Poisoning of electrodes

The term water electrolysis implicitly means that the electrochemical reactor does not contain pure water only. Conventional electrolysis requires that the solution should be electrically conducting for the process to proceed. This implies that an electrolyte should be dissolved in water. Whereas in other cases, for example electrochemical organic or inorganic processes, the presence of an inert electrolyte may constitute a problem for the separation of products, this is not the case for water electrolysis since gaseous products are obtained. Nevertheless, the electrolyte can give other kinds of problems, such as corrosion phenomena, poisoning of electrodes and so on. 

Use of DBLs is effective to prevent metal element diffusion between Ni-based anode and steel. Nevertheless, as the DBLs are porous, they will unlikely contribute to prevent Cr evaporation and poisoning of electrodes in the long run. Besides, it was also shown that high depletion of Cr content in the bulk of... 

Liquid membrane electrodes should only be dipped for short periods of time in solutions which contain hydrophobic compounds, such as proteins or oils, in order to avoid a poisoning of the membrane. Between measurements in such solutions the electrode should be washed with copious amounts of distilled water. Poisoning of the membrane (for example by picrate anion with the potassium electrode) exhibits itself through a sudden loss of Nernstian response to the measured ion. With PVC-stabilized membranes surface impurities can be removed by wiping with a moist paper towel. Poisoning of electrodes with membrane fQters requires replacement of the fQter. 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Figure 6.15 Pulse voltammetry of a Pt(lOO) electrode in 0.5 M H2SO4 + 0.25 M HCOOH. The inset is an expansion of the region between 0.4 and 0.5 V to show the decay in the current due to the poisoning of the surface. During the pulse at high potentials (0.9 V) to remove the poison, the current was not recorded (baseline between pulses). (Adapted from Clavilier [1987].)...

The poisoning of the nickel surface is manifested in a sharp increase of electrode potential relative to the value in the active state. Since at most only part of the surface remains active, the actual current density can be much... 

In classical polarographic techniques, a dropping mercury electrode is used. This is a complex device in which continuously produced small droplets of mercury are used as the active electrode in order to prevent poisoning of the electrode and to provide constant conditions throughout the analysis. For many applications, specifically designed electrodes are available which are simpler to use. 

In the sequence of reactions (2)-(3)-(4) it was assumed that electron exchange takes place without the interaction of the species Ox and Red with the electrode surface. However, it is possible that the exchange of electrons does not occur unless the reagent Ox, or the product Red, is weakly or strongly adsorbed on the electrode surface. It is also possible that the adsorption of the species Ox or Red might cause poisoning of the electrode surface, thus preventing any electron transfer process. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

The electrolysis was carried out with a potential programme including only two plateaux, because of weak poisoning of the electrode surface, with a first, one at 0.2 V/RHE, during 0.2 s for the adsorption of glyoxal, and a second one at 1.9 V/RHE corresponding to the electrolysis potential (20 s). 

Reduction at Pt electrodes and Pt group metals of Ir and Rh in aqueous solution has led to CO production [56-58]. Intermediate-adsorbed species have been studied by Giner in acidic solution at Pt [59], while similar spectroscopic experiments at Pt electrodes have also been conducted by Nicolic et al. [60], It appears that the reduction of C02 occurs by reaction with adsorbed hydrogen atoms to produce both linear and bridge-bonded CO. The reduced C02 species at Pt electrodes, however, caused a rapid poisoning of the electrodes to further C02 reduction [61]. 

Eh can conveniently be measured by inserting a platinum electrode into the soil or sediment and connecting this electrode to a reference electrode, such as the calomel electrode. The electro-motive potential (emf) generated can be measured on a suitable detector and the Eh calculated as the difference between this potential and the electrode potential of the reference electrode. However, the electrode must be kept clean during and after measurements to prevent poisoning of the electrode. This poisoning is due to the formation of a Pt-oxide coating. 

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