Bipolar Membranes (BPM)
Principles of Bipolar Membranes
The conventional method for generating H+ and OH- ions from water uses electrolysis. Electrolysis also generates O2 and H2 and the production of these gases consumes about half of the electrical energy of the process. In contrast, special ion-exchange membranes such as the bipolar membranes are capable of splitting water directly into H+ and OH- ions without generating O2 and H2.
In its simplest form a bipolar membrane is a cation-exchange membrane laminated together with an anion-exchange membrane, through an intermediate layer (the "junction" layer). The intermediate layer is the most important part of the membrane. The principle of "splitting" water by using a bipolar membrane is shown in the Figure.
Water diffuses from across the cation- and anion-exchange
layer to junction layer where it dissociates into H+
and OH- ions. The H+
ions then migrates across the cation-exchange layer towards the negative electrode
while the OH- ions migrate across the anion-exchange
layer to the positive electrode.
Any other anions (X-) are excluded from the junction layer by the cation-exchange layer and any other anions (M+) are excluded from the junction layer by the anion-exchange layer.
The 2 main factors that determine the behavior of bipolar membranes are the structure of the bipolar junction between the 2 ion-exchange layers, and the nature of charged groups which are attached to the polymeric matrix.
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Example Application of Bipolar Membranes
Although the use of bipolar membranes is an emerging technology, several applications of this technology are already used on an industrial scale. An example application is the treatment of concentrated salt solutions such as Na2SO4 to produce H2SO4 and NaOH.
A cell system consisting of an anion-exchange membrane (AEM), a bipolar membrane (BPM), and a cation-exchange membrane (CEM) as a repeating unit is shown in the Figure.
Such configuration is often referred to as a 3-compartment cell. This elementary cell is repeated and placed between 2 electrodes.
The Na2SO4 solution flows between the cation-exchange and anion-exchange membranes (the salt channel), while water flows between the ion-exchange membranes and the bipolar membranes, i.e. in the H2SO4 and NaOH channels.
When direct current is applied, Na+ will be attracted to the cathode while SO42- will be attracted to the anode. Na+ will flow across the CEM into the NaOH channel while SO42- will flow across the AEM into H2SO4 channel. In the bipolar membrane, water will dissociate to form the equivalent number of H+ and OH- ions. The H+ ions will permeate through the cation-exchange side of the BPM and form H2SO4 with the SO42- ions in the H2SO4 channel.
Similarly, the OH- ions will permeate through the anion-exchange side of the BPM and form NaOH with Na+ ions in the NaOH channel. The final result is the production of NaOH and H2SO4 from Na2SO4 solution at a significantly lower cost than by other methods.
In a similar way, the use of bipolar membrane process offers an interesting alternative to the conventional membrane electrolysis for the production of NaOH and HCl from NaCl using the same cell as shown above. The existing method of electrolysis produces chlorine, which is an environmental concern. However, a disadvantage in using bipolar membranes is that the concentration of HCl and NaOH in practical situations is limited to about 20%, while using conventional process this concentration reaches 30%.
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