Effect Of Redox Mediators Research Articles

Molecular Unravelling of the Structural Effect of Quinone Redox Mediators on Oxygen Reduction Reaction in Aprotic Lithium-Oxygen Batteries.

Redox mediators (RMs) provide tantalizing solutions to unlock the energy capabilities of aprotic lithium-oxygen (Li-O2) batteries by driving solution-mediated Li2O2 growth. However, the structural effect of RMs on the catalytic efficiency of the oxygen reduction reaction remains incompletely understood. Herein, we present the interplay between the structure of RMs and their discharge capabilities by a comparative study of model quinone (Q)-based RMs. Specifically, at low current densities, incorporating electron-withdrawing groups onto the Q ring can positively move the discharge potential and deliver larger discharge capacity by extending the lifespan of the LiQO2 intermediate and allowing for Li2O2 growth into deeper electrolyte regions. Conversely, at high current densities, the absence of electron-withdrawing groups facilitates homogeneous reaction kinetics from LiQ to regenerate Q (i.e., decreased lifespan of LiQO2), mitigating electrode potential polarization and preserving catalytic activity of Q for higher discharge capacity. The work establishes structure-property relationships that guide the rational design of RMs toward next-generation Li-O2 batteries.

Effect of Redox Mediators on Corrosion Behavior and Scanning Electrochemical Microscopy Response.

Scanning electrochemical microscopy (SECM) is widely used to measure local electrochemical reactivity of corroding surfaces. A major criticism of using SECM in feedback mode for corrosion studies is the requirement of an external redox mediator (RM) as it could react with the metal and affect the Nernst potential at the metal-solution interface. Consequently, it becomes challenging to differentiate the interference caused by the RM from the local reactivity of the metal. Herein, a multiscale electrochemical approach is presented to investigate the effect of RM choice on the corroding substrate. Two common RMs, ferrocenemethanol and hexaammineruthenium(III) chloride, were used to perform SECM over copper and aluminum. It was found during macroscale electrochemical measurements that Ru(NH)63+ acted as an oxidant and promoted corrosion. The SECM feedback behavior varied for copper depending on the RM used, suggesting that the corrosion reactions controlled the negative feedback mechanism, not the formation of an insulating passive film. The passivated aluminum surface consistently exhibited negative feedback, regardless of the RM used. SECM approach curves also displayed a distortion in the steady state current, which was caused by the deposition of substrate-generated species on the microelectrode. These deviations in feedback response were accounted for during analysis through incorporation into a finite element model to accurately extract the RM kinetic rate constants. The importance of understanding these processes is highlighted to avoid misinterpretation of passive behavior and advances toward a more quantitative use of SECM for corrosion studies.

A lithiated zeolite-based protective layer to boost the cycle performance of lithium−oxygen batteries via redox mediator sieving

Lithium–oxygen (Li–O2) batteries with ultra-high theoretical specific energy (3500 Wh kg−1) have attracted significant attention, but the sluggish electrochemical processes of discharge product Li2O2 lead to poor cycling stability. Redox mediators (RMs) as soluble catalysts are widely used to assist with the electrochemical formation/decomposition of Li2O2. However, the shuttle effect of RMs causes severe deterioration of both RMs and Li metal anodes. Herein, for the first time we synthesize a lithiated zeolite-based protective layer on Li anodes to mitigate the shuttle effect of 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) in Li–O2 batteries. The protective layer successfully blocks the migration of TEMPO toward the Li anode owing to the angstrom-level aperture size of lithiated zeolite. Due to the excellent redox-mediator-sieving capability of the protective layer, the cycle life of the Li−O2 batteries is significantly prolonged more than ten times at a current density of 250 mA g−1 and a limited capacity of 500 mA h g−1. This work demonstrates that the lithiated zeolite-based protective layer capable of molecular sieving is a facile and scalable way to mitigate the shuttle effect of RMs in Li–O2 batteries.

Simultaneously Generating Redox Mediator and Hybrid SEI for Lithium–oxygen Batteries by Lewis Acid Catalyzed Ring‐opening Reaction of Organic Iodine

AbstractFacing high overpotential, severe Li corrosion and degradation of electrolytes caused by the reactive oxygen species, the development of lithium–oxygen batteries is seriously limited. Although the iodine species have been considered to be effective redox mediators (RMs) for lowering the charging overpotential, the shuttling of oxidized I3− may attack the Li metal anode, compromising the number of RMs, cycling stability and energy efficiency. Here the intend to introduce 3‐Iodooxetane (C3H5OI, IOD) into TEGDME‐based electrolyte to form a protective SEI layer on the Li surface for defending against the attack of I3−. However, the iodine in IOD is found difficult to dissociate. To solve this problem, hard Lewis acid, aluminum trichloride (AlCl3) is proposed as the catalytic agent for dissociating the I− and triggering the ring‐opening reaction of the detached C3H5O+ ions. The former can dissociate redox couple I3−/I− while the latter can form oligomers or polymers under the attack of a nucleophile. Meanwhile, AlCl3 can form Al2O3 and LiCl inorganic species. Taking together, the introduction of IOD and AlCl3 into electrolytes can effectively derive reduced overpotential and in situ SEI layers consisting of flexible organics and rigid inorganics, endowing lithium–oxygen batteries over 150 cycles with significantly enhanced stability and lifespan.

The transformations of thiols and their dimers in the redox-mediated thiol-disulfide exchange reaction

A search for new approaches to sulfurous waste utilization is one of the urgent tasks of chemical technology. Thiol-disulfide exchange reaction (TDE) is one of the possible ways to involve technogenic wastes in organic synthesis. Electricity can promote such type of interactions. In this paper, we have studied TDE reactions involving low molecular weight thiols or their dimers under electrochemical conditions. The exchange processes were examined using the model reaction between 1-propanethiol and phenyl disulfide. Electrolysis was performed in the presence of redox mediators such as arylphosphines, substituted amines, o-, p-aminophenols or catechol. These compounds can initiate a TDE process with a formation of unsymmetrical disulfides. 4-Amino-2,6-diphenylphenol was chosen as the most effective redox mediator, which reduces the anodic overvoltage of a thiol oxidation by 1.20 V. The advantage of electrolysis in an undivided cell is the increased yield of target unsymmetrical disulfides due to the possibility of reduction ofhomodimers at the cathode. The involvement of refining waste, such as C3–C4 disulfide oil, in the reaction with substituted thiophenols made it possible to obtain a number of unsymmetrical arylalkyl disulfides with biologically active fragments in a high yield (up to 97%) under indirect electrolysis conditions.

Cooperative Effect of Redox Mediator and Ion Selective Membrane to Inhibit the Shuttle Effect for Li–O2 Battery with Large Cyclic Capacity

AbstractAprotic lithium‐oxygen (Li–O2) batteries hardly cycle at the condition of high area capacity for realizing their high energy density, because the unconducive lithium peroxide (Li2O2) discharge product limits the electron transfer between electrode and O2/Li2O2. Here, it is demonstrated that one of redox mediator (RM), triethylene glycol bis‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl radical (D‐TEMPO), can be effectively used to promote the electron transfer between electrode and Li2O2, which the shuttle effect of RM can be cooperatively inhibited by regulating the size of RM and the thickness of ion‐selective membrane. As a result, the Li–O2 battery coupled with double cathodes, D‐TEMPO, and ion‐selective membrane can be stably operated for 46 days at a capacity of 5 mAh cm−2. The concept in this work provides the cooperative design of a stable solution‐mediated pathway for high‐capacity Li–O2 battery with long cycle stability.

Simultaneous bioenergy production and dairy wastewater treatment by microbial fuel cell using Taguchi method: performance & optimization study

Abstract Microbial fuel cell (MFC) employs microbial communities as biocatalysts to convert chemical energy from organic substrates to electrical energy. The investigation of MFC incorporated with anaerobic mixed cultures and sulfonated polystyrene (SPS) membrane is of interest for this research due to its competency in generating renewable biological energy and wastewater treatment. Methylene blue was an effective redox mediator in this study. The reactor optimization was performed via the Design of Experiments (DOE) approach using Minitab software. The performance of the batch reactor was optimal with the operating conditions of temperature 30 °C, pH 7, and mediator concentration 250 μM. The contour plots and ANOVA specified that mediator concentration was the most influential parameter that affects MFC performance. MFC fed with 250 μM methylene blue concentration generated a maximum voltage of 0.33 V, current (density) of 4.08 A/m2, power (density) of 1.34 W/m2. The COD removal was 82.4 % at the end of the batch cycle of seven days. The data obtained from the experiments showed that small amounts of a mediator (250 μM) in wastewater elevate the bio-electricity output of the MFC reactor by 1.22 folds.

Oxygen Radical Anion Substituted Iron Phthalocyanine as an Effective Redox Mediator for Li-O2 Batteries.

Transition metal phthalocyanines are potential soluble redox mediators for Li-O2 batteries. In this work, effective strategies to control the redox potentials and activities of iron phthalocyanine (FePc) based redox mediators are designed by the introduction of electron-withdrawing or electron-donating groups. Substituted electron-donating groups can shift the oxidation potential of FePc to a higher energy level, consequently reducing the charging voltage of Li-O2 batteries. Especially, oxygen radical anion (-O-) modified FePc (FePc-O-) shows the most significant improvement to the oxygen reduction and evolution reactions of Li-O2 batteries. Electronic analysis indicates that -O- substitution can break the symmetry of electronic structures of FePc which further tunes the reduction of O2 and the oxidation of Li2O2. Detailed reaction mechanisms of (FePc-O-)-mediated Li-O2 batteries are proposed based on first-principles molecular dynamics simulations and thermodynamic free energy calculations.

Completely Eradicating Singlet Oxygen in Li–O2 Battery via Cobalt(II)-Porphyrin Complex-Catalyzed LiOH Chemistry

Li-O2 batteries have an extremely high theoretical specific energy; however, the large charge overpotential and highly reactive singlet oxygen (1O2) are two major obstacles. Porphyrin as a special kind of macrocyclic conjugated aromatic system exhibits excellent redox activity, which can be optimized by introducing a center metal atom. Herein, 5,10,15,20-tetrakis(4-aminophenyl)-porphyrin (TAPP) and 5,10,15,20-tetrakis(4-aminophenyl)-porphyrin-Co(II) (Co-TAP) are applied as effective redox mediators for Li-O2 batteries. The synergistic effects of a center metal atom and organic ligand make Co-TAP more favorable for oxygen reduction and evolution. To understand the fundamental reaction mechanisms with or without TAPP or Co-TAP, the discharge/charge processes and the parasitic reactions have been comprehensively studied. The results reveal that TAPP affects the formation mechanism of Li2O2, while Co-TAP transforms the main discharge product into LiOH without adding extra water. Co-TAP-containing batteries operated via LiOH chemistry completely eradicate 1O2 and significantly alleviate the parasitic reactions associated with 1O2.

Mechanism of norfloxacin transformation by horseradish peroxidase and various redox mediated by humic acid and microplastics

The catalysis of HRP coupling with redox mediator was a feasible technology for the transformation of antibiotics. This work screened three effective redox mediators syringaldehyde (SYR), acetosyringone (AS) and p-coumaric acid (PCA) for the norfloxacin (NOR) transformation in HRP/redox mediator system. Then, compared their transformation characteristics under varying conditions. The molecular docking results indicated HRP catalytic mediator was spontaneous, and the absolute value order of free energy between three redox mediators and HRP was consistent with the order of NOR removal in experiment. The presence of humic acid (HA) and polystyrene (PS) microplastics could block the removal of NOR, and the inhibition effect was proportional to the level of HA and PS particles. Seven and six possible intermediate products were identified by using SYR/AS and PCA as redox mediators, respectively, and potential NOR transformation pathways were proposed. SYR and AS treatment had the same transformation products and pathways due to their similar structure, including defluorination, oxidation, cross-coupled with mediator, demethylation and dehydrogenation. While for the PCA group, NOR not only performed the above action (except defluorination), but also underwent decarbonylation. These findings may expand our knowledge of the conversion and fate of fluoroquinolones through HRP coupled with redox mediator in the environment.

An electrochemical sensor for the voltammetric determination of artemisinin based on carbon materials and cobalt phthalocyanine.

An electrochemical sensor for the determination of artemisinin has been developed based on a glassy carbon electrode modified with hybrid nanocomposites of cobalt phthalocyanine, graphene nanoplatelets, multi-walled carbon nanotubes and ionic liquids(IL). To improve the sensitivity and selectivity of the sensor, cobalt phthalocyanine (CoPc) was used as an effective redox mediator to promote and catalyze the artemisinin reduction. Furthermore, the graphene nanoplatelets and multi-walled carbon nanotubes were used as excellent conducting supporting materials to improve the sensitivity of the electrochemical sensor. Moreover, IL with a surface charge was also employed to prevent aggregation of the graphene nanoplatelets and multi-walled carbon nanotubes. The analytical signal was generated from the reduction of Co(III)Pc generated by artemisinin. The proposed electrochemical sensor was applied tothe detection of artemisinin using differential pulse voltammetry and provided a signal with wide linearity ranging from 1.5-60μM and 60-600μM and a detection limit of 0.70μM (3SD/m). Furthermore, the proposed sensor displayed good repeatability and reproducibility of 2.9-3.0 and 3.1-4.4% RSD, respectively. Applications of the sensor to drug and plant samples demonstrated accuracy in a range of 105-116% recoveries. In addition, the results were in good agreement with those obtained from the HPLC method as a reference technique. Thus, the proposed electrochemical sensor provides a new alternative platform for sensitive and selective determination of artemisinin in the analysis of pharmaceuticals with good precision and accuracy.

Ferrocene-Grafted Carbon Nanotubes for Sensitive Non-Enzymatic Electrochemical Detection of Hydrogen Peroxide.

Sensitive detection of hydrogen peroxide (H2O2) residue in aseptic packaging at point of use is critical to food safety. We present a sensitive non-enzymatic, amperometric H2O2 sensor based on ferrocene-functionalized multi-walled carbon nanotubes (MWCNT-FeC) and facile screen-printed carbon electrodes (SPCEs). The sensor utilizes the covalently grafted ferrocene as an effective redox mediator and the MWCNT networks to provide a large active surface area for efficient electrocatalytic reactions. The electrocatalytic MWCNT-FeC modified electrodes feature a high-efficiency electron transfer and a high electrocatalytic activity towards H2O2 reduction at a low potential of -0.15 V vs. Ag/AgCl. The decreased operating potential improves the selectivity by inherently eliminating the cross-reactivity with other electroactive interferents, such as dopamine, glucose, and ascorbic acid. The sensor exhibits a wide linear detection range from 1 μM to 1 mM with a detection limit of 0.49 μM (S/N=3). The covalently functionalized electrodes offered highly reproducible and reliable detection, providing a robust property for continuous, real-time H2O2 monitoring. Furthermore, the proposed sensor was successfully employed to determine H2O2 levels in spiked packaged milk and apple juice with satisfactory recoveries (94.33-97.62%). The MWCNT-FeC modified SPCEs offered a facile, cost-effective method for highly sensitive and selective point-of-use detection of H2O2.

A TEMPO-grafted multi-functional cathode with strong anchoring ability towards redox mediators for high energy efficiency Li-O2 batteries

Soluble redox mediators (RMs) with favorable contact interface are considered to be an effective approach for high efficient lithium-oxygen (Li-O2) batteries. However, the shuttling effect of oxidized RMs from the cathode would lead to the degradation of the RMs’ functionality and poor cycling stability. Herein, TEMPO was anchored into three-dimensional porous ZnO@C/NF conductive substrate (ZnO@C@PPy-TEMPO/NF) and directly utilized as a self-supporting cathode for Li-O2 battery. This novel cathode serves multiple functions as redox mediator, abundant reaction center for lithium ion, as well as electron conduction matrix. The battery perform with a high discharge specific capacity of 6.9 mAh cm−2 and a low overpotential of 0.6 V similar with 4-OH-TEMPO in the electrolyte. The detailed catalysis mechanism of the immobilized TEMPO in improving the electrochemical performance has also been studied. During discharging, the cation-π effect between the conductive polypyrrole and lithium ions helps to realize the "isomorphic growth" of Li2O2. During charging, the TEMPO-immobilized electrochemical oxidation to TEMPO+ provides more direct contact with the discharge products, resulting in rapid chemical oxidation of Li2O2. The ingenious design and in-depth mechanism study provide a new avenue to address the shuttle effect of RMs and design an advanced cathode for Li-O2 batteries.

Comparison between Dissolved and Coated LiBr As Redox Mediator for Li-Air Batteries

Lithium-air battery (LAB) is one of promising next-generation batteries because of its theoretical energy density as high as 3500 Wh kg-1. However, the battery still has some difficulties to be challenged, among which large overpotential during charging is the most important one for the air electrode (AE). Application of redox mediator (RM) is a common strategy to lower the overpotential. Although RM is usually dissolved into electrolyte solution to make it react uniformly on the surface of the AE, the oxidized product of RM, RM+, diffuses to the Li electrode to be reduced into RM, which is a kind of short-circuit reaction, shuttle effect [1]. Therefore, we fixed LiBr as the RM on the surface of the AE by coating it as a mixture with carbon powder (Ketjen black, KB) and a binder (PVDF) not only to suppress the shuttle effect but also to concentrate the RM on the surface of the AE where it works [2].Three types of LAB cells were used in this study. The first one is “ no RM ” cell as a reference consisting of an air electrode coated on a carbon paper with a slurry of KB and PVDF, an electrolyte solution of 0.2 M lithium bis(trifuluoromethanesulfonyl)imide/diethyleneglycol dimehtyl ether, 0.2 M LiTFSI/G2, and a Li metal negative electrode. The other two contain LiBr as a RM in different ways. The second one is “ RM in EL (electrolyte)” cell which contains 50 mM of LiBr in addition to 0.2 M LiTFSI/G2. The third one is “ RM on AE” cell which contains LiBr in the slurry of KB-PVDF coated on the carbon paper with the same amount as that in the electrolyte solution of the RM in EL cell. Discharge/charge cycle tests were carried out at a current density of 200 mA (g-KB)−1 with a constant capacity of 500 mAh (g-KB)−1 in the range of 2.0 – 4.5 V at 30oC.Figure 1 shows the cycle performance of three types of LAB cells, which presents a better cyclability of the RM on AE cell than the others while that for the RM in EL cell is almost the same as the no RM cell. Figure 2 shows the charge and discharge voltage evolution during cycles as the voltages at midpoint, 250 mAh (g-KB)−1, for each discharge/charge cycle. The RM on AE cell shows the highest discharge voltages, the lowest charge voltages and the longest cycle life between the voltage range of 4.5 to 2.0 V among the three cells. However, the RM in EL cell shows slightly suppressed charge voltage merely early stage of the cycle test, and lower discharge voltages than the others probably due to the shuttle effect of RM, i.e. Br3 -. Therefore, coating of LiBr on AE, RM on AE , is proved to be a better strategy than dissolving LiBr into the electrolyte solution.At the presentation, we will discuss the detail of LiBr reactions as RM and compare the effect of LiBr between RM in EL and RM on AE cells with some analytical data.This study was supported by JST Projest ALCA-SPRING (JPMJAL1301) and NIMS Joint Research Hub Program, Japan.[1] X. G. Wang et al., ChemElectroChem, 4, 2145 (2017).[2] Y. Hayashi et al., J. Electrochem. Soc., 167, 020542 (2020). Figure 1

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator.

Prior to the practical application of rechargeable aprotic Li-O2 batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li2O2, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li2O2 reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li2O2 particles established that both growth and decomposition proceeded at the Li2O2/electrolyte interface in a system containing the Br-/Br3- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li2O2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li2O2/electrolyte interface is critical to improve the cycle life of Li-O2 batteries.

Strategies to suppress the shuttle effect of redox mediators in lithium-oxygen batteries

Various strategies can be used to suppress the shuttle effect of redox mediators in non-aqueous Li-O 2 batteries, including electrolyte additives, protective separators, anode coating and cathode modification. Rechargeable lithium-oxygen (Li-O 2 ) batteries are the next generation energy storage devices due to their ultrahigh theoretical capacity. Redox mediators (RMs) are widely used as a homogenous electrocatalyst in non-aqueous Li-O 2 batteries to enhance their discharge capacity and reduce charge overpotential. However, the shuttle effect of RMs in the electrolyte solution usually leads to corrosion of the Li metal anode and uneven Li deposition on the anode surface, resulting in unwanted consumption of electrocatalysts and deterioration of the cells. It is therefore necessary to take some measures to prevent the shuttle effect of RMs and fully utilize the soluble electrocatalysts. Herein, we summarize the strategies to suppress the RM shuttle effect reported in recent years, including electrolyte additives, protective separators and electrode modification. The mechanisms of these strategies are analyzed and their corresponding requirements are discussed. The electrochemical properties of Li-O 2 batteries with different strategies are summarized and compared. The challenges and perspectives on preventing the shuttle effect of RMs are described for future study. This review provides guidance for achieving shuttle-free redox mediation and for designing Li-O 2 cells with a long cycle life, high energy efficiency and highly reversible electrochemical reactions.

Exploring community evolutionary characteristics of microbial populations with supplementation of Camellia green tea extracts in microbial fuel cells

This first-attempt study deciphered combined characteristics of species evolution and bioelectricity generation of microbial community in microbial fuel cells (MFCs) supplemented with Camellia green tea (GT) extracts for biomass energy extraction. Prior studies indicated that polyphenols-rich extracts as effective redox mediators (RMs) could exhibit significant electrochemical activities to enhance power generation in MFCs. However, the supplementation of Camellia GT extract obtained at room temperature with significant redox capabilities into MFCs unexpectedly exhibited obvious inhibitory effect towards power generation. This systematic study indicated that the presence of antimicrobial components (especially catechins) in GT extract might significantly alter the distribution of microbial community, in particular a decrease of microbial diversity and evenness. For practical applications to different microbial systems, pre-screening criteria of selecting biocompatible RMs should not only consider their promising redox capabilities (abiotic), but also possible inhibitory potency (biotic) to receptor microbes. Although Camellia tea extract was well-characterized as GRAS energy drink, some contents (e.g., catechins) may still express inhibition towards organisms and further assessment upon biotoxicity may be inevitably required for practice.

Boosting the electrochemical performance of Li–O2 batteries with DPPH redox mediator and graphene-luteolin-protected lithium anode

Aprotic Li-O2 batteries with high theoretical energy density are regarded as promising next-generation energy storage devices. However, the accumulation of discharge products (Li2O2) during the discharge/charge processes would lead to high overpotential, low round-trip efficiency and poor cycling stability of the batteries. Soluble redox mediators (RMs) have been proved to be efficient in promoting the oxidation of Li2O2 particles. However, the self-discharge of the electrochemically oxidized RMs (RMsox) on the surface of Li metal would accelerate the formation of lithium dendrites. In this work, nitrogenous radicals, 1,1-Diphenyl-2-picrylhydrazyl radical 2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH), is designed and employed as a RM to lower the overpotential of Li–O2 batteries. Moreover, the self-discharge of the oxidized DPPH on the surface of Li metal and the formation of lithium dendrites are successfully suppressed by the formation of graphene/luteolin protective layer on Li metal. Consequently, an ultrastable cycle stability (>150 cycles) is achieved with DPPH and the stable graphene/luteolin protective layer for the Li metal anodes. These synergistic effect of RMs and stable Li protective layer may inspire the development of sustainable and durable Li–O2 batteries.

Ap-phenylenediamine oligomer-mediated Li–O2battery with an extremely low charge potential of 3.1 V

We report the use of ap-phenylenediamine oligomer (pPO) as an effective dual-functional redox mediator to significantly reduce the charge potential to 3.1 V and the mediation mechanism is disclosed.

Use of redox mediators for the enhanced degradation of selected nitrophenols

In the present study, sequential anaerobic–aerobic treatment was used for the enhanced degradation of selected nitrophenols. Five identical reactors were used to assess the degradation of 2-nitrophenol and 2,4-dinitrophenol. Reactor 1 was used as control, and another four reactors were operated in anaerobic phase followed by sequential aerobic treatment. The performance of reactors was studied for nitrophenol concentrations of 5, 10, 20 and 40 mg/l and in next phase effect of redox mediators [lawsone and anthraquinone sulphonic acid (AQSA)], along with kinetics of degradation studied for 40 mg/l of nitrophenols. More than 98% degradation of nitrophenols was achieved at lower concentrations but for 40 mg/l removal percentage was reduced, and also COD inhibition was observed. However, with the addition of redox mediators, both nitrophenols and COD removal percentage were greater than 95% and 85%, respectively. Kinetics of degradation revealed that the rate of degradation was found to increase with the increase in concentration of redox mediators from 10 to 20 mg/l, and it was found that degradation was faster with the use of AQSA in comparison with lawsone solution. It indicates that use of redox mediators is favourable for the enhanced degradation of nitrophenols. Sequential anaerobic–aerobic treatment is possible potential treatment method for treating nitrophenolic wastewaters.