Electron-Transfer Kinetics and Electric Double Layer Effects in Nanometer-Wide Thin-Layer Cells

Abstract

Redox cycling in nanometer-wide thin-layer cells holds great promise in ultrasensitive voltammetric detection and in probing fast heterogeneous electron-transfer kinetics. Quantitative understanding of the influence of the nanometer gap distance on the redox processes in the thin-layer cells is of crucial importance for reliable data analysis. We present theoretical consideration on the voltammetric behaviors associated with redox cycling of electroactive molecules between two electrodes separated by nanometer widths. Emphasis is placed on the weakness of the commonly used Butler-Volmer theory and the classic Marcus-Hush theory in describing the electrochemical heterogeneous electron-transfer kinetics at potentials significantly removed from the formal potential of redox moieties and, in addition, the effect of the electric-double-layer on the electron-transfer kinetics and mass transport dynamics of charged redox species. The steady-state voltammetric responses, obtained by using the Butler-Volmer and Marcus-Hush models and that predicted by the more realistic electron-transfer kinetics formulism, which is based on the alignments of the density of states between the electrode continuum and the Gaussian distribution of redox agents, and by inclusion of the electric-double-layer effect, are compared through systematic finite element simulations. The effect of the gap width between the electrodes, the standard rate constant and reorganization energy for the electron-transfer reactions, and the charges of the redox moieties are considered. On the basis of the simulation results, the reliability of the conventional voltammetric analysis based on the Butler-Volmer kinetic model and diffusion transport equations is discussed for nanometer-wide thin-layer cells.

Recent Advances in Voltammetry

“…Again, this phenomenon is predicted to be more apparent for small values of the reorganisation energy. Given that this is in general associated with fast kinetics (i.e., large k 0 values), special attention has been paid to the use of nanosize (including nanodiscs and impacting nanoparticles), nanogap, and channel electrodes such that the enhanced mass transport shifts the kinetic‐controlled voltammetric response away from . Thus, it is theoretically possible to observe kinetically‐limited steady‐state currents at large overpotentials in the above systems when the size of the electrode or the gap distance is reduced to the nanometer scale, though in practice this requires that the geometry of the electrode is accurately known and, in the case of electrodes of a few nanometers, to deal with double layer and nonclassical effects.…”

“…for electrode processes with k 0 ≤0.02 cm s −1 by using microelectrodes of 25–50 μm radius, which also allows for the reduction of undesirable ohmic drop and capacitive effects. The use of nanosize or nanogap electrodes would be necessary for faster electron transfers, which presents difficulties in terms of electrode fabrication and characterisation as well as modeling of nonconventional effects.…”

Double layer effects in voltammetric measurements with scanning electrochemical microscopy (SECM)

“…Consequently, surface charge effects can be seen at greater distances than the Debye length. The EDL is increasingly recognised to have significant impact on nanoscale electrochemical systems, for which the characteristic diffusion layer size, for a redox reaction involving solutes, approaches that of the EDL [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The purpose of this paper is to consider the impact of surface charge on scanning electrochemical microscopy (SECM) measurements when configured to create a dual-working electrode thin layer electrochemical cell for kinetic measurements.…”

Impact of Adsorption on Scanning Electrochemical Microscopy Voltammetry and Implications for Nanogap Measurements

“…This has fueled the trend of miniaturizing electrochemical systems, leading to the development of nanoelectrodes [20][21][22][23][24][25][26] and various nanogap systems. [27][28][29][30][31][32][33] When using nanoscale electrochemical systems for quantitative kinetic measurements, precise knowledge of electrode geometry and the physicochemical characteristics of 4 electrochemical cells is imperative. For example, unaccounted for irregularities in the electrode shape from idealized models, 34 tip recession [35][36][37] or 'lagooned' geometries 38,39 may produce highly erroneous determination (overestimation) of ET kinetic parameters.…”