Nano-structured porous carbon-silica composite aerogel derived from low-cost kapok fibers and TEOS

Abstract

This study proposes an ambient-temperature synthesis of highly porous carbon–silica composite aerogels (C-Si aerogel) made using kapok fibers. Effects of the fiber's volume in the C-Si composite and the pyrolysis temperature on the properties of the obtained C-Si aerogel was determined. Initially, tetraethyl orthosilicate (TEOS) in methanol was mixed with 3–20 % v/v of pretreated kapok fibers. After that, the gel underwent incubation, a solvent change, and at-pressure drying. Then, the dried fiber-silica composite aerogels (F-Si) were pyrolyzed under an inert N2 atmosphere at temperatures of 300 °C to 800 °C for 15 min. Analysis of the resulting materials indicated that the highest surface area of 753 m2/g with 5.5 nm pore size was found in the C-Si5400 aerogel sample. Further increases of kapok fiber content above 5 % v/v decrease both surface area and pore volume of the C-Si aerogels. This is because the high volume of kapok fibers has probably blocked the pores or voids of the C-Si aerogel. In comparison, the C-Si5400 aerogel sample had a surface area 21 % higher than a conventional silica aerogel. Notably, carbon–silica composite aerogel can be obtained by pyrolyzing dried fiber-silica composite aerogels in the temperature range from 400 °C to 600 °C. In addition, pyrolysis of C-Si composite at temperatures above 800 °C can remove kapok fibers and yield a pure silica aerogel. However, as the pyrolysis temperatures rise from 500 °C to 800 °C, carbon–silica composite aerogels' pore volume and surface area decrease. All adsorption and desorption isotherms obtained from N2 adsorption experiments on C-Si aerogel composite at −196 °C are type IV isotherms. The mean pore size of these carbon–silica composite aerogels is in the range of 4.2 to 7.8 nm. This study's results show that the degree of the kapok filling volume and pyrolysis temperature play essential roles in the microstructure properties of the obtained C-Si composite materials.