Patrice Huguet

Intensive current transfer in membrane systems: Modelling, mechanisms and application in electrodialysis

Usually in electrochemical systems, the direct current densities not exceeding the limiting current density are applied. However, the recent practice of electrodialysis evidences the interest of other current modes where either the imposed direct current is over the limiting one or a non-constant asymmetrical (such as pulsed) current is used. The paper is devoted to make the mechanisms of mass transfer under these current regimes more clear. The theoretical background for mathematical modelling of mass transfer at overlimiting currents is described. Four effects providing overlimiting current conductance are examined. Two of them are related to water splitting: the appearance of additional charge carriers (H(+) and OH(-) ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. These effects result from formation of concentration gradients (known as concentration polarization) caused by the current flowing under conditions where ionic transport numbers are different in the membrane and solution. Similar effects take place not only in electrodialysis membrane systems, but in electrode ones, in electrophoresis and electrokinetic micro- and nanofluidic devices such as micropumps. The relation of these effects to the properties of the membrane surface (the chemical nature of the fixed groups, the degree of heterogeneity and hydrophobicity, and the geometrical shape of the surface) is analyzed. The interaction between the coupled effects is studied, and the conditions under which one or another effect becomes dominant are discussed. The application of intensive current modes in electrodialysis, the state-of-the-art and perspectives, are considered. It is shown that the intensive current modes are compatible with new trends in water treatment oriented towards Zero Liquid Discharge (ZLD) technologies. The main idea of these hybrid schemes including pressure- and electro-driven processes as well as conventional methods is to provide the precipitation of hardness salts before the membrane modules and that of well dissolved salts after.

3 In situ and operando determination of the water content distribution in proton conducting membranes for fuel cells: a critical review

Proton exchange membrane fuel cells have been recognized as a promising zero-emission power source for portable, mobile and stationary applications. The information of water content distribution in the different components of the cell during operation, particularly the proton conducting membrane, is a critical issue for the validation of mass transfer models, the definition of optimized operating conditions and the development of efficient systems with innovative designs for efficient water management. In order to fully understand the way a fuel cell performs, water transport and distribution have to be investigated in situ and operando. In this review, we critically examine the state-of-the-art of operando diagnostics sensitive to the membrane water content, particularly those techniques able (in principle) to give insights into water transport occurring along both the in- and through-plane directions. Particular attention is devoted to experimental results obtained across the membrane thickness i.e. to the determination of water concentration profiles originating from the water activity and electrical gradients occurring through the working fuel cell. Different operando techniques have been developed for this purpose, from the early 1990s up to the last few years: internal resistance measurements, magnetic resonance and neutron imaging, neutron and X-ray scattering, confocal μ-Raman spectroscopy. These techniques can be roughly separated as either direct (i.e. the water amount can be directly derived from the detected signal, avoiding sometimes arbitrary assumptions during data processing) but intrusive (i.e. they require significant modification of the fuel cell, compared to the current design and materials) or indirect but with a significantly lower intrusiveness. It appears that operando measurements of the membrane water distribution allow a unique picture of how the internal part of the fuel cell works, thus certainly contributing to the development of more effective cell designs and materials in the near future. Nevertheless, improvement in the fundamental understanding of the actual fuel cell requires further efforts to increase spatial and, more particularly, temporal resolution of current operando techniques. Also, the comparison of limitations arising from the basic principles of the different operando approaches suggests that ultimate progress will arise from the combination of complementary techniques for simultaneous measurements.

The crossed interdiffusion of sodium nitrate and sulfate through an anion exchange membrane, as studied by Raman spectroscopy

Raman spectroscopy is applied to visualize concentration profiles in solution adjacent to the top of DSV anion exchange membranes in horizontal positions. The concentration profiles of sulfates, nitrates and their fluxes through the membrane are recorded as a function of time during 13–15 h experiments. A three layer mathematical model based on extended Nernst–Planck equations with a convective term is developed. The membrane’s kinetic and static parameters, such as ionic diffusion and ion exchange coefficients, are determined by separate experiments. Only one parameter, the diffusion boundary layer thickness, δ, is fitted. It is found that the model described fluxes well, especially when the variation of δ is taken into account. At the same time, the model gives nearly linear concentration profiles, whereas the measured profiles are much more complicated. This complicated shape is explained by natural convection and the emergence of the Benard vortexes in the system.

In-situ measurement of electroosmotic drag coefficient in Nafion membrane for the PEMFC.

A new method based on hydrogen pump has been developed to measure the electroosmotic drag coefficient in representative PEMFC operating conditions. It allows eliminating the back-flow of water which leads to some errors in the calculation of this coefficient with previously reported electrochemical methods. Measurements have been performed on 50 μm thick Nafion membranes both extruded and recast. Contrary to what has been described in most of previous published works, the electroosmotic drag coefficient decreases as the membrane water content increases. The same trend is observed for temperatures between 25 and 80 °C. For the same membrane water content, the electroosmotic drag coefficient increases with temperature. In the same condition, there is no difference in drag coefficient for extruded Nafion N112 and recast Nafion NRE212. These results are discussed on the basis of the two commonly accepted proton transport mechanisms, namely, Grotthus and vehicular.

Confocal Raman micro-spectroscopy and electrochemical investigation of anion transport through ion-exchange membranes

Abstract The aim of this study is the determination of the ionic distribution versus the spatial coordinates within the membrane polymers. The confocal Raman spectroscopy is a useful tool for the study because it allows investigation of the species nature within materials. The studied membrane is a AW from Solvay (with a poly-4-vinylpyridine graft and an ethylene tetrafluoroethylene matrix). The first results are the pK value (2.25), verification of the grafts homogeneity in the whole membrane and an initial study about ionic transport within membrane and spectra analysis with factor analysis (principal components analysis and evolving factor analysis with multivariate curve resolution).

Characterisation of cation exchange membrane in hydro-organic media by electrochemistry and Raman spectroscopy

The physicochemical characterisation of a CMX membrane in hydro-organic media was undertaken. N-Methylformamide (NMF) and five other solvents with various relative permittivities were investigated. The water content, electrical resistance and transport numbers allowed us to compare the mobility of protons and sodium ions through the membrane as a function of the nature and amount of the organic solvent in the equilibrating solution. Moreover, Raman spectroscopy proved to be a powerful tool to investigate the solvent sorption within the membrane material.

Raman spectroscopy investigation and improved knowledge on industrial cation-exchange membranes involved in electrodialysis process

Abstract Raman spectra of three specific, industrial, cation-exchange membranes (CEMs) have shown the existence of an extra vibrational band. The relative intensity of this band is different in each membrane spectrum recorded. Chlorosulfonation of polymeric ethylenetrifluoroethylene (ETFE) film grafted with polystyrene chains is used to obtain these CEMs involved in the electrodialysis process. A Raman study of the above reaction has been undertaken and has shown that non-sulfonated polystyrene rings give rise to this extra vibrational band. Different behavior of CEMs synthesized under similar conditions can be explained by a variable amount of non-sulfonated polystyrene rings contained in these materials.

Correction: Asymmetric bi-layer PFSA membranes as model systems for the study of water management in the PEMFC

Correction for ‘Asymmetric bi-layer PFSA membranes as model systems for the study of water management in the PEMFC’ by A. Z. Peng et al., Phys. Chem. Chem. Phys., 2014, 16, 20941–20956.

Determination of shifts by means of the absolute area of the difference spectrum: cases of non-rigorous application of the theory

Abstract The method of determination of spectral shifts by use of the absolute area of the difference spectrum, previously developed in several papers, holds rigorously when the bands to be compared either have same profiles or are both symmetrical. It often happens in practice that none of these conditions is fulfilled and the concept itself of spectral shift is then questionable. However, it is possible to define in any case quantities, which, in the rigorous cases of definition of the spectral shift, are equal to each other and to the spectral shift. These quantities are the shift between the maximum intensity of the bands, the distance between vertical lines sharing the area of the bands into two equal parts, the displacement which must be given to one of the bands to reach the minimum of the absolute area of the difference spectrum. They are determined for a series of couples of model profiles, one of them being more and more dissymmetrical. Their values are compared with the approximate spectral shifts given by the application of the absolute area method. It is shown that up to relatively important dissimilarity of the profiles, the values of all these quantities are very close and may be taken as the spectral shift. Moreover, an extrapolation method giving the shift between the maximum intensities of dissimilar and dissymmetrical bands is given.