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.