Electrode deterioration

Ascorbic acid (1) is most commonly used for testing the performance of electrodes in redox systems. Thus, a Ag-Ag ascorbate selective electrode was constructed with view to use it for vitamin C determination. Its reproducibility and stability was satisfactory and ascorbate ion concentration could be determined in neutral, alkaline and alcoholic media" . A voltametric study was carried out for the evaluation of graphite-epoxy composite (GEC) electrodes for use in the determination of ascorbic acid and hydroquinone. They were compared with mercury and CPE in similar operating conditions of pH and supporting electrolytes. Like all redox electrodes, also GEC electrodes deteriorate on exposure to air or after repeated usage, and the surface had to be renewed for activation. GEC electrodes were found to be adequate for redox system analyses"". The electrocatalytic oxidation of 1 is an amplification method for determination of specific miRNA strands using the An biosensor described in Table 1 . 

Some metals, e.g., lead and iron, will form stable conductive oxide coatings under properly controlled anodic conditions.16 18 Metal electrodes with oxide coatings are economical compared to those with platinum coatings, but they are not reversible. Indeed, reversibility of ED electrodes is exceptional. Care must be exercised to ensure that the polarity of ED electrodes is not inadvertently reversed, because severe electrode deterioration can result. 

Although the reactions at the cathode are not severe in terms of electrode deterioration, fouling problems can be severe at the cathode. The presence of pH-sensitive salts, e.g., Ca(HC03)2 in the feedwater is a matter of concern, because the OH ions generated at the cathode can lead to precipitation. 

As the cell operates, [Zn" ] increases (as the Zn electrode deteriorates) and [Cu ] decreases (as Cu plates out on the Cu electrode). Although the changes during this process occur smoothly, we can identify four general stages of operation. Figure 21.11A shows the first three. The main point to note is as the cell operates, its potential decreases ... 

Popovic et al. proposed the formation of charge traps that act as nonradiative recombination centers in the emissive material as one possible explanation for the relative differences in the electroluminescence and photoluminescence losses in operating OLEDs [27]. Despite the absence of direct evidence of the formation of such traps or their ability to act as nonradiative recombination centers, the proposal was quite plausible, particularly in view of a complementary notion that, in some cases, recombination on highly emissive charge traps is responsible for the predominant emission by the dopants [35,36]. The rise in voltage required to drive a constant current through a device has been cited as possible evidence of the formation of traps, but the voltage rise could be, in fact, attributed to mechanistically imrelated phenomena, such as electrode deterioration. 

The electric field profiling also provides an important insight into injecting electrode deterioration during operation. Because currents in OLED devices are commonly injection-limited, the electrode deterioration results in increasing electric fields in adjacent layers. The total operating voltages and, consequently, power efficiencies, are adversely affected. 

Electrode requirements The calomel reference electrode should be of the sleeve tip rather than the fiber or ceramic plug type and care should be taken with the replacement of the reference electrode solution. The fluoride ion-selective electrode consists of a europium-doped lanthanum fluoride crystal. The emf response of the electrode to standard solutions should not be less than 55 mV over the range of concentrations of 0.2-200 mg dm Values of the emf response lower than this can arise from poor matching of the sensor and reference electrodes, deterioration of the sensor electrode membrane, poor maintenance of the reference electrode, or poor maintenance of the sensor electrode. The analysis should be carried out in plastic vessels. 

Another key fact is that the electroreduction of CO2 may take place at the electrode surface or be mediated by a catalyst attached to the electrode or soluble in the solvent (Scheme 8.1). In case (A) the reaction occurs at the surface of the electrode which directly transfers electrons to CO2 this generally requires high overvoltage and may cause electrode deterioration or even consumption and scarce selectivity. In case (B) the electrode transfers electrons to the catalyst which then interacts with CO2 causing its reduction. In such a case, CO2 is not in contact with the electrode, and this results in better stability of the electrode and its longer life, which may be important in the overall costing determination of a process. 

Imanishi and co-workers of Mie University, Japan, [68] have examined the charge discharge performance of an aqueous lithium-air cell with 0-LATP and saturated LiOH with 10 M LiCl aqueous solution with a third electrode for the OER, as proposed by Stevens [50]. Rechargeable hthium-air batteries require a bi-functional air electrode for the oxygen reduction reaction (ORR) and the OER. When an air electrode is used for the OER, it is exposed to a highly corrosive potential. Arai et al. [69] examined a high surface area carbon black (Ketjen black KB) electrode for the ORR and OER in aqueous 8 M KOH and observed electrode deterioration during the OER, which they claimed was due to a loss of the electrochemicaUy active surface... 

The electrolytes are non-corrosive and the electrodes do not corrode with time. This feature is of special significance when compared with an ordinary liquid resistance starter used commonly for slip-ring motors. Electrolytes do not deteriorate and therefore do not require replacement. The evaporated liquid can be replenished with drinking water when the level of the electrolyte falls as a result of evaporation. In Europe such starters have been used for over 15-20 years. Electrolyte switching is a costlier proposition compared to direct on-line or star/delta switching due to additional shorting contactor and timer, and the cost of electrolyte, its tank and thermostatic control etc. The cost may. 

Deterioration of electrode performance due to corrosion of electrode components is a critical problem. The susceptibility of MHt electrodes to corrosion is essentially determined by two factors surface passivation due to the presence of surface oxides or hydroxides, and the molar volume of hydrogen, VH, in the hydride phase. As pointed out by Willems and Buschow [40], VH is important since it governs alloy expansion and contraction during the charge-discharge cycle. Large volume changes... 

If, after many titrations, the inflexion quality deteriorates, simply peel off the old electrode coating and recoat as described above. 

---