Advancing Sustainable Ammonia Production via Solventless, Robust, and Thermally Conductive Absorbents

Abstract

Sorption-based low-pressure green ammonia synthesis using supported metal halide salts enables efficient interconversion between hydrogen and ammonia, allowing the high hydrogen density and well-established transportation network of ammonia to be used for green energy storage. Magnesium chloride supported on silica gel (MgCl₂/SiO₂) sorbent has been the subject of much investigation owing to its high capacity, selectivity, and reversibility at temperatures close to reactor conditions; however, MgCl₂/SiO₂ suffers from low thermal conductivity, which complicates absorber design at larger scales and prolongs absorption–desorption cycle times. We present a scalable, solventless method for supporting MgCl₂ on thermally conductive aluminum fibers (MgCl₂/Al)─a thermally conductive ammonia sorbent with a high working capacity of ∼220 mg_NH3/g_absorbent. Although the solventless synthesis causes variance in initial-cycle pressure drop and capacity, we show that this stabilizes after cycling. The high thermal conductivity of MgCl₂/Al allows for rapid absorption–desorption cycles, enabling easier scale-up. MgCl₂/Al also maintains its cyclic capacity up to at least 50 cycles.