Perfluorinated Ion Exchange Membranes Research Articles

Multi-length-scale heterogeneous structured ion exchange membranes for cost-effective electrolysis and hydrogen production

The design of an ultrabubble-repellent ion exchange membrane (IEM) with high ion transport efficiency for the electrolysis process remains an ongoing challenge. Herein, a multi-length-scaled heterogeneous ZrO2 coating was successfully introduced onto a sodium perfluorinated ion exchange membrane (SPIEM) surface by a facile coating method. Surface morphology optimization technology is developed to control the mesopore size, density and surface coverage of particles, thus forming a “popcorn-like” hierarchical micro/nanostructured ZrO2 coating (M-Ⅱ), which is conducive to bubble repellency in time and efficient ion transport. By using this membrane in chlor-alkali electrolysis, the power consumption can reach 2037.8 kWh t−1, representing reductions of 166.2 kWh and 113.2 kWh, respectively, for producing one ton of 100% NaOH compared with pristine SPIEM (2204 kWh t−1) and commercial Nafion TM N2030 (2151 kWh t−1) at 6 kA cm−2. As a result, M-Ⅱ can realize the production of sodium hydroxide and hydrogen at low cost. The M-Ⅱ also showed remarkable stability with negligible fluctuation for 360 h. Furthermore, after simple purification, the hydrogen from chlor-alkali electrolysis shows excellent performance in fuel cell tests. This study demonstrates the effectiveness of micro/nanoscale engineering on chlor-alkali electrolysis performance, providing a simple and efficient method for solving the gas adhesion problem in the electrolysis process.

Under-brine superaerophobic perfluorinated ion exchange membrane with re-entrant superficial microstructures for high energy efficiency of NaCl aqueous solution electrolysis

Perfluorinated ion exchange membrane (PIEM) with excellent anti-bubble property is of great significance for high energy efficiency of electrolysis of NaCl aqueous solution (chlor-alkali process). Here, an under-brine superaerophobic perfluorinated ion exchange membrane (SPIEM) with ZrO2 coating is successfully achieved by a facile and efficient spray method. The ZrO2 coating with hierarchical micro/nanoscale surface structure endows the membrane a low bubble adhesive force of 72.81 μN and ultrahigh bubble contact angle of 151.3° in brine solution. The superaerophobic property of the membrane is favorable for reducing bubbles adhesion, maximizing the effective working area of the membrane and promoting ion transport. By employing SPIEM as ion exchange membrane, the electrolysis cell of NaCl aqueous solution shows a low power consumption of 1923 kWh t−1 and it can save about 100 kWh and 90 kWh of power for producing per ton of NaOH than using PIEM (2030 kWh t−1) and Nafion ™ N2030 (2010 kWh t−1) at 4.5 kA m−2. It is believed that this study is beneficial for development of high performance chlor-alkali ion exchange membrane and provides a perspective for other polymer electrolyte membranes.

A NMR study of the hydration of sulfo and carboxyl groups in perfluorinated cation exchange membranes

The mobility of the polymer matrix and the structure of the hydration complex in the acid forms of perfluorinated ion exchange membranes MF-4SK and F-4KF depending on moisture content were studied by 1H, 13C, and 19F solid-state high-resolution NMR. A correlation between an increase in mobility of separate polymer matrix segments under hydration conditions and their hydrophilic properties was observed. Carboxyl groups in F-4KF membranes were shown to exist as dimers C(OOH)2C, and the main hydrated complex form in MF-4SK membrane under low moisture content conditions was the hydroxonium ion H5O2+.

Decomposition of Perfluorinated Ion-Exchange Membrane to Fluoride Ions Using Zerovalent Metals in Subcritical Water

The decomposition of Nafion NRE-212, a typical perfluorinated ion-exchange membrane used for fuel cells, in subcritical water was investigated. This is the first report of the decomposition of a perfluorinated ion-exchange membrane aimed at the development of a technique to recover the fluorine component for waste treatment. Although the membrane showed little decomposition in pure subcritical water, the addition of several zerovalent metals to the reaction system accelerated the membrane decomposition to F− ions, and the acceleration increased in the order Al < no metal < Zn < Cu ≪ Fe. When the membrane and iron powder were heated in subcritical water at 350 °C for 17 h, 73.2% of the fluorine content in the initial membrane was successfully transformed into F− ions. In addition to F− ions, trifluoroacetic acid and HCF(CF3)OC2F4SO3− were also detected in the reaction solutions as intermediates, and CO2 and CF3H were detected in the gas phase. Time profiles of the products suggest that one pathway of the decomposition of the membrane proceeded by decomposition of the pendant-chain part, followed by decomposition of the polymer backbone.

Modeling and Simulation of the Degradation of Perfluorinated Ion-Exchange Membranes in PEM Fuel Cells

A polymer electrolyte membrane (PEM) fuel cell model that incorporates chemical degradation in perfluorinated sulfonic acid membranes is developed. The model is based on conservation principles and includes a detailed description of the transport phenomena. A degradation submodel describes the formation of hydrogen peroxide via distinct mechanisms in the cathode and anode, together with the subsequent formation of radicals via Fenton reactions involving metal-ion impurities. The radicals participate in the decomposition of reactive end groups to form carboxylic acid, hydrogen fluoride, and . Degradation proceeds through unzipping of the polymer backbone and cleavage of the side chains. Simulations are presented, and the numerical code is shown to be extremely time-efficient. Known trends with respect to operating conditions are qualitatively captured, and the exhibited behavior is shown to be robust to changes in the rate constants. The feasibility of a chemical degradation mechanism based on peroxide and radical formation is discussed.

Characterization of Flemion® membranes for PEFC

Perfluorinated ion exchange membranes were studied to clarify the characteristics of membranes required for polymer electrolyte fuel cells (PEFC). The influence of membrane thickness on gas permeability and the influence of incorporation of cations on water content and ac specific resistance of Flemion® and Nafion®117 were estimated. Gas permeation rates of the membranes decreased in inverse proportion to the increase of the membrane thickness and gas permeability coefficients were nearly constant and independent of the thickness. Hydrogen permeation rates of Flemion®S at 70°C were converted to 2.1 mA/cm 2 as current density. Water content changed by only 5% in the region of the ion exchange ratio from 0% to 100% and was independent on the kinds of incorporated cations in the region of the ion exchange ratio under 40% except for K +. Ac specific resistance increased markedly when the ion exchange ratio exceeded 50%. In the case that the ion exchange ratio was under 30%, ac specific resistance increased with decrease of the numbers of protons having no relation with the kinds of cations. Area resistance of Flemion® was smaller because it has higher ion exchange capacity and thinner thickness than Nafion®117.

Electrochemically-aided membrane separation and catalytic processes

Electrochemically driven membrane separations and catalytic processes are interesting research areas which have, to date, received relatively little attention. Research into electrically-aided membrane separation and catalytic processes is currently being carried out in South Africa. The research objective is the development and characterisation of novel composite materials based on solid polyelectrolytes (SPE), containing nanoparticles of catalytically active metals, such as Pt, Ir or Pd, distributed within the polymeric matrices. An example of such an SPE matrix is a perfluorinated ion-exchange membrane. The novel composite materials (in fact, membranes) are both ionically and electronically conductive due to the presence of the metal nanoparticles. The application of potentials to conducting membranes results in the enhancement of catalytic activity as well as the selectivity of separations. The membranes based on SPE can be used for the catalytic processing of petrochemical mixtures, water treatment (disinfection, nitrate removal, etc.), oxygen, hydrogen and ozone generation, and electrically enhanced gas and vapour separations. In this paper Dr Dmitri Bessarabov briefly outlines the current status of the research, available technology and discusses challenges and possible applications.

Studies of Perfluorinated Ion Exchange Membranes for Polymer Electrolyte Fuel Cells

Studies of Perfluorinated Ion Exchange Membranes for Polymer Electrolyte Fuel Cells

Simple device for permeation removal of water vapour from purge gases in the determination of volatile organic compounds in aqueous samples

A commercially available Nafion tube (a perfluorinated ion-exchange resin membrane separator) inserted in a plastic container filled with 5A molecular sieve was tested on dry water-saturated gas streams containing common volatile pollutants of aqueous environmental samples after a purging cycle. The proposed device was shown to remove water vapour effectively from humidified gas streams ( > 90%) even under continuous operation for more than 10 days. The results showed that the system is compatible with the tested compounds (aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons and some sulphur compounds) because these compounds are practically unaffected on passing through the Nafion dryer tube. The losses of most compounds at the μg/kg level are less than 5%. Therefore, the proposed device is suitable for the removal of water vapour from purging streams and can be used in the determination of these volatile organic compounds in aqueous samples using the purge-and-trap technique.

Oxygen permeation through perfluorinated carboxylate ion exchange membranes

Permeation of oxygen through perfluorinated carboxylate membranes (Flemion ®) of different counterions and equivalent weights (EW = 700 and 800) was investigated. Solubilities and diffusion coefficients of oxygen in the membranes were determined by an electrochemical monitoring technique using solid polymer electrolyte composite electrodes. The solubilities were close to the values in perfluorocarbon media rather than to the value in water. The solubility decreased with increasing water content of the membranes. The diffusion coefficient in 700 EW membranes increased with increasing water content, while that in 800 EW membranes decreased slightly. On the basis of these results, the diffusion path of oxygen through the membranes was discussed in terms of morphological properties of perfluorinated ion exchange membranes.

Development of Perfluorinated Ion-exchange Membrane and Chlor-Alkali Electrolysis

Development of Perfluorinated Ion-exchange Membrane and Chlor-Alkali Electrolysis

Preparation of perfluorinated ion exchange membranes and their application in acid recovery

The preirradiation grafting of crosslinked sulphonated styrene monomer (CSS) and a crosslinked aromatic monomer (ARM) onto ethylene-tetrafluoroethylene (ETFE) film was studied. The dependence of the grafting reaction on temperature, concentration of monomer, crosslinking agent, concentration of inhibitor and on the preirradiation dose was investigated. Interaction between monomer diffusibility and the viscosity of the monomer bath controls the grafting reaction. Other factors such as the mutual reactivity of monomer and crosslinking agent contribute to explaining the results of the grafting reaction. The properties of some membranes were also determined. The application of these membranes to acid recovery by dialysis, electrodialysis and electro-electrodialysis (EED) showed their excellent performance.

High Performance Ion Exchange Membrane for Industrial Use

Salt brine electrolysis using perfluorinated ion exchange membranes is now recognized worldwide as the most energy saving process for the production of chlorine and caustic alkali. Membranes are based on perfluorinated polymers having functional groups. Although perfluorosulfonated membranes were first made by the du Pont Company, they were not always found to show sufficient performance in chlor alkali process. Asahi Glass succeeded in the development of new membranes through the first successful synthesis of perfluorocarboxylated polymers. These membranes achieved high permselectivity due to less water uptake of the carboxylates as compared with the sulfonates. Currently, many commercial membranes contain layers of both sulfonate and carboxylate perfluoropolymers. This paper deals with synthetic methods for fluorocarbon acid monomers and polymers, structure and properties of the polymers and membranes made thereof, and industrial applications.

Superparamagnetic properties of small iron hydroxide precipitates in ion exchange membranes

In some pH conditions, the immersion of an iron salt of a perfluorinated ion exchange membrane in an aqueous solution leads to the precipitation of small grains of amorphous iron hydroxide inside the membrane. The structure, dimension and magnetic properties of these small particles vary as a function of the initial water content of the membrane, Mössbauer results show that the variation of the hyperfine field with temperature can be explained by a model of fluctuations of the magnetization. The size of the precipitates varies with a 3 2 power law of the initial water content, whereas the blocking temperature varies linearly with water content. This could indicate that the superparamagnetic relaxation time depends primarily on the surface of these small particles.

SO2 diffusion through acetonitrile-saturated perfluorosulfonic acid membranes

The diffusion coefficient of SO2 and acetonitrile in Nafion perfluorinated ion exchange membranes have been determined from chromatographic measurements of SO2 concentration and the isotopic ratios in acetonitrile solutions. The diffusion coefficients vary with residual water content and attain values about one fifth those reported for water-saturated membranes falling in the range 1–2 × 10−7 cm2 s−1. Values obtained for Teflon reinforced material indicates diffusion takes place over an effective surface more than twice the optically open area.

Bromine Diffusion Through Nafion® Perfluorinated Ion Exchange Membranes

The diffusion coefficient of bromine through Nafion® perfluorinated ion exchange membranes has been determined as a function of the equivalent weight of the membrane, the membrane pretreatment, and the solute concentration. The measured voltage decay of bromine concentration cells is interpreted in terms of a simple mathematical model that takes into account the solution equilibrium between bromine, bromide, and tribromide ions followed by diffusion of bromine or tribromide ions through the membrane. Diffusion coefficients fall in the range of corresponding to bromine fluxes from or current densities from 0.3 to 10 mA/cm2. Bromine fluxes as a function of solute concentration are determined directly by quantitative cathodic reduction of bromine or tribromide after diffusion through the membrane.