Degradation cyclic voltammetry

A fuel cell is a typical electrochemical device. Thus, the electrochemical analysis methods developed for other electrochemical devices can be directly used or modified for diagnosis of fuel cell degradation, including catalyst layer degradation. Cyclic voltammetry and polarization curve analysis are very frequently used in failure analysis and degradation diagnosis. 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

Although the diffusion coefficient (79abts = 3.2 x 10 cm /s) and solubility (>30 mM) of ABTS are much higher than those of competing redox polymers, the compound was found to suffer from oxidative degradation at potentials exceeding 0.92 V vs SHE in pH 7 buffer. Cyclic voltammetry of ABTS detected two oxidation peaks, one at 530 mV that was... 

In general the electrochemical stability of an electrolyte is experimentally evaluated by means of cyclic voltammetry. However, the determination of the electrochemical windows exhibits several problems. First, the electrochemical degradation or breakdown of an electrolyte is an irreversible reaction, thus there is no theoretical redox potential [40, 41], Passivation of the electrodes often makes it difficult to identify the onset of the reaction due to inhibition of further reactions [40, 42],... 

The antitumour action of the natural antibiotic bleomycin is thought to involve the aerobic degradation of DNA by the Fe2+-bleomycin complex. In order to probe the mechanism of antitumour action of bleomycin, the 4-ethylamido[5,(2 -thienyl)-2-thiophene] imidazole iron(II) complex was synthesized [129]. It was studied in non-aqueous solution using cyclic voltammetry and showed antitumour activity in vitro, its action causing cleavage of the double helical DNA. 

The same group highlighted the beneficial effect of adopting a membrane-divided cell with aqueous NaOH anolyte and non-aqueous (ACN) catholyte. The degradation of 1,2,3-TCB was performed on glassy carbon (cyclic voltammetry experiments) and Pd (preparative electrolyses) cathodes (Miyoshi et al. 2004b, c). 

Poly(p-phenylene) also fascinated many workers due to its cheap cost, good mechanical strength and high stability for use in electrochemical systems. In anodic oxidation and over-oxidation studies in aqueous acidic solutions of different nucleophilicity by cyclic voltammetry, two peaks were observed. The reversible first peak was attributed to poly(p-phenylene) oxidation and formation of some insertion compound, whereas the strong and irreversible second peak at higher potentials was attributed to over-oxidation of the polymer [291] and the polymer anode started degrading at acid concentrations below 13 M [292], Accordingly, the microanalytical, IR spectroscopic and acid-base titration studies showed evidence of the formation of aromatic dicarboxylic acid, phenolic compound and carbondioxide via a p-benzoquinone intermediate [291],... 

Cyclic voltammetry is not primarily a quantitative analytical technique. The references at the end of this chapter provide additional guidance to its applications and interpretation. Its real value lies in the ability to establish the nature of the electron transfer reactions—for example, fast and reversible at one extreme, slow and irreversible at the other—and to explore the subsequent reactivity of unstable products formed by the forward sweep. Suffice it to say that such studies are valuable for learning the fate and degradation of such compounds as drugs, insecticides, herbicides, foodstuff contaminants or additives, and pollutants. 

Fig. 20. Cyclic voltammetry of Desulfovibrio africanus fer-redoxin III, showing how cluster transformation may be controlled and monitored. Scan rate 16 mVs", temperature 2°C. Protein 0.1 mM in solution containing 0.1 M NaCl, 20 mM HEPES, 0.1 mM EGTA and 1.1 mM neomycin at pH 7.4. The EGTA sequesters Fe that is released during slow degradation of the protein during handling.

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