Methods for preparing porous carbon hollow spheres are disclosed. The method includes contacting one or more polymer hollow spheres with a SiO2 precursor to form one or more SiO2-containing polymer hollow spheres; carbonizing the one or more SiO2-containing polymer hollow spheres to form one or more SiO2-containing carbon hollow spheres; and removing SiO2 from the one or more SiO2-containing carbon hollow spheres to form one or more porous carbon hollow spheres. The prepared porous carbon hollow spheres may be filled with liquid metal salts and treated at elevated temperatures to form filled porous carbon spheres. Methods of filling the porous carbon hollow spheres and compositions that include the filled porous carbon spheres are also disclosed.