Electrically conductive membranes: synthesis, mechanism of action, mathematical modelling, and applications

An increasingly important and rapidly developing area of modern membrane science is represented by stimuli-responsive membranes. The transport properties of such membranes can be altered in response to changes of temperature, light, electric/magnetic fields, ionic strength, and pH. Over the last decades, a lot of research has been focused on the development of membranes, which can change their transport properties in ionic solutions in response to the electric field. If the pore dimensions are comparable to the Debye length, then the ion transport through a membrane can be regulated by applying a transmembrane potential difference and/or varying the surface charge. In membranes with electrically conductive surface, the surface charge can be addressed by changing the surface potential. It provides a powerful tool for regulating ionic conductivity, ionic selectivity, and ion rejection. Electrically conductive membranes find applications in desalination, heavy metal removal, refractory organics degradation, oil-water separation, and fouling mitigation.

In this work, we provide a concise overview of experimental and theoretical perspectives of electrically conductive membranes. The main synthesis strategies and precursor materials for the preparation of selective layers are discussed. The performance of electrically conductive membranes in electro- and baromembrane processes under the action of potential, species concentration, and pressure gradients is described. A particular attention is focused on electrically assisted nano- and ultrafiltration, where the variation of surface charge by changing the surface potential can significantly improve membrane selectivity without compromising its permeability.

The modelling framework for describing electrically conductive membranes is presented and discussed. It relies on one- and two-dimensional pore flow models, which are based on the Navier- Stokes, Nernst-Planck, and Poisson equations. On the surface of electrically conductive membranes, two types of charges can be present: chemical charge due to dissociation of functional groups or adsorption of ions, and electronic charge due excess or deficiency of electrons. A peculiarity of such membranes relates to the fact that their surface is polarizable, i.e. an electronic charge can be induced on the surface by an external electric field. It results in a number of interesting effects, such as enhancement of membrane potential at zero current and non-linear current / voltage curves due to dependence of ionic conductivity on the applied potential difference. Modelling of electrically-assisted nanofiltration is discussed by describing recently obtained experimental data on salt and dyes rejections with the help of composite carbon-based polymeric membranes.

The work is supported by the Russian Science Foundation, project 23-19-00269.