Non-isothermic cells

The cyclic voltammograms of these systems display quasi-reversible behavior, with AEv/v being increased because of slow electrochemical kinetics. Standard electrochemical rate constants, ( s,h)obs> were obtained from the cyclic voltammograms by matching them with digital simulations. This approach enabled the effects of IR drop (the spatial dependence of potential due to current flow through a resistive solution) to be included in the digital simulation by use of measured solution resistances. These experiments were performed with a non-isothermal cell, in which the reference electrode is maintained at a constant temperature... 

Hr is the enthalpy change in the single interface reaction under study in the lni o vs. T experiment but cannot directly be determined in a thermodynamically rigorous manner. However, with some extrathermodynamic assumption about evaluation of individual heats of ionic solvation,estimates of AH , can be made but they are never very satisfactory. Uncertainties are 20-35 kJ moP However, non-isothermal cell measurements offer an approximate way of obtaining... 

Consequently, two different temperature coefficients of the electrode potential can be obtained the isothermal and the thermal temperature coefficient. The thermodynamic implications of these coefficients were clearly established by de Bethune, who also evaluated the values of the temperature coefficients of a wide variety of reference electrodes. While the use of a non-isothermal cell presents the clear advantage that only the temperature effect on the reaction on the working electrode is evaluated, it also presents the problem that the measurements will be interfered by the appearanee of a thermodiffiision potential, arising from temperature differences within the electrolyte solution. This thermodiffusion potential can be experimentally minimized (by using, for example, a saturated potassium chloride bridge for the liquid unions) or, alternatively, the effect of the thermodiffusion potential can be subtracted by calculating its numerical value from ... 

The groups of Taniguchi and of Hawkridge have each undertaken detailed studies of the influence of temperature, electrolyte composition and pH upon the reduction potential of horse cytochrome c. Reduction potentials were measured by cyclic voltammetry with a non-isothermal cell. With this configuration, in which the reference temperature is kept constant while the sample temperature is varied, data lead directly to the reaction centre entropy change AS° as given in Eq. (6). 

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 isothermal conditions, a classical three electrode cell was used where, the working electrode (WE) was a silver disk (0 = 4 mm). To avoid any salts precipitation, two silver bars of great area were used one as the reference and the other as the counter electrode. The non-isothermal cell (NITC) used in this work has been previously described in (Citti, Aaboubi, Chopart, Gabrielli, Olivier, and Tribollet 1997). 

Electrochemical cells where electrodes are heated, obviously are non-isothermal cells, i.e. there exists a thermal gradient somewhere between working and reference electrodes, respectively. In isothermal cells, such a gradient does not exist since... 

With non-isothermal cells, which are an inherent feature of modem thermoelec-trochemical methods, several phenomena have to be considered which otherwise would not be meaningful. Some of them, like the thermodiffusion (Soret effect) can be interpreted on a thermodynamic basis, others, like diffusion or convection effects, are considered to be dynamic processes and need a specific dynamic interpretation. 

Thermodynamic interpretation of non-isothermal cells directs attention to some old electrochemical principles, which are tmderrepresented in textbooks and should be reconsidered while we are at it. 

The most important conclusion of the above considerations is that measurements with non-isothermal cells may provide true single-electrode potentials (in this case, indeed potentials, not voltages ). The very important entropy of a single electrode Sf. can be determined this way. The corresponding relation is AEjAT=SJzF. 

Single-electrode potentials are important for some fundamental but unmeasurable quantities. The problem has been discussed in literature [2, 3]. Relative potential values (not absolute values which refer to an imaginary point in the universe ) can be calculated. Also single-electrode entropy values can be calculated by means of non-isothermal cells, but it is necessary to make use of some non-thermodynamic assumptions. 

Heat and Entropy Flow in Open Non-isothermal Cells... 

Open electrochemical cells do not have any external connection between electrodes. Consequently, no electrolytic current will flow. In open, non-isothermal cells, nevertheless, some exchange processes will take place. The necessity to maintain a stationary temperature difference, e.g., means that heat is flowing continuously from hot to cool place. Consequently, there must be some transfer of entropy even without any kind of electrolysis. We have to discuss such effects first in terms of thermodynamics. 

Elementary processes of water electrolysis at mercury [268-271] and at platinum [272] electrodes have also been studied in autoclaved aqueous solution. Diffusion coefficients [235, 236] and transfer coefficients [273] have been determined in pressurised aqueous medium. The influence of the phenomenon of thermodiffusion (Soret effect) was followed [217]. This effect generally plays a role if regions of different temperature inside a homogeneous electrolyte phase occur. This is not typical for isothermal systems considered here. In non-isothermal cells (see later below), it should be taken into account. 

In situ electrochemical calorimetry generally makes use of thermometers which are closely connected with the electrode surface studied. This way, single electrode properties can be determined. Cells with electrode-thermometer units are non-isothermal cells where temperature differences between working and reference electrodes are encountered. Their thermal behaviour is similar to that of cells with electrodes which are heated by external devices. Consequently, all the effects characteristic for non-isothermal cells (e.g. the Soret effect) have to be considered. 

Modem thermoelectrochemistry typically works with non-isothermal cells. 

A completely different approach to establish a heated solution region is followed by two techniques which make use of an ordinary electrolysis vessel that is converted to give a non-isothermal cell just by generating a hot spot in close vicinity to the electrode-solution interface. The electrode material itself is not... 

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