Cells under current load

In this section we will explore what happens when we force a current to flow through symmetrical or asymmetrical cells (in the last case in the opposite direction to that of short circuit flow), i.e. we consider electrochemical polarization in the generalized sense. (For reasons of presentation charging processes in galvanic elements will be discussed, together with the discharge processes, in the next section.) Conductivity sensors, electrolysis cells, electrochemical reactors and pumps are all important technological applications of cells under a current load. 

Bulk and boundary conductivity sensors have already been discussed in Chapter 5 in relation to equilibrium defect chemistry, potentiometric sensors in the previous section. Nevertheless, we wish — in view of the importance of this application — to sketch out some of the fundamentals of electrochemical (composition) sensors . The fact that a variation in the chemical composition (ck) ehcits a physical signal is the rule rather than the exception. This is merely a necessary sensor criterion. In addition, it is important that a sensor signal exhibits adequate sensitivity , is sufficiently selective, stable and as free from drift as possible , and displays an ad- 

The semiconductor boundary sensor (surface conductivity sensor in Table 7.2) avoids the disadvantage of the high temperatures that were important for the bulk sensors 

It is merely an explicit time-dependence that is not desired (see second term) in a case where Ck is time-dependent, should naturally also be time-dependent. If, for example, = ack then dS /dt = 0 means that the sensitivity is not time-dependent (see footnote 35). 

The response time (tr) and drift can be meaningftilly separated, if a (pseudo)stationary state ( 00 ), whose TR value varies on a much greater time scale, is reached relatively quickly. Thus, the response time is defined, for example, by (S(tr) — S( oo ))/S( oo ) = 1% and the drift by dS/dt when t tr [570]. 

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