L.E.A. Berlouis

Progress in redox flow batteries, remaining challenges and their applications in energy storage

Redox flow batteries, which have been developed over the last 40 years, are used to store energy on the medium to large scale, particularly in applications such as load levelling, power quality control and facilitating renewable energy deployment. Various electrode materials and cell chemistries have been proposed; some of the successful systems have been demonstrated on a large-scale in the range of 10 kW–10 MW. Enhanced performance is attributable to the improvements in electrodes, separator materials and an increasing awareness of cell design. This comprehensive review provides a summary of the overall development of redox flow battery technology, including proposed chemistries, cell components and recent applications. Remaining challenges and directions for further research are highlighted.

KOtBu: A Privileged Reagent for Electron Transfer Reactions?

Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes, and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction.

Reductions of Challenging Organic Substrates by a Nickel Complex of a Noninnocent Crown Carbene Ligand

The first crown-tetracarbene complex of Ni(II) has been prepared, and its crystal structure determined. The complex can be reduced by Na/Hg, with an uptake of two electrons. The reduced complex reductively cleaves arenesulfonamides, including those derived from secondary aliphatic amines, and effects Birch reduction of anthracenes as well as reductive cleavage of stilbene oxides. Computational studies show that the orbital that receives electrons upon reduction of the complex 2 is predominantly based on the crown carbene ligand and also that the HOMO of the parent complex 2 is based on the ligand.

Difference reflectance spectroscopy of anodic films on copper and copper base alloys

Abstract The development of difference reflectance techniques using conventional spectrophotometers for the study of the growth and the characterisation of anodic films on copper and its alloys with nickel is described. It is shown that Cu2O is the main component of anodic films formed on 90 Cu + 10 Ni and 70 Cu + 30 Ni. CuO appears to be formed when O2 is reduced on Cu2O covered electrodes.

Electrochemical behaviour of aqueous SO2 at polycrystalline gold electrodes in acidic media. A voltammetric and in-situ vibrational study. Part II. Oxidation of SO2 on bare and sulphur-modified electrodes

Ministerio de Educacion y Cultura, Programa F.P.I. y proyecto PB97-0130; Instituto de Cultura J. Gil Albert.

Thermal analysis investigation of hydriding properties of nanocrystalline Mg–Ni- and Mg–Fe-based alloys prepared by high-energy ball milling

The hydrogen loading characteristics of nanocrystalline Mg, Mg–Ni (Ni from 0.1 to 10 at.%), and Mg–Fe (Fe from 1 to 10 at.%) alloys in 3 MPa H2 were examined using high pressure differential scanning calorimetry and thermogravimetric analysis. All samples showed rapid uptake of hydrogen. A decrease in the onset temperature for hydrogen absorption was observed with increasing Ni and Fe alloy content, but the thermal signatures obtained suggested that only Mg was involved in the hydriding reaction; i.e., no clear evidence was found for the intermetallic hydrides Mg2NiH4 and Mg2FeH6. Hydrogen loading capacity decreased with temperature cycling, and this was attributed to a sintering process in the alloy, leading to a reduction in the specific surface available for hydrogen absorption.

A thermal analysis investigation of the hydriding properties of nanocrystalline Mg-Ni based alloys prepared by high energy ball milling

A thermal analysis study of the hydrogen loading characteristics of nanocrystalline Mg-Ni alloys (Ni content ranging from 0.1 at% to 10 at%) has been carried out in 3 MPa hydrogen, employing the techniques of differential scanning calorimetry and thermogravimetric analysis (TGA). The measurements confirmed the nonequilibrium state of the samples as prepared by the mechanical alloying technique. An enthalpy associated with the stabilisation of the alloys on first heating in hydrogen was found for all the samples studied. The magnitude of this enthalpy increased with the nickel content of the alloy. All the samples showed rapid uptake of hydrogen at 3 MPa pressure, indicating that the nickel was thus playing a very active role at the alloy surface in dissociating hydrogen and so enabling more rapid hydride formation by the alloy. This catalytic activity of the nickel decreased with temperature cycling over the range 80°C to 500°C. Although TGA analysis, carried out at the end of the cycling period, gave the hydrogen content as 1.1 wt% to 1.7 wt% for the alloys, this is well short of the theoretical amounts expected (7.6 wt% for MgH2), indicating that the samples had become deactivated during cycling. No evidence was found of the intermetallic Mg,Ni prior to or after hydriding.

The effect of novel processing on hydrogen uptake in FeTi- and magnesium-based alloys

This paper discusses the production and initial evaluation of hydrogen storage alloys produced by physical vapour deposition (PVD) and mechanical alloying (MA). PVD is usually associated with the production of thin films and coatings. However, DERA Farnborough have developed a high rate vapour condensation process to produce bulk deposits, in some cases up to 44 mm thick. Vapour condensation using electron beam evaporation produces the ultimate in cooling rates with extended solid solubility and refinement of microstructure, which produce enhanced physical and mechanical properties. MA is a complimentary technique for processing hydrogen storage materials which has been developed within DERA over the past 3 years. These techniques have been applied to Mg and FeTi alloy systems and it is shown that both methods greatly enhance the amount of hydrogen uptake and the ease of activation.

Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption

The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles.

Hybrid super electron donors - preparation and reactivity

Summary Neutral organic electron donors, featuring pyridinylidene–imidazolylidene, pyridinylidene–benzimidazolylidene and imidazolylidene–benzimidazolylidene linkages are reported. The pyridinylidene–benzimidazolylidene and imidazolylidene–benzimidazolylidene hybrid systems were designed to be the first super electron donors to convert iodoarenes to aryl radicals at room temperature, and indeed both show evidence for significant aryl radical formation at room temperature. The stronger pyridinylidene–imidazolylidene donor converts iodoarenes to aryl anions efficiently under appropriate conditions (3 equiv of donor). The presence of excess sodium hydride base has a very important and selective effect on some of these electron-transfer reactions, and a rationale for this is proposed.