Advances in electrocatalyst design for the ammonia oxidation reaction

The transition towards a hydrogen (H₂)-based economy is significantly constrained by challenges associated with its storage, transportation, and supply-chain infrastructure. These limitations arise from hydrogen's low volumetric energy density and the need for energy-intensive compression or cryogenic liquefaction. To circumvent these issues, liquid-phase energy carriers such as methanol, ethanol, and ammonia have emerged as attractive alternatives, offering significant benefits, including higher energy density, ease of storage and handling, and compatibility with existing fuel distribution infrastructure. Among the available candidates, ammonia (NH₃) has gained considerable attention, particularly for applications in direct ammonia fuel cells (DAFCs). Ammonia has a high hydrogen content (~17.6 wt%) and a relatively high volumetric energy density, enabling efficient hydrogen storage without prior reforming or extraction. Despite these advantages, the practical implementation of DAFCs is hindered by the intrinsically sluggish kinetics of the ammonia oxidation reaction (AOR). In addition, strong adsorption of reaction intermediates on catalyst active sites leads to catalyst poisoning, thereby reducing turnover frequency and overall cell performance. In this review, we present recent advances in electrocatalyst development and engineering strategies to improve AOR activity and durability in DAFC systems. Particular emphasis is placed on the evolution of catalyst materials, spanning platinum-group-metal (PGM)-based systems to hybrid PGM/non-PGM architectures and fully non-PGM catalysts. Furthermore, key design principles, including electronic structure modulation, surface and interface engineering, compositional optimization, and AI/ML approaches, are discussed to provide fundamental insights into AOR electrocatalysts. Finally, we outline the current challenges and highlight future research directions towards the development of next-generation electrocatalysts for efficient and durable DAFC operation in this review article.