Electrode Stimulation: Heterogeneous Electron Transfer Reactions Induced By Modulation of Electrostatic Potentials in the Electrolyte

Introduction The rates of electrode reactions are frequently expressed in terms of the surface overpotential, ηs, defined as “the potential of the working electrode relative to a reference electrode of the same kind placed in the solution adjacent to the surface of the working electrode.”1 Changes in ηs are most commonly effected by means of a potentiostat, a device that allows accurate control of the potential of the working electrode with respect to a reference electrode in solution.2 This communication describes a method that makes it possible to promote heterogeneous electron transfer reactions at a working electrode, WE, polarized at a fixed potential, EWE, by changing the electrostatic potential in the electrolyte “just outside its double layer”, ϕdl, and thus ηs. Experimental The body of the electrochemical cell used in our experiments was a polypropylene centrifuge tube with its bottom end cut off to expose a circular hole ca. 5 mm in diameter. A commercial Au disk microelectrode (12.5 µm diameter), Au µ-el, inserted into the hole of the cell facing upwards, was used as WE, and a carbon rod and a Ag/AgCl as counter and reference electrodes, respectively. The stimulating electrode, SE, was a Pt disk (1.6 mm diameter) placed directly above the WE. The potentials of the WE and SE were controlled independently using a Metrohm Autolab potentiostat. The cell was filled with 0.1 M HClO4 and then purged with Ar prior to the measurements. EWE was fixed at a prescribed value, while the potential of the SE, ESE, was scanned at a rate ν = 50 V/s, starting at 1.2 V, down to -0.25 V and back to 1.2 V for all experiments. Results The cyclic voltammogram of the Au µ-el, WE, recorded at a scan rate ν = 50 V/s in deaerated 0.1 M HClO4 (see insert, Fig. 1), displayed characteristic features ascribed to the formation of Au oxide and its subsequent reduction. ESE was set at 1.2 V for ca. 1 s, and then scanned at ν = 50 V/s down to -0.25 V, and back again to 1.2 V, yielding the “unfolded” voltammogram in the upper panel, Fig. 1. The current response of the Au working µ-el, iWE, during this scan, when fixing EWE= 1.15 V, is shown in green in the lower panel, Fig. 1. As clearly indicated, for ESE < 0.5 V during the scan in the negative direction, both |iSE| and |iWE| markedly increased, albeit in opposite directions, reaching a maximum at ca. 22 ms. Whereas |iSE| decreased by ca. 30% toward the end of the scan in negative direction, |iWE| dropped to a very small value similar to that found prior to the stimulation. These results are consistent with the formation of Au oxide stimulated by a shift in ηs toward positive values induced by iSE. In fact, the sudden drop in iWE signals the end of Au oxide formation at the maximum applied ηs. During the entire scan toward positive potentials, iWE was very small, indicating that the value of ηs achieved was insufficient to effect the reduction of the Au oxide formed. This is in agreement with the cyclic voltammogram of Au, for which the onset of Au oxide reduction is ca. 0.9 V (see insert, Fig. 1). In striking contrast, very small iWE was observed in similar experiments in which EWE = 0.4 V (see black, Fig. 1) regardless of the direction of the scan. We ascribe this behavior to changes in the structure of the interface, which may involve a redistribution of charge in the diffuse double layer, as well as a possible reorientation of water dipoles at the WE surface. Strong evidence that the response of the WE is indeed derived from changes in ϕdl was provided by experiments in which the distance between WE and SE, δ, was varied. As shown in Fig. 2, increasing δ led to a decrease in ηs, thus, in a corresponding decrease in the magnitude of the peak current associated with Au oxide formation. This behavior, as well as the lack of proportionality between the peak current and δ, are both in agreement with theoretical predictions. Acknowledgments This work was supported by a grant from NSF, CHE-1412060. References 1. Newman, J. S.; Thomas-Alyea, K. E., Electrochemical systems. Third ed.; J. Wiley: Hoboken, N.J, 2004. 2. Bard, A. J.; Faulkner, L. R., Electrochemical methods: fundamentals and applications. 2nd ed.; Wiley: New York, 2001. Figure 1