New Cercer Cathodes of Electronic and Protonic Conducting Ceramic Composites for Proton Conducting Solid Oxide Fuel Cells

摘要

Currently investigated cathodes in proton conducting solid oxide fuel cells (PC-SOFC) are principally based on materials employed in oxygen-ion conducting SOFC cathodes. Recently, materials based on ceramic-ceramic composites (cercer) [1-4], combining a proton conducting phase and an electronic conducting phase, have shown appealing electrochemical results. This work presents the electrochemical properties of different mixed-conducting cercer composites as PC-SOFC cathodes for two different kinds of protonic electrolytes: (1) La_(0.8)Sr_(0.2)MnO_(3-δ)-La_(0.995)Ca_(0.005)NbO_(4-δ) (LSM-LCN) cathode on LCN electrolyte. (2) La_(0.8)Sr_(0.2)MnO_(3-δ)-La6WO_(12-δ) (LSM-LWO) cathode on LWO electrolyte. Different ratios of the electronic and the protonic phases have studied in the cathode preparation in order to study the influence of each one on the electrode processes. Symmetrical cell testing was accomplished by means of electrochemical impedance spectroscopy (EIS) in wet air in order to characterize the composite cathodes in the temperature range 700-900°C. Different dilutions on both oxygen partial pressure and water content have been performed as a function of the temperature in order to characterize the processes (surface reaction and charge transport) occurring at the composite electrode under oxidizing conditions. Moreover, the role of the protonic transport has been studied by replacing protonic water by deuterated water. The introduction of a protonic phase in the electronic (LSM) cathode allows the reduction of the polarization resistance (Rp) due to the increase of three phase boundary area along the whole thickness of the cathode. On the other hand, a high amount of protonic phase produces an increase in Rp due to the lowest total conductivity of the cathode. Balanced electrodes (50-50 vol% for LSM-LCN composites and 40-60 vol% for LSM-LWO) show the lowest Rp at any tested temperature in humidified air. Different limiting processes have been identified depending on the electrolyte material. Finally, the effect of the addition of nanodispersed catalysts on the electrode surface has been investigated.