Catalytic effect of ammonia-containing species on water splitting during electrodialysis with ion-exchange membranes.

Abstract It is known that some components of the bathing solution can enhance water splitting at depleted solution/ion-exchange membrane interface via protonation/deprotonation reactions with water. In this paper, we show that not only the presence of such components is important, but also a mechanism ensuring their sufficiently high concentration near the membrane surface. A comparative study of the electrochemical behavior of a Neosepta® homogeneous cation-exchange CMX and an anion-exchange AMX membranes (Astom, Japan) in 0.02 M KCl or 0.02 M NH 4 Cl solutions is carried out. The NH 4 +/NH 3 couple is an effective catalyst of water splitting. The deprotonation reaction rate constant of the NH 4 + ions, which limits the rate of water splitting, is about 10 s−1. It is almost 6 orders of magnitude greater than the rate constant for direct water dissociation in free solution. A comprehensive electrochemical characterization of the membrane systems is made: voltammetry, chronopotentiometry, electrochemical impedancemetry, and pH-metry (including color indication of the pH of the membrane internal solution). It is found that the water splitting rate at the interface of the AMX membrane in the NH 4 Cl solution is essentially higher than in the KCl solution. The difference in the rates of this reaction in KCl and NH 4 Cl solutions at the CMX membrane is insignificant. The reason for the weak effect of the NH 4 +/NH 3 couple on the rate of water splitting at the CMX membrane is that the concentration of both NH 4 + and NH 3 species is very low near its surface if the current density is close to or higher than its limiting value. As for the AMX membrane, we show that the ammonia-containing species can be transported to its depleted surface from the enriched solution by back "facilitated" diffusion through the membrane and this transport plays a crucial role in the occurrence of the rapid water splitting. [ABSTRACT FROM AUTHOR]