Abstract
In this dissertation, silica aerogels, composite aerogels, and transition metal oxide aerogels were synthesized via sol-gel processes with various drying procedures, and further applied in energy saving and storage. Because of the novel chemical and physical properties of aerogels, excellent performances in thermal insulation and specific capacitances were achieved. This dissertation is divided into five main parts, three in energy saving and two in energy storage. In energy saving, pure silica aerogels with low density, good hydrophobicity, low thermal conductivity were successfully synthesized via a sol-gel process with multiple surface modification (MSM) treatments followed by ambient pressure drying. The chemical and physical properties of MSM samples were compared to those of single surface modification (SSM) and non-modification (NSM) samples. Because of their hydrophobic surfaces, these silica aerogels are suitable for long term usage. The fragility of pure silica aerogels however makes it difficult to apply in monolithic form. A facile one-step polymer-incorporation sol-gel process, together with a surface modification and an ambient pressure drying processes, was further developed to prepare silica-poly(vinylpyrrolidone) composite aerogels. These composite aerogels are with high hydrophobicity (static contact angle >120o), good mechanical strength (Young’s modulus of bending >30 MPa), and low high-temperature thermal conductivity (0.063 W/m-K at 300 oC), which are critical characteristics for practical applications of aerogels, particularly in energy saving areas, for long-term usage and large scale production. Furthermore, in order to apply silica aerogels at high temperatures (300oC~500 oC), carbon nanofibers were incorporated into the mesoporous network of silica aerogels to obtain opacified monolithic aerogels at concentrations as high as 20 wt % through an accelerated-gelation sol-gel process. The incorporation of carbon nanofibers enhanced the dimensional stability of the silica aerogels and suppressed the thermal radiations that became dominant at high temperatures, to achieve an ultralow thermal conductivity of 0.050 W/m-K at 500 oC, whereas maintaining a thermal stability above 500 oC (much better than the conventional high-temperature thermal insulation materials: 0.3 W/m-K at 500 oC for glass fibers, 0.1 W/m-K at 527 oC for alumina fused brick, and 1.7 W/m-K at 527 oC for sillimante). In energy storage, metal alkoxide precursors, which were highly sensitive to heat and moisture, were replaced by non-alkoxide precursors (metal nitrate and metal chloride) to prepare transition metal oxide aerogels. High specific surface area and mesoporous structure of aerogels were ideal for supercapacitor applications. This idea was successfully demonstrated for the first time by taking cobalt oxide aerogels as an example. Cobalt oxide aerogels of excellent supercapacitive properties, including high specific capacitances (the highest ever reported for cobalt oxides, >600 F/g at a high mass loading of 1 mg/cm2) and onset frequencies, and excellent reversibility and cycle stability, were successfully synthesized with an epoxide addition procedure by using cobalt nitrate as the precursor. The present development makes possible the low cost production of high performance supercapacitors of the asymmetric type. Mesoporous materials of high specific surface area, porosity, electronic conductivity, as well as electrochemical activity, and multiple oxidation states/structures are desired for next generation supercapacitors. These requirements for material characteristics can be met by the nickel cobaltite (NiCo2O4) aerogels prepared from an epoxide-driven sol-gel process. In this work, nickel cobaltite aerogels of ultrahigh specific capacitance (1400 F/g under a mass loading of 0.4 mg/cm2 at a sweep rate of 25 mV/s from 0.04 to 0.52 V in a 1 M NaOH solution), excellent reversibility, and outstanding cycle stability were synthesized with a chloride based epoxide addition procedure for the first time. The post-gel-drying calcination temperature was found to play a critical role in producing the preferred products. Nickel cobaltite aerogels of outstanding supercapacitive properties were obtained with a starting Ni/Co ratio of 0.5 and post-gel-drying calcination temperature of 200 oC, achieving an optimal combination of composition, crystallinity, specific surface area, pore volume, and pore size. The development of the present work makes possible the low cost production of next generation, ultrahigh performance supercapacitors of the asymmetric type.