However, traditional methods of CEM synthesis via solvent casting are expensive, require selective materials, and involve a considerable amount of solvent, leading to safety concerns. Additionally, some cases may require extra chemicals and further manufacturing steps following solution casting. For example, to fabricate cation exchange membranes, sulfonation of polymer chains is necessary to incorporate negatively charged SO₃H⁻ groups into the structure. The sulfonation step typically involves concentrated sulfuric acid or chlorosulfonic acid—both highly corrosive substances whose disposal is challenging, time-consuming, and expensive. To fully realize the potential of CEMs, a new manufacturing strategy is required to synthesize highly selective, low-resistance, scalable, and affordable ion-exchange membranes (IEMs) by enabling the tuning of membrane chemistry while minimizing material usage.
Interfacial polymerization (IP) of m-phenylene diamine (MPD) and trimesoyl chloride (TMC) onto a support is a well-known technique for the synthesis of polyamide membranes commonly used in pressure-driven desalination processes. The structure of these membranes differs significantly from that of traditional cast membrane structures. Unlike conventional cast membranes, IP-derived membranes feature a bilayer architecture: a robust support layer ensuring mechanical stability and an ultrathin polyamide active layer responsible for ion rejection. The polyamide matrix formed via IP comprises crosslinked networks and linear polymer chains, with the latter containing sparse carboxylate (COOH⁻) groups. This limited number of charged groups results in low permselectivity, making such membranes ineffective for electro-membrane processes like electrodialysis (ED), which demand precise permselectivity. The use of different ranges of monomer (TMC:MPD) ratios can serve as a solution to increase carboxylate groups. However, interfacial polymerization (IP) is a rapid and uncontrollable process where only MPD is mobile in both phases, severely limiting the adjustable range of TMC:MPD ratios available for tuning polyamide chemistry. Therefore, new approaches enabling better control over polyamide chemistry should be explored.
Additive manufacturing through electrospray—a technique that uses a high electric field to atomize liquid monomers into fine droplets—is a promising method for fabricating polyamide membranes. In this process, direct deposition of both monomers onto a support is possible as most of the solvent evaporates during atomization. This method provides unprecedented control over monomer ratios, monomer quantities, and final polyamide thickness while reducing material consumption, unlike IP. In this study, we aim to tune the chemistry of polyamide membranes by utilizing a wide range of TMC: MPD molar ratios via electrospray—a feat that cannot be achieved through IP.
This innovative approach allows us to tailor the permselectivity of polyamide-based CEMs up to ≥90% to meet specific process requirements such as ED while ensuring scalability. In our project, we successfully synthesized CEMs using electrospray technology and evaluated their performance in electrodialysis applications. The results were promising, with our membranes achieving 84% NaCl removal while maintaining high water recovery. This performance is highly competitive, considering that commercially available solvent-cast ion exchange membranes typically achieve 91% NaCl removal. This significant advancement opens up opportunities to implement low-cost thin-film composite (TFC) membrane technology—used in other membrane-based desalination processes for over 40 years—in ED and its derivative electro-membrane desalination technologies as well as energy conversion systems like neutral pH redox flow batteries.