Electrochemical Composition Actors

In this section we will describe what may be called electrochemical composition actors, in which chemical action—and not mechanical action as usually described by the term actor (or actuator)—is caused by electrical excitation. Unlike electrochemical composition sensors which detect chemical composition via electrochemical methods, they cause changes in chemical composition. It shall not remain unmentioned that at present interesting work is pursued also to realize mechanical action on an electrochemical stimulus ( artifical muscles ).48 

Unlike in the case of basic storage devices (cf. the charging process of a battery) which we do not wish to subsume under this term and which we consider in the following section, here we will refer to situations in which the compositional changes caused by electricity, are meant to achieve more specific goals such as 

Electrochemical pumps have also been successfully used to promote reactions catalytically. If oxygen is pumped through a zirconia cell into a reaction chamber which is, e.g., filled with hydrocarbons, not only the oxygen that is transferred, reacts. The anode itself can act catalytically. It seems that this so-called NEMCA effect ( Nonfaradaic Electrochemical Modification of Catalytic Activity 51) relies on a hindered surface reaction as a consequence of which the applied potential is translated into a concentration polarization, as reflected by the enrichment of a not fully oxidized oxygen species (e.g., O or O2 ) at the interface.51 53 For this species, redistribution equilibrium may be assumed that 

From the theory of ambipolar conduction (see, e.g., Ref.56) it follows immediately that given an oxygen potential difference A/i0 between the lhs (u(]) and the right-hand side, an oxygen flux which is of the form 

If we switch from a situation with uniform chemical potential (A/i0 = 0) to a situation in which on one side a different but constant PQ is established, a transient occurs during which the homogeneous stoichiometry profile changes to an approximately linear profile (see chemical polarization, see Appendix 3). As long as the electrode reactions are fast, the emf measured at such a sample is always determined by the invariant boundary values of the oxygen potential (ju0,ju0 + Aju0) but, owing to the internal virtually neutral short-circuit, lower than the Nernst-value. The result is, instead of Eq. (20),56 57 now 

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Polarization cell Measurement of kinetic data by polarization Electrochemical composition actors (electrolyzers, pumps, windows), electrochemical composition sensors (amperometric, conductometric)... 

There is an extended special literature3,10-16 on applications of solid state electrochemistry and even more on electrochemical devices. According to our objective, in this section applications will be emphasized in which migration and diffusion in the solid state are decisive processes (as discussed in Part I2). We intend to subsume such applications under the headlines composition sensors, composition actors, and energy storage or conversion devices. 

We will begin with a description of electrochemical sensors or more specifically composition sensors based on electrochemical principles (i.e., we refer to an electrochemical detection of composition). Another group of applications refers to devices in which the transference of mass and charge is used primarily to change composition or produce chemicals (electrochemical pumps and electrochemical reactors, or electrochemical filters) we will term such devices composition actors. At the end we will discuss energy conversion and storage devices (which we do not subsume under the term composition actors as here the energy aspect is to the fore). 

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