliated into the positively charged CAN-LDH nanosheets(CAN-LDH-NS) and restacking on the surface of negatively charged reduced graphene oxide(rGO) flakes by means of electrostatic interactions to form the CAN-LDH-NS/rGO composite. Powder X-ray diffraction(XRD), fourier transform infrared(FT-IR), transmission electron microscopy(TEM) and Raman spectroscopy(Raman), scanning electron microscopy(SEM) and BET specific surface area were used to characterize the structure and morphology of the obtained sample. The electrochemical performances of the sample were evaluated by using an electrochemical workstation and a LAND-CT2001 A system. As a result, the obtained CAN-LDH-NS/r GO composite display a capacitance of 1296 F g-1 at a current density of 1 A g-1, much higher than that of pure Co Al-LDH. Besides, the cycle life of CAN-LDH-NS/r GO composite matained 90.5% of the intial capacitance after 1000 cycles at a high current density of 15 A g-1, indicating its excellent cycle stability. The rGO with high electrical conductivity and large specific surface area were responsible for the superior electrochemical performance, which can simultaneously facilitate the electron transport and ion diffusion during Faradic reaction.The CoAl-layered double hydroxide/reduced graphene oxide hybrid gel was anchored on the surface of Ni foam directly via a two-step process(denoted as CoAl-LDH/rGO-gel/Ni). Firstly, rGO gel with free-standing structure(denoted as rGO-gel/Ni) was coated on Ni foam by a simple strategy and subsequent electrodeposition of CoAl-LDH nanosheets on it to form CoAl-LDH/rGO-gel/Ni. The structure and morphology of the obtained sample were characterizated by XRD, Raman, TEM and SEM. Electrochemical tests presented a superior electrochemical performance, which demonstrated a capacitance of 0.95 F cm-2(1262 F g-1) at a current density of 2 mA cm-2 for CoAl-LDH/rGO-gel/Ni electrode, much higher than that of pure CoAl-LDH/Ni(0.40 F cm-2, 538.2 F g-1). The enhanced capacitance can be attributed to high electrical conductivity and large specific surface area of rGO, which can provide a fast electron transport and abundant active sites for Faradaic process. Besides, the asymmetric supercapacitor based on active carbon as negative electrode also presents the superior performances, which displays a specific capacitance of 90.4 F g-1 at 2 mA cm-2 and an energy density of 32.14 Wh kg-1. Moreover, it also shows a remarkable cycling stability with 98.05 % retention at a current density of 10 mA cm-2 after 2000 cycles, suggesting its excellent cycle stability.A novel NiP@CoAl-LDH nanotube arrays(NiP@CoAl-LDH NTAs) are successfully designed on Ni foam via template-assisted electrodeposition process. Typically, ZnO nanorod arrays(ZnO NRAs) template was coated on Ni foam by hydrothermal process and followed by electrodeposition of NiP on it to form ZnO@NiP nanorod arrays(ZnO@NiP NRAs). Then, the obtained ZnO@NiP NRAs was immersed into 3 M Na OH to remove ZnO NRAs template and the Ni P nanotube arrays(NiP NTAs) was fabricated. Finally, the NiP@CoAl-LDH NTAs were synthesized by electrodeposition of CoAl-LDH nanosheets on NiP NTAs. The structure and morphology of the obtained sample were characterizated by XRD, XPS, TEM and SEM. Electrochemical result showed better electrochemical performances of Ni P@Co Al-LDH NTAs(1.33 F cm-2, 1112 F g-1 at 1 mA cm-2 with a retention of 74.1% at 20 mA cm-2) than pure NiP NTAs and Co Al-LDH electrode. The superior electrochemical performances can be assigned to the core-shell nanotube structure of Ni P@Co Al-LDH and the pseudocapacitive characteristics of CoAl-LDH and NiP. The asymmetric supercapacitor device based on active carbon as negative electrode also displayed predominant electrochemical properties, such as high capacitance of 97.53 F g-1 at 2 mA cm-2 and energy density of 34.68 Wh kg-1 at a power density of 0.42 kW kg-1. Besides, the capacitance of asymmetric supercapacitor device maintained 95.50 % of the initial capacitance at 6 mA cm-2 after 4000 cycles, indicating its potential applications in long-term cycle life supercapacitor. These results illustrate the promise of Ni P@CoAl-LDH NTAs as electrode materials. |
