Unravelling the reaction mechanism on Ni–YSZ anode supported direct ammonia solid fuel cell: Experimental and theoretical studies

Abstract

While the state-of-the-art nickel–yttria-stabilized zirconia (Ni–YSZ) cermet exhibits a great potential to realize high-efficiency ammonia to power conversion using direct ammonia solid oxide fuel cell (DA-SOFC), the mechanisms of NH₃ cracking and H₂ oxidation reaction are still unclear. In this context, with the objective of enhancing Ni–YSZ anode activity, the performance of DA-SOFC using conventional $NiO-Y_2{O}_3-{ZrO}_2|Y_2{O}_3-{ZrO}_2|{Ce}_{0.8}{Gd}_{0.2}{O}_{1.95}{-La}_{0.6}{Sr}_{0.4}{Co}_{0.2}{Fe}_{0.8}{O}_3$ (Ni–YSZ|YSZ|GDC–LSCF) anode-supported cell is studied through in-depth materials characterisation coupled with density functional theory (DFT) calculations to unveil the reaction’s mechanism on Ni–YSZ(111) anode. The performance of SOFC is examined under the NH₃, H₂, and N₂/H₂ mixtures as fuel at three different temperatures: 700, 750, and 800 °C. In addition to X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM–EDX) characterizations, the electrochemical characterization of Ni–YSZ anode is performed using the electochemical impedance spectroscopy (EIS) technique at 700–800 °C. Notably, polarization resistance is primarily due to the charge transfer reaction and secondarily by the gas diffusion impedance observed. The activation barriers of elementary steps involved in the proposed mechanisms for NH₃ cracking and H₂ oxidation reactions on the Ni–YSZ(111) surface are computed using ab initio calculations. DFT findings revealed that the hydrogenation step of hydroxyl species, accompanied by charge transfer, dominates the hydrogen oxidation reaction. In contrast, the recombination and desorption of nitrogen adatoms act as the rate-controlling step for NH₃ decomposition, demonstrating the highest kinetic barrier of all elementary steps. This study offers a mechanistic understanding that is vital for achieving the rational design of Ni–YSZ-based anode cermet for DA-SOFC.