Hebb-Wagner technique

While in the experiments described so far both ionic and electronic species are transferred, it is also possible to determine the partial electronic conductivity by using ionically blocking electrodes to suppress the ionic transport so that only electrons and holes can pass. This technique is known as the asymmetric polarization or Hebb-Wagner technique. By using a chemically inert electronic conducting material, no ions will be delivered to the electrolyte when a voltage is applied with such a polarization that the mobile ions tend to be depleted at the inert electrode. An electrode used on the other side fixes the chemical potential of the mobile component of the electrolyte by the applied voltage at that phase boundary. 

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 Hebb-Wagner polarization technique has been developed either for the determination of electron and hole conductivity in ionic conductors [Hebb, 1952 Joshi Wagner, 1975 Wagner, 1957] or for the measurement of ionic conductivity in MIECs [Riess, 1996 Wiemhofer et al., 2002]. Basically, the method consists in using a reversible electrode and blocking electrodes to suppress the predominant charge carrier and thus enable measurement of the minority sp>ecies. The main limitations of the method have been reviewed [Riess, 19%] and new experimental set-ups have been proposed. 

The analogous conclusions are arrived at for the ionic conductivity in the case of cell 5. The blocking technique can be extended in order to de-convolute ionic and electronic conductivities for a grain boundary of a bicrystal (see Figure 43)236 or even in composites.240 In both the cases, the relations are substantially more complicated because of local inhomogeneities. Some of the pitfalls of the Wagner-Hebb technique have been described by Riess.241... 

The steady-state result (Eq. (77)) can be directly used to separate ionic and electronic conductivities the disadvantage of the technique is that it presupposes gas-separation. If not special measures are taken, it becomes unreliable for the ionic transport number less than 1%. Thus, this method well complements the Wagner-Hebb method which is very sensitive to small transference numbers. The partial conductivities of PbO shown in Figure 48 have been de-convoluted by the emf technique just described.3... 

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