Study On Nano Carbon Materials/Transitional Metal Oxides For Supercapacitor Electrodes

Supercapacitors have been recognized as unique energy storage devices which have higher energy density than conventional capacitors, and higher power density and longer cyclic life than batteries, filling the gap between conventional dielectric capacitors and batteries. The promising application of supercapacitors in the fields such as mobile telecommunication, consumer electronics, electric vehicle, aviation and national defense, has been attracted more and more attention through out the world. Studies on supercapacitors are mainly focus on the preparation of high performance electrode materials and electrolytes. With high specific surface nano carbon and cheaper oxidated nickel hydroxide and manganese dioxide as electrode materials for supercapacitors, this dissertation has investigated the electrode materials preparation, capacitive property and mechanism of capacitance. The electrodes have been characterized by X-ray diffraction (XRD), X-ray photon spectra (XPS) and scattering electronic microscopy (SEM) , the capacitive characteristics of the electrode has been investigated by galvanostatic charge/ discharge testing. The main results are as follows:1. Activated carbon (AC) for supercapacitors has been prepared from different raw materials, such as coal, coconut shell chars and common AC carbon by activated with KOH through microwave radiation. It is a novel and advanced activation method for AC, and the AC has higher specific capacitance and cyclic stability. Themicrowave power, radiation time, the content of KOH , the arc time and homogeneity have some effect on the properties of AC during the activation. The raw materials of the AC is also the key effect on the capacitance, because the structure and ingredient of the raw materials are different. AC prepared by coconut shell chars has much more excellent stability, while the reactivated carbon has higher specific capacitance. It shows that the specific capacitance of the AC prepared by coconut shell chars is 266.71 F/g when the radiation time, arc time and mass ratio of KOH to AC is around 7min, 5 min and 3:1 respectively, and the capacity retains 98.62% after 200 cycles. The specific capacitance of reactivated carbon is 295.17 F/g when the power, radiation time and mass ratio of KOH to AC is 480 W, 5min and 2:1 respectively, and the capacity retains 98.07% after 100 cycles.2. Oxidated Ni(OH)₂ powder for supercapacitors has been synthesized by oxidation of Ni(OH)₂ with KMnO₄ and then tested by XRD and XPS. Asymmetric capacitors have been fabricated utilizing oxidated Ni(OH)₂ and activated carbon as positive and negative electrode material. It shows that the main phase is β-Ni(OH)₂ , the bonding energy of nickel increases obviously. Matching the positive and negative electrode pairs would increase the efficiency and specific capacitance of electrodes materials because the specific capacitance of oxidated Ni(OH)₂ is much larger than that of AC. The working voltage and specific capacitance of asymmetric capacitor could be up to 1.6 V and 93.78 F/g (double electrodes specific capacitance) respectively under 125 mA/g current when the ratio of activated carbon and oxidated Ni(OH)₂ is around 2.7. It is easier to self discharge.3. The preparation of CeO₂ and Y₂O₃ doped MnO₂ and the heat treatments of α -MnO₂ have been studied. MnO₂ for supercapacitor has been prepared via the reaction of KMnO₄ and Mn(AC)₂. The results show that the material tested by XRD and SEM has α crystal structure with particle diameter range from 50 to 100 nm. Capacitance characterizations were carried out by using galvanostatic charge/discharge. It shows that the α -MnO₂ exhibits good capacitance. The capacity only loses 5.56% after 500 cycles and a maximum specific capacitance of264.50 F/g is obtained under 250mA/g current density. MnO^ doped Co, Ni(OH)2, CeC>2 and Y2O3 have been prepared by chemical co-deposition. It is found that some content of Co, Ni(OH)2, CeC>2 and Y2O3 can improve the capacitive performances of MnC>2. The addition of Co, Y2O3, especially the CeC?2 can improve the cyclic stability. When the mass ratio of CeO2 to MnO2 is 10 wt. %, the capacity only loses 1.18% after 500 cycles and a maximum specific capacitance of 257.68 F/g is obtained under 250mA/g current density. The additive of Co, Ni (Otfh and Y2O3 can increase the specific capacitance of MnCh. The specific capacitance of 15 wt. % Ni(OH)2 doped MnO2 is 328.83 F/g and the working voltage is 1.50 V. The mechanism of the improvement has been discussed. It is considered that the sustained MnCh crystal lattice by doping ion and the combination of manganese is the cause. Moreover, the heat treatment of a -MnO2 has been studied. Temperature and time effect on the capacitive of a -MnC>2> but temperature is the main factor. The results show that the heat treated a -MnO2 exhibits excellent capacitance and stability in 6 mol/L KOH electrolyte. When the temperature and time of heat treatment is 300 °C and 3-hour respectively, the capacity only loses 2.24 % after 500 cycles and a maximum specific capacitance of 302.52 F/g is obtained under 500mA/g current density.