Insights into the formation of metal carbon nanocomposites for energy storage using hybrid NiFe layered double hydroxides as precursors

NiFe-carbon magnetic nanocomposites prepared using hybrid sebacate intercalated layered double hydroxides (LDHs) as precursors are shown to be of interest as supercapacitors. Here, the low-temperature formation mechanism of these materials has been deciphered by means of a combined study using complementary in situ (temperature-dependent) techniques. Specifically, studies involving X-ray powder diffraction, thermogravimetry coupled to mass spectrometry (TG-MS), statistical Raman spectroscopy (SRS), aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) have been carried out. The experimental results confirm the early formation of FeNi3 nanoparticles at ca. 200-250 °C, preceding the concerted collapse of the starting NiFe-LDH laminar structure over just 50 °C (from 350 to 400 °C). At the same time, the catalytic interactions between the metallic atoms and the organic molecules permit the concomitant formation of a graphitic carbon matrix leading to the formation of the final FeNi3-carbon nanocomposite. Furthermore, in situ temperature-dependent experiments in the presence of the intrinsic magnetic field of the STEM-EELS allow observing the complete metal segregation of Ni and Fe even at 400 °C. These results provide fundamental insights into the catalytic formation of carbon-based nanocomposites using LDHs as precursors and pave the way for the fine-tuning of their properties, with special interest in the field of energy storage and conversion. This journal isFinancial support from the European Union (ERC Advanced Grant Mol-2D 788222, ERC Starting Grant 2D-PnictoChem 804110, ERC Proof of Concept Grant Hy-MAC 713704, and COST-Action on Molecular Spintronics (MOLSPIN CA15128)), the Spanish MINECO (Projects MAT2017-89993-R, MAT2015- 66888-C3-3-R, RTI2018-097895-B-C43 co-nanced by FEDER, and the Unit of Excellence “Maria de Maeztu” MDM-2015-0538), and the Generalitat Valenciana (Prometeo Program and iDiFEDER/2018/061 co-nanced by FEDER) is gratefully acknowledged. G. A. acknowledges the support by the Deutsche Forschungsgemeinscha DFG (FLAG-ERA AB694/2-1) and the Generalitat Valenciana (CIDEGENT/2018/001 grant). G. A. received nancial support through the Postdoctoral Junior Leader Fellowship Programme from “la Caixa” Banking Foundation. J. R. thanks the Spanish MINECO for his predoctoral grant. Electron microscopy observations were carried out at the ICTS ELECMI node at Centro Nacional de Microscop´ıa Electr ´onica at the Universidad Complutense de Madrid. The authors thank Dr Mar´ıa Dolores Jord´an Mart´ın for her kind assistance with the XPS measurements