Ammonia to power: Advancing direct ammonia solid oxide fuel cells through experimental and theoretical studies

Achieving net zero emissions by 2050 is an emerging challenge to meet global warming mitigation goals. Ammonia is an outstanding medium for hydrogen storage and a promising carbon-free energy carrier. Furthermore, it presents superiority in storage and transport, which remain critical bottlenecks for hydrogen usage at a broader scale. Direct ammonia solid oxide fuel cells (DA-SOFCs) stand out as a promising technology for converting ammonia to power in a single step, providing a potential decarbonization pathway for several power generation applications currently using fossil fuels. This review aims to present a comprehensive summary of the recent advancements in both experimental and computational facets of DA-SOFC technology. Then, we discuss the various types of DA-SOFCs, all of which are assessed with respect to the materials and process conditions used. The impact of surface modification on DA-SOFC performance via doping of electrolyte and metal infiltration into conventional anode catalysts has also been reviewed. The highest power density of DA-SOFC reported so far is 1330 mW cm(-2) at 650 degrees C, achieved using a very thin electrolyte (similar to 1.1 mu m thick) in a GDC-YSZ-GDC sandwich structure. Realizing the potential of DA-SOFC technology requires overcoming several technological challenges, including nickel nitridation, microstructural deformation, and thermal and chemical strains, highlighted in this review. Though a peltothra of review articles related to DA-SOFC technology are available in the literature, this review focused on elucidating the underlying reaction mechanisms in DA-SOFC at the atomic level using ab initio approaches is lacking despite its significance in designing active cell materials. In this relation, atomistic insights into the reaction mechanisms in DA-SOFCs using density functional theory (DFT) computations have been presented. For instance, DFT computations revealed that the hydrogen spillover from the Ni to Ni-YSZ interface is the most favorable mechanism for H-2 oxidation at the triple phase boundary (TPB) region of Ni/YSZ anode, accounting for an activation barrier of 1.17 eV, whereas hydroxyl and oxygen spillover presented higher kinetic barriers of 2.25 and 1.99 eV, respectively. Finally, this review concludes by discussing the challenges and the future perspectives to advance ammonia SOFC technology to a commercialization level.