Molecular and Cationic Engineering of Fe-Based Layered Double Hydroxides for Water Oxidation
Authors: Seijas Da Silva Alvaro , Sanchis-Gual Roger , Oestreicher Victor , Coronado Eugenio , Abellán Gonzalo 
Published: September 2023

Layered double hydroxides (LDHs) are a class of anionic clays with a layered structure, that can be exfoliated giving place in 2D materials. Recently, these materials are attracting increasing attention from the point of view of their electrochemical performance towards energy storage and conversion applications. [1] In the field of electrocatalysts for the oxygen evolution reaction, Fe-based LDHs stand out as one of the best oxygen evolution reactions (OER) electrocatalysts in alkaline conditions. In the attempt to improve the electrochemical performance of Fe-LDHs-based electrodes, the effect of the microstructure, exfoliation, metal composition, crystallinity, doping, defect engineering and so on has been reported to increase the knowledge of electrochemical behavior and obtained better performances.[2] Beyond these well-studied parameters, interlayer space, Fe-clustering and covalent functionalization have been not explored, being the last two synthetic challenges within the field of LDHs.
On the one hand, to explore the influence of interlayer space in the behaviour of Fe–LDHs as an oxygen evolution reaction (OER) electrocatalyst, we develop a family of NiFe-LDHs with the same metal composition, morphology, and dimensions and different alkyl sulfate to reach extreme ranges of basal spacing (from 8 to 32 Å). Increasing the basal space of the LDH results in a higher electrochemical surface area and a reduction of the resistance related to the chemisorption of oxygen leading to better kinetic behaviour. Indeed, the Tafel slope for the NiFe–LDHs with the highest basal space studied is similar to that obtained for benchmark exfoliated NiFe nanosheets and shows better stability as a consequence of the tridimensional robustness of the hybrid material. However, an excessive increment of the interlayer space compromises the onset potential.[3]
On the other hand, to shed light on the influence of Fe-clustering on this topic, firstly, we have selected MgFe-based LDHs phases as model systems to decipher whether the Fe-clustering exert an effect on the OER performance. By optimization of a hydrothermal method based on triethanolamine different Fe-clustering degree has been achieved, and thanks to the diamagnetic behaviour of Mg, a simple magnetic characterisation allows us to identify the Fe-clustering degree. Nevertheless, both samples behave identically in terms of OER performance when glassy carbon electrodes are used. Surprisingly, when the samples are tested on the most employed electrode, nickel foam, striking differences arise. Specifically, the sample exhibiting the lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA/cm2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based ones during the electrochemical tests.[4]
Last but not least, the covalent functionalization of LDHs has been explored to enhance their processability and stability as an electrocatalyst, for this, we develop for the first time the covalent functionalization of Fe-LDHs beyond silane-based molecules due to the reversible character of this bond in alkaline media. By a modified hydrothermal method from Kuroda et al.,[5] the covalent functionalization of NiFe-LDHs with tris(hydroxymethyl) aminomethane (Tris) was performed. The NiFe@Tris LDH were fully characterized using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis coupled with gas chromatography-mass spectrometry, magnetic measurement, XAS in order to identify the structural differences with a bare-LDH. At the same time, the stability of the covalent functionalization in basic media was proved. Interestingly, the Tris molecule anchored to LDHs increases the electrocatalytic stability in absence of binder material. Thus, after 40 hours of the OER, the overpotential increase is 37% lower in the NiFe@Tris than in the NiFe-Cl@bare. To check the origin of this overpotential decrease, ICP analysis of electrolytes pre-treated with acid after the stability test was performed. A reduction of 27% and 17% of Ni and Fe respectively is observed in the electrolyte for functionalized samples. Consequently, it is demonstrated that, regardless of the decomposition process (detachment or dissolution), the NiFe-LDH functionalized with the tripodal ligand Tris presents greater stability.[6]
Overall, we demonstrated as molecular and cation engineering of LDHs can be used to enhance the electrochemical activity and increase the stability for their use as electrocatalysts for the oxygen evolution reaction.

References

[1] Dionigi, F.; Strasser, P. Advanced Energy Materials, 2016, 6 (23), 1600621.
[2] Yi H.; et al. Advanced Energy Materials, 2021, 11, 2002863.
[3] Carrasco, J.A.; Sanchis-Gual, R.; Seijas-Da Silva, A; Abellán, A.; Coronado, E. Chemestry of Materials, 2019, 31, 6798-6807
[4] Seijas-Da Silva, A; Oestreicher, V.; Coronado, E; Abellán, A. Dalton Transactions, 2022, 51, 4675–4684
[5] Muramatsu, K.; Et al. Inorganic Chemistry, 2020, 59, 6110−6119.
[6] Seijas-Da Silva, A; et al. Manuscript in process.

Figure 1