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.

Non-aqueous redox flow batteries (NRFBs) are emerging technologies that promise higher energy densities than aqueous counterparts. Unfortunately, cell resistance and redox component crossover observed when using ion-exchange membranes (IEMs) hinders NRFB development. The size exclusion approach for polymer-based NRFBs addresses these issues by using macromolecular design to mitigate crossover. Here, we highlight the benefits of this approach using inexpensive nano-porous separators (PS) (Celgard and Daramic). We evaluated these along with an IEM (Fumasep) in a flow cell configuration using a classical redox couple of viologen and ferrocene, both in monomer and polymer forms. High Coulombic efficiencies above 98% and access up to 80% of capacity were observed for the polymer cells. These displayed better performance with PS than with the IEM, which exhibited lower energy efficiencies from higher overpotentials. The monomer equivalent cells with PS resulted in lower efficiencies and rapid decrease in depth of discharge. Post-cycling analysis by ultramicroelectrode voltammetry showed that the small molecules freely crossed PS and to a lesser degree through the IEM. Therefore, here we demonstrate that the combination of redox active polymers and simple PS enables a potential next-generation NRFB system that provides a competitive alternative to the use of IEMs in NRFBs. Renewable energy sources, such as wind and solar, are emerging inputs in the electrical grid.1 However, these sources are inherently intermittent, thus strategies designed to decouple energy production from its use are highly desirable. Redox flow batteries (RFBs) promise to address these challenges by offering convenient charge storage in fluid form with straightforward scalability and deployment, and long-term durability.2-4 In particular, non-aqueous redox flow batteries (NRFBs) promise higher energy densities than aqueous RFBs owing to a wider electrochemical window. 5,6 NRFBs provide a wide range of molecular and electrolyte designs with attractive redox potentials, solubilities and chemical tunability.6-11 Yet, their wide-scale adoption has been hampered by the lack of chemically-suitable ion-exchange membranes (IEMs) that decrease areal resistance while simultaneously preventing material crossover between compartments. 6,8,12,13 Most commercially developed RFBs use IEMs, 14 such as perfluorinated Nafion, 12 due to its robustness, stability and suitable performance in aqueous environments. IEMs are typically the membrane of choice since these are non-porous sheets of crosslinked polyelectrolytes capable of exchanging ions at its interface with low levels of solution mixing.12 Crossover in those cases is typically ascribed to varying levels of ion selectivity 12,14 in the membranes. However, recent reports suggest low conductivity of these membranes in typical non-aqueous battery solvents (e.g. propylene carbonate, acetonitrile, etc.).13,15 A possible solution is to explore membrane chemistries that provide highly-conductive IEMs tailored to the...

Here we show how to design organic redox-active solutions for use in redox-flow batteries, with an emphasis on attaining high volumetric capacity electrodes that minimize active-material crossover through the flow cell’s membrane. Specifically, we advance oligoethylene oxides as versatile core motifs that grant access to liquid redox-active oligomers having infinite miscibility with organic electrolytes. The resulting solutions exhibit order-of-magnitude increases in volumetric capacity and obviate deleterious effects on redox stability. The design is broadly applicable, allowing both low potential and high potential redox centers to be appended to these core motifs, as demonstrated by benzofurazan, nitrobenzene, 2,2,6,6-tetramethylpiperidin-1-yl)­oxyl, and 2,5-di-tert-butyl-1-methoxy-4-(2′-methoxy)­benzene pendants, whose reduction potentials range from −1.87 to 0.76 V vs Ag/Ag+ in acetonitrile. Notably, the oligoethylene oxide scaffold minimizes membrane crossover relative to redox-active small molecules, while also providing mass- and electron-transfer kinetic advantages over other macromolecular architectures. These characteristics collectively point toward new opportunities in grid-scale energy storage using all-organic redox-flow batteries.

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

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.

Few reports to date have focused on the chemical and electrochemical reversibility of redox pendants assembled into soluble redox-active polymers (RAPs). Here we report a series of soluble RAPs for flow battery applications designed with cyclopropenium (CP) pendants. The tether length between CP and a polystyrene backbone was varied and found to influence electrochemical activity and stability. Different tether lengths of x methylene groups (x = 1–7) were simulated, and x = 1, 5, and 7 were synthesized to evaluate experimentally. This study illustrates that polymers with extended tether groups display an improved reversibility in cyclic voltammetry. The behavior is mirrored in the stability of the charged state tested in galvanostatic half-cells. When paired with a viologen polymer, these CP-based polymers produce a 1.55 V nonaqueous flow battery. The capacity decays for the polymers were structure-dependent, which provides empirical insight into materials design for high potential catholyte polymers.

Donor–acceptor (D–A) polymers are promising materials for organic electronics because of high charge carrier mobilities and narrow band gaps. Despite recent progress, there is incomplete understanding of the mechanisms underlying charge transport in these materials. In this work, we use single molecule techniques to study intrachain charge transport in D–A oligomers containing alternating diketopyrrolopyrrole (DPP) acceptor and bithiophene donor units. Interestingly, at high applied bias, longer DPPTT oligomers exhibit substantially higher conductance compared to shorter oligomers, which is interpreted using density functional theory (DFT) simulations. Overall, this work provides an increased understanding of intrachain charge transport along D–A oligomers.

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