Classical Molecular Dynamics Investigation of Polymeric Membranes for Organic Redox Flow Batteries

Organic Redox flow batteries (ORFBs) are prime candidates for large-scale energy storage due to their modular design and scalability, flexible operation, and ability to separately adjust the voltage and capacity1. Design and optimization of ORFBs are challenging tasks and require extensive research. A major issue with ORFBs is the crossover of redox-active species which can cause decaying performances 2. Computational methods can facilitate the selection of membranes and help in their characterization by allowing the direct examination of their nano-structural, mechanical, and transport properties in various electrolyte solutions, along with their interfacial interactions. In this study, we use classical molecular dynamics to explain and quantify the properties of a potential membrane for ORFBs. Figure 1: Simulation stages of PDADMA-TFSI/DMF molecules (a) Random positions of PDADMA-TFSI/DMF molecules in the simulation box. (b) Simulation box after the solution has relaxed. (c) Solid membrane after the evaporation of DMF. Only the polymer chains are displayed for the sake of clarity. The ionic polymer membrane investigated in this study consists of Polydiallyldimethylammonium (PDADMA+n) polymeric cation combined with bis-(trifluoromethanesulfonyl) imide (TFSI-) anion 3. Dimethylformamide (DMF) was used as a solvent for casting the membrane. All the simulations conducted in this research utilized the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), exploiting the non-polarizable OPLS-AA and CL&P force field for simulation of interatomic interactions 4,5. The simulation workflow in this research was initiated by reproducing the DMF (solvent) properties such as density and diffusivity to ensure the reliability of the simulation protocols. Then, the solution of PDADMA-TFSI / DMF (containing 30 decamer chains) was equilibrated to reach a stable state. In the next step, the solvent was evaporated to obtain the solid structure of the membrane. Evaporation is simulated by randomly removing DMF molecules from the bulk solution of PDADMA-TFSI/DMF. The simulation box is then adjusted, and remeshing is performed for calculating the Coulombic interaction. Our aim is to demonstrate how changes in equilibration and evaporation conditions can affect the final membrane nanostructure and transport properties.