Thermoelectrochemical TEC Transfer Function

The technique introduced by Citti et al. and then by Rotenberg used a vertical electrode heated by a laser beam or by an infrared diode. In this case, the same order of magnitude is obtained for the free convection and the thermal convection, and experiments can be performed with a sufficient accuracy. A fast redox reaction at a heated vertical electrode is considered. The motion of the solution is spontaneous and arises due to forces originating from heterogeneous reactions. Such forces result from density variation in the solution produced by both thermal and concentration gradients near the electrode. 

A perturbation of the electrode temperature generates a perturbation of the velocity field and then a perturbation of the concentration field near the electrode. According to Pick s law 

Motion of the solution in thermal laminar free convection is spontaneous and arises due to forces originating from heterogeneous reactions and from the release of heat from the electrode. Such forces follow the modification of the solution density caused by two phenomena. The concentration in the proximity of the reaction surface cheinges in the course of heterogeneous reaction and leads to changes in the density of solution. In addition, the release of heat induces variations of solution density from point to point as a result of nonuniform changes in the temperature of the solution. The density of the solution is a function of concentration and temperature and can be expressed by 

The body force acting on a unit of fluid volume is equal to pg and changes from point to point in the solution. It is natural to consider that most of the change in concentration occurs in a very thin layer and most of the change in temperature occurs in a larger, but still thin layer. The changes in temperature and in concentration are the causes of fluid motion. Therefore, one can safely assume that fluid motion also occurs in this layer. Thus, the theories for the hydrodynamic boxmdary layer can also be applied to fluid motion in thermal free convection. In this case, the hydrod3mamic boundary layer coincides with the thermal diffusion layer. 

The concentration distribution of the electroactive species is determined from the solution of the convective-diffusion equation 

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ABSTRACT Voltammetric and thermoelectrochemical (TEC) transfer function measurements have been carried out to study the eleetrodeposition of silver from nitric and tartaric solutions. For an isothermal cell, the observed increase of the limiting current is due to the diffusion coefficient increase and to the mass transport boundary layer decrease when bath temperature increases. In a non-isothermal cell, through the use of sine wave temperature modulation, the TEC transfer function measurements show a typical mass transport responses and typical adsorption relaxation in middle frequency domain. The experimental data are in good accordance with previously developed model and permit to determine the diffusion activation energy and the densification coefficients of silver ions in this media. 

In previous works we have shown the great importance to control the working electrode temperature (non-isothermal process) and developed a new transfer function based on the sine wave modulation of the electrode temperature, (Olivier, Merienne, Chopart, and Aaboubi 1992). Thus the thermoelectrochemical (TEC) transfer function has been experimentally measured and compared with theoretical models for mass transport controlled systems or charge... 

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