Influence of Electrode Roughness on Double Layer Formation in Ionic Liquids

Abstract

We have used broadband electrochemical impedance spectroscopy for characterizing double layer formation at the interface between the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide [Py1,4]TFSI and three gold electrodes with different surface structure and roughness. Two alternative approaches for analyzing the data were compared: a) A fit in the complex impedance plane using a resistor and a constant phase element (CPE) connected in series; b) A fit in the complex capacitance plane using a Cole–Cole function. In the complex capacitance plane, a high-frequency semicircle due to double layer formation could be clearly distinguished from other capacitive or Faradaic processes detected at lower frequencies. The Cole–Cole fit of the high-frequency semicircle revealed that this semicircle is almost unsuppressed with α values close to unity, even for the rough polycrystalline Au electrode. In contrast, the CPE exponent depends much more strongly on electrode potential and electrode roughness. We show that this strong dependence is closely related to the existence of slower capacitive or Faradaic processes, and is not caused by nonideal double layer formation.