Carbon electrode surfaces

The extent of the irreversible charge losses due to film formation depends to a first approximation on the surface area of the lithiated carbon which is wetted by the electrolyte [36, 66, 120-124]. Electrode manufacturing parameters influencing the pore size distribution within the electrode [36, 121, 124, 125] and the coverage of the individual particles by a binder [124, 126] have an additional influence on the carbon electrode surface exposed to the electro-... 

Figure 3. Cyclic voltammograms of ascorbic acid at a freshly polished, active (a) and a deactivated (b) glassy carbon electrode surface. See text for details.

C.H. Lei and J.Q. Deng, Hydrogen peroxide sensor based on coimmobilized methylene green and horseradish peroxidase in the same montonorillonite-modified bovine serum albumin-glutaraldehyde matrix on a glassy carbon electrode surface. Anal. Chem. 68, 3344—3349 (1996). 

In most applications, the electrochemical compounds are usually oxidized, yielding one or more electrons per molecule reacted. The oxidized form is usually unstable and reacts further to form a stable compound that flows past the carbon electrode surface. Unfortunately, this is not always the case, with the stable oxidized form occasionally building up at the surfaces of the carbon electrode. This creates sensitivity problems and decreases the efficiency of the detector. However, the problem is usually overcome by regularly cleaning the carbon electrode surfaces, removing any oxidizable products. Eluents for EC detection must be electrochemically conductive, which is achieved by the addition of inert electrolytes (to maintain a baseline current) such as phosphate or acetate. All solvents and buffers used in preparation of an eluent must be relatively pure and selected so as to not undergo electrochemical changes at the applied electrode potentials. 

A special case is when the electrochem-ically active components are attached to the metal or carbon (electrode) surface in the form of mono- or multilayers, for example, oxides, hydroxides, insoluble salts, metalloorganic compounds, transition-metal hexacyanides, clays, zeolites containing polyoxianions or cations, intercalative systems. The submonolayers of adatoms formed by underpotential deposition are neglected, since in this case, the peak potentials are determined by the substrate-adatom interactions (compound formation). From the ideal surface cyclic voltammetric responses, E° can also be calculated as... 

The oxidized forms of these two sandwich-type POMs are stable from pH 3 to at least pH 7. Their characterization by cyclic voltammetry revealed the stepwise reduction of the Gu " " centers within the POMs, before Gu" deposition on the glassy carbon electrode surface [115]. Phenomena are described mainly for the P-derivative and are the same for the As-analog. 

Other Substrates Deposition of cadmium was also studied on Bi, Sn and Pb [303], Ni [304], reticulated vitreous carbon [305], Ti [306], and indium tin oxide [307]. UPD of Cd on tellurium results in CdTe formation [270, 308]. Electrodes coated with conducting polymers were also used to deposit cadmium electrochemi-cally. In the case of polyaniline, the metal reduction potential corresponds to the neutral (nonconducting) state of the polymer, therefore cadmium was found to deposit on the substrate-glassy carbon electrode surface, in the open pores of the polymer film [309, 310]. 

Biosensors constructed for ethanol and D-glucose measurements in beverages were built using ferrocene compounds as electron transfer mediators between the coenzyme PQQ of immobilized enzymes glucose (GDH) and alcohol (ADH) dehydrogenases and a carbon electrode surface <2003JOM(668)83>. 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

The concept of using the functional groups of electrode surfaces themselves to attach reagents by means of covalent bonding offers synthetic diversity and has been developed for mono- and multi-layer modifications. The electrode surface can be activated by reagents such as organosilanes [5] which can be used to covalently bond electroactive species to the activated electrode surface. Recently, thermally induced free-radical polymerization reactions at the surfaces of silica gel have been demonstrated [21]. This procedure has been applied to Pt and carbon electrode surfaces. These thermally initiated polymer macromolecules have the surface Of the electrode as one of their terminal groups. Preliminary studies indicate that the... 

Cyclic voltammetry was carried out in the presence of penta- and hexacyano-ferrate complexes in order to probe the homogeneity and conductivity of the TRPyPz/CuTSPc films (125), (Fig. 36). When the potentials are scanned from 0.40 to 1.2 V in the presence of [Fe (CN)6] and [Fe CN)5(NH3)] complexes, no electrochemical response was observed at their normal redox potentials (i.e., 0.42 and 0.33 V), respectively. However, a rather sharp and intense anodic peak appears at the onset of the broad oxidation wave, 0.70 V. The current intensity of this electrochemical process is proportional to the square root of the scan rate, as expected for a diffusion-controlled oxidation reaction at the modified electrode surface. The results are consistent with an electrochemical process mediated by the porphyrazine film, which act as a physical barrier for the approach of the cyanoferrate complexes from the glassy carbon electrode surface. 

High quality IR spectra of different carbon surfaces were obtained by photo-thermal beam deflection spectroscopy (IR-PBDS) [123,124]. This technique was developed with the intention of providing an IR technique that could be used to study the surface properties of materials that are difficult or impossible to examine by conventional means. Recently, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been successfully applied to study the effect of different pretreatments on the surface functional groups of carbon materials [101,125-128]. Several studies aiming to improve the characterization of the carbon electrode surface and the electrode-electrolyte interface have been carried out using various in situ IR techniques [14,128-132]. The development of in situ spec-troelectrochemical methods has made it possible to detect changes in the surface oxides in electrolyte solutions during electrochemical actions. 

The formation of grafted films on carbon electrode surfaces is attained by electrooxidation of arylacetates under Kolbe electrolysis conditions [64]. 

Figure 8.37. In situ Raman spectra of dopamine oxidation in I M HBr at a carbon electrode surface. An IPDA detector was gated for 50 msec time increments ending at the times indicated after initiation of DA oxidation. The band at 1572 cm is from the electrogenerated orthoquinone, that at 1539 cm is from brominated quinone. (Adapted from Reference 33 with permission.)...

Figure 6. SEM image of glassy carbon electrode surface modified with gold nanoparticles prepared with the seed-mediated growth approach. Reproduced from [32], copyright 2007, with permission from the Japan Society for Analytical Chemistry.

Schreurs, J., Van den Berg, J., Wonders, A., Barendrecht, E., Characterization of a Glassy-Carbon-Electrode Surface Pretreated with rf-Plasma , Reel. Thav. Chim. Pays-Bas 103 (1984) 251-259. 

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