(Invited) Voltammetric Examination of Carbon Nanoelectrodes Under Mixed Diffusion - Migration Mass Transport Conditions
Authors: Hyk Wojciech 
Published: September 2023

In general, experiments carried out with nanoelectrodes are similar to those with microelectrodes. However, the extremely increased efficiency of the mass transport to nanoelectrodes results in steady-state current conditions in ultra-short times and high current densities. The transport of electroactive species to the working electrode surface is mainly realized by diffusion. The contribution of migration to the total mass transport is usually eliminated by shortening the thickness of the electrical double layer (EDL) achieved by the addition of supporting electrolyte excess. This simplifies the theoretical description of mass transport to pure diffusion but on the other hand, modifies physicochemical properties, including transport parameters of the analyzed environment. In systems containing a deficiency of supporting electrolyte, migration contributes to mass transport. The transport studies in these systems require the employment of small-sized electrodes (micro- / nanoelectrodes) and potentiostats capable of measuring ultralow currents.
Migration introduces an additional variable (electrostatic potential) to the transport expressed by the Nernst-Planck equation. Under the low ionic strength conditions, the transport dynamics depends on the concentration profiles of a substrate, product, cations and anions of supporting electrolyte, and an uncompensated electrostatic potential gradient. In practice, the contribution of migration can be quantitatively related to the support ratio parameter (ξ), defined as the ratio of the concentrations of electroinactive charge carriers (i.e. not involved in the electrode reaction) to the concentration of the electrode reaction substrate. In terms of the ξ parameter, migration has a negligible effect on mass transport under the conditions of high values of the support ratio (ξ > 100). As the support ratio decreases (ξ < 1) the potential gradient occurs due to increased resistivity of the system and, consequently, the total mass transport is affected by the migrational contribution.
The problem of diffusion-migration transport in voltammetric experiments involving microelectrodes was treated theoretically and the derived models allow one to predict the voltammograms for various types of electrode processes carried out under the conditions of various concentration of supporting electrolyte. The major factor complicating the theoretical description of mass transport induced by migration is the influence of an EDL. In the system containing a deficiency of supporting electrolyte, EDL may occupy a significant part of the diffusion layers of the substrate and the product of the electrode process. With the decrease of the electrode radius (below 100 nm), the EDL start to overlap with the diffusion layer and significantly affects the transport even in the system containing substantial concentrations of supporting electrolyte.
To explore new research domains for nanoelectrodes, the procedures for the fabrication of nanoelectrodes of reproducible parameters and the theoretical description of the mass transport to nanoelectrodes under mixed diffusion-migration transport conditions have to be developed.
The ultrasmall electrodes are highly susceptible to electrostatic mechanical and electrochemical damage, leading to the formation of recessed or protruding electrodes with altered current response, which is an additional impediment in the studies with their use. As a result of the damage causing recession, the electrodes lose their current response and give low feedback in their use as probes in scanning electrochemical microscopy (SECM). The damage caused by electrostatic discharging can be avoided by handling and storing the electrodes grounded together with objects in the vicinity.
This work demonstrates the voltammetric behavior of the fabricated carbon nanoelectrodes in the presence and absence of supporting electrolyte. The ferrocene derivatives of various charges served as redox probes for the studies of diffusion-migration transport to nanoelectrodes. The obtained results are confronted to the theoretical predictions derived for microelectrodes. They revealed that migration of electroactive species to nanoelectrodes does not introduce significant changes to voltammetric signal as much as for bigger microelectrodes. Moreover, the elimination of supporting electrolyte in most cases led to the poor development of the voltammetric waves at nanoelectrodes. The possible reason may be related to the breakdown of the electroneutrality in the region adjacent to the nanoelectrode, since the electrical double layer for nanoelectrodes may occupy a significant fraction of the transport depletion layer. The uncertainty in the concentrations of ionic impurities (that determine the actual support ratio) in the layer adjacent to the electrode is another factor that may produce unpredictable and, to some extent, random voltammetric behavior of charged redox probes at nanoelectrodes in the absence of supporting electrolyte.