Increased electron transfer kinetics and thermally treated graphite stability through improved tunneling paths (original) (raw)
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Thermal Modification of Graphite for Fast Electron Transport and Increased Capacitance
ACS applied nano materials, 2018
On thermal treatment, eight different graphite materials became resistant to air aging for at least nine weeks compared to the usual time of hours to a few days when assayed in mM ferriferrocyanide solution. In addition, resistance to aging lasted at least seven 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 non-perturbative 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.
Chemical Oxidation of Graphite: Evolution of the Structure and Properties
The Journal of Physical Chemistry C, 2017
Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graphite and the electrical resistivity increased by seven orders of magnitude. After 4 days of chemical processing, the expanded structure of graphite oxide became unstable and spontaneously exfoliated; more than 2 weeks resulted in a significant decrease in the oxygen content accompanied by reaggregation of the GO sheets. These correlated measurements allow us to offer a comprehensive view into the complex oxidation process.
Electron Transfer Kinetics on Mono- and Multilayer Graphene
ACS Nano, 2014
Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized.
The Journal of Physical Chemistry, 1994
Electron-transfer rates for 17 inorganic redox systems plus methyl viologen were determined on highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC). Provided the HOPG defect density is low, the electrontransfer rates of all systems are much slower on the basal plane of HOPG than on GC. The slow rates on HOPG show a trend with the homogenous self-exchange rate constants, but in all cases the HOPG rate constants are substantially lower than that calculated via Marcus theory from self-exchange rates. The low HOPG rates do not exhibit any trends with redox system charge or E1/2, as might be expected in the presence of double-layer or hydrophobic effects. The results are consistent with the semimetal properties of HOPG, which have been invoked to explain its low interfacial capacitance. Both the density of electronic states (DOS) and carrier density for HOPG are much lower than those for metals. By analogy to theories developed for electron transfer at semiconductor electrodes, the rate depends on an effectively bimolecular reaction between the redox system and carriers in the electrode. The low DOS and carrier density of HOPG leads to low electron-transfer rates compared to those of metals, or to those predicted from exchange rates. Disorder in the graphite increases electron-transfer rates and the DOS, thus yielding much faster rates on both G C and defective HOPG. For the 14 outer-sphere systems studied here, this electronic factor is much more important than any interaction with specific surface sites present at defects. The evidence indicates that, for Fe(CN)i3I4, Eu:tJt3, Fet2It3, and V:l/+3, specific surface interactions provide inner-sphere routes which have a large effect on the observed rate constant. aq
Electrochemically oxidised graphite
Carbon, 2000
An evaluation of some of the properties of electrochemically oxidised graphite has been carried out. These studies include textural characterisation, magic angle spinning NMR, ESR and ion exchange properties. A study of the surface morphology has also been carried out using high-resolution transmission electron microscopy and identification of surface groups confirmed by FTIR spectroscopy. The electrochemical method of preparation is shown to confer, to the porous graphite oxide obtained, different surface chemical groups that can be used for ion exchange purposes. ESR shows that Cu(II) is coordinated to the oxidised graphite.
High Temperature Oxidation of Graphite
Nanomaterials and Energy, 2018
Graphite is used in extreme environments, such as high-temperature gas-cooled reactors and anodes for hightemperature fuel cells, because of its outstanding irradiation performance and oxidation resistance. Its oxidation resistance at high temperature depends on the porosity, ash content, oxidizer concentration, burn-off degree and degree of graphitization. The rate of oxidation was determined for different graphite grades (GLM50, GLM and SPSS) at the isothermal temperature range from 773 to 1273 K in a controlled dry oxygen environment. The degree of graphitization was quantitatively measured using selected area electron diffraction patterns and Raman spectra and was shown to affect the oxidation resistance of the graphite grade. The lowest degree of graphitization (0•67) and a high average pore radius (15•4 nm) in GLM decrease the rate of oxidation due to limited active sites for oxidation. The pore radius and the high graphitic structure reduce the average activation energy of GLM50 (41•8 kJ/mol) compared to those of GLM (48•5 kJ/mol) and SPSS (58•5 kJ/mol) in the boundary-layer-controlled regime in the temperature range 1073-1273 K. Notation A pre-exponential factor A S unit surface area E a activation energy of oxidation I D D-band Raman intensity I G G-band Raman intensity k isothermal rate constant at temperature T R universal gas constant Dm/A S normalized weight loss Cite this article
Graphite photoelectrochemistry
Journal of Electroanalytical Chemistry, 2000
Photocurrent generation by UV-vis light irradiation of glassy carbon, boronated glassy carbon, carbon-black and carbon-fiber electrodes was studied in comparison with the behavior of HOPG basal plane and edge plane electrodes. The presence of a space charge layer at these electrodes was manifested by photogeneration of electrochemical currents. Both anodic and cathodic photocurrents were observed for all the forms of graphitic materials investigated. An exponential dependence of photocurrent magnitude on electrode potential and on photon energy was found for all the forms of graphitic materials. The exponential response was explained by a model involving the generation of a hot charge carrier by photoexcitation in the space charge layer, transport to the solid-electrolyte interface and subsequent interfacial charge transfer reactions. Studies of photocurrent-potential dependence performed with glassy carbon, boronated glassy carbon and carbon-black electrodes in highly acidic to highly basic aqueous solutions (pH 0-14) revealed a linear dependence of the flat bands potential on pH with a slope of approximately 0.05 V per unit of pH.