Expanding the Dimensionality of Polymers Populated with Organic Robust Radicals toward Flow Cell Application: Synthesis of TEMPO-Crowded Bottlebrush Polymers Using Anionic Polymerization and ROMP

Recent Developments on Electroactive Organic Electrolytes for Non-Aqueous Redox Flow Batteries: Current Status, Challenges, and Prospects

The ever-increasing threat of climate change and the depletion of fossil fuel resources necessitate the use of solar- and wind-based renewable energy sources. Large-scale energy storage technologies, such as redox flow batteries (RFBs), offer a continuous supply of energy. Depending on the nature of the electrolytes used, RFBs are broadly categorized into aqueous redox flow batteries (ARFBs) and non-aqueous redox flow batteries (NARFBs). ARFBs suffer from various problems, including low conductivity of electrolytes, inferior charge/discharge current densities, high-capacity fading, and lower energy densities. NARFBs offer a wider potential window and range of operating temperatures, faster electron transfer kinetics, and higher energy densities. In this review article, a critical analysis is provided on the design of organic electroactive molecules, their physiochemical/electrochemical properties, and various organic solvents used in NARFBs. Furthermore, various redox-active organic materials, such as metal-based coordination complexes, quinones, radicals, polymers, and miscellaneous electroactive species, explored for NARFBs during 2012-2023 are discussed. Finally, the current challenges and prospects of NARFBs are summarized.

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices

Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.

Multimodal investigation of electronic transport in PTMA and its impact on organic radical battery performance

Organic radical batteries (ORBs) represent a viable pathway to a more sustainable energy storage technology compared to conventional Li-ion batteries. For further materials and cell development towards competitive energy and power densities, a deeper understanding of electron transport and conductivity in organic radical polymer cathodes is required. Such electron transport is characterised by electron hopping processes, which depend on the presence of closely spaced hopping sites. Using a combination of electrochemical, electron paramagnetic resonance (EPR) spectroscopic, and theoretical molecular dynamics as well as density functional theory modelling techniques, we explored how compositional characteristics of cross-linked poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) polymers govern electron hopping and rationalise their impact on ORB performance. Electrochemistry and EPR spectroscopy not only show a correlation between capacity and the total number of radicals in an ORB using a PTMA cathode, but also indicates that the state-of-health degrades about twice as fast if the amount of radical is reduced by 15%. The presence of up to 3% free monomer radicals did not improve fast charging capabilities. Pulsed EPR indicated that these radicals readily dissolve into the electrolyte but a direct effect on battery degradation could not be shown. However, a qualitative impact cannot be excluded either. The work further illustrates that nitroxide units have a high affinity to the carbon black conductive additive, indicating the possibility of its participation in electron hopping. At the same time, the polymers attempt to adopt a compact conformation to increase radical-radical contact. Hence, a kinetic competition exists, which might gradually be altered towards a thermodynamically more stable configuration by repeated cycling, yet further investigations are required for its characterisation.

Design of Microstructure-Engineered Polymers for Energy and Environmental Conservation

With the ever-growing demand for sustainability, designing polymeric materials using readily accessible feedstocks provides potential solutions to address the challenges in energy and environmental conservation. Complementing the prevailing strategy of varying chemical composition, engineering microstructures of polymer chains by precisely controlling their chain length distribution, main chain regio-/stereoregularity, monomer or segment sequence, and architecture creates a powerful toolbox to rapidly access diversified material properties. In this Perspective, we lay out recent advances in utilizing appropriately designed polymers in a wide range of applications such as plastic recycling, water purification, and solar energy storage and conversion. With decoupled structural parameters, these studies have established various microstructure-function relationships. Given the progress outlined here, we envision that the microstructure-engineering strategy will accelerate the design and optimization of polymeric materials to meet sustainability criteria.

Charge-transport kinetics of dissolved redox-active polymers for rational design of flow batteries

Charge-transport kinetics of redox-active polymers is essential in designing electrochemical devices. We formulate the homogeneous and heterogeneous charge-transfer processes of the redox-active polymers dissolved in electrolytes. The critical electrochemical parameters, the apparent diffusion coefficient of charge transport (D _app) and standard electrochemical reaction constant (k ⁰), are estimated by considering the physical diffusion D _phys of polymer chains (D _app, k ⁰ ∝ D _phys). The models are validated with previously reported compounds and newly synthesized hydrophilic macromolecules. Solution-type cells are examined to analyze their primary responses from the electrochemical viewpoints.

Key Features of TEMPO-Containing Polymers for Energy Storage and Catalytic Systems

The need for environmentally benign portable energy storage drives research on organic batteries and catalytic systems. These systems are a promising replacement for commonly used energy storage devices that rely on limited resources such as lithium and rare earth metals. The redox-active TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl) fragment is a popular component of organic systems, as its benefits include remarkable electrochemical performance and decent physical properties. TEMPO is also known to be an efficient catalyst for alcohol oxidation, oxygen reduction, and various complex organic reactions. It can be attached to various aliphatic and conductive polymers to form high-loading catalysis systems. The performance and efficiency of TEMPO-containing materials strongly depend on the molecular structure, and thus rational design of such compounds is vital for successful implementation. We discuss synthetic approaches for producing electroactive polymers based on conductive and non-conductive backbones with organic radical substituents, fundamental aspects of electrochemistry of such materials, and their application in energy storage devices, such as batteries, redox-flow cells, and electrocatalytic systems. We compare the performance of the materials with different architectures, providing an overview of diverse charge interactions for hybrid materials, and presenting promising research opportunities for the future of this area.

Electronic and Spintronic Open-Shell Macromolecules, Quo Vadis?

Open-shell macromolecules (i.e., polymers containing radical sites either along their backbones or at the pendant sites of repeat units) have attracted significant attention owing to their intriguing chemical and physical (e.g., redox, optoelectronic, and magnetic) properties, and they have been proposed and/or implemented in a wide range of potential applications (e.g., energy storage devices, electronic systems, and spintronic modules). These successes span multiple disciplines that range from advanced macromolecular chemistry through nanoscale structural characterization and on to next-generation solid-state physics and the associated devices. In turn, this has allowed different scientific communities to expand the palette of radical-containing polymers relatively quickly. However, critical gaps remain on many fronts, especially regarding the elucidation of key structure-property-function relationships that govern the underlying electrochemical, optoelectronic, and spin phenomena in these materials systems. Here, we highlight vital developments in the history of open-shell macromolecules to explain the current state of the art in the field. Moreover, we provide a critical review of the successes and bring forward open opportunities that, if solved, could propel this class of materials in a meaningful manner. Finally, we provide an outlook to address where it seems most likely that open-shell macromolecules will go in the coming years. Our considered view is that the future of radical-containing polymers is extremely bright and the addition of talented researchers with diverse skills to the field will allow these materials and their end-use devices to have a positive impact on the global science and technology enterprise in a relatively rapid manner.

Development of high-voltage bipolar redox-active organic molecules through the electronic coupling of catholyte and anolyte structures

All-organic non-aqueous redox flow batteries (O-NRFBs) are a promising technology for grid-scale energy storage. However, most examples of high-voltage (>2 V) O-NRFBs rely upon the use of distinct anolytes and catholytes separated by a membrane or porous separator which can result in crossover of redox active material from one side of the battery to the other. The resulting electrolyte mixing leads to irreversible reductions in energy density and capacity. A potentially attractive solution to overcome this crossover issue is the implementation of symmetric flow batteries where a single bipolar molecule functions as both an anolyte and a catholyte. Herein, we report the development of a new class of bipolar redox active materials for use in such symmetric flow batteries through the electronic coupling of phenothiazine catholytes and phthalimide anolytes. Such a strategy results in hybrid molecules possessing higher cell voltages than what could be obtained together by their uncoupled building blocks. Performance in flow batteries is demonstrated for two members of this new class of molecules, with the highest performing candidate featuring a ΔE of 2.31 V and demonstrating 93.6% average coulombic efficiency, 86.8% energy efficiency, and 68.6% capacity retention over the course of 275 charge–discharge cycles and 5 cell polarity reversals. Finally, the superior performance of symmetric O-NRFBs is experimentally confirmed by comparing these results to an asymmetric flow battery constructed with a distinct phenothiazine catholyte and a distinct phthalimide anolyte on opposing sides of the cell.

A new class of bipolar redox active molecules with enhanced voltages is reported via the electronic coupling of phthalimide anolytes and phenothiazine catholytes. Their performance is tested under relevant nonaqueous redox flow battery conditions.

Nitroxide radical polymers for emerging plastic energy storage and organic electronics: fundamentals, materials, and applications

Increasing demand for portable and flexible electronic devices requires seamless integration of the energy storage system with other electronic components. This ever-growing area has urged on the rapid development of new electroactive materials that not only possess excellent electrochemical properties but hold capabilities to be fabricated to desired shapes. Ideally, these new materials should have minimal impact on the environment at the end of their life. Nitroxide radical polymers (NRPs) with their remarkable electrochemical and physical properties stand out from diverse organic redox systems and have attracted tremendous attention for their identified applications in plastic energy storage and organic devices. In this review, we present a comprehensive summary of NRPs with respect to the fundamental electrochemical properties, design principles and fabrication methods for different types of energy storage systems and organic electronic devices. While highlighting some exciting progress on charge transfer theory and emerging applications, we end up with a discussion on the challenges and opportunities regarding the future directions of this field.

Redox-Active Aqueous Microgels for Energy Storage Applications

The search for new environmental-friendly materials for energy storage is ongoing. In the presented paper, we propose polymer microgels as a new class of redox-active colloids (RACs). The microgel stable colloids are perspective low-viscosity fluids for advanced flow batteries with high volumetric energy density. In this research, we describe the procedure for the anchoring of 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) redox-active sites to the polymeric chains of water-soluble microgels based on poly(N-isopropylacrylamide)-poly(acrylic acid) interpenetrating networks. Using cyclic voltammetry and EPR spectroscopy, we show that ca. 14% of 4-amino-TEMPO groups retain electroactive properties and demonstrate the reversible redox response. It allows achieving a stable capacity of 2.5 mAh/g, enabling the low-viscous catholyte with a capacity of more than 100 mAh/L.

Design, Synthesis, and Self-Assembly of Janus Bottlebrush Polymers

Janus bottlebrush polymers are a class of special molecular brushes, which have two immiscible side chains on the repeating unit of the backbone. The characteristic architectures of Janus bottlebrush polymers enable unique self-assembly properties and broad applications. Recently, remarkable advances of Janus bottlebrush polymers have been achieved for polymer chemistry and material science. This review summarizes the synthetic strategies of Janus bottlebrush polymers, and highlights the self-assembly applications. Finally, the challenges and opportunities are proposed for the further development.

A Methoxyamine-Protecting Group for Organic Radical Battery Materials-An Alternative Approach

An alternative synthetic route towards the widely employed electroactive poly(TEMPO methacrylate) (PTMA) via a thermally robust methoxyamine-protecting group is demonstrated herein. Protection of the radical moiety of hydroxy-TEMPO with a methyl functionality and subsequent esterification with methacrylic anhydride allows the high-yielding formation of the novel monomer methyl-TEMPO methacrylate (MTMA). The polymerization of MTMA to poly(MTMA) (PMTMA) is investigated via free radical polymerization and reversible addition-fragmentation chain-transfer polymerization (RAFT), a reversible-deactivation radical polymerization technique. Cleavage of the temperature-stable methoxyamine functionality by oxidative treatment of PMTMA with meta-chloroperbenzoic acid (mCPBA) releases the electroactive PTMA. The redox activity of PTMA was confirmed by cyclic voltammetry in lithium-ion coin cells.

Benzoquinone- and Naphthoquinone-Bearing Polymers Synthesized by Ring-Opening Metathesis Polymerization as Cathode Materials for Lithium-Ion Batteries

Organic electrode materials have attracted great interest for next-generation lithium-ion batteries owing to their merits of low cost, resource sustainability, and environmental friendliness. Dissolution in organic electrolyte is one of critical factors that limit their development, and constructing corresponding polymers is an effective way to prevent it. Herein, the synthesis of benzoquinone- and naphthoquinone-bearing polymers by ring-opening metathesis polymerization of monomers with an exo-type four-membered ring between polymerizable norbornene and redox-active quinone units is reported. They exhibit significantly reduced solubility and clearly enhanced electrochemical performance. In particular, a high capacity (189.7 mAh g^-1 at 0.1 C, 1 C=216.1 mA g^-1 ), stable cycling (75.6 % capacity retention after 500 cycles at 2 C), and good rate capability (retaining 80.4 % from 0.1 to 2 C) were obtained for the naphthoquinone-bearing polymer, which stand out among naphthoquinone-bearing polymer electrode materials. This work offers rational molecular design and a new polymerization strategy to construct high-performance polymer electrode materials.

Synthesis of a TEMPO-Substituted Polyacrylamide Bearing a Sulfonate Sodium Pendant and Its Properties in an Organic Radical Battery

A novel nitroxyl radical polymer poly(TEMPO-acrylamide-co-sodium styrene sulfonate) (abbreviated as poly(TAm-co-SSS)) was synthesized using 4-acrylamido-2,2,6,6- tetramethylpiperidine (AATP) copolymerized with styrene sulfonate sodium (SSS). AATP was synthesized through a substitution reaction of acryloyl chloride. Meanwhile, poly(4-acrylamido-2,2,6,6-tetramethylpiperidine-1-nitroxyl radical) (PTAm) was prepared as a control sample. Then, the structures of products were characterized by nuclear magnetic resonance spectroscopy (1H-NMR), Fourier transform infrared spectroscopy (FT-IR), high performance liquid chromatography-mass spectrometry (HPLC-MS), differential scanning calorimetry (DSC), X-Ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR), respectively. Additionally, the electrochemical impedance spectra (EIS) and the charge-discharge cycling properties were studied. The results demonstrated that the poly(TAm-co-SSS) with the side group of sodium sulfonate adjacent to TEMPO group exhibits a better charge-discharge cycling stability than that of the PTAm. Moreover, the charge specific capacity of the poly(TAm-co-SSS) is larger than that of the PTAm. Besides, the first coulombic efficiency of poly(TAm-co-SSS) is higher in comparison with that of PTAm. These superior electrochemical performances were ascribed to the synergistic effect of sulfonate ions group and nitroxyl radical structure, which benefits the improvement of charge carrier transportation of the nitroxyl radical polymers. Consequently, the nitroxyl radical poly(TAm-co-SSS) is promising for use in organic radical battery materials, based on the good electrochemical properties.

Grubbs' and Schrock's Catalysts, Ring Opening Metathesis Polymerization and Molecular Brushes-Synthesis, Characterization, Properties and Applications

In this review, molecular brushes and other macromolecular architectures bearing a bottlebrush segment where the main chain is synthesized by ring opening metathesis polymerization (ROMP) mediated by Mo or Ru metal complexes are considered. A brief review of metathesis and ROMP is presented in order to understand the problems and the solutions provided through the years. The synthetic strategies towards bottlebrush copolymers are demonstrated and each one discussed separately. The initiators/catalysts for the synthesis of the backbone with ROMP are discussed. Syntheses of molecular brushes are presented. The most interesting properties of the bottlebrushes are detailed. Finally, the applications studied by different groups are presented.

Long-Life and High-Power Binder-Free Cathode Based on One-Step Synthesis of Radical Polymers with Multi-Pendant Groups

The main bottlenecks for the widespread application of radical polymers in organic radical batteries are poor cycling stability, due to the dissolution of radical polymers into the electrolyte, and the low efficiency of multi-step synthesis strategies. Herein, a kind of electrolyte-resistant radical polymer bearing multi-pendant groups (poly(ethylene-alt-TEMPO maleate) (PETM)) is designed and synthesized through a one-step esterification reaction to graft 4-hydroxy-2,2,6,6-teramethylpiperidinyl-1-oxy into the commercially available poly(ethylene-alt-maleic anhydride). Interestingly, PETM is hardly soluble in the ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate-based electrolyte, showing an extremely low solubility of 0.59 mg mL^-1 , but is easily soluble in tetrahydrofuran and N-Methyl pyrrolidone. The derived binder-free PETM cathode exhibits nearly 100% utilization of the grafted nitroxide radicals (88 mA h g^-1 ) and excellent rate capability with almost invariant capacitance from 10 C to 40 C. Significantly, the PETM cathodes retain 94% of the initial capacity after 1000 cycles, outperforming most reported radical polymer-based cathodes.

Biodegradable Nanocomposite Antimicrobials for the Eradication of Multidrug-Resistant Bacterial Biofilms without Accumulated Resistance

Infections caused by multidrug-resistant (MDR) bacteria are a rapidly growing threat to human health, in many cases exacerbated by their presence in biofilms. We report here a biocompatible oil-in-water cross-linked polymeric nanocomposite that degrades in the presence of physiologically relevant biomolecules. These degradable nanocomposites demonstrated broad-spectrum penetration and elimination of MDR bacteria, eliminating biofilms with no toxicity to cocultured mammalian fibroblast cells. Notably, serial passaging revealed that bacteria were unable to develop resistance toward these nanocomposites, highlighting the therapeutic promise of this platform.

Molecular engineering of organic electroactive materials for redox flow batteries

With high scalability and independent control over energy and power, redox flow batteries (RFBs) stand out as an important large-scale energy storage system.

Nitroxide radical polymers – a versatile material class for high-tech applications

A comprehensive summary of synthetic strategies for the preparation of nitroxide radical polymer materials and a state-of-the-art perspective on their latest and most exciting applications.

Liquid Quinones for Solvent-Free Redox Flow Batteries

Liquid benzoquinone and naphthoquinone having diethylene glycol monomethyl ether groups are designed and synthesized as redox active materials that dissolve supporting electrolytes. The Li-ion batteries based on the liquid quinones using LiBF₄ /PC show good performance in terms of voltage, capacity, energy efficiency, and cyclability in both static and flow modes. A battery is constructed without using intentionally added organic solvent, and its high energy density (264 W h L^-1 ) demonstrates the potential of solvent-free organic redox flow batteries using liquid active materials.

The impact of the molecular weight on the electrochemical properties of poly(TEMPO methacrylate)

This work reports the synthesis of high molecular weight poly(TEMPO methacrylate) and the molecular weight influence on electrochemical properties.

Synthetic approaches towards structurally-defined electrochemically and (photo)redox-active polymer architectures

This review details synthetic strategies leading to structurally-defined electrochemically and (photo)redox-active polymer architectures,e.g.block, graft and end functionalized (co)polymers.

Redox-Flow Batteries: From Metals to Organic Redox-Active Materials

Research on redox-flow batteries (RFBs) is currently experiencing a significant upturn, stimulated by the growing need to store increasing quantities of sustainably generated electrical energy. RFBs are promising candidates for the creation of smart grids, particularly when combined with photovoltaics and wind farms. To achieve the goal of "green", safe, and cost-efficient energy storage, research has shifted from metal-based materials to organic active materials in recent years. This Review presents an overview of various flow-battery systems. Relevant studies concerning their history are discussed as well as their development over the last few years from the classical inorganic, to organic/inorganic, to RFBs with organic redox-active cathode and anode materials. Available technologies are analyzed in terms of their technical, economic, and environmental aspects; the advantages and limitations of these systems are also discussed. Further technological challenges and prospective research possibilities are highlighted.

Poly(TEMPO)/Zinc Hybrid-Flow Battery: A Novel, "Green," High Voltage, and Safe Energy Storage System

The combination of a polymer-based 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) catholyte and a zinc anode, together with a cost-efficient size-exclusion membrane, builds a new type of semi-organic, "green," hybrid-flow battery, which features a high potential range of up to 2 V, high efficiencies, and a long life time.

Modular Design of Poly(norbornenes) for Organelle-Specific Imaging in Tumor Cells

Through modular ROMP (ring-opening metathesis polymerization) directly from monomeric norbornenes of bioactive peptides, rhodamine B chromophore, and PEG solubilizer, we designed and synthesized a series of water-soluble poly(norbornenes) with organelle-specific imaging capability in tumor cells. For the selection of FxrFxK, TAT, and SV40 peptide sequences, these fluorescence probes exhibited different targeting specificity toward mitochondria, lysosome, and nucleolus, respectively, based on the same poly(norbornene) backbonds. More importantly, the ROMP strategy enables selective combination from various monomers and allows programmable biofunctionalization via peptide sequence permutations, which would greatly extend the biomedical applications such as imaging, diagnosis, and therapy for these synthetic polymers.

Stable organic radical polymers: synthesis and applications

We present an overview of the synthetic strategies and methodologies for stable organic radical polymers, and summarise their applications in diverse areas.

Polymer/zinc hybrid-flow battery using block copolymer micelles featuring a TEMPO corona as catholyte

A poly(TEMPO methacrylate)-poly(styrene) block copolymer was utilised as catholyte in polymer/zinc hybrid flow batteries.

Switchable two-photon imaging of RGD-functionalized polynorbornenes with enhanced cellular uptake in living cells

Two-photon imaging polynorbornenes were fabricated directly from photochromic spiropyran, RGD peptides and hydrophilic PEG monomers via modular ROMP.