Preparation And Properties Of Carbon Aerogels And Its Composite By Ambient Pressure Drying

Carbon aerogel (CA), a kind of light, porous, amorphous nano-carbon material, according to its unique structure and excellent properties, has great application potential in heat insulation, energy storage, catalysis, adsorption, and other fields. Nowadays, the high preparation cost seriously restricted the process of CA industrialization. The drying of wet gel is one of the crucial steps in CA preparation, which is commonly performed by supercritical drying method. Although the aerogels obtained by supercritical drying method have excellent properties, the high preparation cost, risk and poor repeatability always prevent the large-scale preparation of aerogel. Therefore, applying productive low cost ambient pressure drying (APD) to replace the traditional supercritical drying and optimizing the preparation conditions to minimize the contraction and collapse of nanopores in CA will make important research significance to promote the CA industrialization process.In this thesis RF organic and carbon aerogels by ambient pressure drying and sol-gel method were prepared, and the influence of different preparation parameters (initial solution pH, concentration of precursors, exchanging solvent and gelation temperature) on the aerogel structure and physical properties were investigated in detail. By optimizing the preparation conditions, we obtained the suitable parameters for ambient pressure drying method. Results show that when the initial solution pH is in the range of 6.0-6.4 and the concentration of precursors is 25%～40%, organic and carbon aerogels with low density and good pore structures can be obtained. Especially when pH is 6.0 and the precursors concentration is 30%, the density of RF sample can be as low as 0.3 g cm" 3, and the pore diameter can be within 100 nm. Organic solvent of RF wet gel displacement can significantly reduce the subsidence and contraction of gel, which can greatly reduce the density of aerogel. By studying the influence of different organic solvents on the RF aerogel microstructure and physical properties, considering preparation period and preparation cost, ethanol can be used as suitable RF aerogel exchanging solvent.According to the modification APD methods of SiO2 aerogel, we used some drying control chemical additives (formamide, ethylene glycol and glycerol) to modify RF aerogels, and found that only the addition of formamide (FA) can significantly improve the pore properties of aerogels; meanwhile, the aerogel density and drying shrinkage can be reduced. According to the test results and analysis on the mechanism of FA, we can make speculation that the reduction of cross-linking and branching degree is mainly caused by barrier effect of FA, which is benefit for the growth of RF particles. In addition, based on the improved pore structure, the modified CA was used as electrode materials for electric double-layer capacitors (EDLCs). The results exhibit that the addition of FA can obviously improve the specific capacitance of carbon aerogels. When the mole ratio of FA and resorcinol is 0.5, the capacitance of modified CA electrode can increase from 60 F g-1 (original CA) to 96 F g-1, with 60% improvement. Moreover, the capacity retention rate of modified CA is as high as 93%, indicating its excellent rate performance.Ascribed to the brittleness of pure CA, it is difficult to obtain monolithic aerogels with large-bulk and no cracking. Herein, we prepared large-bulk CAs by using carbon fiber (CF) felts (PAN and activated viscose) as reinforcements. By optimizing the preparation condition of RF aerogels, the RF parameters, solution pH of 6.2 and the precursor concentration of 40%, were chosen to prepare composite aerogels. The resulted composite CAs have low density (～0.4 g cm-3) and small pore size (-50 nm). The microscopic morphology results show that carbon fibers are well embedded in RF aerogels. The thermal conductivity at room temperature of activated viscose CF felt reinforced organic and carbon aerogals is 0.039 W m-1 k-1 and 0.048 W m-1 k-1, respectively, implying excellent thermal insulation properties. In addition, CF/CA composite, as EDLCs electrode materials, has high specific capacity (> 80 F g-1) and excellent cycle stability.In order to strengthen the nanostructure and suppress the collapse of nanopores of resorcinol-formaldehyde (RF) aerogels during the APD process, the influence of graphene oxide (GO) addition on the aerogel shrinkage was investigated. Results show that GO can significantly decrease the aerogel density and shrinkage. According to the test results and analysis on the mechanism of GO, we can make speculation that GO, attributed to the good dispersibility and the strong interaction, can significantly reinforce the network of gels and improve the resistance to collapse during the drying process. Then, by optimizing preparation condition and carbonization, graphene (GNSs)/CA composite with ultra-low density of 0.182 g cm-3, high specific surface aero of 533 m2 g-1, and small pore size (< 100 nm) were prepared; Meanwhile, this composite aerogel has excellent thermal insulation property with thermal conductivity of 0.028 W m-1 k-1. In addition, the capacity retention rate of GNSs/CA composite electrode is as high as 96%, indicating outstanding capacitance performance. When used as anode materials for lithium ion batteries, the sheet-like GNSs/CA composite exhibits high rate capability (436 mAh g-1 and 146 mAh g-1 at 50 mA g-1 and 2 A g-1, respectively)...