Repurposing Waste Chemicals to Engineer Redox Mediators for Selective Active Species Modulation in Photo-Fenton-Like Systems.

The effects of different redox mediators on the oxygen reduction reaction (ORR) catalyzed by an iron porphyrin complex, iron(III) meso‐tetra(N‐methyl‐4‐pyridyl)porphine chloride [FeIIITMPyP], in 0.1 M triflic acid were investigated by cyclic voltammetry (CV) and spectroelectrochemistry in conjunction with density functional theory (DFT) calculations. The formal potentials of the FeIIITMPyP catalyst and the redox mediators, as well as the half‐wave potentials for the ORR, were determined by CV in the absence and presence of oxygen in acidic solutions. UV/Vis spectroscopic and spectroelectrochemical studies confirmed that only the 2,2′‐azino‐bis(3‐ethylbenzothiazioline‐6‐sulfonic acid)diammonium salt (C18H24N6O6S4) showed effective interactions with FeIIITMPyP during the ORR. DFT calculations suggested strong interaction between FeIIITMPyP and the C18H24N6O6S4 redox mediator. The redox mediator caused lengthening of the dioxygen iron bond, which thus suggested easier dioxygen reduction. Consistent results were observed in electrochemical impedance spectroscopic measurements for which the electron‐transfer kinetics were also evaluated.

We investigated the biological decolourisation of dyes with different molecular structures. The kinetic constant values (k1) achieved with azo dye Reactive Red 120 were 7.6 and 10.1 times higher in the presence of RM (redox mediators) AQDS and riboflavin, respectively, than the assays lacking RM. The kinetic constant achieved with the azo dye Congo Red was 42 times higher than that obtained with the anthraquinone dye Reactive Blue 4. The effect of RM on dye reduction was more evident for azo dyes resistant to reductive processes, and ineffective for anthraquinone dyes because of the structural stability of the latter.

Nonaqueous lithium-oxygen (Li-O2) batteries have attracted intensive research interest owing to their potential to provide gravimetric energy density 3-5 times that of conventional Li-ion batteries.1-2 The practical application of this technology has been hindered by poor cycle life (<100 cycles) and low round-trip efficiency (65-70%). These challenges are related to sluggish reaction kinetics and side reactions at the oxygen electrode.3-4 Applying redox mediator has been demonstrated an effective strategy to improve the reaction kinetics of Li-O2 reactions.5-8 However, limited understanding is available about the effect of redox mediator on the reaction stability of Li-O2 batteries. In this work we will study the effect of redox mediator on the reaction stability of the Li-O2batteries. We first studied the instability in cycling a Li-O2 cell without redox mediator. Applying high-temporal resolution on-line electrochemical mass spectrometry (OEMS), we show that the charging process of the Li-O2 cell consists of several oxygen-evolving stages, followed by CO2 evolution. First, the oxygen gas is evolved at ~2e-/O2 between 3.0 – 3.5 V vs. Li (VLi). As the charge voltage reaches 3.5 VLi, the rate of oxygen evolution starts to decrease accompanied with hydrogen evolution, as shown in Figure 1a. We further characterize the cycled O2-electrode and the electrolyte with 1H NMR. As shown in Figure 1b, the formation of side products including formic acid, acetic acid and acetone is confirmed. It was reported that generation of acetone can be mitigated by cycling with a redox mediator.9 Therefore, it is critical to investigate if and how the redox mediator influence the reaction stability of Li-O2reactions. The effect of redox mediator on charging and cycling instabilities will be discussed. Origin responsible for the instabilities as well as the mitigation mechanism of redox mediator will be discussed. Figure caption: Fig. 1. (a)Voltage and gas evolution profiles during charging of a Li-O2 battery without redox mediator after discharged to 1000 mAh/g. (b) 1H NMR spectrum of the carbon cathode and separator after 5 cycles of galvanostatic discharge and charge without redox mediator. Acknowledgements This work is substantially supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HKSAR), China, under Theme-based Research Scheme through Project No. T23-407/13-N, and partially supported by a RGC project No. CUHK24200414.

Effect of redox mediators in pharmaceuticals degradation by laccase: A comparative study

Azo and anthraquinone dye decolorization in relation to its molecular structure using soluble Trichosanthes dioica peroxidase supplemented with redox mediator

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.

Effects of quinoid redox mediators on the activity of anammox biomass

Electron transfer tuning for persulfate activation via the radical and non-radical pathways with biochar mediator.

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

Development of bioreactor systems with functional bio-carrier modified by disperse turquoise blue S-GL for disperse scarlet S-BWFL decolorization

Chemistries that can demonstrate tolerance to both moisture and atmospheric carbon dioxide have the power to advance the technology readiness level of non-aqueous lithium air batteries. Previously published results have found that lithium iodide as redox mediator in dimethoxyethane solvent containing high water content affords reversible cycling of a lithium oxygen battery via lithium hydroxide[1]. In collaboration with the authors of the previous work, this concept has been extended to explore additional redox mediators and alternative electrolyte mixtures. Quantitative oxygen evolution for the new chemistries was studied using continuous differential electrochemical mass spectroscopy. Oxygen evolution was found for some of the new chemistries employed. Faradaic efficiency for oxygen evolution and Tafel analysis shows complex behaviour. Discharge product analysis shows that some of the new chemistries which afford oxygen evolution also cycle via lithium hydroxide.[1] Liu, et al. Science, 2015, 350, 6260, p530.

Effects of redox mediators on nitrogen removal performance by denitrifying biomass and the activity of Nar and Nir

The effect of redox mediators in the dye decolorization by two laccase isoenzymes from Trametes versicolor cultures supplemented with barley bran has been investigated. All the redox mediators tested, 1-hydroxybenzotriazole (HBT), promazine (PZ), para-hydroxybenzoic acid (pHBA) and 1-nitroso-2-naphthol-3,6-disulfonic acid (NNDS), led to higher dye decolorization than those obtained without mediator addition. Among the different tested mediators, PZ was the most effective one at a low range of concentration (0.5–50 µM) and the natural mediator employed, pHBA did not improve significantly the degree of decolorization, and was slightly inhibitory.

Synthetic dyes are very difficult to degrade for their complex structure. During dyeing process in industry, a huge amount of dyes are lost as effluents. Degradation of these effluents by physical and chemical processes is the most problematic issues nowadays. Enzymatic oxidation of these dyes using oxidoreductase such as laccase has received a great attention in recent years for decolorization of dyes in effluent. In the present study, an isolated fungus Aspergillus flavus PUF5 was used to produce laccase enzyme and employed for degradation of five commercially used textile dyes, phenol red, methyl orange, malachite green, bromophenol blue and Congo red. The investigations were made with the effect of redox mediators (1 mM), temperature (25–45 °C), pH (4–8) and incubation time (0–24 h) in the dye decolorization by laccase. Highest efficiency in dye decolorization was found with redox mediator 1-hydroxy-benzotriazole (HBT). The interactions between four variable factors were statistically studied using response surface methodology. Optimized states of selected variables were dye concentration (6.04 µM), enzyme concentration (78.8 U/ml), pH (5.6) and redox mediator (1.07 mM) with predicted and observed activity of laccase 85.94 and 86.3 U/ml, respectively. These results suggest that laccase is a potential enzyme for removal of dyes present in wastewater of textile industry.

The increase in demand for energy has made Photosystem I (PSI), which can convert solar energy into a charge separation with an efficiency near unity, a topic of great research interest. Redox mediators play an important role in the optimization of PSI based electrode systems. Profound understanding about how these mediators impact the photo-induced current of these bio-hybrid systems is necessary. Here we report the investigation of various mediators with formal potentials (E0′) that span the two electron transfer (ET) sites of PSI (E0′ from 0.48 to −0.65 V vs. Ag/AgCl). The present results have demonstrated that mediators with more positive formal potentials produced larger photocurrents. Ferricyanide was found to be the most effective mediator in this system at equilibrium potential, with a photocurrent density of 900 nA/cm2. Additionally, the effects of the mediators’ light absorbance and concentration properties have been studied. The mediators with light absorbance that overlap with the Qy band were found to have a more significant effect on photocurrent generation than mediators with absorbance overlap with the Soret band. This thorough study of these mediators facilitates PSI based applications, and offers a method to optimize bio-circuit devices that involve redox mediators.