Water-coupled monovalent and divalent ion transport in polyviologen networks

Abstract

Redox-active polymers (RAPs) are of interest as environmentally friendly and earth-abundant energy storage materials. Polyviologens are promising RAPs, but they tend to dissolve during operation. Further, the two-electron redox reaction for polyviologens in various electrolytes is not always reversible, highlighting the need for a deeper understanding of the redox mechanism. Here, the energy storage mechanism for a cross-linked viologen (PTPM) is demonstrated using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), comparing NaCl and Na₂SO₄ aqueous electrolytes. E-QCMD reveals that the ion-electron transport mechanism is strongly dependent on the valency of the anion. More sudden and dramatic changes in the electrode's mass were observed for the divalent sulfate ion as compared to the smooth mass transitions associated with the monovalent chloride ion. Meanwhile, there was marked hysteresis in the mass transfer profile for NaCl, but little hysteresis for Na₂SO₄. Our results demonstrate that electrolyte design, and specifically ion valency, will have a large impact on the nature of mass transport in polymer-based electrodes. This work enables electrolyte selection for the next generation polymer batteries with improved performance.