In this study, we report a novel approach for suppressing coke formation in direct-methane solid oxide fuel cells (SOFCs) with a conventional nickel cermet anode by simply adding ammonia to the fuel gas. Because ammonia preferentially occupies the acidic sites of the anode catalyst materials, a significant decrease in the coke formation rate is realized by introducing ammonia into the methane gas. In addition, hydrogen, a decomposition product of ammonia, also acts as an additional fuel for the SOFCs, resulting in high cell performance. At 700 degrees C, the coke formation rate over the Ni-YSZ anode is suppressed by 71% after the addition of 333% NH3 into CH4. Suppressed coke formation is also observed for other Ni catalysts such as Ni/Al2O3, a common catalyst for methane reforming that has been successfully used as the anode catalyst layer for SOFCs operating on methane, which suggests that introducing NH3 as an additive gas is a general method for suppressing the coke formation. The addition of ammonia can also effectively improve the power output and operational stability and offers a novel means for developing new coke-resistant SOFCs operating on widely available hydrocarbons for clean power generation to realize a sustainable future. (C) 2013 Published by Elsevier B.V.