The phase transition, dynamics and transport of water and electrolyte solutions in nano-pores and/or membranes is of high relevance in new technologies for energy management. We study these issues in poly-electrolyte membranes and solid-states nanopores, with various approach including neutron diffration, spectroscopy, forced wetting, surface force dynamics and electrical measurements.
Water transport at subzero temperatures in a polymeric electrolyte membrane
(Marie PLAZANET in coll. with F. Sacchetti (Univ. Perugia) and R. Torre
Nafion is a perfluorosulphonated polymer that adopts a complex structural organisation resulting from the nanosegregation between hydrophobic backbones and hydrophilic acid groups. Upon hydration, water-filled channels and cavities appear in the polymer matrix. Below 273 K, a phenomenon of water sorption/desorption competes with the freezing of water in this confined environment. Combining neutron diffraction and Transient Grating spectroscopy, we eventually disentangled the two phenomena and provided a model in quantitative agreement with the data that reconciles both confinement and entropic effects. The study has more general implications on the behaviour of water in a soft confinement at subzero temperatures.
Forced intrusion and spontaneous extrusion cycles of a nonwetting liquid in a pulverulent nanoporous material with large specific area of the order of 1000 m2/cm3 are envisioned for energy applications. When the pressure is increased, the liquid penetrates into the pores at a well-defined intrusion pressure Pint. The global volume of the system decreases and energy is stored at interfaces. When the pressure is released, the liquid is expelled at an extrusion pressure Pext, lower than Pint for nanometric pores (blue curve of interest for damping application) equal to Pint for sub-nanometric pores (red curve, of interest for reversible energy storage). Sub-nanometric pores can also be used as molecular sieve. With salt solutions, an additional osmotic contribution results from the separation of pure water entering the pore from salt molecules retained out of the pores. This seperation process, in volume, leads to a translation of intrusion/extrusion plateau equal to the osmotic pressure of the solution. Giant osmotic pressure larger than 100 MPa can be sustain beacuse of the « in volume » nature of the extraction.
Hydrophobic nanopores of controlled geometry are an ideal nano-laboratory to study liquid transport in strong hydrophobic confinement. Slow logarithmic dependance of the intrusion and extrusion pressures are observed according to the intrusion or extrusion duration. The dynamical extrusion is controlled by the nucleation of nano-bubbles in hydrophobic environment. We use kinetic studies to determine the critical volume and energy barrier of vapor nucleii in nanopores, and have evidenced the role of line tension in the physics of surface nano-bubbles. Line tension value of the contact line surrounding the nucleation bubble can be measured.
Below the micrometer scale, flows interact strongly with surfaces. We study the coupling between interfacial hydrodynamics, electro- and diffusio-kinetic transport, and their applications for energy recovery.
This topic is financed by the ANR program Blue Energy, anb by the program ARC -Energy of the Rhône-Alpes-Auvergne region.
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