Fabrication of PVDF cation exchange membrane for electrodialysis: Influence and mechanism of crystallinity and leakage of co-ions

The influence of crystallization behavior on membrane structure and performance has been extensively studied in many membrane fields. Some studies of proton exchange membranes used in fuel cells have involved the influence of crystallization. However, crystallization of ion exchange membranes (IEMs) used in electrodialysis (ED) has been rarely studied. Co-ion leakage in ED process is adverse, but there have been few mechanism studies regarding how co-ions cross IEM surfaces on the concentration chamber side and migrate into the dilution chamber to reduce current efficiency. Polyvinylidene fluoride is a kind of good crystalline material. In this study, sodium dodecyl benzene sulfonate was introduced as the functional material, with polyvinylpyrrolidone (PVP) as the hydrophilizing agent. They were blended to prepare cation exchange membranes (CEMs). Amorphous PVP was used to regulate membrane crystallization behavior. The results showed that, when the ion exchange capacity is basically equal, surface roughness of the prepared crystalline CEMs was low and the structure dense. The ED efficiency significantly surpassed amorphous CEMs. The co-ion leakage of amorphous CEMs was greater than those of crystalline CEMs due to the rough and non-dense surfaces of amorphous CEMs. However, after densification modification, although surface resistance increased, co-ion leakage was effectively suppressed. Combining simulation and theoretical analysis, it was presumed that dense membrane surfaces on the concentrated chamber side was very important for suppressing co-ion leakage. This study on the influence and mechanism of crystallinity and co-ion leakage was helpful for optimizing CEM mass transfer and improving current efficiency.