# Modeling Zero‐Gap Saltwater Electrolysis With Advective Flow Through a Thin‐Film Composite Membrane

## TL;DR

This study models how water and ions move through a membrane during saltwater electrolysis, showing that fluid flow significantly affects ion transport.

## Contribution

The paper introduces a model that incorporates advective flow in thin-film composite membranes for zero-gap saltwater electrolysis.

## Key findings

- Including advective flow in the model improved agreement with experimental ion crossover data.
- Reversing membrane orientation changed the dominant ion and direction of fluid flow.
- Convective flux is critical, as its removal caused up to a 740% change in predicted ion crossover.

## Abstract

In zero‐gap saltwater electrolysis, ion transport is influenced by convective forces, but their effects have not been examined when using thin‐film composite (TFC) membranes with advective flow through the membrane. In this study, we adapted a one‐dimensional solution‐friction transport model for a zero‐gap electrolyzer to incorporate measured water flux across a TFC membrane. Open‐circuit or electrolysis (20 mA cm–2) experiments quantified ion transport with and without electrochemical reactions. Water velocity, estimated from volume changes in the anolyte and the catholyte, was used to infer convective contributions to ion transport. Ion‐specific friction coefficients were determined using open‐circuit data. Using the fitted friction factors and incorporating water flux, the modeled ion crossover concentration showed good agreement with electrolysis data, including changes caused by reversing the membrane orientation. Removing the convective flux from the model showed up to a 740% change in predicted ion crossover and worsened agreement with experimental data. The strong correlation between the fraction of charge carried by major salt ions and the measured water flux suggests that electroosmotic drag could be one of the main mechanisms responsible for the observed water flux. These results highlight the importance of incorporating solution convection when modeling ion behavior in zero‐gap systems using TFC membranes.

During saltwater electrolysis with a thin‐film composite membrane, ion and water transport are coupled. The direction of water flux follows that of the dominant salt ion carrying charge. When Na+ transport from the anode to the cathode is dominant, fluid flow occurs in the same direction. Reversing the membrane orientation shifts NO3– to be the major salt charge carrier from the cathode to the anode, with fluid flow in the same direction.© 2026 WILEY‐VCH GmbH