Measurement Of Current Distribution Research Articles

A review of current, temperature and water distribution diagnosis technique for PEMFC

Diagnosis technology is a key means for understanding the electrochemical reaction process of fuel cell. With the increasing demand for high-power stacks in the market, Membrane Electrode Assemblies (MEA) with large active area have been a hot topic. Obtaining the current, temperature, and water distribution information over the entire active area is critical to structural design and performance control of fuel cell. This paper reviews the diagnosis technique that can be used for in-suit measurement of current, temperature, and water distribution. The applicability of various techniques and specific applications in fuel cell diagnosis are summarized, and future developing trend is prospected.

Pulsed-Current Distribution in Electromagnetic Rail Launchers

This article deals with the measurement of current distribution through the rail cross section of an electromagnetic rail launcher. A measurement approach based on a loop system magnetically coupled to the launcher that is placed along the rails is proposed. The estimation of the current distribution in the rail, following the basic rules of the partial-inductance method, is discussed in this article. To test the proposed method, an experimental analysis on an “ad-hoc”-developed multiconductor rail prototype supplied by a high-voltage capacitor bank is presented. The experimental results obtained by the proposed method were compared with the direct-current measurements achieved by a set of Rogowski coils showing good agreement for different current values.

Advancement of Segmented Cell Technology in Low Temperature Hydrogen Technologies

The durability and performance of electrochemical energy converters, such as fuel cells and electrolysers, are not only dependent on the properties and the quality of the used materials. They strongly depend on the operational conditions. Variations in external parameters, such as flow, pressure, temperature and, obviously, load, can lead to significant local changes in current density, even local transients. Segmented cell technology was developed with the purpose to gain insight into the local operational conditions in electrochemical cells during operation. The operando measurement of the local current density and temperature distribution allows effective improvement of operation conditions, mitigation of potentially critical events and assessment of the performance of new materials. The segmented cell, which can replace a regular bipolar plate in the current state of the technology, can be used as a monitoring tool and for targeted developments. This article gives an overview of the development and applications of this technology, such as for water management or fault recognition. Recent advancements towards locally resolved monitoring of humidity and to current distributions in electrolysers are outlined.

Spatially-Resolved Measurement of Current Distribution in Polymer Electrolyte Water Electrolyzers

Operational cost, primarily power consumption, is the one of the main obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [1], thus overall electrochemical efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Conventional research and diagnostic techniques, such as polarization curves and EIS measurements, predominantly probe macroscopic performance characteristics without any spatial resolution of electrochemical performance. These offer limited insight into the in-situ behaviors of the PEWE and its overpotentials. The work of Schuler, et al. [2] gives a detailed understanding of transport mechanisms inside a PEWE by drawing extensively on ex-situ techniques. Work carried out by Zhang et al. [3] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; while insightful, such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [4, 5]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer. Thus the distributed current measurement occurs passively while a potentiostat controls the electrolyzer, without interrupting its operation. This setup is capable of using flow fields of both 5cm2 and 25cm2 active area. The 5 cm2 flow field has 16 segments while the 25cm2 has 100 segments that are electronically isolated, yielding high spatial resolution of the current distribution. The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b and 1c. Preliminary current distribution experiments were carried out at 85⁰C, water flow rate of 0.5 to 3 mL/min/cm2, at a cell compression under bolt torques of 5 Nm and 10 Nm. Experiments were also carried out with two sets of liquid-gas diffusion layers: iridium-coated titanium and bare titanium felts. The results show a relatively uniform current distribution across the electrolyzer with no significant impact of flow rate. The experiments also revealed that the spatial current distribution is highly sensitive to local cell compression, which is not captured in the overall cell performance of a polarization curve. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offer which would otherwise be impossible to probe.

Resolving the Discrepancy in Tortuosity Determination for Battery Porous Electrodes Via a Numerical Approach

The tortuosity factor of porous electrode microstructure is a crucial input parameter for numerical models of batteries as it strongly influences the electrode performance. As such, it is very important to have a method to determine this parameter accurately, based on a definition that reflects the design of the cell.Various experimental methods have been developed for either directly measuring or indirectly inferring the tortuosity factor; however, numerical approaches, based on 3D image data, are now gaining interest in the battery community, due to the advances in nanoscale tomographic imaging methods. The standard definition of the tortuosity factor solves the Fick diffusion equation at steady-state, i.e., between two parallel constant-value boundaries. Although this approach has been widely used for porous materials, including both electronic insulators (e.g., a battery separator), and electronic conductors (e.g., battery porous electrodes), it may be the case that the definition needs to be adjusted depending on the scenario being observed.In this study, we intend to give an insight into the appropriate way to determine the tortuosity factor of battery porous electrodes and the impact of various tortuosity determination methods is investigated. An additional module that relies on the symmetric cell method [1] [2] was implemented in the TauFactor software package [3] to compare with the already-implemented diffusion-based method [4]. This symmetric cell method refers to the measurement of the ionic current distribution inside the pores using AC impedance based on a symmetric cell setup. Figure 1 shows the workflow for tortuosity determination applied in this study. The integration of this module in TauFactor might be interesting for tortuosity determination at the microscale since it is the same method as at macroscopic scale observed in various experimental approaches. Figure 1 . Illustration of the workflow for tortuosity determination applied in this work. The module recently implemented in TauFactor allows calculation based directly on tomographic data in symmetric cell configuration, and generates a simulated impedance spectrum. A macroscopic model such as TLM or Newman’s model is used to extract the tortuosity value of the electrode.

Vanadium Flow Battery Electrochemistry and Fluid Dynamics Model with in-Situ Current Distribution Validation

It has been estimated that high penetration of renewable electricity generation (68%-73% from solar and wind) could be possible in the United States by 2050 [1]. To achieve this goal, large-scale energy storage technologies will play an important role. Energy storage technologies can improve electricity service stability, flexibility, reliability, and resilience on the grid. Among energy storage methods, vanadium redox flow batteries (VRFBs) have been a sustained focus of interest due to distinct advantages, e.g. flexibility and scalability, high columbic efficiency, and long cycle life. However, high system cost is a major obstacle for widespread commercialization of VRFBs; cell component costs (membranes and chemicals) tend to dominate overall system costs. It is possible to improve electrolyte utilization and system efficiency, leading to reductions in both component costs. This can be achieved through increasing vanadium solubility in the solvent, modifying membranes to be more ionically conductive, minimizing crossover, facilitating electrochemical kinetics, improving mass transport, and reducing parasitic pump losses. The VRFB is a complex system with multiple interrelated parameters (especially mass transport and electrochemical reactions) affecting electrochemical cell performance; these interrelated parameters are difficult to disentangle experimentally. Comprehensive and suitably validated mathematical simulations can both help to understand complex phenomena inside VRFBs and provide practical knowledge for controlling and optimizing VRFB systems. In this study, a three dimensional, steady-state multi-physics model is developed for VRFBs with strip cell architecture under the dilute solution theory assumption. The simulation domain includes a central membrane, both electrodes, flow plates, and current collectors. Continuum relationships including conservation of mass, momentum, species and charge coupled with Butler-Volmer kinetics are employed.In this study, the impacts of various electrochemical and transport parameters on the electrochemical performance and current distribution are investigated. It is found that the diffusion coefficient of the vanadium species is the most significant parameter affecting the current distribution along the channel. The model successfully predicts both the charge-discharge polarization curve and the current distribution with the fitted diffusion coefficient parameter. The diffusion coefficient of the vanadium species was found to be order of magnitude higher than the experimentally-measured values found in the literature [2].Polarization curve analysis and fully segmented, printed circuit board (PCB)-based, localized current distribution measurements are employed to validate the mathematical model. All tests are conducted with a simplified test bed with a segmented strip cell architecture, having only one straight channel and a total of 1 cm2 active area. Strip cell architecture effectively eliminates higher-dimensional behaviors (e.g. channel hopping, bypass at channel switchbacks, and potential fluid short circuits) and provide straightforward systems for phenomenological as well as comparative and detailed model validation studies [3].

The Role of Multiple Fluctuation Events in NbN and NbTiN Superconducting Nanostrip Single-Photon Detectors

We report on measurements of the switching current distributions on two-dimensional NbN superconducting nanostrip single-photon detectors (SNSPD), 5 nm thick and 80 nm wide, in an interval of temperatures from 6 K down to 0.3 K and compare the data with those obtained for similar NbTiN nanostrips. The standard deviations of the switching distributions show an extended region at high temperatures where multiple phase slip switching events occur. This is probably related to a decreasing critical current and an increasing electron and phonon heat capacities. In this temperature region, the width of the switching distribution, and therefore the dark count rate, is considerably reduced down to values below those observed at the lowest temperature. Finally, we also quantify the energy scale of the fluctuation phenomena. The proposed experimental approach may result in a powerful tool for the diagnostic of SNSPD operation mode.

Beam diagnostics on the electron beam welding installation at Budker Institute of Nuclear Physics

This paper describes the equipment and methods of the beam diagnostics, namely, the beam current density distribution measuring and the beam position and inclines angle determination. Such a diagnosis is aimed at applying on the electron-beam technological installations, in particular, on the electron-beam welding installations. The paper presents the results of the beam current density distribution measurement, describes the design of the beam position and inclines angle measurement system, and presents the results of preliminary tests of the prototype of this system.

Local impact of load cycling on degradation in polymer electrolyte fuel cells

Degradation mechanisms occurring during proton exchange membrane fuel cells (PEMFC) operation critically depend on the applied electrical load profile. In this work, durability tests were performed using different load cycling ranges. The tests include refresh procedures in order to recover temporary performance losses. Operando current density distribution measurements as well as electrochemical characterization techniques were applied to obtain essential data for understanding degradation behavior of the components of the membrane electrode assembly (MEA). The analysis is supported by scanning electron microscopy (SEM) and energy dispersive x-ray diffraction (EDX) analysis of MEA components. Dynamic load cycling has proven to lead to lower performance losses as compared to constant load operation. Moreover, reversible performance losses were associated with an increased heterogeneity of the current density distribution along the flow field. Thereby, areas with high local current density exhibit particularly strong Pt band formation in the membrane linked with a thinning of the cathode catalyst layer.

Practical Current Distribution Measurement Systems for Lead Cells

In this paper, the current distribution of an experimental lead cell is estimated using two methods and the results are compared. Noninvasive measurements are obtained using our magnetic tomography system, consisting of an array of magnetic sensors and a solver algorithm that is specially adapted for application to lead cells. Verification of this magnetic tomography system is achieved by comparing the results with data from a new invasive measurement instrument also reported in this paper. This new instrument is an array of ferrous cored current transducers submerged in the cell electrolyte, providing a direct spatially and temporally resolved measurement of the ionic current in a lead cell. Comparisons between the techniques and data are drawn. The new flowthrough internal sensor array has been designed specifically for this application and represents an improvement over a similar system reported in the literature. In addition, some auxiliary features of the experimental setup are designed to enable useful magnetic measurements to be taken by minimizing the complexity of the setup, and therefore the complexity of the models needed to interpret the data. Example results are presented for a typical automotive lead cell operating at a low state of charge and in a fatigued condition, to demonstrate the ability of both measurement systems to produce spatially and temporally resolved images of current distribution. The methods are evaluated, and the future work is recommended.

Effects of the grid geometry on the performances of a triode-type corona electrode system

The triode-type electrode system is frequently employed for accurately controlling the corona charging of insulating materials. The primarily aim of this paper is to characterize the effects of grid geometry of the triode-type corona electrode system by current-voltage and current density distribution measurements. The ionizing element of the electrode system is a thin Tungsten wire (diameter: 200 μm) attached to a cylinder (diameter: 26 mm) and distanced at 34 mm from its axis. The wire-cylinder assembly is a “dual” electrode, facing a grid electrode, connected to a well-defined potential, and a grounded plate electrode. The geometry of the grid has a very significant influence on the current-voltage characteristics of the triode system, on the corona onset voltage value and on the repartition of the current density. The experimental results obtained for different geometries of the grid electrode system are interpreted in order to make some recommendations regarding its design.

Operando and Ex-Situ Investigation of PEMFC Degradation

Longevity is one of key requirements of PEMFC for automotive applications. Hence, understanding degradation phenomena and durability limitations is fundamental to increase PEMFC life time and to make this technology competitive In this contribution a selection of our recent work on these topics is presented. In particular, attention in dedicated to study effect of performance limitations at low Pt loadings and to investigate impact of operation conditions on local degradation phenomena such as drying, contaminations or heterogeneities during dynamic operation. Moreover, specific operando accelerated stress tests are being developed and the relative impact on various stressors on individual MEA components is studied. Eventually, we present results related to on-line fault monitoring using current density distribution measurements linked with impedance spectroscopy.

Superconductor to resistive state switching by multiple fluctuation events in NbTiN nanostrips

We report on measurements of the switching current distributions on two-dimensional superconducting NbTiN strips that are 5 nm thick and 80 nm wide. We observe that the width of the switching current distributions has a non-monotonous temperature dependence, where it is constant at the lowest temperatures up to about 1.5 K, after which it increases with temperature until 2.2 K. Above 2.5 K any increase in temperature decreases the distribution width which at 4.0 K is smaller than half the width observed at 0.3 K. By using a careful analysis of the higher order moments of the switching distribution, we show that this temperature dependence is caused by switching due to multiple fluctuations. We also find that the onset of switching by multiple events causes the current dependence of the switching rate to develop a characteristic deviation from a pure exponential increase, that becomes more pronounced at higher temperatures, due to the inclusion of higher order terms.

Isolation of Mass Transport and Current Distribution in Vanadium Flow Batteries Via Segmented Strip Cell

Vanadium redox flow batteries (VRFBs) have drawn considerable attention due to advantageous characteristics including flexible design, high coulombic efficiency, and long cycle life. But a number of challenges must be overcome for commercial implementation [1]. At the cell level, mass transport limitations can contribute to performance losses, limiting VRFB performance and depth of discharge, especially for cells operating at high power densities [2–4]. Mass transport losses are associated with inadequate delivery of active species to electrode surfaces and are controlled by multiple physicochemical phenomena. Therefore, understanding mass transport mechanisms in the porous electrode is crucial to understanding pathways to achieve optimal performance and a higher depth of discharge. In this study, bulk motion and concentration-driven mass transport mechanisms are qualitatively investigated utilizing a simplified test bed with a segmented strip cell architecture, having only one straight channel and a total of 1 cm2 active area as it is shown in Figure. The simple strip cell design has no gap between cell components [5], allowing the control and isolation of mass transport mechanisms by suppressing higher-dimensional behaviors, and measurement of localized current along the channel path. Thus, the impact of individual transport mechanisms on current distribution can be evaluated with precision. The diffusion-dominated and convection-dominated mass transport regimes are achieved by employing various channel depths (0.25 mm, 0.5 mm, 1 mm, 2.5 mm). Fully segmented, printed circuit board (PCB)-based, localized current distribution measurement is implemented on all strip cell configurations to obtain current distribution along the channel. Major parameters affecting current distribution considered in this work include constant voltage operation, channel depth, and flow rate. It is shown that charging is not a mass transport-limited process for VRFBs. However, once the mass transport-limited region is reached during discharge, non-uniform current distributions are observed. The highest current deviation down the channel is obtained for the deepest channel (2.5 mm) cell configuration. This type of current distribution behavior is attributed to concentration gradient-driven mass transport dominating in the electrode. As channel depth decreases, current distribution is flattened from inlet to outlet as a result of improved bulk motion of the active species in the electrode. It is also observed that the flow rate contribution for improving bulk motion of the active species in the electrode is more distinct in convection-dominated cells (0.25 mm, 0.5 mm) than diffusion-dominated cells (2.5 mm, 1 mm). This work reveals that it is possible to achieve enhanced electrochemical performance with reduced pressure drop in convection-dominated cells. Gandomi, Y. A., Aaron, D. S., Houser, J. R., Daugherty, M. C., Clement, J. T., Pezeshki, A. M., Ertugrul, T. Y., Moseley, D. P., and Mench, M. M. “Critical Review — Experimental Diagnostics and Material Characterization Techniques Used on Redox Flow Batteries” J. Electrochem. Soc. 165, no. 5 (2018): 970–1010. doi:10.1149/2.0601805jesHouser, J., Clement, J., Pezeshki, A., and Mench, M. M. “Influence of Architecture and Material Properties on Vanadium Redox Flow Battery Performance” Journal of Power Sources 302, (2016): 369–377. doi:10.1016/j.jpowsour.2015.09.095, Available at http://dx.doi.org/10.1016/j.jpowsour.2015.09.095Clement, J. T., Aaron, D. S., and Mench, M. M. “In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries” Journal of The Electrochemical Society 163, no. 1 (2016): A5220–A5228. doi:10.1149/2.029305jesHouser, J., Pezeshki, A., Clement, J. T., Aaron, D., and Mench, M. M. “Architecture for Improved Mass Transport and System Performance in Redox Fl Ow Batteries” Journal of Power Sources 351, (2017): 96–105. doi:10.1016/j.jpowsour.2017.03.083, Available at http://dx.doi.org/10.1016/j.jpowsour.2017.03.083Aaron, D. S., Liu, Q., Tang, Z., Grim, G. M., Papandrew, A. B., Turhan, A., Zawodzinski, T. A., and Mench, M. M. “Dramatic Performance Gains in Vanadium Redox Flow Batteries through Modified Cell Architecture” Journal of Power Sources 206, (2012): 450–453. doi:10.1016/j.jpowsour.2011.12.026, Available at http://dx.doi.org/10.1016/j.jpowsour.2011.12.026 Figure 1

Investigation of the effect of cathode stoichiometry of proton exchange membrane fuel cell using localized electrochemical impedance spectroscopy based on print circuit board

Proton Exchange Membrane Fuel Cell can have a large active area, and the working condition in different areas can be entirely different. Localized electrochemical impedance spectroscopy can directly observe the proton exchange membrane fuel cell internal reaction conditions. In this work, localized electrochemical impedance spectroscopy test system based on print circuit board is implemented in a 50 cm2 multi-channel serpentine flow fields. The localized electrochemical impedance spectroscopy performances of different segments with different cathode stoichiometry (1.8, 2.3 and 2.8) at different current density (100 mA cm−2, 500 mA cm−2 and 900 mA cm−2) are studied. The result demonstrates that the fuel cell may suffer from local drying and flooding at the same time. To make full use of the potential of a fuel cell, a suitable cathode stoichiometry should be identified to control the drying of the inlet and the flooding of the outlet at the same time. It is shown that a cathode stoichiometry of 2.3 is close to the optimum cathode stoichiometry to keep the fuel cell in good consistency without gas waste. Besides, a current density distribution measurement is performed to verify the conclusions of this work.

Evaluation of Two Water Transports through Electrolyte Membrane of Polymer Electrolyte Fuel Cell Based on Water Visualization and Current Measurement

AbstractTo alleviate membrane dehydration in polymer electrolyte fuel cells (PEFCs) during low‐humidity operations, it is essential to deeply understand two water crossover phenomena through electrolyte membranes. This study presented the quantitative method for separately evaluating the electro‐osmotic and back‐diffusion water transports in the electrolyte of an operating PEFC based on the measurements of water vapor and current distributions. Firstly, the water vapor distribution in the anode flow field was investigated by the visualization technique using humidity test paper (HTP). Subsequently, the detailed transport processes of water through the electrolyte membrane can be numerically estimated from the measurement results of water and current profiles by using a simple fuel cell model. In this work, the effects of inlet gas humidification, microporous layer (MPL) addition and flow configuration on the two water transports in the membrane were demonstrated under low‐humidity conditions to verify the validity of the proposed methodology. Results revealed that the introduction of MPL on the cathode side increases the water concentration in the anode channel due to the back‐diffusion effect. The counter‐flow configuration effectively improves the membrane hydration at the anode inlet because of the strong back‐diffusion from the wetted cathode outlet.

Analysis of Influence of Impurities in Hydrogen on PEFC Performance, Based on Measurement of Current Distribution Using Electrochemical Impedance Distribution Analyzer

Polymer electrolyte fuel cells (PEFC) systems have attracted considerable attention in that they can be powered by the hydrogen that is produced as a by-product of various industrial processes, such as caustic soda production and petrochemical processes. Most of the by-product hydrogen is discarded as a waste stream or used for heat production. Therefore, such plants could realize significant energy savings by installing PEFC systems, allowing them to generate power with a high level of efficiency by using the by-product hydrogen to fuel PEFC systems. However, the by-product hydrogen produced by these plants typically contains impurities. Although most of these impurities can be removed, a small amount inevitably remains in the hydrogen and can cause the performance of a PEFC system to degrade. Therefore, the influence of these impurities must be clarified before installing a PEFC system. Given the above, we have been developing a means of quickly evaluating the influence of impurities on a PEFC. The measurement of the current distribution is an important analysis approach that provides information on the local electrochemical reaction conditions in each of the cells of a PEFC. These cells are composed of an electrolytic membrane, a catalyst layer, a diffusion layer, and a flow channel. We have devised a means of measuring the current distribution using current distribution measurement sensors (S++ Simulation Services, Germany) which are inserted between the gas flow channel and current collector of either electrode. This method of measuring the current distribution allows us to quickly analyze the influence of impurities on the PEFC performance, relative to conventional voltage measurements, and can clarify which areas in a cell have degraded because the voltage change is the average of the values for every area in a cell, whereas the current change appears as an absolute value in a local area. Electrochemical impedance spectroscopy (EIS) is a valuable electrochemical analysis tool that can be applied to many kinds of electrochemical devices, including fuel cells. EIS can provide much useful information on the electrolyte membrane, the catalyst layer, and the reactant transfer in a PEFC cell. Although the measurement of the electrochemical impedance distribution in a cell provides a useful means of analyzing the influence of impurities on the PEFC performance, to the best of our knowledge, no previous studies have attempted a high-resolution analysis of the distribution. Therefore, we have developed a new measurement system that enables us to measure both the electrochemical impedance distribution and current distribution. The measurement system is an improved current distribution measurement sensor, intended for application to a JARI-standard cell, with an active area of 25 cm2 and which has 81 segmented internal current shunt resistances for measuring DC current and 81 segmented external connectors for measuring AC current signals. In the present study, to simulate impurities in the hydrogen, we added NH3 (25 ppm) and analyzed its influence on the PEFC performance. We used the measurement system to measure the change in the electrochemical impedance distribution and current distribution over time. The operating temperature, humidification temperatures of the anode and cathode, and current density of the cell were set to 65 ºC, 65 ºC, and 0.3 A/cm2 respectively. The AC current for measuring the electrochemical impedance was 1 Ap-p and the frequency range was 100 mHz–100 kHz. The current distribution measurement (Fig. 1) revealed that the influence of the NH3 was greatest at the inlet to the cell. Furthermore, from the results of the electrochemical impedance distribution measurement, it was found that the current distribution and some aspects of the resistance distribution, as determined from the electrochemical impedance, were correlated. Meanwhile, the most significant factor influencing the effect of NH3 on the PEFC performance is the increase in the activation overvoltage at the inlet of the cell. In addition, it was also thought that the cathode reaction resistance of the cell increases, which is caused by the catalyst-specific surface area decreasing at the inlet of the cell. In conclusion, our new measurement method provides an effective means of determining the largest degraded area and the largest degradation factor affecting PEFC performance, as a result of the presence of impurities in hydrogen. Therefore, relative to conventional measurements, we can quickly evaluate the influence of impurities on the PEFC performance by focusing on and analyzing these areas and factors. In the future, we will apply our new measurement method to the evaluation of the influence of impurities on the PEFC performance by acquiring information on the influence of such impurities on PEFC performance. Figure 1

(Invited) Durability of Polymer Electrolyte Membranes for Automotive Applications

There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical, mechanical and thermal degradation. Chemical and mechanical degradation can occur during normal fuel cell operation whereas thermal degradation should only occur at extreme temperatures such as those that can be induced by membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation with focus on the effect of simultaneous chemical and mechanical degradation. Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. For example, porous polymer supports such as (expanded polytetrafluoroethlene) ePTFE can be used to make PFSA membrane which exceed US Department of Energy mechanical durability requirements and hydroxyl radical scavenging additives such as Ce3+ and Mn2+ can enable PFSA membranes to meet chemical stability requirements. Accelerated stress tests (ASTs) have been developed that can be used to screen various membrane types and mitigation strategies, and effective in-situ diagnostics have been developed to track degradation of PFSA membranes during fuel cell operation. The failures observed in these accelerated tests as seen by postmortem analysis mimic the failures of PFSA membranes in field tests. A new highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. The HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the gas outlets. HASTs were conducted using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art robust membrane containing both an ePTFE mechanical support and a Ce-based chemical stabilizing additive. Result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. Further results will be presented that decouple the cerium migration effect from the mechanical/chemical synergistic degradation effect.

In Operando Cell Diagnostic with MRI

One of the biggest obstacles in the way of battery technology development is the limited amount of information that one can obtain from the battery mechanism without taking it apart. Here we are presenting a novel non-destructive battery assessment technology that is capable of obtaining crucial information from batteries even when the cells are encased in conductive material. One important application is commercial-type cell-phone batteries, which can be analyzed with this method. The technique is based on measuring the magnetic susceptibility of active ingredients inside an electrochemical cell that is affected by the oxidation state of the materials to give insights into the state of charge (SOC) of the battery, its failure mechanisms and the current distributions. The technique has been demonstrated on various cell types, defects, chemistries, and current level. The measurement is based on Magnetic Resonance Imaging (MRI) techniques, is fast (a couple of seconds), and could be adapted to a range of cell types. The figure shows the schematic setup and examples of images of different cells which are used to diagnose the SOC and the state of health. The technology represents a new opportunity for nondestructive cell diagnostics, and hence could be of great help in the development of next-generation batteries. We demonstrate the identification of defect, state of charge, and classification of cells based on different types of features. The method can also provide localized current distribution measurements. Figure caption: Magnetic field map measurements for the fully charged cells. (A)-(C) sample placement, and image orientation; (D)-(E) Field maps measured for the cells. Field maps are referenced to the empty holder, giving an absolute field map for the Li-ion cell. (F) Magnetic field maps for the defect cells, with the mean and standard deviation indicated (taken over all of the voxels in each image). The fields are given relative to one of the non-defect cells (not shown). Figure 1

Measurements of the Ion-Beam Current Distribution over a Target Surface under a High Bias Potential

A technique for direct measurements of the current distribution over the surface of a target that is at a negative bias potential relative to the ground was developed and tested. Measurements were performed using a six-channel meter that transmitted the measured data through a wireless Bluetooth acquisition system. The trace of an ion flow on the target surface was visualized by the method of luminous beams. Examples of the measured current distributions on a target surface depending on the geometric and physical parameters of the ion-optical system of a Penning plasma source are presented.