Single-atom Fe in N-doped hierarchical carbonaceous structure derived from MOF: A pathway to high-performance CDI and MCDI

Abstract

Capacitive deionization (CDI) is an efficient method for brackish water desalination with low energy consumption. The effectiveness of CDI relies on the optimal engineering of electrode materials to enhance salt removal capacity (SRC). Carbon-based materials, through morphological optimization and heteroatom doping, have shown improved performance. Iron-based carbonaceous structures are promising candidates for enhancing electrode capacitance in CDI and membrane capacitive deionization (MCDI) systems. This study investigated the synthesis of nitrogen and metal co-doped porous carbons via carbonization using a metal-organic framework (MOF). The performance of single-atom iron embedded in nitrogen-doped carbon (SAFe-N-C) was compared to iron nanoparticles integrated into nitrogen-doped carbon (NPFe-N-C) and nitrogen-doped carbon (N-C). Electrochemical evaluations in a 1.0 M NaCl electrolyte demonstrated that SAFe-N-C, as the optimized electrode, benefited from uniform adsorption sites, reduced internal resistance, excellent stability, and a specific capacitance of 117 F g^-1 at 2 mV s^-1. It also achieved SRCs of 36.7 mg g^-1 and 57.8 mg g^-1 from a 500 mg L^-1 NaCl feed solution at 1.2 V during CDI and MCDI tests, respectively. Therefore, atomically dispersed Fe in the hierarchical porous N-doped structure can be a promising alternative for enhancing CDI and MCDI performance. Density functional theory (DFT) calculations indicated that effective Fe–N groups serve as active sites, promoting charge density redistribution and improving ion affinity, leading to substantial SRC. This study provides insights into the critical role of Fe–N sites in carbon-based CDI and MCDI enhancement.