Tuning Carbon Microstructure for Advancing Electrochemical Energy Storage

A novel process illustrating the chemically induced graphitization of activated carbon materials is herein presented. Activated carbon surfaces were progressively doped with surface oxygen groups (SOGs) by applying sequential oxidation treatments of 10% v/v nitric acid (HNO3). Significant crystallite enlargement was found to be dependent on total surface acidity as indicated from progressive oxidation treatments. The restructuring of π-electron density indicated by Raman and DRIFTS, was found to significantly increase the graphite-like order and system conjugation attributed to electron withdrawing effects of specific SOGs. The chemical graphitization mechanism resulted from the electron-withdrawing character of chemically attached SOGs--inducing electron holes within the graphene basal lattice, increasing potential energy, and consequently enabling the aggregation of independent single graphene layers into elongated, graphitically stacked crystallite formations--elucidated through powder X-ray diffraction. Substrates with extremely high density of total acidity (DTA) displayed graphitic disorder upon progressive oxidation treatment, whereas low DTA carbons allow for chemical graphitization. Oxidized carbons were analyzed for their respective solid electrolyte interface (SEI) layers formed under Li-insertion conditions. A long chained fluorinated surfactant was also applied to the oxidized carbons in order to further assess the impact of surface passivation. Surface treatments displayed the suppression of pore-impeding propylene glycol and lithium carbonate formation, increasing Li-ion pore-accessibility due to the development of a more homogenous, lithium ethylene dicarbonate, SEI layer. Reversible loading capacities of the treated carbon electrodes increased over 1000% after extended charge and discharge cycles--illustrating the electrochemical utility in the combined graphitization / surfactant passivation techniques developed. Glassy carbon was also utilized for modeling the mechanism of both SOG and SNG formation on carbon substrates--with carboxyl-carbonate and nitronium-ion species revealed as precursor species, respectively, under HNO3 oxidation conditions. The structural impact of the sp2 C-C polarizing SNG integration revealed dramatic increases in fullerene-type crystallites, as indicated by XRPD, TEM, SEM and Raman analysis. The graphene-layer ordering effects of electron-donating SNG integration displayed an elegant contrast to the chemical graphitization mechanisms outlined for electron-withdrawing, SOG integration. Further examination of the oxygen-reduction reaction (ORR) effects of the functionalized carbon substrates were elucidated via micro-powder electrode cyclic voltammetry