Scanning Electrochemical Microscopy with Forced Convection Introduced by High-Precision Stirring

Abstract

In the present report the well-known advantages of hydrodynamic mass transport in electrochemical systems are used in combination with scanning electrochemical microscopy (SECM). The hydrodynamic SECM system integrates a high-precision stirring device into the experimental setup. The well-defined stirring of the SECM electrolyte results in steady state diffusion layer characteristics in the vicinity of large substrate electrodes operated in chronoamperometric measuring mode. For a range of rotation frequencies of a rotating cylinder the thickness and the stability of the diffusion layer was studied by hydrodynamic SECM in the substrate generation/tip collection (SG/TC) as well as in the competition mode using ferrocene methanol as redox mediator. Different UME probe dimensions ranging from Pt diameters of 20 μm down to 0.6 μm were used. The smallest probe with d = 0.6 μm was found most suitable for these studies due to the almost convection-independent amperometric response associated with subμm electrodes. Additionally, preliminary studies of hydrodynamic SECM imaging of a 2 mm Pt disk electrode surface in the SG/TC mode based on in situ produced hydrogen as a mediator are presented. Comparative images measured in the conventional positive feedback mode in quiescent solution show that hydrodynamic SECM offers attractive complementary information.

Impacts of Forced Convection Generated via High Precision Stirring on Scanning Electrochemical Microscopy Experiments in Feedback Mode

“…Within a single image, either a constant stirrer rotation frequency was applied or different rotation frequencies were applied subsequently. The same observation can be made from the top view presented in Figure 2 (c) which showed that, at a height of 10 mm, in agreement with literature [22] a quite small but constant velocity was predicted within the relevant region above the substrate. An UME (3) was used as working electrode, while a platinum wire (4) acted as counter electrode and pseudo reference electrode.…”

“…An electrical high precision stirrer (2250S012BX4 CSD 3830, BL-DC-Motor with integrated motion controller, Dr. Fritz Faulhaber GmbH & Co. KG, Schö naich, Germany) in combination with a laboratory-constructed rotating cylinder (PEEK; diameter d = 3 mm with a beveled edge (458)) was integrated into the SECM setup as previously described [22]. A laboratory-constructed electrochemical cell made out of polytetrafluoroethylene was placed inside a Faraday cage on a damper plate to protect the system from electrical noise and vibrations.…”

“…UMEs with diameters (d probe ) of 25 mm and 12.5 mm were prepared according to literature [24,25] with certain modifications as previously described [22,26]. The quality and size of the UMEs were checked prior to each measuring session.…”

“…In this manner, the effects of the convection on imaging in feedback mode were investigated. It was moved into the convection which means in a direction opposite to the flow of the fluid generated by stirring the solution [22]. Within a single image, either a constant stirrer rotation frequency was applied or different rotation frequencies were applied subsequently.…”

“…Only a few experiments with exact temperature control are described in literature for example by Schuhmann [17], Komatsu [18], Amemiya [19] and coworkers, respectively. As recently shown by the Matysik group [22,23], it is possible to introduce external forced convection to the SECM system by integrating an electrical high-precision stirrer unit into the setup or by a flow of the mediator solution. Therefore, the application of external forced convection in SECM can be a feasible approach to minimize the effects of natural convection.…”