Mechanistic Insights on Direct Ammonia Solid Fuel Cell Using Ni-YSZ Anode: Experimental and Density Functional Theory Studies

While Ni-Yttria-stabilized zirconia (Ni-YSZ) cermet exhibits a great potential to realize high-efficiency ammonia to power using direct ammonia solid oxide fuel cell (DA-SOFC), the mechanisms of NH 3 cracking and H 2 oxidation reactions as well as direct electro-oxidation of ammonia are still unclear. In this context, with the motivation, the DA-SOFC performance of fuel electrode (Ni-YSZ) using conventional anode-supported cell (Ni-YSZ|YSZ|GDC-LSCF) is studied through microscale observation coupled with density functional theory (DFT) calculations to unravel the reaction mechanisms on Ni-YSZ(111) anode. The performance of DA-SOFC is examined using NH 3 , H 2 and N 2 /H 2 mixture as fuel at the temperature range of 700-800℃. In addition to XRD and SEM-EDX characterizations, the electrochemical characterization is performed using the impedance spectroscopy (EIS) measurements at three different temperatures of 700, 750 and 800 °C. Interestingly, the experimentally observed trend in Open-circuit voltage (OCV) implies that direct electro-oxidation of ammonia is the dominant reaction mechanism. Notably, polarization resistance is primarily contributed by ammonia anodic reaction and secondarily by the gas diffusion impedance. DFT results revealed that ammonia adsorbs favorably on the Ni cluster, and hydrogen follows a dissociative mode of adsorption on the Ni cluster. The activation barriers of elementary steps involved in the proposed mechanisms for fuel cracking and oxidation reactions are computed. This work is anticipated to provide valuable insights into the rational design of Ni-YSZ-based fuel electrodes.