Due to their wide electrochemical potential window, ionic liquids (ILs) permit the electrodeposition of very unnoble materials. Hydrogen evolution as a side reaction does not occur. Further advantages of the ionic liquids are their low vapor pressure, the often low toxicity, and the non-flammability. A disadvantage compared to water is the much higher viscosity, that however strongly decreases already with small temperature increase. Metal ions with higher oxidation states are often reduced stepwise, where stable intermediates may be formed. The fundamentals of electrochemical deposition from ionic liquids are described in the reference works of H. Ohno  and F. Endres et al. . Several reviews showed the large potential of ILs for the deposition of unnoble metals and semiconductors [3-5]. The structure at the phase boundaries electrode / ionic liquid and the elemental processes of deposition are not explored to the same extent and those in aqueous solutions. In that regard, especially scanning probe microscopy techniques have shown large potential for researching these important properties [6-12], and resulted in first details of a rather complex interfacial structure.
Since about ten years, the electrodeposition of refractory metals at moderate temperatures is under study, with an emphasis on titanium and tantalum. Also aluminium and noble metals have been studied.
Tantalum. In 2005, Zein El Abedin et al. studied the electrodeposition of Ta on Pt and Au at different temperatures from a solution of TaF5 in (BMP)Tf2N . While the deposition at room temperature led only to ultrathin layers, temperatures of 200°C permitted the deposition of metallic Ta with a layer thickness of at most 1 µm at low current densities. However, also Ta subhalides were deposited. Later, the deposition was carried out under similar conditions on a Ni-Ti alloy . After removal of a subhalide layer a much improved corrosion resistance of the coated alloy was observed. The avoidance of subhalide layer formation is one of the large challenges in refractory metal deposition . Arnould et al. studied constant current deposition of thin Ta layers on Ti substrates . Borisenko et al. performed fundamental investigations on Ta electrodeposition on gold at room temperature using scanning tunneling microscopy (STM) and EQCM . They could show that for slow deposition rate and in voltammetry at low potentials already at room temperature metallic Ta is formed. For larger currents or in the presence of thicker layers an enrichment of fluoride ions is observed, and subhalide formation prevails. From 1-ethyl-3-methylimidazolium Tf2N, ((EMIm)Tf2N) elemental Ta could not be deposited, and thicker layers prepared in (BMP)Tf2N showed cracks . Opposite to (EMIm)Tf2N, Ta deposition worked from (PMIm)Tf2N, but only at elevated temperatures in a narrow potential range, and not at constant current . It was not possible to prepare well-adherent, crack free layers. The application of pulsing led to an improvement, though. In most cases there was still ionic liquid embedded in the layers. A systematic study of the influence of pulse parameters was carried out for the deposition of Ta on Ti in (BMP)Tf2N at room temperature . Conditions for the deposition of homogeneous layers could be identified that however still were not crack-free. Maho et al. deposited Ta on the shape memory alloy Nitinol from (BMP)Tf2N + TaF5 + LiF at room temperature with a current density of -100 μA/cm2 [20, 21]. This and well adherent but porous layers were obtained. Endres and Bund recently summarized some results on the fundamentals of Ta deposition from ionic liquids  and demonstrate the large influence of solvate layers.
Titanium. First attempts to deposition Ti from other media than salt melts were carried out in 1993 by Abbott et al. using aromatic solvents. Ag or Cu were added in order to facilitate nucleation. The formed layers showed dendritic or granular morphologies . Endres et al. studied the deposition from the halides (TiCl4, TiF4, TiI4) in (EMIm)Tf2N, (BMP)Tf2N and trihexyltetradecyl-phosphonium Tf2N ((P14,6,6,6)Tf2N) . Subhalides were deposited instead of metal. Even the use of lithium as a reductant was not successful. Xiao-Ying Zhang et al. studied the deposition of Ti from TiO2 in [BMIm]Cl/AlCl3 at 70°C . By direct electrochemical reduction metallic Ti was obtained, but only identified by ex-situ XPS measurements.
For Al deposition usually the system AlCl3-EMImCl is used. However, deposition is also possible from many other ILs. A general issue is the moisture sensitivity of the ILs, especially the chloride based ones. Many attempts have been undertaken to employ less moisture and air sensitive systems, for instance by replacing the chloride by Tf2N anions. However, the also very hygroscopic AlCl3 still had to be used .
- H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Inc., Hoboken, New Jersey, 2011.
- F. Endres, D. MacFarlane, A. Abbott (Eds.), Electrodeposition from Ionic Liquids, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008.
- A.P. Abbott, I. Dalrymple, F. Endres, D.R. Macfarlane, Why use Ionic Liquids for Electrodeposition?, in: Electrodeposition from Ionic Liquids, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 1-13.
- S. Zein El Abedin, F. Endres, Electrodeposition of Metals and Semiconductors in Air- and Water-Stable Ionic Liquids, Chemphyschem : a European journal of chemical physics and physical chemistry, 7 (2006) 58-61.
- F. Endres, Ionic Liquids: Solvents for the Electrodeposition of Metals and Semiconductors, Chemphyschem : a European journal of chemical physics and physical chemistry, 3 (2002) 144 - 154.
- B. Uhl, T. Cremer, M. Roos, F. Maier, H.-P. Steinruck, R.J. Behm, At the ionic liquid|metal interface: structure formation and temperature dependent behavior of an ionic liquid adlayer on Au(111), Physical Chemistry Chemical Physics, 15 (2013) 17295-17302.
- A. Elbourne, S. McDonald, K. Voïchovsky, F. Endres, G.G. Warr, R. Atkin, Nanostructure of the Ionic Liquid–Graphite Stern Layer, ACS Nano, 9 (2015) 7608–7620.
- T. Carstens, A. Ispas, N. Borisenko, R. Atkin, A. Bund, F. Endres, In situ STM, AFM and EQCM studies of the electrochemical deposition of tantalum in two different ionic liquids with the 1-butyl-1-methylpyrrolidinium cation, Electrochimica Acta, 197 (2016) 374-387.
- N. Borisenko, R. Atkin, A. Lahiri, S.Z.E. Abedin, F. Endres, Effect of dissolved LiCl on the ionic liquid–Au(111) interface: an in situ STM study, Journal of Physics: Condensed Matter, 26 (2014) 284111.
- R. Atkin, N. Borisenko, M. Druschler, F. Endres, R. Hayes, B. Huber, B. Roling, Structure and dynamics of the interfacial layer between ionic liquids and electrode materials, J. Mol. Liq., 192 (2014) 44-54.
- F. Endres, N. Borisenko, S.Z. El Abedin, R. Hayes, R. Atkin, The interface ionic liquid(s)/electrode(s): In situ STM and AFM measurements, Faraday Discussions, 154 (2012) 221-233.
- R. Wen, B. Rahn, O.M. Magnussen, Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale In Situ Video-STM Study, Angewandte Chemie International Edition, 54 (2015) 6062-6066.
- S. Zein El Abedin, H.K. Farag, E.M. Moustafa, U. Welz-Biermann, F. Endres, Electroreduction of tantalum fluoride in a room temperature ionic liquid at variable temperatures, Physical Chemistry Chemical Physics, 7 (2005) 2333-2339.
- S. Zein El Abedin, U. Welz-Biermann, F. Endres, A study on the electrodeposition of tantalum on NiTi alloy in an ionic liquid and corrosion behaviour of the coated alloy, Electrochemistry Communications, 7 (2005) 941-946.
- C. Arnould, J. Delhalle, Z. Mekhalif, Multifunctional hybrid coating on titanium towards hydroxyapatite growth: Electrodeposition of tantalum and its molecular functionalization with organophosphonic acids films, Electrochimica Acta, 53 (2008) 5632-5638
- N. Borisenko, A. Ispas, E. Zschippang, Q. Liu, S. Zein El Abedin, A. Bund, F. Endres, In situ STM and EQCM studies of tantalum electrodeposition from TaF5 in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, Electrochimica Acta, 54 (2009) 1519-1528.
- A. Ispas, A. Bund, F. Endres, Application of the Electrochemical Quartz Crystal Microbalance for the Investigation of Metal Depositions from Ionic Liquids, ECS Transactions, 16 (2009) 411-420.
- A. Ispas, B. Adolphi, A. Bund, F. Endres, On the electrodeposition of tantalum from three different ionic liquids with the bis(trifluoromethyl sulfonyl) amide anion, Physical Chemistry Chemical Physics, 12 (2010) 1793-1803.
- A. Ispas, A. Bund, Pulse plating of tantalum from 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide ionic liquids, Transactions of the Institute of Metal Finishing, 90 (2012) 298-304.
- A. Maho, J. Delhalle, Z. Mekhalif, Study of the formation process and the characteristics of tantalum layers electrodeposited on Nitinol plates in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid, Electrochimica Acta, 89 (2013) 346-358.
- A. Maho, F. Kanoufi, C. Combellas, J. Delhalle, Z. Mekhalif, Electrochemical Investigation of Nitinol/Tantalum Hybrid Surfaces Modified by Alkylphosphonic Self-Assembled Monolayers, Electrochimica Acta, 116 (2014) 78-88.
- A.P. Abbott, A. Bettley, D.J. Schiffrin, Titanium electrodeposition from aromatic solvents, Journal of Electroanalytical Chemistry, 347 (1993) 153-164.
- F. Endres, S. Zein El Abedin, A.Y. Saad, E.M. Moustafa, N. Borissenko, W.E. Price, G.G. Wallace, D.R. MacFarlane, P.J. Newman, A. Bund, On the electrodeposition of titanium in ionic liquids, Physical Chemistry Chemical Physics, 10 (2008) 2189-2199.
- X.-Y. Zhang, Y.-X. Hua, C.-Y. Xu, Q.-B. Zhang, X.-B. Cong, N. Xu, Direct electrochemical reduction of titanium dioxide in Lewis basic AlCl3–1-butyl-3-methylimidizolium ionic liquid, Electrochimica Acta, 56 (2011) 8530-8533.
- S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Electrodeposition of Nano- and Microcrystalline Aluminium in Three Different Air and Water Stable Ionic Liquids, Chemphyschem : a European journal of chemical physics and physical chemistry, 7 (2006) 1535-1543.