Carbon electrodes, anode effect

The electrochemical 2-chlorophenol and 2,6-dichlorophenol removal from aqueous solutions using porous carbon felt (Polcaro and Palmas 1997) or a fixed bed of carbon pellets (Polcaro et al. 2000) as three-dimensional electrodes was investigated by Polcaro s group. The group s experimental setup consisted of a two-compartment electrochemical cell separated by an anionic membrane where the carbon felt or pellets could be lodged and the solution was recirculated by peristaltic pumps. Both carbon-based anodes effectively removed the chlorophenols as well as their reaction... 

In addition to the stabilizing effect of cathode particles on the electrolyte solutions at elevated temperatures, graphite-like carbon electrodes (anodes) were also foimd to reduce the thermal decomposition of bulk LiPFe electrolyte solutions. However, the reduction of bulk electrolyte decomposition coincided with reactions of the electrolyte with the anode. The surface of the carbon electrode was covered with the products of the electrolyte reduction, which formed a protective solid electrolyte interface (SEl) layer [35-37], The stabilizing effect of these anodes (e.g., based on lithiated mesocarbon microbeads, MCMB) on the electrolyte was proposed to relate to the degradation of the solid-electrolyte interphase (SEl) in LiPFe-based electrolytes at elevated temperatures [32,38], The loss of capacity and power from lithium-ion cells undergoing accelerated aging experiments has been attributed to the presence of thermal decomposition products of the electrolyte in the anode SEl [32],... 

Now let us consider a model for a SC device that comprises two electrodes (anode and cathode), each of them being electrically connected to a current collector fabricated of A1 foil. Let two of such collectors have a certain thickness of SAi- As an electrode material, an activated carbon powder is considered below. Anode and cathode are interposed with a separator of thickness Ss. The electrodes and separator are impregnated with electrolyte. In this paper we mostly focus on the optimization of SC performance by varying the electrode thickness, while some other effects will briefly be considered in the next section. 

Scott et al. [33] designed a DMFC with stainless steel mesh as the anode FF plate that was able to remove the carbon dioxide gas effectively. Later, the same research group was able to demonstrate that using similar meshes as DLs in the anode side also improved the overall gas removal [26,34] (wet-proofed CFP was used as the DL on the cathode side). These meshes were used on the anode side and were made out of catalyzed Ti because similar meshes have been used extensively as catalyzed electrodes in other industries, such as the chlor-alkali industry [26]. 

In equation 5, C is amorphous carbon and CF2 changes to many perfluorocarbons, such as CF4, C2F6, etc., by secondary reactions. The surface coverage of graphite fluoride on the anode depends on the relative reaction rates of equations 4 and 5. Equation 6 has been introduced to analyze the wettability of the carbon surface with graphite fluoride formed on it.2 It shows the relationship between the fraction of effective surface for equation 3 per unit surface area of carbon (a) and the contact angle (0) of a fluorine gas bubble on the surface of the carbon electrode.2... 

The arc discharge was observed on the carbon anode when the anode effect occurred, and the contact angle of fluorine gas on the surface of the carbon electrode was found to be 180".2 From equation 6, therefore a can be calculated as zero. 

As CXF is the material which contributes to the anode effect, the carbon electrode with CXF formed on it is easily polarized. The elimination of traces of water from the crude bath by the hydrolysis of water with fluorine is important for the stable operation of the fluorine cell. 

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

The effects of mild ECP on carbon fibers appear to be quite similar to that on macro GC electrodes. Anodization forms surface oxides and eventually a graphite oxide film. The oxide layer or film preferentially adsorbs or ion exchanges cations. In the case of in vivo analysis, this leads to enhanced sensitivity for cationic dopamine over the ascorbate anion [5,6,54], There does not appear to be a standard procedure for mild ECP, but most workers alter the process to improve performance for a particular analytical target. 

Philips has taken a different direction in the field of electrochemical fluorination. The Philips ECF process uses porous carbon electrodes and KF 2 HF as an electrolyte. Here, fluorination is effected in the pores of the anodes by electrochemically produced elemental fluorine. The process is therefore suitable for low-boiling products which are substantially insoluble in the electrolyte. The process, which has been successfully tested on the pilot scale, is reviewed by W. V. Childs in 75). 

Electrode surface activation can be improved simply by electrochemical pretreatment. Determination of nitroaromatic compounds in water and soil spiked samples have been reported at electrochemically activated carbon-fiber microelectrodes. No interference was found from compounds such as hydrazine, phenolic compounds, carbamates, triazines or surfactants. The detection limit obtained can be approximately 0.03 iigml-1 for all the nitroaromatic compounds (Agui et al. 2005). Chen and coworkers reported an effective field-deployable tool for detecting nitroaromatic compounds with an electrochemically pre-anodized screen-printed carbon electrode (SPE) (Chen et al. 2006). 

Along with the anode reaction, the so-called anode effect, a phenomenon often observed in fused salt electrolysis (see Chapter 4), may occur. In the present case, it may be due to a surface film of the type CVX formed on the anode material. This film on the one hand protects the carbon against destruction (and is the reason for high anodic overpotentials) in normal operation and, on the other hand, under more or less known conditions may block electron transfer completely. These conditions depend strongly on the electrolyte composition (purity) [46,47]. Additives, such as lithium fluoride, may be helpful in preventing the anode effect by wetting the electrode material. 

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