Redox-active Materials Research Articles

One‐Step Electrodeposition of Iron Oxyhydroxide Onto 3D Porous Graphene Substrates for on Chip Asymmetric Micro‐Supercapacitors

AbstractElectrochemical capacitors based on redox active materials can achieve greater capacitance values than traditional electric double layer composites. Herein, electrodeposition of iron oxyhydroxide from a mildly acidic acetate precursor is reported. The one‐step deposition resulted in a submicron film composed of FeOOH phase, which was confirmed via Raman and x‐ray photoelectron spectroscopy. The capacitance increased linearly with loading amount and achieved a maximum at 1600 mC deposition with 120 mF cm−2 at 25 mV s−1 after which the film became more resistive, limiting electrolyte access to the porous graphene substrate. The deposited FeOOH demonstrated promising rate capability and good cycling stability, without phase changes, retaining 82 % of the initial capacitance after 5000 consecutive charge/discharge cycles. The charge storage mechanism of FeOOH was determined via in situ Raman spectroscopy, which followed reversible iron oxygen vibration changes upon cycling which become more intense upon reduction as a result of sodium ion intercalation. Furthermore, an asymmetric configuration full cell combining FeOOH/MnO2 allowed the working voltage to be extended to 2 V, maintaining an ideal capacitor behaviour, and achieving a maximum energy and power density of 21 μWh cm−2 and 2.5 mW cm−2 respectively.

Oxygen-Stable Electrochemical CO2 Capture using Redox-Active Heterocyclic Benzodithiophene Quinone.

Electrochemical carbon capture offers a promising alternative to thermal amine technology, which serves as the traditional benchmark method for CO2 capture. Despite its technological maturity, the widespread deployment of thermal amine technologies is hindered by high energy consumption and sorbent degradation. In contrast, electrochemical methods, with their inherently isothermal operation, address these challenges, offering enhanced energy efficiency and robustness. Among emerging strategies, electrochemical carbon capture systems using redox-active materials such as quinones stand out for their potential to capture CO2. However, their practical application is currently limited by their low stability in the presence of oxygen. We demonstrate that benzodithiophene quinone (BDT-Q), a heterocyclic quinone, exhibits high stability in electrochemical carbon capture processes with oxygen-containing feed gas. Conducted in a cyclic flow system with a simulated flue gas mixture containing 13 % CO2 and 3.5 % O2 for over 100 hours, the process demonstrates high oxygen stability with an electron utilization of 0.83 without significant degradation, indicating a promising approach for real world applications. Our study explores the potential of new heterocyclic quinone compounds in the context of carbon capture technologies.

GdVO4:Eu3+ and LaVO4:Eu3+ Nanoparticles Exacerbate Oxidative Stress in L929 Cells: Potential Implications for Cancer Therapy

The therapeutic potential of redox-active nanoscale materials as antioxidant- or reactive oxygen species (ROS)-inducing agents was intensely studied. Herein, we demonstrate that the synthesized and characterized GdVO4:Eu3+ and LaVO4:Eu3+ nanoparticles, which have been already shown to have redox-active, anti-inflammatory, antibacterial, and wound healing properties, both in vitro and in vivo, worsen oxidative stress of L929 cells triggered by hydrogen peroxide or tert-butyl hydroperoxide (tBuOOH) at the concentrations that are safe for intact L929 cells. This effect was observed upon internalization of the investigated nanosized materials and is associated with the cleavage of caspase-3 and caspase-9 without recruitment of caspase-8. Such changes in the caspase cascade indicate activation of the intrinsic caspase-9-dependent mitochondrial but not the extrinsic death, receptor-mediated, and caspase-8-dependent apoptotic pathway. The GdVO4:Eu3+ and LaVO4:Eu3+ nanoparticle-induced apoptosis of oxidatively compromised L929 cells is mediated by ROS overgeneration, Ca2+ overload, endoplasmic reticulum stress-associated JNK (c-Jun N-terminal kinase), and DNA damage-inducible transcript 3 (DDIT3). Our findings demonstrate that GdVO4:Eu3+ and LaVO4:Eu3+ nanoparticles aggravate the oxidative stress-induced damage to L929 cells, indicating that they might potentially be applied as anti-cancer agents.

New Redox Chemistries of Halogens in Aqueous Batteries.

Halogen-based redox-active materials represent an important class of materials in aqueous electrochemistry. The existence of versatile halogen species and their rich bonding coordination create great flexibility in designing new redox couples. Novel redox reaction mechanisms and electrochemical reversibility can be unlocked in specifically configurated electrolyte environments and electrodes. In this review, the halogen-based redox couples and their appealing redox chemistries in aqueous batteries, including redox flow batteries and traditional static batteries, that have been studied in recent years are discussed. New aqueous electrochemistry provides hope to outperform the state-of-the-art materials and systems that are facing resources and performance limitation, and to enrich the existing battery chemistries.

Designing Alkylammonium Cations for Enhanced Solubility of Anionic Active Materials in Redox Flow Batteries: The Role of Bulk and Chain Length.

Advancing grid-scale energy storage technologies is crucial for realizing a fully renewable energy landscape, with non-aqueous redox flow batteries (NRFBs) presenting a promising solution. One of the current challenges in NRFBs stems from the low energy density of redox active materials, primarily due to their limited solubility in non-aqueous solvents. Herein, this study explores the solubility of vanadium(IV/V) bis-hydroxyiminodiacetate (VBH) crystals in acetonitrile, aiming to use them as anionic catholytes in NRFBs. We focused on enhancing VBH solubility by modifying the structure of the alkylammonium cation. Employing periodic density functional theory and a solvation model, we calculated the dissolution free energy ([[EQUATION]]), which includes sublimation ([[EQUATION]]) and solvation ([[EQUATION]]) energies. Our results indicate that neither elongating straight-chain alkyl groups beyond a tetrabutylammonium baseline nor introducing bulky substituents at the nitrogen center significantly enhances solubility. However, the introduction of carbon spacers combined with terminal bulky substituents markedly improves solubility by favorably altering both [[EQUATION]] and [[EQUATION]]. These findings underline the nuanced impact of cation structure on solubility and suggest a viable approach to optimize VBH-based anionic catholytes. This advancement promises to enhance NRFB efficiency and sustainability, marking a significant step forward in energy storage technology.

High-voltage and highly reversible redox chemistry of in situ cross-linked polydiphenylamine as an aqueous zinc battery cathode

Organic redox-active materials are promising electrode candidates for the next generation of rechargeable batteries owing to their versatile redox chemistry, sustainability, and potential low cost. Here, we present polydiphenylamine as a moderately high-voltage (1.25 V vs. Zn) and highly reversible organic cathode for aqueous zinc batteries. As revealed by cyclic voltammetry and spectroscopic analysis, the chemical oxidation–derived diphenylamine dimer electrochemically transforms into a cross-linked polymeric material during the first electrochemical oxidation (charge), which then operates by p-doping/dedoping redox of the amine center and a pseudocapacitor-like solid-state charge storage mechanism during the subsequent discharge-charge cycles. With an 80 wt% active loading, the polymer cathode delivers a specific capacity of 138 mAh/g at 100 mA/g, with 73% retention over 1000 cycles at a 99.8% Coulombic efficiency. Excellent rate capability is evidenced by 87 mAh/g capacity at 400 mA/g and highly stable cycling of over 1200 cycles under variable current rates. Facile one-step synthesis, fast charge storage kinetics, and attractive electrochemical performance establish polydiphenylamine as a notable organic cathode candidate for aqueous zinc batteries.

NaLiFe(C2O4)2: A polyanionic Li/Na-ion battery cathode exhibiting cationic and anionic redox

Recently, polyanionic compounds have received great interest as alternative cathode materials to conventional oxides due to their different advantages in cost, safety, structural stability, as well as being environmentally friendly. However, the vast majority of polyanionic materials reported so far rely exclusively upon the redox reaction of the transition metal for lithium/sodium transfer.The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is a promising strategy to enhance the energy density of polyanionic cathode materials for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution.Herein, we report NaLiFe(C2O4)2 as a new positive electrode and use different characterization techniques such as Raman spectroscopy and Mössbauer analyses to characterise this dual-ion redox process experimentally. First-principles calculations also help to understand the interactions between the transition metal and the oxalate group as the main factor that modulates the cationic and polyanionic redox couples in these materials.

Interface engineering of laminated redox-copolymers for electrochemical remediation of perfluoroalkyl substances

Redox polymers now show promise for selective removal of perfluoroalkyl substances from a complex aqueous matrix at high energy efficiency and no secondary pollution. However, self-agglomeration of redox polymers is a dominating barrier to constructing a low-cost, stable system in practical applications. Herein, we introduce polymer interface engineering strategy to enhance the activity of amine functionalities and nitrous oxide radicals in copolymers composed of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl and 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine. The grafted laminated redox polymer on multi-walled carbon nanotubes (termed MWCNT-g-P(TMPMA-co-TMA)) via free-radical polymerization demonstrated a 19.1–59.9 % increase in the electrosorption capacity with a maximum value (Qm) of 725.09 mg g−1 and a pseudo-first-order rate constant k1 = 1.46 min−1 and sustainable regeneration efficiency at ∼80 %. The core–shell structure of MWCNT-g-P(TMPMA-co-TMA) with a laminated redox polymer layer of ∼5 nm processed a low charge transfer resistance, which is expected to contribute to the enhanced capture performance and stability. An integrated system consisting of a MWCNT-g-P(TMPMA-co-TMA) electrode and a boron-doped diamond electrode can efficiently separate and degrade trace-concentration PFOA from industrial wastewater at high mineralization efficiency. This work provides insight into the design and optimization of redox active materials for water purification and beyond.

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.

Spontaneous Oxidation of Organic Matter and Ammonium Uptake from Manure Wastewater by Redox-Active Materials

Spontaneous Oxidation of Organic Matter and Ammonium Uptake from Manure Wastewater by Redox-Active Materials

High-Stable All-Iron Redox Flow Battery with Innovative Anolyte based on Steric Hindrance Regulation.

All-soluble all-iron redox flow batteries (AIRFBs) are an innovative energy storage technology that offer significant financial benefits. Stable and affordable redox-active materials are essential for the commercialization of AIRFBs, yet the battery stability must be significantly improved to achieve practical value. Herein, ferrous complexes combined with the triisopropanolamine (TIPA) ligand are identified as promising anolytes to extend battery life by reducing cross-contamination due to a pronounced steric hindrance effect. The coordination structure and failure mechanism of our Fe-TIPA complexes were determined by molecular dynamics simulation and spectroscopic experiments. By coupling with [Fe(CN)6]4-/3-, Fe-TIPA/Fe-CN AIRFBs retained excellent stability exceeding 1831 cycles at 80 mA ⋅ cm-2, yielding an energy efficiency of ~80 % and maintaining a steady discharge capacity. Moreover, the all-soluble electrolyte was tested in an industrial-scale Fe-TIPA/Fe-CN AIRFB prototype energy storage system, where an energy efficiency of 81.3 % was attained. Given the abundance of iron resources, we model the TIPA AIRFB electrolyte cost to be as low as 32.37 $/kWh, which is significantly cheaper than the current commercial level. This work demonstrates that steric hindrance is an effective measure to extended battery life, facilitating the commercial development of affordable flow batteries.

A flexible solid-state asymmetric supercapacitor device comprising cobalt hydroxide and biomass-derived porous carbon.

Development in the field of alternative and renewable energy sources is becoming necessary considering the current energy demands of the growing technologies. The main challenge associated with the produced energy is to store it for future use, such that it can be used when needed. Supercapacitors are among the electrochemical energy storage systems that provides higher power density, faster charging-discharging, high specific capacitance (C S), and long cycling life. Herein, the fabrication of a flexible solid-state asymmetric supercapacitor (ASC) device is reported, where Co(OH)2 hollow spheres and biomass-derived porous carbon (PC) are the cathode and anode, respectively. Co(OH)2 is a highly redox active material, whereas PC is an electric double-layer capacitive (EDLC) material. In this device, aqueous KOH solution (electrolyte) encapsulated in PVA gel (separator) was used to bind the electrodes. This Co(OH)2//PC ASC device exhibited a high C S of 260 F g-1 (at 2 A g-1). It retained ∼91% of the initial C S value (at 6 A g-1) till ∼5000 cycles. Electrochemical impedance spectroscopy (EIS) study confirmed low internal resistance (0.95 Ω) and charge transfer resistance (1.41 Ω) values of Co(OH)2//PC. These results indicate that the high electron transfer process in the electrode-electrolyte interface during the electrochemical reaction, which is responsible for the excellent performance of this ASC device. The high-performance Co(OH)2//PC ASC device exhibited an energy density of 76.7 W h kg-1 at a power density of 1416.9 W kg-1. To demonstrate its practical use, LED lights were illuminated using this Co(OH)2//PC ASC device.

Aniline tetramer conjugated N-doped graphene aerogel enabling efficient pH-universal aqueous supercapacitors

The development of novel negative electrode (anode) materials for efficient aqueous supercapacitors (SCs) remains appealing yet significantly challenging. Here we propose an aniline tetramer conjugated nitrogen-doped graphene aerogel (AT−NGA) as the anode material, exhibiting a maximum capacitance of 699.1F g−1 under 1 A/g in 1 M H2SO4 as well as a long lifespan of 6,000 cycles at all pH levels. In particular, its capacitive contribution is 94.1 %, superior to the best pseudocapacitive materials known. To evaluate its pH-universality, we assembled three asymmetric SCs, namely, AT−NGA//1 M H2SO4//graphene aerogel, AT−NGA//1 M Na2SO4//NaMnO2-x and AT−NGA//1 M KOH//NiCoFe layered double hydroxide. The acid device delivers maximal energy and power densities of 35.8 mWh g−1 and 13.0 W/g, the neutral device achieves a maximal energy and power densities of 71.8 mWh g−1 and 33.0 W/g, and the base device exhibits a maximal energy and power densities of 48.2 mWh g−1 and 18.0 W/g, respectively. All the SCs display an outstanding cycling performance over 5,000 cycles (especially, 96 % capacitance retention for the acidic device after 12,000 cycles). Our design can also be expanded to prepare other redox-active anode materials for efficient aqueous SC applications.

(Invited) Recent Progress in Organic-Based Aqueous Flow Batteries

We have developed high performance flow batteries based on the aqueous redox behavior of small organic and metalorganic monomers composed of earth-abundant elements. These redox active materials can be inexpensive and exhibit rapid redox kinetics and high solubilities, potentially enabling rapid scaling of flow batteries at reduced cost. Adequate molecular lifetime has been one of the most challenging requirements to satisfy. We have shown that the amount of lost capacity is determined by the molecular calendar life, which can depend on state of charge, but is independent of the number of charge-discharge cycles imposed. I will discuss recent progress in molecule development and in measuring extremely small capacity fade rates and will discuss how an understanding of molecular decomposition mechanisms has permitted us to design molecules with decadal projected lifetimes and even to reverse capacity fade by recomposing decomposed molecules within the functioning battery electrolyte.

Operando Visualization of Redox Flow Batteries Using Fluorescence Spectroscopy with Alizarin Red S Electrolyte

Redox flow batteries (RFBs) charge and discharge by transferring electrons between two pairs of redox species separated by a proton exchange membrane. Unlike traditional batteries that store energy in redox-active materials within an enclosed cell, RFBs store energy in liquid electrolytes housed in external reservoirs1. These electrolytes are pumped through the cell, enabling redox reactions on the surface of electrodes. The energy storage capacity of RFBs is determined by the concentration of the active species and the size of the reservoirs, making them a promising technology for large-scale energy storage applications2. While RFBs possess advantages such as long lifetime, flexible design for battery capacity and power, and rapid response, they suffer from drawbacks like low energy density, power density, and energy efficiency3. Efforts to address these issues have primarily focused on modifications to the electrode materials or the flow field4 , 5. However, evidence supporting performance enhancements often relies on global parameters such as energy efficiency, energy density, and voltage drop, lacking detailed mechanistic and microscopic studies. Real-time observation of the electrode-electrolyte interface during redox reactions and the transport of redox species in the flow field holds the potential to significantly contribute to understanding the electrode structure and mass transport phenomena in RFBs6. This understanding, in turn, facilitates prediction and purposeful enhancement of battery performance by developing desirable structures for both electrode and flow fields. Microscopy, particularly confocal microscopy, serves as a powerful tool for observing microscopic structures. Confocal microscopes yield sharper images with higher resolution than widefield microscopy, enabling rapid acquisition of high-resolution images.In this study, we employed a confocal laser scanning microscope for operando visualization of RFBs. We designed a transparent framework for the battery, allowing light to pass through, and covered its surface with a glass cover to visualize the porous carbon electrode. We selected a fluorescent redox-active species, Alizarin Red S to enable visualization using fluorescence spectroscopy. Leveraging its unique fluorescence properties, we established a correlation between fluorescence intensity and the state of charge. This innovative approach enables us to observe the distribution of electrolytes, the dynamics of redox reactions on the electrode surface (with fast speed of imaging ＜0.2s), the mass transfer behavior of the electrolyte on the electrode surface, and the uniformity of electrolyte flow in the flow field, including areas of stagnation or recirculation. Zhang, J. et al. An all-aqueous redox flow battery with unprecedented energy density. Energy Environ. Sci. 11, 2010–2015 (2018).Rubio-Garcia, J., Kucernak, A. & Charleson, A. Direct visualization of reactant transport in forced convection electrochemical cells and its application to redox flow batteries. Electrochem. commun. 93, 128–132 (2018).Tolmachev, Y. V. Flow Batteries From 1879 To 2022 And Beyond. Qeios 1–79 (2023).Wang, R. et al. Gradient-distributed NiCo2O4 nanorod electrode for redox flow batteries: Establishing the ordered reaction interface to meet the anisotropic mass transport. Appl. Catal. B Environ. 332, 122773 (2023).Kumar, S. & Jayanti, S. Effect of flow field on the performance of an all-vanadium redox flow battery. J. Power Sources 307, 782–787 (2016).Wong, A. A., Rubinstein, S. M. & Aziz, M. J. Direct visualization of electrochemical reactions and heterogeneous transport within porous electrodes in operando by fluorescence microscopy. Cell Reports Phys. Sci. 2, 100388 (2021).

Proton Activity and Pathway in Aqueous Organic Redox Flow Battery Electrolyte

Aqueous soluble organic (ASO) redox-active materials have recently shown great promise as alternatives to transition metal ions employed as energy-bearing active materials in redox flow batteries for large-scale energy storage because of their structural tunability, cost-effectiveness, availability, and safety features. However, development so far has been limited to a small palette of organics that are aqueous soluble. This presentation will use fluorenone as an example to showcase how a natively redox-inactive molecule can be tuned to possess two-electron redox reversibility through hydrogenation and dehydrogenation. The modified fluorenone molecules demonstrated high energy density and recorded stable cycling. Furthermore, research has shown the unique chemical-electrochemical coupled redox reactions mechanism; thus, the system rate capabilities can be improved by incorporating suitable hydrogen acceptors that regulate the proton pathway for faster kinetics.Reference:Feng et al., Science 372, 836–840, 2021Feng et al., Joule 7, 1–14, 2023

(Invited) Computational Framework and Insights Towards Electrolyte Design for Redox Flow Batteries

Achieving a fully renewable energy future requires the development of grid-scale energy storage technologies, and non-aqueous redox flow batteries (NRFBs) hold considerable potential.1 However, the redox-active materials (RAMs) currently in consideration as lead candidates for NRFBs exhibit shortcomings, including low solubility, impaired transport properties, and low reduction potentials. Utilizing theory and computation is invaluable in predicting the structure-property relationship of RAM as NRFB active materials. The numerous components within NRFB electrolytes, spanning solvents, active materials, and supporting salts, suggest that material discovery could experience a substantial acceleration through computational and machine-learning approaches.This presentation will provide a progress update on our efforts towards a computational framework involving quantum mechanical (QM), molecular dynamics (MD), and machine learning methods to predict fundamental chemical and physicochemical properties of redox-active materials for energy storage systems. We used an extremely stable, bio-inspired vanadium(4+) bis-hydroxyiminodiacetic acid (VBH) as the scaffold to explore the large parameter space of NRFB systems.2–4 First, we show our framework based on density functional theory (DFT) for calculating solubilities, incorporating both lattice and solvation energies, that can be calibrated and used with synthetic verifications. We found that lattice free energy, which has previously been neglected, has a significant role in tuning electrolyte solubility. Second, we describe our MD simulations on the concentration and composition dependence of transport properties, such as viscosity, using DFT-derived force field bonded parameters for the novel [VBH]2- anion to understand the interactions between the ions and the solvent. Third, we demonstrate that QM molecular design could finely adjust the reduction potential. These fundamental investigations would offer insights into electrolyte design and how molecular properties dictate the macroscopic properties that contribute to specific performance aspects of advanced battery electrolytes.1 Y. Huang, S. Gu, Y. Yan and S. F. Y. Li, Current Opinion in Chemical Engineering, 2015, 8, 105–113.2 H. Huang, R. Howland, E. Agar, M. Nourani, J. A. Golen and P. J. Cappillino, J. Mater. Chem. A, 2017, 5, 11586–11591.3 S. K. Pahari, T. C. Gokoglan, B. R. B. Visayas, J. Woehl, J. A. Golen, R. Howland, M. L. Mayes, E. Agar and P. J. Cappillino, RSC Adv., 2021, 11, 5432–5443.4 B. R. B. Visayas, S. K. Pahari, T. C. Gokoglan, J. A. Golen, E. Agar, P. J. Cappillino and M. L. Mayes, Chem. Sci., 2021, 12, 15892–15907.

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

The Challenge of Polyanion Redox Process in Oxalates

Lately, polyanionic compounds have received great interest as alternative cathode materials to conventional oxides due to their different advantages in cost, safety, structural stability, as well as being environmentally friendly. However, the vast majority of polyanionic cathodes reported so far rely primarily upon the redox reaction of the transition metal for lithium/sodium transfer. The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is considered to be a promising strategy to successfully enhance the energy density of polyanionic cathodes for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group itself also shows redox behaviour enabling reversible charge/discharge and high capacity without gas evolution.Recently, our group reported dual ion redox in the lithium iron oxalate, in the potassium iron oxalate and in the sodium lithium iron oxalate, Li2Fe(C2O4)2, K2Fe(C2O4)2 & NaLiFe(C2O4)2 1 ,2,3 in which both the iron and the oxalate group appear to exhibit reversible redox activity. We also found this phenomenon present in the lithium-rich KLi3Fe(C2O4)3.4 Different characterisation techniques such as Raman spectroscopy or Synchrotron HAXPES and XAS analyses make it possible to show this phenomenon experimentally. First-principles calculations also help to understand the interactions between the transition metal and the oxalate group as the main factor that modulates the cationic and polyanionic redox couples in these materials.Our current results suggest that this phenomenon could be widespread among transition metal oxalates.Moreover, the oxalate anion is a particularly versatile species and may be monodentate, bidentate, tridentate, or even tetradentate, giving rise to a huge range of possible compounds and a very rich structural chemistry. It is important to establish the structural requirements that give rise to polyanion redox behaviour and a range of materials are being synthesised and characterised by our group. In addition to pure oxalates, it is possible to prepare materials with mixed polyanions e.g., oxalate/phosphate.In our work, we demonstrate that oxalate has a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.(1) Yao, W.; Armstrong, A. R.; Zhou, X.; Sougrati, M. T.; Kidkhunthod, P.; Tunmee, S.; Sun, C.; Sattayaporn, S.; Lightfoot, P.; Ji, B.; Jiang, C.; Wu, N.; Tang, Y.; Cheng, H. M. An Oxalate Cathode for Lithium Ion Batteries with Combined Cationic and Polyanionic Redox. Nat. Commun. 2019, 10 (1), 1–9.(2) Pramanik, A.; Manche, A. G.; Sougrati, M. T.; Chadwick, A. G.; Lightfoot, P.; Armstrong, A. R. K2Fe(C2O4)2: a New Oxalate Cathode for Li/Na ion Batteries Exhibiting a Combination of Multielectron Cation and Anion Redox. Chem.of Mat. 2023, 35, 2600–2611.(3) Manche, A. G.; Pramanik, A.; Lindgren, F.; Ericsson, T.; Häggström, L.; Armstrong, A. R. NaLiFe(C2O4)2: A New Iron-Based Polyanionic Cathode for Next-Generation Li/Na-ion Battery Exhibiting Reversible Cationic and Anionic Redox Behaviour. Energy Storage Mat. 2024, In review (4) Pramanik, A.; Manche, A. G.; Clulow, R.; Lightfoot, P.; Armstrong, A. R. Exploiting Anion and Cation Redox Chemistry in Lithium-Rich Perovskite Oxalate: A Novel next-Generation Li/Na-Ion Battery Electrode. Dalt. Trans. 2022, 51 (33), 12467–12475.

Sodium Polysulfides As Catholytes for Redox Flow Batteries

Redox flow batteries based on sodium polysulfide (Na2Sx) show promise due to sodium's high solubility in organic solvents (e.g., diglyme. However, when x is lower than 5, sodium polysulfide exhibits low solubility (less than 0.1 molal), limiting the nominal capacity of this redox-active material. Sodium phosphorothioate complexes, Na2P2Sx, have been reported as an alternative to Na2Sx. Incorporating phosphorus into the polysulfide structure improves their solubility in organic solvents, specifically in diglyme.This presentation will describe the synthesis and characterization of sodium phosphorothioate complexes (Na2P2Sx, where x = 6, 7, 8, and 9) for nonaqueous redox flow batteries. The structure of these materials is explored using 31P NMR and 23Na NMR spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. Overall, 31P NMR results indicate that P5+ is tetrahedrally coordinated in all structures and that Na-P-S compounds have a common PS3- moiety. Ongoing efforts are aimed at providing more precise assignments to the 31P bands and correlating these findings with complementary structural information obtained from 23Na NMR and Raman spectroscopy.Electrochemical properties of sodium polysulfide and sodium thiophosphate catholytes, specifically Na2S8 and Na2P2S8, were also compared using CV. The voltammogram of Na2S8 revealed two clear redox reactions in the 1.5-3 V range vs. Na/Na+ due to a multi-step electron transfer process involving the S2-/S0 redox center. In contrast, Na2P2S8 exhibited a larger peak separation (~2 V) and lower peak currents at a given scan rate, potentially indicating a significant activation barrier associated with the rearrangement of the thiophosphate’s polyanionic structure during reduction/oxidation. Despite the substantial voltage hysteresis, Na2P2S8 displayed stable cycling performance, with a slight increase in peak current after 100 cycles. This behavior is consistent with measurements on RFBs containing a Na2P2Sx catholyte, where an electrode activation process increased the cell’s reversible capacity, likely due to subtle changes in the electrode’s surface chemistry during repeated oxidation/reduction. Finally, preliminary flow cell measurements on Na2P2S7 catholyte show reversible voltage profiles after the formation step with deep discharge. Overall, this presentation demonstrates that Na-P-S catholytes represent a promising material for high energy, low-cost redox flow batteries.AcknowledgementsThis research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program in the Office of Electricity.