Redox-active Organic Materials Research Articles

Quantifying concentration distributions in redox flow batteries with neutron radiography

The continued advancement of electrochemical technologies requires an increasingly detailed understanding of the microscopic processes that control their performance, inspiring the development of new multi-modal diagnostic techniques. Here, we introduce a neutron imaging approach to enable the quantification of spatial and temporal variations in species concentrations within an operating redox flow cell. Specifically, we leverage the high attenuation of redox-active organic materials (high hydrogen content) and supporting electrolytes (boron-containing) in solution and perform subtractive neutron imaging of active species and supporting electrolyte. To resolve the concentration profiles across the electrodes, we employ an in-plane imaging configuration and correlate the concentration profiles to cell performance with polarization experiments under different operating conditions. Finally, we use time-of-flight neutron imaging to deconvolute concentrations of active species and supporting electrolyte during operation. Using this approach, we evaluate the influence of cell polarity, voltage bias and flow rate on the concentration distribution within the flow cell and correlate these with the macroscopic performance, thus obtaining an unprecedented level of insight into reactive mass transport. Ultimately, this diagnostic technique can be applied to a range of (electro)chemical technologies and may accelerate the development of new materials and reactor designs.

Rechargeable Batteries with Earth Abundant Elements – Organic Redox Polymers for Li-Organic Batteries

The energy storage needs of the 21st century demand solutions that are both 'efficient' and 'sustainable.' The research explores alternatives to transition metal-based electrode materials traditionally used in LIBs, with a particular focus on redox-active organic materials, such as ample availability of precursors, structural diversity, non-toxicity, and low cost.[1] Conductive polymers, organosulfur compounds, organic radicals, small carbonyl compounds, carbonyl polymers, imine and azo compounds are the few classes of organics explored. This study focuses on the development of rechargeable batteries using earth-abundant elements, with a specific emphasis on organic redox polymers for Li-ion batteries (LIBs).[2] The investigation focuses on poly(3-vinyl-N-methylphenothiazine) (PVMPT), a p-type material, showcasing its excellent rate capability, cyclic stability, and reasonable theoretical capacity (112 mAh g−1). Despite the promising electrochemical performance of PVMPT, the electrode processing methods involve the use of toxic and flammable solvents, such as N-methyl-2-pyrrolidone (NMP), and expensive binders like polyvinylidene fluoride (PVDF).[3] To address these challenges, this study proposes the use of green solvents (Cyrene, Ƴ-valerolactone, and Dimethyl sulfoxide) for electrode processing, eliminating the need for toxic solvents and promoting the sustainability and safety of organic materials.[4] A shift to sustainable aqueous binders, such as carboxymethyl cellulose (CMC), sodium alginate (SA), and styrene-butadiene rubber (SBR),[5] is explored (Figure 1a-c), with the CMC system showing superior capacity retention.Additionally, the study delves into the Phenoxazine class of redoxomers, particularly polyvinylphenoxazine (PVMPO), a fast-charging cathode with a theoretical capacity of 120mAh g−1.[6] Challenges related to the dissolution of the oxidized state in the carbonate electrolyte are addressed by employing highly porous conductive additives and exploring different electrolyte compositions. A shift from toxic PVDF/NMP binder systems to aqueous binders (CMC, SBR, SA) is implemented, with the CMC-based electrodes demonstrating superior adhesion even after 10,000 cycles at 1C rate (Figure 1d-f).To conclude, the study believes that the strategic selection of conductive additives and electrolytes, coupled with the use of green solvents and aqueous binders, will contribute to the development of eco-friendly and sustainable energy storage systems. The comprehensive performance evaluation of the proposed systems is presented in detail, utilizing various characterization tools such as XRD, FT-IR, UV-VIS, and SEM.References Lee et al., Adv Mater., 2018, 42,1704682.Dühnen et al., Small Methods, 2020, 4, 2000039.Kolek et al., Energy Environ. Sci., 2017, 10, 2334.Ravikumar et al., ACS Appl. Energy Mater., 2021, 4, 696−703Bresser et al., Energy Environ. Sci., 2018,11, 3096-3127.Otteny et al., ACS Sustainable Chem. Eng., 2020, 8, 238−247 Figure 1

Regulating intramolecular hydrogen bonds of p-phenylenediimidazole-based small-molecule compounds towards the enhanced lithium storage capacity

The use of redox-active organic electrode materials in energy storage is restricted due to their inferior solvent resistance, abysmal conductivity, and the resultant low practical capacity. To address these issues, a class of bipolar p-phenylenediimidazole-based small-molecule compounds are designed and fabricated. The π-conjugated backbone of these small molecules allows for electron delocalization on a big conjugation plane, endowing them with good conductivity and reaction reversibility. Furthermore, when the para-positions of phenylene are occupied by hydroxyl groups, as-formed intramolecular hydrogen bonds (N–H…O) between phenolic hydroxyl groups and the –NH groups of imidazole rings further enhance the structural planarity, resulting in higher π-conjugation degree and better conductivity, and thus higher utilization of active sites and electrode capacity, proved by both experimental results and theoretical calculations. The optimized composite electrode DBNQ@rGO-45 shows a high specific capacity (∼308 mA h g−1 at 100 mA g−1) and a long cycling stability (112.9 mA h g−1 after 6000 cycles at 2000 mA g−1). The significantly better electrochemical properties for hydroxyl group-containing compounds than those without hydroxyl groups attributed to intramolecular hydrogen bond-induced conjugation enhancement will inspire the structure design of organic electrodes for better energy storage.

Layer-by-layer assembling redox wood electrodes for efficient energy storage

The exploration of redox-active organic materials and low tortuous thick-electrodes is attractive for energy storage. The in-situ valorized lignin on raw wood surface accompanied by layer-by-layer deposition of electro-active materials endow such spatially distributed wood electrodes with high specific capacitance. Here, we report a layer-by-layer assembled ca.1.5 mm-thick redox wood hybrid electrode with 20 mg cm-2 electro-active mass loading for efficient energy storage. The in-situ modified surface lignin in treated wood (TrW) holds promise as redox-active material with enriched nanoporosity, carbonyl functionalities, and multi-phase ionic transport structure. The carbon nanotubes (CNTs) networking with in-situ polymerized polypyrrole (PPy) nanorods three-dimensionally in the lumen of TrW afford a wool-like, highly porous nanostructure. Such a hierarchical structured PPy@CNTs@TrW electrode offers a high areal capacitance of 1.46 F cm-2 with an extraordinary energy density of 0.983 mWh cm-3 (3.68 Wh kg-1) and power density of 5.4 mW cm-3 (20.25 W kg-1). Here, the valorized surface lignin contributes contributes to electrochemical energy storage accompanied by spatially distributed PPy@CNTs in low tortuous electrodes. The electrode offers extremely low electrochemical impedance of 0.61 Ω electrode resistance and 1.57 Ω electrolyte resistance. The hybrid wood electrode showcases even higher conductivity and energy/power density than thin carbonized wood and other state-of-the-art thin electrodes made of highly conductive three-dimensional networks. This work highlights the potential of in-situ valorized lignin in developing high-performance eco-friendly thick-electrodes for electrochemical energy storage applications.

14-Electron Redox Chemistry Enabled by Salen-Based π-Conjugated Framework Polymer Boosting High-Performance Lithium-Ion Storage.

A paucity of redox centers, poor charge transport properties, and low structural stability of organic materials obstruct their use in practical applications. Herein, these issues have been addressed through the use of a redox-active salen-based framework polymer (RSFP) containing multiple redox-active centers in π-conjugated configuration for applications in lithium-ion batteries (LIBs). Based on its unique architecture, RSFP exhibits a superior reversible capacity of 671.8 mAh g-1 at 0.05 A g-1 after 168 charge-discharge cycles. Importantly, the lithiation/de-lithiation performance is enhanced during operation, leading to an unprecedented reversible capacity of 946.2 mAh g-1 after 3500 cycles at 2 A g-1. The structural evolution of RSFP is studied ex situ using X-ray photoelectron spectroscopy, revealing multiple active C═N, C─O, and C═O sites and aromatic sites such as benzene rings. Remarkably, the emergence of C═O originated from C─O is triggered by an electrochemical process, which is beneficial for improving reversible lithiation/delithiation behavior. Furthermore, the respective strong and weak binding interactions between redox centers and lithium ions, corresponding to theoretical capacities of 670.1 and 938.2 mAh g-1, have been identified by density functional theory calculations manifesting 14-electron redox reactions. This work sheds new light on routes for the development of redox-active organic materials for energy storage applications.

Non-Metal Ion Storage in Zinc-Organic Batteries.

Zinc-organic batteries (ZOBs) are receiving widespread attention as up-and-coming energy-storage systems due to their sustainability, operational safety and low cost. Charge carrier is one of the critical factors affecting the redox kinetics and electrochemical performances of ZOBs. Compared with conventional large-sized and sluggish Zn2+ storage, non-metallic charge carriers with small hydrated size and light weight show accelerated interfacial dehydration and fast reaction kinetics, enabling superior electrochemical metrics for ZOBs. Thus, it is valuable and ongoing works to build better ZOBs with non-metallic ion storage. In this review, versatile non-metallic cationic (H+, NH4 +) and anionic (Cl-, OH-, CF3SO3 -, SO4 2-) charge carriers of ZOBs are first categorized with a brief comparison of their respective physicochemical properties and chemical interactions with redox-active organic materials. Furthermore, this work highlights the implementation effectiveness of non-metallic ions in ZOBs, giving insights into the impact of ion types on the metrics (capacity, rate capability, operation voltage, and cycle life) of organic cathodes. Finally, the challenges and perspectives of non-metal-ion-based ZOBs are outlined to guild the future development of next-generation energy communities.

Synergistic π-Conjugation Organic Cathode for Ultra-Stable Aqueous Aluminum Batteries.

Rechargeable aqueous aluminum batteries (AABs) are promising energy storage technologies owing to their high safety and ultra-high energy-to-price ratio. However, either the strong electrostatic forces between high-charge-density Al3+ and host lattice, or sluggish large carrier-ion diffusion toward the conventional inorganic cathodes generates inferior cycling stability and low rate-capacity. To overcome these inherent confinements, a series of promising redox-active organic materials (ROMs) are investigated and a π-conjugated structure ROMs with synergistic C═O and C═N groups is optimized as the new cathode in AABs. Benefiting from the joint utilization of multi-redox centers and rich π-π intermolecular interactions, the optimized ROMs with unique ion coordination storage mechanism facilely accommodate complex active ions with mitigated coulombic repulsion and robust lattice structure, which is further validated via theoretical simulations. Thus, the cathode achieves enhanced rate performance (153.9mAhg-1 at 2.0Ag-1) and one of the best long-term stabilities (125.7mAhg-1 after 4,000 cycles at 1.0Ag-1) in AABs. Via molecular exploitation, this work paves the new direction toward high-performance cathode materials in aqueous multivalent-ion battery systems.

Redox-Active Organic Materials: From Energy Storage to Redox Catalysis.

Electroactive materials are central to myriad applications, including energy storage, sensing, and catalysis. Compared to traditional inorganic electrode materials, redox-active organic materials such as porous organic polymers (POPs) and covalent organic frameworks (COFs) are emerging as promising alternatives due to their structural tunability, flexibility, sustainability, and compatibility with a range of electrolytes. Herein, we discuss the challenges and opportunities available for the use of redox-active organic materials in organoelectrochemistry, an emerging area in fine chemical synthesis. In particular, we highlight the utility of organic electrode materials in photoredox catalysis, electrochemical energy storage, and electrocatalysis and point to new directions needed to unlock their potential utility for organic synthesis. This Perspective aims to bring together the organic, electrochemistry, and polymer communities to design new heterogeneous electrocatalysts for the sustainable synthesis of complex molecules.

Micellar Solubilization for High-Energy-Density and Long-Cycle-Life Aqueous Organic Redox-Flow Batteries

Functional group control has been the most widely used strategy to enhance the solubility of redox-active organic materials (ROMs) in aqueous media for aqueous organic redox-flow batteries. However, this strategy inevitably requires complicated organic synthesis, typically involving 2–4 steps with high cost and under harsh conditions, reducing the full potential of ROMs. In this study, to achieve higher ROM solubility, we introduced surfactant molecules to provide a nonpolar microenvironment via micellization. The hexadecyltrimethylammonium chloride surfactant enabled micellar solubilization, resulting in an order-of-magnitude enhancement of the solubility of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). In addition, the micellar solubilization elevated the redox potential of TEMPO, increasing the energy density by more than 10 times compared with that in the bare electrolyte. Furthermore, the micellar solubilization improved the cycle stability due to not only mitigated crossover but also enhanced chemical stability of TEMPO+. This strategy can be applied to various ROMs and is expected to assist in achieving practical application of organic redox-flow batteries.

Redox Active Organic Liquids for Energy Storage

The necessity to move to renewable energy sources has increased over the last few years. However, to ensure a good transition and effective energy supply, efficient, cheap, and green technologies are required as grid-level storage.1 Redox-Active Organic Materials (ROMs) are a fast-growing research topic in different electrochemical storage devices. Their advantageous characteristics of being formed from highly abundant elements, having a high electrochemical tunability, ease of handling and device processing, and high versatility for a variety of energy storage applications make ROMs attractive.1–3 Redox-Active Liquids (RALs) are themselves a new trend within the category of ROMs because the physical properties of organic liquids are advantageous for some storage devices.4–6 Those advantages include no requirement for materials processing after synthesis, high miscibility and/or high solubility with battery electrolytes, the ability to be used as a solvent themselves without any additional additives,4,5 and the avoidance of surface-related changes e.g., dendrites.7 RALs have been studied as mediators or active materials in the context of different applications such as supercapacitors,6,8 conventional,4,5,9 and hybrid-flow batteries.10,11 In this work, we present the synthesis and physio-chemical characterization of different RALs that can be used in such energy storage devices. The characterization results supported by computational analyses were used to investigate the correlation between the chemical structure and the electrochemical properties of the compounds. The obtained information was then further used to inform the design of further compounds with enhanced electrochemical properties.(1) Schon, T. B.; McAllister, B. T.; Li, P. F.; Seferos, D. S. The Rise of Organic Electrode Materials for Energy Storage. Chem Soc Rev 2016, 45 (22), 6345–6404. https://doi.org/10.1039/c6cs00173d.(2) Kwon, G.; Ko, Y.; Kim, Y.; Kim, K.; Kang, K. Versatile Redox-Active Organic Materials for Rechargeable Energy Storage. Acc Chem Res 2021, 54 (23), 4423–4433. https://doi.org/10.1021/acs.accounts.1c00590.(3) Lee, S.; Hong, J.; Kang, K. Redox-Active Organic Compounds for Future Sustainable Energy Storage System. Advanced Energy Materials. Wiley-VCH Verlag August 1, 2020. https://doi.org/10.1002/aenm.202001445.(4) Zhao, Y.; Zhang, J.; Agarwal, G.; Yu, Z.; Corman, R. E.; Wang, Y.; Robertson, L. A.; Shi, Z.; Doan, H. A.; Ewoldt, R. H.; Shkrob, I. A.; Assary, R. S.; Cheng, L.; Srinivasan, V.; Babinec, S. J.; Zhang, L. TEMPO Allegro: Liquid Catholyte Redoxmers for Nonaqueous Redox Flow Batteries. J Mater Chem A Mater 2021, 9 (31), 16769–16775. https://doi.org/10.1039/d1ta04297a.(5) Robertson, L.; Udin, M. A.; Shlrob, I. A.; Moore, J. S.; Zhang, L. Liquid Redoxmers for Nonaqueous Redox Flow Batteries. ChemSusChem 2023, e202300043. https://doi.org/10.1002/CSSC.202300043.(6) Fontaine, O. A Deeper Understanding of the Electron Transfer Is the Key to the Success of Biredox Ionic Liquids. Energy Storage Mater 2019, 21, 240–245. https://doi.org/10.1016/J.ENSM.2019.06.023.(7) Bao, J.; Li, C.; Zhang, F.; Wang, P.; Zhang, X.; He, P.; Zhou, H. A Liquid Anode of Lithium Biphenyl for Highly Safe Lithium-Air Battery with Hybrid Electrolyte. Batter Supercaps 2020, 3 (8), 708–712. https://doi.org/10.1002/BATT.202000092.(8) Mourad, E.; Coustan, L.; Lannelongue, P.; Zigah, D.; Mehdi, A.; Vioux, A.; Freunberger, S. A.; Favier, F.; Fontaine, O. Biredox Ionic Liquids with Solid-like Redox Density in the Liquid State for High-Energy Supercapacitors. Nature Materials 2016 16:4 2016, 16 (4), 446–453. https://doi.org/10.1038/nmat4808.(9) Chen, H.; Niu, Z.; Zhao, Y. Redox-Active Binary Eutectics: Preparation and Their Electrochemical Properties. Electrochem commun 2021, 126. https://doi.org/10.1016/j.elecom.2021.107028.(10) Shimizu, A.; Takenaka, K.; Handa, N.; Nokami, T.; Itoh, T.; Yoshida, J.-I.; Shimizu, A.; Takenaka, K.; Yoshida, J.; Handa, N.; Nokami, T.; Itoh, T. Liquid Quinones for Solvent-Free Redox Flow Batteries. Advanced Materials 2017, 29 (41), 1606592. https://doi.org/10.1002/ADMA.201606592.(11) Chen, N.; Chen, D.; Wu, J.; Lai, Y.; Chen, D. Polyethylene Glycol Modified Tetrathiafulvalene for High Energy Density Non-Aqueous Catholyte of Hybrid Redox Flow Batteries. Chemical Engineering Journal 2023, 462. https://doi.org/10.1016/j.cej.2023.141996.

Fabrication and Performance of Symmetric Flow Battery Systems Incorporating Novel Pyridinium Anolytes

The push to bring more renewable energy sources onto the grid has spawned a flurry of work to develop viable options for long duration energy storage. Redox flow batteries (RFBs) represent one of the most attractive possibilities, and research toward employing redox-active organic materials to improve or replace the incumbent commercially available vanadium-based RFB technology is burgeoning. In particular, numerous pyridinium-based salts have been extensively studied as anolytes in both aqueous and nonaqueous RFBs. We have recently reported a series of readily synthetically accessible 2,6-dimethyl-4-arylpyridinium compounds that display quite negative reduction potentials (ca. -1.55 to -1.8 V vs Fc/Fc+), along with high solubility and excellent electrochemical kinetics. Some of these materials are also extremely persistent in the reduced state, suggesting that they may be good candidates for RFB applications.In this presentation, we will describe the preparation and properties of new pyridinium compounds from the corresponding pyrylium precursors, as well as the fabrication of symmetric RFB systems in which these materials are covalently linked to phenothiazine or ferrocene catholyte moieties. Synthesis, solubility, and electrochemical performance (including preliminary RFB cycling data) of these materials will also be discussed.

From Squaric Acid Amides (SQAs) to Quinoxaline-Based SQAs─Evolution of a Redox-Active Cathode Material for Organic Polymer Batteries.

The search for new redox-active organic materials (ROMs) is essential for the development of sustainable energy-storage solutions. In this study, we present a new class of cyclobuta[b]quinoxaline-1,2-diones or squaric acid quinoxalines (SQXs) as highly promising candidates for ROMs featuring exceptional stability and high redox potentials. While simple 1,2- and 1,3-squaric acid amides (SQAs), initially reported by Hünig and coworkers decades ago, turned out to exhibit low stability in their radical cation oxidation states, we demonstrate that embedding the nitrogen atoms into a quinoxaline heterocycle leads to robust two-electron SQX redox systems. A series of SQX compounds, as well as their corresponding radical cations, were prepared and fully characterized, including EPR spectroscopy, UV-vis spectroscopy, and X-ray diffraction. Based on the promising electrochemical properties and high stability of the new ROM, we developed SQX-functionalized polymers and investigated their physical and electrochemical properties for energy-storage applications. These polymers showed remarkable thermal stability well above 200 °C with reversible redox properties and potentials of about 3.6 V vs Li+/Li. By testing the galvanostatic cycling performance in half-cells with lithium-metal counter electrodes, a styrene-based polymer with SQX redox side groups showed stable cycling for single-electron oxidation for more than 100 cycles. These findings render this new class of redox-active polymers as highly promising materials for future energy-storage applications.

N-Isobutylphenothiazine as a reversible and stable catholyte in non-aqueous organic redox flow batteries

Nonaqueous organic redox flow batteries (NAORFBs) are the focus of current research because redox-active organic materials can be synthesized from abundant elements, and nonaqueous electrolytes offer higher open circuit voltage (OCV). The development of NAORFBs requires stable redox active materials in neutral forms and radical ions. N-substituted phenothiazines are organic catholytes with high electrochemical reversibility and stability. In this study, N-isobutylphenothiazine (iBuPT) was successfully synthesized in a single step from inexpensive precursors of 10H-phenothiazine and 1-bromo-2-methylpropane and used as catholyte material for NAORFB. iBuPT, a member of the N-substituted phenothiazine family, has a half-wave redox potential of 0.432 V, higher than most N-substituted phenothiazines. When iBuPT is coupled with anolyte 2,1,3-benzothiadiazole (BTD) with a half-wave potential of −1.82 V, the OCV value reaches 2.25 V. The half-wave potential of phenothiazine increases by 115 mV when an isobutyl group replaces the N‑hydrogen, and the stability of the radical cation formed during electrochemical oxidation improves. The steric hindrance can be used to tune redox potential and stability. The stability of the iBuPT radical cation is attributed to the steric shielding and resonance stabilization provided by the one isobutyl group and two phenyl rings attached to the redox-active center, N, from which electron transfer occurs. iBuPT is a promising catholyte candidate for NAORFBs.

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.

Bipolar Isoindoline Nitroxide for Nonaqueous Symmetrical Redox Flow Battery

AbstractCrossover in non‐aqueous redox flow batteries remains a critical challenge to the cycle stability of these devices. The use of bipolar redox‐active organic materials (ROM) is an emerging strategy for mitigating crossover. Herein we report the first example of a bipolar ROM derived from an isoindoline nitroxide, a ring class which gives a number of advantages over the more commonly employed piperidines, including greater stability and a 200 mV higher oxidation potential. Through facile synthetic transformation, the unsubstituted isoindoline nitroxide was nitrated to give a novel bipolar molecule, 5‐nitro‐1,1,3,3‐tetramethylisoindoline‐2‐yloxyl (NTMIO). This material was investigated electrochemically, revealing a reversible oxidation and quasi‐reversible reduction giving a cell potential of 2.07 V. NTMIO was then assessed as an active material in both a static and flow battery model, where cycling was observed for both oxidative and reductive redox couples for over 70 and 20 cycles respectively.

Confinement Effects on the Rate Performance of Redox Active Molecules for Pseudocapacitive Flowable Electrodes

The pseudocapacitive flowable electrodes typically show high energy density because of the contribution of the faradaic charge of redox-active organic materials and the electric double layer charge of carbon materials. However, the redox reaction kinetics of organic molecules are slow due to poor diffusion kinetics. We recently reported that a pseudocapacitive flowable electrode exhibited bell-shaped cyclic voltammograms (peak separation (ΔEp) = 0 mV); specifically, the molecules were confined within slit-shaped graphitic micropores of activated carbon (AC). Herein, we studied the relationship between charge storage and the reaction mechanism to tailor the electrochemical performance of a pseudocapacitive flowable electrode by half-cell study. The results show that the redox reaction of the confined molecules entailed a charge-transfer-controlled mechanism, while the unconfined molecules exhibited a mass-transfer-controlled system. This difference inhibited the fast charging and discharging of the pseudocapacitive flowable electrode. This study demonstrates that half-cell studies are crucial for clarifying the relationship between the charge storage and rate performances of pseudocapacitive flowable electrodes.

Extended Conjugation Acceptors Increase Specific Energy Densities in π-Conjugated Redox Polymers

Organic materials are a sustainable alternative to replace inorganic transition metal-based cathodes in rechargeable intercalation batteries. Among the possible redox active organic materials, conjugated polymers with multiple redox sites per repeat unit are expected to afford high energy and power densities while being resistant to dissolution when in contact with battery electrolytes. However, accessing the full capacity of polymeric electrodes while ensuring electrochemical reversibility has been challenging. Using diketopyrrolopyrrole (DPP)-based donor–acceptor (D–A) polymers as model systems and complementary electrochemical experiments and first-principles calculations, we show that conjugated backbone moieties that minimize charge localization on the electron accepting repeat units lead to near theoretical discharge capacities. Further, the capacity enhancement is associated with better rate performance and improved electrochemical stability of the polymer over prolonged cycling. Our work suggests that charge density on the electron accepting moiety is a potential descriptor for rationally designing redox-active polymer electrodes that afford high discharge capacities along with a long cycle life.

2-aminoanthraquinone anchored on N-doped reduced graphene oxide for symmetric supercapacitor with boosting energy density

Redox-active organic materials with the advantages of light weight, flexibility, and low-cost are promising alternatives to high-performance green energy storage devices due to their high theoretical capacitance and availability from renewable resources. Therefore, in this paper, we successfully synthesized a novel 2-aminoanthraquinone (AAQ) organic molecules non-covalent anchored on N-doped reduced graphene oxide nanocomposites (N-RGO@AAQ) by a simple improved solvothermal method. Mainly benefiting from the enhanced ion/electron transport kinetics, the introduction of pseudocapacitance, and the stable nanostructure that facilitate fast ion intercalation and de-intercalation, the synthesized N-RGO@AAQ nanocomposite exhibits excellent electrochemical performances as an electrode material. A satisfactory high specific capacity of 532.48 C g−1 at 0.6 A g−1 and excellent rate capability (60.42%) from 0.6 to 60 A g−1 are achieved. Moreover, the assembled symmetrical supercapacitor device (SSD) assembled by using N-RGO@AAQ electrode delivers a maximum energy density of 21.13 W h kg−1 at a power density of 750 W kg−1 and an outstanding ultra-long cycling durability (maintain 94.67% of the initial capacity after 18 000 cycles at 30 mV s−1). This study provides a strategy and a potential electrode material for the development and application of green energy storage devices with high-performance.

Grafting and Solubilization of Redox-Active Organic Materials for Aqueous Redox Flow Batteries.

This study concerns the development of sustainable design strategies of aqueous electrolytes for redox flow batteries using redox-active organic materials. A green spontaneous grafting reaction occurs between a redox-active organic radical and an electrochemically activated structural modifier at room temperature through a simple mixing step. Then, a physical mixing method is used to formulate a structured aqueous electrolyte and enables aqueous solubilization of the organic solute from below 0.5 to 1.5 m beyond the conventional dissolution limit. The as-obtained concentrated mixture can be readily used as catholyte for a redox flow battery. A record high discharge cell voltage (1.6 V onset output voltage) in aqueous non-hybrid flow cell is attained by using the studied electrolytes.

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.