Phenol electrode passivation

Gattrell, M. and Kirk, D. (1993) A study of electrode passivation during aqueous phenol electrolysis. J. Electrochem. Soc., 140, 903 -911. 

Ferreira, M., Varela, H., Torres , R.M., and Tremiliosi-Filho, G. (2006) Electrode passivation caused by polymerization of different phenolic compounds. Electrochim. Acta, 52 (2), 434-442. 

Electroanalysis of phenol derivatives including CPs is commonly problematic at most solid electrodes mainly due to the electrode passivation resulting from oxidation products. The studies on reaction mechanism of electrooxidation of phe-noi5,43,44 j pg45,46 cidic media at BDD electrodes revealed that in the... 

Anodic oxidation of phenols gave the corresponding poly(1,4-phenyleneoxide)s by selecting the electrolysis conditions to prevent passivation of the electrode. 

However the formation of thin polymer film on the electrode, i.e. passivation of the electrode, resulted in cessation of the polymerization, which restricted the electro-oxidation as a polymerization procedure. The electro-oxidative polymerization as a method of producing poly(phenyleneoxide)s had not been reported except in one old patent, in which a copper-amine complex was added as an electron-mediator during the electrolysis (4). The authors recently found that phenols are electro-oxidatively polymerized to yield poly-(2,6-disubstituted phenyleneoxide)s, by selecting the electrolysis conditions This electro-oxidative polymerization is described in the present paper. 

Although the literature on electrodeposited electroactive and passivating polymers is vast, surprisingly few studies exist on the solid-state electrical properties of such films, with a focus on systems derived from phenolic monomers, - and apparently none exist on the use of such films as solid polymer electrolytes. To characterize the nature of ultrathin electrodeposited polymers as dielectrics and electrolytes, solid-state electrical measurements are made by electrodeposition of pofy(phenylene oxide) and related polymers onto planar ITO or Au substrates and then using a two-electrode configuration with a soft ohmic contact as the top electrode (see Figure 27). Both dc and ac measurements are taken to determine the electrical and ionic conductivities and the breakdown voltage of the film. 

It is well known that the oxidation of phenolic compounds at solid electrodes produces phenoxy radicals, which couple to form a passivating polymeric film on the electrode surfaces [20,21]. The anodic reaction proceeds through an initial one-electron step to form phenoxy radicals, which subsequently can undergo either polymerization or further oxidation with the transfer of oxygen from hydroxyl radicals at the electrode... 

Reductants. Reductants can break down passive oxide films because thev favor reductive dissolution and lower the electrode potential. Naturally x-curring organic substances, including fulvic and humic acids and phenols, tan reduce Fe(III) (hydr)oxides. Especially detrimental are H2S and S(—II) compounds. S(-II) has a strong affinity to Fe(III) (Figure 3) (55). 

There remain some problems to be solved for this electrolytic oxidation of phenols. (i) Considerable magnitude of overvoltage and low current efficiency. (ii) Poly(phenylene oxide) formed deposits on the electrode surface as a thin, insulating film passivating of the electrode. (iii) The side reaction which forms b iphenoquinone. 

Nitrobenzoate amines and nitrous derivatives of azoles form a separate group of Cl. Owing to their high oxidizing capacity, they accelerate the cathodic process, which leads to shifting of the electrode potential of the corrosive system to values maintaining the passive state of the metal [47,61]. Contact Cl of nitrated paraffins, SFA, alkyl phenols, mineral oils, oxidized and nitrated petrolatum and ceresin are also widely used today [62]. 

Regardless to the material used, electrochemical sensors suffer from several drawbacks, one of the most known is the passivation of its surface [31]. The oxidation of electroactive substances involves the formation of oxidized material at the electrode surface which blocks further reaction at the electrode [32]. For instance, during the electrochemical detection of biological analytes in matrices with a high content of phenolic compounds, the surface of the electrode is contaminated and the subsequent analysis is compromised [33]. 

With reference to electrochemical pol3merization, 21 shows the current-potential curve for the oxidation of a Fe microanode in alkaline water when phenol 0.5 M is added of to this system can observe that a comidete passivation of the electrode results after anodic polarization, up to very positive potentials. 

The very fast passivation of a metal electrode whidi results on anodizing phenols in suitable media may be explained in this way in a nwdium containing a base alai e number of phencA molecules are deprotonated to phenoxide anions which are stored on the positive electrode. Phenol molecules fiirthermore, owing to the conjugated n-systems which may interact with the ions of a metallic lattice, are likely to adsorb... 

To prevent the passivation, the successful strategy for BDD electrodes includes the oxidation at highly anodic potential in the region of water decomposition. Hydroxyl radicals produced by the high applied potential are believed to be responsible for the oxidation of the passivating layer. This concept was successfully applied also for selected dichloro- and trichlorophenols, " which are presumably more prone to inactivation of electrode surface than mono-substituted CPs. For them the detection at BDD electrodes is possible even without any electrochemical remediation of the electrode surface as demonstrated for phenol, 2-chlorophenol (2-CP), and 4-chlorophenol (4-CP). ... 

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