Ammonia as hydrogen carrier: Advances in ammonia decomposition catalysts for promising hydrogen production

On-site ammonia decomposition has been considered as a potential candidate to alleviate the challenges of hydrogen storage and transportation by utilizing NH3 (with a high hydrogen content of 17.6 wt%) as hydrogen carrier, along with the flourish of renewable energy. Although the decomposition of NH3 into H2 is thermodynamically favorable at above 400 °C, the reaction kinetics remains sluggish due to the high activation energy for N–H bond cleavage and N2 desorption. This motivates the design and construction of functional catalysts with high-efficiency and low-cost. To date, a variety of metal-based catalysts have been investigated for NH3 decomposition, among which Ru often shows the highest activity due to its optimal metal–nitrogen binding energy. Efforts are being devoted to the further improvement of catalytic performance through tuning the morphology, electronic structure, defect/doping and metal–support interaction. Meanwhile, advanced techniques are employed to disclose the structure–performance relationship of catalyst. Herein, this review identifies the fundamental principles of catalytic NH3 decomposition, addresses the advances on current catalyst design, summarizes the strategies to enhancing efficiency, and provides recommendations for further material design. A comprehensive consideration of recent development of noble metal and transition metal catalysts are provided, as well as the promoters and supports. Moreover, the advantage of using bimetallic materials, which offers a synergy to modulate the electronic structure and improve the intrinsic activity, is emphasized. This review may serve as an informative work to inspire the future development of ammonia decomposition catalyst for practical application.