Gel-type Membrane Research Articles

Evolution of organic gel fouling resistance in constant pressure and constant flux dead-end ultrafiltration: Differences and similarities

A common type of ultrafiltration (UF) membrane fouling is due to organic gel, encountered in the treatment of various effluents and feed waters to desalination plants; polysaccharides are a major component of such fouling layers. In standard methods, dead-end UF tests under constant pressure are used to determine membrane-fouling propensity, but the results are applied with questionable success to the case of constant flux filtration, favored in practice. Observed differences between fouling resistances from the two modes of UF filtration have not been adequately explained yet. The temporal evolution of specific resistance α (considered as the most representative fouling parameter) is investigated herein, aiming to elucidate differences and possible similarities in the different filtration modes. During alginate-solution ultrafiltration, typical gel-type membrane fouling layers are formed with time-varying deposited mass density m (g/cm2) depending on permeate flux; thus, the temporal variation of specific resistance α, of pressure difference across the fouling layer/cake ΔPc, and of pressure drop per unit depth of cake (the latter proportional to ΔPc/m) are studied for both constant pressure P and constant flux J operation, within P and J ranges of practical interest. These detailed data suggest that permeation flux is the process variable controlling the development of resistance α. Specifically, for constant-flux filtration, the resistance α data are well correlated with flux J (as also shown recently), whereas in the constant-pressure mode the commonly observed constant resistance α, over a rather broad range of declining flux, is due to the high flux level prevailing in the early stage of filtration. A successful approach is demonstrated to compare and correlate the specific resistance α data obtained from the two different filtration modes. The usefulness of the new results in practice is also discussed.

Performance Improvement of Polyvinyl Formal Based Gel Polymer Electrolyte for Lithium-Ion Batteries by Coating Al<sub>2</sub>O<sub>3</sub>

Al2O3/PVFM/Al2O3 trilayer membranes are prepared by means of simple coating of PVA-Al2O3 solution onto both sides of PVFM thin membranes, which is prepared via phase inversion method. The characteristics of the trilayer membranes and gel polymer electrolytes are investigated using FESEM, tensile testing apparatus, thermal shrinkage test, EIS and charge-discharge test. When inorganic Al2O3 particles are used to coat the PVFM membrane, drawbacks associated with gel-type membranes, namely, poor mechanical strength and thermal stability are greatly improved. Lithium ion cell with the Al2O3/PVFM/Al2O3 based GPE matched with LiFePO4 shows excellent electrochemical performance.

In-Situ gelled Polymer Electrolytes for Advanced Lithium Ion Batteries

The activities aimed at reducing the safety hazard in lithium batteries have successfully considered the replacement of the conventional, volatile and flammable carbonate-based, liquid electrolytes with more stable polymer electrolytes, obtained in the form of free standing membranes. With respect to other solid polymer electrolytes, such as those based on poly(ethylene oxide), gelled polymer electrolytes (GPEs) have the great advantage of offering high conductivity, i.e. of the order of 10−3 S cm−1, over a wide temperature range, combined with a large electrochemical stability, i.e. extending up to 5V versus Li+/Li [1-3].Poly(vinylidene fluoride) (PVdF)-based, gel-type membranes have been widely investigated as Li-conducting systems and prepared by conventional solvent-casting procedures [4] or electro-spinning techniques [5].In this work we report on originally developed, PVdF-based GPEs obtained by in-situ formation of the gel matrix during cell assembly, followed by a lithium salt infiltration step [6]. Particular emphasis was devoted to the study and the optimization of the GPE composition. The substitution of the commonly used LiPF6salt with the fluorine free, bis(oxalato)borate (LiBOB) was investigated.A detailed thermal and dynamical mechanical characterization was adopted to elucidate the properties of the proposed systems over a wide temperature range. Vibrational spectroscopy revealed interesting molecular interactions among components, mainly. polymer and salt, strongly affecting the stability of the membranes. The electrochemical investigation confirmed high ionic conductivity, a controlled interfacial resistivity versus lithium metal and a good electrochemical stability window.The feasibility of the in-situ gelled, PVdF-based polymer electrolyte containing the LiBOB salt was demonstrated by successful galvanostatic cyclations of the quoted membrane in different lithium-ion cell configurations, obtained by coupling a Sn-C alloying anode with an high voltage spinel or a LiFePO4cathode. Acknowledgments The results of this work have been obtained by the financial support of the European Community within the Seventh Framework Programme APPLES (Advanced, High Performance, Polymer Lithium Batteries for Electrochemical Storage) Project (contract number 265644).

Preparation of a trilayer separator and its application to lithium-ion batteries

Inorganic particulate film/poly(methyl methacrylate) (PMMA)/inorganic particulate film trilayer separators are prepared by means of simple dip-coating of inorganic particle layers on to both sides of PMMA thin films. The mechanical, thermal and electrochemical characteristics of the trilayer are investigated using scanning electron microscopy, a universal tensile machine, a thermal shrinkage test and a charge–discharge test. As a polymer matrix, PMMA has exceptional compatibility with the carbonate-based liquid electrolyte, which can result in improved battery/cell performance. When inorganic Al 2O 3 particles are used to coat the PMMA film, drawbacks associated with gel-type membranes, namely, poor dimensional stability and thermal stability are greatly improved. This inorganic trilayer membrane is believed to be an inexpensive, novel separator for application in lithium-ion batteries.

Aprotic ionic liquids as electrolyte components in protonic membranes

In this paper we describe the preparation and the properties of a series of aprotic ionic liquid-based, proton-conducting membranes. The ionic liquids (ILs) 1,2-dimethyl-3-n-propylimidazolium bis(trifluoromethanesulfonyl)imide and the 3-methyl-1-n-propylpyridinium bis(trifluoromethanesulfonyl)imide are used as the casting solvents of PVdF gel-type membranes; the proton conductivity is achieved by the addition of a superacid component, namely, trifluoromethanesulfonic acid (HTf) or N,N-bis(trifluoromethanesulfonyl)imide (HTFSI). The polymer electrolytes showed good thermal and electrochemical properties in the temperature range of interest for PEMFC applications. The strong coordination between the ILs and the HTFSI, which have the same anion, improves the thermal stability of this kind of membrane, but lowers the chemical properties and the conductivity, due to an increase in viscosity. HTf-added samples have an ionic conductivity of 2 × 10−2 S cm−1 at 100 °C, showing the best overall properties and making these membranes of interest applications in fuel cells.

New membranes based on ionic liquids for PEM fuel cells at elevated temperatures

Proton exchange membrane (PEM) fuel cells operating at elevated temperature, above 120 °C, will yield significant benefits but face big challenges for the development of suitable PEMs. The objectives of this research are to demonstrate the feasibility of the concept and realize [acid/ionic liquid/polymer] composite gel-type membranes as such PEMs. Novel membranes consisting of anhydrous proton solvent H 3PO 4, the protic ionic liquid PMIH 2PO 4, and polybenzimidazole (PBI) as a matrix have been prepared and characterized for PEM fuel cells intended for operation at elevated temperature (120–150 °C). Physical and electrochemical analyses have demonstrated promising characteristics of these H 3PO 4/PMIH 2PO 4/PBI membranes at elevated temperature. The proton transport mechanism in these new membranes has been investigated by Fourier transform infrared and nuclear magnetic resonance spectroscopic methods.

Investigation of new types of lithium-ion battery materials

This paper reports part of the activities in progress in our laboratory in the investigation of electrode and electrolyte materials which may be of interest for the development of lithium-ion batteries with improved characteristics and performances. This investigation has been directed to both anode and cathode materials, with particular attention to convertible oxides and defect spinel-framework Li-insertion compounds in the anode area and layered mixed lithium–nickel–cobalt oxide and high voltage, metal type oxides in the cathode area. As for the electrolyte materials, we have concentrated the efforts on composite polymer electrolytes and gel-type membranes. In this work we report the physical, chemical and electrochemical properties of the defect spinel-framework Li-insertion anodes and of the high voltage, mixed metal type oxide cathodes, by describing their electrochemical properties in cells using either “standard” liquid electrolytes and “advanced” gel-type, polymer electrolytes. The results illustrated here demonstrate that the spinel-framework anodes of the Li[Li 1/3Ti 5/3]O 4 type can be combined with high voltage cathodes of the Li 2M x Mn (4− x) O 8 family for the fabrication of new types of lithium-ion battery systems cycling around 3.5 V. The development of this interesting concept is however still limited by the availability of highly stable electrolytes.

Novel types of lithium-ion polymer electrolyte batteries

Three new types of polymer electrolyte lithium-ion batteries are presented and discussed. These batteries have been prepared by using gel-type, poly(acrylonitrile), PAN-based membranes as the electrolyte separators of various electrode ombinations. The latter include ‘standard’ materials such as a graphite Li x C 6 anode and a manganese spinel LiMn 2O 4 cathode, as well as more innovative electrodes such as KC 8 and SnO 2 anodes, and LiCr y Mn 2− y O 4 and LiNi y Co 1− y O 2 cathodes. The results, although preliminary, are convincing in demonstrating the feasibility of these new concepts and ultimately, that PAN-based, gel-type membranes are suitable for the development of plastic-like, lithium-ion batteries of practical interest.

Plastic power sources

Lithium ion polymer batteries and laminated solid-state redox supercapacitors, formed by placing a highly conducting gel-type membrane electrolyte between a graphite film and a composite cathode film and between a poly(pyrrole)–poly(aniline) electrode combination, respectively, have been fabricated and tested. The preliminary results are encouraging in suggesting that these plastic power sources may be particularly advantageous for mobile electronic products and for zero emission electric vehicles.

Silver ion coordination in membranes for facilitated olefin transport

Silver K-edge XAFS (X-ray absorption fine structure) was used to determine the coordination of silver ions in synthetic polymeric membranes of interest for facilitated olefin transport. The Ag[sup +] ions in membranes treated with aqueous solutions of AgNO[sub 3], AgClO[sub 4], AgBF[sub 4], and AgF are three-coordinate with an average Ag-O bond length (2.22 [plus minus] 0.04 [angstrom]) that is independent of the nature of the counteranions (i.e., nitrate, perchlorate, tetrafluoroborate, and fluoride) and significantly shorter than that (2.34 [plus minus] 0.04 [angstrom]) for four-coordinate Ag[sup +] in aqueous solution. Similarly, the silver ion coordination is independent of the nature (i.e., composition, thickness, manufacturer, etc.) of the membrane. The aqueous environment in these geltype membranes is sufficiently different from bulk water, such that it causes a change of coordination of the Ag[sup +] ions. In contrast, the coordination of the linear [NC-Ag-CN][sup [minus]] anion is the same in both bulk solution and membrane.

Material transport in hyperfiltration

Transport equations for hyperfiltration, based on the frictional formalism of non-equilibrium thermodynamics, are derived for a gel-type membrane. Taking the concentration distribution in the membrane into account and allowing for possible non-equilibrium partition effects at the membrane boundaries, the flow equations are integrated and equations for solute rejection in dilute solutions are derived. It is shown that the main parameters determining solute rejection are the solute partition coefficient on the high-pressure side of the membrane and a frictional factor, involving the solute and solvent friction coefficients. Partition effects on the low-pressure side are unimportant for processes at high flow rates.