Origin of Temperature-Dependent Overpotential of Co Electrodeposition in an Ionic Liquid

Electrodeposition in ionic liquids is a promising process for surface functionalization as well as material recycling with a high current efficiency. For metal electrodeposition in ionic liquids, overpotentials, which were problematically high at room temperature (RT), significantly decreased with elevating temperatures. To reveal the microscopic origin of such a remarkable temperature dependence, we investigated the interfacial structures of 1-propyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide [C(3)mpyr][TFSA] under Co electrodeposition reaction on an Au electrode by using surface-enhanced infrared absorption spectroscopy (SEIRAS) at various temperatures. In situ interfacial observation indicated that the onset potentials of anion-to-cation replacement in the first ionic layer showed a more positive onset at higher temperatures. Even with such a modification, the onsets of the first ionic layer replacement still agree with those of Co electrodeposition in the entire temperature range between RT and 370 K. This agreement validates that the interfacial structure dependence of the electrodeposition reactivity is the origin of the high electrodeposition overpotential, as previously discussed at RT.