High Frequency Resistance Measurements Research Articles

Temperature Dependence of Water Crossover in Proton Exchange Membrane Water Electrolysis

The importance of hydrogen production by proton exchange membrane water electrolysis (PEMWE) in the energy sector makes it worthwhile to understand the system behaviour in failure cases and possible recovery procedures. In particular, insufficient feed water supply to the anode can be caused by inhomogeneous distribution in the stack, pump failure or erroneous operation strategy. For the PEMWE, a distinct turnover point has been reported if feed water supply is reduced in isothermal conditions.[1] Further reduction of the water supply results in higher ohmic resistance due to gradual membrane dehydration and increased mass transport losses.[2] Here, we will use the term “water starvation” to describe this event. The drainage of water from the anode can be explained by oxygen evolution reaction (OER), humidification of oxygen gas and water crossover to the cathode. The last one was measured for well hydrated membranes at atmospheric pressure [3] or below typical operating temperatures [4].In the first part of this work, we present the water crossover in a PEMWE on single cell level to understand their dependence on the operating parameters such as temperature and pressure. Media were supplied and conditioned to a 5 cm2 single cell by using an in-house test bench and a potentiostate with booster. The electrochemical measurements were performed with a commercially available catalyst coated perfluorosulfonic acid membranes (loading of 0.3 mg/cm2 geo Pt/C on cathode and 1 mg/ cm2 geo Ir-Oxide on anode). The anode was kept at atmospheric pressure, whereas the cathode pressure was varied from atmospheric to 400 kPaabs at various current densities and cell temperatures of 40 – 80 °C. To determine the water crossover under several operating conditions, the water vapour was condensed to liquid phase in a heat exchanger and measured by a gravimetric method.Furthermore, the measured water crossover is used to force a water starvation in the second part of this work. After cell assembly, a conditioning procedure was performed until steady cell performance was achieved. Cell performance was evaluated using polarization curve, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and water crossover. The last two are of particular interest, because they provide information about membrane humidification. The Tafel slope and mass transport overvoltage were evaluated from the polarization curves and high frequency resistance (HFR). Water starvation experiments were then carried out at a constant cell voltage by reducing the feed water supply, resulting in a dramatic decay of the current density. The analysis of Tafel slope and EIS data reveals an increase of ohmic and mass transport resistances. Furthermore, the HFR measurements were correlated with water crossover as a function of membrane hydration. We observed that the specific water crossover per current density decreases as a consequence of membrane dehydration. After this step, feed water supply was increased to an unrestricting level for membrane rehydration and performance recovery. For this recovery procedure, different operation modes were employed by varying the current densities. At constant current density, membrane hydration is influenced by the ionic current from the cathode to the anode via proton drag. EIS and water crossover measurements allow us to determine the influence of operation on the dynamics of the membrane rehydration process. Together with the polarization curves, cyclic voltammetry and HFR data, reversible and irreversible degradation processes caused by the water starvation can be identified.Altogether, we present a deeper understanding of water crossover for a wide operating parameter range and provide a guideline for recovery of the performance for PEMWE.[1] Christoph Immerz et al 2020 Meet. Abstr. MA2020-02 2456[2] C. Immerz, B. Bensmann, P. Trinke, M. Suermann, R. Hanke-Rauschenbach, J. Electrochem. Soc. 2018, 165, F1292-F1299.[3] K. Onda, T. Murakami, T. Hikosaka, M. Kobayashi, R. Notu, K. Ito, J. Electrochem. Soc. 2002, 149, A1069.[4] P. Medina, M. Santarelli, International Journal of Hydrogen Energy 2010, 35, 5173.

Control Method of the Dual-Winding Motor for Online High-Frequency Resistance Measurement in Fuel Cell Vehicle.

The dual-winding motor drive has recently been proposed in the field of fuel cell vehicles due to its performance and high robust advantages. Efforts for this new topology have been made by many researchers. However, the high-frequency resistance measurement of a proton exchange membrane fuel cell based on dual-winding motor drive architecture, which is important for water management to optimize the lifespan of fuel cells, was not employed in earlier works. In this paper, a new control method of the dual-winding motor is proposed by introducing a dc input current control to realize high-frequency resistance measurement and normal drive control simultaneously, without using extra dc-dc converter. On the basis of the revealed energy exchange principles among electrical ports and mechanical port of the dual-winding motor, the load ripple caused by high-frequency current perturbation is optimized based on the q-axis current distribution between two winding sets. The decoupling control algorithm for the coupling effect within and across windings is also discussed to improve the dynamic response during high-frequency resistance measurement. Finally, simulation results verify the effectiveness and improvement of the proposed method. Fast Fourier transform results indicated that the total harmonic distortion of the dc input current was reduced from 22.53% to 4.47% of the fundamental, and the torque ripple was suppressed from about ±4.5 Nm to ±0.5 Nm at the given operation points.

Prediction of high frequency resistance in polymer electrolyte membrane fuel cells using long short term memory based model

High-frequency resistance (HFR) is a critical quantity strongly related to a fuel cell system's performance. It is beneficial to estimate the fuel cell system's HFR from the measurable operating conditions without resorting to costly HFR measurement devices. In this study, we propose a data-driven approach for a real-time prediction of HFR. Specifically, we use a long short-term memory (LSTM) based machine learning model that takes into account both the current and past states of the fuel cell, as characterized through a set of sensors. These sensor signals form the input to the LSTM. The data is experimentally collected from a vehicle lab that operates a 100 kW automotive fuel cell stack running on an automotive-scale test station. Our current results indicate that our prediction model achieves high accuracy HFR predictions and outperforms other frequently used regression models. We also study the effect of the extracted features generated by our LSTM model. Our study finds that only very few dimensions of the extracted feature are influential in HFR prediction. The study highlights the potential to monitor HFR condition accurately and timely on a car.

Exploring Anion Exchange Membrane Fuel Cell (AEMFC) Performance with Different Classes of Cathode Electrocatalysts

AEMFCs as an alternative to acidic polymer electrolyte fuel cells have the potential to reduce cost and precious metal requirements of fuel cell systems.1 In the past decade, several highly conductive polymers have been developed and a number of these have demonstrated more than 1W∙cm-2 peak power density with hundreds of hours stability; however, most of these studies have focused on Pt containing cathodes with high loadings.2,3 While the promise of Pt and Pt group metal (PGM) free electrocatalysis has been a major selling point of AEMFCs, more work exploring the potential of PGM-free cathode electrocatalysis is needed. In this study we explore and compare the performance of three leading cathode electrocatalysts: Pt/C, Ag/C, and Fe-N-C. These electrocatalysts are tested in AEMFCs cathodes using both powder- and dispersion-based PF AEM Gen 2 ionomers.4 The ability to test the different catalyst and ionomer combinations leads to additional insight into performance and catalyst-ionomer interactions.The performance trends of the tested MEAs will be presented. Pt/C has been found to be slightly superior to the performance of the non-Pt electrocatalysts. The performance of both metal nanoparticle catalysts, Pt/C and Ag/C, exhibit strong differences in performance depending on whether or not a powder or dispersion-based ionomer is used, with the powder-based approach showing much higher performance. Unlike the metal electrocatalysts, Fe-N-C catalysts show almost no change of performance between powder and dispersion-based ionomers.These performance metrics are also reflected in high frequency resistance measurements (Figure 1) investigated as a function of relative humidity at 300 mA·cm- 2. This data clearly shows the expected trend of increasing HFR with decreasing humidification of reactant gasses. The Fe-N-C data shows the lowest HFR values and very small changes between powder and dispersion-based ionomers. Both, Pt/C and Ag/C show dramatic increases in HFR for the dispersion-based polymer compared to the powder-based ionomer. These large changes in HFR are unexpected and cannot be fully explained just by changes happening within the electrodes. They suggest that not only do the electrodes undergo variable electrochemical properties due to this processing, but water management within the cell is also impacted by the type of catalyst and ionomer used in processing the electrodes. These features and the impact they have on the performance of different AEMFC cathodes will be presented and discussed.

Fault Detection and Isolation for Proton Exchange Membrane Fuel Cell Using Impedance Measurements and Multiphysics Modeling

AbstractThis study proposes a fault detection and isolation tool for proton exchange membrane fuel cell (PEMFC) operating in embedded applications. A model‐based approach, taking partially into account degradation phenomena, is proposed in order to increase the robustness of the tool regarding transient operations and stack ageing. The considered faults are the abnormal operating conditions that can decrease the fuel cell lifetime. The fault detection approach is based on residual generation using both voltage and high frequency resistance measurements and thus combining the advantages of knowledge‐based model and electrochemical impedance spectroscopy (EIS) diagnosis approaches. To that end, a multi‐physics fuel cell model has been used. This model computes not only the stack voltage but also the high frequency resistance in dynamic conditions. Additionally, the model is modified to take into account the ageing of the fuel cell. Validation is carried out on experimental characterizations during 1,000 h ageing. The results on a new fuel cell stack show a score of 91% for fault isolation. However, without any adaptation, this score drops dramatically as the stack ages. Finally, thanks to ageing modeling and to the proposed adaptation of the detection/isolation procedure, the diagnosis performance remains reliable during fuel cell stack ageing.

Perforated Anode Gas Diffusion Layer for Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) are very promising electrochemical power sources. They are very suitable for portable electronics and transportation vehicles and considered as a potential alternative to lithium-ion batteries. Methanol is easily stored and transported and can be supplied in liquid form to a low temperature/pressure operating fuel cell. Direct operation on liquid fuels is attractive because of the exceptionally high energy density and convenience of liquid fuels [1]. However, DMFC’s suffer from two major obstacles, namely, the sluggish reaction at the anode and the methanol cross over to the cathode. The sluggish reaction at the anode is due to many individual steps required for methanol electrooxidation. The formation of CO2 as a product and its removability also contributes to the sluggishness of the reaction. CO2 bubbles, that can block reaction sites from reactants, must be transported from catalyst layer to flow channels and out of the cell as fast as possible. Most research of DMFCs is primarily focused on two aspects: 1. developing a new catalyst or reduce the use of precious metals. 2. Developing a more methanol resistant membranes to reduce the methanol cross over to the cathode that leads to useless mixed potentials at the cathode. If the anode structure is well designed in a way that prevents large amounts of methanol to transport through the membrane, the cell efficiency will be improved. The design and optimization of the anode electrode is very crucial in the development of DMFC’s. In this work, anode gas diffusion layer is modified by laser perforation with 500μm diameter holes along the channels. The perforation creates hydrophilic regions around the holes due to the loss of the hydrophobic PTFE material after being exposed to the hot laser beam. Therefore, the perforation not only creates pathways for reactants and products to transported but also change the wettability of the gas diffusion layer. The change in wettability and the pathways created by the laser reduce kinetic, ohmic and concentration polarization compared to the gas diffusion layer without perforation. Figure1 shows polarization curves comparison between a perforated anode gas diffusion layer and a non-perforated one (virgin GDL). High frequency resistance measurements and EIS (electrochemical impedance spectroscopy) of the anode and cathode of the virgin gas diffusion layer and the new perforated gas diffusion layer have also been studied. The results show that the difference in performance is attributed to the changed structure of the anode gas diffusion layer, when taking EIS of the anode at relatively low current density, the charge transport resistance arc (magnitude) of the perforated anode gas diffusion layer is much less than the virgin gas diffusion layer. To confirm that the results are solely due to the anode structure, the cathode impedance at low current density was also obtained and showed no difference in the charge transport resistance between the two tested gas diffusion layers. It was also noticed that the high frequency resistance (HFR) changed when increasing the current density; the perforated GDL maintain lower HFR for tested regions of the current density. This finding could be explained by the ability of better hydration of the membrane that perforated pathways create. Figure 1

Degradation of Proton Exchange Membrane (PEM) Water Electrolysis Cells: Looking Beyond the Cell Voltage Increase

The degradation of proton exchange membrane water electrolysis cells is usually measured in a temporal increase of the cell voltage. Although this is sufficient to evaluate the stability of a system, it is less suitable for targeted material development. Thus, an overpotential-specific and temporally resolved electrochemical characterization protocol is proposed. In this the ohmic overpotential is determined with high frequency resistance measurements. These are also used in combination with polarization curves to distinguish between the kinetic and mass transport overpotentials and to determine kinetic key parameters, according to the Butler-Volmer and transition state theory. Complementary electrochemical impedance spectroscopy measurements further unravel the individual resistances. On this basis, the following statements can already be issued. The major share of the measured cell voltage increase, i.e. degradation, is of apparent nature as it is recovered once lower potentials are applied. It is suggested that this is due to changes in the oxidation states of the iridium-based catalyst. Real degradation occurs in the ohmic and mass transport overpotential mainly at higher current densities and longer operating times. The increasing kinetic overpotential with increasing operating time is primarily potential-driven. Interestingly, both the Tafel slope and the apparent exchange current density slightly increase over time.

An innovative membrane-electrode assembly for efficient and durable polymer electrolyte membrane fuel cell operations

An innovative membrane-electrode assembly, based on a polyoxometalate (POM)-modified low-Pt loading cathode and a sulphated titania (S-TiO2)-doped Nafion membrane, is evaluated in a polymer electrolyte membrane fuel cell. The modification of fuel cell cathode with Cs3HPMo11VO40 polyoxometalate is performed to enhance particles dispersion and increase active area, allowing low Pt loading while maintaining performance. The POM's high surface acidity favors kinetics of oxygen reduction reaction. The mesoporous features of POM allow the embedding of Pt inside the micro-mesopores, avoiding the Pt aggregation during fuel cell operation and delaying the aging process, with consequent increase of lifetime. On the other hands, commercial Nafion is modified with superacidic sulphated titanium oxide nanoparticles, allowing operation at low relative humidity and controlled polarization of the MEA. Further MEAs, formed by unmodified Nafion membrane and the POM-based cathode, as well as sulphated titanium-added Nafion and commercial Pt-based electrodes, are used as terms of comparison. The cell performances are studied by polarization curves, electrochemical impedance spectroscopy, Tafel plot analysis and high frequency resistance measurements. The dependence of cell performances on relative humidity is also studied. The catalytic and transport properties are improved using the coupled system, despite the reduced Pt loading, thanks to rich proton environment provided by cathode and membrane.

Local electrochemical characteristics at various operating pressure and temperature values using a segmented polymer electrolyte membrane fuel cell

The pressurization of reactant gases is one of the solutions for generating considerable power in a polymer electrolyte membrane fuel cell with a restricted size. Electrochemical phenomena, such as current density distribution and ohmic resistance distribution, were observed to validate the effects of operating pressure and temperature on cell performance. The test was conducted in galvanostatic mode, and an inhomogeneous current distribution was observed under a high-pressure condition, except at a high temperature. High-frequency resistance measurement was also conducted to observe local ohmic resistance. Result showed that high pressure and temperature reduced ohmic loss and improved overall cell performance.

Cell Performance Determining Parameters in High Pressure Water Electrolysis

When hydrogen is used for energy applications, then often elevated pressure is required, i.e. up to 1000bar for hydrogen refueling stations. When polymer electrolyte electrolyzers are operated at elevated pressures, to reduce external mechanical compression needs, the thermodynamically expected cell voltage increase with pressure is not generally observed. Consequently, beneficial processes have to compensate for the increased work performed. In this paper, the influence of the operating pressure up to 100bar on the cell voltage behavior is investigated in detail. Data from galvanotstatic polarization curves up to 4A/cm2 combined with high frequency resistance measurements are analyzed using a zero-dimensional Tafel model. It shows that beneficial processes with increasing pressure are related to both the kinetic and mass transport overpotentials. At current densities above 1A/cm2 about two thirds of the overpotential gain can be related to reaction kinetics. Based on the Tafel model approach an increase of the apparent exchange current density by a factor of 4 to 7 is observed for the pressure increase from 10 to 100 bar. In contrast, the Tafel slope is independent of pressure and only the expected increase with temperature (73 to 81mV/dec) for 30 to 70°C is observed. Mass transport losses decrease with pressure. At 50°C and 2A/cm2, increasing the pressure from 1 to 100bar results in a decrease of the mass transport overpotential from 130 to 80mV.

Experimental Investigation of the Influence of Pressure on the Performance of Polymer Electrolyte Water Electrolysis Cells

Fluctuating electric energy production from renewable sources is increasing. While this is desired from an environmental point of view because production does not match consumption, storage is required to avoid curtailment [1]. Dynamically operable polymer electrolyte water electrolysis (PEEC) may be used to convert excess electric energy from fluctuating sources into chemical energy, i.e. hydrogen and oxygen [2]. For storage, transportation or methanation of hydrogen and optionally oxygen, pressure levels up to 300 bar, for mobility up to 800 bar are needed. Conventional, mechanical compression of hydrogen to the above mentioned pressure levels is inefficient, noisy and many compression stages are required which cause high investment costs and intensive maintenance. In contrast, electrochemical compression in high pressure (HP) PEEC promises to be more efficient, silent and low maintenance. On the one hand theoretically higher pressure leads to higher thermodynamic voltages, while on the other hand it may decreases the transport losses due to smaller gaseous bubbles, especially at high current densities [3]. Experimental results of HP-PEEC up to several hundred bar are discussed, investigating the influence of HP on transport losses at high current densities. The evolution of the different overvoltages (kinetic, ohmic, mass transport), determined using high frequency resistance measurements and tafel extrapolation, is discussed as a function of gas pressure. In detail the influence of different porous titanium sinter current collector media on mass transport losses is discussed as function of temperature, pressure and current density. It is shown that there seems to be an optimum average pore diameter at about 10 µm (Fig. 1). Literature: [1] C. J. Barnhart, et al., Energy Environ. Sci., 6 2804-2810 (2013). [2] F. Barbir, Solar Energy, 78(5) 661-669 (2005). [3] S. A. Grigoriev, et al., International Journal of Hydrogen Energy, 34(14) 5986-5991 (2009) Figure 1

Effective Ionic Resistance in Battery Separators

A key to understanding the limitations of lithium ion batteries and for modelling battery performance is the knowledge of ohmic losses in the porous battery components. In the electrolyte filled pores of a separator, ohmic losses arise from ionic resistivity. In a porous electrode they originate from the network of ionic and electronic resistances. We apply impedance spectroscopy to measure the high frequency resistance of separators and other battery components. In the literature, many values for effective ionic resistance of separators can be found [1,2,3,4], but their usefulness is sometimes diminished by unclear definitions of tortuosity as well as by the frequent lack of experimental details such as cell setup, electrolyte, and temperature. In general the high frequency resistance (HFR) of a porous separator can be described by: HFR = d τ/ε 1/(A κ) + Rcontact , where d is the thickness, ε is the porosity, τ is the tortuosity, κ is the conductivity of the electrolyte and A is the area. We aim to give a theoretical background on HFR measurements in general and discuss setups that allow us to measure the ionic resistance (and thus tortuosity) in a porous separator with high accuracy. We will discuss sources of error such as contact resistance, Rcontact, and inductive noise, showing how 4-point-probe measurements, proper shielding of the cables, and well-defined geometry can substantially decrease measurement errors. With regards to determining the tortuosity of porous separators, two measurement setups have been applied. The first consists of two Cu blocks with a defined effective area. The separators are placed between the blocks and wetted with electrolyte for measurement. The other approach uses a three layer Cu/separator/Cu pouch bag cell with 2x2 cm² electrodes, filled with electrolyte and vacuum sealed. The HFR between the Cu electrodes is then measured carefully in an electrolyte of EC:EMC 3:7 (wt%) with 1M LiPF6 and 2 wt% vinylene carbonate (LP572, BASF) at 25°C. Some results for several layers of a 20 µm PP/PE/PP trilayer separator similar to Celgard C480 are shown in Figure 1. Repetitive measurements yield very similar results and also the two completely different setups do not deviate by more than a few percent. The slope of a linear regression fit gives the ionic resistance of one separator layer, amounting to 1.566 Ω cm2. From this and the porosity of ε = 0.47, we can calculate a tortuosity of τ = 3.41 or an equivalent MacMullin number of 7.26. We will present data for a range of typical battery separators, showing that the commonly used Bruggeman estimation of the tortuosity (τ = ε-0.5 equating to τ = 1.46 for this separator) is not valid for typical separators, as shown before in literature [5]. We hope that this updated separator data will be helpful for battery modelling in the future. In our contribution, we will also present an impedance based method to quantify the ionic resistance in porous electrodes and its dependence on electrode morphology and compression.

Polymer Electrolyte Membrane Fuel Cells: Characterization and Diagnostics

The normal operation of proton exchange membrane fuel cells (PEMFCs) can induce significant temperature, humidity, pressure or concentration gradients across the cell’s active area. The overall performance and stability can be affected negatively by these gradients. For example, localized transient changes in environmental conditions may contribute to material degradation, and ultimately, to cell failure.We report on new diagnostic techniques and methodologies that have been developed to characterise PEMFC material properties, and the inhomogeneities across the active area of a working cell. Specifically, we will describe ex situ techniques for membrane electrode assembly (MEA) materials characterisation, and in situ techniques for the spatially resolved characterization of PEMFC performance. These techniques incorporate independent control over local potentials with concurrent membrane water content characterization (via high-frequency resistance measurements). Our testing hardware features sixteen fully isolated segments over a 42 cm2 active area, reference electrode capabilities, and individual segment control. This configuration enables the measurement of localised anode and cathode overpotentials separately.The technique’s versatility will be illustrated by effective platinum surface area (EPSA) measurements across the active under accelerated stress tests (ASTs) conducive to platinum dissolution. Each segment in a membrane electrode assembly was exposed to 10,000 cycles of Pt dissolution AST separately. The beginning-of-life EPSA was calculated via cyclic voltammetry, and the catalyst layer morphology and properties were found to change during the AST. Impedance measurements were obtained in H2/N2 conditions in order to map the ionic conductivity and the polarization resistance of the catalyst layer across the segmented cell. Higher EPSA and performance loss were observed in segments near the fuel cell outlet compared to those near the reactant inlets.

Polymer electrolyte fuel cell performance degradation at different synchrotron beam intensities

The degradation of cell performance of polymer electrolyte fuel cells under monochromatic X-ray irradiation at 13.5 keV was studied in galvanostatic and potentiostatic operation modes in a through-plane imaging direction over a range of two orders of magnitude beam intensity at the TOMCAT beamline of the Swiss Light Source. The performance degradation was found to be a function of X-ray dose and independent of beam intensity, whereas the degradation rate correlates with beam intensity. The cell performance was more sensitive to X-ray irradiation at higher temperature and gas feed humidity. High-frequency resistance measurements and the analysis of product water allow conclusions to be drawn on the dominating degradation processes, namely change of hydrophobicity of the electrode and sulfate contamination of the electrocatalyst.

Optimization of Passive Air Breathing Fuel Cell Cathodes

Air-breathing polymer electrolyte membrane fuel cells (ABFCs) use free convection airflow to supply oxygen to their cathodes. These cells are typically characterized by low output power densities compared with forced-convection fuel cells. Because ABFC designs rely on natural convection air delivery, cathode performance is often the performance bottleneck. This paper specifically examines the tradeoff between mass transport losses and ohmic electrical resistance losses for an optimal ABFC cathode design. Optimization is nontrivial because the simultaneous requirements for excellent cell compression, current collection, and gas access are often in contradiction. Simple scaling analysis and experimental observations suggest that the tradeoff between lateral mass transport resistance losses and cathode/gas diffusion layer (GDL) contact resistance losses determines the optimal ABFC cathode design. In order to quantitatively study these effects, we have tested a series of different cathode geometries in a standardized ABFC. Using high frequency resistance measurements and fast-scan polarization measurements, we have been able to interrogate both the ohmic and mass transport losses associated with each cathode configuration. We have also used pressure sensitive foils to examine the pressure distribution for representative configurations, providing a quantitative link between pressure distribution and cell resistance. Finally, we have studied the effect of deploying a current collecting contact layer between the cathode and the GDL. Results indicate that the deployment of a sufficiently stiff yet highly porous contact layer significantly reduces contact resistance losses while imposing minimal additional mass transport losses. A stiff yet porous contact layer reduces the contact resistance losses by increasing the total contact surface area and providing a more even distribution of pressure across the face of the cell. By minimizing contact resistance losses, this strategy enables the deployment of ABFC cathode structures with greater than 90% open area, thereby leading to an enhanced ABFC performance, particularly at high current densities.

Impedance Response of the Proton Exchange Membrane in Polymer Electrolyte Fuel Cells

The characteristics of the impedance response of a Nafion 112 membrane in an operating polymer electrolyte fuel cell (PEFC) were investigated under subsaturated and fully humidified operation conditions using current step and ac impedance measurements in combination with high time resolution–high frequency resistance (HFR) measurements. The impedance as obtained from HFR measurements shows the behavior of a pure resistor only under fully humidified conditions. At low humidity, the impedance spectrum exhibits an inductive loop at low modulation frequencies below . The formation of this loop is associated with the slow uptake and release of water by the bulk of the solid polymer electrolyte with ac current. The results show that the appearance of a low-frequency inductive loop in the impedance spectrum of a low-humidity PEFC is essentially the result of this effect and, therefore, indicative of a limitation of cell performance by the hydration of the ionomer by the product water formed at the cathode. This effect must be taken into account when interpreting low-frequency inductive loops in PEFC impedance spectra, in addition to other explanations proposed in the literature, which have primarily focused on reaction steps in the oxygen reduction reaction and platinum oxidation/dissolution.

Measurement of Water Transport Properties Through Membrane-Electrode Assemblies: I. Membranes

A method using a hydrogen pumping cell to measure the electro-osmotic drag (EOD) coefficient of Gore-Select membranes is described. This technique was verified with the measurement of Nafion membranes, yielding results comparable with those reported in the literature and hence supporting its validity for various types of membranes. The EOD coefficient of Gore-Select membranes was measured to be 1.07 over the wide relative humidity range between and 95%. An approach was also proposed to characterize the water concentration at the interface between the membrane and the cathode catalyst layer using high-frequency resistance measurement in an operating fuel cell. Based on the information of water concentrations on both sides of the membrane and the water diffusive flux across the membrane, the water diffusion coefficient of Gore-Select membranes was experimentally determined. It was found that the water diffusion coefficient of Gore-Select membranes is roughly half that of Nafion membranes based on the best fit of our available data in this work.

Electrical test methods for on-line fuel cell ohmic resistance measurement

The principles and trade-offs of four electrical test methods suitable for on-line measurement of the ohmic resistance ( R Ω) of fuel cells is presented: current interrupt, AC resistance, high frequency resistance (HFR), and electrochemical impedance spectroscopy (EIS). The internal resistance of a proton exchange membrane (PEM) fuel cell determined with the current interrupt, HFR and EIS techniques is compared. The influence of the AC amplitude and frequency of the HFR measurement on the observed ohmic resistance is examined, as is the ohmic resistance extracted from the EIS data by modeling the spectra with a transmission line model for porous electrodes. The ohmic resistance of a H 2/O 2 PEM fuel cell determined via the three methods was within 10–30% of each other. The current interrupt technique consistently produced measured cell resistances that exceeded those of the other two techniques. For the HFR technique, the frequency at which the measurement was conducted influenced the measured resistance (i.e., higher frequency providing smaller R Ω), whereas the AC amplitude did not effect the observed value. The difference in measured ohmic resistance between these techniques exceeds that reasonably accounted for by measurement error. The source of the discrepancy between current interrupt and impedance-based methods is attributed to the difference in the response of a non-uniformly polarized electrode, such as a porous electrode with non-negligible ohmic resistance, to a large perturbation (current interrupt event) as compared to a small perturbation (impedance measurement).

High-frequency resistance measurement by the use of a variable mutual inductance

The paper deals with a method of high-frequency resistance measurement employing a variable mutual inductance. The sources and magnitude of the errors likely to occur are dealt with mathematically. The results of coil resistance measurements are given and compared with the calculated values of resistance and with the results obtained by the “ resistance variation ” method. The application to condenser resistance measurement is considered.

High-frequency resistance measurement by the use of a variable mutual inductance

The paper deals with a method of high-frequency resistance measurement employing a variable mutual inductance. The sources and magnitude of the errors likely to occur are dealt with mathematically. The results of coil resistance measurements are given and compared with the calculated values of resistance and with the results obtained by the “resistance variation” method. The application to condenser resistance measurement is considered.