Potentiostatic electrolysis

From an exhaustive potentiostatic electrolysis, the product(s) formed at the selected electrode potential can be isolated. Preparative and analytical techniques are available to determine the composition of the product mixture and the structure of its components. Mechanistic reasoning will often allow defining the reaction steps. Even more information about the reaction can be gained from electrolysis experiments at various defined potentials, for example, after each peak in the cyclic voltammogram of the substrate. 

Figure 6.22. Variation of current with time during potentiostatic electrolysis.

Table 19.2 Results of potentiostatic electrolysis (1 h) using a modified Pt electrode in a pH 1.0 aqueous solution...

Co t) decreases exponentially with time during electrolysis, and the same is true for the potentiostatic electrolysis current, i t). The expression for i t) is shown in Eq. 99, where /l is the limiting current at the start of the electrolysis t 0). 

Although main reduction product of CO2 on Ag under galvano/potentiostatic electrolysis is CO, CH4 was found to be effectively formed under the pulsed electrolysis at 17=-2.25V and V, =-0.6 -0.4V. The present result shows that the peak potential at which 77 (CH4) and 77 (CO) become a maximum shifts positively. A maximum faradaic efficiency of about 30% for hydrocarbonization reaction on Ag electrode was achieved as shown in Table 1. The yields of hydrocarbons increased while 77 (CO) decreased. 

In the extremely negative conditions (i.e., < 0.3V) however, hydrogen evolution occurred simultaneously and disturbed again the Tc deposition reaction. In the case of a carbon electrode which had no region in 3 mol/dm nitric acid solution, reduction of TCO4 to TcO was the most significant in the potentiostatic electrolysis at (]) e= 0.3 V or l.OV (V vs. Ag/AgCl) [8]. 

However, when dimesylates (MsO) are submitted to electrolysis, product distribution becomes dramatically different from that corresponding to ditosylates already described. As a matter of fact, the reduction of cyclopentane and cyclohexane 1,3-bismethanesulpho-nate (divided cell, Pt cathode and anode, potentiostatic electrolysis) now leads to a cyclopropane ring and not to the expected four-membered ring ether (Scheme 3). 

The potentiostatic electrolysis at a platinum electrode at +1-0 V results in a polymer layer on the electrode surface. After an induction period the electrode surface layer gives an ESR signal with one line of 0.15 mT linewidth (Fig.lO). The linewidth is independent on the radical concentration and the type of the cation in the supporting electrolyte. If the polyaniline film is in contact with aniline in the abscence of an applied potential, the line intensity decrease with time indicating a further reaction of the polymer with the aniline. Under UV-irradiation the free spin can be renewed without reaching its initial intensity (Fig.lOb). 

Polypyrrole can be synthesized by the galvanostatic or potentiostatic electrolysis of nonaqueous or aqueous solution containing pyrrole monomer and supporting electrolyte. Metals, such as Pt and Au, carbon, or semiconductors can be used as electrode materials. Pt is most often used for electrochemical measurements of polypyrrole film electrodes. The physical and electrochemical properties of synthesized polypyrrole are greatly influenced by the electrolysis current, potential conditions, electrode materials, concentrations of the pyrrole monomer and supporting electrolyte, electrolyte (dopant) and solvent materials, temperature, and so on. 

Pauporte T, Mendoza L, Cassir M, Galtayries A and Chivot J (2005), Direct low temperature deposition of crystallized C03O4 films by potentiostatic electrolysis , J Electrochem Soc, 152, C49-C53. 

In cases where more than one product may be formed, particuarly when the relative yield is potential dependent, potentiostatic electrolysis is particularly desirable. If a controlled constant current electrolysis is carried out in such a case, the potential will generally increase with time as the reactant is consumed, so that eventually the diffusion-limited current is reached, which itself eventually becomes smaller and smaller and the remaining fraction of the current is then passed by decomposition of the solvent or supporting electrolyte, usually with evolution of hydrogen (cathodic processes) or oxygen (anodically) in the case of an aqueous medium. 

It is evident that controlled potential electrolysis is, in principle, the preferred technique. In large-scale preparations, potentiostatic electrolysis with wholly electronic equipment may be economically unfeasible, but electromechanical systems may be constructed more easily. 

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