Tuning defect nonequilibrium of brownmillerite Sr1+xY2-xO4+δ for rich-oxygen-vacancy direct ammonia solid oxide fuel cells cathode

We prepared brownmillerite SrR2O4+δ (SRO, R = Y, Yb, Gd, Sm) with n-type semiconductors, where SYO is the most negative in conduction band and the smallest in band gap. And it is easy for the electrons to overcome energy barrier. As a result, SYO-based solid oxide fuel cells (SOFC) can offer a maximum power density (MPD) of 1.03 W/cm−2 at 800 °C, which is higher than that based on other three SRO oxides. Due to the enlargement of SYO unit cells and reduction of bond energy, the introduction of Sr2+ at B sites of Sr1+xY2-xO4+δ [SYO(x)] causes decrease of band gap, resulting in a 4-fold increase of electronic conductivity. The foreign Sr2+ tunes oxygen non-stoichiometry and creates surface oxygen vacancies to boost interfacial transport. The measurement of oxygen transport reveals that SYO(0.10) exhibits a bulk diffusion coefficient 500 folds higher than that of La0.7Sr0.3MnO3 (LSM). An anode supported Ni-YSZ|YSZ|SYO(0.10)-60YSZ direct ammonia solid oxide fuel cells (DA-SOFC) yields an MPD of 0.24 W/cm2 at 600 °C and 1.21 W/cm2 at 800 °C, about 1.73- and 1.29-folds higher than that of LSM-based SOFC, respectively. SYO(0.10)-based DA-SOFC can continuously operate at 800 °C for 100 h without significant degradation, displaying high thermal and operation stability.