Effects of Hierarchical Architecture on Electronic and Mechanical Properties of Nanocast Monolithic Porous Carbons and Carbon−Carbon Nanocomposites

Abstract

This study compares the effects of meso- and macroporosity and the influence of nanocomposite structure on textural, electronic, and mechanical properties of monolithic carbon samples. Glassy carbon monoliths with three-dimensionally ordered macropores and walls containing mesopores (3DOM/m C) were synthesized by nanocasting from monolithic silica with hierarchical pore structure. The porous silica monoliths (3DOM/m SiO2) were prepared by combining colloidal crystal templating with surfactant templating. These preforms were infiltrated with a phenolic resin through a gas-phase process. After carbonization and HF extraction of silica, the resulting carbon monoliths maintained the open, interconnected macropore structure of the preform and the mesoporosity of the skeleton, which provided a high surface area >1200 m2/g to the material. Subsequent introduction of more graphitic, nitrogen-doped carbon into the mesopores by chemical vapor deposition produced a monolithic nanocomposite material (3DOM/m C/C). The materials were characterized in detail by powder X-ray diffraction, Raman spectroscopy, small-angle X-ray scattering, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, nitrogen-adsorption measurements, depth-sensing indentation, and electrochemical measurements. The mechanical strength, electronic conductivity, and capacity for lithiation of 3DOM/m C, 3DOM/m C/C, and a 3DOM carbon prepared from resorcinol-formaldehyde precursors without templated mesopores (3DOM RFC) were compared to evaluate the effects of the wall nanostructure and composition on these properties. The mechanical strength and electronic conductivity of the nanocomposite were significantly higher than those before addition of the second carbon phase. The nanocomposite suppressed formation of a solid-electrolyte interface layer during lithiation and had higher lithiation capacity than 3DOM RFC at high discharge rates, but not at low rates.