Pseudocapacitive Fe4N nanoparticles encapsulated within interconnected graphene cavities as efficient binder-free capacitive deionization anodes

Capacitive Deionization (CDI) has emerged as an ideal alternative strategy for energy-efficient brine water treatment. However, as an essential component of the CDI electrode system, anode materials with efficient ability for anion capture have not been adequately addressed. Herein, we have successfully developed a kind of iron nitride (Fe₄N) material with a unique pseudocapacitive nature into CDI anodes. Specifically, the Fe₄N nanoparticles are controllably encapsulated within interconnected graphene cavities to form binder-free monolithic electrodes (GF@FeN) through stepwise assembly strategies. The intrinsic electrochemical desalination potential of the Fe₄N species can be effectively unlocked with the assistance of the graphene porous frameworks. For the typical Cl⁻ anions, the optimized GF@FeN anode exhibits competitive dechlorination capabilities of ~140.32 mg·g⁻¹ (~ 231.23 mg·g⁻¹ for NaCl) and long-term cycling stability (over 88 % retention rate after 50 cycles). Mechanistic discussions indicate that the excellent Cl⁻ capture ability of the Fe₄N species mainly stems from the reversible redox reactions involving the divalent and trivalent iron (Fe²⁺/Fe³⁺) components on its surface. Furthermore, the GF@FeN electrodes can also effectively capture other representative toxic anions, such as F⁻ and CrO₄²⁻, and achieve very high removal rates (~ 99 %) under certain conditions, demonstrating their broad applicability as CDI anodes. This work establishes the feasibility of iron nitride as a promising CDI anode, which not only provides alternative electrode materials for effective anion removal but also opens up vast opportunities for the design of high-performance hybrid CDI electrode systems.