Electrode deactivation

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

The EOTR between the organic compound (R) and the hydroxyl radicals take place at the electrode surface (both adsorbed) according to a Langmuir-Hinshelwood type mechanism. This process has been extensively studied mainly for fuel cell applications. However, as it has. been reported in Sect. 1.2, it is limited for simple Ci organic compounds (methanol, formic acid). Furthermore, there are problems with electrode deactivation due to CO chemisorption on the electrode active sites. 

With galvanostatic instead of potentiostatic control, the potential would vary along a CER, which could compound the problem of selectivity variation caused by concentration changes. In both potentiostatic and galvanostatic operation, the nonuniform conditions may promote side reactions, including local electrode deactivation processes. 

Nogami et al. and Augustynski et al. showed that the activity of copper electrode, deactivated during the CO2 reduction, is recovered by anodic polarization of the electrode. Periodic anodic pulses are effective to maintain the electrocatalytic activity in prolonged electrolysis of CO2 reduction. 

Slow degradation of the conducting carbonaceous phase, resulting in electrode deactivation, also occurs when oxidation of the redox species proceeds at the surface of ion-implanted polymers. 

Scott, K., 1986, Electrolytic reduction of oxalic acid to glyoxylic acid A problem of electrode deactivation, Chem. Eng. Res. Des. 64 266-271. 

Solvent cleaning, as applied to other carbon electrodes, is expected to be an effective, nondamaging method of electrode pretreatment. Several different solvents can be used to clean the electrode surface including acetonitrile, isopropanol, dichloromethane, and toluene. The solvents should first be distilled for purification. Reagent-grade solvents often contain impurities at levels that can cause significant electrode deactivation. AC can be added to the distilled solvents for additional purification. Soak times of 20-30 min should be adequate. The solvent cleaning can also be performed by Soxhlet extraction. 

NADH is a cofactor in a large number of dehydrogenase-based biosensors. However, bare glassy carbon (GC) and other electrodes deactivate rapidly during the determination of NADH due to the irreversible and strong adsorption of NAD+, an oxidation product [26]. A disadvantage with the modified-electrodes is the influence of oxygen present in the solution. The use of the diamond... 

Furthermore, there is no indication of electrode deactivation during 3-MP oxidation under these experimental conditions. 

This anodic reaction can induce polymerization, resulting in the deposition of an adherent polymeric material on the electrode surface. The formation of this polymeric material results in electrode deactivation [6,14-15]. 

The electrode deactivation by polymeric materials and reactivation at high anodic potentials can be illustrated using phenol as a model phenolic compound. Fig. 20.10 shows typical cyclic voltammetric curves for BDD electrodes obtained in a solution containing 2.5 mM of phenol in 1 M HCIO4 at a scan rate of 100 mV s. ... 

The electrochemical oxidation of a large number of organic compounds (Table 20.1) at high anodic potentials (close to the potential region of supporting electrolyte/water decomposition) on BDD has shown that the oxidation can be achieved at high current efficiency without any indication of electrode deactivation (this was the case for phenolic compounds at low anodic potentials) [6,14-15]. 

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