Commun car The Response of an Electrochromic Display with Viologens on a Potential Step

The response of an electrochemical process on a potentiostatic voltage pulse is known from literature, e.g., (1). There the current was expected to be pro-portional to t-1/2, a diffusion-limited process. How-ever, in a recent paper (5) it was concluded that con-vection predominates this diffusion l imitation already after 10O msec. Since it could be expected that the dis-play resistance might also play some role, the current-time response of an electrochromic device on a voltage step was investigated. The electrochromic material we used was an aque-ous solution of 10-2M diheptyl viologen dibromide (ABr2) and 0.3M potassium bromide. The reduction reaction is schematically given by A 2+ + e ~--- A +', followed by the chemical reaction A + " + Br- ~ ABr$ where a violet colored layer is deposited (2, 3). The electrochemical cell used had a vertically positioned transparent SnO2 electrode (10 ~/[:], 0.4 cm 2) as a working electrode, a standard Ag/AgC1 electrode with a Luggin capillary as a reference electrode, and a large Pt foil separated from the cell solution by a glass frit as a counterelectrode. This three electrode system was driven by a potentiostat (Wenking St 72), and the voltage pulse was delivered by a single shot pulse generator. The r sponse of the current o a voltage step was recorded with a storage oscilloscope. The current-t ime r sponse measured with several po-tential steps is shown in Fig. 1. The observed current-time transient can be divided into three regions. First is the time region directly after t _-- 0. In this region a peak current is observed which is mainly due to double layer charging. This time interval, which varies from 10 to 40 msec depending on the overpotential, wa