Ion selectivity and electrosorption of pseudocapacitive NiFe₂O₄ nanocrystals

Pseudocapacitive NiFe₂O₄ (NFO) nanocrystals and reduced graphene oxide (rGO) nanosheets were synthesized, and their ion selectivity and electrosorption performance were evaluated. The NFO cathode-anode pair demonstrated a remarkable preference for SO₄²⁻ over Cl⁻, achieving a selectivity coefficient 76 within the first 30 min. This pronounced selectivity for anion SO₄²⁻ was primarily attributed to its higher divalent negative charge, which induced a stronger electrostatic attraction to the positively charged NFO electrode surface (+15.2 mV). Both materials delivered comparable salt adsorption capacities (SAC) of 231 mg g⁻¹ for NFO and 243 mg g⁻¹ for rGO in Al(NO₃)₃·9H₂O solution. The superior electrosorption performance of the NFO electrode was not attributed to its surface area, which was very low due to the non-porous material; instead, it was due to NFO's well-defined crystalline lattice spacing and available interstitial spaces enabled efficient ion intercalation and electrosorption. A key advantage of NFO was its exceptional divalent anion selectivity, where the electrode adsorbed 2.84 times more SO₄²⁻ ions than Cl⁻ ions, demonstrating a high selectivity that usually cannot be achieved by electric double-layer (EDL)-based electrodes. These results highlight NFO electrodes as a promising alternative electrode material for compact and energy-efficient capacitive deionization (CDI) systems, particularly for applications demanding superior ion selective separation.