Beyond Fertilizers: NH₄ZnPO₄ for the Reversible Chemical Storage of Ammonia

Enhancing NH₃ as a carbon-free energy carrier of H₂ and next-generation fuel is a promising approach for a sustainable society. Chemically storing NH₃ molecules in crystal structures offers better selectivity and reusability than storage in traditional porous materials based on physicochemical adsorption; however, designing materials that can be reversibly stored in structural gaps is still a significant challenge. Herein, the use of NH₄ZnPO₄, which is previously used as a fertilizer, is proposed as an NH₃ uptake material through a chemical storage mechanism. The NH₄ZnPO₄ particles synthesized by a wet mechanochemical method with monoclinic and hexagonal crystal structures can incorporate NH₃ molecules and directly transform them into NH₄Zn(NH₃)PO₄ without producing byproducts. The chemical storage mechanism depends on the particle morphology; therefore, the uptake amount per surface area surpasses that of porous materials. NH₄ZnPO₄ exhibited excellent cycling performance due to its reusability, which is regenerated by releasing NH₃ from NH₄Zn(NH₃)PO₄ when heated in air at ≈100 °C. Taking inspiration from previously used and familiar fertilizers further extends this new area of innovative materials that can be used for the reversible storage of low-molecular-weight gases.