Room temperature ionic liquids possess negligible vapor pressure that allows us to put the liquids in a vacuum chamber without any vaporization of the liquid. This unique property enables to conduct electrochemical reactions in vacuum condition. Fortunately it has been found that observation of ionic liquid by a scanning electron microscope (SEM) is possible without any charging of the liquid. Then, the first idea for developing this in situ observation was just implementation of electrochemical reactions in a vacuum chamber of a SEM. However, several improvements including development of specifically-designed cells were required to observe electrochemically induced changes in electrode surfaces.
We examined Saito's equation for diffusion-controlled current at a microdisk electrode. We measured steady-state currents of a ferrocenyl derivative at glass-coated Pt electrodes with various sizes, of which radii were evaluated with a scanning electron microscope. The relation between the currents and the radii less than 2 μm deviated downward from the proportionality, whereas the proportionality was maintained accurately for the radii more than 5 μm. Radii of invisible electrodes which have been so far evaluated from the steady-state currents should be smaller than the geometrical values. The lower deviation of the proportionality was explained in terms of memory diffusion of the Fick's first law under the steady state.