Direct　ammonia　solid　oxide　fuel　cells　(DA-SOFCs)　offer　a　promising　pathway　for　the　efficient　utilization　of　carbon-free　ammonia　fuel.　However,　the　nitridation　of　nickel-based　cermet　anodes　in　ammonia　causes　rapid　microstructural　coarsening,　leading　to　durability　problems.　Herein,　an　efficient,　ammonia-tolerant　Fe-modified　Ni-Gd0.1Ce0.9O1.95　(NiFe-GDC)　nanocomposite　anode　is　developed　by　coupling　a　self-assembly　synthesis　process　with　a　sintering-free　electrode　fabrication　technique.　The　as-synthesized　nanocomposite　oxides　self-assemble　into　multiple　phases,　with　GDC　firmly　grown　on　preformed　NiO　and　NiFe2O4　nanoparticles,　which　are　subsequently　in　situ　alloyed　in　a　reducing　atmosphere　to　form　a　unique　NiFe@GDC　encapsulation　structure　with　strong　metal–oxide　interactions.　This　NiFe-GDC　nanocomposite　not　only　provides　abundant　active　sites　for　ammonia　decomposition　and　electrochemical　oxidation,　but　also　exhibits　exceptional　resistance　to　nitridation　and　microstructural　coarsening.　Density　functional　theory　calculations　reveal　that　in　situ-formed　NiFe　alloy　lowers　the　energy　barriers　for　ammonia　adsorption　and　dehydrogenation　while　enhancing　the　nitrogen　desorption　process.　An　electrolyte-supported　DA-SOFC　with　the　NiFe-GDC　nanocomposite　anode　achieves　a　peak　power　density　of　0.61 W　cm−2　at　800 °C　and　exhibits　outstanding　operational　stability　for　100 h.　This　work　offers　new　insights　into　the　development　of　active　and　durable　nickel-based　nanocomposite　anodes　for　DA-SOFCs.　©　2025　Wiley-VCH　GmbH.