Manuel Maréchal

Insights into the interplay between molecular structure and diffusional motion in 1-alkyl-3-methylimidazolium ionic liquids: a combined PFG NMR and X-ray scattering study

We report on how the local structure and the diffusional motion change upon increasing the alkyl chain length in 1-alkyl-3-methylimidazolium cation ionic liquids. This study has been performed by combining pulse field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS) experiments. The cationic side chain length varies from ethyl (n = 2) to hexadodecyl (n = 16), while the anion is always bis(trifluoromethanesulfonyl)imide (TFSI). We find that the self-diffusivity of the individual ionic species is correlated to the local structure in the corresponding ionic liquid, namely the nano-segregation into polar and non-polar domains. In agreement with previous results, we observe that for relatively short alkyl chains the cations diffuse faster than the anions; however we also note that this difference becomes less evident for longer alkyl chains and a cross-over is identified at n ≈ 8 with the anions diffusing faster than the cations. Our results indicate that this controversial behavior can be rationalized in terms of different types of cation-cation and anion-anion orderings, as revealed by a detailed analysis of the correlation lengths and their dispersion curves obtained from SAXS data. We also discuss the validity of the Stokes-Einstein relation for these ionic liquids and the evolution of the extrapolated cationic radius that was found to depend non-strictly linearly on n, in agreement with the cation-cation correlation lengths.

Characterization of strain recovery and “self-healing” in a self-assembled metallo-gel

We report a self-assembled metallo suprapolymer gel exhibiting remarkable self-healing features. The Ni2BTC metallo suprapolymer gels result from the complexation of Ni(2+) metal ions by a tritopic ligand (bis-terpyridine cyclam) in dimethylformamide (DMF) and an annealing step at 50 °C for 24 hours. The self-healing properties are characterized by visual inspection, rheological and impedance spectroscopy measurements: the results are compared with those of a fatty acid-based molecular organogel chosen as a reference system. The creep-recovery analysis uses the Burgers model for low strains and characterizes a recovery capability of up to 72% of the deformation in Ni2BTC gels while it is only 32% for the fatty acid organogel. At very large strains, the impedance spectroscopy confirms the slow repairing process consistently with the visual observations. Rheological measurements demonstrate the restructuring of the fractured networks. The fatigue of the self-healed gel networks undergoing long sequences of strain-relaxation steps is characterized.

Sulfonic and Phosphonic Acid and Bifunctional Organic–Inorganic Hybrid Membranes and Their Proton Conduction Properties

Hybrid organic-inorganic approaches are used for the synthesis of bifunctional proton exchange membrane fuel cell (PEMFC) membranes owing to their ability to combine the properties of a functionalized inorganic network and an organic thermostable polymer. We report the synthesis of both sulfonic and phosphonic acid functionalized mesostructured silica networks into a poly(vinylidenefluoride-co-hexafluoropropylene) (poly(VDF-co-HFP) copolymer. These membranes, containing different amounts of phosphonic acid and sulfonic acid groups, have been characterized using FTIR and NMR spectroscopy, SA-XRD, SAXS, and electrochemical techniques. The proton conductivity of the bifunctional hybrid membranes depends strongly on hydration, increasing by two orders of magnitude over the relative humidity (RH) range of 20 to 100%, up to a maximum of 0.031 S cm(-1) at 60 °C and 100% RH. This value is interesting as only half of the membrane conducts protons. This approach allows the synthesis of a porous SiO(2) network with two different functions, having -SO(3)H and -PO(3)H(2) embedded in a thermostable polymer matrix.

Hierarchical aggregation mechanism in heat-set metallosupramolecular gels using a tritopic functional ligand exhibiting temperature-triggered cis-to-trans molecular conversions

We present the aggregation mechanism in a novel class of heat-set metallosupramolecular Ni2BTC gels formed by complexation of a bis-terpyridine-cyclam (BTC), a tritopic ligand, with nickel ions (II). Viscosimetry, rheology, small-angle X-ray scattering, dynamic light scattering and impedance spectroscopy measurements are used to investigate the aggregative properties. Gels are obtained by heating the homogeneous solutions (Ni2BTC in dimethylformamide) from room temperature to 50 °C for 24 hours, which is a remarkable phenomenology in the class of organic molecular gels. The hierarchical aggregation mechanism relies on a temperature-triggered cis-to-trans molecular conversion of the tritopic ligand at the central cyclam unit. At the nanoscale, the expansion of the coil-like species in the liquid phase to rodcoils allows, beyond a critical concentration, their percolation into transient gel networks. The use of such tritopic ligands gives access to homo- or hetero-metallogels that adapt their properties to the environmental conditions (temperature, pH, mechanical stress and applied electrical potential). Various molecular adjustment parameters (metal ions, metal sequence, metal–ligand stoichiometry, cyclam substituents) can be used to tune the aggregative properties of the system.

Ionic liquids–lignin combination: an innovative way to improve mechanical behaviour and water vapour permeability of eco-designed biodegradable polymer blends

In this work, the potential use of lignin combined with ionic liquids (ILs) has been investigated on the final properties of biodegradable polymer blends. Poly(butylene-adipate-co-terephtalate)–polylactide–lignin (PBAT–PLA–Lig) were melted blending in one pot by using phosphonium ionic liquids as new additives. The effect of incorporating 16 wt% of lignin and PLA and 1 wt% of ILs into PBAT matrix has been explored on the mechanical behaviour, the water vapour permeability, the thermal stability as well as the morphologies of PBAT–PLA–Lig–IL blends. In all cases, the lignin–ionic liquid combination leads to a good mechanical performance coupled with a dramatic increase in water barrier properties. In addition, transmission electron microscopy (TEM) analysis has been used to investigate the influence of ILs on the different morphologies of these sustainable polymer blends.

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.

The properties of new epoxy networks swollen with ionic liquids

In this work, ionic liquids based on trihexyl(tetradecyl)phosphonium associated with bis(2,4,4-trimethylpentyl)phosphinate [TMP] and dicyanamide [DCA] counter anions were used as reactive additives within epoxy/amine networks. We have demonstrated that the addition of ionic liquids plays a key role on the kinetics of polymerization but also on the final properties of the epoxy networks. Structurations at the nanoscale combined with excellent thermal stability and good mechanical properties have been highlighted. By processing sub-stoichiometric epoxy–amine networks and thanks to the reactivity of phosphinate counter-anions towards glycidyl moieties, more than 70 percent of the ionic liquid was introduced which opens new perspectives in the development of high performance gel electrolytes for the safety improvement of electrochemical devices such as lithium ion batteries.

Proton Diffusion Coefficient in Electrospun Hybrid Membranes by Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) was, for the first time, used to estimate the global transverse proton diffusion coefficient, D(H+)(EHM), in electrospun hybrid conducting membranes (EHMs). In contrast to conventional impedance spectroscopy, EIS measurements were performed at room temperature with a liquid interface. In this configuration, the measure of the bulk proton transport is influenced by the kinetics of the transfer of proton at the solid/liquid interface. We demonstrated that the use of additives in the process of the membrane impacts the organization of the hydrophilic domains and also the proton transport. The D(H+)(EHM) is close to 1.10(-7) cm(2) s(-1) (± 0.1.10(-7) cm(2) s(-1)) for the EHMs without additive, whereas it is 4.10(-6) cm(2) s(-1) (± 0.4.10(-6) cm(2) s(-1)) for EHMs with additives.

Self-assembled highly ordered acid layers in precisely sulfonated polyethylene produce efficient proton transport

Recent advances in polymer synthesis have allowed remarkable control over chain microstructure and conformation. Capitalizing on such developments, here we create well-controlled chain folding in sulfonated polyethylene, leading to highly uniform hydrated acid layers of subnanometre thickness with high proton conductivity. The linear polyethylene contains sulfonic acid groups pendant to precisely every twenty-first carbon atom that induce tight chain folds to form the hydrated layers, while the methylene segments crystallize. The proton conductivity is on par with Nafion 117, the benchmark for fuel cell membranes. We demonstrate that well-controlled hairpin chain folding can be utilized for proton conductivity within a crystalline polymer structure, and we project that this structure could be adapted for ion transport. This layered polyethylene-based structure is an innovative and versatile design paradigm for functional polymer membranes, opening doors to efficient and selective transport of other ions and small molecules on appropriate selection of functional groups.Polymer synthesis can provide control over chain microstructure and conformation. Well-controlled chain folding in sulfonated polyethylene, leading to highly uniform hydrated acid layers of subnanometre thickness with high proton conductivity, is demonstrated.

A long-chain protic ionic liquid inside silica nanopores: Enhanced proton mobility due to efficient self-assembly and decoupled proton transport

We report enhanced protonic and ionic dynamics in an imidazole/protic ionic liquid mixture confined within the nanopores of silica particles. The ionic liquid is 1-octylimidazolium bis(trifluoromethanesulfonyl)imide ([HC8Im][TFSI]), while the silica particles are microsized and characterized by internal well connected nanopores. We demonstrate that the addition of imidazole is crucial to promote a proton motion decoupled from molecular diffusion, which occurs due to the establishment of new N-HN hydrogen bonds and fast proton exchange events in the ionic domains, as evidenced by both infrared and 1H NMR spectroscopy. An additional reason for the decoupled motion of protons is the nanosegregated structure adopted by the liquid imidazole/[HC8Im][TFSI] mixture, with segregated polar and non-polar nano-domains, as clearly shown by WAXS data. This arrangement, promoted by the length of the octyl group and thus by significant chain-chain interactions, reduces the mobility of molecules (Dmol) more than that of protons (DH), which is manifested by DH/Dmol ratios greater than three. Once included into the nanopores of hydrophobic silica microparticles, the nanostructure of the liquid mixture is preserved with slightly larger ionic domains, but effects on the non-polar ones are unclear. This results in a further enhancement of proton motion with localised paths of conduction. These findings demonstrate significant progress in the design of proton conducting materials via tailor-made molecular structures as well as by smart exploitation of confinement effects. Compared to other imidazole-based proton conducting materials that are crystalline up to 90 °C or above, the gel materials that we propose are useful for applications at room temperature, and can thus find applications in e.g. intermediate temperature proton exchange fuel cells.