Redox-active Polymers Research Articles

Unexpected Direct Synthesis of Tunable Redox-Active Benzil-Linked Polymers via the Benzoin Reaction.

Strategies for the sustainable synthesis of redox-active organic polymers could lead to next-generation organic electrode materials for electrochemical energy storage, electrocatalysis, and electro-swing chemical separations. Among redox-active moieties, benzils or aromatic 1,2-diones are particularly attractive due to their high theoretical gravimetric capacities and fast charge/discharge rates. Herein, we demonstrate that the cyanide-catalyzed polymerization of simple dialdehyde monomers unexpectedly leads to insoluble redox-active benzil-linked polymers instead of the expected benzoin polymers, as supported by solid-state nuclear magnetic resonance spectroscopy and electrochemical characterization. Mechanistic studies suggest that cyanide-mediated benzoin oxidation occurs by hydride transfer to the solvent, and that the insolubility of the benzil-linked polymers protects them from subsequent cyanolysis. The thiophene-based polymer poly(BTDA) is an intriguing organic electrode material that demonstrates two reversible one-electron reductions with monovalent cations such as Li+ and Na+ but one two-electron reduction with divalent Mg2+. As such, the tandem benzoin-oxidation polymerization reported herein represents a sustainable method for the synthesis of highly tunable and redox-active organic materials.

Mechanistic Insights into Oxidative Molecular Layer Deposition of Conjugated Polymers

Oxidative molecular layer deposition (oMLD) promises to enable molecular-level control of polymer structure through monomer-by-monomer growth via sequential, self-limiting, gas-phase surface reactions of monomer(s) and oxidant(s). However, only a few oMLD growth chemistries have been demonstrated to date, and limited mechanistic understanding is impairing progress in this field. Here, we examine oMLD growth using 3,4-ethylenedioxythiophene (EDOT), pyrrole (Py), p-phenylenediamine (PDA), thiophene (Thi), and furan (Fu) monomers. We establish key insights into the surface reaction mechanisms underlying oMLD growth. We specifically identify the importance of a two-electron chemical oxidant with sufficient oxidation strength to oxidize both a surface and a gas-phase monomer to enable oMLD growth. The mechanistic insights we report enable rational molecular assembly of copolymer structures to improve electrochemical capacity. This work is foundational to unlock molecular-level control of redox-active polymer structure and will enable the study of previously intractable questions regarding the molecular origins of polymer properties, allowing us to control and optimize polymer properties for energy storage, water desalination, and sensors.

A novel approach to redox active polymers: Decolorization of methylene blue by heterogeneous reduction with redox active azo polymer

The existing reducing agents used in the literature to decolorization of methylene blue are soluble in water and the reduction process is carried out in a homogeneous redox reaction medium. After the decolorization process, the reducing agents and its by-products remain in a homogeneous redox reaction medium. Many of these reducing agents are toxic and cause chemical pollution of textile wastewater. Therefore, there is a need for reducing agents that can be mixed heterogeneously with water from which the reducing agent can be easily removed from a redox reaction medium. For this purpose, water-insoluble redox active azo polymer was synthesized from redox active 1,4-dihydroxybenzene and p-phenylenediamine. The synthesized redox active azo polymer was used for decolorization of methylene blue by heterogeneous reduction and various kinetic parameters were investigated for this redox reaction. At the same time, the investigated results of azo polymer and monomer (1,4-dihydroxybenzene) were compared.

Ternary nanocomposite of GQDs-polyFc/Fe3O4/PANI: Design, synthesis, and applied for electrochemical energy storage

Redox-active and conductive polymers and transition metal oxides are appearing group of faradaic battery-type electrode materials that exhibit improved electrochemical properties in supercapacitors. Herein, we report a new class of ferrocenyl-based nanocomposite electrodes for the development of high-performance supercapacitors using a simple physical mixing method of redox active ferrocenyl modified GQDs with conductive polymer including PANI and Fe3O4. Having the popular redox-active organometallic compound like (Fc), PANI, and Fe3O4 as integrated constituent components, the obtained electrode materials exhibit remarkably charge storage and delivery capabilities. The structural properties of the electrode materials investigate by appropriate analyses. The highest capacity, energy density, and power density are to be 295 mAh g−1, 62.1 Wh kg−1 and 4140 W kg−1 for the nanocomposite, respectively. These results which can be attributed to the synergistic behavior of the different types of electrode materials in the hybrid nanocomposites, leading to the improved electrochemical performance. In addition, the cycling performance is tested with 5000 long cycles. The retention of charge storage (91.45%) after 5000 cycles at a current density of 1 A g−1 shows the great potential of GQDs-PolyFc/Fe3O4/PANI nanocomposites as electrode materials for future energy storage technologies. Finally, we conclude with some suggestions for future work.

Bioanalytical System for Determining the Phenol Index Based on Pseudomonas putida BS394(pBS216) Bacteria Immobilized in a Redox-Active Biocompatible Composite Polymer "Bovine Serum Albumin-Ferrocene-Carbon Nanotubes".

The possibility of using the microorganisms Pseudomonas sp. 7p-81, Pseudomonas putida BS394(pBS216), Rhodococcus erythropolis s67, Rhodococcus pyridinivorans 5Ap, Rhodococcus erythropolis X5, Rhodococcus pyridinivorans F5 and Pseudomonas veronii DSM 11331T as the basis of a biosensor for the phenol index to assess water environments was studied. The adaptation of microorganisms to phenol during growth was carried out to increase the selectivity of the analytical system. The most promising microorganisms for biosensor formation were the bacteria P. putida BS394(pBS216). Cells were immobilized in redox-active polymers based on bovine serum albumin modified by ferrocenecarboxaldehyde and based on a composite with a carbon nanotube to increase sensitivity. The rate constants of the interaction of the redox-active polymer and the composite based on it with the biomaterial were 193.8 and 502.8 dm3/(g·s) respectively. For the biosensor created using hydrogel bovine serum albumin-ferrocene-carbon nanotubes, the lower limit of the determined phenol concentrations was 1 × 10-3 mg/dm3, the sensitivity coefficient was (5.8 ± 0.2)∙10-3 μA·dm3/mg, Michaelis constant KM = 230 mg/dm3, the maximum rate of the enzymatic reaction Rmax = 217 µA and the long-term stability of the bioanalyzer was 11 days. As a result of approbation, it was found that the urban water phenol content differed insignificantly, measured by creating a biosensor and using the standard photometric method.

Combining Deep Eutectic Solvents with TEMPO-based Polymer Electrodes: Influence of Molar Ratio on Electrode Performance.

For sustainable energy storage, all-organic batteries based on redox-active polymers promise to become an alternative to lithium ion batteries. Yet, polymers contribute to the goal of an all-organic cell as electrodes or as solid electrolytes. Here, we replace the electrolyte with a deep eutectic solvent (DES) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA), while using poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) as cathode. The successful combination of a DES with a polymer electrode is reported here for the first time. The electrochemical stability of PTMA electrodes in the DES at the eutectic molar ratio of 1 : 6 is comparable to conventional battery electrolytes. More viscous electrolytes with higher salt concentration can hinder cycling at high rates. Lower salt concentration leads to decreasing capacities and faster decomposition. The eutectic mixture of 1 : 6 is best suited uniting high stability and moderate viscosity.

Electrolyte initiated instaneous in-situ chemical polymerization of organic cathodes for ultralong-cycling magnesium ion batteries

Redox-active organic compounds are promising electrode materials for rechargeable batteries owing to their diverse structures, light weight, resource recyclability and low cost, but the solubility and side reactions in electrolytes impede their practical applications. Herein, we report that pyrrole-based redox-active organic monomers can be instantly converted into π-conjugated polymer derivatives via a convenient and efficient in-situ chemical polymerization method during battery assembly without tedious fabrication procedures, which, in essence, is to use the electrolyte originally containing bis(hexamethyldisilazido)magnesium (Mg(HMDS)2) to induce the instantaneous in-situ polymerization of 2-pyrrolanthraquinone (2-PyAQ). Benefited from the intrinsic high ionic mobility and minimal electrolyte solubility of the redox-active π-conjugated polymer chains, the as-obtained poly(N-anthraquinoyl pyrrole) (PAQPy) cathode material exhibit high specific capacity, good rate performance and ultralong cycling stability over 4,000 cycles (with a capacity decay rate of only 0.00098% per cycle). Electrochemical kinetics analyses revealed a high Mg-ion diffusivity in the order of 10−11 cm2 s−1 and a high total capacity originated from both capacitive and diffusion-controlled contributions. A series of in-situ and ex-situ spectroscopic studies verified an extraordinary dual-ion storage mechanism based on both the insertion/extraction processes of Mg2+ and MgCl+ cations on anthraquinone side groups and the doping/de-doping processes of HMDS- anions on polypyrrole chains. This work demonstrates an effective and universal stabilization strategy of redox-active organic electrode materials via electrolyte initiated in-situ chemical polymerization to propel the exploration of high-performance secondary batteries.

Tailored Electrolytes for New Redox-Active Polymers

Radical polymers are gaining increasing interest as active materials for batteries owing to their great potential for fast charging, and the fact that no critical elements are involved. PTMA [poly(2,2,6,6- tetramethylpiperidinyloxy-4-yl methacrylate)] is one of the best-investigated radical polymers for lithium batteries, providing high redox activity, fast kinetics and great reversibility during cycling.1 However, the electrochemically active nitroxyl radicals are embedded in a six-membered ring with four methyl groups attached to stabilize the radical, which limits its theoretical capacity due to the rather high molecular weight of the overall functional moiety. To increase the theoretical capacity, a decrease of the “unnecessary” mass is therefore desirable. Smaller redox-active molecules, however, commonly show a high solubility in liquid electrolytes, which prevents stable long-term cycling.Herein, we present a new redox-active polymer based on low molecular weight amides, for which the solubility issue has been overcome by proper design. We show that the electrolyte composition (salt concentration and the choice of solvents as well as their ratio) plays a decisive role for (i) the redox behavior and kinetics (broad vs. sharp redox peaks and the redox peak separation), (ii) the eventual redox potential, which is greater than for PTMA, and (iii) the long-term stable cycling at high dis-/charge rates. As a result, the rational optimization of the electrolyte composition enables achieving excellent electrochemical performance, rendering this new class of polymers highly promising for high-performance rechargeable batteries. Finally, we unveil the underlying phenomena by means of complementary ex situ and in situ characterization techniques to provide an in-depth understanding and, thus, allow for the tailored design of suitable electrolyte compositions for this new class of redox-active polymers and, potentially, polymer electrode materials in general.(1) Kim, J.-K.; Kim, Y.; Park, S.; Ko, H.; Kim, Y. Encapsulation of Organic Active Materials in Carbon Nanotubes for Application to High-Electrochemical-Performance Sodium Batteries. Energy Environ. Sci. 2016, 9 (4), 1264–1269. https://doi.org/10.1039/C5EE02806J.

(Invited) Synergy of Carbonyl and Azo Chemistries for Wide-Temperature-Range Rechargeable Aluminum Organic Batteries

Developing low-cost, sustainable, and all-climate energy storage devices as alternatives to Li-ion batteries (LIBs) is critical for the global application of electric vehicles, grid-scale electrical energy storages. Among various emerging battery systems beyond LIBs, rechargeable aluminum organic batteries (RAOBs) stand out because of the high theoretical capacity, low cost, abundance, high sustainability, and high safety of Al and organic resources. However, the large ion size of Al complex ions, such as AlCl4 -, AlCl2 + and AlCl2+, or the strong Coulombic interaction between Al3+ ions and active materials, as well as the high diffusion energy barrier of Al3+ ion, lead to poor cyclic stability and sluggish reaction kinetics. Moreover, the reaction mechanism, interphase structure, and the impact of multi-functional groups in redox-active organic materials to the electrochemical performance of RAOBs remains elusive. To address these challenges, we designed and synthesized a redox-active polymer bearing carbonyl and azo groups as the cathode to achieve high-performance and wide-temperature-range RAOBs. The polymeric cathode exhibits a high reversible specific capacity, superior cyclic stability, fast charging capability, and a wide operation temperature range (-40oC to 100oC). X-ray photoelectron spectroscopy (XPS), pair distribution function (PDF) analysis, and soft X-ray absorption near edge structure (XANES) were employed to gain fundamental insight into the carbonyl and azo chemistries in RAOBs, as well as the cathode electrolyte interphase (CEI) structure. We demonstrated a step-by-step alumination/de-alumination reaction for carbonyl and azo groups in the polymer cathode and unravel a Al2O3- and AlN-rich CEI, which is critical for the impressive performance of RAOBs.

Tuning the Azo Location in Conjugated Polymers Toward High-Performance Lithium-Ion Batteries

Redox-active π-conjugated polymers exhibit great potential in energy storage applications. However, how the location of redox-active sites affects the electrochemistry and battery performance remains elusive. In this study, three isomeric conjugated polymers with redox-active azo units linked with thiophene were designed and synthesized. The impact of the location of azo units (para vs meta linkage, main chain vs side chain) in the conjugated polymers on the properties of electrochemistry and battery performance were systematically studied. Experimental and theoretical studies clearly demonstrate that “redox-pendant” and “meta junction” are two effective features to design redox-conjugated polymers for high-performance energy storage. By merging the two features, the polymer exhibits a high azo utilization of close to 100% and displays the highest specific capacity of 215 mAh g–1 at 0.1 A g–1, along with a long and flat charge/discharge plateau appearing at 1.7 V. Moreover, the as-fabricated full battery using polymers with such a design as the anode coupled with LiFePO4 or LiCoO2 as the cathode delivers satisfactory discharge specific capacities of 110 and 95 mA h g–1 at 0.1 A g–1, along with output voltages of 1.9 and 2.3 V, respectively. The practical application of the full battery to power an LED bulb was also demonstrated. The present study provides useful insights into the tuning of the structures for high-performance batteries.

Accelerating Charge/Discharge of Lithium Iron Phosphate by Charge Mediation Reaction of Poly(dimethylfluoflavin-substituted norbornene)

Charging and discharging reactions of lithium metal oxides in lithium-ion batteries are accelerated by adding a catalytic amount of redox-active polymer, poly(dimethylfluoflavin-substituted norbornene). The potential difference between the polymer (E1/2 = 3.5 and 4.1 V vs. Li/Li+) and metal oxides induce redox mediation reactions. Their heterogeneous charge-transfer processes are analyzed to estimate the acceleration effects of mediation.

Solution-Processable Redox-Active Polymers of Intrinsic Microporosity for Electrochemical Energy Storage.

Redox-active organic materials have emerged as promising alternatives to conventional inorganic electrode materials in electrochemical devices for energy storage. However, the deployment of redox-active organic materials in practical lithium-ion battery devices is hindered by their undesired solubility in electrolyte solvents, sluggish charge transfer and mass transport, as well as processing complexity. Here, we report a new molecular engineering approach to prepare redox-active polymers of intrinsic microporosity (PIMs) that possess an open network of subnanometer pores and abundant accessible carbonyl-based redox sites for fast lithium-ion transport and storage. Redox-active PIMs can be solution-processed into thin films and polymer–carbon composites with a homogeneously dispersed microstructure while remaining insoluble in electrolyte solvents. Solution-processed redox-active PIM electrodes demonstrate improved cycling performance in lithium-ion batteries with no apparent capacity decay. Redox-active PIMs with combined properties of intrinsic microporosity, reversible redox activity, and solution processability may have broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Manufacturing of ultra-thin redox-active polymer films using the layer-by-layer method and co-polymerization of vinyl viologen units

Manufacturing of ultra-thin redox-active polymer films using the layer-by-layer method and co-polymerization of vinyl viologen units

Integrating multiple redox-active sites and universal electrode-active features into covalent triazine frameworks for organic alkali metal-ion batteries

The advantages of redox-active polymers with multiple redox-active sites and multiple functions stem from the controllability of organic synthesis, which makes it possible to construct universal organic alkali metal-ion batteries. Herein, triazine-bridged hexaazatrinaphthylene polymer (HATN-CTF) was synthesized and then first served as both cathode and anode materials for lithium organic batteries. Interestingly, we realized the controllable construction of crystalline and amorphous HATN-CTF (cHATN-CTF and aHATN-CTF) and give a systematic study to show the superiority of aHATN-CTF resulted from the more abundant active sites and shortened ion diffusion paths. Most importantly, the integration of hexaazatrinaphthylene core and peripheral triazine moieties endows aHATN-CTF with abundant active centers (CN and CC groups), the ability to absorb cations and anions and universal electrode-active features at high/low potential (cathode/anode side), which have been confirmed by the electrochemical analysis, characterizations and theoretical calculations. Furthermore, aHATN-CTF also shows potential applications in all-organic symmetric batteries (AOBs), flexible AOBs (FAOBs) and other rechargeable alkali metal-ion batteries. Our findings highlight the importance of redox-active polymers with multiple redox-active sites and multiple electrode-active features and open a new avenue to construct organic alkali metal-ion batteries.

Molecular engineering of dihydroxyanthraquinone-based electrolytes for high-capacity aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) are a promising technology for large-scale electricity energy storage to realize efficient utilization of intermittent renewable energy. In particular, organic molecules are a class of metal-free compounds that consist of earth-abundant elements with good synthetic tunability, electrochemical reversibility and reaction rates. However, the short cycle lifetime and low capacity of AORFBs act as stumbling blocks for their practical deployment. To circumvent these issues, here, we report molecular engineered dihydroxyanthraquinone (DHAQ)-based alkaline electrolytes. Via computational studies and operando measurements, we initially demonstrate the presence of a hydrogen bond-mediated degradation mechanism of DHAQ molecules during electrochemical reactions. Afterwards, we apply a molecular engineering strategy based on redox-active polymers to develop capacity-boosting composite electrolytes. Indeed, by coupling a 1,5-DHAQ/poly(anthraquinonyl sulfide)/carbon black anolyte and a [Fe(CN)6]3−/4− alkaline catholyte, we report an AORFB capable of delivering a stable cell discharge capacity of about 573 mAh at 20 mA/cm2 after 1100 h of cycling and an average cell discharge voltage of about 0.89 V at the same current density.

Nontraditional Redox Active Aliphatic Luminescent Polymer for Ratiometric pH Sensing and Sensing-Removal-Reduction of Cu(II): Strategic Optimization of Composition.

Here, redox active aliphatic luminescent polymers (ALPs) are synthesized via polymerization of N,N-dimethyl-2-propenamide (DMPA) and 2-methyl-2-propenoic acid (MPA). The structures and properties of the optimum ALP3, ALP3-aggregateand Cu(I)-ALP3, ratiometric pH sensing, redox activity, aggregation enhanced emission (AEE), Stokes shift, and oxygen-donor selective coordination-reduction of Cu(II) to Cu(I) are explored via spectroscopic, microscopic, density functional theory-reduced density gradient (DFT-RDG), fluorescence quenching, adsorption isotherm-thermodynamics, and electrochemical methods. The intense blue and green fluorescence of ALP3 emerges at pH = 7.0 and 9.0, respectively, due to alteration of fluorophores from -C(═O)N(CH3 )2 / -C(═O)OH to -C(O- )═N+ (CH3 )2 / -C(═O)O- , inferred from binding energies at 401.32eV (-C(O- )═N+ (CH3 )2 ) and 533.08eV (-C(═O)O- ), significant red shifting in absorption and emission spectra, and peak at 2154cm-1 . The n-π* communications in ALP3-aggregate, hydrogen bondings within 2.34-2.93 Å (intramolecular) in ALP3 and within 1.66-2.89 Å (intermolecular) in ALP3-aggregate, respectively, contribute significantly in fluorescence, confirmed from NMR titration, ratiometric pH sensing, AEE, excitation dependent emission, and Stokes shift and DFT-RDG analyses. For ALP3, Stokes shift, excellent limit of detection, adsorption capacity, and redox potentials are 13561 cm-1 /1.68eV, 0.137 ppb, 122.93mg g-1 , and 0.33/-1.04V at pH 7.0, respectively.

3D microstructure characterization of polymer battery electrodes by statistical image analysis based on synchrotron X-ray tomography

Polymer-based batteries represent a promising concept for next-generation energy storage due to their potentially higher power densities and smaller ecological footprint, compared to classical Li-ion batteries. Since the microstructure of electrodes is a key factor for the performance of battery cells, a detailed understanding of this microstructure is essential for the improvement of manufacturing processes. In the present contribution, the 3D microstructure of electrodes for polymer-based batteries is quantitatively characterized for the first time, where synchrotron X-ray tomography is combined with statistical image analysis. In particular, 3D imaging is performed for two porous electrodes, which both consist of the redox-active polymer PTMA as well as conductive additives, but differ regarding their binder materials. The focus is put on local heterogeneity of volume fractions of the constituents, surface area per unit volume of the polymer phase and the length of shortest transportation paths through both, polymer and binder-additive phase. It is shown that using different binder materials leads to significant differences regarding the 3D electrode microstructures. In this way, statistical analysis of image data helps to gain further insight into the influence of manufacturing processes on electrode microstructures and thus, on the performance of battery cells.

Light-Induced and Thermal Isomerization of Azobenzenes on Immobilized Gold Nanoparticle Aggregates.

Morphologically different gold nanoparticle (AuNP) aggregates were prepared on macroscopic surfaces covered with a layer of polydopamine (PDA). The extent of particle aggregation and the particle size distribution could be controlled by the Au(III) reduction times, while the reduction process was triggered solely by the redox active polymer. Shorter reaction times led to smaller particles along with lower levels of aggregation, while longer reductions resulted in larger average particle diameter and heavier aggregation. The prepared surfaces were characterized by UV-Vis, AFM and KPFM techniques. These surfaces were used as solvent-free condensed phases to probe the photochemical and thermal isomerization processes of attached azobenzenes with different spacer lengths. Fast and reversible light-induced switching was observed in each case. The thermal cis-to-trans isomerization was found to be accelerated for particle-bound azobenzenes compared to those in solution.

Ionic Effect on Electrochemical Behavior of Water-Soluble Radical Polyelectrolytes

Water-soluble redox-active polymers (RAPs) have emerged as attractive electroactive materials for aqueous redox flow batteries because these systems rely on readily available size-exclusion membranes rather than expensive ion-selective membranes. While incorporating ionic units is the most effective strategy for forming water-soluble redox polyelectrolytes, little is known about how charges dictate their electrochemical behavior. Here, we design a series of water-soluble TEMPO (2,2,6,6-tetramethylpiperidyl-1-oxy) radical polyelectrolytes with identical radical distribution but various ionic groups through a sequential postmodification method. Physical and electrochemical characterizations show disparate diffusion coefficients (D) and charge transfer kinetics (k0) of these radical polyelectrolytes at various pH. Particularly, pH exerts a strong impact on k0 of the negatively charged polymer (i.e., with carboxylic acid groups). The bimolecular reaction rate kex determined from redox-induced polymer films shows electrostatic interactions between charged segments can enhance the electron self-exchange reaction rate by ten folds. The results suggest that charge effects are of great importance when designing water-soluble redox polymers for electrochemical applications.

Crystal Structure and Electrochemical and Charge Transfer Properties in Redox-Active Coordination Polymers Based on a Truncated Tetrathiafulvalene Linker

Crystal Structure and Electrochemical and Charge Transfer Properties in Redox-Active Coordination Polymers Based on a Truncated Tetrathiafulvalene Linker