Nitrogen-doped carbon nanotube encapsulated Fe7S8 nanoparticles for the high-efficiency and selective removal of Pb2+ by pseudocapacitive coupling

Abstract

Lead (Pb) is one of the most common public hazards in drinking water, and it widely derives from mining, batteries, the waste from dye manufacturing, and so on. To eliminate this critical environmental issue, capacitive deionization (CDI) technology has been recognized as a potential alternative for the highly-efficient capture of Pb ions, owing to its operational convenience, environmental-friendliness, and low cost. In this work, we utilized a melamine foam frame to prepare nitrogen-doped carbon nanotube encapsulated Fe₇S₈ nanoparticles (Fe₇S₈@NCNT) via three hydrothermal, carbonization, and sulfidation steps for the pseudocapacitive removal of Pb²⁺ ions from water. The synergistic effect derived from Fe²⁺/Fe³⁺ redox pairs and specific structural design endowed the Fe₇S₈@NCNT electrode with excellent pseudocapacitance and enhanced CDI performance in comparison with Fe₃C@NCNT and Fe₇S₈@NF. As a result, the exceptional Pb²⁺ removal capacity reached an experimental maximum value of 223.1 mg g⁻¹ at 1.2 V with superior cycling stability. The formation of a Pb–S band after the pseudocapacitive coupling of Pb²⁺ ions on the surface of the electrode was confirmed based on ex situ Raman and XPS studies, suggesting a deep pseudocapacitance electrosorption mechanism for Pb²⁺ ions. Furthermore, Fe₇S₈@NCNT showed high selectivity toward Pb²⁺ under competition from ubiquitous Na⁺ and Ca²⁺ ions at relatively high concentrations, considering practical applications, and this is also supported by density functional theory (DFT) calculations. Thus, Fe₇S₈@NCNT can show great promise as a cathode for removing heavy metal Pb²⁺ ions from water.