Thermal Modification of Graphite for Fast Electron Transport and Increased Capacitance

Abstract

On thermal treatment, eight different graphite materials became resistant to air aging for at least 9 weeks compared to the usual time of hours to a few days when assayed in 1 mM ferri-ferrocyanide solution. In addition, resistance to aging lasted at least 7 days when immersed in 1 mM ferri-ferrocyanide solution compared to the frequently reported few minutes to hours. Experimental results confirm that with heat treatment, HOPG-ZYH, graphite rods, pyrolytic graphites, graphite felts, and natural and artificial graphites undergo structural reorganization that leads to restructuring of their electronic nature. This electronic restructuring enhances and sustains their electrochemical properties. The extent of reorganization is dependent on the initial disordered state, which in turn is important to the final structural and electronic conditions. These results strongly suggest that the primary factor enhancing the electronic response of heat-treated materials is from an overall higher density of states (DOS) localized on delocalizing πbonds compared to their controls. This structural reorganization of the graphites also supports a degree of crystallinity along the lattice sites that enables carrier hopping irrespective of adventitious oxygen-containing and hydrocarbon moieties that are associated with aging-induced sluggish electron transfer kinetics. The attributes of this electronic structure demonstrate a strongly correlated system that exhibits a nonperturbative behavior. A one-dimensional Hubbard model describes this behavior to explain the surface-to-electronic chemistry of treated graphites by addressing both their enhanced electrochemical performance and their delayed or reduced aging effects.