Rational Design of Fe-Based Layered Double Hydroxide for Water Oxidation: Molecular and Cationic Engineering

Abstract

Nowadays, 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, many approaches have been reported.[1] Beyond these, 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 exfoliated NiFe nanosheets and shows better stability as a consequence of the tridimensional robustness of the hybrid material.[2]On the other hand, to shed light on the influence of Fe-clustering on the OER performance, firstly, we have selected MgFe-based LDHs phases as model systems. 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, striking differences arise when the samples are tested on nickel foam. 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. [3] To extend our study to NiFe-LDH, we have successfully synthesized high-clustering nanometric NiFe-LDHs using a scalable synthesis approach. This synthetic protocol has allowed us to obtain a significantly higher concentration of approximately 100 g/L of our materials, which is almost two orders of magnitude larger than current state-of-the-art synthetic methods. Importantly, the Fe-clustering of this scale-up NiFe-LDH results in improved electrochemical performance compared to traditionally synthesized NiFe-LDH/FeOOH across various current density ranges and different electrochemical setups, including rotatory disk electrode (RDE) and Anion Exchange Membrane Water Electrolyzer (AEMWE), with testing conducted up to a current density of 3 A·cm-2. For instance, the overpotential at 2.5 A·cm-2 for the scale-up NiFe-LDHs is measured at 1.96 V, whereas the traditionally synthesized NiFe-LDH/FeOOH exhibit a higher overpotential of 2.18 V. [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 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 presents greater stability.[6]Overall, we demonstrated that the rational design of LDHs can enhance catalytic activity and stability for their use as electrocatalysts for the oxygen evolution reaction.