Redox

About: Redox is a research topic. Over the lifetime, 26853 publications have been published within this topic receiving 862368 citations. The topic is also known as: reduction-oxidation & reduction-oxidation reaction.

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

Abstract: Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

Electrocatalytic reduction of nitrite and nitric oxide to ammonia with iron-substituted polyoxotungstates

Abstract: Heteropolytungstates in which one of the positions normally occupied by a tungsten cation is occupied instead by an iron cation are shown to be catalysts for the electroreduction of nitrite to ammonia. The lacunary derivatives in which the empty tungsten site is unoccupied show no catalytic activity. The catalytic mechanism involves the intermediate formation of a nitrosyl complex of the Fe(II) form of the catalyst. The pH dependence of the rate of formation of the nitrosyl complex shows that nitrous acid is the reactive form of nitrite between pH 2 and 8. The catalyzed reduction does not produce hydroxylamine as an intermediate and appears to depend upon the ability of the multiply reduced heteropolytungstates to deliver electrons to the NO group bound to the iron center in a concerted, multiple-electron step. The iron-substituted heteropolytungstates are not degraded by repeated cycling between their oxidized and reduced states. A particularly valuable feature of the heteropolytungstate is the ease with which the formal potentials of the several redox couples they exhibit may be shifted by changing the identity of the central heteroatom. Exploitation of this feature provides diagnostic information that can be decisive in establishing the mechanism of electrocatalytic processes.

Plasma-liquid electrochemistry: Rapid synthesis of colloidal metal nanoparticles by microplasma reduction of aqueous cations

Abstract: We demonstrate operation of an aqueous electrochemical cell with an atmospheric-pressure microplasma cathode and a solid metal anode for the rapid production of colloidal metal nanoparticles. Microplasmas are miniaturized versions of low-pressure glow discharges that operate nonthermally at high pressures (∼1atm) and contain energetic electrons. Aqueous metal cations are directly reduced by electrons in the microplasma without the presence of a solid cathode or chemical reducing agents. Kinetic studies performed by UV-visible absorbance spectroscopy suggest that particle nucleation and growth are coupled to redox reactions initiated in the electrochemical cell through plasma-liquid interactions.

Active-site probes of flavoproteins

Abstract: Flavins are very versatile coenzymes, functioning with considerable efficiency in a wide variety of enzymic reactions involving either two-electron or one-electron transfers, with both functions often catalysed by the same enzyme. They are responsible for catalysing the dehydrogenation of many different types of compounds, including dithiols, reduced nicotinamide nuclwtides, alcohols and a-hydroxy acids, amines and a-amino acids, and even saturated C-C bonds, provided that a suitable activating group such as a carbonyl residue is situated a/% to the bond to be oxidized. In the process of catalysing these dehydrogenation reactions, the flavin is itself reduced, and, in order to function catalytically, the oxidized form must be regenerated at the expense of reduction of some acceptor. The acceptor may be in some cases the oxidized form of the same type of compound that serves as reducing substrate, e.g. it might be a disulphide, an oxidized nicotinamide nucleotide or an unsaturated compound such as fumarate or crotonylCoA. In such a case the enzyme might conveniently be classified as a transhydrogenase and subclassified as a C-C, C-S, C-N or N-N transhydrogenase, depending on the nature of the atoms acting as hydrogen donor and hydrogen acceptor (Hemmerich & Massey, 1979). In most cases, however, the acceptor molecule will be molecular 0, or another redox protein, such as an iron-sulphur protein or a cytochrome. In the latter cases the flavoprotein necessarily acts as a mediator between two-electron and one-electron transfers. Flavoproteins fill a unique spot in biochemistry with this capacity. An equal richness of possibilities exists in the reactions of different flavoproteins with molecular 0,. In some cases one-electron transfer is carried out, with superoxide (02-) and flavin semiquinone as the immediate products of the reaction. In other cases a direct two-electron reduction of 0, to H,O, appears to occur, and in another class of flavoproteins, the mono-oxygenases, one atom of the 0, molecule is incorporated into H20 and the other is incorporated into another substrate of the enzyme, to form an oxygenated product.

Redox and fluorophore functionalization of water-soluble, Tiopronin- protected gold clusters

Abstract: Place-exchange and amide-forming coupling reactions represent two facile and efficient routes to poly-functionalization of water-soluble nanoparticles. In this paper, place-exchange and amide-forming coupling reactions with water-soluble tiopronin-MPCs are described and their products characterized by 1H and 31P NMR, capillary electrophoresis, electrochemistry, and fluorescence spectroscopy. Place-exchange reactions of ligands with tiopronin-MPCs yield products with about half of the ligand exchange expected on the basis of solution stoichiometry and a nonselective exchange and were not noticeably affected by steric encumbrances. Tiopronin-MPCs to which viologens are coupled (avg 36/MPC) adsorb as monolayers on Au electrodes as shown in electrochemical quartz crystal microbalance experiments. Multilayer adsorption occurs on long experimental time scales. The strong adsorption of the viologen-functionalized clusters is ascribed to increased interaction and stability in the viologen reduction products. Capi...