Theory of ion and water transport in electron-conducting membrane pores with pH-dependent chemical charge

TL;DR

This paper develops an extended uniform potential model for ion and water transport in electron-conducting membrane pores, incorporating pH-dependent chemical charge and electronic charge, and validates it against experimental data.

Contribution

It introduces a novel extended UP model that accounts for both electronic and chemical charges in membrane pores, enhancing understanding of their combined effects.

Findings

Model agrees with experimental membrane potential data at zero current

The ratio of electronic to chemical charge is characterized by key dimensionless parameters

Tuning electronic charge can improve osmotic energy conversion efficiency

Abstract

In this work, we develop an extended uniform potential (UP) model for a membrane nanopore by including two different charging mechanisms of the pore walls, namely by electronic charge and by chemical charge. These two charging mechanisms will generally occur in polymeric membranes with conducting agents, or membranes made of conducting materials like carbon nanotubes with surface ionizable groups. The electronic charge redistributes along the pore in response to the gradient of electric potential in the pore, while the chemical charge depends on the local pH via a Langmuir-type isotherm. The extended UP model shows good agreement with experimental data for membrane potential measured at zero current condition. When both types of charge are present, the ratio of the electronic charge to the chemical charge can be characterized by the dimensionless number of surface groups and the dimensionless capacitance of the dielectric Stern layer. The performance of the membrane pore in converting osmotic energy from a salt concentration difference into electrical power can be improved by tuning the electronic charge.