Scalable fabrication and electrochemical characterization of binder-free Fe₃O₄@C films on a Cu current collector

Here we present a scalable strategy for fabrication of nanostructured magnetite (Fe₃O₄) films directly on a copper (Cu) current collector as potential anodes for energy storage devices. Hydrothermal synthesis was employed in the first step to realize Fe₃O₄ films which were subsequently encapsulated in a carbon shell in a controlled way using chemical vapor deposition (CVD) techniques. Through systematic investigations, it was revealed that lower hydrothermal temperature (80 °C) results in rhombus-shaped nanoarchitectures which were primarily composed of α-FeOOH. At higher temperature (120 °C), three-dimensional (3D) superstructures comprising of Fe₃O₄/iron carbonate (FeCO₃) composite nanolayers were found. Furthermore, it was revealed that, irrespective of the composition of the hydrothermally synthesized film, each one converted upon calcination to cubic Fe₃O₄ without any significant changes in the morphology. Conformal carbon encapsulation allowed the formation of Fe₃O₄@C core–shell structures having tunable shell thickness and strong interfacial bonding (Fe–O–C). Although structural investigations showed improvement in crystallinity with carbon encapsulation by both CVD techniques, it was more pronounced in the case of microwave plasma CVD (MP-CVD), which also induced partial reduction of Fe₃O₄ to metallic iron (Fe). The films showed pseudocapacitive behavior in 1 M Na₂SO₄ during electrochemical evaluation, and stored charge at lower scan rates predominantly by a diffusion-controlled process, which decreased with increasing scan rate, leading to a dominant capacitive process for charge storage. The developed binder-free films have great potential as highly robust anodes for lithium and post-lithium ion batteries (LIBs), supercapacitors, and battery–supercapacitor hybrid devices.