the Donnan exclusion. However, the proton selectivity is Proton transport phenomena across a composite membrane considered to be very high in a mixed solution of trivalent (PBM), where an anion exchange layer composed of poly(sulfone) ions such as Al 3/ and protons if the anion exchange layer modified with quaternary amino groups was pasted on a cation is composed of poly(sulfone) with a quaternary amino group exchange layer composed of a sulfonated copolymer of polyand the thickness is about 1/10th that of the cation exchange (styrene) and poly(divinylbenzene), were investigated. This layer which is 100 mm (4). The thickness of an anioncomposite membrane shows the characteristics of a bipolar memexchange layer of 10 mm is large enough to exclude the brane based on current-voltage measurements. If the voltage is cations. Therefore, it is very difficult to consider that the applied from the anion-exchange layer, water is dissociated into anion exchange layer has high proton selectivity.
H / and OH 0 , and then OH 0 is transported to the anode through the poly(sulfone) layer, which suggests the possibility of polymer
The bipolar membrane is a layered structure involving a decomposition of the anion-exchange layer by the base. For the cation-selective membrane joined to an anion-selective separation of H / and Al 3/ from their mixed solution by the PBM, membrane. It is well-known that rectification is observed in the voltage is always applied from the anion exchange layer to the the current and voltage properties and that water splitting cation exchange layer to cause water splitting. However, there is occurs at the boundary between the anion and cation layers no evidence of poly(sulfone) decomposition during H / and Al 3/ (5-7). The water splitting mechanism in a bipolar memseparation though the reverse bias voltage is applied to this membrane has been discussed for a long time. Mafe ´et al. exbrane. H / is transported from the external solution and combined plained the water splitting using the Donnan effect and the with dissociated OH 0 from the intermediate surface in the anion Nernst-Planck equation; water molecules dissociate into exchange layer to produce water. The dissociated H / in the intercations (H / ) and anions (OH 0 ) in accordance with the mediate surface is transported to the cation exchange layer side.
second Wien effect (8,9). The efficiency of the water split-Therefore, the poly(sulfone) layer is always in a neutral state, which implies that PBM has resistance to the alkaline atmosphere. ting is explained as a function of the dielectric constant at ᭧ 1998 Academic Press the boundary surface. On the other hand, Simons suggested Key Words: bipolar membrane; water splitting; rectification efthat the water splitting was caused by the chemical reaction fect; poly(sulfone); proton transport. of binary or ternary amino groups with water molecules (10-14). Such discussion tells us that the intermediate boundary region is responsible for water splitting.