THE POSSIBLE USE OF MIXED IONIC ELECTRONIC CONDUCTORS INSTEAD OF ELECTROLYTES IN FUEL-CELLS

Various types of mixed ionic electronic conductors (MIECs) are considered for replacing pure ionic conductors, i.e. solid electrolytes (SEs), in fuel cells (FC). It is shown that a rather high electronic conductivity, which originates from a change in stoichiometry under reducing or oxidizing conditions at the electrodes, can be tolerated provided the FC is operated close to its maximum power output. Thus a MIEC can replace an SE in an FC that is intended to operate under those conditions. The operation conditions in the electrolysis process of water are different. Under these conditions a MIEC cannot replace an SE. The analysis leading to the acceptance of mixed conductivity suggests also that electrodes can be prepared by change of stoichiometry or by doping of the surface region of the SE or MIEC used. The stoichiometry change and the doping will turn the surface regions into semiconductors which can serve as electrodes. The use of a protective thin SE layer on a MIEC is analyzed. It is not needed for an FC operated under maximum power output condition. It may, however, be useful for low-power operation conditions and for electrolysis.