An in situ formed MnO–Co composite catalyst layer over Ni–Ce0.8Sm0.2O2−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 Mn1.5Co1.5O4 spinel, over Ni–Ce0.8Sm0.2O2−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 Mn1.5Co1.5O4 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.