Elucidating the Phase Transformation of Proteins into Graphitic Carbon Aerogels

Abstract

Graphitic carbon aerogels (GCAs) can be synthesized through pyrolyzing protein precursors in N2 atmosphere at high temperatures to form a porous structure with multilayer graphene sheets surrounded by a carbon fiber network. GCAs have high surface areas and high electrical conductivities, making them promising new materials for energy storage and catalysis applications. However, some precursors such as bovine serum albumin (BSA) and pasteurized egg whites (PEW) give rise to larger graphene sheets within a regular carbon fiber network, while others such as α-lactalbumin and β- lactoglobulin form GCAs with smaller graphene sheets or amorphous carbon structures. It is not fully understood what causes these differences in GCA morphology, and thus the purpose of this work is to further investigate the mechanism of phase transformation from proteins into GCAs to better understand these differences. Thermal analysis of the pyrolysis process was performed, with the differential scanning calorimetry results showing a decomposition temperature of 290ºC in BSA. Thermogravimetric analysis (TGA) with mass spectroscopy showed that CO2 is released after 300ºC as the material starts to decompose. It is theorized that as the gases are released, there is bubble formation of the trapped gases that ultimately leads to the porous GCA final structure. This is further supported by in situ scanning electron microscopy data that showed bubble formation in the PEW after 300ºC. The TGA showed that both α-lactalbumin and β-lactoglobulin undergo a biphasic decomposition and retain less than 10% of their original weight after pyrolysis, which could be cause for their smaller graphene sheets and more amorphous structures, although more research would be needed to further investigate this.