Redox Active Polymers as Soluble Nanomaterials for Energy Storage

Abstract

It is an exciting time for exploring the synergism between the chemical and dimensional properties of redox nanomaterials for addressing the manifold performance demands faced by energy storage technologies. The call for widespread adoption of alternative energy sources requires the combination of emerging chemical concepts with redesigned battery formats. Our groups are interested in the development and implementation of a new strategy for nonaqueous flow batteries (NRFBs) for grid energy storage. Our motivation is to solve major challenges in NRFBs, such as the lack of membranes that simultaneously allow fast ion transport while minimizing redox active species crossover between anolyte (negative electrolyte) and catholyte (positive electrolyte) compartments. This pervasive crossover leads to deleterious capacity fade and materials underutilization. In this Account, we highlight redox active polymers (RAPs) and related polymer colloids as soluble nanoscopic energy storing units that enable the simple but powerful size-exclusion concept for NRFBs. Crossover of the redox component is suppressed by matching high molecular weight RAPs with simple and inexpensive nanoporous commercial separators. In contrast to the vast literature on the redox chemistry of electrode-confined polymer films, studies on the electrochemistry of solubilized RAPs are incipient. This is due in part to challenges in finding suitable solvents that enable systematic studies on high polymers. Here, viologen-, ferrocene- and nitrostyrene-based polymers in various formats exhibit properties that make amenable their electrochemical exploration as solution-phase redox couples. A main finding is that RAP solutions store energy efficiently and reversibly while offering chemical modularity and size versatility. Beyond the practicality toward their use in NRFBs, the fundamental electrochemistry exhibited by RAPs is fascinating, showing clear distinctions in behavior from that of small molecules. Whereas RAPs conveniently translate the redox properties of small molecules into a nanostructure, they give rise to charge transfer mechanisms and electrolyte interactions that elicit distinct electrochemical responses. To understand how the electrochemical characteristics of RAPs depend on molecular features, including redox moiety, macromolecular size, and backbone structure, a range of techniques has been employed by our groups, including voltammetry at macro- and microelectrodes, rotating disk electrode voltammetry, bulk electrolysis, and scanning electrochemical microscopy. RAPs rely on three-dimensional charge transfer within their inner bulk, which is an efficient process and allows quantitative electrolysis of particles of up to ∼800 nm in radius. Interestingly, we find that interactions between neighboring pendants create unique opportunities for fine-tuning their electrochemical reactivity. Furthermore, RAP interrogation toward the single particle limit promises to shed light on fundamental charge storage mechanisms.

Cation‐Anion Redox Switching in an All‐Ferrocene Salt

“…Investigations into ferrocene compounds as redox active materials continue to grow. [1][2][3][4][5][6][7][8] Recently, attention has turned to the use of ferrocenes as possible components in redox flow battery (RFB) applications, [9][10][11][12][13][14][15][16][17][18][19] arising from the desire to produce new chemical compounds for the low cost and scalable storage of energy. Among transition metal-based compounds, ferrocenes have many notable properties as RFB components, ranging from their relative low cost and ease of synthesis to their stabilities and electrochemical reversibility in a variety of media.…”

Review Article: Flow battery systems with solid electroactive materials

“…77 More detailed discussion on organic active materials and redox polymers can be found in recent reviews. 22,65,73,207 One challenge facing dissolved organic molecules as active species is membrane crossover, which reduces cell capacity and operating lifetime. One option to mitigate this issue is to design new membrane materials; however, Montoto et al proposed instead using redox active colloids (RAC) as the active materials to address this issue.…”

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

“…[2,[7][8][9] These properties have driven a 'renaissance' in flow battery development. [2,8,11] Redox active colloids (RACs), are emerging energy storage materials for size-exclusion redox flow batteries (SE-RFBs). [2,8,11] Redox active colloids (RACs), are emerging energy storage materials for size-exclusion redox flow batteries (SE-RFBs).…”

“…[10] Much of the recent effort has focused on new materials and system designs that enable a transition into organic solvents with higher energy densities. [7,[11][12][13][14] With billions of redox sites per particle, RACs attain high capacity per molecule and high structural tunability while exhibiting molecular-like electrochemical properties. [7,[11][12][13][14] With billions of redox sites per particle, RACs attain high capacity per molecule and high structural tunability while exhibiting molecular-like electrochemical properties.…”

“…[12] In SE-RFBs, RAC size is leveraged towards decreasing species crossover across nanoporous separators while alleviating the poor conductivity displayed by nonaqueous solvents. [11,16] Here we elucidate the properties of RACs for applications in SE-RFBs via single particle experiments. However, the benefits and limitations of using a flowable particle for energy storage have not been fully elucidated.…”