Elucidating Degradation Behavior of Ni in Direct-Ammonia Solid Oxide Fuel Cells

Direct-ammonia solid oxide fuel cells (SOFCs) offer a cost-effective and efficient avenue for accelerating the use of hydrogen as a major energy vector. The effect of directly feeding ammonia as a fuel into SOFCs and their associated degradation behavior, however, needs to be fully understood in order to design robust cells capable of operating with good performance and durability under the required operating conditions for extended periods of time. Here, using nanoscale characterization techniques, we examined the degradation behavior of Ni–YSZ anodes utilizing ammonia as a fuel. Nickel nitridation behavior could be readily confirmed on the surfaces of model Ni metal samples annealed in ammonia at sufficiently low temperatures; however, its occurrence seemed remarkably low for typical Ni–YSZ-supported cells operated at SOFC conditions. In our experimental conditions, the formation of NiO on Ni surfaces was observed to accelerate in the presence of water with ammonia in the fuel composition. This was also accompanied by significant changes in Ni wetting behavior that resulted in preferential spreading within the pore regions of the anode support. Both factors are considered to contribute more significantly to the observed degradation of electrochemical performance of the Ni–YSZ anodes rather than any influence of nickel nitridation per se.