Characterization of electrokinetic and membrane properties of nanoporous polyimide films via measurements of time-resolved pressure-induced potential

Abstract

Charged nanoporous films have interesting ion-separation properties essentially controlled by pore-surface charge density. This (or a related zeta-potential) is often estimated from the so-called streaming potential. However, with nanoporous membranes in sufficiently dilute electrolyte solutions, the pressure-induced electrical response is due not only to streaming potential but also to an electrical response to concentration gradients arising because of a partial salt rejection by charged nanopores. In this study, we refine the methodology of interpretation of measurements of transient filtration potential developed by us previously and apply it to the determination of electrokinetic and membrane properties (in particular, diffusion permeance) of nanoporous polyimide films developed for uses as battery/supercapacitor separators. Diffusion permeance is directly related to electrical conductance, which is an important property of battery/supercapacitor separators.We observe that in dilute electrolyte solutions (<2 mM KCl), the electrical response to an applied pressure difference is dominated by the concentration potential and not by the classical streaming potential contrary to what is often tacitly assumed. The development of concentration gradients across and around the film takes some time, which enables separate determination of instantaneous (streaming potential) and time-delayed (concentration potential) components. An improved interpretation procedure developed in this study enables estimates of impact of non-linear and osmotic corrections and makes the interpretation more reliable. The results of this study are important for the identification and optimization of novel innovative applications for nanoporous films.