Electrochemical Poly(vinylidene fluoride) (PVDF) Membranes Using Polyethylenimine Cross-Linked Polydopamine-Bound Carbon Nanotubes

Electrochemical membranes (ECMs) are an emerging multifunctional separation technology that enables simultaneous contaminant filtration and reaction due to their electrically conductive porous surface coatings. Physical coating stability remains a technical challenge for ECMs, which are largely based on carbonaceous nanomaterials or metallic thin films. In this research, binding chemistry based on polydopamine (PDA) and polyethylenimine (PEI) was developed to prepare physically stable ECMs. Poly(vinylidene fluoride) (PVDF) ultrafiltration membranes were coated with PEI cross-linked PDA followed by the deposition of carboxyl-functionalized single/double-walled carbon nanotubes (SW/DWCNTs-COOH). Fabricated membranes were characterized for their structural, physicochemical, electrochemical, and separation properties. In a batch electrochemical system, the membranes achieved >99.2% electrochemical reduction of methyl orange (MO) in 120 min. Results revealed that the PDA/PEI intermediate layer can significantly enhance the adhesion between the SW/DWCNTs and the underlying polymer membrane without a substantial reduction (<10%) in water permeability. Response surface methodology (RSM) was employed to optimize the permeability and surface electrical conductivity of ECMs by studying the influence of PDA concentration, PEI concentration, and the branched-amine content of PEI in the coating solution. RSM analysis demonstrated two factorial interactions between PDA and PEI concentrations, as well as PEI concentration and PEI branch molecular weight. Our optimization study revealed that the use of a 1:1 ratio of PDA/PEI at low concentrations (∼2 mg/mL) and high PEI branch Mw (∼1200) was the ideal preparation condition within the tested design space to maximize both the water permeability (∼895 L/m2/h/bar) and electrical conductivity (∼29,761 S/m). This optimized cross-linking chemistry demonstrates the ability to make practical and physically stable ECMs on chemically inert PVDF membranes, expanding the range of membranes that can be used to create electrochemical membranes.