Blocking electrodes, ionic conductivity

What has been ignored so far and will only be briefly mentioned is that a stoichiometric polarization is also caused by grain boundaries if the ratio of ionic and electronic conductivities differs from the bulk value, as it is usually the case.230 Figure 41 gives a clear example of this. In the general case of blocking electrodes and grain boundaries we expect even two stoichiometry polarization processes. 

So far, the ionic conductivity of most ILs has been measured by the complex impedance method [116], In this method, charge transfer between carrier ions and electrode is not necessary. Therefore platinum and stainless steel are frequently used as blocking electrodes. However, it is often difficult to distinguish the resistance and dielectric properties from Nyquist plots obtained by the impedance measurement. In order to clarify this, additional measurements using non-blocking electrodes or DC polarization measurement are needed. 

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

The Rb based on the sample cannot be calculated correctly, since the electric charge transfer resistance and the electric double layer in an electrode interface are also detected as a resistance, even if bias voltage is impressed to the measurement cell in order to measure the ionic conductivity. For the ionic conductivity measurement, a dc four-probe method, or the complex-impedance method, is used to separate sample bulk and electrode interface [4]. In particular, the complex-impedance method has the advantage that it can be performed with both nonblocking electrodes (the same element for carrier ion and metal M) and blocking electrodes (usually platinum and stainless steel were used where charge cannot be transferred between the electrode and carrier ions). The two-probe cell, where the sample is sandwiched between two pohshed and washed parallel flat electrodes, is used in the ionic conductivity measurement by complex-impedance method as shown in Figure 6.1. 

It is important to stress one major difference between ac and dc modes. In the ac mode a conducting substrate participates in the conduction process and reduces the solution resistance observed. On the contrary, an insulating substrate blocks the ionic pathway and increases the solution resistance observed. In the dc mode both conducting (provided the applied potential is sufficiently small that no Faradaic process occurs on the substrate) and insulating substrates block the conduction pathway and increase the resistance between the tip and the auxiliary electrode (see Fig. 5). 

The ionic conductivity is due to both cation and anion species. The cationic transference number = (o / o. ). The anionic transference number x = 1 - x. Different ionic and electronic contributions to the total conductivity can be measured by an electrolysis experiment with the use of selective blocking electrodes which is for blocking all the conducting ion species but the desired one [16, 22].This is a direct current (d.c.) experiment. 

It may be helpful at this point to ejcplain why the formidable "central problem" discussed above doesn t show up in the traditional theory of mixed conduction as developed by Wagner. He only treats two situations ion blocking electrode conditons and open circuit conditions. When the electrodes are ion blocking, l vanishes in Equation, 32 (for each ionic species) in which case Equation 32 is easy to integrate. 

The partial ionic or electronic currents and conductivities can also be measured by setting the driving force for the other partial current to zero. If one examines the Hebb-Wagner method, it becomes apparent that this nullification also occurs there. Because of the extreme resistance of the ion-blocking electrode to ionic current, the driving force for ionic motion in the MI EC vanishes (V/iion = 0). 

Kreuer et al. (1981) have used ionically blocking electrodes to show negligible electronic conduction in lithium hydrazinium sulphate, HUP and HUAs. Lyon Frey (1985) and Slade et al. (1987) have used a similar method on HUP and on hydrous oxides". 

Reiss, I., Measurements of electronic and ionic partial conductivities in mixed conductors, with the use of blocking electrodes. Solid State Ionics, 44, 207-214 (1991). 

This point can be easily located on a diagram similar to that shown in figure 4. In principle, this provides an easy way to determine the final state reached by the material. If the electrode is ideally blocked l = 0 and ti = 0. Then the electronic conductivity in the final state is necessarily much greater than the ionic conductivity. 

Excessive binder content PVDF in the electrode leads to a decrease in the ionic conductivity of the electrode due to the ion-blocking property of the ionic insulating binder. However, decreasing the binder content beyond a certain limit (specific to the materials used in the electrode) causes a decrease in the electronic conductivity and cycle fife of the electrode due to the poor physical connection between the electrode particles and the current collecting foil. 

The ionic conductivity of the membranes sandwiched between two stainless steel blocking electrodes (1 cm diameter) was measured using an electrochemical impedance... 

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