Importance of the double layer structure in the electrochemical deposition of Co from soluble Co2+ - based precursors in Ionic Liquid media

Electrodeposition in Ionic Liquid (IL) media is being widely used to obtain different metals, alloys and more recently binary compounds. However, the understanding of this process is still slim due to poor knowledge in the chemistry of ILs and also the complex structure of the double layer at the electrode surface. It is then difficult to predict deposition conditions when trying to synthesize a desired deposit. In this work, we provide insight into the processes taking place at the electrode by detailed studies of Co2+ reduction in ionic liquid media at elevated temperature, which have revealed an unusual Cyclic Voltammetry profile. A drastic drop in the reaction rate was observed, together with a well-shaped reduction peak during the oxidation sweep. Using Electrochemical Quartz Crystal Microbalance, Impedance Spectroscopy measurements and varying bath parameters, we ascribed those phenomena to the reconstruction of the double layer structure upon imposed polarization. Similar behaviors were observed for other systems (Mn2+, Ni2+, Zn2+). Finally, this work provides new insight into the electrochemical processes in ILs. We anticipate that it will also result in significant improvement in preparing metal and other deposits in the ionic liquid media.