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 π bonds 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.

Electrochemical stability and capacitance of in-situ synthesized Prussian blue on thermally-activated graphite

“…S1a and e) to allude to morphological differences as the reason for the preferential PBNP deposition on the treated rather than the pristine GFA3. These results suggest that the thermal treatment altered the crystallinity and electronic properties of the graphites without any visible morphological changes, as earlier reported [45]. This implies that the treated GFA3 fibers are more receptive to the PBNP formation and growth (because of its enhanced crystallinity) compared to the pristine GFA3 fibers.…”

“…S7) with and without PBNP show that thermal treatment improved both the kinetics and capacitance even without PBNP. This improvement is due to the increased crystallinity and proper electronic tuning of the treated graphite as discussed [45]. In the presence of PBNP (2× and 3× dipped), the treated GFA3 capacitance doubled while those of the pristine GFA3-PBNP nearly doubled by the third dipping cycle (Table 1).…”

“…To interrogate these electronic properties, an analysis of the π-band peak intensities over those of the σ bands (Fig. 6) were calculated as already explained elsewhere [45]. The results show that treated graphites and graphite-PBNP had about the same signal intensities and peak values up to the E F region.…”

“…The thermal treatment protocol is as reported elsewhere [45]. Briefly, samples were loaded onto a ceramic block and inserted into a quartz tube furnace (Lindberg/Blue M type model, Thermo Scientific, Asheville NC, USA) pre-heated to 650 °C with both ends open to air.…”

“…Raman spectroscopy as previously described [45] utilized a WITec™ alpha300 R(aman) instrument (GmbH, Germany) with a 100-mW, frequency-doubled, Nd:YAG (λ = 532.5 nm) laser focused through a 20× Nikon objective (NA = 0.4, WD = 3.9 mm) for an excitation spot of ~ 10 μm diameter. Laser power attenuated manually to reduce fluorescence and localized thermal effects during sampling produced a final incident irradiation power of the order of 10 mW.…”