Measurement of membrane conductivities using an open-ended coaxial probe

doi:10.1016/0022-0728(95)04072-V

Journal of Electroanalytical Chemistry

Volume 395, Issues 1–2, 10 October 1995, Pages 67-73

https://doi.org/10.1016/0022-0728(95)04072-V Get rights and content

Abstract

In this article, we describe the use of an open-ended coaxial probe to measure the conductivity of an ionic membrane. The technique is simple and it allows measurements to be made quickly. Control of physical parameters, such as the water content of the membrane, is easily achieved as access is only required to one side of the membrane. The technique also has the advantage that very small probes can be constructed allowing the measurement of the conductivity of local areas of the membrane.

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The high ionic conductivity was attributed to non-uniform electric fields appearing at the edges of the electrodes. Researchers that have used open-end coaxial probes to measure local ionic conductivities of Nafion membranes have also encountered non-uniform electric field effects, which appeared when the membrane thickness was larger than the separation between the inner and outer conductors in the coaxial line [65,66]. Since the TP measurements use a parallel plate electrode geometry, inhomogeneous electric field lines at the edge of the electrodes create a fringing electric field.
Accurately measuring the ionic conductivity of membranes is important both for assessing the performance of new membranes as well as for advancing the fundamental understanding of electric field-driven ion transport in membranes. Yet, despite a considerable number of reports on the topic, there is no standardized method for ionic conductivity measurements. One of the main challenges with ionic conductivity measurements is isolating the membrane resistance from the other resistances in the electrochemical cell. In this study, we present a detailed discussion of factors that affect ionic conductivity measurements with the commonly used direct contact method, in which the membrane contacts the electrodes. Such factors include external resistances due to imperfect contact between the membrane and electrodes, residual impedances due to the sample holder and wiring, and fringe effects. We also present and validate simple methods for isolating the true membrane ionic conductivity in both the through-plane and in-plane orientations. These methods can be implemented with virtually any membrane to obtain accurate and reproducible ionic conductivity values. Finally, we outline recommendations for performing ionic conductivity measurements with the direct contact method and a set of parameters to verify when reporting these results.
A review of membranes in proton exchange membrane fuel cells: Transport phenomena, performance and durability
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Citation Excerpt :
However, AC Impedance measurement is time-consuming and expensive. Gardner et al. [107] first applied the open-ended coaxial probe measurement technology in electrochemical research in 1995 to experimentally determine the ionic conductivity of membranes. This technique is simple, fast, and only needs to enter one side of the membrane.
Membrane is one of the most important components in proton exchange membrane fuel cells (PEMFCs), which determines the transport phenomena, performance, and durability. With the rapid development of novel membranes, many transport coefficients in membranes applied in numerical studies are outdated due to the lack of experimental data for new membranes. In this review, the fundamentals of commercially available membranes are scrutinized, followed by the fundamental working mechanisms. A detailed examination of the transport phenomena within the membranes, including transport mechanisms, mathematical description, and experimental methods, is conducted for protonic conduction, electro-osmosis drag, diffusion, hydraulic permeation, and gas crossover, which are urgently needed for theoretical and numerical studies. It is found that various empirical or analytical correlations have been established to predict the transport coefficients of the membranes. However, empirical models may not be accurate for all types of membranes since there is no sufficient experimental data for a solid correlation and validation. The experimental methods reviewed in the present study can be applied for new membranes, which is essential to quantify the transport phenomena and its further impact on cell performance and durability. The key transport-phenomena-related factors that affect the performance and failure modes of membranes are also reviewed in this study, which helps to develop strategies in improving membranes’ performance and durability during operation. This review deepens the understanding of the short-term and long-term performance of the membrane in PEMFCs and provides important insights into the further design of novel membranes.
Benefits of platinum deposited in the polymer membrane subsurface on the operational flexibility of hydrogen fuel cells
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The effects of electroless Pt layers (<80 μg_Pt cm⁻²) in the membrane subsurface on hydrogen crossover, ORR kinetics, and overall fuel cell performance are closely investigated. The electroless Pt layer in the membrane subsurface reduces gas crossover for a 17 μm thick membrane in an inversely proportional relationship to the loading, yielding up to >65% reduction. High electroless Pt layer loading reduces proton conductivity and results in less optimal ORR kinetic performance. Given the relationship between kinetic performance, exchange current density, and proton concentration, the impact of electroless Pt layer loading on proton concentration is determined. This correlation sheds light on the relationship between proton concentration, proton conductivity, and Pt loading, and can be useful as a probe for proton concentration diagnostic examinations. Hydrogen fuel cell tests reveal that with an additional membrane Pt loading of <20 μg_Pt cm⁻², the performance of GDE-based MEAs at low humidity are improved in both polarization and long-term humidity cycling tests, while the performance under wet conditions is still nearly identical to the baseline. This suggests that the addition of electroless Pt in the membrane subsurface at such loadings enhances the operational flexibility of hydrogen fuel cells and reduces hydrogen crossover effects.
Modeling cell pair resistance and spacer shadow factors in electro-separation processes
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The electrical resistance of the stack is one of the most important parameters that affects the performance of reverse electrodialysis (RED¹) and other electro-separation processes. The stack is made up of many cell pairs, each cell pair consisting of alternating anion and cation exchange membranes separated by spacer-filled channels which allow fluid to flow across the membrane surfaces, provide mechanical support and maintain intermembrane distance. The presence of non-conducting mesh in the spacer-filled channels increases the electrical resistance of the cell pairs; this so-called shadow effect can be characterized by the shadow factor (β) of the mesh. The shadow factor has not been determined experimentally so far. Furthermore, the extent of the shadow effect on the flow channels and membranes has not been investigated in much detail and the proposed analytical models in the literature are based only on assumptions. In this paper, we used a non-contact resistance (NCR²) method to measure the resistances of mesh filled spacer channels, membranes and sandwiched unit cells separately in high (1 M) and low (0.1 M) salinity NaCl solutions. The shadow factors of four different spacer mesh samples were determined experimentally. The existing methods of modeling the cell pair resistance were evaluated and the most suitable functional form for the resistances in series (RIS³) model was determined. While the mesh shadow factors were found to be independent of solution concentration, the membrane resistances were found to decrease with an increase in solution concentration. The mesh shadow factor was found to have little influence on the active area of the membranes. The NCR method could expedite the process of down selecting appropriate low resistance spacer meshes and membranes for the RED process. The validated RIS model can be used in techno-economic models for designing electro-separation processes for commercial scale plants.
Ion conduction and chemical and physical properties of sulfonated polyetherimide as membrane: Theoretical study
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Citation Excerpt :
The values of the Nafion-115 and Nafion-117 were obtained by Sone et al. [42] and by Arbaugh et al. [43], respectively. We observe that the values of σ determined in this work are about an order of magnitude (n = 5), two order of magnitude (n = 3) and three order of magnitude (n = 2, 4) lower than those reported by some authors for Nafion [11–13,44–46]. These results indicate that the position of the sulfonic acid groups in the repeat unit, and the degree of hydration of the membrane largely determine the ionic conductivity; i.e our results show that 5-sPEI membrane (0.021 S cm−1) has better conductivity than that 2-sPEI membrane (0.00016 S cm−1), 3-sPEI membrane (0.0067 S cm−1), and 4-sPEI membrane (0.00029 S cm−1).
Molecular dynamics and mechanical simulations were performed to investigate the mechanism of ionic conductivity in sulfonated polyether imides membranes with various sulfonyl groups (n-sPEI), where n is the number of sulfonyl groups in the polymer backbone. A comparative theoretical study was performed to explore the effect of sulfonating degree (n = 2, 3, 4 and 5) on the proton exchange in sulfonated membranes of polyether imides at the hydrated state. Geometry optimization of the membranes in the presence of 200 water molecules revealed that the membrane with n = 5 has a higher ionic conductivity than other cases. We also calculate some chemical and physical properties of each n-sPEI polymer using advanced Quantitative Structure-Property Relationships (QSPRs) which rapidly screened candidate polymers for a broad range of properties, and allowed the property prediction of copolymer blends. All calculations using the QSPRs method were performed with dry molecular structures. The drive mechanism was determined and analyzed, finding that the sulfonyl groups position in the polymer backbone greatly influences in the value of ionic conductivity. The membrane model was built using polymeric chains oriented in three dimensions. The ionic species used in the simulations were hydronium and hydroxyl. The theoretical results are compared with experimental results reported in the literature. The study aimed at finding the polymer that presented the best conduction mechanism to enhance the efficiency of Proton Exchange Membrane Fuel Cell (PEMFC). The ionic conductivity parameter does not necessarily have a linear relationship with the number of sulfonated groups.
Influence of temperature on the electrokinetic properties and power generation efficiency of Nafion<sup>®</sup> 117 membranes
2014, Journal of Power Sources
In the present study we investigate the transport properties of Nafion^® 117 membranes in temperatures ranging from ambient temperature up to 70 °C. The hydraulic permeability, streaming potential and ion conductivity have been measured as function of temperature in 0.03 M LiCl solutions in purposely designed, non-conductive set-ups. In particular, the apparent activation energies of the processes have been retrieved: 29.4 kJ mol⁻¹, 9.3 kJ mol⁻¹ and 22.9 kJ mol⁻¹ for the hydraulic permeability, streaming potential coefficient and ion conductivity respectively. Based on the knowledge of the temperature dependence of these three independent properties the figure-of-merit of the electrokinetic energy conversion process has been calculated obtaining a monotonous increase of the efficiency with temperature. At 70 °C the electrokinetic efficiency is rather high about 26.6%:50% higher with respect to the one found at room temperature. The electrokinetic transport properties were also used to esteem the average pore size of the water channels in the polymer matrix resulting in pore diameters ranging approximately from 2.0 (25 °C) to 2.8 nm (70 °C).

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Regular paperMeasurement of membrane conductivities using an open-ended coaxial probe

Abstract

J. Electroanal. Chem.

Solid State Ionics

Canadian solid polymer fuel cell development

J. Electrochem. Soc.

Chem. Ing. Tech.

J. Phys. Chem.

J. Electrochem. Soc.

J. Electrochem. Soc.

IEEE Trans. Instrumentation Measurement

Regular paper
Measurement of membrane conductivities using an open-ended coaxial probe