Graphite electrochemical properties

Although some nanostructured carbons are able to store reversibly higher amounts of lithium than graphite [2], they generally demonstrate a high hysteresis (see for example Figure 2) that still precludes their use in lithium-ion batteries. In order to improve their electrochemical properties as electrode materials, a current effort is made to better understand the... 

Thus, the electrochemical properties of the individual carbon materials are not so high as to enable their commercial usage in Li-ion batteries. In order to improve the performance, we started making composite materials from two individual carbon ingredients. Figure 1 shows a typical result of electrochemical tests of an electrode made of a blend of graphite and soft carbon treated at 1100°C (Cl 100) in comparison with the discharge curves of the individual constituents. 

The early patent disclosures have claimed the application of a wide spectrum of gas-evolving ingredients and phosphorus-based organic molecules as flame retarding additives in the electrolytes. Pyrocarbonates and phosphate esters were typical examples of such compounds. The former have a strong tendency to release CO2, which hopefully could serve as both flame suppressant and SEI formation additive, while the latter represent the major candidates that have been well-known to the polymer material and fireproofing industries.The electrochemical properties of these flame retardants in lithium ion environments were not described in these disclosures, but a close correlation was established between the low flammability and low reactivity toward metallic lithium electrodes for some of these compounds. Further research published later confirmed that any reduction of flammability almost always leads to an improvement in thermal stability on a graphitic anode or metal oxide cathode. 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

Carbon fibers have not been studied as extensively as GC or graphite, and in most cases the fiber is pretreated. Thus most of the electrochemical properties of fibers are discussed in the next section, Preparation. A few general points are useful here, dealing with size and resistance. Since the majority of carbon fibers are 5-15 pm in diameter, they will exhibit nonplanar diffusion under most conditions, whether they are used as disks or as cylinders. For example, VDt for a typical analyte (D = 5 x 10 6 cm2/s) equals 2.2 pm at 10 ms. This is a significant fraction of a typical fiber diameter, so diffusion will become nonplanar even at short times. Thus any experiment lasting more than a few milliseconds will deviate from a response predicted for planar diffusion. Note that the deviation depends on whether the fiber end is used as a disk electrode or an exposed fiber is used as a cylinder, but quantitative theories have been presented for both cases [48]. 

The diphenylalanine nanotube sensors were based on the observation that peptide nanotubes improve the electrochemical properties of graphite and gold electrodes when deposited directly onto the electrode surface (Yemini et al., 2005b). The high surface area of the nanotubes and the potential alignment of aromatic residues are thought to contribute to the observed increase in conductivity. This property makes nanotube-coated electrodes and hydrophobin-coated electrodes suitable for use as amperometric biosensors that produce a current in response to an electrical potential across two electrodes. 

The pioneering studies of capacitance exhibited by basal and edge planes of carbons by Yeager and coworkers showed abnormal behavior, which led Randin and Yeager [151] to conclude that the space charge characteristics of graphite must be taken into account in examining the electrochemical properties of this material. ... 

Because of the complex behaviour to be expected for natural nucleic acids, it is only natural that considerable effort has been devoted to studies of the electrochemical properties of their monomeric units, and defined analogues of these, as well as of synthetic oligo- and polynucleotides. A variety of techniques has been applied for this purpose, and some of the details and findings are covered in several reviews 19 24). Most investigations have dealt with electroreduction processes 15 20,24,25). Only relatively recently has attention been directed to possible electrooxidation of nucleic acids and their constituents with the aid of the graphite electrode which, in comparison with the mercury electrode, possesses a much greater accessible range of positive potentials 26 29). 

Carbonaceous substrates (graphite and glassy carbon) are generally preferred because of their mechanical, chemical, and electrochemical properties. Excellent results are also obtained by chemically modified platinum (154,156,179) and tin(IV) oxide electrodes (155). For example, glucose oxidase has been successfully immobilized by cross-linking the enzyme with BSA and GA onto an electrochemically oxidized platinum surface, with silanization using 3-amino-propyltriethoxysilane ... 

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