Molecular Mechanism\nof Mechanical Breathing in Organic\nMixed Ionic-Electronic Conductors

Mechanical breathing of organic mixed\nionic-electronic\nconductors\n(OMIECs) is a structural response upon redox reactions. The breathing\nstrain is often in the range of a few percentages to a few hundred\npercentages in different OMIECs operated in different chemical environments.\nSuch mechanical activation needs to be tailored such as to be maximized\nin actuators and to be minimized in increasing the device reliability.\nWe perform atomistic modeling to understand the molecular mechanism\nof mechanical swelling of OMIECs immersed in various electrolytes\nand at different oxidation states. We study poly(3,4-propylenedioxythiophene)\n(PProDOT) which is widely used in organic electrochromic devices as\na model system and compare its swelling behavior in electrolytes of\ndifferent salt concentrations, solvents, and anions. PProDOT deforms\nmore in the electrolyte of lower salt concentration and larger anions.\nMass transport of the electrolyte, especially the organic solvent,\nis dominant in regulating mechanical swelling of PProDOT compared\nto the electrostatic interactions. We examine the evolution of microstructural\nfeatures and local bonding environments associated with the mixed\nconduction upon oxidation. We calculate the diffusion coefficients\nof the cation, anions, and solvents in the mixture of PProDOT and\nthe electrolyte, which inform the swelling kinetics of PProDOT and\nthe solvation structure in the electrolyte. The results are further\nvalidated by different self-aggregated PProDOT configurations and\nmodeling protocols. The finding is in good agreement with the experiments\nand provides fundamental understanding of the molecular motifs underpinning\nthe breathing strain in OMIECs.