Efficient Removal of Chlorine Ions by Ultrafine Fe₃C Nanoparticles Encapsulated in a Graphene/N-Doped Carbon Hybrid Electrode: Redox and Confinement Effect

Developing a high-performance Cl-storage electrode is a crucial issue for capacitive deionization (CDI). Iron-/nitrogen-doped carbon hybrid composites with densely dispersed ultrafine Fe-based nanoparticles are promising candidates for Cl-storage electrodes, yet further improvement of Fe-based nanoparticles prone to agglomeration is strongly desired. Hereby, a hybrid electrode with ultrafine iron carbide nanoparticles encapsulated in graphene/chitosan-derived N-doped carbon (Fe3C@GNC) is successfully constructed via facile one-step pyrolysis of aerogel composites. The encapsulation effect of graphene can effectively confine Fe3C nanoparticles in the carbon matrix, enabling stable and dispersive ultrafine Fe3C nanoparticles, and chitosan also enables N-doping. Also, a satisfactory conductive system with synergistically long- and short-range conductive networks is successfully generated by the graphene/N-doped carbon matrix. The Fe3C@GNC electrode exhibits a typical pseudocapacitive behavior, with a specific capacitance of up to 305.33 F g(-1) and a dominant capacitive contribution of up to 96%. As a Cl-storage electrode for CDI, it delivers a Cl- adsorption capacity as high as 82.08 mg g(-1) with a retention rate of 74.2% for 150 cycles. Furthermore, it is revealed that the Cl- storage mechanism of Fe3C@ GNC is a pseudocapacitance effect induced by the reversible Fe2+/Fe3+ redox couple, which can achieve fast reaction kinetics and structural stability in the CDI process.