Novel Electronic Structures within Electrochemical Cells

Electrochemical reactions have been studied since the early eighteenth century [1,2].  Two-half cells are typically connected via a permeable membrane, which enables the passage of ions, yet, limits the flow of the bulk electrolyte molecules.  

We wish to electrically control the ion flow inside an electrochemical cell.  One way to do so is to externally vary the potential between the anode and the cathode electrodes.  However, such approach affects the surface potential of both electrodes and, thus changes overall reaction rate.  This crude approach is not suitable for control since the ion current depends on the external bias nonlinearly as may be observed by Tafel plots.

We would like to operate the cell at a fix external bias, yet to be able to control only the cell’s current.  In our approach a third, permeable electrode (a gate electrode) is placed between the anode and the cathode [3,4].  This mid-electrode presents an electrical barrier to the flowing ions. By applying bias to the gate electrode one may increase, or decrease this barrier, hence controlling the ion flow and as a consequence, the cell’s external current.  Examples of relatively low oxidizing, permeable, gate electrode are: graphene – monolayer or a few layers of graphite – and various configurations of functionalized carbon nanotube (CNT) films.  From electronic point of view, an electrochemical cell (e.g., a battery) is a two-terminal device.  We turn it into a three-terminal device by introducing a central electrode – the gate electrode.  Unlike other channel devices, the ions pass across the electrode in what amounts to a true large area (2-D) structure.  This area may be on the orders of many cm2 or even larger.  We are dealing with a multi-component current species containing both positive and negative charges.

Affecting the ionic current in electrochemical cells may be using the configuration shown in the figure.  Two graphite rods are immersed in a 1% NaCl solution.  A voltage is applied between the rods, Ecell.  The permeable gate electrode is made of two layers of functionalized carbon nanotubes: one layer is made p-type, while the other is made n-type.  This composite gate electrode constitutes a permeable p-n junction.  We bias the junction(s) with a bias potential V.  We measure the cell’s current Icell as a function of V at a given Ecell. 

In talk we will report on cyclic voltammetry (CV) as well as current-voltage experiments.

1.   Galvani, L.  (1791) Commentarii Bononiesi, 7, p. 363.

2.   Volta, A., (1800) Phil. Mag., 8, pp. 289-311.

3.   Amrita Banerjee and Haim Grebel, Electrochem. Commun. (2010), doi:10.1016/ j.elecom.2009.12.013.

4.   Joel Grebel, Amrita Banerjee and Haim Grebel, Electrochimica Acta, 95 308–312 (2013).