Tunable Sol-gel Preparation And Properties Of Porous Advanced Energy Materials

Porous materials are widely used for adsorption, filtration, sound and heat insulation, energy storage and catalytic applications because of their low relative density and high specific surface area. These characteristics (such as pore structure, specific surface area, mechanical properties and so on) of porous materials mainly depend on the preparation methods, and thus determine the performance of porous materials. Among the many methods of preparing porous materials, sol-gel method is widely used because of the advantages of easy to obtain desired homogeneous multi-component system, low synthesis temperature and good controllability.However, there are several problems urged to be solved in the state of art sol-gel process for preparing porous materials. First, porous zirconia and nickel oxide materials which attracted much attention for energy devices in recent years, are completely different from the silica system in which three-dimensional continuous gel network structure can be formed by self polycondensation reaction during the sol-gel process. Up to now, most researches used epoxy compounds as gel accelerator to synthesize gel network structure in order to prepare nano-porous materials of these compounds; but epoxy compound have shortcomings such as toxicity, flammability and explosibility. Thus it is necessary to develop new green sol-gel process. Secondly, for the porous carbon materials which have been widely used, the present sol gel methods are complicated. Activation process is needed in most current methods to obtain microporous structure and large specific surface area. In addition, one-dimensional carbon nanofiber porous structure, which is propitious to improve the electrical conductivity, has not been prepared yet by the sol-gel process. So it is necessary to develop a new sol-gel preparation process for such desired one dimensional porous carbon structure. Also, few reports were focused on the tuning of pore structure and the modulating of elastic modulus of porous piezoelectric polymer materials during sol-gel process to match the mechanical properties of bio organism. In view of the above problems, an environment-friendly method for preparation of metal oxide aerogels using citric acid as gel accelerator was first proposed in this dissertation. The mechanism of such sol-gel process was thoroughly studied. Secondly, porous carbon nanofibers were prepared by sol-gel process using sustainable Agarose as carbon precursor and zinc acetate as sol-gel catalyst. The performance of the porous carbon nanofibers as electrochemical capacitor electrodes was studied. Finally, porous poly (vinylidene fluoride) (PVDF) based composites were prepared by sol-gel technique combined with phase-separation mechanism. The mechanical properties of such PVDF based porous materials matches with the bio organism. The piezoelectric energy harvesting device for simulated human blood vessel was fabricated, and the feasibility of converting bio-mechanical energy to electrical energy of such device is investigated.The contents and results of this dissertation are as follows:(1) A new and tunable sol-gel method for the preparation of metal oxide aerogels using citric acid as gelation agent was proposed. The gelation mechanism by citric acid as gel accelerator was studied using zirconia aerogels as an example. This method overcomes the disadvantages of the toxicity of epoxide in conventional sol gel process. The zirconia aerogels prepared by this method have good thermal stability. Based on this, nickel oxide aerogels were also prepared by such method. The results show that nickel aerogel annealed at 250? has high specific capacitance of 914 F·g-1, good rate capability (The specific capacitance is 436 F·g-1 when the current density was 20 A·g-1), and good cycle stability (the capacitance can maintain 93% of its original value after 3000 cycles at the current density of 10 A·g-1).(2) Porous carbon nanofiberswere prepared by sol-gel method using Agarose as precursor and zinc acetate as sol gel catalyst. The diameter of the porous carbon nanofibers prepared by this method was about 15-20 nm. Raman characterization results show that the carbon nanofibers have high graphitization degree. The results of electrochemical tests showed that the optimized sample has specific capacitance as high as 157 F·g-1 and good stability (the capacitance can maintain 95% of its original value after 2000 cycles at the current density of 10 A·g-1).(3) ? phase porous PVDF materials with different pore structure were prepared by the sol-gel method combined with phase separation mechanism. Transparent and flexible PVDF-PDMS composites were prepared by immersing polydimethylsiloxane (PDMS) into the porous PVDF. PVDF-PDMS composites with small tensile modulus (close to that of human blood vessel) were obtained by controlling the PVDF content. The PVDF-PDMS composite with 4% of PVDF in volume has the maximum voltage output (2.87 V), and the peak-to-peak voltage was 0.35 V under the simulated pressure change of heart pulse. This device can be embedded in biological organs for harvesting the biological mechanical energy for the bio-electronic self-energy supply.