Elecrochemical Generation and Coulometric Methods

Elecrochemical generation is important for introducing precise known amounts of chemicals to a solution through coulometric methods. These coulometric methods are ideal for adding small amounts of desired chemicals in a nanosystem, but unwanted products are generated at the counter electrode because the systems are small and cannot use cell separators that other systems use. The authors propose to solve this problem with a capacitive electrode.

They also examine a different use for the capacitive counter electrode where the electrode would be continuously renewed and issue of charging with electrode emersion would be addressed. To do this, a single faradaic electrode cell was set-up. They took Pt wire sealed in glass and used it as the faradaic electrode. Deionized (DI) water was placed on the SiO2 film on a Si wafer that had an insulating film of Sio2 to serve as the capacitive electrode. The tube was overfilled with water in order to get a water drop at the top to make contact with the SiO2 surface. The tip was attached to a translation stage for versatility in movement. The drop moved horizontally to make contact with the surface generated by moving the tip laterally. Pt wire was also used in the ECL experiments. The Pt wire was coated and polished with epoxy cement to expose area of it towards the direction of the photomultiplier tube and was bent at a right angle. Si/SiO2 in an aqueous solution with buffer in it served as the counter electrode. An autolab potentiostat was used as a working electrode and The Si back contact served as a counter/reference electrode. Potential pulses were applied to the Pt/solution/SiO2/Si and ECL emission and current was recorded at the same time. In their results, the current as a function of time decreased. Their figure illustrated that the charges brought to the SiO2 interface changed slightly at the open circuit. Current was steady when the tip was not moving. The current increased when the tip was moved laterally, but reached a steady state provided by the action of having a continuous contact with fresh surface. The steady-state current depended on how fast the surface was contacted. When the charging current increased as the water drop moved, the steady-state current was produced and decreased when the drop was not moving.

The authors confirmed that their findings of charge restoration based on the charge at the spots the water drop had already been were in agreement with the results of previous studies. ECL was detected immediately after the potential pulse was applied. ECL emission positively correlated with decreasing charging current, meaning that it also decreased as the current decreased. It was successfully demonstrated through the production of ECL that faradaic process in a single electrode electrochemical system is possible and that ECL can be generated without interference from counter electrode reactions. The authors included diagrams to illustrate the set-up and use of their method in which allowed for easy reference as to what they were doing. The theory behind the logic of their method was mathematically illustrated through formulas in addition to providing a section for additional considerations pertaining to the study. These characteristics made the paper overall effective and fairly simple to read.

Liu, C.; Bard, A. J. Anal. Chem. 2005, 77, 5339-5343.