Ion-exchanges membranes (IEMs) have many applications beyond fuel cells -- they can also be used to synthesize all types of compounds that are used in various industries. The most popular IEMs consist of polymeric resins with charged functional groups based upon their ion selectivity, they are referred to as anion-exchange (AEM) and cation-exchange (CEM) membranes. AEMs have positively charged functional groups and allow for the passage of anions. In contrast, CEMs have negatively charged functional groups and allow for the passage of cations. A third membrane type is the bipolar membrane (BPM), which can generate hydroxyl ions and protons, and are particularly useful in the synthesis of acids and bases. BPMs consist of both a cation-exchange layer and an anion exchange layer, with a thin intermediate in-between the two layers. The intermediate layer sometimes consists of a weak acid or catalyst to enhance the desired reaction. The ideal BPM layer is non-permeable for salt ions because high concentrations of salt ions often result in membrane contamination.
IEMS can be used for membrane electrolysis (ME), electro-electrodialysis (EED), electrodialysis metathesis (ED-M), ion-substitution electrodialysis (ISED) and electrodialysis with bipolar membrane (BMED). ME allows for the electrosynthesis of inorganic and organic compounds by a redox (reduction or oxidation) reaction where the products are obtained. In EED, ED-M, and ISED, ion replacement reactions of inorganic and organic salts allow the synthesis of acids, bases and ionic liquids. The major role of IEM in these processes are the membranes’ selectivity towards cations and anions to allow synthesis of chemical compounds with high-purity. The main limitation is often the electroosmotic solvent transport which limits the product concentration at the localized sites.
There are numerous chemistry products that can be created using IEMs via electrochemical salt splitting (i.e., MX where M is a cation such as Li+, Na+, K+, or ammonium and X is an anion such as phosphate, borate, acetate, or nitrate). To demonstrate how this process works, we will show how sodium sulfate is split to form sulfuric acid and sodium hydroxide in this blog post.
Sodium sulfate can be split into sulfuric acid and sodium hydroxide using an electrochemical cell similar to a battery. A sodium sulfate solution is passed through the central part of the cell in Figure 1. Under an electric field, the sulfate is transported through the anion permeable membrane into the anolyte and sodium ions pass through the cation permeable membrane into the catholyte. The reactions at the anode and the cathode generate protons and hydroxides, which cause the sulfuric acid and sodium hydroxide to accumulate in the anolyte and catholyte.
Figure 1. The Salt Splitting of Sodium Sulfate.
The design of this type of cell and the type of membrane selected is critical because as the acid concentration increases in the anolyte part of the cell, the current efficiency drops due to a back migration of protons from the anolyte compartment through the anion exchange membrane to the central portion of the cell. This, in turn, makes the pH in the central electrolyte stream more acidic and reduces the current efficiency of the sodium hydroxide. The back migration of the protons across the anion exchange membrane is well-known and there are several types of membranes that are designed to minimize the proton back migration.
In addition to the issue of back migration, a lower pH in the central portion of the cell also limits the type of cation exchange membrane that can be used. Bilayer cation exchange membranes, such as Nafion® 902 contain perfluorinated polymers with sulfonic acid exchange groups on one side and carboxylic acid exchange groups on the other side. The chemistry of the membrane limits the back migration of hydroxide ions, allowing higher basic concentrations to be reached with good efficiency. There are other Nafion® membrane types that can be used, depending upon the acidity of the central portion of the cell.
Bipolar membranes, like those used for fuel cells, can also be used for salt splitting (Figure 2). These membranes allow for the formation of hydroxide and protons without the coproduction of hydrogen and oxygen. A salt splitting stack can have a few cells or hundreds of cells membranes containing bipolar, anion, and/or cation membranes between a pair of electrodes. Depending upon the design, the salt splitting stacks can yield efficient and cost-effective designs. However, these designs also have stability problems such as unwanted transport across the membrane interface.
Figure 2. The Salt Splitting of Sodium Sulfate Using a Bipolar Membrane.
Many types of IEMS can be used effectively in chemical synthesis for salt splitting to generate hydrogen (H+) or hydroxyl (OH-) ions for synthesis of acids or bases. Many types of bipolar membranes allow for salt conversion, both in aqueous and non-aqueous solutions, with the solvent (water, alcohol) being split into a proton and a corresponding anion. This allows both inorganic and organic salts to be effectively converted to acids and bases. The main limitation lies in the membrane stability towards concentrated acids or alkaline solutions. Our example of splitting sodium sulfate into sulfuric acid and sodium hydroxide is just one chemical that can be applied for synthesis, reuse or recovery of inorganic and organic acids or bases from the reaction mixtures.