Oxygen Nonstoichiometry and Cathodic Property of Ce0.6Mn0.3Fe0.1O2-δ for High Temperature Steam Electrolysis Cell Using LaGaO3-Based Oxide Electrolyte

The reverse operation of SOFCs, so called solid oxide electrolysis cells (SOECs), can efficiently produce hydrogen from high temperature steam. Ni-based metal, which are widely used for anode of the conventional SOFCs, are also widely used for the cathode of SOECs due to its high catalytic activity and high electronic conductivity. However,  Ni-based cathodes frequently showed low stability against oxidation resulting in deactivation under SOECs condition. As a result, Ni-based electrodes are easily aggregated and also delaminated from electrolyte. Therefore, it is essentially required to recycle a part of produced hydrogen to the Ni based electrodes for preventing the oxidation of Ni, however, this recycle of the part of hydrogen formed requires the complicated system and also decrease the electrolysis efficiency. In order to solve these problems, recently, oxide cathode have been attracting much attention for the cathod of SOECs instead of Ni-based metal. Since oxide cathode is stable in redox cycles, recycles of hydrogen is not required. However, oxide cathode have some disadvantages, for instance, low catalytic activity and also low electronic conductivity. In this study,  Mn- and Fe-doped CeO₂ (Ce_0.6Mn_0.3Fe_0.1O₂, CMF) was investigated as a new oxide cathode for SOECs. CMF is a mixed ionic and electronic conductor (MIEC), and has been reported as a potential anode catalyst for SOFCs. Although its electrical conductivity is lower than that of the typical perovskites oxide electrode materials (10^-1 S/cm at 900 °C), its catalytic activity for electrode reaction was enough high when it is used in SOFCs. 

Oxygen nonstoichiometry in this CMF was studied by thermogravimetry.  It was found that higher oxidation state of Mn and Fe was observed resulting in hole conduction.  The origin of this hole conduction was further studied by XPS measurement.  On the other hand, in reducing atmosphere, because of the reduction of Ce, n-type semiconduction is observed. Therefore, oxygen nonstoichiometry of CMF is discussed in details from valence number of Ce, Mn, and Fe.

On the other hand, CMF cathode showed almost the same performance with that of  Ni-SDC cathode in steam electrolysis and  hydrogen evolution rate well obeys the Faraday’s law in temperature range of 900-700°C and stable electrolysis performance was observed. The XRD pattern and the SEM images of CMF cathode after the electrolysis measurement revealed that the CMF phase is stably remained and aggregation or delamination were hardly observed. These results suggest that CMF is highly promising as oxide cathode for SOECs