The development of direct methane solid oxide fuel cells (SOFCs) is severely hindered by the deactivation of conventional Ni-based anodes due to carbon fouling. Here, a Ce0.8Ni0.2O2-delta (CNO) internal reforming layer is imposed on conventional Ni-Sm0.2Ce0.8O2-x (SDC) anodes for direct methane SOFCs. In CNO, there are two types of Ni species which are segregated NiO dispersed over the CNO and incorporated Ni2+ in the ceria lattice, respectively. The Ni2+ dopants are stable in wet hydrogen at 650 degrees C; however, the segregated NiO is reduced into Ni under the same conditions. With the doping of Ni2+ into the ceria lattice, surface oxygen vacancies are generated in CNO. For the stability testing in wet methane (similar to 3 mol% H-2O in methane) at 650 degrees C and 0.2 A cm(-2), the voltage of the conventional Ni-SDC anode decreases by 43.1% in approximately 26 h, whereas the CNO internal reforming layer operates stably for 40 h. In wet methane at 650 degrees C, with the addition of the CNO internal reforming layer, the polarization resistance of the Ni-SDC anode reduces by 22.3% from 0.0917 to 0.0712 U cm(2), and the maximum current density of it increases from 614 to 664mWcm(-2). (C) 2018 Elsevier Ltd. All rights reserved.