Niobium Electroreduction from Ionic Liquids Using Fluoride Based Precursors
Authors: Yesilbas Göktug , Seidl Lukas , Martens Sladjana , Schneider Oliver 
Published: February 2020

Niobium belongs to the group of refractory metals, and shows advanced properties such as a high melting point and a good hardness, due to the strong interatomic bonds [1,2]. Furthermore, niobium exhibits excellent chemical and corrosion resistance [3]. This makes niobium coatings very attractive for a broad spectrum of applications [4].
Due to the low standard potential, it is not possible to electrodeposit niobium from aqueous solutions. Ionic liquids (ILs) are a good alternative, and first studies have demonstrated the general feasibility of Nb metal deposition from NbF5 containing ILs [5-7]. However, layers deposited from standard ionic liquids like 1-butyl-1-methylpyrrolidinium bistrifluoromethylsulfonyl imide (BMP TFSI) often still are not dense, show cracks, have residual ionic liquids incorporated, and contain Nb subhalides [5]. The use of different precursors like NbCl5 and other ionic liquids, particularly those based on a triflate anion, has been reported [8-10]. In our own research, we recently studied in detail the electrochemical reduction of NbCl5 in TFSI-based ionic liquids [11].
In this work, the electrochemical behavior of NbF5 in different ionic liquids was studied with the electrochemical quartz crystal microbalance technique (EQCM). A network analyzer was used to measure the admittance between the two Au electrodes (one serving as working electrode) of 10 MHz quartz resonators in parallel to the electrochemical measurements. This permitted to extract both changes in resonance frequency and damping of the quartz. The latter is particularly important in ionic liquids, as local changes in the electrolyte composition due to a reduction process are often associated with damping changes. Experiments were carried out at different temperatures, using pulsed electrodeposition and pulsed ultrasound techniques. The application of ultrasound can significantly enhance mass transport and thereby prevent precursor depletion and fluoride enrichment at the surface. On the downside, there is a risk of accelerated electrolyte degradation [11, 12]. Figure 1a shows an example for a successful deposition from a 0.25 M NbF5 solution in an alkylammonium IL electrolyte. A reasonably well adhering layer was obtained even at room temperature that however still contained some fluoride ions at the surface, and was very rough. According to the XPS measurements in Figure 1b and c, an average composition of NbF0.6 was obtained at the surface.

Figure 1. a) Photograph of layer deposited from 0.25 M NbF5 b) XPS spectrum (F1s) c) XPS spectrum (Nb 3p5/2,3/2)

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Figure 1