Switchable ionic selectivity of membranes with electrically conductive surface: Theory and experiment

Nanoporous membranes with electrically conductive surface represent an important class of stimuli? responsive materials. The variation of surface potential provides a powerful tool for adjusting ionic selectivity, conductivity, and rejection. This work is devoted to the theoretical and experimental study of switchable ionic selectivity. The one?dimensional Space charge (SC) and two?dimensional Uniform potential (UP) models are first generalized to constant surface potential case taking into account the Stern layer with inner (iHp) and outer (oHp) Helmholtz planes. The ionic selectivity is investigated experimentally by measuring the membrane potential at zero current for C?Nafen membranes prepared from alumina nanofibers with conductive carbon coating. The evolution of charging current is used to determine the dependence of surface charge density and differential capacitance on the applied potential. These data are fitted by the UP and SC models to find the Stern layer capacitance. It is shown that the variation of surface potential results in a continuous change of ionic selectivity from anion to cation. The membrane potential data are fitted by the UP and SC models using the chemical charge density and concentration boundary layer thickness as fitting parameters. It allows to determine the potential, at which the membrane becomes non?selective. The SC and UP models provide close results for membrane potential and surface charge density and demonstrate a good agreement with the experimental data. The UP model overestimates the solution velocity and ion concentrations at the membrane surface, while it underestimates the ion fluxes and iHp/oHp potentials. This work essentially extends our understanding of ion transport in stimuli?responsive membranes operated by the electric field. The results can be applied in the area of nanofiltration, (reverse) electrodialysis, electrochemical sensors, and nanofluidic devices. Nanoporous membranes with electrically conductive surface represent an important class of stimuli? responsive materials. The variation of surface potential provides a powerful tool for adjusting ionic selectivity, conductivity, and rejection. This work is devoted to the theoretical and experimental study of switchable ionic selectivity. The one?dimensional Space charge (SC) and two?dimensional Uniform potential (UP) models are first generalized to constant surface potential case taking into account the Stern layer with inner (iHp) and outer (oHp) Helmholtz planes. The ionic selectivity is investigated experimentally by measuring the membrane potential at zero current for C?Nafen membranes prepared from alumina nanofibers with conductive carbon coating. The evolution of charging current is used to determine the dependence of surface charge density and differential capacitance on the applied potential. These data are fitted by the UP and SC models to find the Stern layer capacitance. It is shown that the variation of surface potential results in a continuous change of ionic selectivity from anion to cation. The membrane potential data are fitted by the UP and SC models using the chemical charge density and concentration boundary layer thickness as fitting parameters. It allows to determine the potential, at which the membrane becomes non?selective. The SC and UP models provide close results for membrane potential and surface charge density and demonstrate a good agreement with the experimental data. The UP model overestimates the solution velocity and ion concentrations at the membrane surface, while it underestimates the ion fluxes and iHp/oHp potentials. This work essentially extends our understanding of ion transport in stimuli?responsive membranes operated by the electric field. The results can be applied in the area of nanofiltration, (reverse) electrodialysis, electrochemical sensors, and nanofluidic devices. ? 2021 Elsevier Ltd. All rights reserved.

Automated summary

Nanoporous membranes with electrically conductive surface are an important class of stimuli-responsive materials. The variation of surface potential allows for adjusting ionic selectivity, conductivity, and rejection. This work focuses on the theoretical and experimental study of switchable ionic selectivity, providing insights into ion transport in stimuli-responsive membranes operated by electric field.