Electrochemical properties of composite fuel cell cathodes for La5.5WO12−δproton conducting electrolytes

Abstract

New composite cathodes for proton conducting solid oxide fuel cells (PC-SOFCs) based on the novel La_5.5WO_12−δ (LWO) electrolyte have been developed. First the applicability of LWO as a protonic electrolyte has been proved by recording the OCV in a Pt/LWO/Pt cell as a function of the temperature, matching the expected Nernst voltage. In order to improve the electrode performance on LWO PC-SOFCs, composite cathodes have been prepared by mixing the La_0.8Sr_0.2MnO_3−δ (LSM) electronic phase with the LWO protonic phase. The ceramic–ceramic (cer–cer) composites have been electrochemically studied as cathodes on LWO dense electrolytes in symmetrical cells. Different ratios of both phases and two different electrode sintering temperatures (1050 and 1150 °C) have been studied. Electrochemical impedance spectroscopy (EIS) analysis has been carried out in the temperature range 700–900 °C under moist (2.5% H₂O) atmospheres. Different oxygen partial pressures (pO₂) have been employed in order to characterize the processes (surface reaction and charge transport) taking place at the composite cathode. A substantial improvement in the cathode performance has been attained by the addition of the LWO protonic phase into the LSM electronic material. From the electrochemical analysis it can be inferred that electrode enhancement is principally ascribed to the increase in the three-phase-boundary length, which enables electrochemical reactions to occur along the thickness of the electrode.