Scanning Electrochemical Microscopy and Hydrodynamic Voltammetry Investigation of Charge Transfer Mechanisms on Redox Active Polymers

Abstract

We recently showed that viologen-based redox active polymers (RAPs) with molecular weights between 21 and 318 kDa are attractive charge storage materials as anolytes for size-selective non-aqueous redox flow batteries. Here, we characterize the electron transfer mechanisms of these RAPs, as well as a ferrocene based catholyte RAP, in acetonitrile/Li + electrolyte. We utilized scanning electrochemical microscopy (SECM) and rotating disk electrode (RDE) voltammetry to measure the rate of electron transfer and the rate of charge hopping between neighboring pendants along the insulating backbone of RAPs. The electron transfer kinetics of a 271 kDa ferrocene RAP mimic the facile kinetics of its monomer repeating unit. In contrast, viologen RAPs displayed RDE and SECM signatures that suggest a preceding chemical step to electron transfer. Viologen RAPs adsorb strongly to the electrode surface and create a redox active film that controls the rate of electron transfer via self-exchange. In addition, finite element simulations including a preceding chemical step demonstrated that a purely mass-transfer limited model is insufficient to recreate the viologen RAP feedback SECM response. Non-aqueous redox flow batteries (NRFBs) are emerging technologies for electrical energy storage, and are an attractive alternative to their aqueous counterparts. [1][2][3][4][5][6] The choice of organic solvents with voltage windows larger than that required for the electrolysis of water enables the use of a larger variety of redox molecules and electrolyte systems, where the combination of high solubility and higher reaction potentials leads to increased energy density. A major challenge in NRFBs is to increase the conductivity of the electrolyte through the commonly used ion exchange membrane. In response to this challenge, our groups recently reported on a new size-selective concept in flow batteries where a porous separator replaces poorly performing ion exchange membranes.7-9 These porous separators are coupled to high energy density redox active polymers (RAPs) that replaced small molecules as charge storage media. RAPs consist of an unconjugated polymer backbone densely decorated with redox active pendants. We previously reported on the electrochemical characterization of viologen-based RAPs with molecular weight between 21 and 318 kDa, which displayed hydrodynamic radii between 4 and 7 nm. Increasing the size of RAPs decreased transport through Celgard which was used as a porous separator. However, we are also interested in understanding the impact of size on the rate and mechanisms of charge transfer and transport on RAPs to determine their ultimate performance limit. 10RAPs displayed attractive electrochemical properties such as similar standard reduction potentials than those of the parent monomer, efficient bulk electrolysis with up to +98% of redox groups reversibly accessible, and high solubility of up to 2.8 M with quantitative electrode reactivity even when at 1.0 M. Despite these promising features, little is known about el...

Role of the Supporting Electrolyte Ions and Additives on the Electron Transport Properties of Electrografted Films Bearing Ferrocenyl Moieties

“…Considering the peak currents at 0.1 and 1 V s −1 (4.6 and 39.9 μA), as well as the quotient between 1 and 2, D e was estimated to be about 4.86×10 −11 cm 2 s −1 . This value is in the order of magnitude for different ferrocene polymers for which this parameter has been reported to be between 10 −9 to 10 −12 cm 2 s −1 , depending on the film thickness and structure [54–56] . More interconnected structures like metal‐organic frameworks have values in a wider range (10 −7 to 10 −12 cm 2 s −1 ) [57,58] …”

Redox-Active Polymers for Energy Storage Nanoarchitectonics

“…There have been not enough systematic studies, however, on the influence of the polymer solubility on battery performance. 107 Most recently in 2017, Jia et al reported the effect of molecular weight on the electrochemical properties of a redox-active polymer, poly(TEMPO methacrylate) (PTMA). 108 In this study, single electron transfer-living radical polymerization (SET-LRP) was utilized to synthesize poly(2,2,6,6-tetramethylpiperidine methacrylate) (PTMPM) with controllable molecular weight and low dispersity (Figure 6A).…”

Impact of Charge Transport Dynamics and Conditioning on Cycling Efficiency within Single Redox Active Colloids

“…[12,17,18] Charge hopping on redox-active polymers has been investigated in solution, [19][20][21] as films on modified electrodes, [21,22] and with isolated molecules. Electron transfer at the electrode-particle interface and subsequent charge hopping enables particle electrolysis via the movement of charge between the oxidized (V 2 + ), and the reduced state (V + ).…”