Ammonia (NH3) has 17.8 wt% hydrogen and is easily liquified at 25°C and 8 bar pressure. Ammonia is carbon-free and can be produced sustainably at large scale and low cost. Solid oxide fuel cells generate electricity with efficiencies greater than 60% and can use ammonia as fuel without the need for external cracking. In this work, a single-cell SOFC was characterized using the in-situ ammonia decomposition reaction (Int-ADR) and compared with the ex-situ ammonia decomposition reaction (Ext-ADR), and pure hydrogen (H2 100%), between temperatures of 750°C and 850°C. Constant load tests performed at 800°C with 84% fuel utilization reached the LHV efficiency of 58%. The open-circuit voltage (OCV) of Int-ADR was similar to that of Ext-ADR, confirming that usage of ammonia as fuel in the Ni-YSZ anode involved two steps (i) ammonia decomposition into nitrogen and hydrogen and (ii) electrochemical conversion of hydrogen into steam.