Selective Ion Transport through Copolymer Membranes Functionalized with Imidazole Ligands

Lithium-ion batteries are crucial for the transition to green energy. Recycling lithium-ion batteries is increasingly important to safely dispose of this hazardous waste as well as to supplement the supply of valuable and rare metals such as cobalt and lithium. Nanofiltration membranes can separate dissolved molecules from each other through a combination of electrostatic and steric hindrance in an energy-efficient manner. While small molecules pass through these membranes, larger particles cannot and thus a separation occurs. If membrane ligands that selectively interact with one metal over another could be developed, then these membranes could separate similarly-sized and similarly-charged ions from each other. Functionalized membranes could then replace complex and waste-intensive solvent extraction methods (hydrometallurgy and pyrometallurgy) used in lithium-ion battery recycling processes. This project aims to create metal-selective membranes by using imidazole ligands to selectively partition a target metal into a copolymer membrane–through ligand-metal complexes–while excluding metals with lower affinity for the ligand.

Ligands with an imidazole moiety were investigated due to their well-known complexation with transition metals. To begin, the strength of the interaction between the ligand and metal was measured in solution. Specifically, propargyl 1h-imidazole-1-carboxylate was dissolved in water and mixed with varying levels of copper or cobalt. The absorption change in the ultraviolet-visible light spectra correlated to the imidazole complexing with copper while no spectroscopic effects were observed in this region of light for cobalt-imidazole complexes. The imidazole ligand was attached to the membrane through a copper-catalyzed azide alkyne coupling (CuAAC) click reaction. The success of the reaction was confirmed using Fourier-transform infrared spectroscopy. X-ray fluorescence spectroscopy was used to demonstrate that membranes exposed to a solution with equimolar concentrations of both copper and cobalt, bound copper selectively. Copper diffusion cell experiments demonstrated that the imidazole-functionalized membranes had twice the copper permeability as non-functionalized membranes. Diffusion cell experiments with equimolar mixtures of copper and cobalt demonstrated modest selectivity which we attribute to the large pore size of the current membrane.

Current membrane pore sizes–around 3 nm diameter–were quantified using neutral solute rejections experiments with poly(ethylene) glycol molecules of varying size. This study will continue to alter different aspects of the membrane fabrication process to produce membranes with smaller pores.