Rectification effects of Nafion-backed micropore-voltammograms by difference in migrational modes

Abstract

Rectified current-voltage curves were observed by micro-hole voltammetry in various concentrations of sodium ion electrolyte for transport through a micro-hole and a cationic exchange membrane (Nafion). Data suggest a linear current-concentration relation, implying electric migration-control. The rectification is observed as difference in current-potential slopes at positive and at negative applied voltages. The voltammograms are analyzed quantitatively in the context of Ohm's law as a function of both the geometry of the micro-hole and of the entire cell. The conductivity in the direction of the ion transport from the micro-hole to Nafion, corresponding to the lower conductance (closed diode), is proportional both to the cross-section area of the micro-hole and to the inverse micro-hole length. This is consistent with control by electro-migration within the micro-hole. In contrast, the conductivity in the opposite cation transport direction (open diode) is almost independent of the micro-hole geometry but varies with the length of the glass tube of the cell. The current-potential slope is consistent with the value calculated for the NaCl concentration and the geometry of the measurement cell. Under these conditions, the micro-hole is filled with a high concentration of NaCl as the flux of Na+ from the Nafion to the hole, and the high concentration causes the migration current larger that the current in the opposite direction. Consequently, the rectification ratio can be enhanced by careful design of both (i) the cell geometry including ion-exchange membranes and (ii) the micro-hole geometry rather than nano-scaled chemistry in holes.