Tan, Sze-yin, Perry, David and Unwin, Patrick R. (2018) Double layer effects in voltammetric measurements with scanning electrochemical microscopy (SECM). Journal of Electroanalytical Chemistry, 819 . pp. 240-250. doi:10.1016/j.jelechem.2017.10.044

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Abstract

Scanning electrochemical microscopy (SECM) operating as a variable gap ultra-thin layer twin-working electrode cell, has long been recognised as a powerful technique for investigating fast kinetics (heterogeneous electron transfer and homogeneous reactions coupled to electron transfer) as a consequence of high mass transport rates between the working electrodes when biased to promote redox shuttling. Recently, SECM has advanced technically and nanogap cells with dimensions on the 10s of nm scale have been reported. In this paper, we consider double layer effects on voltammetric measurements in this configuration, outlining a comprehensive model that solves the Nernst-Planck equation and Poisson equation with charged interfaces. For supporting electrolyte concentrations that have been used for such measurements (50 mM and 100 mM), it is shown that for typical electrode charges and charge on the glass insulator that encases the ultramicroelectrode (UME) tip used in SECM, there are profound effects on the voltammetric wave-shapes for redox reactions of charged redox couples, in the common modes used to study electron transfer kinetics, namely the tip-voltammetry (feedback) mode and substrate-voltammetry (substrate-generation/tip-collection and competition) modes. Using the reduction and oxidation of a singly charged redox species to a neutral and doubly charged species, respectively, as exemplar systems, it is shown that the charge on the electrodes can greatly distort the voltammetric wave-shape, while charge on the glass that surrounds the UME tip can affect the limiting current. Analysis of SECM voltammograms using methods that do not account for double layer effects will thus result in significant error in the kinetic values derived and tip-substrate distances that have to be estimated from limiting currents in SECM. The model herein provides a framework that could be developed for further studies with nanogap-SECM (e.g. consideration of alternative models for the electrical double layer, other supporting electrolyte concentrations, potential of zero charge on the electrodes and charges on the redox couples). The model results presented are shown to qualitatively match to SECM voltammetric features from experimental data in the literature, and are further supported by experimental data for redox processes of tetrathiafulvalene (TTF), namely the TTF/TTF+ and TTF+/TTF2+ redox couples. This serves to demonstrate the immediate practical application of some of the ideas presented herein. For future applications of SECM, the use of different supporting electrolyte concentrations and a range of tip-substrate separations may allow the determination of both electron transfer kinetics and double layer properties.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science > Chemistry
Library of Congress Subject Headings (LCSH):	Scanning electrochemical microscopy, Charge exchange, Mass transfer, Electrolytes
Journal or Publication Title:	Journal of Electroanalytical Chemistry
Publisher:	Elsevier Science SA
ISSN:	1572-6657
Official Date:	15 June 2018
Dates:	Date Event 15 June 2018 Published 24 October 2017 Available 19 October 2017 Accepted
Volume:	819
Page Range:	pp. 240-250
DOI:	10.1016/j.jelechem.2017.10.044
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
RIOXX Funder/Project Grant:	Project/Grant ID RIOXX Funder Name Funder ID UNSPECIFIED University of Warwick http://dx.doi.org/10.13039/501100000741 UNSPECIFIED Leverhulme Trust http://dx.doi.org/10.13039/501100000275 Wolfson Research Merit Award Royal Society http://dx.doi.org/10.13039/501100000288

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