Christian Beauger

Cellulose–silica aerogels

Aerogels based on interpenetrated cellulose-silica networks were prepared and characterised. Wet coagulated cellulose was impregnated with silica phase, polyethoxydisiloxane, using two methods: (i) molecular diffusion and (ii) forced flow induced by pressure difference. The latter allowed an enormous decrease in the impregnation times, by almost three orders of magnitude, for a sample with the same geometry. In both cases, nanostructured silica gel was in situ formed inside cellulose matrix. Nitrogen adsorption analysis revealed an almost threefold increase in pores specific surface area, from cellulose aerogel alone to organic-inorganic composite. Morphology, thermal conductivity and mechanical properties under uniaxial compression were investigated. Thermal conductivity of composite aerogels was lower than that of cellulose aerogel due to the formation of superinsulating mesoporous silica inside cellulose pores. Furthermore, composite aerogels were stiffer than each of reference aerogels.

Niobium- and antimony-doped tin dioxide aerogels as new catalyst supports for PEM fuel cells

In order to tackle the problem of low durability, tin dioxide was studied to replace carbon black as a catalyst support in proton exchange membrane fuel cells (PEMFCs). SnO2 is a well-known n-type semi-conductor whose electronic conductivity can be improved by doping with hypervalent cations such as Nb5+ or Sb5+. In addition, as a catalyst support, this material has to develop a high specific surface area with an adequate mesoporous morphology to allow both good dispersion and activity of the catalyst (Pt). To this end, our objective was to develop doped SnO2 aerogels in order to gather in a same material both a high electronic conductivity and an adapted morphology. In this study, SnO2 xerogels and aerogels were successfully synthesized following an acid-catalyzed sol–gel route starting with metal alkoxides as precursors. Dried gels were calcined for 5xa0h at 600xa0°C in flowing air. The effect on both the structure and the morphology of the material resulting from doping with niobium or antimony was investigated by XRD, SEM, and nitrogen sorption. The electronic conductivity of pure and doped SnO2 materials was obtained from impedance spectroscopy and resistance measurements. Our materials showed a very interesting airy morphology adapted for the foreseen application: a reasonable specific surface area (80–90xa0m2/g) with a bimodal pore size distribution centered on around 25 and 45xa0nm. Moreover, all Sb-doped samples exhibited significant improvement in electronic conductivity. 5xa0at.% Sb-doped SnO2 even showed an electronic conductivity of 1xa0S/cm, very similar to that of Vulcan XC-72 (4xa0S/cm) and representing a 5 orders of magnitude increase compared to that of pure SnO2.

Xerocellulose: lightweight, porous and hydrophobic cellulose prepared via ambient drying

AbstractnLow density, highly porous and hydrophobic cellulose-based new material, Xerocellulose, was prepared and characterised. First, tritylcellulose with different degrees of substitution (DS) was synthesised in homogeneous conditions. Xerocellulose was then prepared from tritylcellulose via dissolution–coagulation–drying route, similar to other polysaccharide-based aerogels, but drying was performed in ambient room conditions. The new material has a density between 0.1 and 0.2xa0g/cm3 and is highly hydrophobic with contact angle 140° for DSxa0=xa00.72. Compared with cellulose aerogel and pristine microcrystalline cellulose, Xerocellulose obtained from tritylcellulose with DSxa0=xa00.72 showed a drastically decreased water vapour uptake. The evolution of Xerocellulose density and morphology as a function of the DS is presented and discussed.

Improving the activity and stability of Ir catalysts for PEM electrolyzer anodes by SnO2:Sb aerogel supports: does V addition play an active role in electrocatalysis?

Low Ir loading oxygen evolution reaction (OER) catalysts with superior activity and durability for proton exchange membrane (PEM) electrolyzers are an important topic in industry and academia. One possible strategy for addressing this challenge is the use of support materials that are stable under highly corrosive acidic environments at a high working potential (>1.4 V). Moreover, highly porous structure is another key criteria for OER catalyst support to achieve a high electrochemical surface area. Here, we report a novel Ir supported on a SnO2:Sb aerogel OER catalyst (Ir/SnO2:Sb-mod-V), which was prepared under ambient pressure by using vanadium additives. It shows an unrivaled activity and enhanced stability, on which vanadium does not play any active role but demonstrates the influences that changes the porosity of the aerogel support and affects the impurity content of the chlorine. By taking advantage of the high porosity of the aerogel substrate, Ir/SnO2:Sb-mod-V allows a decrease of more than 70 wt% for precious metal usage in the catalyst layer while keeping a similar OER activity compared to its unsupported counterpart.

Pt Nanoparticles Supported on Niobium-Doped Tin Dioxide: Impact of the Support Morphology on Pt Utilization and Electrocatalytic Activity

AbstractTwo synthesis routes were used to design high surface area niobium-doped tin dioxide (Nb-doped SnO2, NTO) nanostructures with either loose-tube (fibre-in-tube) morphology using electrospinning or aerogel morphology using a sol-gel process. A higher specific surface area but a lower apparent electrical conductivity was obtained on the NTO aerogel compared to the loose tubes. The NTO aerogels and loose tubes and two reference materials (undoped SnO2 aerogel and Vulcan XC72) were platinized with a single colloidal suspension and tested as oxygen reduction reaction (ORR) electrocatalysts for proton-exchange membrane fuel cell (PEMFC) applications. The specific surface area of the supports strongly influenced the mass fraction of deposited Pt nanoparticles (NPs) and their degree of agglomeration. The apparent electrical conductivity of the supports determined the electrochemically active surface area (ECSA) and the catalytic activity of the Pt NPs for the ORR. Based on these findings, electrospinning appears to be the preferred route to synthesize NTO supports for PEMFC cathode application.n Graphical AbstractOn top : SEM images of the synthesized supports : 5.0 at.% Nb-doped SnO2 aerogel (NTO-AG) and loose tubes (NTO-LT) - At the bottom : specific activity (SA0.90) and mass activity (MA0.90) of the synthesized electrocatalysts for the oxygen reduction reaction (ORR) determined at E = 0.90 V vs. RHE as a function of the conductivity of the supports

SYNTHESIS AND MICROSTRUCTURE OF A NOVEL TiO2 AEROGEL–TiO2 NANOWIRE COMPOSITE

TiO2 aerogel-10 mol% TiO2 nanowire composite was prepared by a sol-gel technique with the addition of TiO2 nanowires to TiO2 sol, followed by supercritical drying in CO2. TiO2 nanowires (anatase with minor rutile phases) as dispersoid were prepared by a hydrothermal process followed by calcination in air at 600°C. The TiO2 nanowires were dispersed in a 2-propanol/H2O/HNO3 solution, and the mixture was added drop by drop to a tetrabutyl orthotitanate [i.e. Ti (IV) n-butoxide] solution in 2-propanol. After gelation, the TiO2 alcogel-TiO2 nanowire composite was dried in supercritical CO2 to obtain the final, TiO2 aerogel-TiO2 nanowire composite. TEM analysis revealed that a unique nanowire network structure was formed withinthe mesoporous aerogel matrix. The aerogel-TiO2 nanowire composite had a relatively large surface area 427 m2/g, with mesopores ~ 16 nm in diameter and a pore of volume of 1.63 cm3/g.