The Studies Of Supercapacitive Behaviors Of Metal Oxide/carbon Microbead Composites

Supercapacitor (also called electrochemical capacitor) combines the advantages of both conventional capacitors and rechargeable batteries. It has not only much more energy density than conventional capacitors, but also much higher power density than rechargeable batteries. Therefore, supercapacitors exhibit broad prospects in power source applications such as electric vehicles, spatial, military, mobile telecommunication, and consumer electronics. Electrode material is a key component of the supercapacitor and it determines the performance of supercapacitor. The research and development of electrode materials have become a hot topic in both academic and industrial field.In this study, carbon microbeads (CMB) were prepared by inverse emulsion polymerization and ambient drying technique. Compared with supercritical drying technique, new method for preparing CMB under ambient pressure drying conditions resolved the problems, such as complex process, expensive cost and performance was difficult to control. The influence of preparation conditions on the morphology and performance was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 sorption isotherm and cyclic voltammetry (CV). The results showed that the optimal stirring speed and V_s/V_h ratio were 480 rpm and 0.01, respectively. The CMB prepared at 800℃is a typical mesoporous carbon material with partially graphitized structure. In order to improve the surface structure and electrochemical performances, CMB was activated in 6 M HNO3, and the physicochemical and electrochemical performances of activated CMB (ACMB) were characteristized by Fourier transform infrared (FT-IR) spectrum, CV, galvanostatic charge-discharge, and electrochemical impedance spectrum (EIS). It was found that the activation caused a slight enhancement in specific surface area and pore size. Besides, the oxygen and nitrogen functional groups on the surface of CMB were detected, which could improve the hydrophilicity of CMB and the capability to form electric double-layer. Thus ACMB has better electrochemical properties. The specific capacitance of electrode increases from 196.8 to 246 F g^-1 at a current density of 1 A g^-1 in 6 M KOH and its energy density is 8.5 Wh kg^-1. Furthermore, ACMB shows good rate capability, the specific capacitance has tiny decrease even at large current density.Co₃O₄/CMB, Mn₂O₃/CMB, and NiO/CMB composites for supercapacitor electrode materials were synthesized by in situ coating method and their supercapacitive behaviors were investigated. It was found that both double-layer capacitance and Faradic pseudo-capacitance existed in the metal oxide/CMB composites. The metal oxide/CMB composites showed better electrochemical performance than pure CMB since nano-sized metal oxides homogenously encapsulated on the surface of CMB. The optimum content of Co₃O₄ in Co₃O₄/CMB composite is 10 wt.%. The specific capacitance of the 10%-Co₃O₄/CMB electrode is up to 350.2 F g^-1 at a current density of 1 A g^-1. Moreover, 10%-Co₃O₄/CMB composite supercapacitor exhibits good high-rate capability and excellent cycle life. Mn₂O₃ is another electrode material which can provide high redox pseudo-capacitance. When 10 wt.% Mn₂O₃ is coated on the surface of CMB , the specific capacitance of electrode is up to 333.8 F g^-1 at a current density of 1 A g^-1. Nanowhisker-like NiO was coated on the surface of CMB and the appearance of NiO/CMB composites become sea urchin-like morphology with core-shell structure. The 15%-NiO/CMB composite exhibits the best capacitive properties, the specific capacitance of electrode is up to 356.2 F g^-1. Besides, the symmetric supercapacitor using 15%-NiO/CMB composite as the electrode active material shows stable cycling performance.Co_xNi_1-x oxides were successfully synthesized by chemical coprecipitation method. The results of EDX and CV showed that Co_0.70Ni_0.30 oxide was obtained when the optimum molar ratio of the Co: Ni was 2:3. The Co_0.70Ni_0.30 oxide shows the spherical morphology with mesoporous structure, which is consisted of many interleaving thin nano?akes. An asymmetric supercapacitor (Co_0.70Ni_0.30 oxide/KOH/ACMB) was fabricated with Co_0.70Ni_0.30 oxide as the positive electrode and ACMB as the negative electrode. The asymmetric supercapacitor represents both double-layer capacitance and Faraday pseudo-capacitance, and its work voltage is extended to 1.6 V. The specific energy is 27.5 Wh kg^-1 at a current density of 1 A g^-1 and still keeps 24.4 Wh kg^-1 even at a current density of 5 A g^-1. The specific energy is three times higher than that of a symmetric ACMB capacitor. The hybrid supercapacitor also demonstrates good cycling performance.A symmetrical button cell supercapacitor was assembled with ACMB as the active material and 1 M Et₄NBF₄/AN as electrolyte. The work voltage of organic capacitor is increased up to 3.0 V. The specific energy is 37.4 Wh kg^-1 at a current density of 1 A g^-1 and still keeps 29.4 Wh kg^-1 even at a current density of 5 A g^-1, which is much more than that of in 6 M KOH. Organic capacitor exhibits a low leakage current and good voltage retention rate. The capacitance retention of supercapacitor remained 95% after 5000 cycles. Besides, Organic capacitor showed good capacitive behavior in the range of temperature from 0 to 70℃.Regularity of electrochemical impedance spectra for ACMB was studied in systems, AC impedance measurements show that the ACMB electrode has typical characteristic of porous electrode and the ohmic resistance (Rs) is about 2.0Ω. The capacitor shows typical double-layer capacitive behavior and the diffusion resistance decreases with the increase of T from 0 to 55℃. When the T≥65℃, at fixed voltage, charge-transfer resistance (R_ct) increases with temperature; the increase in R_ct with voltage at fixed T clearly indicates the occurrence of faradaic processes. The phase angle |δ| values are between 45°⁹0°, indicating the mixed kinetic processes of adsorption-limited process and semi-infinite diffusion-like limited step. The time constant is 3.5 s at 0℃, andÏ„_R keeps about 1.0 s at the range of 25⁶5℃. When T > 65℃,Ï„_R begins to increase. The supercapacitor balances between the ideal capacitor and ideal resistive behavior: resistive at high frequencies and capacitive at low frequencies. The maximum capacitance of supercapacitor calculated from low-frequency impedance data is 28.4 F g^-1. The energy density and power density of capacitor can be achieved 35.5 Wh kg^-1 and 112.5 kW kg^-1, respectively.