Radical Polymer Research Articles

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.

Three-dimensional porous radical polymer/reduced graphene oxide composite with two-electron redox reactions as high-performance cathode for lithium-ion batteries

Organic cathode materials, especially stable radical polymers, have been considered as ideal substitutes to inorganic cathode for rechargeable lithium batteries (LIBs) due to their fast electron transfer kinetics, tunable electrochemical properties, facile solution processing and environmental sustainability. Unfortunately, unsatisfied electronic conductivity of polymers leads to poor practical capacity, low utilization efficiency, and hinder their application for LIBs. In this work, a three-dimensional porous electrode composed by poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) and reduced graphene oxide (PTMA/rGO) is prepared via a simple dissolution-assembly-depositing method. Since the 3D porous structure effectively promotes the electrolytes penetration and facilitates electron conduction pathways, satisfactorily, the PTMA/rGO electrode shows an obvious two-electron redox reactions and delivers the highest reversible capacity of 153 mA h g−1 at 20 mA g−1 with an ultrahigh utilization efficiency of 98.8%, which is much higher than that of PTMA/CB electrode prepared by the traditional physical mixing method (104 mA h g−1, utilization efficiency of 66.1%). Meanwhile, the PTMA/rGO electrode also exhibits excellent rate performance and cycle stability. This kind of high performance PTMA/rGO electrode with simple preparation method provides a broad application prospect of the next generation of high performance LIBs based on organic electrode.

Multiple-stimuli-responsiveness and conformational inversion of smart supramolecular nanoparticles assembled from spin labeled amphiphilic random copolymers

HypothesisOrganic radical polymers with tailored pendant functionalities have emerged as exciting and promising materials for their application versatility. Moreover, eco-friendly polymer-based organic nanomaterials with redox-active pendant side groups can replace the harmful heavy metal-based inorganic materials. On the other hand, self-assembled nanomaterials are of great interest and attracted more attention recently for their promising application in different advanced fields, but it is yet challenging to predict suitable hydrophilic-lipophilic balance (HLB) for stimuli-responsive random copolymers assembly due to structural irregularity. Among several experimental techniques, electron paramagnetic resonance (EPR) spectroscopy plays a unique and promising role in revealing structural and dynamic information of nanostructured radical containing materials. ExperimentsIn this study, a series of spin labeled amphiphilic random copolymers poly(methyl methacrylate-co-acrylic acid) have been synthesized and characterized by FT-IR, UV–Vis spectroscopies, TGA, DSC and water contact angle (CA) techniques. Their electrochemical properties have been determined by cyclic voltammetry (CV) in different organic solvents. EPR spectroscopy has been applied with other analytical techniques to elucidate the smart supramolecular nanoparticles (SNPs) formation, stimuli-responsiveness and structural changes through the dynamics of different molecular interactions. FindingsThe structural and dynamic information of self-assembled nanoparticles have been observed to be dependent on multiple-stimuli-responsiveness in different microenvironments by applying physiological and chemical parameters such as the different concentration of radicals, pH, temperature, nature of the solvent and reducing agent. The obtained results reveal the knowledge to understand insight into the mechanism for the formation of stimuli-responsive colloidal nanoparticles assembled from amphiphilic random copolymers with apt HLB value. The CV results reveal that the charge transfer process of the nanoparticles in solution was diffusion regulated and depended on the accessibility of radicals. The radical (spin labeled) polymers offer a broad way to develop stimuli-responsive materials in various colloidal nanostructures by changing the microenvironment, appreciating their potential advanced applications in electronic devices, catalysis, stimuli-triggered drug/gene delivery and reactive oxygen species (ROS) scavenger.

Triazole-enabled small TEMPO cathodes for lithium-organic batteries

The use of redox-active organic compounds to make rechargeable batteries is a promising strategy for future energy storage especially from a resource and environmental sustainability point of view. While many organic structures have been identified as potential electroactive materials, their direct application in solid-state energy storage is limited. This is due to their high solubility in common organic electrolytes, resulting in poor long-term battery performance. In this work, we overcome this significant problem by synthesizing very low soluble nitroxide radicals with theoretical capacities of 111–131 mAh g−1, equal to or higher than most commonly used nitroxide radical polymers. These new structures consist of TEMPO (i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy) radicals and triazole ring linkages synthesized by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) ‘click’ reaction. Analysis of electrode morphology and electrochemical performance at increasing mass loading of the nitroxide radicals demonstrate the importance of lower solubility, preventing the shuttle effect observed for common organic electrode materials. The electrodes with the content of active materials up to 80 wt% can still maintain a good interaction with the carbon and conductivity throughout the electrode and deliver a capacity greater than 60 mAh g−1 over 200 cycles.

Radical polymer-grafted carbon nanotubes as high-performance cathode materials for lithium organic batteries with promoted n-/p-type redox reactions

Here we present the covalently grafting organic radical polymer, poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) (PTMA), onto the surface of multi-walled carbon nanotubes (MWNT) via free radical polymerization, affording MWNT-g-PTMA. The free-standing MWNT-g-PTMA-based electrode exhibits a high specific capacity of 262.9 mAh g−1 and a >60% capacity retention (144 mAh g−1) after 250 cycles, superior to that of physically mixed MWNT/PTMA electrode. Investigating the charge/discharge process by ex situ electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS), the excellent electrochemical performance of MWNT-g-PTMA-based electrode is attributed to the mutual participation of both p- and n-type redox reactions of PTMA with both cationic and anionic ions insertion/extraction into/from the electrode. Moreover, the capacity fading mainly results from irreversible side reactions during the n-type doping process of PTMA as investigating the charge/discharge process in two separate voltage ranges. This study may contribute to the understanding of energy storage mechanisms of radical polymers in the right way.

A Review of Battery Materials as CDI Electrodes for Desalination

The world is suffering from chronic water shortage due to the increasing population, water pollution and industrialization. Desalinating saline water offers a rational choice to produce fresh water thus resolving the crisis. Among various kinds of desalination technologies, capacitive deionization (CDI) is of significant potential owing to the facile process, low energy consumption, mild working conditions, easy regeneration, low cost and the absence of secondary pollution. The electrode material is an essential component for desalination performance. The most used electrode material is carbon-based material, which suffers from low desalination capacity (under 15 mg·g−1). However, the desalination of saline water with the CDI method is usually the charging process of a battery or supercapacitor. The electrochemical capacity of battery electrode material is relatively high because of the larger scale of charge transfer due to the redox reaction, thus leading to a larger desalination capacity in the CDI system. A variety of battery materials have been developed due to the urgent demand for energy storage, which increases the choices of CDI electrode materials largely. Sodium-ion battery materials, lithium-ion battery materials, chloride-ion battery materials, conducting polymers, radical polymers, and flow battery electrode materials have appeared in the literature of CDI research, many of which enhanced the deionization performances of CDI, revealing a bright future of integrating battery materials with CDI technology.

Redox chemistry of nitrogen-doped CNT-encapsulated nitroxide radical polymers for high energy density and rate-capability organic batteries

To enhance the electrochemical performance and identify the redox reaction of a nitroxide radical-based organic battery, poly(2,2,6,6-tetramethylpiperidinyloxy-4-vinylmethacrylate) (PTMA) as nitroxide radical polymer is synthesized and used to fill the central pore of nitrogen-doped carbon nanotubes (NCNTs). The PTMA-filled NCNTs demonstrate high rate capability, stable cycling, and high electrode stability, owing to their high charge transfer and ion diffusion; further, they prevent the dissolution of organic active material. The nitrogen doping on CNTs improves the electrochemical properties by created defects. The PTMA-filled NCNT cells have a high reversible capacity of 199.8 mAh g−1 and high energy density of 489.4 Wh kg−1 on electrode level at 0.2C with a two-step redox couple. Moreover, the cells with the two-step redox couple exhibit excellent stable cycle performance with high capacity retention and Coulombic efficiency for 3000 cycles. The nitroxide radical of PTMA can be converted to either an oxoammonium cation or an aminoxy anion; further, the redox reaction of each step exhibits different performance. The nitroxide radical–oxoammonium cation redox step in NCNTs shows a high rate capability owing to the high charge transfer rate and ion diffusion coefficient. The unusual results will help further improve the properties and commercialization of rechargeable organic batteries.

Conductive nonconjugated radical polymer as high capacity organic cathode material for high-energy Li/Na ion batteries

A nonconjugated radical polymer poly (4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) is explored as organic cathode material in Li and Na ion batteries for the first time. The PTEO exhibits a high electrical conductivity of 0.13 S m −1, far exceeding other organic cathode materials ranging from 10 to 10 to 10-5 S m −1. It offers high redox potentials with platueas at ~3.6 V vs. Li +/Li and 3.3 V vs. Na +/Na, respectively. The PTEO cathode with only 30 wt% carbon additive agent delivers a capacity of over 200 mAh g-1 and energy density of more than 500 Wh kg-1 for both Li/Na ion batteries. Reaction mechanism of the PTEO proceeds through the neutral state nitroxide radical (N–O·) to oxoammonium cation (+NO) and the aminoxy anion (N–O -). The PTEO is the only radical polymer that can deliver a capacity over 200 mAh g −1 for organic cathodes with no more than 50% carbon additive agent, making it a promising organic cathode material for Li/Na ion batteries.

Impact of PSBpin Content on the Electrochemical Properties of PTMA-PSBpin Copolymer Cathodes

The class of radical polymers is one of the most appealing electrode materials in organic radical batteries (ORBs). Herein, we report a series of copolymers poly(2,2,6,6-tetramethylpiperidinyloxyl-...

Single- and double-redox reaction of poly(2,2,6,6-tetramethylpiperidinyloxy-4-vinylmethacrylate)/ordered mesoporous carbon composite nitroxide radical polymer battery

Rechargeable organic batteries constitute a promising alternative to lithium ion batteries because of the abundance of the necessary raw materials, their lightness, environmental friendliness, high flexibility, and high capacity. However, organic batteries present problems of low conductivity and the instability of their liquid electrolytes. PTMA-impregnated CMK-3 (PTMA/CMK-3) is successfully used as a novel electrode to solve the problems associated with organic batteries. The PTMA/CMK-3 has a theoretical capacity of 111 mAh g−1 with a single-step, single-redox reaction between the nitroxide radical and oxoammonium cation (3.0–4.0 V cut-off) and a theoretical capacity of 222 mAh g−1 with a two-step double-redox reaction between the aminoxy anion and oxoammonium cation (1.5–4.0 V cut-off). Although the PTMA/CMK-3 cells deliver an initial discharge capacity of 110.9 mAh g−1 with the single-redox reaction and 219.8 mAh g−1 with the double-redox at 1 C-rate (which is almost equal to the theoretical capacity), the capacity retentions at the 1000th cycle are 80.7% and 29.3%, respectively. The high degradation associated with the double-redox reaction is ascribed to the irreversible reaction of the aminoxy anion. The resolution of the irreversibility of the aminoxy anion could lead to the commercialization of nitroxide radical-based organic batteries.

Radical Polymer Grafting on the Surface of Single-Walled Carbon Nanotubes Enhances Photoluminescence in the Near-Infrared Region: Implications for Bioimaging and Biosensing

Photoluminescence (PL) of single-walled carbon nanotubes (SWCNTs) in the near-infrared (NIR) region (E11 emission) is attractive, especially for bioimaging and biosensing in vivo because of the goo...

TEMPO‐Contained Polymer Grafted onto Graphene Oxide via Click Chemistry as Cathode Materials for Organic Battery

AbstractPoly(2,2,6,6‐tetramethylpiperidin‐1‐oxyl‐4‐yl methacrylate) (PTMA) is a typical organic battery material containing nitrogen and oxygen radicals. The grafting of PTMA onto graphene oxide to obtain composite cathode material poly(2,2,6,6‐tetramethylpiperidin‐1‐oxyl‐4‐yl methacrylate)‐Graphene Oxide (PTMA‐GO) via click chemistry is reported. The method of preparing PTMA‐GO is completed at room temperature, which can retain the oxygen‐containing functional groups on the surface of graphene oxide, and the process is simple and easy to conduct. The composite materials are characterized by infrared, X‐ray diffraction, thermogravimetry, and electron paramagnetic resonance. The cathode material is assembled in a button battery to measure its cyclic voltammogram, electrochemical impedance spectroscopy, and charge and discharge capacity cycles. It is found that the PTMA‐GO electrode is twice as conductive as the PTMA electrode alone. The specific capacity of the PTMA‐GO electrode is nearly 2.3 times higher than that of the PTMA electrode after 300 charge–discharge cycles. This represents an important reference value for the development of organic batteries, especially for the research of nitroxide radical polymer electrode materials.

Monoradically luminescent polymers by a super acid-catalyzed polymerization and deep-red electroluminescence

Luminescent radicals have received great attention recently due to their idiographic luminescent properties and potential 100% utilization efficiency of doublets under electrical excitation. However, luminescent radical polymers are rarely explored owing to their challenging molecular design and synthesis. Herein, we report an efficient approach to construct luminescent radical polymers by a super acid-catalyzed polymerization reaction proceeded at room temperature, which intrinsically avoid the heavy metal catalyst. The obtained polymers exhibit the unique paramagnetic signals, good thermal properties, and excellent photo-stability. Moreover, the quantifiable electroluminescence of such radical polymers was demonstrated for the first time.

Air-Stable Organic Radicals: New-Generation Materials for Flexible Electronics?

In the last few years, air-stable organic radicals and radical polymers have attracted tremendous attention due to their outstanding performance in flexible electronic devices, including transistors, batteries, light-emitting diodes, thermoelectric and photothermal conversion devices, and among many others. The main issue of radicals from laboratory studies to real-world applications is that the number of known air-stable radicals is very limited, and the radicals that have been used as materials are even less. Here, the known and newly developed air-stable organic radicals are summarized, generalizing the way of observing air-stable radicals. The special electric and photophysical properties of organic radicals and radical polymers are interpreted, which give radicals a wide scope for various of potential applications. Finally, the exciting applications of radicals that have been achieved in flexible electronic devices are summarized. The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.

Mixed Ionic and Electronic Conduction in Radical Polymers

Organic mixed ionic and electronic conductors typically have heterogeneous conjugated macromolecular backbones and ether-based pendant groups to transport ions and charges. Moving from this archety...

100th Anniversary of Macromolecular Science Viewpoint: Fundamentals for the Future of Macromolecular Nitroxide Radicals.

Macromolecular radicals, radical polymers, and polyradicals bear unique functionalities derived from their pendant radical groups. The increasing need for organic functional materials is driving the growth in research interest in macromolecular radicals for batteries, electronics, memory, and imaging. This Viewpoint summarizes the current state-of-knowledge regarding the macromolecular nitroxide radicals' redox mechanism, conductivity, chain conformation, controlled polymerization, network structure, conjugated forms, and applications. The nitroxide radical group is the focus because it is the most widely studied. Although most literature focuses upon applications, an emerging body of work is highlighting the fundamental physicochemical properties of macromolecular radicals. To this end, this Viewpoint recommends areas of opportunity in fundamental studies and best practices in reporting.

Novel membrane-free hybrid capacitive deionization with a radical polymer anode for stable desalination

Currently, limited charge efficiency and poor long-term desalination stability remain the major challenges for the practical application of membrane-free capacitive deionization (CDI). Hybrid capacitive deionization (HCDI) is suggested to effectively address these issues. Although cathode-hybridized HCDI has been studied widely, anode-hybridized HCDI has been seldom reported to date. In this work, a radical polymer, poly (2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), with good anion-storage capability was used as the anode of the membrane-free HCDI (denoted as A-HCDI) for the first time. PTMA restrains the undesirable faradaic reactions that occur on the conventional carbon anodes and suppresses the cation expulsion effect that dominates the charging process of the carbon anode after the long-term oxidation, degrading the membrane-free CDI process. Compared with AC anode, PTMA anode improves the average charge efficiency from 18.7% to 48.6% in the first 20 cycles at 1.2 V. Moreover, A-HCDI exhibits a long-term stable salt removal capacity of 13.9 mg g−1 during charging/discharging at 1.2/0 V for 500 h, while membrane-free CDI only remains effective before 70 h. The novel A-HCDI provides a new approach for efficient and stable membrane-free electrochemical desalination.

Aprotic and Protic Ionic Liquids as Electrolytes for Organic Radical Polymers

In this study we systematically investigated the impact of aprotic and protic ionic liquids on the electrochemical behavior of poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) based electrodes. We showed that the use of aprotic ILs based on the FSI− anion enables the realization of electrodes able to display high capacity, good capacity retention at high current density (42% at 50 C), low self-discharge and high stability during float tests. We also reported, for the first time, that it is possible to successfully use PTMA-based electrodes in combination with PILs. We showed that the PTMA electrodes display good capacity but a limited cycling stability in these ionic liquids. This investigation confirms that ionic liquids are very interesting candidates for the realization of high performance PTMA-based electrodes. A deeper understanding of the interactions taking place between ionic liquids and organic active materials will be required to finely tune the properties of these promising electrolytes for an application in organic radical polymer-based energy storage systems.

Redox polymers incorporating pendant 6‐oxoverdazyl and nitronyl nitroxide radicals

ABSTRACTPolymers comprised of redox‐active organic radicals have emerged as promising materials for use in a variety of organic electronics, including fast‐charging batteries. Despite these advances, relatively little attention has been focused on the diversification of the families of radicals that are commonly incorporated into polymer frameworks, with most radical polymers being comprised of nitroxide radicals. Here, we report two new examples prepared via ring‐opening methathesis polymerization containing 6‐oxoverdazyl and nitronyl nitroxide radicals appended to their backbones. The polymerization reaction and optoelectronic properties were explored in detail, revealing high radical content and redox activity that may be advantageous for their use as semiconducting thin films. Initial studies revealed that current–voltage curves obtained from thin films of the title polymers exhibited memory effects making them excellent candidates for use in resistive memory applications. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 309–319

Quantifying internal charge transfer and mixed ion-electron transfer in conjugated radical polymers.

Macromolecular radicals are receiving growing interest as functional materials in energy storage devices and in electronics. With the need for enhanced conductivity, researchers have turned to macromolecular radicals bearing conjugated backbones, but results thus far have yielded conjugated radical polymers that are inferior in comparison to their non-conjugated partners. The emerging explanation is that the radical unit and the conjugated backbone (both being redox active) transfer electrons between each other, essentially “quenching” conductivity or capacity. Here, the internal charge transfer process is quantified using a polythiophene loaded with 0, 25, or 100% nitroxide radicals (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO]). Importantly, deconvolution of the cyclic voltammograms shows mixed faradaic and non-faradaic contributions that contribute to the internal charge transfer process. Further, mixed ion-electron transfer is determined for the 100% TEMPO-loaded conjugated radical polymer, from which it is estimated that one triflate anion and one propylene carbone molecule are exchanged for every electron. Although these findings indicate the reason behind their poor conductivity and capacity, they point to how these materials might be used as voltage regulators in the future.