An in situ formed MnO-Co composite catalyst layer over Ni-Ce_0.8Sm_0.2O_2−x anodes for direct methane solid oxide fuel cells

The development of direct methane solid oxide fuel cells is greatly impeded by the problem of carbon deposition on conventional Ni-based anodes. Here, we report a MnO-Co composite catalyst layer, formed by the in situ reduction of Mn_1.5Co_1.5O₄ spinel, over Ni-Ce_0.8Sm_0.2O_2−x (SDC) anodes for direct methane solid oxide fuel cells (SOFCs). Transmission electron microscopy (TEM)-energy dispersive spectroscopy (EDS) results demonstrate that Co beads are extracted from the Mn_1.5Co_1.5O₄ structure and distributed over the MnO surface after reduction in H₂. X-ray photoelectron spectroscopy (XPS) results show that the intensity of surface hydroxyl groups/absorbed oxygen species is almost the same as that of lattice oxygen species due to the Co enrichment on the MnO-Co composite surface. With the addition of the MnO-Co catalyst layer, the stability of Ni-SDC anode-supported SOFCs is improved in wet methane (∼3 mol% H₂O in methane), but their electrochemical performance is worsened due to the increase of mass transport resistance. However, with the addition of the SDC promoter to the MnO-Co catalyst layer, not only is the excellent stability retained but also the electrochemical performance is improved. The performance of the MnO-Co-SDC catalyst layer is also compared with that of 2MnO-Co-SDC and MnO-2Co-SDC catalyst layers in wet methane at 650 °C. The SOFC with the MnO-Co-SDC catalyst layer exhibits the biggest maximum power density and the smallest polarization resistance, and operates stably for over 900 min at 0.2 A cm⁻². The maximum power densities of SOFCs with the MnO-Co-SDC catalyst layer were 361, 701 and 849 mW cm⁻² at 600, 650 and 700 °C in wet methane, respectively.