Advances in Gaseous Ammonia Decomposition for Hydrogen Production: Catalysts and Emerging Pathways

Ammonia (NH₃) is a compelling carbon-free hydrogen carrier. Its catalytic decomposition to produce a hydrogen/nitrogen (H₂/N₂) gas stream is central to the “NH₃-H₂” clean energy cycle, provided that residual NH₃ is removed to fuel-cell-grade purity downstream. This review integrates advances from the past five years across four major catalytic NH₃ decomposition pathways, encompassing conventional thermocatalysis, plasma-catalytic, photo(thermal), and electrically driven catalysis, within a unified mechanistic and practical framework, distinguishing it from existing single-pathway reviews. Noble metal catalysts, particularly Ru-based systems, achieve superior low-temperature activity through support engineering, promoter effects, and active-site construction. However, our analysis reveals that non-noble metal (Fe, Co, Ni) catalysts and their alloys, nitrides, and carbides have made substantial progress, with certain Co-based and bimetallic systems approaching Ru-level performance via interfacial oxygen vacancy engineering and electronic structure modulation. Emerging non-thermal routes effectively overcome thermodynamic barriers, enabling operation at temperatures 200–300 °C below conventional thermal requirements, though each faces distinct challenges in energy efficiency, stability, and scalability. Key challenges remaining across all pathways to practical implementation, including residual NH₃ removal and H₂ purification, catalyst deactivation and stability, heat management and energy efficiency, start-up/shut-down dynamics, as well as system integration and economics, are critically assessed. This review provides theoretical guidance and practical recommendations for developing scalable, low-temperature NH₃ decomposition technologies.