In this study, we prepared SrR2O4+δ (SRO, R=Y, Yb, Gd, Sm) of brownmillerite structure. Among the four n-type SRO semiconductors, SYO is the most negative in conduction band and the smallest in band gap. As a result, the SYO-based SOFC can offer a maximum power density (MPD) of 1.03W/cm-2 at 800°C, which is higher than that based on the other three SRO oxides. The introduction of larger Sr2+ at the 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+ 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 LSM. An anode supported Ni-YSZ|YSZ|SYO(0.10)-60YSZ DA-SOFC yields an MPD of 0.24W/cm2 at 600°C and 1.21 W/cm2 at 800°C with remarkable stability, about 1.73- and 1.29-folds higher than that of LSM-based SOFC, respectively.