Sol−Gel-Derived Carbon Aerogels and Xerogels: Design of Experiments Approach to Materials Synthesis

Abstract

A design of experiments approach was conducted to determine the effects of the gel pH (5.5 and 7.0), weight percentage of solids (5 and 20%), pyrolysis temperature (800 and 1050 °C), and gel type (aerogel or xerogel) on the surface area, pore volume, and electrochemical double-layer capacitance of carbonized resorcinol/formaldehyde resins. The full 24 factorial design carried out on 16 different materials (8 carbon aerogels and 8 carbon xerogels) and the full 23 factorial designs carried out on the carbon aerogels and carbon xerogels individually revealed significant differences between these two types of gels, with specific two-factor interactions observed that could not have been easily resolved with the traditional approach of changing one variable at a time. The gel pH was found to be the most dominant factor (parameter) affecting not only the surface area and pore volumes of both types of gels but also the capacitance, more so for the carbon aerogels in all cases. On average, the carbon aerogels exhibited higher surface areas, pore volumes, and capacitances than the carbon xerogels; in general, higher surface areas correlated with higher pore volumes, and higher capacitances correlated with higher surface areas and also with higher pore volumes, but not as closely. The properties of the carbon aerogels were also more sensitive to the synthesis and processing conditions than those of the carbon xerogels, indicating a broader range of applications for the former but more controllability of synthesis and processing for the latter, and vice versa.