STM and QCM investigation of refractory metal electrodeposition in ionic liquids: influence of the ionic liquid on the electroreduction processes of the respective refractory metal halides

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In this research project selected air and water stable ionic liquids shall be employed to investigate the electrodeposition of the refractory metals vanadium, niobium and tantalum. Recently we found at the example of aluminium that the cation of ultra pure ionic liquids has a strong effect on metal electrodeposition. We could also show that tantalum can be electrodeposited in 1-Butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide, [BMP]TFSI, in thin up to 1 ¿m thick layers as a crystalline phase. A side reaction, which is not yet understood, is that the electrodeposition of tantalum can be accompanied by remarkable amounts of nonstoichiometric tantalum subhalides. We assume that the cation of the ionic liquid plays ¿ as in the case of aluminium deposition ¿ a crucial role in the electrodeposition of refractory metals. Whereas in 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [EMIm]TFSI, thin non-stoichiometric titanium subhalides are obtained by electroreduction of TiCl4 and TiF4, [BMP]TFSI suppresses deposition to such an extent that it can only be probed by in situ Scanning Tunneling Microscopy (STM) on the nanoscale. Even in mixtures of [BMP]TFSI and [EMIm]TFSI this effect is observed. In this project we aim to investigate with the in situ STM and with the electrochemical quartz crystal microbalance (EQCM) the electrodeposition of vanadium, niobium and tantalum in [BMP]TFSI, [EMIm]TFSI and mixtures of them. With the STM we would like to understand how the ionic liquid influences the initial states of electrodeposition on the nanoscale. EQCM measurements shall reveal information about the rheological and tribological properties of the interface during the electrodeposition processes and the molar masses of the involved species.