Scanning electrochemical microscopy. 60. Quantitative calibration of the SECM substrate generation/tip collection mode and its use for the study of the oxygen reduction mechanism.
The substrate generation/tip collection (SG/TC) mode of scanning electrochemical microscopy (SECM) coupled with linear voltammetry is proposed as a way to quantify reaction intermediates generated in the solution at small substrates (100 mum diameter). The collection efficiency (CE) for SG/TC mode depends on the collector tip radius (a), the tip/substrate distance (d), and the size of the insulating glass sheath surrounding the collector tip (RG). In this work, we present experimental and simulated calibration CE values for different SG/TC geometries. Results of digital simulations in axial 2-D symmetry with the tip approaching a planar substrate are shown and fit experimental results obtained using ferrocenemethanol as a redox mediator very well. This model assumes that the mediator reacts under stationary-state conditions and undergoes diffusion-controlled electron transfer without any heterogeneous or homogeneous kinetic complications. Empirical equations for all SG/TC geometries reported here are provided as a convenient way to predict the maximum CE value for any given distance within the calibration range. Hydrogen peroxide quantification during the oxygen reduction reaction (ORR) at a Hg on Au electrode in acid pH was carried out using the SG/TC mode of SECM to demonstrate the utility of this technique in determining the number of electrons transferred (n) in the ORR. The results (n = 2.12-2.19) clearly point out the predominance of the two-electron pathway over the four-electron pathway when ORR takes place at this electrode material. Therefore, this work presents a powerful alternative to the rotating ring-disk electrode (RRDE) as means of obtaining mechanistic information by calculating the number of electrons transferred during an electrochemical reaction.
Sánchez-Sánchez
Carlos M
CM
Center for Electrochemistry, Department of Chemistry and Biochemistry, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, USA.
Rodríguez-López
Joaquín
J
Bard
Allen J
AJ
D002138:Calibration; D003198:Computer Simulation; D004058:Diffusion; D004563:Electrochemistry / Q000379:methods*; D004566:Electrodes; D005296:Ferrous Compounds / Q000737:chemistry; D006046:Gold / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry; D007700:Kinetics; D008855:Microscopy, Electron, Scanning / Q000379:methods*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010984:Platinum / Q000737:chemistry
D005296:Ferrous Compounds; C071550:ferrocenemethanol; D010984:Platinum; D006046:Gold; D006861:Hydrogen Peroxide; D010100:Oxygen