Electrochemistry of Mixed Ionic–Electronic Conductors
The materials of main interest in the field of electrochemistry of mixed conductors are ionic compounds. Solid mixed ionic–electronic conductors (MIEC) exhibit both ionic and electronic conductivity. Electronic conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. The electronic current can be eliminated as mentioned before using an electron-blocking electrode or by short circuiting. The application of four electrodes on a MIEC is not a trivial modification of the two-electrode configuration. To suppress the electronic current, at least three of the electrodes must be electron blocking. Then the voltage of the MIEC proper vanishes, as under short-circuit conditions. The galvanic cells considered so far were assumed to have a uniform temperature. Temperature gradients exert driving forces on both the electronic and the ionic defects. The MIECs are mainly characterized by the concentration of the different defects, their relations, and by the conductivities of the mobile species.