Organic Redox Research Articles

Non-aqueous organic redox active materials for a bicontinuous microemulsion-based redox flow battery

To mitigate fluctuations in renewable energy sources, redox flow batteries (RFBs) attract much attention for stationary energy storage applications by taking advantage of decoupling power and energy. However, there are still some limitations for conventional RFBs, including low energy density, a narrow electrochemical window (∼1.23 V) for an aqueous electrolyte, and low ionic conductivity for a non-aqueous electrolyte. Herein, we demonstrate a bicontinuous microemulsion-based RFB using methyl-p-benzoquinone and 2,2,6,6-Tetramethyl-1-piperidinyloxy as anolyte and catholyte redox active materials, respectively. By stabilizing an aqueous and organic medium at a nanometer scale through a surfactant, the microemulsion electrolyte exhibits a wider electrochemical window and good ionic conductivity, a promising alternative to increase the energy density by applying non-aqueous redox species with high solubility and high redox potential. The resulting cell exhibits Coulombic efficiency of 80%–85% and capacity retention of 50% over 30 cycles. We provide a systemic analysis of performance limitation and capacity decay and proposed a possible dimerization mechanism for an anolyte through a density functional theorycalculation, ending with a discussion of the efforts needed to realize the application of microemulsion-based RFBs.

Development of organic redox‐active materials in aqueous flow batteries: Current strategies and future perspectives

AbstractAqueous redox flow batteries, by using redox‐active molecules dissolved in nonflammable water solutions as electrolytes, are a promising technology for grid‐scale energy storage. Organic redox‐active materials offer a new opportunity for the construction of advanced flow batteries due to their advantages of potentially low cost, extensive structural diversity, tunable electrochemical properties, and high natural abundance. In this review, we present the emergence and development of organic redox‐active materials for aqueous organic redox flow batteries (AORFBs), in particular, molecular engineering concepts and strategies of organic redox‐active molecules. The typical design strategies based on organic redox species for high‐capacity, high‐stability, and high‐voltage AORFBs are outlined and discussed. Molecular engineering of organic redox‐active molecules for high aqueous solubility, high chemical/electrochemical stability, and multiple electron numbers as well as satisfactory redox potential gap between the redox pair is essential to realizing high‐performance AORFBs. Beyond molecular engineering, the redox‐targeting strategy is an effective way to obtain high‐capacity AORFBs. We further discuss and analyze the redox reaction mechanisms of organic redox species based on a series of electrochemical and spectroscopic approaches, and succinctly summarize the capacity degradation mechanisms of AORFBs. Furthermore, the current challenges, opportunities, and future directions of organic redox‐active materials for AORFBs are presented in detail.

Mass Transfer Behaviors and Battery Performance of a Ferrocyanide-Based Organic Redox Flow Battery with Different Electrode Shapes

The ferrocyanide-based organic redox flow battery (ferrocyanide-based ORFB), based on electrochemistry, has become a potential energy storage technology due to its low price, eco-friendliness, safety, and convenience. However, its low efficiency and poor mass transfer performance hinder the application of the ORFB. The influence of the electrode shape (trapezoid, sector, and rectangle) on the mass transfer and battery performance are studied based on a numerical model, which is verified by the experiments. The results show that battery performance of the trapezoid electrode is better than that of the sector and rectangle electrode. The discharge voltage of the rectangle battery is the lowest, and the discharge voltage of the trapezoid battery is the highest. The discharge voltage of the rectangle battery is 4.47% lower than that of the trapezoid battery. The uniformity factor value of the trapezoid battery is 26.9% higher than that of the rectangle battery. The trapezoid shape is the best design for the electrode, contributing to the application of the ferrocyanide-based ORFBs.

Palaeoenvironment of the Albian‐Cenomanian Mundeck Formation in the Douala Basin (SW, Cameroon): Evidence from facies analysis and geochemistry

The Lower Cretaceous syn‐rift deposits of the Douala Basin (Mundeck Formation), located in the equatorial West African margin were investigated in order to determine the palaeoenvironment and organic matter source, redox conditions, palaeoclimate, and tectonic settings, based on facies, organic and inorganic geochemical analyses. The sediments consist of pebble, coarse, medium‐ to fine‐grained sandstone, siltstone, and grey to black fossiliferous shales. The shales are characterized by an average total organic carbon (TOC) of 1.02%, fair to poor source generative potential with present‐day hydrogen index values, which range from 31 to 350 mg HC/g TOC suggesting that the shale contains Type III kerogens with mainly gas generative potential. The organic particles are dominated by phytoclasts (40% on average) and non‐fluorescent amorphous organic matter (34% on average). The palynomorphs represent about 26% on average with a large amount of pollen grain and a minor amount of dinoflagellate cysts and microforaminifera test linings. The inferred depositional environment of the studied sediments is an alluvial fan to fluvio‐lacustrine with shallow marine incursion. This depositional environment received the contribution of aquatic algae and microorganisms with a large amount of terrestrial organic matter, evidenced by the presence of some dinoflagellate cysts and microforaminifera test linings, and relatively higher amounts of SiO2, Al2O3, and phytoclasts. The presence of trace elements such as Ba, Sr, V, Ni, Co and Cr in the analysed samples and their ratio, suggest suboxic to anoxic conditions of deposition with low salinity. The warm to cold and dry climatic conditions prevailed during the deposition of the Mundeck deposits and sediments from active continental margin setting during the lower Cretaceous periods in the Douala Basin as confirmed by inorganic geochemical data. This study sheds light on the early stage of the evolution of the Douala Basin which could be integrated into the context of a mid‐Jurassic to lower Cretaceous regional evolution of the opening of a passive margin. This evolution is characterized by an alluvial fan, fluviatile, lacustrine, lagoon and marine depositional system. This depositional system is recognized as part of the petroleum system recognized within the syn‐rift deposits in the others basins along the West African margin, where giant hydrocarbons have been discovered.

Nitrogen-Doped 4H Silicon Carbide Single-Crystal Electrode for Selective Electrochemical Sensing of Dopamine.

In this work, we designed, fabricated, and characterized the first nitrogen (N)-doped single-crystalline 4H silicon carbide (4H-SiC) electrode for sensing the neurotransmitter dopamine. This N-doped 4H-SiC electrode showed good selectivity for redox reactions of dopamine in comparison with uric acid (UA), ascorbic acid (AA), and common cationic ([Ru(NH3)6]3+), anionic ([Fe(CN)6]3-), and organic (methylene blue) redox molecules. The mechanisms of this unique selectivity are rationalized by the unique negative Si valency and adsorption properties of the analytes on the N-doped 4H-SiC surface. Quantitative electrochemical detection of dopamine by the 4H-SiC electrode was achieved in the linear range from 50 nM to 10 μM with a detection limit of 0.05 μM and a sensitivity of 3.2 nA.μM-1 in a pH = 7.4 phosphate buffer solution. In addition, the N-doped 4H-SiC electrode demonstrated excellent electrochemical stability. This work forms the foundation for developing 4H-SiC as the next-generation robust and biocompatible neurointerface material for a broad range of applications such as the in vivo sensing of neurotransmitters.

The origins of photosynthetic systems: Clues from the phosphorus and sulphur chemical scenarios

Photosynthesis is the predominant biochemical process of carbon dioxide assimilation in the biosphere. To reduce carbon dioxide into organic compounds, photosynthetic organisms have one or two distinct photochemical reaction centre complexes with which they capture solar energy and generate ATP and reducing power. The core polypeptides of the photosynthetic reaction centres show low homologies but share overlapping structural folds, overall architecture, similar functional properties and highly conserved positions in protein sequences suggesting a common ancestry. However, the other biochemical components of photosynthetic apparatus appear to be a mosaic resulting from different evolutionary trajectories. The current proposal focusses on the nature and biosynthetic pathways of some organic redox cofactors that participate in the photosynthetic systems: quinones, chlorophyll and heme rings and their attached isoprenoid side chains, as well as on the coupled proton motive forces and associated carbon fixation pathways. This perspective highlights clues about the involvement of the phosphorus and sulphur chemistries that would have shaped the different types of photosynthetic systems.

Cation‐Anion Redox Active Organic Complex for High Performance Aqueous Zinc Ion Battery

Organic redox compounds are attractive cathode materials in aqueous zinc‐ion batteries owing to their low cost, environmental friendliness, multiple‐electron‐transfer reactions, and resource sustainability. However, the realized energy density is constrained by the limited capacity and low voltage. Herein, copper‐tetracyanoquinodimethane (CuTCNQ), an organic charge–transfer complex is evaluated as a zinc‐ion battery cathode owing to the good electron acceptation ability in the cyano groups that improves the voltage output. Through electrochemical activation, electrolyte optimization, and adoption of graphene‐based separator, CuTCNQ‐based aqueous zinc‐ion batteries deliver much improved rate performance and cycling stability with anti‐self‐discharge properties. The structural evolution of CuTCNQ during discharge/charge are investigated by ex situ Fourier transform infra‐red (FT‐IR) spectra, ex situ X‐ray photoelectron spectroscopy (XPS), and in situ ultraviolet visible spectroscopy (UV–vis), revealing reversible redox reactions in both cuprous cations (Cu+) and organic anions (TCNQx‐1), thus delivering a high voltage output of 1.0 V and excellent discharge capacity of 158 mAh g−1. The remarkable electrochemical performance in Zn//CuTCNQ is ascribed to the strong inductive effect of cyano groups in CuTCNQ that elevated the voltage output and the graphene‐modified separator that inhibited CuTCNQ dissolution and shuttle effect in aqueous electrolytes.

Battery performance promotion and mass transfer enhancement of organic redox flow battery by a novel spindle electrode design

Due to the safety, economy, flexible design and eco-friendliness, the organic redox flow battery (ORFB) was regarded as one of the huge development potential technologies, which was limited by the poor mass transfer behavior. A novel electrode structure like the spindle was designed, which could enhance mass transfer and promote the battery performance. Based on a three-dimensional steady-state numerical model, the influences of the novel electrode on the battery performance and mass transfer behaviors in different state of charge (SOC) were investigated. The results showed that the spindle electrodes could increase the discharge voltages by 8% and reduce the charge voltage 5.5% compared with the conventional cuboid electrode. The average concentration of 2,6-reDHAQ in the spindle electrode was 107% higher than that in the cuboid electrode and for the uniformity factor the promotion was 46% higher. It can be concluded that the spindle electrode can improve the battery performance of ORFB and provided a new design idea for the electrode, which was beneficial to the industrialization and commercialization of ORFBs.

Sulfonated Diels-Alder Poly(phenylene) Membrane for Efficient Ion-Selective Transport in Aqueous Metalorganic and Organic Redox Flow Batteries

Redox flow batteries (RFBs) can achieve long lifetimes and high performance when employing highly selective and conductive membranes. Neutral and alkaline RFBs suffer from higher resistances due to lower cation conductivity, compared to acidic RFBs utilizing proton transport. We report the use of a sulfonated Diels-Alder poly(phenylene) membrane that exhibits low and stable potassium area specific resistance and high efficiency RFB cycling relative to Nafion, as well as undetectable ferricyanide crossover. An alkaline (pH 12) organic anthraquinone derivative RFB using this membrane demonstrates over 10 days of cycling without capacity loss from crossover. A neutral chelated chromium complex RFB using this membrane demonstrates a peak discharge power of 1.23 W cm−2, and 80% energy efficiency (EE) cycling at an average discharge power density of 446.3 mW cm−2. Finally, the membrane exhibits similar favorable conductivity for many monovalent cations, opening the opportunity to improve the cycling and crossover performance of other acidic, neutral, and alkaline RFBs.

A Long-Lived Water-Soluble Phenazine Radical Cation.

Long-lived water-soluble organic radical species have long been desired for applications in bioimaging and aqueous energy storage technologies. In the present work, we report a phenazine radical cation sodium 3,3'-(phenazine-5,10-diyl)bis(propane-1-sulfonate) (PSPR) with a high solubility of 1.4 M and high stability in water. Collaboratively demonstrated by experiments and theoretical calculations, PSPR is not prone to undergo dimerization or disproportionation reactions, and its appropriate electron density avoids reactions with oxygen or water, which contribute together to its long lifetime in water under air. With an open-shell configuration, PSPR shows interesting magnetic activity with a narrow linewidth in the electron paramagnetic resonance spectra and a magnetic circular dichroism response. PSPR exhibits an ambipolar redox activity in water. By pairing with a cheap zinc negative electrolyte, a high-performance aqueous organic redox flow battery based on PSPR as a positive electrolyte with an open-circuit voltage of 1.0 V is established, which shows no obvious capacity fade after cycling for 2500 cycles (∼27 days), demonstrating the great promise of PSPR for large-scale energy-storage technology.

Effect of variable viscosity of electrolytes on mass transport and electrochemical reactions in aqueous organic redox flow battery

In this work, the viscosity of an organic electrolyte solution is quantified and its effect is investigated on the performance of an aqueous organic redox flow battery (AORFB). For a viologen derivative, the solution viscosity is found to be highly dependent on both the state of charge (SOC) and the concentration, which can be attributed to the strong intermolecular interaction in solution. The correlation between the viscosity and the SOC is then developed for different concentrations of the electrolytes. A 2D model with the effect of variable viscosity is developed to elucidate the mass transport and electrochemical reaction processes in the flow battery. It is found that the variable viscosity yields non-uniform distribution of the local velocity and concentration in the porous electrode and lowers the performance of the flow battery. Based on the above findings, the electrode porosity is optimized and a gradient porosity is developed to match the mass transport with the electrochemical reaction in flow batteries. This work not only elucidates the key role of viscosity in the flow battery for the first time, but also provides a rational design of the electrode structure to balance the mass transfer and electrochemical reactions.

Ion-Selective Microporous Polymer Membranes with Hydrogen-Bond and Salt-Bridge Networks for Aqueous Organic Redox Flow Batteries.

Redox flow batteries (RFBs) have great potential for long-duration grid-scale energy storage. Ion-conducting membranes are a crucial component in RFBs, allowing charge-carrying ions to transport while preventing the cross-mixing of redox couples. Commercial Nafion membranes are widely used in RFBs, but their unsatisfactory ionic and molecular selectivity, as well as high costs, limit the performance and the widespread deployment of this technology. To extend the longevity and reduce the cost of RFB systems, inexpensive ion-selective membranes that concurrently deliver low ionic resistance and high selectivity toward redox-active species are highly desired. Here, high-performance RFB membranes are fabricated from blends of carboxylate- and amidoxime-functionalized polymers of intrinsic microporosity, which exploit the beneficial properties of both polymers. The enthalpy-driven formation of cohesive interchain interactions, including hydrogen bonds and salt bridges, facilitates the microscopic miscibility of the blends, while ionizable functional groups within the sub-nanometer pores allow optimization of membrane ion-transport functions. The resulting microporous membranes demonstrate fast cation conduction with low crossover of redox-active molecular species, enabling improved power ratings and reduced capacity fade in aqueous RFBs using anthraquinone and ferrocyanide as redox couples.

N-methylphenothiazine as stable and low-cost catholyte for nonaqueous organic redox flow battery

This work uses a highly durable, environmentally friendly, and low-cost catholyte of N-methylphenothiazine (MPT), which is a heterocyclic compound containing nitrogen and sulfur for a non-aqueous organic redox flow battery (NAORFB). By coupling MPT with 2,1,3-benzothiadiazole (BTD)-based electrolyte (BTD/MPT), it delivers the open-circuit voltage (OCV) of 2.23 V. The average coulombic efficiency of the NAORFB using redox-active materials of BTD/MPT reaches 99.5% over 50 cycles. This high coulombic efficiency is the best indicator of the stability of such redox-active materials of BTD/MPT. The outstanding stability is attributed to two benzene rings and one methyl group attached to the N-atom active center of N-methylphenothiazine. These groups stabilize the radical cation by releasing electrons and shielding against nucleophilic attack.

Concentration-dependent Cycling of Phenothiazine-based Electrolytes in Nonaqueous Redox Flow Cells.

Increasing redox-active species concentrations can improve viability for organic redox flow batteries by enabling higher energy densities, but the required concentrated solutions can become viscous and less conductive, leading to inefficient electrochemical cycling and low material utilization at higher current densities. To better understand these tradeoffs in a model system, we study a highly soluble and stable redox-active couple, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI). We measure the physicochemical properties of electrolytes containing 0.2-1 M active species and connect these to symmetric cell cycling behavior, achieving robust cycling performance. Specifically, for a 1 M electrolyte concentration, we demonstrate 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This demonstration helps to establish potential for high-performing, concentrated nonaqueous electrolytes and highlights possible failure modes in such systems.

An Extremely Stable, Highly Soluble Monosubstituted Anthraquinone for Aqueous Redox Flow Batteries

AbstractAn extremely stable, energy‐dense (53.6 Ah L−1, 2 m transferrable electrons), low crossover (permeability of <1 × 10−13 cm2 s−1 using Nafion 212 (Nafion is a trademark polymer from DuPont)), and potentially inexpensive anthraquinone with 2‐2‐propionate ether anthraquinone structure (abbreviated 2‐2PEAQ) is synthesized and extensively evaluated under practically relevant conditions for use in the negolyte of an aqueous redox flow battery. 2‐2PEAQ shows a high stability with a fade rate of 0.03–0.05% per day at different applied current densities, cut‐off voltage windows, and concentrations (0.1 and 1.0 m) in both a full cell paired with a ferro/ferricyanide posolyte as well as a symmetric cell. 2‐2PEAQ is further shown to have extreme long‐term stability, losing only ≈0.01% per day when an electrochemical rejuvenation strategy is employed. From post‐mortem analysis (nuclear magnetic resonance (NMR), liquid chromatography–mass spectrometry (LC‐MS), and cyclic voltammetry (CV)) two degradation mechanisms are deduced: side chain loss and anthrone formation. 2‐2PEAQ with the ether linkages attached on carbons non‐adjacent to the central ring is found to have three times lower fade rate compared to its isomer with ether linkages on the carbon adjacent to the central quinone ring. The present study introduces a viable negolyte candidate for grid‐scale aqueous organic redox flow batteries.

Vertical organic electrochemical transistors for complementary circuits

Organic electrochemical transistors (OECTs) and OECT-based circuitry offer great potential in bioelectronics, wearable electronics and artificial neuromorphic electronics because of their exceptionally low driving voltages (<1 V), low power consumption (<1 µW), high transconductances (>10 mS) and biocompatibility1–5. However, the successful realization of critical complementary logic OECTs is currently limited by temporal and/or operational instability, slow redox processes and/or switching, incompatibility with high-density monolithic integration and inferior n-type OECT performance6–8. Here we demonstrate p- and n-type vertical OECTs with balanced and ultra-high performance by blending redox-active semiconducting polymers with a redox-inactive photocurable and/or photopatternable polymer to form an ion-permeable semiconducting channel, implemented in a simple, scalable vertical architecture that has a dense, impermeable top contact. Footprint current densities exceeding 1 kA cm−2 at less than ±0.7 V, transconductances of 0.2–0.4 S, short transient times of less than 1 ms and ultra-stable switching (>50,000 cycles) are achieved in, to our knowledge, the first vertically stacked complementary vertical OECT logic circuits. This architecture opens many possibilities for fundamental studies of organic semiconductor redox chemistry and physics in nanoscopically confined spaces, without macroscopic electrolyte contact, as well as wearable and implantable device applications.

A Highly Stable, Capacity Dense Carboxylate Viologen Anolyte towards Long‐Duration Energy Storage

AbstractAqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium, (CBu)2V, as a highly stable, high capacity anolyte material under near pH neutral conditions. (CBu)2V can achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of (CBu)2V is attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)2V/(NH)4Fe(CN)6 AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L−1 for 1000 cycles, power densities of up to 85 mW cm−2, and an energy efficiency of 70 % at 60 mA cm−2. (CBu)2V not only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date.

A Highly Stable, Capacity Dense Carboxylate Viologen Anolyte towards Long-Duration Energy Storage.

Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid-scale renewable energy storage. However, physicochemical properties of redox-active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N, N'-dibutyrate-4,4'-bipyridinium,(CBu)2V, as a highly stable, high capacity anolyte material under near pH neutral conditions.(CBu)2Vcan achieve more than 2.1 M solubility and display a reversible, kinetically fast reduction at -0.43 V vs NHE at pH 9.DFT studies revealed that the high solubility of(CBu)2Vis attributed to its high molecular polarity while its negative reduction potential is benefitted from electron-donating carboxylate groups. A 0.89 V (CBu)2V/(NH)4Fe(CN)6AORFB demonstrated exceptional energy storage performance, specifically, 100% capacity retention with a utilized energy density of 9.5 Wh/L for 1000 cycles, power densities of up to 85 mW/cm2, and an energy efficiency of 70% at 60 mA/cm2.(CBu)2Vnot only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days), most stable flow battery performance to date.

Stable Operation of Aqueous Organic Redox Flow Batteries in Air Atmosphere

AbstractAs a green route for large‐scale energy storage, aqueous organic redox flow batteries (AORFBs) are attracting extensive attention. However, most of the reported AORFBs were operated in an inert atmosphere. Herein, we clarify this issue by using the reported AORFB (i.e., 3, 3′‐(9,10‐anthraquinone‐diyl)bis(3‐methylbutanoicacid) (DPivOHAQ)||Ferrocyanide) as an example. We demonstrate that the dissolved O2 can oxidize the discharged DPivOHAQ in anolyte, leading to capacity‐imbalance between anolyte and catholyte. Therefore, this cell shows continuous capacity fading when operated in an air atmosphere. We propose a simple strategy for this challenge, in which the oxygen evolution reaction (OER) in catholyte is employed to balance oxygen reduction reaction (ORR) in anolyte. When using the Ni(OH)2‐modifed carbon felt (CF) as a current collector for catholyte, this cell shows an excellent stability in air atmosphere because the Ni(OH)2‐induced OER capacity in catholyte exactly balances the ORR capacity in anolyte. Such O2‐balance strategy facilitates AORFBs’ practical application.

Stable Operation of Aqueous Organic Redox Flow Batteries in Air Atmosphere.

As a green route for large-scale energy storage, aqueous organic redox flow batteries (AORFBs) are attracting extensive attention. However, most of the reported AORFBs were operated in an inert atmosphere. Herein, we clarify this issue by using the reported AORFB (i.e., 3, 3'-(9,10-anthraquinone-diyl)bis(3-methylbutanoicacid) (DPivOHAQ)||Ferrocyanide) as an example. We demonstrate that the dissolved O2 can oxidize the discharged DPivOHAQ in anolyte, leading to capacity-imbalance between anolyte and catholyte. Therefore, this cell shows continuous capacity fading when operated in an air atmosphere. We propose a simple strategy for this challenge, in which the oxygen evolution reaction (OER) in catholyte is employed to balance oxygen reduction reaction (ORR) in anolyte. When using the Ni(OH)2 -modifed carbon felt (CF) as a current collector for catholyte, this cell shows an excellent stability in air atmosphere because the Ni(OH)2 -induced OER capacity in catholyte exactly balances the ORR capacity in anolyte. Such O2 -balance strategy facilitates AORFBs' practical application.