Effect of Hydrodynamic Interactions and Flow on Charge Transport in Redox-Active Polymer Solutions

Redox-active polymer-grafted particles as redox mediators for enhanced charge transport in solution-state electrochemical systems

Efficient charge transport pathways in solutions of redox-active polymers are essential for advancing next-generation energy storage systems. Herein, we report the grafting of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) polymer brushes onto silica particles with different molecular weights and grafting densities, and the impact of these composite particles in solutions of PTMA. The polymer-grafted particles are characterized using Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) techniques. The grafted polymers have molecular weights of 2.5 kDa and 5.0 kDa, with corresponding grafting densities of 0.688 and 0.378 chains nm-2 for SiO2-PTMA-2.5k and SiO2-PTMA-5k, respectively, with the grafting density decreasing with increasing graft length. To investigate the effect of these composite particles on charge transport in solutions of PTMA, different concentrations of the grafted particles were added to solutions of PTMA of different concentrations (near overlap concentration, C*) in 0.1 M LiTFSI in acetonitrile. Electrochemical analysis reveals that below C* the addition of SiO2-PTMA-5k increases the apparent diffusion coefficient (D app) 15.2% to 1.041 × 10-6 cm2 s-1, the exchange rate constant (k ex,app) by 9.5% to 1.546 × 1011 L mol-1 s-1, and the heterogeneous electron transfer rate constant (k 0) by 24.6%, to 5.526 × 10-4 cm s-1. These results indicate that the synergistic interactions between unbound PTMA polymer chains in solution and PTMA-grafted particles facilitate interchain charge transfer kinetics. This highlights that grafted redox-active particles can enhance charge transport without the limitations of polymer-only solutions (e.g., chain entanglement) and presents a promising design strategy for high-performance electrochemical applications, such as redox flow batteries (RFBs).

Effect of Flow on Charge Transport in Semidilute Redox-Active Polymer Solutions

Redox-active polymers (RAPs) are polyelectrolytes that can undergo redox self-exchange reactions and can thus store charge. This ability makes them of interest as an electrolyte material in redox flow batteries due to their molecular size, chemical modularity, and ability to quickly charge and discharge. It is therefore important to understand how charge is transported at the molecular level, under different conditions such as RAP concentration, flow type, and flow strength. While previous efforts have explored these mechanisms in detail, they have primarily focused on charge transport dynamics in equilibrium or in dilute nonequilibrium situations. In this work, we seek to build upon these previous models by accounting for both nonequilibrium dynamics into semidilute RAP solutions and showing ways in which intermolecular interactions couple to strong flows to affect charge transport. Using recent advances in modeling multichain systems in flow, we show that for a single charge, both extensional and shear flow promote charge transport by extending the polymer conformation. This allows the charge to hop along a longer path along the same chain while also increasing the number of chain-chain collisions needed for interchain hopping. We also show that, when multiple charges are present in equilibrium, the charge transport decreases monotonically at all polymer concentrations but decreases the most below the overlap concentration. We attribute this to a decreased probability of chain-to-chain collisions, leading to a concomitant decrease in charge hopping with increasing charge fraction. Overall, we show how charge fraction, concentration, and flow strength couple to produce enhanced charge transport in concentrated solutions in both extensional and shear flows at semidilute concentrations.

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.

Assessing molecular doping efficiency in organic semiconductors with reactive Monte Carlo

The addition of molecular dopants into organic semiconductors (OSCs) is a ubiquitous augmentation strategy to enhance the electrical conductivity of OSCs. Although the importance of optimizing OSC-dopant interactions is well-recognized, chemically generalizable structure-function relationships are difficult to extract due to the sensitivity and dependence of doping efficiency on chemistry, processing conditions, and morphology. Computational modeling for an integrated OSC-dopant design is an attractive approach to systematically isolate fundamental relationships, but requires the challenging simultaneous treatment of molecular reactivity and morphology evolution. We present the first computational study to couple molecular reactivity with morphology evolution in a molecularly doped OSC. Reactive Monte Carlo is employed to examine the evolution of OSC-dopant morphologies and doping efficiency with respect to dielectric, the thermodynamic driving for the doping reaction, and dopant aggregation. We observe that for well-mixed systems with experimentally relevant dielectric constants, doping efficiency is near unity with a very weak dependence on the ionization potential and electron affinity of OSC and dopant, respectively. At experimental dielectric constants, reaction-induced aggregation is observed, corresponding to the well-known insolubility of solution-doped materials. Simulations are qualitatively consistent with a number of experimental studies showing a decrease of doping efficiency with increasing dopant concentration. Finally, we observe that the aggregation of dopants lowers doping efficiency and thus presents a rational design strategy for maximizing doping efficiency in molecularly doped OSCs. This work represents an important first step toward the systematic integration of molecular reactivity and morphology evolution into the characterization of multi-scale structure-function relationships in molecularly doped OSCs.