Factors governing surface reactions leading to nitridation of nickel catalysts in ammonia-fueled solid oxide fuel cells

Abstract

Solid oxide fuel cells that directly utilize ammonia as a hydrogen storage medium typically employ nickel-based materials, such as Ni/yttria-stabilized zirconia, as anode catalysts. While solid oxide fuel cells operate at high temperatures with high energy efficiency, durability issues with constituent materials necessitate lower-temperature operation. However, at low temperatures, incomplete ammonia decomposition promotes nickel nitridation, reducing catalytic efficiency. Addressing this nitridation issue is essential for enabling the commercialization of low-temperature solid oxide fuel cells. In this study, we examine nitrogen-nickel surface interactions and determine the thermodynamic and kinetic mechanisms driving nitridation under different process conditions by performing a series of density-functional theory calculations. We find that adsorbed ammonia decomposition intermediates do not promote nitridation of the nickel surface. Instead, our calculations reveal that nitridation is driven by surface saturation of adsorbed nitrogen atoms, leading to nitrogen penetration beneath the surface. The incorporated nitrogen atoms induce lattice distortion in the surrounding nickel structure, causing the resulting nickel nitrides to exhibit thermodynamic instability and decompose readily in reducing atmospheres or at elevated temperatures, leaving behind vacancies in the nickel lattice. These vacancy defects within the nickel modulate surface-nitrogen interactions, ultimately promoting the formation of nickel nitrides. Finally, we determine the nitridation tendency of different nickel crystal planes by examining how atomic nitrogen saturation ratios vary across different surfaces and demonstrate that nickel surfaces with high-index crystal planes show higher nitrogen saturation concentrations, creating favorable conditions for initial nitridation. The theoretical insights gained from this study provide valuable guidance for solving the nitridation problem in ammonia-fueled solid oxide fuel cells.