Probing the dynamics of non-faradaic processes in Ionic liquids at extended time and length scales using XPS with AC modulation

Charging dynamics of ionic liquid (IL) electrolytes play important roles in various aspects of electrochemical processes. However, the precise understanding of such processes at extended time and length scales is incomplete due to the experimental difficulties in probing the electrochemical potential and other relevant parameters. In principle, such shortcomings should not apply to theoretical/computational approaches; however, existing works have mostly concentrated on or around electrode/electrolyte interfaces and short timescales due mostly to prohibitive demands on computational efforts. To fill this gap, we have utilized X-ray photoelectron spectroscopy to study the charging dynamics of ILs in contact with two wire electrodes under AC square wave excitation, with frequencies ranging from hundreds of kHz down to the mHz region. Using the changes in the binding energy position of the IL-related core-level peaks, electrical potential profiles along the lines in between the electrodes and on the entire surface of the electrolyte have been investigated in situ. From these results, we identify two widely different time constants. The timescale of the fast process was shown to be on the order of RC time constant, while the slow process takes place on a timescale of seconds. Our method in the present study is expected to open up a new way for extracting novel dynamic information for gaining a better understanding of such processes and designing efficient IL-based electrochemical devices with a novel perspective on the charging.