Mechanisms of Diffusive Charge Transport in Redox-Active Polymer Solutions

Abstract

Redox-active polymers (RAPs) have pendant groups that can change their charge state due to an electrochemical driving force. There has been an interest in using RAPs as a charge storage medium in redox flow batteries due to their ability to take on charge combined with their large macromolecular size. The performance of these batteries is in part tied to the transport of charge within these RAP solutions, and consequently there has been a recent effort to understand the physics governing charge diffusion in RAP systems. These efforts have highlighted the key role of both intra- and intermolecular charge transport mechanisms in governing their electrochemical response; however, little is known about how the molecular structure of polyelectrolyte solutions affects these proposed mechanisms. In this paper, we develop a coarse-grained, hybrid Brownian dynamics and kinetic Monte Carlo simulation to study charge transport in RAP solutions. We show how a number of different transport mechanisms interplay, including the intrapolymer transport of charge both along the chain via self-exchange transport and polymer segmental motions as well as hopping due to interpolymer collisions and translational diffusion of the chains themselves. We provide theoretical arguments to describe the diffusive motion of charge via these mechanisms, which match well with simulation results. Our predictions suggest the existence of three distinct regimes of charge transport, which distinguish between inter- and intramolecular processes and dilute and semidilute solutions.

Sequence-Controlled Electron Transport in Conjugated Copolymers

“…The localized polaron hopping regime assumes that intermonomer electronic couplings are weak relative to both electron−phonon coupling and site energy disorder, leading to charge carriers localized on individual monomers, with thermally activated hopping events driving transport. While this regime is likely applicable to polymers lacking full conjugation along the backbone, 36 in the absence of severe conformational disorder, the intermonomer electronic couplings in conjugated polymers typically exceed both electron−phonon couplings and site energy disorder, resulting in charge transport states delocalized over multiple monomers. 37 When transitions between delocalized eigenstates of a polymer chain are promoted via thermal fluctuations, we refer to this regime as delocalized polaron hopping.…”

Hybrid Amyloid‐Based Redox Hydrogel for Bioelectrocatalytic H2 Oxidation

“…Nonetheless, the implied D CT is still 2 to 3 orders of magnitude lower than what reported for a methyl viologen‐polyethylenimine hydrogel ( D CT =4.7×10 −9 cm 2 s −1 ) [29a] . A possible explanation involve inter‐wire charge transfer being hindered by limited surface contact and/or high distance; and the limited Brownian motion of large rigid nanofibers diminishing the collision frequency of redox sites [43, 44] …”

Hybrid Amyloid‐Based Redox Hydrogel for Bioelectrocatalytic H2 Oxidation

“…[29a] Ap ossible explanation involve inter-wire charge transfer being hindered by limited surface contact and/ or high distance;a nd the limited Brownian motion of large rigid nanofibers diminishing the collision frequency of redox sites. [43,44] Ad ifferent behavior was observed for modified EPPG electrodes (Figure 5b,d): compared to analogous GC electrodes,h igher current densities were recorded at all scanning rates and the amount of faradaic charge exchanged during the reduction of Vio ++ moieties was 8t imes higher (v = 100 mV s À1 ).…”

“…[29a] A possible explanation involve inter-wire charge transfer being hindered by limited surface contact and/or high distance; and the limited Brownian motion of large rigid nanofibers diminishing the collision frequency of redox sites. [43][44] A different behaviour was observed for modified EPPG electrodes (Figure 5 b, d): compared to analogous GC electrodes, higher current densities were recorded at all scanning rates and the amount of faradaic charge exchanged during the reduction of Vio ++ moieties was 8 times higher (v = 100 mV s -1 ). Also, linear regression between peak current density J and v ½ is less convincing (Figure S14b): instead J scales tightly with v up 100 mV s -1 as in a surface-confined electron transfer process (Figure 5 d), but the relationship breaks down at higher scanning rates.…”