Electrochemical cells cell geometry effect

As we have seen in the previous sections, our understanding of SOFC cathode mechanisms often hinges on interpretation on the magnitude and time scale of electrochemical characteristics. However, these characteristics are often strongly influenced by factors that have nothing to do with the electrode reaction itself but rather the setup of the experiment. In this section we point out two commonly observed effects that can potentially lead to experimental artifacts in electrochemical measurements (1) polarization resistance caused gas-phase diffusion and (2) artifacts related to the cell geometry. As we will... 

A semi-infinite transport geometry implies that, for the duration of the experiment, effects generated at the working electrode do not reach the other electrode or the cell walls, so that the transport coordinate behaves as if it were infinite in extent. The prefix semi reflects the fact that the geometry appears infinite in one direction (away from the electrode) but not in the other (towards the electrode). Because of the limited duration of most electrochemical experiments, cells may be tiny and yet be appropriately described as having a semi-infinite transport geometry. 

Ultrasonic baths will be familiar from their everyday use in the laboratory where they are commonly used for cleaning surfaces and to aid dissolution. A bath essentially comprises a number of transducers of fixed frequency, commonly 20-100 kHz, attached beneath the physical exterior of the bath unit. Baths typically deliver ultrasonic intensities between 1 and 10 W cm to the reaction medium. For sonovoltammetry (or sonoelectrosynthesis) the bath may be filled with distilled water and a conventional electrochemical cell is placed inside the bath at a fixed position (Walton et al., 1995) so that the cell is electrically isolated from the sound source. Alternatively, the internal metal casing of the bath can be coated so that the full volume is available to use as an electrochemical cell (Huck, 1987). For both arrangements results can be highly sensitive to positioning and/or cell geometry effects. 

The characteristics of the ESR cavity (Fig.l) implies the basic restrictions in the construction of an electrochemical cell for ESR-measurements. Like any other probes the electrochemical cell is to be mounted in the center of the resonant cavity where the magnetic field has its maximum. Lossy samples like electrolyte solutions have to be restricted into the z-direction to avoid high dielectric absorption which lowers the quality-factor of the cavity and the sensitivity of the measurement or makes the measurements impossible at high absorption. Therefore a flat cell with a 0.3 to 0.5 mm thickness of the solution layer must be used. The cell has to be made of quartz because of the "sucking in effect of that material which improves the sensitivity by a factor of 2. This geometry gives the limitations of the electrochemical conditions low electrolyte volume, high cell resistance and small electrodes. For the last fact even further restrictions exist. 

S. Primdahl and M. Mogensen [1998] Gas Conversion Impedance A Test Geometry Effect in Characterization of Solid Oxide Enel Cell Anodes, J. Electrochem. Soc. 145, 2431-2438. 

The detail of bubble evolution from electrodes is a topic of practical importance. The size of a bubble at detachment is dependent on the gas and solution density, the cell geometry, hydrodynamics, interfacial tensions between the bubble, electrode and solution, and the roughness of the electrode. The way in which bubbles detach affects the resistance to current flow to the electrode, and rising bubbles effectively stir the electrolyte. Electrochemical noise measurements have been used to characterize the evolution of chlorine [87], oxygen [88], and hydrogen [89]. 

The second problem associated with electrochemical synthesis of polypyrroles seems to be related to die difficulties found in correlating the polymer s properties with the conditions of synthesis. This has been derived from the widely spread idea of an overall understanding of the electrochemical mechanism. However, even the most simple electrochemical process of pyrrole electropolymerization involves different experimental variables in order to optimize polymer properties. These variables can be chemical, such as solvent or reactants (monomer and dopant salt), or physical, such as temperature, nature and shape of die electrodes, cell geometry or electrical conditions during synthesis. In addition, commonly the effects of all these variables are interdependent. 

Table 2 Simulation of Effect of Electrochemical Cell Geometry on Performance ...

The extent to which a solution is able to respond to these gravitational tendencies depends on the geometry of the cell (see Sect. 4). Even under the worst conditions, however, the driving force for natural convection is small and it takes a considerable amount of time for this small force to overcome the inertia of the solution mass. Accordingly, natural convection is unimportant in rapid experiments. In fact, its effect seldom appears before about 10 or 20 s, which is long in relation to most electrochemical experiments. 

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