Redox-active Organic Materials Research Articles

Design strategy for redox-active organic materials derived from N-heterocyclic carbenes

Redox-active organic molecules (ROMs) that are highly stable and soluble in various oxidation states have been designed to further develop high-energy batteries. A new ROM design strategy perspective is provided by the molecular engineering of N-heterocyclic carbenes in a manner that stabilizes unstable substances.

(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.

High-Energy and Long-Lasting Organic Electrode for a Rechargeable Aqueous Battery

Redox-active organic materials (ROMs) hold great promise as potential electrode materials for eco-friendly, cost-effective, and sustainable batteries; however, the poor cycle stability arising from the chronic dissolution issue of the ROMs in generic battery systems has impeded their practical employment. Herein, we present that a rational selection of electrolytes considering the solubility tendency can unlock the hidden full redox capability of the DMPZ electrode (i.e., 5,10-dihydro-5,10-dimethylphenazine) with unprecedentedly high reversibility. It is demonstrated that a multiredox activity of DMPZ/DMPZ+/DMPZ2+, which has been previously regarded to degrade with repeated cycles, in the newly designed electrolyte can be utilized with surprisingly robust cycle stability over 1000 cycles at 1C. This work signifies that tailoring the electrode–electrolyte compatibility can possibly unleash the hidden potential of many common ROMs, catalyzing the rediscovery of organic electrodes with long-lasting and high energy density.

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.

Surficial amide-enabled integrated organic anode–binder electrode for electrochemical reversibility and fast redox kinetics in lithium–ion batteries

Binders have been focused on the improvement of electrode stability. However, they have great potential to enhance the electrochemical properties by introducing additional interaction with functional groups of organic electrodes. In this study, we propose an integrated organic anode–binder system from small molecule to polymer. It comprises a redox-active benzene group for highly stable and fast lithium-ion batteries via chemical cross-linking of the aromatic redox-active molecule with the poly(acrylic acid) binder. The surficial amide linkage not only suppresses electrolyte dissolution but also increases the lithium-ion reaction kinetics of the benzene ring with high reversible capacity. The lowered lowest unoccupied molecular orbital level and lithium-ion induction effect reduce the charge transfer and interface resistance, significantly outperforming the traditional electrode system with a poly(vinylidene fluoride) binder. This strategy is successfully expanded to polymeric system of polyimide microparticles possessing additional redox-active imide moiety along with benzene rings in their backbone. This work suggests a new strategy of providing additional capacity through reaction kinetics enhancement for multiscale redox-active organic materials.

Fluorination Enables Simultaneous Improvements of a Dialkoxybenzene-Based Redoxmer for Nonaqueous Redox Flow Batteries.

Redoxmers or redox-active organic materials, are one critical component for nonaqueous redox flow batteries (RFBs), which hold high promise in enabling the time domain of the grid. While tuning redox potentials of redoxmers is a very effective way to enhance energy densities of NRFBs, those improvements often accompany accelerated kinetics of the charged species, undermining stability and cycling performance. Herein, a strategy for designing redoxmers with simultaneous improvements in redox potential and stability is proposed. Specifically, the redoxmer 1,4-di-tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene (ANL-C46) is developed by incorporating fluorinated substitutions into the dialkoxybenzene-based platform. Compared to the non-fluorinated analogue, ANL-C46 demonstrates not only an increased (∼0.41 V) redox potential but also much enhanced stability (1.6 times) and cyclability (4 times) evidenced by electron paramagnetic resonance kinetic study, H-cell and flow cell cycling. In fact, the cycling performance of ANL-C46 is among the best of high potential (>1.0 V vs Ag/Ag+) redoxmers ever reported. Density functional theory calculations suggest that while the introduced fluorine substitutions elevate the redox potentials, they also help to depress the decomposition reactions of the charged redoxmers, affording excellent stability. The findings represent an interesting strategy for simultaneously improving energy density and stability, which could further prompt the development of high-performance redoxmers.

High-Voltage Organic Cathodes for Zinc-Ion Batteries through Electron Cloud and Solvation Structure Regulation.

Redox-active organic materials, as a new generation of sustainable resources, are receiving increasing attention in zinc-ion batteries (ZIBs) due to their resource abundance and tunable structure. However, organic molecules with high potential are rare, and the voltage of most reported organic cathode-based ZIBs is less than 1.2 V. Herein, we explored the redox process of p-type organics and figured out the relationship between energy change and voltage output during the process. Then, we proposed a dual-step strategy to effectively tune the energy change and eventually improve the output voltage of the organic electrode. Combining the regulation of the electron cloud of organic molecules and the manipulation of the solvation structure, the output voltage of an organosulfur compound based ZIB was greatly increased from 0.8 V to 1.7 V. Our results put forward a specific pathway to improve the working voltage and lay the foundation for the practical application of organic electrodes.

P‐Type Redox‐Active Organic Electrode Materials for Next‐Generation Rechargeable Batteries

p‐Type redox‐active organic materials (ROMs) draw increasing attention as a promising alternative to conventional inorganic electrode materials in secondary batteries due to high redox voltage, fast rate capability, environment friendliness, and abundance. First, fundamental properties of the p‐type ROMs regarding the energy levels and the anion‐related chemistry are briefly introduced. Then, the development progress of the p‐type ROMs is outlined in this review by classifying them according to their redox centers. The molecular design strategies employed for improving their electrochemical performance are discussed to guide further research. Finally, a summary of the electrochemical performance achieved, regarding voltage, specific energy with power, and cycle stability, is provided with perspectives.

Graphene and Polyethyleneimine Bilayer Wrapping onto Quinone Molecular Crystal Cathode Materials for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries are promising large-scale stationary batteries, owing to their safety and environmental friendliness. For cathode materials, the utilization of redox-active organic materials instead of conventional transition metal oxides is considered to be an effective approach to enhance specific capacity and eliminate resource constraints. However, the poor cyclability of these materials due to their poor electrical conductivity and high solubility hinders their practical application, calling for in-depth investigations to overcome these limitations. Herein, we demonstrate a facile graphene wrapping method for organic quinone materials. We resolve the strong Coulombic repulsion between graphene and quinone by using polyethyleneimine as an interlayer. The graphene- and polyethyleneimine-wrapped quinone exhibits superior cyclability in aqueous zinc-ion batteries by suppressing elution into the electrolyte and crystal growth of quinone during charge/discharge cycles. This bilayer wrapping concept can be widely applied to various organic materials to achieve an electrical conductivity and cyclability with high capacity.

Prediction of Pourbaix diagrams of quinones for redox flow battery by COSMO-RS

Redox-flow batteries are relevant to store energy from intermittent sources such as solar panels or wind turbines, thereby smoothing their energy supply. Up to now, most redox-flow batteries are based on vanadium. Vanadium is a rare and expensive material, thus recent research has focused on redox-flow batteries based on organic compounds and, in particular, anthraquinones as electroactive materials. However, the tunability of organic chemistry poses a needle-in-haystack challenge as the structures exhibiting the most desirable electrochemical properties may be hard to pinpoint. Moreover, the low water solubility of the most readily available anthraquinones may hinder their use as battery electrolytes. To aid in such endeavor, a theoretical approach is proposed to predict Pourbaix diagrams of redox-active organic compounds, allowing in silico anticipation of their electrochemical behavior in the entire pH range. DFT/COSMO-RS predicted pKa and reduction potentials are in good agreement with experimental data, and the resulting calculated Pourbaix diagrams are also in agreement with 4 experimental ones from literature, proving the reliability of the method. Finally, the effect of nature and position of some functional groups on the anthraquinone backbone is discussed, illustrating the power of the method to both understand and quantify the electrochemical activity of redox active organic materials.

Pyromellitic diimide based bipolar molecule for total organic symmetric redox flow battery

Non-aqueous redox flow batteries based on total organic electrolytes, consisting of earth-abundant elements, i.e., C, H, N, O, are regarded as a promising technology for sustainable and large-scale energy storage. In particular, bipolar redox-active organic materials (BROMs) are distinctly fascinating as electroactive species because they have multiple oxidation states and can undergo multiple redox processes. By virtue of this unique characteristic, BROMs can be utilized as both anolyte and catholyte in symmetric flow batteries, consequently, helping alleviate cross-contamination issues. Herein we report an all-organic symmetric redox flow battery based on diimide molecule as a bipolar electroactive material. The potential interval between the cathodic peak and the inner and outer anodic peaks led to a promising cell voltage of 2.22 V. The symmetric battery can be operated at a current density of 20 mA cm−2, with a coulombic efficiency of 90% for over 100 cycles. This work proposes an alternative approach to designing multi-electron organic redox active materials to facilitate the advancement of high-density symmetric flow batteries.

Versatile Redox-Active Organic Materials for Rechargeable Energy Storage.

With the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metal-based electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions.In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal-air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.

Combined Computational and Experimental Approach to Determine and Understand the Solubility of Phenothiazines as Redoxmers

Due to their property of decoupled power and capacity, non-aqueous redox flow batteries (NAqRFBs) provide an alternate solution to address the increase needed in grid storage. Phenothiazine (PT) class of derivatives possess desired properties such as high potential and chemical stability for redox-active catholyte materials for NAqRFBs. Nevertheless, most of the currently studied redox-active organic materials (ROMs), including certain phenothiazines suffer from low solubility specifically in their charged states and how the solubility trends change in charged states remains a mystery. Improving the solubility of ROMs without sacrificing stability or the redox potential requires a molecular-level understanding of how solubility changes in ROMs in general or for a specific class of materials. In fact, procurement of a universal understanding and ability to predict molecular solubility would serve the scientific community in well as solubility plays such an important role in almost every sub-discipline in chemistry, physics, engineering and, medicine. Quantitative structure-property relationships (QSPR) are known to be highly efficient due to their simplicity and ease of use. QSPR are mathematical relationships that link chemical structure and a chemical property (i.e. solubility) in a quantitative manner for a series of compounds. Hence, we used an approach of combining experiments and computational simulations with the aid of QSPR with the two major objectives of (1) Numerically predicting solubility of PTs at different states of charge and solution environments and (2) Understanding solubility trends and their physiochemical basis. These predictive models were constructed utilizing numerous experimental and computed molecular descriptors. A database of experimental, electronic structure, geometrical, topological descriptors containing around 1900 descriptors for neutral and radical cation forms of PT derivatives was constructed and the best descriptors were selected using various data science methods. Herein, we were able to establish a computational workflow that is robust and efficient in the optimization of conformationally flexible PT derivatives followed by parameter extraction.The major focus in the QSPR study was on the radical cation systems since the solubility usually suffers when going from neutral to charged state which is undesirable when considering NAqRFBs. QSPR models that can capture the solubility trends and predict the solubility of the validation set of compounds were validated with different statistical validation techniques. The statistical models developed were able to predict the solubility of compounds and were comparable with the experiments. Interestingly, these models were able to predict the solubility of the radical cation form of the compound secBuPT in 0.5 M tetraethylammonium tetrafluoroborate-acetonitrile (TEA/BF4-ACN) and pure acetonitrile (ACN) within a 10% deviation from the experimental value before experimental testing. Moreover, the analysis of molecular descriptors that were selected by successful models elucidates the contribution of factors such as having a surface area with delocalized positive charges, the effect of asymmetry and entropy, the susceptibility of the N atom on the PT core, the contribution of substituents at 3,7 positions and the effectiveness of these contributions with respect to strain/steric and charge hindrances. These findings were in agreement with our experimental results and MD simulations performed in similar efforts by the group and aid us in bringing more meaningful discussion on the solubility of these systems.In summary, the results and insights gained herein would set foundations on understanding and predicting the solubility of phenothiazine derivatives thus help experimentalists design better materials for non-aqueous redox flow batteries. Figure 1

Viologen Derivatives in Deep Eutectic Solvents for Energy Storage

Redox flow batteries (RFBs) serve as one of the most promising battery technologies to meet the grid-scale energy storage requirements, owing to their scalable nature and acceptable redox chemistry. To achieve economical and sustainable energy storage, there has been a rapid transition from redox-active inorganic materials to the earth-abundant and tunable redox-active organic compounds for RFBs. Among explored redox-active organic materials, viologen derivatives show promise for battery applications due to their two successive redox reactions and low cost. Taking advantage of both redox reactions, however, may not be possible in aqueous electrolytes due to the insolubility of the charge-neutral viologen in aqueous solutions.1 Deep eutectic solvents (DESs) can offer an inexpensive electrolyte for energy storage which could potentially solve the solubility issues associated with these materials.2 Owing to their low vapor pressure, DESs can be also utilized at elevated temperatures for lower viscosity and enhanced conductivity.3 In an attempt to identify a high-concentrated and reversible redox-active DES, the electrochemical properties and solubility of methyl, ethyl, and benzyl viologens in choline chloride (ChCl) and ethylene glycol (EG) (1:4 molar ratio) mixture were investigated. To better qualify these redox-active molecules as an anolyte material for RFBs, the redox reactions were studied to assess the redox reversibility, the potential separation between the consecutive electron transfer reactions, and the redox kinetics. The electron-transfer rate constants were estimated using linear sweep voltammetry (LSV) at a glassy carbon rotating-disk electrode in a three-electrodes configuration cell. Diffusion coefficients were also calculated from the steady-state diffusion-limited current using the rotating-disk electrode as well as a microelectrode. Elucidating the kinetic parameters, diffusion coefficients, and transport properties of the viologen derivatives in DESs will bring insight into designing a concentrated redox-active DES for energy storage. B. Hu and T. L. Liu, J. Energy Chem., 27, 1326 (2018).B. Chen, S. Mitchell, N. Sinclair, J. Wainright, E. Pentzer, and B. Gurkan, Mol. Syst. Des. Eng., 5, 1147 (2020).N. S. Sinclair, D. Poe, R. F. Savinell, E. J. Maginn, and J. S. Wainright, J. Electrochem. Soc., 168, 020527 (2021).

3D Network of Sepia Melanin and N‐ and, S‐Doped Graphitic Carbon Quantum Dots for Sustainable Electrochemical Capacitors

AbstractOrganic electrode materials operating in aqueous electrolytes offer the opportunity to avoid toxic, critical, and expensive materials for electrochemical energy storage. When deposited on carbon current collectors, redox active organic materials add faradaic to electrostatic capacitance contribution to the electrodes. Here, a 3D network electrode material is reported upon, based on sepia melanin, a quinone macromolecule, and nitrogen‐ and sulfur‐doped graphitic carbon quantum dots (N,S GCQDs) designed to achieve good electronic conductivity and electrolyte wettability. The effect of various undoped and doped carbon quantum dots is also investigated, synthesized from acetic acid and sucrose instead of graphite, on the electrochemical performance of sepia melanin. Sepia/N,S GCQD shows optimum areal capacitance (≈180 mF cm−2) that is about twice as high as sepia (≈77 mF cm−2) with lower charge transfer resistance (1 ohm for sepia/N,S GCQDs compared to 10 ohms for sepia). The sepia/N,S GCQD symmetric supercapacitor in 0.5 m Na2SO4(aq) exhibits promising capacitance retention ≈92% after 10 000 cycles at 5 A g−1, 100% coulombic efficiency, 11 µW h cm−2 and 102 mW cm−2 maximum energy and power densities. The work paves the way for stable and potentially biodegradable supercapacitor electrode materials for environmentally benign electrochemical energy storage.

Highly persistent triphenylamine-based catholyte for durable organic redox flow batteries

Organic redox flow batteries (ORFBs) have recently drawn significant attention as cost-effective and scalable energy storage systems, utilizing the distinct features of redox-active organic materials (ROMs), which offer chemical diversity and potential mass-scalability. Nevertheless, their inferior cycle performance compared with that of conventional inorganic-based RFBs is one of the major drawbacks limiting their practical application, which is often attributable to the chemical instability of charged ROMs during long-term operation. Herein, we present an ultra-stable triphenylamine-based molecule, tris(4-methoxyphenyl)amine or 3MTPA, which exhibits a highly stable redox reaction and rapid kinetics as a catholyte in non-aqueous media. It is demonstrated that the 3MTPA catholyte exhibits exceptionally robust radical cation stability, enabling its retention of near the theoretical capacity even after 168 h of high temperature storage in a fully charged state, while most known ROMs have not been able to achieve it and suffer from the significant degradation. Moreover, a flow cell exploiting 3MTPA is capable of delivering an unprecedentedly high capacity retention of 99.998% per cycle over 1400 cycles, opening up a new pathway toward highly durable ORFBs.

Rechargeable Aqueous Aluminum Organic Batteries.

Aqueous aluminum-ion batteries (AABs) are regarded as promising next-generation energy storage devices, and the current reported cathodes for AABs mainly focused on inorganic materials which usually implement a typical Al3+ ions (de)insertion mechanism. However, the strong electrostatic forces between Al3+ and the host materials usually lead to sluggish kinetics, poor reversibility and inferior cycling stability. Herein, we employ an organic compound with redox-active moieties, phenazine (PZ), as the cathode material in AABs. Different from conventional inorganic materials confined by limited lattice spacing and rigid structure, the flexible organic molecules allow a large-size Al-complex co-intercalation through reversible redox active centers (-C=N-) of PZ. This co-intercalation behavior can effectively reduce desolvation penalty, and substantially lower the Coulombic repulsion during the ion (de)insertion process. Consequently, this organic cathode exhibits a high capacity and excellent cyclability, which exceeds those of most reported electrode materials for AABs. This work highlights the anion co-intercalation chemistry of redox-active organic materials, which is expected to boost the development of high-performance multivalent-ion battery systems.

N-alkyl-carboxylate-functionalized anthraquinone for long-cycling aqueous redox flow batteries

Aqueous redox flow batteries (ARFBs) are regarded as promising candidates for large-scale energy storage. In particular, ARFBs based on organic redox-active species can largely reduce the capital cost and mineral resource requirements. However, the design of redox-active organic materials with desirable electrochemical performances needs to be further explored. Herein, we report a green and low-cost synthesis of a highly soluble N-alkyl-carboxylate-functionalized derivative of 2,6-diaminoanthraquinone, namely, N,N'-(9,10-anthraquinone-2,6-diyl)-di-β-alanine (DAEAQ), which exhibits a high voltage plateau and long cycling life when serving as an anolyte in ARFBs. The alkaline ARFBs based on DAEAQ anolyte and potassium ferrocyanide (K4Fe(CN)6) catholyte deliver a high open-circuit voltage of 1.12 V at pH ≥12. When sandwiched with a custom highly-sulfonated and nonfluorinated cation exchange membrane, the as-prepared ARFBs achieve an excellent maximum power density of 0.34 W cm−2 and an optimal capacity retention rate of 99.86% day−1. This work highlights the great potential of rationally designed anthraquinone derivatives in ARFBs for grid-scale renewable and sustainable energy storage applications.

A Method for Evaluating Soluble Redox Couple Stability Using Microelectrode Voltammetry

Soluble, redox-active organic materials hold promise as charge-storage species for flow batteries; however, chemical stability during extended operation remains a key challenge. While a number of spectroscopic and electrochemical techniques are currently used to probe these complex and often ill-defined decay pathways, these techniques have limitations, including lack of accessibility and potentially cumbersome preparatory steps. Here, we use microelectrode voltammetry to observe nonaqueous flow battery electrolytes directly, simultaneously identifying the rate of self-discharge (reversible material loss) and material decay (irreversible material loss). We validate this technique using ferrocene as a stable model redox couple, examine and address sources of error, and, finally, demonstrate its capability by assessing the decay of a well-studied and moderately-stable substituted dialkoxybenzene [2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene]. These results suggest that microelectrodes may have utility for rapid assessment of redox electrolyte state-of-charge and state-of-health, both in operando and post mortem.

Polycyclic Quinone and Its Dithiin Derivative As Cathode Materials for Sustainable Aqueous Zn-Organic Secondary Batteries

Aqueous Zinc-ion (Zn-ion) batteries are considered as attractive energy storage devices for large scale applications owing to their intrinsic safety, low cost and robustness1. However, their implementation is held back by sluggish diffusion kinetics of Zn2+ into inorganic host materials. Therefore, the focus is on redox active organic materials particularly quinone derivatives. These materials show moderate coulombic repulsions to the Zn2+ diffusion by having big tunnels in the crystal lattice2. Yet, very few organic materials have been reported for aqueous Zn-ion batteries. In the contextual importance of exploring new organic materials that can reversibly host Zn2+. Herein, we have explored highly conjugated pentacene-5,7,12,14-tetraone (PT) as cathode material for aqueous ZIBs for the first time. The versatile features such as strong π-stacking, high theoretical capacity (i.e. 317 mAh g-1 for 4e- transfer), structural diversity, inherent insolubility and non-defamation upon electrochemical process unlike simple quinones make PT a promising candidate to study. Moreover, we used filter paper as a separator instead of nafion membrane and ZnSO4 as supporting electrolyte salt which makes this battery more environmentally friendly and cost effective. In order to improve the rate capability and cycling stability, PT was incorporated in the mesoporous nanonetwork of CMK-3 carbon. In addition, the discharge voltage of the cell was enhanced by introducing sulfur atoms in the PT skeleton. Figure 1