Highly efficient ammonium ion removal via a hybrid capacitive deionization platform with low-gap and structurally stable MIL-101(Fe)@cellulose nanocrystals

Abstract

Metal-organic frameworks (MOFs) have surfaced as exceptionally promising porous materials, primarily because of their ability to form stable structures with remarkable properties. These frameworks possess numerous pores that enhance ion transport in capacitive deionization (CDI) applications. However, significant hurdles persist with MOFs, including a scarcity of active sites and complications related to water stability, which impede their broader usage. Additionally, the charge-discharge cycles that take place during operation can induce lattice distortions, a phenomenon associated with the Jahn-Teller effect that is intrinsic to MOF structures. To address this challenge, we have synthesized hybrid nanoparticles composed of MIL-101(Fe) and cellulose nanocrystals (CNCs), referred to as MCNCs, featuring a stable structure that integrates a π-conjugated framework characterized by extensive electron delocalization and a narrowed HOMO-LUMO gap. Additionally, the high density of active sites provided by CO groups promotes efficient redox reactions, enhancing the NH₄⁺ capture capability. The produced MCNCs polymer demonstrates remarkable electro-adsorption properties and significant structural durability, achieving an impressive specific capacitance of 119.05 F g⁻¹ at 0.5 A g⁻¹. Leveraging the exceptional pseudocapacitive attributes of the MCNCs, we have engineered a hybrid capacitive deionization system that not only demonstrates an extraordinary ammonium removal capacity of 81.29 mg g⁻¹ and a swift average removal rate of 3.75 mg g⁻¹ min⁻¹ but also retains 93.79% of its initial removal capacity for ammonium ions after 100 cycles, offering a fast, effective, and sustainable method for treating water.