Ion Exchange Membranes for Sustainability

Greenhouse gases (GHG) represent one of the main causes of global warming. Carbon dioxide emissions are related to fossil fuels, that supply over the 50% of the global energy production. Renewable resources are the pathway to reduce GHG emissions, but currently they can not satisfy the energy demand due to high fluctuations in the power generated. Salinity gradient power (SGP), or blue energy, allows to produce energy from mixing water solutions that have different salt concentrations, including seawater and fresh water. This process has gathered more attention over the years since it allows to employ an unlimited and sustainable source like seawater. Electrodialysis (ED) technology is used for water desalination, water treatment and other cases where valuable chemicals can be recovered. The process relies on the use of ion exchange membranes (IEMs), materials that separate ions from a solution. Reverse electrodialysis is the complementary process of ED and allows to produce electrical energy from the salinity gradient between two solutions. For these technologies IEMs need to present high permselectivity, ionic resistance and mechanical stability, requiring high cost polymers and processes. New materials have emerged for ion separation and in particular 2D materials, which show promising properties in this field. The first part of this thesis aims to study the performances of graphene oxide-based IEMs, using para-aramid fibers as reinforcing phase. Graphene oxide (GO) is an interesting material for its high mechanical stability and ion selectivity. However this material is subject to swelling when in contact with water, reducing its performances and durability over time in a RED process. Aramid fibers are known for their outstanding mechanical properties, therefore they can stabilize a matrix when used as a second phase. By dispersing the fibers in alkaline dimethyl sulfoxide (DMSO) it was possible to produce stable nanofibers (ANFs), whose dimensions facilitate their uniform dispersion among the GO flakes. The GO was then added to the solution, creating a mixture with a water:DMSO ratio equal to 1:50. The liquid dispersion was filtrated by dead-end filtration, obtaining a membrane. Different membranes were fabricated varying the weight of GO or the content of nanofibers, in order to see the effect of those parameters on the structure and performances. Field emission scanning electron microscopy and Fourier transform infrared spectroscopy measurements were performed to see the morphology of the nanocomposite and the functional groups of the membranes. Finally, electrochemical characterization experiments evaluated the permselectivity and the ionic resistance of the membranes. The second part of the thesis focuses on the characterization of commercial bipolar membranes for electrodialysis applications. Bipolar membranes are composed by an anion exchange membrane (AEM) a cation exchange membrane (CEM) and an interfacial layer (IL) between the two. A bipolar membrane allows to dissociate water into protons and hydroxyl ions. This phenomenon occurs at the interface between the AEM and the CEM when the potential across the bipolar membrane reaches the theoretical water dissociation potential (0.83 V). The dissociation is also promoted by the functional groups of the IL material. The applications involved in this study were acid and base recovery from a corresponding salt (e.g. NaOH and HCl from NaCl) and the regeneration of an alkaline adsorbent used for carbon dioxide capture.