Bipolar membrane (BPM) is a special ion exchange membrane. It is composed of a cation exchange layer, an interface hydrophilic layer (catalytic layer) and an anion exchange layer. The thickness of the intermediate interface layer is nanometers. Under the action of a direct current electric field, the bipolar membrane can be hydrolyzed to obtain H+ and OH- on both sides of the membrane.
Fig.1 Schematic drawing of the water splitting function of a BPM(J. Balster, et al., 2010)
Preparation of Bipolar membranes
There are many methods for preparing bipolar membranes. First, two oppositely charged ion exchange resins in an inert matrix can be bonded together by heating and pressing or using an adhesive. In addition, anion and cation exchange membranes can be fused together by heating and pressure to form a bipolar membrane. Finally, the casting method can also be used for preparation. The bipolar membrane prepared by the casting method has good mechanical strength, high current density operation ability, high permeability selectivity, low potential drop and stable performance. It is the main preparation method for the commercialization of bipolar membranes. A key factor of the casting method is the degree of close contact between the anion and cation layers. The degree of contact must be controlled so that the layers do not penetrate each other to a certain degree, otherwise it will cause high resistance between the membranes. Therefore, an interface layer was introduced. The number of functional groups and the water content of the interface layer also have a great influence on the resistance of the bipolar membrane.(Ji Hua, Hao, et al., 2001)
Bipolar membranes can be used for water splitting in photoelectrochemical devices by maintaining a stable pH gradient. In artificial photosynthesis equipment, the influence of pH is very important. Silicon as a photocathode is unstable under alkaline conditions, while most metal oxide photoanodes will dissolve under acidic conditions. Therefore, it is necessary to allow the oxidation and reduction components of solar-powered water splitting components to operate at their respective optimal pH values. The operation of solar-driven water electrolyzers with transient pH gradients has been used to reduce the photoelectrolysis voltage, but this method is only suitable for short-term operation, because the pH gradient will quickly dissipate. The bipolar membrane can maintain a large steady-state pH difference, making the photoelectrochemical device run better.(Mcdonald M B, et al., 2015)
- Improvement of juice quality
- Chlor-alkali production
- Bromine Extraction from Seawater
- Heavy metal wastewater treatment
- Waste gas treatment
- J. Balster, et al.Tailoring the interface layer of the bipolar membrane[J]. Journal of Membrane Science,2010.
- Ji Hua, Hao, et al. Preparation of bipolar membranes (I)[J]. Journal of Applied Polymer Science, 2001.
- Mcdonald M B, et al. Use of Bipolar Membranes for Maintaining Steady‐State pH Gradients in Membrane‐Supported, Solar-Driven Water Splitting[J]. Chemsuschem, 2015.