Solvent-Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes.

The latest developments of localized high-concentration electrolytes (LHCEs) shed light on stabilizing the high-energy-density lithium (Li) metal batteries. It is generally considered that the nonsolvating diluents introduced into the LHCEs improve the viscosity and wettability of high-concentration electrolytes (HCEs) without changing their inner solvation structures, thus maintaining the highly coordinated contact ion pairs (CIPs) and ionic aggregates (AGGs) of the precursor HCEs with limited free solvent numbers and high Coulombic efficiency (CE) of Li metal anodes. Herein, we show an unexpected effect of the diluent amount on the solvation structures of the LHCEs: as the diluent amount increases, the proportions of free solvent molecules and CIPs rise up simultaneously. The latter is probably due to the partial splits of the AGGs via the dipole-dipole interactions between the diluent and solvent molecules. Accordingly, a moderately diluted LHCE shows the best Coulombic efficiency of Li metal anodes (99.6%), compared with the precursor HCE (97.4%) or highly diluted LHCE (99.0%). This work reveals a new criterion of the LHCE chemical formulation for the designing of advanced electrolytes for high-energy-density batteries.

Nanostructured Carbon-Nitrogen-Sulfur-Nickel Networks Derived From Polyaniline as Bifunctional Catalysts for Water Splitting.

The development of reliable production routes for sustainable hydrogen (H2), which is an essential feedstock for industrial processes and energy carrier for fuel cells, is needed. It appears to be an unavoidable alternative to significantly reduce the dependence on conventional energy sources based on fossil fuels without increasing the atmospheric CO2 levels. Among the different power-to-X scenarios to access high purity H2, the electrochemical approach based on electrolysis looks to be a promising sustainable solution at both the small and large industrial scales. However, the practical realization of this important opportunity faces several challenges, including the efficient design of cost-effective catalytic materials to be used as a cathode with improved intrinsic and durable activity. In this contribution, we report the design and development of efficient nanostructured catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in aqueous media, whereby noble metal-free elements are embedded in a matrix of a conducting polymer, polyaniline (PANI). To increase the electrical conductivity and further the electrocatalytic ability toward HER of the chemically polymerized PANI in the presence of nickel (II) salt (nitrate), the PANI-based materials have first been stabilized at a mild temperature of 250-350°C in air and then carbonized at 800-1,000°C under nitrogen gas to convert the chemical species into nitrogen, sulfur, nickel, and carbon nanostructured networks (CNNs). Different physicochemical (TGA-DSC, Raman spectroscopy, XRD, SEM, EDX, ICP, CHNS, BET, and XPS) and electrochemical (voltammetry and electrochemical impedance spectrometry) methods have been integrated to characterize the as-synthesized CNNs materials and interrogate the relationship of material-to-performance. It has been found that those synthesis conditions allow for the substantial increase of the electrocatalytic performance toward HER and OER in alkaline media in terms of the onset potential and charge transfer resistance and overpotential at the specific activity of 10 milliamps per square centimeter, thus ranking the present materials among the most efficient noble metal-free catalysts and making them possible candidates for integration in practical low-energy consumption alkaline electrolyzers.

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.

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.

Probing proton dissociation in ionic polymers by means of in situ ATR-FTIR spectroscopy.

The hydration process of cationic membrane protogenic groups was investigated using in situ ATR-FTIR spectroscopy. The aim of this study is to provide a relationship between the hydration degree of the membrane and the dissociation state of exchange sites inside the polymer material. IR spectra were recorded by means of an environmental device specifically manufactured to allow the control of water vapour pressure in equilibrium with the sample. The behaviour of Nafion 112 and sulfonated poly(ether ether ketone) (S-PEEK), in both proton and sodium forms, was compared. IR data, analyzed and fitted in the 800-1850 cm(-1) spectral range, gave precise information on the assignment of sulfonic group vibrational modes. The results of this study improve the understanding of the transition phenomena between dissociated and undissociated states of the grafted sites in protonic conductors.