Preparation-morphology-performance relationships in cobalt aerogels as supercapacitors

The ability to direct the morphology of cobalt sol-gel materials by using the simple synthetic parameters in epoxide-driven polycondensations has been dramatically demonstrated, and the influence of such morphological differences upon the supercapacity of the materials has been explored. Precursor salt, epoxide, and solvent all influence the speed of the sol-gel transition and the size and shape of the features observed in the as-prepared materials, thereby leading to highly varied microstructures including spheres, sponge-like networks, and plate assemblies of varied size. These morphological features of the as-prepared cobalt aerogels were observed for the first time by high resolution scanning electron microscopy (HRSEM). The as-prepared aerogel materials were identified by powder X-ray diffraction and thermogravimetry as weakly crystalline or amorphous cobalt basic salts with the general formula Co(OH)_2-nX_n where X = Cl or NO₃ according to the precursor salt used in the synthesis. For all samples, the morphology was preserved through mild calcining to afford spinel phase Co₃O ₄ in a variety of microstructures. Wide-ranging specific surface areas were determined for the as-prepared and calcined phases by physisorption analysis in agreement with the morphologies observed by HRSEM. The Co ₃O₄ aerogels were evaluated for their supercapacitive performance by cyclic voltammetry. The various specimens exhibit capacitances ranging from 110 to 550 F g^-1 depending upon the attributes of the particular aerogel material, and the best specimen was found to have good cycle stability. These results highlight the epoxide-driven sol-gel condensation as a versatile preparative route that provides wide scope in materials' properties and enables the analysis of structure-performance relationships in metal oxide materials.