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.　©　2022