Low-temperature ammonia adsorption-desorption performance of Al2O3-supported, graphite-casted metal chloride pellets

Ammonia (NH₃) is widely recognized as a carbon-free hydrogen carrier with high energy density, making it a promising energy vector for sustainable energy systems. However, its efficient storage and release remain technical challenges. In this study, the low-temperature NH₃ adsorption-desorption performance of four pure metal chlorides (CaCl₂, MgCl₂, MnCl₂, and CuCl₂) was systematically evaluated. Among them, CuCl₂ and MnCl₂ exhibited superior NH₃ uptake capacities, but also suffered from volume expansion and particle agglomeration during repeated cycling. To overcome these limitations, composite sorbent pellets were developed by supporting metal chlorides on Al₂O₃ via a graphite-casting method. The resulting CuCl₂-based composite pellet achieved an optimized balance of high NH₃ adsorption capacity (∼ 0.36  g NH₃/g), excellent regeneration efficiency (> 96 %), and adequate mechanical strength. Excessive CuCl₂ loading reduced both regeneration efficiency and mechanical stability, while lower loading enhanced active phase dispersion and sorbent performance. 30 wt% CuCl₂ offered the best performance in terms of adsorption, regeneration, and structural durability. Additionally, the optimized Cu₃₀-Al-C sorbent maintained stable NH₃ adsorption capacity over 20 cycles, confirming excellent reusability. The enhanced performance was attributed to the synergistic effects of Al₂O₃ support and the hydrophobic nature of the graphite-casting process, which enabled uniform distribution of active compound and mitigated structural deterioration during cycling. These results demonstrate the feasibility of graphite-casted, Al₂O₃-supported metal chlorides as structurally stable and regenerable NH₃ sorbents for low-temperature ammonia storage applications.