Despite years of research questions still remain regarding the underlying physicochemical bases of charge storage in electrochemical capacitors. Much of the controversy centers around the nature of the processes responsible for this phenomenon. Work in our laboratories has focused on dynamic aspects of charge injection into IrOx(hyd) in strongly acid aqueous media, as monitored simultaneously by chronocoulometric and time-resolved differential reflectance techniques. We have taken full advantage of the electrochromic properties of these films which makes it possible to monitor changes in the oxidation state of the Ir sites in the films and thus resolve the electrochemical response in terms of its purely capacitive and pseudocapacitive components.

Shown in the figure below are plots of ΔR/R vs time (left ordinate) and the integrated charge, Q, based on the integration of the current (right ordinate), following potential steps between (E_ini → E_fin) (left panels) and (E_fin → E_ini) (right panels) for E_ini = 0.20 V and E_fin = 0.67 (upper panels) and E_fin = 0.60 V (lower panels). The ordinates in the two curves in each panel were adjusted so as to match the initial and final values of ΔR/R and Q. It is important to stress is that the measured time constants for both the charge and optical measurements are on the order of a few tenths of seconds, and thus much longer than the RC time constant estimated based on the electrode dimensions and conductivity of the electrolyte (see above), which would yield values on the order a fraction of a ms.

Careful inspection of these curves (as well as those involving all other values of E_fin examined, revealed two important features:

1. the oxidation process is faster than the subsequent reduction, and, most importantly,
2. the rate at which total charge injection and release occurs, as judged by the electrochemical data, appears faster than that at which the Ir sites undergo redox transitions, as evidenced from the optical data. This phenomenon strongly suggests that charge can be stored in the film involving both non-redox (fast) and redox (slow) sites.