Effects of Electrolytes on the Transport Phenomena in a Cation-Exchange Membrane.

The effect of electrolyte solutions on the characteristics of the current-voltage (I-V) curve in a cation-exchange membrane (CMX membrane, Tokuyama Soda, Inc.) was studied based on the concentration polarization and electroconvection theory. The study includes the limiting current density (LCD), plateau length, and the ratio of resistance of region III to region I of the I-V curve (R(3rd)/R(1st)). Different electrolyte solutions, HCl, LiCl, NaCl, KCl, CaCl(2), MgCl(2), and AlCl(3), were used in this study. The LCD values of the electrolytes were correlated with the diffusion coefficient of the cation (D(+)) and valence of the cation and anion (z(+), z(-)). Except for the HCl solution, the LCD values of the electrolytes increased linearly with D(+)(1-z(+)/z(-)), implying that the current in this region was governed by the concentration polarization phenomena. The deviation of the HCl solution from the linearity is due to a particular transport mechanism of the proton called the Grotthuss-type transport. The differences in the plateau length and the resistance ratio, R(3rd)/R(1st), with the electrolytes were explained by the Péclet number (Pe) representing a transport pattern in the electroconvection theory. The Péclet number is proportional to the Stokes radius of an ion. An electrolyte with a large Stokes radius has a shorter plateau length and a lower ratio of R(3rd)/R(1st) than those of an electrolyte with a small Stokes radius. Water-splitting measurements for the different electrolyte solutions in the CMX membrane revealed that the contribution of water splitting to the overlimting current was insignificant regardless of the electrolytes used in this study. However, when metal hydroxides, such as Al(OH)(3), formed on the surface of the membrane, significant water splitting was observed. Copyright 2001 Academic Press.