Current density electrosynthesis

Details of many synthetic processes are never reported and, hence, as noticed by Pletcher and Walsh [10], any contribution of electrosynthesis remains speculative. Crucial factors are generally the availability and costs of the starting materials, the material yield, a simple product isolation, the stability of the electrolysis medium and acceptable current densities. 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

Electrocatalysis is a heterogeneous process that involves the adsorption and the chemisorption of reactants or intermediates at the interface. These phenomena are encountered, for example, in fuel cells (FCs) or in organic electrosynthesis. The electrocatalytic activity of a given electrode for a certain reaction may be characterized by the current density at a chosen potential, which is proportional to the specific activity, when referred to the effective active surface. As shown in Figure 21.2, the role of a heterogeneous catalyst is to adsorb the electro-reactive species (reactant and intermediate) and transform it to another compound that can more readily undergo the desired... 

Whenever currents are passed, there is always a potential control error due to the uncompensated resistance. It was seen in Section 1.3.4 to be iR. If a cathodic current flows, the true working electrode potential is less negative than the nominal value by that amount. The opposite holds for an anodic current. Even small values of such as 1 to 10 n, can cause a large control error when substantial currents flow. This is one reason why large-scale electrosynthesis is not usually carried out potentiostatically. In that instance, controlling the current density is often more practical. 

Figure 6.7 SEM and TEM (a, inset) images of as-prepared PAN and PANI-V2O5 nanocomposites deposited for 30 min as a function of current densities of (mA cm ) (b) 1, (c) 3, and (d) 5. (Reprinted with permission from Chemistry of Materials, Electrosynthesis of Macroporous Polyaniline-V205 Nanocomposites and Their Unusual Magnetic Properties by I. Karatchevtseva, Z. Zhang, J. Hanna and V. Luca, 18, 20. Copyright (2006) American Chemical Society)...

The ohmic drop frequently represents an important fraction of the cell voltage and plays a major role in the heat generation terms. This is especially true in organic electrosynthesis where current densities are high and electrolyte conductivities low. A major concern in electrochemical engineering is clearly linked to the design of cells with minimum ohmic drops. 

The dependence of Kolbe products on anode potential and electrode material has been the subject of considerable study. Only a few examples from recent studies will be discussed here since the interest is in mechanism determination and not electrosynthesis. Dickinson and Wynne-Jones analyzed the composition of gases evolved from acetate solutions under various conditions and their results are presented in Table 5. They also observed that with a current density of 30mA/cm in citrate and phthalate solutions, oxygen was evolved at current efficiencies of not less than 90% on the metals they examined (Pt, Ir, Pd, Au, and Ni). The results of Conway and Dzieciuch with aqueous potassium formate are seen in Table 6. These results agree with those of Dickinson and Wynne-Jones, showing that the decarboxylation reaction is almost completely inhibited by the oxygen evolution process. The anodic reaction at Pd and Au in aqueous potassium trifluoro-acetate was... 

The direct electrooxidation of aqueous E>-g]uconic acid to l>arabinose on graphite has been performed in a very simple apparatus which may be suitable for practical application. The electrocatalytic oxidation of sucrose on smooth, lead-modified platinum electrodes has been examined with a view to finding experimental conditions for the selective electrosynthesis of value-added compounds. A paper in Bulgarian on the electrooxidation of diacetone-L-sorbose at low current densities in a nickel oxide electrolizer has been publi ed. The influence of the rize of palladium particles and their location on the support on their activity in the oxidation of glucose has been examined. An investigation of the effect of tonperature and pH on the platinum-catalysed oxidation of sucrose showed that changes in temperature affect mainly the reaction rate, where changes in pH alter the selectivity. ... 

Selectivity is often an important factor in electrosynthesis low values may result in increased reagent costs and higher dowMtmmi separation costs. Thus the running and capital costs of a process may be significantly affected by selectivity which, in turn, is a function of current density. 

This corresponds to 3.5 years Of course this negiects convection effects, but it is nonetheiess true that aithough diffusionai rates and current densities are iarge at microeiectrodes, absoiute currents are smaii and the eiectroiysis essentiaiiy does not perturb the ceii contents. If electrosynthesis is to be attempted, a large electrode (and probably large concentrations) are essential. 

A similar approach was followed by Chen and coworkers in 2003 [325] in the preparahon of copper nanorods. These authors used a controlled-current electrochemical method and showed how the shape and yield of the nanorods depended on the current density applied during the electrosynthesis process. 

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