Electrochemical properties of composite cathodes for La_(0.995)Ca_(0.005)NbO_(4-δ)-based proton conducting fuel cells

摘要

The electrochemical properties of mixed-conducting ceramic-ceramic (cer-cer) composites for proton-conducting solid oxide fuel cells (PC-SOFCs) based on La_(0.995)Ca_(0.005)NbO_(4-δ)(LCN) have been investigated. Different ratios of La_(0.8)Sr_(0.2)MnO_(3-δ)/La_(0.995)Ca_(0.005)NbO_(4-δ) (LSM/LCN) composites have been tested as cathodes in symmetrical cells based on La_(0.995)Ca_(0.005)NbO_(4-δ) dense electrolytes while two different electrode sintering temperatures (1050 and 1150°C) have been studied. Additionally, different LCN doped materials (Pr, Ce and Mn), which present a different conduction behavior, have been used as components in composite cathodes (mixtures of LSM/doped-LCN 50/50 vol.%). Electrochemical impedance spectroscopy analysis has been carried out in the temperature range 700-900 C under moist (2.5%) atmospheres. Different oxygen partial pressures (pO_2) have been employed in order to characterize the processes (surface reaction and charge transport) occurring at the composite electrode under oxidizing conditions. The main outcome of the present study is that the mixture of LSM (electronic phase) and LCN (protonic phase) enables to decrease substantially the electrode polarization resistance. This is ascribed to the increase in the three-phase-boundary length and therefore it allows electrochemical reactions to occur in a larger region (thickness) of the electrode.