Effect of Gate Electrode in Electrochemical Cells

Electrochemical cells consist of two half-cells containing each an anode and a cathode.  There is an ionic contact between the two half-cells to maintain the flow of ionic charge.  In order to maintain the charge flow in a voltaic cell, a salt bridge is placed between the two half-cells.  We replaced the salt-bridge by a third electrode - a conductive and permeable gate electrode.  A bias potential was then applied to this gate electrode.  In that way, we were able to control the external current of the cell.  This would be the first step towards the realization of ion transistors.  

In preliminary studies, several gate electrodes were considered: layers of functionalized carbon nanotubes (CNT) and metal plate capacitors.  Cyclic voltammetry and electrochemical impedance spectroscopy revealed the effect of the gate bias on the effective capacitance and impedance of the cell.  

Current-Voltage measurements (I-V curves) were made on layered structures, which were made of p-type and n-type CNT.  These data clearly exhibited the formation of barrier(s) between the two layers.  Cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation (CHI760C, Electrochemical Instrument) at room temperature and using 0.01 M solution of NaCl as electrolyte.  Various scan rates of 0.01, 0.05, 0.1 and 0.5 Vs^-1 were assessed.  Two graphite rods, one serving as a working and the other as a counter electrode, were used.  Ag/AgCl (3M KCl filled) electrode served as the reference.  The gate electrode was placed in the middle of the cell, between the working and counter electrodes.  The corresponding specific capacitances were calculated from the CV data.

Electrochemical impedance spectroscopy (EIS) was performed for each of the gate electrodes, as well.  The data were taken at a dc bias of 0.24 V with sinusoidal signal of small magnitude over sub to kilo Hertz frequency range. The capacitive behavior was more pronounced at low frequencies. Membranes composed of p-i-n structures, a p-type layer of CNT, followed by an insulating layer of MMA and finished by an n-type CNT all deposited on a 10-micron Teflon filter substrate exhibited the larger capacitive behavior compared to p-n layered membranes or metallic capacitor. 

Chrono-potentiometry method was performed for gate electrodes. The curve within a potential window indicated from the time constant better capacitive behavior in p-i-n than other two gate electrodes. The value indicated in milli farad magnitude which is similar to the range of values found in cyclic voltammetry and electrochemical impedance spectroscopy techniques.     

References:

[1] Y. Zhang, H. Grebel, “Controlling Ionic Currents with Transistor-like Structure”, ECS Transactions 2 (18). 2007

[2] S. Sreevatsa, H. Grebel, “Carbon Nanotube Structures as Ionic Barriers: A New Corrosion Prevention Concept”, ECS Transactions 19 (29) 91-100. 2009