Membrane Resistance Measurements Research Articles

RAMMS - A High-Throughput Membrane Measurement System for Fuel Cell Manufacturing

The objective of this work was to develop a high-throughput polymer electrolyte membrane (PEM) resistance measurement device to enhance manufacturing quality control (QC). The principal outcome was the Rapid Membrane Measurement System (RAMMS), an automated, off-line instrument that can be used by membrane manufacturers for QC during production and by membrane users for lost acceptance of a critical component of PEM fuel cell systems. Principal components of the RAMMS were a novel humidification and environmental control system; integrated sample chamber, electrode and actuator system; impedance-based membrane resistance measurement instrumentation, and custom software. Testing and evaluation revealed that electrode durability was the most significant technical challenge and performance limitation of the RAMMS. Acceptable durability was unobtainable with any of the platinum deposition methods and post-deposition treatments designed to improve coating adhesion. Porous platinum electrodes exhibited acceptable durability. The prototype unit came close to achieving all performance criteria and specifications.

Preparation and characterization of positively charged polysulfone nanofiltration membranes

Abstract Charged nanofiltration (NF) membranes were prepared from aminated polysulfone (APSF) via a phase inversion technique, by varying the polymer and solvent weight ratios. A two-step phase inversion technique, adopted for the preparation of NF membranes, involved chloromethylation, and subsequent amination of the polymer, polysulfone (PSF). Fourier-transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopic analyses of the prepared chloromethylated PSF (CPSF) membrane confirmed the incorporation of chloromethylated groups on the polymer backbone. Also, the separation capability of the prepared membranes was evaluated by compaction studies, pure water fluxes (PWF) at different pressures, and hydraulic membrane resistance measurements. Furthermore, scanning electron microscopic (SEM) and atomic force microscopic (AFM) images revealed that the pore size and pore distribution decreased with increase in the polymer weight ratio. In addition, the functionality of the prepared membranes as a nano-filter was analyzed by constructing a stirred batch NF cell and evaluated towards a feed solution containing different concentrations of Cr (III) ions. The performance of the prepared membrane was compared with commercially available (NFT-50) NF membrane.

The electrical response of bilayers to the bee venom toxin melittin: Evidence for transient bilayer permeabilization

Melittin is a 26-residue bee venom peptide that folds into amphipathic α-helix and causes membrane permeabilization via a mechanism that is still disputed. While an equilibrium transmembrane pore model has been a central part of the mechanistic dialogue for decades, there is growing evidence that a transmembrane pore is not required for melittin's activity. In part, the controversy is due to limited experimental tools to probe the bilayer's response to melittin. Electrochemical impedance spectroscopy (EIS) is a technique that can reveal details of molecular mechanism of peptide activity, as it yields direct, real-time measurements of membrane resistance and capacitance of supported bilayers. In this work, EIS was used in conjunction with vesicle leakage studies to characterize the response of bilayers of different lipid compositions to melittin. Experiments were carried out at low peptide to lipid ratios between 1:5000 and 1:100. The results directly demonstrate that the response of the bilayer to melittin at these concentrations cannot be explained by an equilibrium transmembrane pore model.

Electrical Response of Bilayers to the Bee Venom Toxin Melittin

Melittin is a 26-residue amphipathic alpha-helix that causes membrane destabilization via a mechanism that is still disputed. While a transmembrane pore model has been a central part of the mechanistic dialogue for decades, there is growing evidence that a transmembrane pore is not required for melittin activity. In part, the controversy is due to limited experimental tools to probe the bilayer's response to melittin. Electrochemical impedance spectroscopy (EIS) is a technique that can reveal details of molecular mechanism of peptide activity, as it yields direct measurements of membrane resistance and capacitance of supported bilayers. In the work presented here, this technique was used to study the response of surface-supported bilayers of different lipid compositions to melittin. The EIS results directly demonstrate that the response of a bilayer to melittin cannot be explained by a simple transmembrane pore model.

Adapted Flow Field Structures for PEFC

This study investigates the effects of the channel and rib dimensions of the flow field on the performance of polymer electrolyte fuel cells (PEFCs). The development of a channel/rib geometry, where dimensions are adapted to local conditions along the reaction pathway, is described. The optimal channel/rib sizes at a given position along the channel is based on the results of local current density and membrane resistance measurements on the submillimeter scale. The performance of single cells shows that under certain operating conditions, a properly adapted channel/rib dimensioning can lead to an improvement of the cell performance compared to designs with constant channel and rib dimensions. An optimized adapted geometry has narrow channels in the inlet region to prevent membrane drying and narrow ribs toward the outlet to reduce transport losses in the porous gas diffusion layer at conditions when the reactants are saturated and depleted. The improvement of integral cell performance is highest with low cathodic stoichiometries.

Through-Plane Proton Transport Resistance of Membrane and Ohmic Resistance Distribution in Fuel Cells

Ohmic losses in fuel cells contain both ionic (mainly protonic) and electronic contributions. In this work, we developed a method to distinguish the resistance contributions from individual components. Proton transport resistance of proton exchange membranes (PEMs) in the through-plane direction, the major proton transport direction in in situ fuel cells, was measured using electrochemical impedance spectroscopy combined with compression-controlled fuel cell hardware. Membrane resistance was obtained by subtracting the nonmembrane contributions from the total resistance. Proton conductivities of PEMs with different equivalent weights, calculated from through-plane resistance measurements at various relative humidity conditions, were compared with good agreement to the in-plane measurements. Fuel cell electronic resistance and membrane-electrode interfacial resistance were also evaluated using both ex situ and in situ methods. The membrane-electrode interface resistance was nearly constant over a range of relative humidity conditions. The effects of test procedure and cell build strategy were investigated and had a significant impact on the membrane resistance measurements.

Effect of water transport properties on a PEM fuel cell operating with dry hydrogen

In this work, membrane resistance measurement and water balance experiment were implemented to investigate the feasibility for a PEM fuel cell operating with dry hydrogen. The results showed that when a thin membrane was used in a cell the performance and the membrane resistance changed a little while the anode humidity changed from saturated to dry. Comparing with the anode humidity, the influence of the cathode humidity was serious on the cell performance. The water balance experiments showed that the net water transport coefficient was negative even the anode was humidified and liquid water existed not only in the cathode but also in the anode. High cathode humidity was disadvantage for the removal of water both in the anode and the cathode.

Preparation of polyvinyl alcohol–silica hybrid heterogeneous anion-exchange membranes by sol–gel method and their characterization

Polyvinyl alcohol–silica hybrid heterogeneous anion-exchange membranes were prepared by sol–gel method by dispersing the anion-exchange resin in the gel formed by acid/base hydrolysis of tetraethylorthosilicate in polyvinyl alcohol solution. The effect of acid and base-catalyzed hydrolysis on membrane preparation and extent of resin loading was studied. Various membranes were prepared with 60% (w/w) anion-exchange resin (Indoin FFIP) loaded with different extents of silica. Membrane potential and membrane resistance measurements have been carried out with different counter-ion to investigate the relationship between ionic migration and silica content in the membrane forming material. Counter-ion transport number and permselectivity of these membranes were found to very due to the silica content and nature of counter-ion. Data indicates that, with increase in compactness of the membranes, its ability to discriminate between counter-ions also increases. Decrease in relative transport of bulkier ions are useful, especially, desirable for selective removal of these type of ions or membrane based electrochemical sensors.

Mathematical Model and Characterization of the Transient Behavior of a PEM Fuel Cell

Modeling of fuel cells is getting more and more important as powerful fuel cell stacks are getting available and have to be integrated into power systems. In this paper, the governing equations of the transient behavior of a proton exchange membrane fuel cell are presented. They show the influence of the operating conditions and the current density on internal parameters, especially the ohmic resistance. A method and specially designed electronic load are presented for in-situ membrane resistance measurements. This resistance is plotted in transient operation during parameter changes and current variations. The results are compared to the simulated values to explain the internal phenomena of the fuel cell. Special attention is given to the water management, one of the most important challenges in low temperature fuel cell operation where drying or flooding can decrease the performance or even lead to destruction.

Performance Evaluation of Platinum Dispersed Self-humidifying Polymer Electrolyte Membrane Prepared by Using RF Magnetron Sputter

The performance evaluation on Pt loading in the self-humidifying polymer electrolyte membrane for Polymer Electrolyte Mem Brane Fuel Cell(PEMFC) was investigated by using single cell test and measurement of membrane resistance. The self-humidifying membrane comprised two membranes made of perfluorosulfonylfluroride copolymer resin and fine Pt particles lying between them, coated by sputtering. From the results of performance characteristics of self-humidifying membrane cell with different Pt loading, a single cell using self-humidifying membrane with 0.15 mg/㎠ Pt loading showed better performance than that with the others over entire current density. Also, a single cell with 0.15 mg/㎠ Pt loading had a lower resistance value than the other cells under externally nonhumidifying condition. It is indicated that the water produced in the membrane cell with 0.15 mg/㎠ Pt loading showed a higher provision to maintain ionic conductivity of the membrane than the other cells. The optimum amount of Pt particles embedded in the membrane for self-humidifying PEMFC was determined to be about 0.15 mg/㎠.

Comparison of different cells for resistance determination of freely standing polymer membranes developed for direct methanol fuel cell (DMFC) applications

Cation conductive membranes, especially highly proton conductive membranes, are of interest not only for chlor-alkali electrolysis but for polymer electrolyte fuel cells as well. The very challenge for electrochemical characterization in this case is the low specific resistance of the polymer required for such applications, which in turn makes membrane resistance measurement a non-trivial problem. We investigate the different possibilities to characterize such membranes. For that purpose, a series of membranes made from sulfonated poly (phenylene oxide) (SPPO, degree of sulfonation (DS) from 7% up to 40%) was prepared. Commercially available membranes Nafion 117 as a reference are investigated, too. This contribution summarizes our new results concerning impedance spectroscopy measurements, using different measuring cells.

Two-dimensional finite-element method study of the resistance of membranes in polymer electrolyte fuel cells

A two-dimensional model of fluid flow, mass transport and electrochemistry is presented to examine the effect of current density and cell pressure on the resistance of Nafion 117 membranes in polymer electrolyte fuel cells. The finite element method is used to solve the continuity, potential and Stefan–Maxwell equations in the flow channel and gas diffusion electrode regions and the concentrated solution theory equations in the membrane region. Model calculations for concurrent flow indicate that current density and water flux through the membrane are non-uniform and reach a minimum at the bottom of the membrane near the gas exit. A comparison of model results to membrane resistance measurements in the literature is discussed. The multidimensional nature of transport within the fuel cell is described and plots of flow streamlines, gas mole fractions and membrane water content are shown. The boundary condition for water transport across the active catalyst layer is examined.

Impedance measurements of polymer electrolyte membrane fuel cells running on constant load

A new method to measure the impedance of fuel cells connected to a constant load is developed. This method is used to measure the impedance of a polymer electrolyte membrane fuel cell. It is shown that in situ membrane resistance measurements can be made by this new method.

The new EPR molecular oxygen probe fusinite is not toxic to cells

The possible cytotoxic effects of fusinite, a new charcoal-like electron paramagnetic resonance (EPR) oxygen probe, were evaluated in three cell types with very different characteristics and growth features: K562 (an erythroleukemic cell line which grows in suspension), A431 (an epidermal carcinoma cell line which grows in monolayer) and primary cultures of murine fibroblasts (which also grow in adhesion culture) utilizing morphological and functional studies as well as growth analyses. Scanning and transmission electron microscopy as well as fluorescence microscopy were used for the morphological analyses while conductometric relaxation studies in the radiowave frequency range, membrane resistance measurements and adenine nucleotide levels were utilized for the more subtle functional evaluation of cell parameters. The results show that the presence of fusinite particles, even after long internalization times, does not induce any cytotoxic effects in the cells studied. Thus, from these results, it can be deduced that fusinite is non-toxic as well as highly stable, inert and very sensitive to oxygen, and can be used with great success for cell studies where determination of oxygen concentration is important.

A modified Maxwell-Stefan model for transport through inert membranes: the binary friction model

This paper focuses mainly on the development of a model for permeation through inert membranes, as encountered in many cases in ultrafiltration and in gas permeation through inert porous plugs. The ultrafiltration model is made up of a boundary layer transport model and a porous membrane model in series, which are connected by an equilibrium relation. The boundary layer model is developed with the Vieth approximation for turbulent diffusivity. For the internal membrane transport, a modification of the Maxwell-Stefan-Lightfoot equation is derived (the binary friction model), which in a natural way includes both interspecies (diffusive) and species-wall forces. Application for the partial separation of PEG-3400 from aqueous solution shows that membrane friction coefficients can simply be estimated from membrane resistance measurements and mixture viscosity data. The only adjustable parameter to be determined is the distribution coefficient between the free solution and the membrane pores. The differences between the Lightfoot approach and the dusty gas model (DGM) are shown to stem from errors in the drivations of the latter, thus invalidating the dusty gas approach in the normal region in which viscous friction effects become important. For gases, the binary friction model is developed to include Knudsen and viscous wall friction terms as well as intermolecular diffusion. It is shown to give excellent coverage of the He-Ar diffusion data of Evans et al. ( J. Appl. Phys., 33 (1962) 2682; 34 (1963) 2020), with wall friction coefficients derived directly from Knudsen coefficients and gas viscosity data. The apparent success of the DGM in describing the same phenomena is shown to be caused by the relatively small importance of the wall friction forces at elevated pressures, and by the correct transition to Knudsen flow at low pressures. In addition, it is shown that diffusive slip phenomena in capillaries can be described well by the binary friction model.

In Situ Membrane Resistance Measurements in Polymer Electrolyte Fuel Cells by Fast Auxiliary Current Pulses

A solid‐state current pulse generator for in situ membrane resistance measurements by superimposed square current pulses in polymer electrolyte fuel cells was designed and built. The choice of the measuring technique and of parameters of the instrumentation was based on a critical analysis of the relevant electrochemical and physical processes. The inductance of the current pulse path is very low (≈5 nH), because the last stage of the generator is directly attached to the fuel cell. This low inductance permits the generation of 5 A pulses with extremely fast (decay time ≤5 ns) trailing edges (accompanied by a moderate ringing), which in turn makes it possible to measure the voltage transient induced by the current decay, with gigahertz resolution. The voltage transient is analyzed in a time window of 200 to 700 ns after the end of the pulse. By measurements in this time window, it is possible to separate accurately the ohmic series resistance of the cell (membrane resistance) from the other overpotentials at the electrochemical interfaces. Because the pulse current path is independent of the dc loop, the resistance can be measured independently of the dc value, i.e., at open circuit and under high current density conditions. The instrument was tested, and the results were analyzed for accuracy. Resistances down to 2 mΩ can be measured with an error of <5%. The influence of the pulse length and pulse amplitude on the cell voltage response was also investigated. For cell resistances in the order of few milliohms, a current pulse amplitude of 5 A is the minimum requirement for accurate measurements.

A novel method for rapid measurement of membrane resistance, capacitance, and access resistance

During patch clamp recordings, measurement of passive parameters such as access resistance (Ra), membrane resistance (Rm), and membrane capacitance (Cm) often provides useful information regarding physiological changes in the cell. In particular, an increase in capacitance may indicate vesicle fusion events as occurs during exocytosis. Rapid capacitance changes are usually measured with a phase-sensitive detector set to a phase angle that is independent of resistance changes. However, this angle changes over time and may be difficult to determine in cells with a low membrane resistance. The present paper describes a technique for rapidly measuring Ra, Rm, and Cm by simultaneously applying two harmonic frequencies and calculating the passive parameters from the resultant electrode current. Calibration and operation are independent of the compensation circuit settings that may be set to null most of the electrode current. The technique may be implemented on a standard patch clamp setup without other specialized equipment and should be particularly useful for the study of cells that have low Rm or undergo rapid changes in Ra or Rm.

Presynaptic enhancement of signal transients in photoreceptor terminals in the compound eye

Intracellular recordings show that photoreceptor axons in the first optic ganglion of the blowfly have characteristic electrical properties not seen in the soma. In the axon, a fast depolarizing transient (FDT) occurs on the rising phase of the photoresponse. The FDT is elicited by depolarizing current injections, can follow a sudden relaxation from hyperpolarization, has a distinct voltage threshold, and is accompanied by a rapid transient increase in membrane conductance. Blocking voltage-sensitive potassium conductances in photoreceptors with TEA unmasks spontaneous, long-lasting (ca. 50-150 ms) action potentials. These voltage-sensitive effects and measurements of membrane resistance suggest that the FDT is caused by a voltage-dependent increase in conductance, localized in the photoreceptor axon terminals. The transient amplifies the peak presynaptic response at least twofold, and appears to generate the fast, initial component of the transient postsynaptic response. Thus the FDT contributes to the synaptic amplification and boosting of high frequencies that enhance signals at the visual system's first synapse.

Influence of Operating Conditions on the Selectivity Factor of the Nickel/Sodium Electrodialysis Separation

The separation by electrodialysis is studied as a function of several operating conditions such as current density, Na/Ni concentration ratio, and temperature. The experimental separation factors are discussed, assuming a membrane‐controlled process, in terms of Hittorf transport numbers and electro‐osmotic coefficients for the electrically driven transport of both cations through the cationic membrane (Nafion 125 from du Pont) which is assumed to be the selective component in the cell stack. Activation energy for the separation is calculated and the obtained data are compared with the values derived from membrane resistance measurements.

Parotid acinar cells: ionic dependence of isoprenaline-evoked membrane potential changes.

The effect of microionophoretic application of isoprenaline on membrane potential and resistance of mouse parotid acinar cells was investigated. For measurements of membrane resistance and the isoprenaline equilibrium potential (Eiso), two microelectrodes were inserted into neighbouring communicating cells. Passing direct current through one of these electrodes, the resting potential could be set at desired levels and Eiso was determined by plotting the relation between the size of the isoprenaline-evoked potential change and the resting potential. Simple depolarizations were found at relatively high resting potentials, while biphasic potential changes in response to isoprenaline (hyperpolarization followed by depolarization) were observed at low resting potentials. Both depolarizing and hyperpolarizing responses to isoprenaline were accompanied by a reduction of membrane resistance. The isoprenaline equilibrium potential in the initial phase of the response was about -53 mV, but had a value of about -24mV in the delayed phase. The initial isoprenaline-evoked potential change was sensitive to alterations in extracellular Na, K and Cl concentrations. The delayed depolarizing response to isoprenaline was markedly reduced by replacing extracellular Na by Tris or extracellular Cl by SO4. These results indicate that isoprenaline opens up conductance pathways permeable to Na and K.