Effect of air addition to methane on performance stability and coking over NiO–YSZ anodes of SOFC

Highlights: • Air addition strategy was tested for operation of SOFC directly fed by methane. • The optimal 1:5 air to methane ratio was proved after 100 h SOFC operation. • SOFC structure was verified through EIS and SEM analysis. - Abstract: The use of natural gas as fuel for solid oxide fuel cell is one of main potentials of this technology to be exploited as an efficient and profitable future power generation source. However, using direct methane (main component of natural gas) in conventional nickel-based fuel cells leads to carbon deposition problem which causes performance failure even in short period (24 h). According to thermodynamic principles, fuel addition with oxygen carriers is a good solution to prevent carbon deposition problem. Among the different options, a deep investigation is here presented for the air addition case. Through experimental activity under different operating conditions and suitable performance and structural cell characterization, the 1:5 optimal air addition to methane is determined, providing outcomes of interest for SOFC operation optimization in case of direct methane feeding. In fact, through impedance spectroscopy analysis and voltage measurements, as well as ex-post structural analysis, it is proved that in these conditions both carbon deposition and anode layers delamination are avoided, also after 100 h operation; moreover a cell stable operation at 0.6 V is guaranteed. The proposed operation mode, therefore, represents a promising solution, to be deeply investigated in the future at stack level, for SOFCs directly fed with natural gas.