Inventory of Common Questions About Bipolar Membranes

Bipolar membranes (BPMs) have emerged as a promising technology in the field of membrane separation processes due to their ability to simultaneously perform anion and cation transport. This unique property allows BPMs to effectively separate and concentrate ions, making them valuable in various applications such as water treatment, desalination, and electrodialysis.

What Are Bipolar Membranes? How Do They Work?

BPMs are composite membranes consisting of an anion exchange membrane (AEM) and a cation exchange membrane (CEM) with an interface between them. These membranes facilitate the electrolysis of water by creating a localized pH gradient at the junction interface when an electric current is applied. The AEM selectively allows anions to pass through, while the CEM enables the transport of cations. This results in the splitting of water into protons (H⁺) and hydroxide ions (OH^-) at the interface.

It is worth mentioning that bipolar membrane electrodialysis (BMED) is the most commonly used technology based on BPM. It combines the principles of electrodialysis and water electrolysis to effectively remove salts, acids and alkalis from aqueous solutions.

What Are the Applications of BPMs?

• Electrodialysis: BPMs are commonly used in electrodialysis processes for desalination and water treatment.
• pH Regulation: BPMs are utilized in pH regulation processes where they enable the controlled addition or removal of protons (H⁺) and hydroxide ions (OH^-) to adjust and maintain the pH levels of solutions.
• Metal Recovery: In metal recovery applications, BPMs can selectively separating metal ions from complex solutions, allowing for the efficient recovery and purification of valuable metals.
• Electrosynthesis: BPMs are employed in electrosynthesis processes for the production of chemicals and fuels through electrochemical reactions.
• Wastewater Treatment: Bipolar membranes are used in wastewater treatment plants for the removal of contaminants, heavy metals, and organic pollutants from industrial wastewater.
• Energy Conversion: In energy-related applications, bipolar membranes are utilized in processes such as water electrolysis for hydrogen production, fuel cells for energy storage, and redox flow batteries for energy conversion and storage.

What characteristics should an ideal bipolar membrane have?

An ideal bipolar membrane should possess several key characteristics to ensure optimal performance in electrochemical processes.

• High conductivity of the individual bulk layers.
• Fast chemical (dissociation or association of water) kinetics at the interface.
• High water permeability.
• Long lifetime under operational current densities.
• Low parasitic (ion) crossover.

The ideal BPM components. [1]

BPMs from Alfa Chemistry

What factors should be considered when selecting BPMs for specific application?

When selecting a bipolar membrane for a particular application, several factors must be considered to meet the process requirements effectively. These factors include:

• Membrane selectivity;
• Operational pH range;
• Temperature stability;
• Resistance to fouling;
• Membrane thickness;
• Cost-effectiveness;
• Compatibility with the target ions in the solution.

Understanding the specific needs of the application and evaluating these factors can guide the selection of the most suitable bipolar membrane for optimal performance.

Reference

1. Marijn A. Blommaert, et al. ACS Energy Lett. 2021, 6(7), 2539-2548.