We report a QENS study of the molecular motions in a perfluorinated ionomer membrane, Nafion, under increasing hydration levels from almost dry to fully saturated. Combined experiments performed on time-of-flight and backscattering spectrometers have been used to investigate the picosecond to the nanosecond dynamic behavior of water. The experimental spectra have been simulated over the whole Q range from 0.34 to 2.25 Å-1 by a single theoretical model taking into account the localized motions within confining domains, the microscopic features of the elementary jump process, and the long-range diffusion mechanism. The diffusion in a restricted geometry with ill-defined boundaries has been described by Gaussian statistics, contrary to the popular diffusion inside an impermeable sphere model where the boundaries are well defined. Evaluation of the spectra reveals the existence of two populations of protons in Nafion at all hydrations that are nonexchangeable on the nanosecond time-scale. A first population of three protons per ionic group is involved in a slow jump mechanism on characteristic length-scales of 2 to 4 Å and typical times ranging from 500 to 150 ps when increasing the water content in the membrane. This slow population, already present in the dried state, is presumably composed of the protons of the hydronium ions. The second fast population is composed of the additional hydrating water molecule protons. Between low hydration (3 H2O/SO3 -) and saturation (17.5 H2O/SO3 -), these protons are involved in a faster localized motion on roughly the same length scale, i.e., in the same water droplet as the hydronium ion. Long-range diffusion of these protons between neighboring domains of restricted motions is observed, even at very low hydration. As the number of water molecules in the membrane increases, a general finding is that the characteristic sizes increase and the characteristic times decrease, approaching asymptotic values at saturation. This is further reflected by the behavior of the local diffusion coefficient (inside a droplet) and the long-range diffusion coefficient (from one droplet to another) that vary, respectively, from 0.45 to 2 (10-5 cm2/s), and 0.1 to 0.58 (10-5 cm2/s), for λ ∼ 3 to 17.5. Overall, a molecular scenario for the proton motions among the different hydration steps has been proposed on the basis of the quantification of the dynamics on different length scales and time scales:  below 10 H2O/SO3 - the hydration protons diffuse faster and faster in ionic clusters of growing size. Above this first hydration regime, the asymptotic upper limit with increased hydration is reached:  the water molecules locally display a bulk-like behavior within the hydrophilic domains. The long-range diffusion appears to be correlated to the enhancement of hydronium mobility with water loading. These findings, that qualitatively confirm the results of a previous similar study, bring a significant improvement to the description of the experimental data and new quantitative information concerning the ...

Through a tight collaboration between chemical engineers, polymer scientists, and electrochemists, we address the degradation mechanisms of membrane electrode assemblies (MEAs) during proton exchange membrane fuel cell (PEMFC) operation in real life (industrial stacks). A special attention is paid to the heterogeneous nature of the aging and performances degradation in view of the hardware geometry of the stack and MEA. Macroscopically, the MEA is not fuelled evenly by the bipolar plates and severe degradations occur during start‐up and shut‐down events in the region that remains/becomes transiently starved in hydrogen. Such transients are dramatic to the cathode catalyst layer, especially for the carbon substrate supporting the Pt‐based nanoparticles. Another level of heterogeneity is observed between the channel and land areas of the cathode catalyst layer. The degradation of Pt3Co/C nanocrystallites employed at the cathode cannot be avoided in stationary operation either. In addition to the electrochemical Ostwald ripening and to crystallite migration, these nanomaterials undergo severe corrosion of their high surface area carbon support. The mother Pt3Co/C nanocrystallites are continuously depleted in Co, generating Co2+ cations that pollute the ionomer and depreciate the performance of the cathode. Such cationic pollution has also a negative effect on the physicochemical properties of the proton‐exchange membrane (proton conductivity and resistance to fracture), eventually leading to hole formation. These defects were localized with the help of an infrared camera. The mechanical fracture‐resistance of various perfluorosulfonated membranes further demonstrated that polytetrafluoroethylene‐reinforced membranes better resist hole formation, due to their high resistance to crack initiation and propagation. WIREs Energy Environ 2014, 3:540–560. doi: 10.1002/wene.113 This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Fuel Cells and Hydrogen > Systems and Infrastructure Energy Research & Innovation > Science and Materials

This article presents the results of CO 2 /brine two-phase flow experiments in rocks at reservoir conditions. X-ray CT scanning is used to determine CO 2 saturation at a fine scale with a resolution of a few pore volumes and provide 3D porosity and saturation maps that can be use to correlate CO 2 saturations and rock properties. The study highlights the strong influence of sub-core scale heterogeneities on the spatial distribution of CO 2 at steady state and provides useful relative permeability data on a sample originated from an actual storage site (CO2CRC-Otway project, Victoria, South-West Australia). Two different samples tested, although different in nature, present strong heterogeneities, but differ in the detail of the connectivity of high porosity layers. In both samples, the results of the investigations show that sub-core scale heterogeneities control the sweep efficiency and may cause channeling through the porous medium. In one of the samples, CO 2 saturation appears uncorrelated to porosity close to the outlet end of the core. This observation is understood as a result of the position and the orientation of high porosity layers with respect to the inlet face of the core. Finally, in the operating conditions of the two experiments, the saturation maps demonstrate that gravity does not play a major role since no detectable buoyancy driven flow is observed.

No abstract

No abstract

A simple model based on Gaussian statistics, aimed at describing localized diffusive translational motion in one, two, and three dimensions is presented and used to calculate the corresponding incoherent neutron scattering laws. In the time domain, these laws are closed form mathematical functions. In the frequency domain, some of these laws can be expressed as an infinite series depending on one single index. Owing to this relative simplicity, such a model can advantageously replace previous models such as diffusion on a segment, inside a circle and inside a sphere with an impermeable surface, to analyze neutron quasielastic scattering data associated with molecular motions in confined media. It may also be more realistic when the confinement is defined by soft, ill-defined boundaries.

No abstract

Saline-aquifer storage of carbon dioxide (CO 2 ) has become recognized as an important strategy for climate-change mitigation. Saline aquifers have very large estimated storage capacities, are distributed broadly across the globe, and have the potential for geologic-scale retention times. Many of these storage sites are not well characterized, and it is critical to conduct detailed experiments and analysis to understand how features such as heterogeneity can influence the theoretical storage capacity, spatial extent of plume migration, and secondary trapping processes. Coreflooding experiments are used routinely by the oil and gas industry for such analysis and provide a very useful tool for studying saline-aquifer formations also. Numerical simulations of these coreflooding experiments can provide insight beyond the experimental measurements themselves, such as numerically studying how properties such as relative permeability and capillary pressure affect CO 2 distribution in these systems under various flow conditions. However, accurate subcore-scale simulations of these experiments have remained a challenge, and the issue of how to represent subcorescale permeability has not been resolved previously.Laboratory coreflooding experiments injecting CO 2 into a saline-water-saturated Berea sandstone core have been conducted at reservoir conditions. Computed-tomography (CT) scans of the core show large spatial variations of CO 2 saturation, even within a relatively homogeneous core. Numerical simulations of the experiment have been conducted to study the effect of subcorescale heterogeneity and the role of permeability in determining the subcore-scale CO 2 distribution in the core to explain these very large spatial variations in CO 2 saturation.Numerical simulations of the experiment consistently showed that use of traditional methods for estimating subcore-scale permeability, typically based solely on porosity distributions, results in subcore-scale saturation distributions that do not match experimental measurements. In this paper, we develop a new method for calculating subcore-scale permeability distributions on the basis of capillary pressure measurements and porosity distributions as an alternative to the traditional porosity-only-based models. Using experimentally measured saturation and porosity distributions and capillary pressure data to calculate permeability, simulations based on this new method show a substantial improvement both in the absolute value and in the spatial distribution of predicted CO 2saturation values. With this technique for accurately calculating permeability distributions, it is possible to study subcore-scale multiphase flow of brine and CO 2 to understand how small-scale heterogeneities influence the spatial distribution of CO 2 saturation and to improve our ability to predict the fate of stored CO 2 .