Controlling Charge Percolation in Solutions of Metal Redox Active Polymers: Implications of Microscopic Polyelectrolyte Dynamics on Macroscopic Energy Storage

Soluble redox-active polymers (RAPs) enable size-exclusion nonaqueous redox flow batteries (NaRFBs) which promise high energy density. Pendants along the RAPs not only store charge but also engage in electron transfer to varying extents based on their designs. Here, we explore these phenomena in Metal-containing Redox Active Polymers (M-RAPs, M = Ru, Fe, Co). We assess by using cyclic voltammetry and chronoamperometry with ultramicroelectrodes the current response to electrolyte concentration spanning 3 orders of magnitude. Currents scaled as Ru-RAP > Fe-RAP ≫ Co-RAP, consistent with electron self-exchange trends in the small molecule analogues of the MII/III redox pair. Varying the ionic strength of the electrolyte also revealed nonmonotonic behavior, evidencing the impact of polyelectrolytic dynamics on M-RAP redox response. We developed a model to account for the behavior by combining kinetic Monte Carlo and Brownian dynamics near a boundary representing an electrode. While 1D pendant-to-pendant charge transfer along the chain is not a strong function of electrolyte concentration, the microstructure of the RAP at different electrolyte concentrations is decisively impacted, yielding qualitative trends to those observed experimentally. M-RAP size-exclusion NaRFBs using a poly viologen as negolyte varied in average potential with ∼1.54 V for Ru-RAP, ∼1.37 V for Fe-RAP, and ∼0.52 V for Co-RAP. Comparison of batteries at their optimal and suboptimal solution conditions as gauged from analytical experiments showed clear correlations in performance. This work provides a blueprint for understanding the factors underpinning charge transfer in solutions of RAPs for batteries and beyond.