Defect-engineered MOF-801 as a redox-active intercalation battery-type capacitive deionization cathode: mechanistic insights into selective calcium ion removal

Abstract

Capacitive deionization (CDI) employing battery-type faradaic cathodes based on redox-active intercalation principles demonstrates potential to surpass reverse osmosis (RO). The technology's exceptional prospects lie particularly in its highly selective adsorption of specific cations, which could significantly enhance the novelty of CDI systems. The crux of constructing such redox-intercalation battery-type cathodes crucially hinges on two scientific challenges: (1) developing enhanced adsorption channels to accelerate hydrated cation mass transfer and (2) intensifying the redox reactions of desolvated ions to achieve superior adsorption capacity. Herein, we present a defect-engineering strategy where abundant structural defects were intentionally created on highly hydrophilic MOF-801 through modulator-assisted synthesis. Subsequently, the defect-rich MOF-801 was electro-polymerized with pyrrole and surfactants on graphite substrates, yielding a composite cathode (denoted as SS-MOF-801@PPy) for selective calcium ion removal. Structure–performance characterization and electrochemical behavior analysis reveal that Ca²⁺ undergoes intercalation/deintercalation redox reactions within SS-MOF-801@PPy, demonstrating a diffusion-controlled battery-type behavior. Remarkably, the optimized cathode achieves a Ca²⁺ adsorption capacity of 70 mg g⁻¹ and maintains 34 mg g⁻¹ selective adsorption capacity for Ca²⁺ in ternary solutions (Ca : Na : K = 1 : 1 : 1 molar ratio), approaching the threshold for industrial applications. DFT calculations elucidate the fundamental mechanism: the hydrated calcium ions (Ca²⁺·nH₂O) first undergo desolvation through interactions with the hydrophilic MOF-801 within the open-channel defects, where the adsorption energy surpasses that of hydrated Na⁺. The desolvated Ca²⁺ then intercalates into oxygen-functionalized active sites, initiating favorable redox reactions.