Modeling Ion Transport across Polyamide Membranes during Saltwater Electrolysis

Electrolysis powered by renewable energy could produce carbon neutral hydrogen for uses in agriculture and future energy storage, but the technology is currently hindered by a reliance on ultra-pure water and high capital costs. To eliminate the need for an ultra-pure water feed, we are studying the direct electrolysis of salty and impure water, which exists abundantly around the world. Additionally, thin-film composite polyamide membranes could replace expensive cation exchange membranes (CEMs) in electrolysis with a contained anolyte and saltwater catholyte because they can facilitate small water ion transport (proton and hydroxide ions) while size selectively hindering salt ion transport between compartments. Chloride ions present in seawater must be prevented from reaching the anode, where the chlorine evolution reaction competes with the oxygen evolution reaction, potentially producing chlorine gas and other oxidized chlorine species that can damage electrolyzer components. Optimizing membrane performance requires a fundamental understanding of membrane properties that impact ion retention and transport during electrolysis. Thin-film composite polyamide membranes are typically used for reverse osmosis processes, so there is limited understanding about what controls their performance during electrolysis.

While modeling has been useful in understanding ion transport across thin-film composite membranes during filtration experiments, it has never been used for these membranes in electrochemical systems with a potential gradient driving ion transport in the absence of hydraulic pressure. Nernst-Planck based modeling with electroneutrality and Donnan, steric, and dielectric partitioning conditions was used in COMSOL to predict ion transport across a reverse osmosis membrane between a contained anolyte and saltwater catholyte during electrolysis. The ion crossover between the anolyte and catholyte was experimentally measured for a range of feed concentrations (1 M, 0.8 M, and 0.6 M), and for several different constant current densities (10 mA-cm^-2 and 14 mA-cm^-2). A model was fit to the ion transport data during electrolysis of 0.6 M electrolytes at 10 mA-cm^-2, and it was validated with the remaining ion transport data at multiple concentrations at current densities. The modeling provided insights into selective water ion and salt ion transport across these membranes during electrolysis. As protons are generated at the anode and hydroxide ions at the cathode, their concentrations increased at the membrane surface causing these ions to carry a larger fraction of charge across the membrane over time. Water ions are minimally hindered by the thin-film composite membrane and have higher diffusion coefficients in water than salt ions, allowing them to transport rapidly across the membrane and decreasing the potential drop across the membrane. The changing potential drop across the membrane decreases the migration driving force for salt ions, changing their fluxes across the membrane over time. Fitting a model to experimental data provides an opportunity to better visualize ion transport across thin-film composite membranes and predict performance at a range of operating conditions.