Reversible Type Solid Oxide Fuel Cells Using Ni-Fe-CeO2 Based Cermet Fuel Electrode and Applied for Metal-Air Rechargeable Battery

 Solid Oxide Fuel Cells (SOFC) is expecting as a highly efficient energy convertor.  On the other hand, recently, there are strong interests on high temperature steam electrolysis because of efficient hydrogen production method.   In addition, reversible operation of SOFCs is also interesting as an energy storage method. Therefore, recently, there is also high interest on reversible operation type SOFC so-called SORC.  At present, for SORC, similar materials for conventional SOFC are widely used, i.e., Y₂O₃ stabilized ZrO₂ and Ni based cermet for electrolyte and fuel electrode, respectively.  However, because of high temperature operation, energy efficiency as well as stability is still not high as an energy storage process.  

   In this study, we investigated intermediate temperature SORC using LaGaO₃ electrolyte. For reversible type operation, aggregation of Ni is easily occurred but it was found that addition of small amount of Fe is effective for preventing aggregation of Ni.  The optimized composition for fuel electrode is Ni-Fe(9:1) from stability.   Effects of mixed conducting oxide added for Ni-Fe were studied.  For SOFC operation, power density is larger as the following order, Ce(Mn,Fe)O₂<NiFe=La(Sr)Ga(Mg)O ₃<Ce_0.8Sm_0.2O₂< La(Sr)Fe(Mn)O₃. On the other hand, for SOFC operation, the electrolysis current density is increased as the following order,  Ce(Mn,Fe)O₂<NiFe<La(Sr)Ga(Mg)O₃<< La(Sr)Fe(Mn)O₃ <Ce_0.8Sm_0.2O_2.  Therefore, for reversible operation, NiFe-SDC shows the most active.   Degradation of the cell using NiFe-SDC and Sm(Sr)CoO₃ for fuel and air electrode, respectively, was studied at 1073 K, 100mA/cm².  The observed terminal potential was 1.13 and 1.025 V for SOEC and SOFC respectively at initial cycle and the observed potential slightly increased for SOFC and decreased for SOEC with cycle number, however, after 50 cycles, the observed terminal potential was 1.15 and 1.0 V for SOFC and SOFC, respectively.   Therefore, the degradation is hardly observed for the reversible operation of the cell. 

Combination of SORC with metal oxidation could open a new application area for fuel cell. In this study, application of various metals like Fe, Sn, Al, and Mg to SORC were studied by using CaO Stabilized ZrO₂ for electrolyte  and it became clear that redox of metal proceeded in SORC electrochemically resulting in the all solid state metal-air rechargeable battery.  The observed open circuit potential is well agreed with theoretical P_O2 from Gibbs free energy of metal oxidation.  By decreasing operating temperature, P_O2 range of electrolyte domain can be expanded.  Application of LSGM for electrolyte of this new concept of oxygen redox shuttle battery will also be discussed.