Structure and Composition of Bipolar Membrane
Bipolar membranes (BPMs) typically consist of multiple layers with alternating positively charged (cationic) and negatively charged (anionic) regions, resulting from the combination of anion exchange layer (AEL) and cation exchange layer (CEL). The AEL is a polymer matrix containing positively charged functional groups, such as quaternary ammonium ions, while the AEL incorporates negatively charged functional groups, such as sulfonic and carboxylic acid moieties. The middle zone, called the interface, is a water layer maintained by hydrophilic components of the AEL and CEL.
The structure of a bipolar membrane depends largely on the method of membrane production.[1]
The interface between AEL and CEL can be divided into:
- Smooth interface: It is characteristic of homogeneous BPMs.
- Corrugated interface: It is formed by mechanically processing, with enhanced adhesion of the monopolar layers.
- Heterogeneous interface: It is a characteristic of BPM obtained by hot pressing.
Interface Structure of BPMs. [1]
How Bipolar Membranes Work
The main process based on bipolar membranes is bipolar membrane electrodialysis (BMED). In BPM, the thickness of the intermediate interface layer is on the nanometer scale. Under the action of a DC electric field, the water in the middle interface layer dissociates, and hydrogen ions and hydroxide ions are obtained on both sides of the membrane. Therefore, the function of the bipolar membrane is to provide a source of H+ ions and OH- ions under the action of electric field force. Based on the above principles, a bipolar membrane electrodialysis system composed of a bipolar membrane and other anion and cation exchange membranes can convert salts in aqueous solutions into corresponding acids and bases without introducing new components. [1, 2]
The working principle of BMED. [1]
Wide Range of Applications of Bipolar Membranes
In recent years, bipolar membranes have developed rapidly. They can realize acid-base separation at low cost and have excellent application prospects in many subdivided fields such as high-salt wastewater resource recovery, acid-base separation, and organic acid recovery. In addition, bipolar membranes also show great potential in the field of environmental protection, such as CO2 and SO2 capture and flue gas desulfurization.
- Bipolar membranes for acid and alkali production and recycling
Using BMED technology, inorganic acids and bases can be generated from corresponding salt solutions, such as sodium chloride, NaNO3, Na2SO4, Na3PO4, NH4F and other salts.
The BMED process is also used to produce and recover organic acids, including formic acid, acetic acid, propionic acid, succinic acid, citric acid, lactic acid, malic acid, tartaric acid, gluconic acid, lactobionic acid, vitamin C, amino acids, salicylic acid and petroleum acid.
- Bipolar membrane for CO2 capture
BMED can use renewable electricity to split water into acids and bases to regenerate bicarbonate-based CO2 capture solutions such as direct air capture (DAC). Research by Justin C. Bui et al. shows that for DAC, pH fluctuations caused by BPM water dissociation drive the formation of CO2 at the cation exchange layer- catholyte interface with an energy intensity of less than 150 kJ mol−1. [3]
- Bipolar membrane for flue gas desulfurization
Traditional desulfurization processes, such as the wet limestone-gypsum method, consume a large amount of alkali, and the by-products after desulfurization cannot be used, causing secondary pollution. The bipolar membrane electrodialysis technology not only does not require the input of alkaline substances, but also can turn sulfur dioxide into treasure.
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References
- Pärnamäe, R., et al. Journal of Membrane Science, 2021, 617, 118538.
- Pourcelly, G. Russian Journal of Electrochemistry, 2002, 38, 919-926.
- Bui, Justin C., et al. Energy & Environmental Science, 2023, 16(11), 5076-5095.
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