(Invited) Electrodeposition of Nickel Based Nanostructures from Deep Eutectic Solvent / Water Mixtures As Electrocatalysts for the Oxygen Evolution Reaction
Authors: Mernissi Cherigui El Amine , Łukaczyńska Monika , Mamme Mesfin Haile , Terryn Herman , Bals Sara , Ustarroz Jon 
Published: February 2020

Supported nanostructured materials play a key role in the improvement of energy conversion and storage technologies, such as fuel cells, electrolyzers, batteries and supercapacitors. Nickel-based nanoparticles (NPs) are ideal for a wide range of (electro)catalytical applications, more specifically for the oxygen evolution reaction (OER), where a stable, highly active and cost effective electrocatalyst is needed to overcome the sluggish kinetics.
Electrodeposition permits the growth of the NPs directly on the support of interest, enabling this way an increase in their electroactivity. However, for that purpose, the electrochemical processes occurring during nanoscale electrodeposition need to be understood. In this context, Deep Eutectic Solvents (DESs) have generated great enthusiasm as a new generation of non-aqueous electrolytes [1] for electrochemical deposition. By adding water to the DESs, the electrolyte behavior has been found to be remarkably different [2], opening an interesting line of investigation for nanoscale electrodeposition investigations.
Here, we present our studies on the electrodeposition of nickel nanostructures from several choline-chloride based DESs with different hydrogen bond donors and different amounts of added water. By combining electrochemical techniques, with FE-SEM, XPS, HAADF-STEM, and EDX, the electrochemical processes occurring during nickel deposition and the effect of added water are now better understood. UV-Vis spectroscopy and Molecular Dynamics (MD) help to clarify the intriguing electrochemical behavior that can be seen for very small amounts of water in the DESs [3-4].
Our studies, show that the ability of understanding the effect (and controlling the amount) of water in DES is essential to tune the chemical and morphological nature of the electrodeposited Ni based nanostructures, thereby obtaining highly electroactive NPs [5-6]. At sufficiently negative potentials, Ni growth is halted due to water splitting and the (electro)chemical reduction of the DES components. Under certain conditions, the formation of a mixed layer of Ni/Ni(OH)2(ads) is favored [5-6] and further 3D growth of the Ni containing nanostructures can be halted. Hence, Ni nanostructures are embedded in a 2D crystalline Ni containing network that is formed in the inter-particle region [7]. The presence of a mixed layer (Ni/NiOx(OH)2(1-x)) and a 2D network rich in oxide and hydroxide species is shown to enhance the electrocatalytic activity of the nickel based nanomaterials towards the OER.
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