Tuning the Structure of Layered Hydroxides for Boosting Energy Application Performance
Authors: Diouane Youssra , Oestreicher Victor , Seijas Da Silva Alvaro , Abellán Gonzalo 
Published: September 2023

We are currently on a race against the clock to protect our planet from greenhouse gas emissions. The solution could be inspired from mother nature talking about renewable energies or so-called green energies including wind, solar, hydraulic, biomass and so on, which allows us to reduce the fossil fuels consumption. However, these kinds of energy devices should be coupled to an energy storage system such as batteries and supercapacitors to compensate the absence of wind and sun. For this purpose, we are looking forward to developing strong energy and power application systems by focusing on the development of novel electrode materials.
Among 2D materials, layered hydroxides emerge as one of the most attractive electrode materials based on earth-abundant metals, at the same time inside of this family, three type can distinguish: the first one is the Brucite-like structure1 (β-structure) where the metal is arranged in the octahedral position and the interlayer space is defined by Van-Der-Waals interaction. The second one is the Hydrotalcite-like structure also called Layered Double Hydroxide (LDH) recognized by the following formula MII
1-x MIII
x (OH)2 Ax. xH2O. The LDH regroups in the same layer a divalent and trivalent metals and are both arranged in the octahedral position2. The Simonkolleite-like structure3 (α-structure) with M1-x
Oh
Mx
Td (OH)2-x Ax. yH2O as formula, is the third type which only contains divalent metals arranged in tetrahedral (Td) and octahedral (Oh) position.
Layer double hydroxides are explored materials in the energy field. Multiple studies have been reported on the behaviour of the layered hydroxides materials in energy storage4 and conversion5. This material is very appealing for researchers worldwide due to their good electrochemical behaviour, chemical versatility, electrochemical stability, and their low cost as well6. For instance, LDHs based on earth-abundant elements are good candidates for electrode materials for oxygen evolution reaction (OER) in alkaline condition, as well as for supercapacitors. However, one of the major drawback of these materials is their low electrical conductivity, which represents a limitation in the optimization of the electrochemical performance and the assemble of devices.
Among the different approximation to overcome this issue, we are intensively working on the modulation of the electronic properties of LHs. In this line, Co-based α-layered hydroxides, exhibiting simonkolleite-like structure, can be an interesting case of study since the covalent bond between the hydroxylated layer and the anion (figure 1). Indeed, we have recently demonstrated that the electrical properties can be modulated by halide substitution6, where the band gap can be reduced by ligand to metal charge transfer as demonstrated by DFT+U simulations. Considering these previous results, we decided to analyse the role of these chemical modifications in terms of the electrochemical properties.
Interestingly, our results evidence a better electrochemical activity of Co-based α-LHs in comparison with the typical β-LHs or LDHs analogues, in both OER and specific capacitance performance7. Moreover, in the case of α-LHs the values can be further enhanced by tuning the nature of the anion: for instance, structures containing iodide overpass in more than 40% those ones containing chloride, as mentioned by DFT+U simulation (figure 2).
Hence, this work positions α-CoII hydroxides as key phases among the layered hydroxides family as electrode materials for energy storage and conversion, where the conductivity can be improved by anion substitution8.

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2 Du, Y.; O’Hare, D. Synthesis, Morphology, Structure, and Magnetic Characterization of Layered Cobalt Hydroxyisocyanates. Inorg. Chem.
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3 Oestreicher, V.; Dolle, C.; Hunt, D.; Fickert, M.; Abellán, G. Room Temperature Synthesis of Two-Dimensional Multilayer Magnets Based on α-CoII Layered Hydroxides. Nano Mater. Sci.
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4 Zhang, D.; Cao, J.; Zhang, X.; Zeng, Z.; Insin, N.; Qin, J.; Huang, Y. Modification Strategies of Layered Double Hydroxides for Superior Supercapacitors. Adv. Energy Sustain. Res.
2022, 3 (3), 2100183.

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6 Oestreicher, V.; Hunt, D.; Torres-Cavanillas, R.; Abellán, G.; Scherlis, D. A.; Jobbágy, M. Halide-Mediated Modification of Magnetism and Electronic Structure of α-Co(II) Hydroxides: Synthesis, Characterization, and DFT+U Simulations. Inorg. Chem.
2019, 58 (14), 9414–9424.

7 R. Sanchis-Gual, V. Oestreicher, G. Abellan et al. Submitted.

8 Y. Diouane, V.Oestreicher, G. Abellan et al, manuscript in preparation

Figure 1