Core-Shell Redox-Nanoparticles Integrate High Ammonium Selectivity with Long-Term Stability

Cu-based Prussian blue analogues (PBAs) are promising material platforms for selective ammonium intercalation due to their vacancy-rich framework. However, their limited cycling stability has hindered practical applications. Here, we design core-shell heterostructured PBAs using copper hexacyanoferrate (CuHCF) as an ammonium-selective core, with a nickel hexacyanoferrate (NiHCF) as an outer shell that enhances electrochemical stability. The nanostructural design of these mixed core-shell particles preserved ammonium selectivity while extending cycling longevity. The CuHCF@NiHCF nanoparticles were synthesized by a two-step coprecipitation, with the NiHCF shell thickness precisely controlled to 10 and 20 nm. Both core-shell particles retained over 98% capacity after 1000 charge-discharge cycles at 1 A g^-1. Electrosorption with the 10 nm shell particles achieved a high separation factor of 9.2 for NH₄⁺ over Na⁺ with 0.42 mmol g^-1 NH₄⁺ uptake, whereas the 20 nm shell particles exhibited a lower selectivity of 3.9. Furthermore, the selectivity of the 10 nm shell particles was enhanced to exceed 20 through voltage control. Electrosorption in a flow cell using the 10 nm shell particles demonstrated efficient NH₄⁺ extraction from municipal wastewater, enriching the molar % of NH₄⁺ among total cations from 29% to 61%. This work establishes shell thickness control as a strategy to couple high NH₄⁺ selectivity with stability for sustainable resource recovery.