The trade-off between membrane permselectivity and conductivity: A percolation simulation of mass transport

Abstract

Ion exchange membranes (IEMs) have been widely applied in water desalination, separation and sustainable energy generation. In these applications, ionic conductivity and permselectivity draw the most attention from researchers. However, theoretical guidance is still lacking on where to find the optimized membrane properties in applications that are sensitive to both conductance and permselectivity. In this study, we have proposed a novel model by combining the simplified three-phases of an IEM and the percolation theory. Using open and closed sites to represent the polymeric and interstitial electrolyte phases of the membrane matrix on a 3D lattice, membrane permselectivity and conductivity can be successfully modeled and simulated on a set of sulfonated poly(2,6,-dimethyl-1, 4-phenylene oxide) (SPPO) membranes prepared using different organic solvents (N-Methyl-2-pyrrolidone, dimethylformamide and dimethyl sulfoxide), and commercial Fumatech FKS membranes. The resulting simulation provides good agreement with experimental observations. Notably, the decrease in permselectivity upon decreasing membrane thickness is well explained by the model considering the membrane void ratio and the percolative states of the lattice structure. When the membrane void ratio is larger than the critical threshold of 0.31, it renders the membrane low permselectivity while high conductance. This work potentially provides fundamental guidance in developing high-performance IEMs.