Tuning defect nonequilibrium of brownmillerite Sr_1+xY_2-xO₄₊ for rich-oxygen-vacancy direct ammonia solid oxide fuel cells cathode

We prepared brownmillerite SrR2O4+delta (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 degrees 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+delta [SYO(x)] causes decrease of band gap, resulting in a 4-fold increase of electronic conductivity. The foreign Sr2+ tunes oxygen nonstoichiometry 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 vertical bar YSZ vertical bar SYO(0.10)-60YSZ direct ammonia solid oxide fuel cells (DA-SOFC) yields an MPD of 0.24 W/cm(2) at 600 C and 1.21 W/cm(2) at 800 degrees 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.