Compatibility and performance of SOFCs based on lanthanum tungstates

Abstract

Rare-earth tungstates with general composition “Ln6WO12” have attracted great attention in last few years due to their relatively high mixed proton-electron conductivity [1, 2]. One of the main ad-vantages of these electrolytes, compared to the traditional perovskites based on BaCeO3, is that they exhibit high tolerance towards CO2 and H2S environments. Therefore, this material is a potential electrolyte for proton conducting solid oxide fuel cells (PC-SOFC). In this work, the lanthanum tungstate with com-position La27W4NbO55-δ (LWNO) has been tested as proton conductor electrolyte [3]. For this purpose, different electrodes and composite electrodes have been considered, including: La0.8Sr0.2MnO3-δ, La0.6Sr0.4Co1-xFexO3-δ, La0.5Sr0.5Cr0.5Mn0.5O3-δ, SrFe0.75Nb0.25O3-δ and NiO. Chemical compatibility between the cell compo-nents is investigated by X-ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS). Furthermore, area specific resistance (ASR) of the different electrodes is determined in symmetrical cells by impedance spectroscopy. XRPD and EDS analysis do not reveal significant bulk reactivity between most of these electrodes and LWNO electrolyte in the typical operating temperature range of a SOFC (600-900 ºC). However, minor interdiffusion of elements at the electrolyte/electrode interface affects both the ohmic losses and electrode polarization of the symmetric cells. ASR values are significantly improved by using a buffer layer of Ce0.8Gd0.2O1.9, between the electrolyte and electrode materials, to prevent reactivity. A single cell with 350 µm thick electrolyte, NiO-Ce0.8Gd0.2O1.9 anode and La0.6Sr0.4Co0.8Fe0.2O3-δ cathode, generates maximum power densities of 140 and 18 mWcm-2 at 900 and 650 ºC, respectively. Hence, lanthanum tungstates could be competitive proton conductors for PC-SOFCs with similar performance to those based on BaZrO3 if thin film electrolytes are used.