Modification And Electrochemical Properties Of Carbon Material For Anode Of Ultrabattery

Due to energy crisis and environmental pollution, new energy vehicles caused many countries'high value and vigorously develop in recent years. As one of the key components of new energy vehicles－ower battery influences the development of new energy vehicles. Ultrabattery is a new kind of improved lead acid battery, its negative electrode adopts both the Pb negative of the lead-acid battery and the carbon electrode of supercapacitors. The carbon electrode provides charges in starting procedure and recycles charges in braking, so it can reduce the sulfuric acid salinization of the lead electrode, and improve the battery life. The performance of carbon electrode in ultrabattery plays a vital role. In the paper, the electrochemical performance of the carbon electrodes in the working environment of lead acid battery was firstly studied, problems were found, and then modification for carbon electrode and doping for electrolytes were proceed. Finally, the electrochemical performance of the hybrid capacitor PbO2|AC (as part of ultrabattery) was investigated, and the performance of modified activated carbon electrode and electrolyte with additive was discussed in PbO2|AC. The following conclusions were obtained:(1) The electrochemical characteristics of activated carbon in the working environment of lead acid batteries. Activated carbon shows different capacity play and hydrogen evolution behavior in different voltage interval and different concentrations of sulfuric acid. It plays a larger specific capacitance in a relatively positive work interval (vs. SCE), and in the lead acid battery negative working voltage range (－0.9～－0.4 V), the specific capacitance was relatively lower. The concentration of the sulphuric acid also influenced the specific capacitance, the superior specific capacitance of the activated carbon shows in moderate levels of sulfuric acid (3 M). In 5 M sulfuric acid, and which is the electrolyte of the lead acid baterry, the specific capacitance was lower, and the evolution value of the hydrogen increased with increasing the sulphuric acid concentration. Therefore, the using of activated carbon in the working system of lead acid battery can not fully play its capacitance, yet bring the hydrogen evolution problem. So the inhibition of hydrogen evolution of activated carbon is the main way to improve the using life of ultrabattery and the key to its success use in ultrabattery. The parallel electrode of Pb and activated carbon showed both the effects of faraday and double layer characters, this indicated that the activated carbon electrode can absorb and release charge, and plays a buffer role in ultrabattery.(2) The modified activated carbon. Based on modified activated carbon by Pb ion and adding the addition of ZnO, CaSO4, CaO, A12O3, Bi2O3 to the activated carbon electrode, it was found that the modification of Pb ion, adding 5% ZnO,10% Bi2O3 to the electrode and 50ppm Bi2O3 to the electrolyte respectivity can improve the capacitance of activated carbon and inhibit the hydrogen evolution. PbSO4 played the role in Pb ion modification, as it appeared with the transformation peak of Pb and PbSO4, the capacitance of activated carbon was improved. Due to the high hydrogen evolution potential of PbSO4, the modification by Pb ion inhibited the evolution of hydrogen on activated carbon. The adding of 5% ZnO to the electrode shows the best effect in inhibiting hydrogen, but the effect was inferior to that of Pb ion modification. Meanwhile, the capacitance was improved by doping 5% ZnO to the electrode. The role of adding Bi2O3 in electrolyte and to the electrode was not the same, adding Bi2O3 to the electrolye has an obvious effect in inhibiting the hydrogen evolution, and adding Bi2O3 to the electrode plays a main role in improving the capacitance.(3) The performance of PbO2|AC capacitors. The discharging voltage of PbO2|AC capacitor was low. The specific capacity of the activated carbon in the lead acid baterry' working conditions was small, which less than 10mAh/g at 1 A/g. The internal resistance in cycling process will increase, which maybe caused by the fall of the positive active material. The capacitors with the carbon electrodes modified by Pb ion,5% ZnO,10% Bi2O3 respectively shows the higher specific capacity than that of pure carbon electrode at 1.7～2.4 V. The results of the cycle life test of 10000 cycles indicated that the electrodes modified by Pb ion and 10% Bi2O3 demonstrated better performances. The doping agent made the charge voltage reduced, for electrode modified by 10% Bi2O3, the charge voltage dropped from 2.7 V to 2.4 V, and the hydrogen evolution reduced at the same time. Meanwhile, the adsorption status of H+ at the negative electrode change by the doping agent, the discharge voltage increased, which means the increasing of the discharge capacity. The discharge voltage increased from 0.8 V to 1.4 V and 1.6 V by the Pb ion and 10% Bi2O3 modification respectively. The activated carbon electrode showed best stability and lifetime by modified with 10% Bi2O3. The capacitor with doping 50 ppm Bi2O3 in electrolyte showd lower charge voltage than that of doping additives in electrodes, but the discharging voltage was also lower, which indicated that low in electrolyte that doping 50 ppm Bi2O3 to electrolyte can reduce the hydrogen evolution of the activated carbon materials, but with no capacity improved.