Versatile and readily available battery materials compatible with a range of electrode configurations and cell designs are desirable for renewable energy storage. Here we report a promising class of materials based on redox active colloids (RACs) that are inherently modular in their design and overcome challenges faced by small-molecule organic materials for battery applications, such as crossover and chemical/morphological stability. RACs are cross-linked polymer spheres, synthesized with uniform diameters between 80 and 800 nm, and exhibit reversible redox activity as single particles, as monolayer films, and in the form of flowable dispersions. Viologen-based RACs display reversible cycling, accessing up to 99% of their capacity and 99 ± 1% Coulombic efficiency over 50 cycles by bulk electrolysis owing to efficient, long-distance intraparticle charge transfer. Ferrocene-based RACs paired with viologen-based RACs cycled efficiently in a nonaqueous redox flow battery employing a simple size-selective separator, thus demonstrating a possible application that benefits from their colloidal dimensions. The unprecedented versatility in RAC synthetic and electrochemical design opens new avenues for energy storage.
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.
A fundamental question facing electrodes made out of few layers of graphene (FLG) is if they display chemical properties that are different to their bulk graphite counterpart. Here, we show evidence that suggests that lithium ion intercalation on FLG, as measured via stationary voltammetry, shows a strong dependence on the number of layers of graphene that compose the electrode. Despite its extreme thinness and turbostratic structure, Li ion intercalation into FLG still proceeds through a staging process, albeit with different signatures than bulk graphite or multilayer graphene. Single-layer graphene does not show any evidence of ion intercalation, while FLG with four graphene layers displays limited staging peaks, which broaden and increase in number as the layer number increases to six. Despite these mechanistic differences on ion intercalation, the formation of a solid-electrolyte interphase (SEI) was observed on all electrodes. Scanning electrochemical microscopy (SECM) in the feedback mode was used to demonstrate changes in the surface conductivity of FLG during SEI evolution. Observation of ion intercalation on large area FLG was conditioned to the fabrication of "ionic channels" on the electrode. SECM measurements using a recently developed Li-ion sensitive imaging technique evidenced the role of these channels in enabling Li-ion intercalation through localized flux measurements. This work highlights the impact of nanostructure and microstructure on macroscopic electrochemical behavior and provides guidance to the mechanistic control of ion intercalation using graphene, an atomically thin interface where surface and bulk reactivity converge.
The design of chemically stable and electrochemically reversible redox active polymers (RAPs) is of great interest for energy storage technologies. Particularly, RAPs are new players for flow batteries relying on a size-exclusion based mechanism of electrolyte separation, but few studies have provided detailed molecular understanding of redox polymers in solution. Here, we use a systematic molecular design approach to investigate the impact of linker and redox-pendant electronic interactions on the performance of viologen RAPs. We used scanning electrochemical microscopy, cyclic voltammetry, bulk electrolysis, temperature-dependent absorbance, and spectroelectrochemistry to study the redox properties, charge transfer kinetics, and self-exchange of electrons through redox active dimers and their equivalent polymers. Stark contrast was observed between the electrochemical properties of viologen dimers and their corresponding polymers. Electron self-exchange kinetics in redox active dimers that only differ by their tether length and rigidity influences their charge transfer properties. Predictions from the Marcus–Hush theory were consistent with observations in redox active dimers, but they failed to fully capture the behavior of macromolecular systems. For example, polymer bound viologen pendants, if too close in proximity, do not retain chemical reversibility. In contrast to polymer films, small modifications to the backbone structure decisively impact the bulk electrolysis of polymer solutions. This first comprehensive study highlights the careful balance between electronic interactions and backbone rigidity required to design RAPs with superior electrochemical performance.
Redox active polymers (RAPs) are electrochemically versatile materials that find key applications in energy storage, sensing, and surface modification. In spite of the ubiquity of RAP-modified electrodes, a critical knowledge gap exists in the understanding of the electrochemistry of soluble RAPs and their relation to polyelectrolyte dynamics. Here, we explore for the first time the intersection between polyelectrolyte behavior and the electrochemical response that highly soluble and highly substituted RAPs with viologen, ferrocene, and nitrostyrene moieties elicit at electrodes. This comprehensive study of RAP electrolytes over several orders of magnitude in concentration and ionic strength reveals distinct signatures consistent with surface confined, colloidal, and bulk-like electrochemical behavior. These differences emerge across polyelectrolyte packing regimes and are strongly modulated by changes in RAP coil size and electrostatic interactions with the electrode. We found that, unlike monomer motifs, simple changes in the ionic strength caused variations over 1 order of magnitude in the current measured at the electrode. In addition, the thermodynamics of adsorbed RAP films were also affected, giving rise to standard reduction potential shifts leading to redox kinetic effects as a result of the mediating nature of the RAP film in equilibrium with the solution. Full electrochemical characterization via transient and steady-state techniques, including the use of ultramicroelectrodes and the rotating disk electrode, were correlated to dynamic light scattering, ellipsometry, and viscometric analysis. These methods helped elucidate the relationship between electrochemical behavior and RAP coil size, film thickness, and polyelectrolyte packing regime. This study underscores the role of electrostatics in modulating the reactivity of redox polyelectrolytes.
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