Olivier Fontaine

Charging a Li–O2 battery using a redox mediator

The non-aqueous Li-air (O2) battery is receiving intense interest because its theoretical specific energy exceeds that of Li-ion batteries. Recharging the Li-O2 battery depends on oxidizing solid lithium peroxide (Li2O2), which is formed on discharge within the porous cathode. However, transporting charge between Li2O2 particles and the solid electrode surface is at best very difficult and leads to voltage polarization on charging, even at modest rates. This is a significant problem facing the non-aqueous Li-O2 battery. Here we show that incorporation of a redox mediator, tetrathiafulvalene (TTF), enables recharging at rates that are impossible for the cell in the absence of the mediator. On charging, TTF is oxidized to TTF(+) at the cathode surface; TTF(+) in turn oxidizes the solid Li2O2, which results in the regeneration of TTF. The mediator acts as an electron-hole transfer agent that permits efficient oxidation of solid Li2O2. The cell with the mediator demonstrated 100 charge/discharge cycles.

Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium.

The electrochemical reduction of 4-nitrophenyl diazonium, NPD, in different ionic liquids presenting different viscosities has been investigated. The electrochemical studies show that the reduction of diazonium leading to the formation of its corresponding radical occurs whatever the viscosity of the grafting media. Following that, the presence of an organic layer attached to the electrode after electrochemical treatment was evidenced by cyclic voltammetry (CV) in acidic media thanks to the presence of nitro groups. Moreover, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) confirm the presence of a nitrophenyl (NP) layer attached to the electrode material. Next, the examination of the electrochemical data through the measurement of the charge, corresponding to the reduction of the attached nitrophenyl (NP) moieties, shows that the surface concentration of NP, Γ(NP), decreases when the viscosity, η, of the grafting media increases. Additionally, in the case of the more viscous ionic liquid, N-tributyl-N-methylammonium bis(trifluoromethylsulfonyl)imide [Bu(3)MeN] [NTf(2)], a cosolvent has been added leading to fine decrease of the viscosity. The IR and CV investigations of the modified electrodes demonstrate the decrease of the amount of the attached molecules when the viscosity of the grafting media increases. In addition, a correlation between Γ(NP) as function of 1/η was observed. Finally, XPS and AFM experiments lead to an estimate of the thickness of the attached layer. As a result, both methods are in perfect agreement and thicknesses of 4 and 1 nm are measured after grafting in acetonitrile and in pure ionic liquid [Bu(3)MeN] [NTf(2)], respectively. By comparison with classical solvent, the use of viscous ionic liquid for the grafting leads to a decrease in the amount of the attached molecules and conduce to the formation of thinner or less dense layer.

Electropolymerization of Polypyrrole by Bipolar Electrochemistry in an Ionic Liquid

Bipolar electrochemistry has been recently explored for the modification of conducting micro- and nanoobjects with various surface layers. So far, it has been assumed that such processes should be carried out in low-conductivity electrolytes in order to be efficient. We report here the first bipolar electrochemistry experiment carried out in an ionic liquid, which by definition shows a relatively high conductivity. Pyrrole has been electropolymerized on a bipolar electrode, either in ionic liquid or in acetonitrile. The resulting polymer films were characterized by scanning electron microscopy and by contact profilometry. We demonstrate that the films obtained in an ionic liquid are thinner and smoother than the films synthesized in acetonitrile. Furthermore, a well-defined band of polypyrrole can be obtained in ionic liquid, in contrast to acetonitrile for which the polypyrrole film is present on the whole anodic part of the bipolar electrode.

Chemical Recognition of Active Oxygen Species on the Surface of Oxygen Evolution Reaction Electrocatalysts

Abstract Owing to the transient nature of the intermediates formed during the oxygen evolution reaction (OER) on the surface of transition metal oxides, their nature remains largely elusive by the means of simple techniques. The use of chemical probes is proposed, which, owing to their specific affinities towards different oxygen species, unravel the role played by these species on the OER mechanism. For that, tetraalkylammonium (TAA) cations, previously known for their surfactant properties, are introduced, which interact with the active oxygen sites and modify the hydrogen bond network on the surface of OER catalysts. Combining chemical probes with isotopic and pH‐dependent measurements, it is further demonstrated that the introduction of iron into amorphous Ni oxyhydroxide films used as model catalysts deeply modifies the proton exchange properties, and therefore the OER mechanism and activity.

Single-ion conductor nanocomposite organic–inorganic hybrid membranes for lithium batteries

A modified sol–gel synthesis of di-urethanosil resins provides an easy preparation of single-ion conductor membranes, by combining in one pot the in situ formation of oligosilsesquioxane nanofillers, the cross-linking of PEO chains and the covalent grafting of anion groups. Prototypal membranes demonstrated promising combination of thermal stability, flexibility and lithium ion conductivity performance.

Sol-gel route to zirconia-Pt-nanoelectrode arrays 8 nm in radius: their geometrical impact in mass transport.

The fabrication of advanced nanoelectrode arrays and their electrochemical characterization are presented. These nanoelectrode arrays are constituted of nanoperforations of 8 nm in radius leading to platinum and protected by an inorganic matrix made of crystalline zirconia. These nanoelectrodes arrays provide a ceramic support with a high thermal and chemical stability. These devices present a well characterized structure with a control of size, shape, and spacing of the nanoelectrodes, allowing studying in depth both the mass transport and the charge transfer properties in the nanometer range. The radial diffusion occurs when the experimental scan rate is superior to a theoretical scan rate estimated from the model proposed by Amatore and colleagues. The coupling between electrochemical analysis and nanoscale structural characterizations successfully demonstrates that the theory defined for microelectrode arrays can be directly transposed for well-defined metal-ceramic nanocomposite nanoelectrodes.

Synthesis of Titania@Carbon Nanocomposite from Urea‐Impregnated Cellulose for Efficient Lithium and Sodium Batteries

Nanostructured TiO2 and TiO2@C nanocomposites were prepared directly from urea-impregnated cellulose by a simple reaction/diffusion process and evaluated as negative electrode materials for Li and Na batteries. By direct treatment with TiCl4 under anhydrous conditions, the urea impregnation of cellulose impacts both the TiO2 morphology and the carbon left by cellulose after pyrolysis. Hierarchical TiO2 structures with a flower-like morphology grown from-and-at the surface of the cellulose fibers are obtained without any directing agent. The resulting TiO2/cellulose composite is then transformed either into pure TiO2 flowers by calcination in air at 600 °C, or into TiO2@C nanocomposites by pyrolysis under Ar at 600 °C. Electrochemical studies demonstrate that both samples can (de)insert lithium and sodium ions and are promising electrode materials.

Multiwalled Carbon Nanotube/Cellulose Composite: From Aqueous Dispersions to Pickering Emulsions

A mild and simple way to prepare stable aqueous colloidal suspensions of composite particles made of a cellulosic material (Sigmacell cellulose) and multiwalled carbon nanotubes (MWCNTs) is reported. These suspensions can be dried and redispersed in water at pH 10.5. Starting with rather crude initial materials, commercial Sigmacell cellulose and MWCNTs, a significant fraction of composite dispersed in water could be obtained. The solid composites and their colloidal suspensions were characterized by electronic microscopy, thermal analyses, FTIR and Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and light scattering. The composite particles consist of tenuous aggregates of CNTs and cellulose, several hundred nanometers large, and are composed of 55 wt % cellulose and 45 wt % CNTs. Such particles were shown to stabilize cyclohexane-in-water emulsions. The adsorption and the elasticity of the layer they form at interface were characterized by the pendant drop method. The stability of the oil-in-water emulsions was attributed to the formation of an elastic network of composite particles at interface. Cyclohexane droplet diameters could be tuned from 20 to 100 μm by adjusting the concentration of composite particles. This behavior was attributed to the limited coalescence phenomenon, just as expected for Pickering emulsions. Interestingly, cyclohexane droplets were stable over time and sustained pH modifications over a wide range, although acidic pH induced accelerated creaming. This study points out the possibility of combining crude cellulose and MWCNTs through a simple process to obtain colloidal systems of interest for the design of functional conductive materials.

A one-pot route to prepare class II hybrid ionogel electrolytes

A new system based on class II hybrid ionogels (Si-IL gels) has been developed for overcoming leaking problems associated with liquid electrolytes in dye-sensitized solar cells (DSSCs). We co-condensed a silica precursor with an alkoxysilane functionalized ionic liquid precursor, under acidic conditions, to obtain a hybrid ionogel where ionic liquid is covalently linked to silica domains. The morphology and the microstructure of the Si-IL xerogels were explored using NMR, SAXS and field emission scanning electron microscopy (FE-SEM). Cyclic voltammetry experiments were carried out to measure the apparent diffusion coefficient of the redox couple I−/I3− into these Si-IL gels. The diffusion coefficient of triiodide in the gel is comparable to the one observed in a solvent-free based ionic liquid electrolyte. These Si-IL gels were evaluated as electrolytes in quasi-solid-state DSSCs. For this purpose, various DSSCs have been fabricated. The cells containing Si-IL ionogels with 50 wt% of a silica modified liquid ionic precursor exhibit a short-circuit photocurrent of 2.8 mA cm−2, an open circuit voltage of 680 mV, a fill factor of 0.65, and an overall efficiency of 1.25%. Accordingly, this work constitutes a proof of concept.

Nanocomposites with both structural and porous hierarchy synthesized from Pickering emulsions

Commercial carboxymethylcellulose was used to prepare dispersible multi-walled carbon nanotubes-based composites. These composites were employed to prepare Pickering oil-in-water emulsions. Emulsion-templated macroporous materials were then prepared by embedding the oil droplets into a polymer resin arising from the polycondensation of furfural and phloroglucinol within the continuous aqueous phase in the presence of FeCl3 as catalyst. Polymerization afforded organic–inorganic nanocomposite materials in the form of capsules. After pyrolysis, highly microporous, magnetic and electrically conductive micrometric capsules could be obtained. This approach opens interesting prospects for catalysis, separation and electrochemistry applications.