Abstract
Transport through vanadium redox-flow-battery membranes strongly influences cell performance. In this work, we use a multicomponent concentrated-solution model of transport and thermodynamics in phase-separated cation-exchange membranes, the most common separator type, to develop structure-performance relationships. The model incorporates species partitioning into the membrane, thermodynamic nonidealities, and Stefan-Maxwell-Onsager frictions between species. Molecular-thermodynamics and -transport theories parameterize the model. We validate the calculations against measured Coulombic and voltage efficiencies of a vanadium flow battery as a function of current density. Our model shows that species transport is the result of collective interactions between all species present in the system. The magnitude of coupling suggests that predictions made using dilute-solution theory for transport in these systems will be misleading in many situations. As a demonstration of the capabilities of the model, we predict cell performance, incorporating these interactions, as a function of electrolyte concentration and composition and membrane equivalent weight and backbone modulus. We find that electrolytes with high sulfuric acid concentrations provide the greatest cell performance (quantified by maximizing power density at a target energy efficiency). In the case of membrane properties, low equivalent-weight polymers perform better; at high equivalent weights, a low membrane modulus is preferred.
Highlights
This work develops a multicomponent, concentrated-solution model of transport in vanadium redox-flow batteries (VRFB) membranes that accounts for StefanMaxwell-Onsager transport couplings between species and thermodynamic nonidealities
The molecular-thermodynamics model outlined in Part I calculates ion and water partitioning into the membrane from the electrolyte in the electrodes and provides thermodynamic factors that influence species diffusion
Water concentration gradients play a dominant role in vanadium crossover
Summary
The contributions from water concentration gradients are large.[44] The calculations show that in cases that require only semi-quantitative predictions, neglecting the terms Dij ¶cj,M for j 1 i, 0 will cause ∼25% error to the calculated fluxes at OCP with the error decreasing with increasing current density.
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