Sr-free cathodes for solid oxide fuel cells operating under CO2-rich atmospheres: performance and structural stability

CO2-tolerant cathodes are required to successfully integrate SOFCs into new-generation oxy-fired hybrid power production cycles. Sr-free rare-earth nickelates are valid alternatives to state-of-the-art Sr-based perovskites, which are prone to deactivation via carbonate formation. Accordingly, we tested planar 25 cm2 SOFCs (SolydEra) with YSZ electrolyte and supporting Ni-YSZ anode, equipped with two different types of bilayer cathodes, either La2NiO4+δ/LaNi0.6Fe0.4O3-δ (LNO/LNFO) or Pr2NiO4+δ/Pr2Ni0.9Fe0.1O4+δ (PNO/PNFO) cathode. The SOFCs performance was evaluated at 700°C collecting I/V curves and impedance spectra, supplying the cathode either with air or with a 21% O2 79% CO2 mixture, while feeding the anode with 7% humidified H2. When substituting air with the O2/CO2 mixture, the current density at 0.7 V decreased by 40% (from 537 to 324 mA/cm2) on the Pr-based SOFC, while on the La-based SOFC by 29% (from 150 to 107 mA/cm2). Post-test XRD and in situ synchrotron XRD showed that CO2 severely accelerates the phase decomposition of PNO and PNFO, while LNO and LNFO remain structurally stable. On exposure to CO2, thermogravimetric analyses and in situ XPS at near-ambient-pressure confirmed that carbonation occurred in all the samples, with the concentration of oxygen sites (either vacancies or interstitials) playing a major role in establishing the adsorption capability. Despite the absence of Sr, La- and Pr-based nickelates still adsorb CO2, which kinetically affects the SOFC performance. Compared to PNO/PNFO, LNO/LNFO revealed more promising thanks to superior stability and lower CO2 uptake.