Toward predictive permeabilities: Experimental measurements and multiscale simulation of methanol transport in Nafion

A polymer membrane's permeability to solutes determines its suitability for various applications: a permeability value is essential for predicting performance in diverse contexts. Using aqueous methanol permeation through Nafion as an example, we describe a methodology for determining membrane permeability that accounts for boundary layer effects and the possibility of swelling. For the materials and apparatus used herein, analysis of a permeance measurement and computational fluid dynamics simulations show that the mass transfer boundary layer is on the order of ones to tens of microns. The data are used to develop and validate a multiscale model describing solute permeation through a hydrated membrane as a series of physical mechanistic steps: reversible adsorption from solution at the membrane interface, diffusion driven by a concentration gradient within the membrane, and reversible desorption into solution at the opposite membrane interface. The validated model is used to predict methanol transport across a solar-driven CO₂ reduction device and to assess the impact of polymer changes on the measured value. The approach of combining experimental data, computational fluid dynamics, and the mechanistic multiscale model is expected to provide more accurate analysis of membrane permeation data in cases with polymer swelling or unusual device geometries, among others.     

Composite Nafion membrane embedded with hybrid nanofillers for promoting direct methanol fuel cell performance

A special type of hybrid nano-particles was incorporated into the Nafion^® matrix to form a composite membrane. These nano-particles possessed a core–shell structure consisting of silica core (<10 nm) and a densely grafted oligomeric ionmer layer, which was synthesized via atom transfer radical polymerization (ATRP) on the particles’ surface. Besides considerable improvement in the proton conductivity of the membrane, the presence of these hybrid nano-particles in the Nafion^® matrix also repressed its methanol permeability by almost four times. The composite membrane also demonstrated superior performance when tested in a single cell membrane-electrolyte assembly (MEA) under direct methanol fuel cell (DMFC) operating condition. It was found that the composite membrane enabled a power density output that was 1.5 times greater than that of pristine Nafion^®.     

Permeability of the Perindopril Arginine under In Vitro Conditions across Caco-2 Monolayer and Biomimetic Phospholipid Membrane

Perindopril arginine (PA) as an angiotensin-converting enzyme (ACE) inhibitor is widely used in cardiovascular diseases, especially in systemic hypertension and heart failure. Although the pharmacokinetics of PA are well documented, there is no available detailed data on its permeation in in vitro conditions. The present study aimed to assess the transport of PA across both biological membranes and artificial biomimetic ones. For the determination of PA transport, the Caco-2 cell line was selected as a reliable in vitro model of gastrointestinal biological barriers. Additionally, a novel 96-well plate with phospholipid membrane PermeaPad was used to evaluate the transport of PA by passive diffusion. We confirmed that PA is relatively poorly permeable across the Caco-2 monolayer. The permeability results obtained from the non-cell-based model demonstrated higher transport of PA as compared to that of Caco-2. Thus, PA transport across the biological membranes might be suggested to be regulated by the membrane transporters.     

Zirconium meta-sulfonphenyl phosphonic acid-incorporated Nafion membranes for reduction of methanol permeability

Zirconium meta-sulfonphenyl phosphonic acid (Zr-msPPA)/Nafion^® composite membranes were prepared to reduce methanol permeability of the Nafion^® 117 membrane in direct methanol fuel cell (DMFC) applications. Zr-msPPA crystalline nano proton conductors were synthesized inside the membranes via the reaction of zirconium chloride octahydrate and meta-sulfonphenyl phosphonic acid that had been soaked prior. Synthesis of the Zr-msPPA in the membranes was identified from a series of chemical and physical structure characterizations using FTIR, NMR, EDS, and XRD spectroscopy. The thermal stability of the composite membranes was enhanced by addition of the Zr-msPPA, with considerable reduction in methanol permeability with increasing Zr-msPPA content, as the Zr-msPPA nano conductors acted as crystalline barriers to methanol permeation. The ion conductivity also decreased with increasing Zr-msPPA content, but its effect was not as strong as with methanol permeation given the innate, high conductivity of Zr-msPPA.     

Water and methanol permeation through short-side-chain perfluorosulphonic acid ionomeric membranes

The water and methanol transport into a short-side-chain perfluorosulphonic acid ionomeric (PFSI) membrane suitable for application in proton exchange membrane fuel cells (PEMFC), namely Hyflon^® Ion, was studied between 35 and 65 °C. In particular, the permeabilities of pure water, pure methanol and their mixtures at different temperatures were measured through pervaporation experiments, at various values of feed composition. Due to the presence of mutual interactions between permeants as well as among penetrants and polymeric matrix, the composition of the feed solution affects the membrane permeability in a way which cannot be predicted on the basis of permeability data of the pure liquid components alone. It has been found in particular that the presence of the water in the mixture enhances the methanol permeability, due to the positive effects of matrix plasticization and favourable energetic interactions. In turn, by considering water permeability data in the presence of a poorly permeating component such as glycol, it can be concluded that also water permeation is enhanced by the presence of methanol, although to a lower extent.     

Proton and methanol transport behavior of SPEEK/TPA/MCM-41 composite membranes for fuel cell application

This paper reports proton and methanol transport behavior of composite membranes prepared for use in the direct methanol fuel cell (DMFC). The composite membranes were prepared by embedding various proportions (10–30 wt.%) of inorganic proton conducting material (tungstophosphoric acid (TPA)/MCM-41) into sulfonated poly(ether ether ketone) (SPEEK) polymer matrix. The results indicate that the proton conductivity of the membranes increases with increasing loading of solid proton conducting material. The highest conductivity value of 2.75 mS/cm was obtained for the SPEEK composite membrane containing 30 wt.% solid proton conducting material (50 wt.% TPA in MCM-41). The methanol permeability and crossover flux were also found to increase with increasing loading of the solid proton conducting material. Lowest permeability value of 5.7 × 10⁻⁹ cm² s⁻¹ was obtained for composite membrane with 10 wt.% of the solid proton conducting material (40 wt.% TPA in MCM-41). However, all the composite membranes showed higher selectivity (ratio between the proton conductivity and the methanol permeability) compared to the pure SPEEK membrane. In addition, the membranes are thermally stable up to 160 °C. Thus, these membranes have potential to be considered for use in direct methanol fuel cell.     

The effect of polymer swelling and resistance to flow on solvent diffusion and permeability

The main purpose of this paper is to test the model of molecular sorption [Vesely D. Polymer 2001;42:4417-22] for Case II type diffusion by measuring the effect of sorption/swelling and resistance to flow through the swollen region on the mass transport of solvents in glassy amorphous polymer. The system of methanol and polymethylmethacrylate (PMMA) has been selected for easy comparison with the existing literature data.The weight loss of penetrant permeating through the polymer has been monitored using a permeability cell placed on a balance (gravimetry). The rate of diffusion and swelling has been measured using light microscopy on samples cut after different elapsed time exposure to the solvent.The contribution of polymer swelling and resistance to flow has been evaluated by comparing the mass transport during diffusion and permeation processes. It is shown that for thin films the thickness independent component of the mass transport process (swelling) makes a significant contribution to the diffusion rate. For thicker samples the thickness dependent component (the resistance to flow through the swollen polymer) dominates both, diffusion and permeation.     

Composite sulfonated cation-exchange membranes modified with polyaniline and applied to salt solution concentration by electrodialysis

A method is developed for obtaining anisotropic composites based on the sulfonated cation-exchange MF-4SK and MK-40 membranes and the electroactive polymer polyaniline (PANI). The kinetics of aniline polymerization by successive diffusion in these membranes is investigated, and differences in the transport characteristics of the resulting MF-4SK/PANI and MK-40/PANI composites are identified. It is established from results of electroosmotic and diffusion experiments that the composite MF-4SK/PANI-1 membrane (after 1 h of aniline polymerization) suppresses electrolyte and water flow the most. Diffusion permeability drops by an order of magnitude, and water transport numbers are reduced by 50–70%. In the process of sodium chloride concentration by electrodialysis, the salt content of the concentrate increases by 50–70% with the composite MF-4SK/PANI-1 membrane compared to the base MF-4SK membrane and by 15–20% compared to the electrodialysis MK-40 membrane. Transport characteristics of the membrane pairs under investigation are calculated from the model of limiting concentration by electrodialysis: current efficiency, water transport numbers, osmotic and diffusion permeability. The dominant influence of the electroosmotic mechanism of water transport on the effect of salt solution concentration is established.     

Proton-conducting polymer membranes for direct methanol fuel cells

Three methods to block the methanol transport through proton-conducting polymer membranes while maintaining the proton conductivity unchanged have been conducted; 1) selective layer formation on the surface of the membrane, 2) prearation of nanoclay composite membrane providing tortuous pathway of methanol, 3) control and fixation of the proton transport channels. The methanol permeability through the membranes decreased significantly at the expense of the small decrease in the proton conductivity. It is thus concluded that both the structure and the fixation of the proton transport channels are crucial in optimizinging proton conducting membranes for direct methanol fuel cells.     

Enhanced response of microbial fuel cell using sulfonated poly ether ether ketone membrane as a biochemical oxygen demand sensor

The present study is focused on the development of single chamber microbial fuel cell (SCMFC) using sulfonated poly ether ether ketone (SPEEK) membrane to determine the biochemical oxygen demand (BOD) matter present in artificial wastewater (AW). The biosensor produces a good linear relationship with the BOD concentration up to 650 ppm when using artificial wastewater. This sensing range was 62.5% higher than that of Nafion^®. The most serious problem in using MFC as a BOD sensor is the oxygen diffusion into the anode compartment, which consumes electrons in the anode compartment, thereby reducing the coulomb yield and reducing the electrical signal from the MFC. SPEEK exhibited one order lesser oxygen permeability than Nafion^®, resulting in low internal resistance and substrate loss, thus improving the sensing range of BOD. The system was further improved by making a double membrane electrode assembly (MEA) with an increased electrode surface area which provide high surface area for electrically active bacteria.     

Proton exchange membrane using partially sulfonated polystyrene-b-poly(dimethylsiloxane) for direct methanol fuel cell

A diblock copolymer ionomer containing a rubbery poly(dimethylsiloxane) block has been developed as a proton exchange membrane for direct methanol fuel cell (DMFC). The partially sulfonated polystyrene-b-poly(dimethylsiloxane) (sPS-b-PDMS) membrane with 38% sulfonation degree exhibited 3 times lower methanol permeability and 2.6 times higher membrane selectivity (proton conductivity/methanol permeability) compared to Nafion^® 115 at 25 °C. Coexistence of microphase domains and ionic clusters was confirmed from the morphological studies by small-angle X-ray scattering and tapping-mode atomic force microscopy. Gas chromatographic analysis revealed that water/methanol selectivity of sPS-b-PDMS was 20 times higher than that of Nafion^® 115. Such a high water/methanol selectivity can be attributed to the existence of PDMS microdomains minimizing methanol permeation through hydrophilic ion channels. sPS-b-PDMS membranes were fabricated into membrane electrode assembly (MEA), and air-breathing DMFC test for these MEAs showed a better performance compared to the MEA composed of Nafion^® 115.     

Self-assembled polyelectrolyte multilayered films on Nafion with lowered methanol cross-over for DMFC applications

The paper is concerned with the deposition of self-assembled polyelectrolyte multilayer on Nafion membrane by layer-by-layer (LbL) technique with lowered methanol cross-over for direct methanol fuel cell (DMFC) applications. The formation of self-assembled multilayered film on Nafion was characterized by UV–vis spectroscopy and it was found that the polyelectrolyte layers growth on the Nafion surface regularly. Furthermore, the proton conductivity and methanol cross-over measurements were carried out for characterization of the LbL self-assembled composite membranes. The results showed that the concentration and pH of the polyelectrolytes significantly affect the proton conductivity and methanol barrier properties of the composite membranes. 10⁻¹ monomol polyelectrolyte concentration and pH 1.8 was found to be optimum deposition conditions considering proton conductivity and methanol permeation properties of the LbL self-assembled composite membranes. The methanol permeability of the 10 bi-layers of PAH1.8/PSS1.8 deposited LbL self-assembly composite membrane was significantly suppressed and found to be 4.41 × 10⁻⁷ cm²/s while the proton conductivity value is in acceptable range for fuel cell applications.     

Sulfonated poly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells

This paper reports on the preparation and characterization of sulfonated poly(ether ether ketone) (sPEEK)-based mixed matrix membranes. The inorganic matrix consisted of silica: Aerosil^®380, tetraethoxysilane (TEOS) or a combination of both to obtain an interconnected silica network. The behavior of these membranes in ethanol–water systems was studied for application in a direct ethanol fuel cell (DEFC). Uptake measurements showed that the converted TEOS content had a strong influence on the hydrophilicity of the membranes. Proton conductivity was strongly related to the water content in the membrane, but the proton diffusion coefficients of membranes with various Aerosil^®380–TEOS combinations were similar. Dynamic measurements in liquid–liquid (L–L) and liquid–gas (L–G) systems were performed to study the ethanol transport through the membrane. No reduction in ethanol permeability was obtained in the L–L system, but a remarkable reduction was obtained in the L–G system when 2 M ethanol was applied. The reinforcing characteristic of the combined Aerosil^®380–TEOS-system were best observed at 40 °C with 4 M ethanol. The fuel cell performance prediction based on the selectivity of proton diffusion coefficient to ethanol permeability coefficient showed for nearly all composite membranes an improvement with respect to the polymeric reference. The presence of an inorganic phase led to relatively constant proton diffusion coefficients and lower ethanol permeability coefficients in comparison with the polymeric reference.     

ESI-MS study of the mechanism of glycyl-l-histidyl-l-lysine-Cu(II) complex transport through model membrane of stratum corneum

In mammalian organisms copper can be found mainly in the form of complex with specific tripeptide, GHK-Cu (glycyl-l-histidyl-l-lysine-Cu(II)). GHK-Cu is the basic form in which copper is transported in tissues and permeates through cell membranes. The penetration ability of GHK-Cu through the stratum corneum and its role in copper ions transport process is the key issue for its cosmetic and pharmaceutical activity. The permeability phenomenon was studied by use in vitro model system—Flynn diffusion cell with the liposome membrane.The earlier studies on the influence of different ligands on the migration rate of copper ions through model membrane provide evidence for hampering role of ligands structure and pH of formulations in this process.Structures of copper complexes formed in solutions of different pH media were evaluated by use of ESI-MS. The permeability coefficients of copper complexes increase with increasing pH. It was proved that only tripeptide GHK and its complexes with copper: GHK-Cu and (GHK)₂-Cu are able to migrate through membrane model of stratum corneum.     

An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes

Methanol diffusion in two polymer electrolyte membranes, Nafion 117 and BPSH 40 (a 40% disulfonated wholly aromatic polyarylene ether sulfone), was measured using a modified pulsed field gradient NMR method. This method allowed for the diffusion coefficient of methanol within the membrane to be determined while immersed in a methanol solution of known concentration. A second set of gradient pulses suppressed the signal from the solvent in solution, thus allowing the methanol within the membrane to be monitored unambiguously. Over a methanol concentration range of 0.5–8 M, methanol diffusion coefficients in Nafion 117 were found to increase from 2.9 × 10⁻⁶ to 4.0 × 10⁻⁶ cm² s⁻¹. For BPSH 40, the diffusion coefficient dropped significantly over the same concentration range, from 7.7 × 10⁻⁶ to 2.5 × 10⁻⁶cm² s⁻¹. The difference in diffusion behavior is largely related to the amount of solvent sorbed by the membranes. Increasing the methanol concentration results in an increase in solvent uptake for Nafion 117, while BPSH 40 actually excludes the solvent at higher concentrations. In contrast, diffusion of methanol measured via permeability measurements (assuming a partition coefficient of 1) was lower (1.3 × 10⁻⁶ and 6.4 × 10⁻⁷ cm² s⁻¹ for Nafion 117 and BPSH 40 respectively) and showed no concentration dependence. The differences observed between the two techniques are related to the length scale over which diffusion is monitored and the partition coefficient, or solubility, of methanol in the membranes as a function of concentration. For the permeability measurements, this length is equal to the thickness of the membrane (178 and 132 μm for Nafion 117 and BPSH 40 respectively) whereas the NMR method observes diffusion over a length of approximately 4–8 μm. Regardless of the measurement technique, BPSH 40 is a greater barrier to methanol permeability at high methanol concentrations.     

Liquid-supported membranes in chromium(VI) optical sensing: transport modelling

Flat sheet liquid-supported membranes (FSLSM) containing Aliquat 336 as a carrier have been evaluated as sample interface in an optical sensor for Cr(VI) monitoring. A model describing the transport mechanism of Cr(VI) through the membrane is reported. The model considers a diffusion process through a feed aqueous diffusion layer, a fast interfacial chemical reaction and a diffusion of ALQHCrO₄ and (ALQ)₂CrO₄ species through the membrane (Aliquat 336, ALQ). The mathematical equations describing the transport rate are derived and they correlate the membrane permeability coefficient to diffusional and equilibrium parameters as well as to the chemical composition of the system, i.e. extractant concentration in the membrane phase and acidity in the feed phase. The experimental data are explained by the derived equations and the diffusion resistances to mass transfer are evaluated. The influence of other experimental parameters, such as stirring speed in the feed phase and nature of the diluent and stripping agent on the transport is also discussed. Experiments with optical detection demonstrate the suitability of liquid-supported membranes (LSM) containing ALQ as interfaces for optical sensing.     

Physicochemical characterization of ion-exchange membranes in water-methanol mixtures

A complete physicochemical characterization of two ion-exchange membranes—CM2 and Nafion^®117—used in electrodialysis and in direct methanol fuel cells (DMFC) has been carried out. For each membrane, in different methanol-water mixtures—0%, 20%, 40%, 60%, 80% and 100%—and at different temperatures (25.0; 40.0 et 55.0 °C), we have measured the variations of the geometrical dimensions, the proton electrical conductivity, the swelling rate and the amount of methanol in the membrane. The FTIR analysis of Nafion^®117 was performed at different methanol contents of the external solution.The results show that the CM2 membrane presents the best geometrical stability, and the lowest conductivity at any methanol content. At high methanol contents, Nafion^®117 is 10 times more conductive than the CM2 membrane. It was found that the methanol is absorbed more by Nafion^®117, and its effect is more noticeable on the microstructure of this membrane, under standard conditions. The high methanol permeability of these membranes, particularly of the Nafion^®117, induces bad cell efficiencies and lifetimes.     

Effect of sorbed methanol, current, and temperature on multicomponent transport in nafion-based direct methanol fuel cells

The CO2 in the cathode exhaust of a liquid feed direct methanol fuel cell (DMFC) has two sources: methanol diffuses through the membrane electrode assembly (MEA) to the cathode where it is catalytically oxidized to CO2; additionally, a portion of the CO2 produced at the anode diffuses through the MEA to the cathode. The potential-dependent CO2 exhaust from the cathode was monitored by online electrochemical mass spectrometry (ECMS) with air and with H2 at the cathode. The precise determination of the crossover rates of methanol and CO2, enabled by the subtractive normalization of the methanol/air to the methanol/H2 ECMS data, shows that methanol decreases the membrane viscosity and thus increases the diffusion coefficients of sorbed membrane components. The crossover of CO2 initially increases linearly with the Faradaic oxidation of methanol, reaches a temperature-dependent maximum, and then decreases. The membrane viscosity progressively increases as methanol is electrochemically depleted from the anode/electrolyte interface. The crossover maximum occurs when the current dependence of the diffusion coefficients and membrane CO2 solubility dominate over the Faradaic production of CO2. The plasticizing effect of methanol is corroborated by measurements of the rotational diffusion of TEMPONE (2,2,6,6-tetramethyl-4-piperidone N-oxide) spin probe by electron spin resonance spectroscopy. A linear inverse relationship between the methanol crossover rate and current density confirms the absence of methanol electro-osmotic drag at concentrations relevant to operating DMFCs. The purely diffusive transport of methanol is explained in terms of current proton solvation and methanol-water incomplete mixing theories.     

Oxygen/nitrogen separation by polycarbonate/Co(SalPr) complex membranes

An oxygen carrier, cobalt di-(salicylal)-3,3′-diimino-di-n-propylamine (Co(SalPr)), was added into a polycarbonate membrane for improving its oxygen/nitrogen selectivity. Both the oxygen permeability and oxygen/nitrogen selectivity increased when only 3 wt% of Co(SalPr) was added. The permeability kept increasing but the selectivity decreased when more than 3 wt% of Co(SalPr) was added. The oxygen to nitrogen solubility ratio decreased when 3 wt% of Co(SalPr) was added. Further increase in Co(SalPr) content led to an increase in oxygen/nitrogen solubility ratio. It was astonishing to know that the effect of Co(SalPr) content on the oxygen/nitrogen solubility ratio was totally opposite to that on the oxygen/nitrogen selectivity. A membrane gas transport model which combines the dual mobility model with pore model was adopted to explain the above phenomenon. The specific volume measurement implied that the pore diffusion was responsible for this behavior. The contribution of sorption-diffusion type transport was also investigated by examining the transport behavior of the 3 wt% Co(SalPr) containing membrane through which the pore diffusion is relatively low. The effect of upstream pressure on the oxygen permeability and solubility implied that the diffusivity of Henry's mode was much higher than that of Langmuir's mode. It was also found that the effects of upstream pressure and operating temperature on the oxygen/nitrogen selectivity were both in accordance with those on the Henry's mode solubility ratio. The above information suggested that in addition to the pore diffusion the ratio of Henry's mode diffusion dominated the O₂/N₂ separation instead of the overall O₂ to N₂ solubility ratio.     

Proton conductive thin films prepared by plasma polymerization

Proton conductive membranes were prepared as thin films of about 10 μm thickness by an ion beam assisted plasma polymerization process. Argon ions were generated in a high frequency plasma and accelerated towards a PTFE target where CF fragments were released as a consequence of the ion impact. Various sulfur components (SO₂, CF₃SO₃H or ClSO₃H) were added to achieve proton conductivity by the formation of sulfonic acid groups. The CF fragments combined with the sulfur components to form a coherent thin film on a substrate. Mass spectrometric investigations revealed, however, that sulfur oxygen compounds were extremely delicate towards reduction to sulfur carbon compounds like CS₂ or SCF₂. The best membrane conductivities (>10⁻⁴ S/cm) and highest ion exchange capacities (0.15 mmol/g) were achieved with chlorosulfonic acid involved in the plasma polymerization process. Ultra-thin layers of these of these plasma polymers (ca. 300 nm) were subsequently deposited onto Nafion^® membranes in order to suppress methanol permeation for a potential application in a direct methanol fuel cell (DMFC). The ratio of proton conductivity and methanol diffusion coefficient was employed for an assessment of the transport characteristics of the coated membrane. Diffusion coefficients were determined in a flow cell coupled to a mass spectrometer. The plasma polymer coating decreased both the methanol permeation and the proton conductivity. With a proton conductive plasma polymer coating the decrease of methanol diffusion could outweigh the loss of proton conductivity. Plasma coating offers a way to suppress methanol crossover in DMFCs and to maintaining the proton conductivity.