Defect-Induced Pyrochlore Pr ₂Zr ₂O ₇ Cathode Rich in Oxygen Vacancies for Direct Ammonia Solid Oxide Fuel Cells

Abstract

With “Carbon Neutrality” as target, direct ammonia solid oxide fuel cell (DA-SOFC) is of significance for carbon-free power generation. The rare-earth Pr can offer unique electronic structure to confer variable valences conducive to high electron transport. In this study, pyrochlore Pr₂B₂O₇ oxides (B=Zr,ZrSn,Sn,PZO/PZSO/PSO) and perovskite (B=Ti,PTO) were prepared as DA-SOFC cathode with ORR induced by B-site cationic defects. Compared with PTO, the pyrochlore PBO with unoccupied 8a-oxygen sites are high in inherent oxygen vacancies (ca.12.5%), leading to enhanced migration of lattice oxygen anions. Among the n-type semiconductors, PZO is with negative flat-band potential (E_fb)and is more effective in terms of overcoming energy barriers. As a result, the conductivity of PZO (2.58×10^-3 S·cm^-1) is two orders of magnitude higher than that of PTO (5.82×10^-5 S·cm^-1) at 800°C. The oxygen transport performance reveals that the surface exchange coefficient (K_chem) of PZO is about one order of magnitude higher than that of La_0.7Sr_0.3MnO_3-δ (LSM) at 900°C. An anode supported DA-SOFC using the PZO-based cathode can offer a power density of 1.22W·cm^-2 at 800°C and 0.25W·cm^-2 at 600°C. The electrochemical performance is 2.3 folds higher than those of SOFCs using other PBO-based cathode, and higher than most reported SOFCs with the cathodes using A-site Pr. The stability test reveals that the PZO-based DA-SOFC could
operate continuously over 100 h without obvious degradation.

Keywords: Direct ammonia solid oxide fuel cell (DA-SOFC), pyrochlore, Pr2B2O7, B-site cation-deficient, oxygen reduction reaction, oxygen transport