Structural Dynamics of the Electrical Double Layer during Capacitive Charging/Discharging Processes

Transitional structures of Cs+ at the outer Helmholtz plane (OHP) have been determined using time-resolved X-ray diffraction during the double-layer charging/discharging on the Ag(100) electrode in CsBr solution. At the double-layer potential region at which c(2 X 2)-Br is formed on Ag(100), the transient current comprises two exponential terms with different time scales: a rapid and a slow one are due to the dielectric polarization of water molecules and the transfer of Cs+, respectively. The slow term is composed of different dynamic processes of Cs+ during charging and discharging. When the potential is stepped in the positive direction, the coverage of Cs+ at the OHP decreases. In this step, the transient X-ray intensity at the (0 0 1) reflection, which is sensitive to the OHP structure, shows that Cs+ is released from the OHP according to exponential function of time. The decay of transient intensity of X-ray has a time scale similar to that of the current transient measurement. On the other hand, the accumulation process of Cs+ from the diffuse double layer to the OHP comprises two different kinetic processes after a potential step in the negative direction: a rapid one is the accumulation of Cs+ near the outer layer, and a slow one is the structural stabilization of the Cs+ layer.