An in-Situ Study of the Transformation of Hybrid Layered Hydroxides into Metallic Nanocomposites with Interest in Energy Storage Systems

The high increase of energy demand in recent decade and the necessity of reduce the fossil fuel consumption have triggered the interest of the scientific community in the search of 'green' alternatives. Due to the characteristic intermittence of solar and wind technologies, there is a huge effort on the development energy storage systems. Among them hybrid capacitors (HCs) are promising systems due its high energy and power density values, in comparison with Li-ion batteries, that make them ideal for industrial and automobile applications. Battery-like materials such as nanocomposites (NCs) based on metallic nanoparticles (working as pseudo-capacitor based in redox reactions) with a carbon matrix (working as electric double layer capacitors) are key HCs systems itself. These NCs exhibits a notorious enhancement on their electrochemical performance by applying an external magnetic field. [1] Typically, NCs are obtained by the thermal decomposition of hybrid materials under inert atmosphere, as it has been demonstrated for the case of layered double hydroxides (LDHs), layered hydroxides (α-LHs) and metal organic frameworks (MOFs). [1-3] However, there is a lack of information in terms how the thermal transformation takes places. In this work, we present for the first-time an in-situ study about the transformation of hybrid layered hydroxides into NCs. For that, synchrotron measurements of X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) are performed for different hybrid layered hydroxides to follow the transformation in terms of oxidation state and crystallinity. These hybrid layered hydroxides will variate on: (i) the layered hydroxide structure and the anion-layer interactions (LDHs vs α-LHs + electrostatic vs covalent), (ii) the cation nature, and (iii) the identity of the organic linkers, to name a few. For instance, in the case of α-LHs, while the LHs structure defines the thermal decomposition, and the organic molecule length controls the quality and quantity of the final carbon matrix at the NCs. [4] Interestingly, in function of the amount of carbon present on the organic molecule, the oxidation state and the formation of the carbon matrix on the final material are conditioned, which imply that there is a minimum amount of carbon to obtain the NCs. In the other hand, for the previous NCs obtained, the electrochemical characterization is performed in pouch cells, applying an external magnetic field throughout the galvanostatic cycles. This assembly prevents the loss of material along the time and during the process, where it is evidenced how the magnetic field improves the electrochemical performance with an increase in the specific capacity of the NCs. In overall, this work introduces fundamental knowledge on the rational design of hybrid layered hydroxides and the subsequent nanocomposites and its performance in energy storage systems. References [1] Romero, J.; et al. Giant Enhancement in the Supercapacitance of NiFe–Graphene Nanocomposites Induced by a Magnetic Field. Adv. Mater., 2019 31 (28), 1900189.. [2] Lopez-Cabrelles, J.; et al. Solvent-Free Synthesis of ZIFs: A Route toward the Elusive Fe(II). Analogue of ZIF 8. J. Am. Chem. Soc., 2019 141, 7173−7180. [3] Romero, J.; et al. Insights into the formation of metal carbon nanocomposites for energy storage using hybrid NiFe layered double hydroxides as precursors Chem. Sci. 2020, 11 (29), 7626–7633. [4] Oestreicher, V.; et al., The Role of Covalent Functionalization in the Thermal Stability and Decomposition of Hybrid Layered Hydroxides. Phys. Status Solidi RRL, 2020 14: 2000380 Figure 1