Current Density Distribution Measurements Research Articles

EWTN: Quantifying Mass Transport to Enable Water Electrolyzer Architectures with Low Flow-Rate Sensitivity

To develop cost-effective and high-performing polymer electrolyte water electrolyzers (PEWEs) for gigawatt-scale applications, researchers have focused on reducing precious metal catalyst loadings and optimizing porous transport layers. However, the performance of PEWEs is also affected by system architecture. Mass transport losses are dependent on localized architecture and material interactions. In-situ measurements, such as current density distribution maps have demonstrated advantages in understanding the intricate characteristics and influence of two-phase flow within PEWEs. This study proposes the parameter of effective water transport number (EWTN) as a quantitative tool to investigate such current density distribution (CDD) measurements for PEWEs. Results show that higher flow-rates have EWTN values of 0.95 and above, indicating no mass transport limitations; while lower flow-rates with large gradients CDD have EWTN values between 0.6-0.8, indicating mass transport limited conditions. The new analysis also identified a correlation between mass transport losses due to bubble accumulation, membrane hydration, and ohmic overpotentials. To address these limitations, an unitized pin-type LGDL/flow-field design was developed, which effectively prevents local gas phase accumulation, resulting in improved mass transport characteristics. The results of this work show reduced flow-rate sensitivity with the pin-type architecture and ∼13% increased performance at 0.24ml/min/cm2 .

Operando analysis of in-plane heterogeneity for the PEM electrolyzer cell: Mappings of temperature and current density

The internal multi-physical fields of a proton exchange membrane electrolyzer cell (PEMEC) are affected by various factors, exhibiting an intricate coupling state. During electrolysis process, the complexity manifests as heterogeneous distribution of in-plane temperature and current density, potentially leading to local hot spots and performance degradation. Such challenges pose threats to the lifespan and operational safety of PEMEC. This study developed temperature measurement endplates and segmented printed circuit board (PCB), enabling operando measurement of in-plane temperature and current density distribution inside a 25 cm2 lab-scale PEMEC. The mappings of temperature and current density were plotted, and the in-plane heterogeneity was assessed by the introduced indexes. The results indicate that variations in operational parameters affect the balance among cell heat production, heat dissipation, and heat input in the flow area, leading to temperature distribution heterogeneity. The migration mechanism of high-temperature zone is elucidated. Factors such as water-gas flow and local conduction contribute to the heterogeneity in current density distribution. Additionally, specific operating conditions including cold and warm start-up, and water starvation are taken as cases, and a detailed analysis is conducted on the dynamic behaviors of current and temperature distribution and the heterogeneity evolution. This work not only provides experimental evidence for the electrical-thermal characteristics inside the PEMEC but also offers insights for local performance evaluation and operation fault diagnosis in practical applications.

Current density and temperature distribution measurement and homogeneity analysis for a large-area proton exchange membrane fuel cell

Homogeneous distribution of electro-chemical reaction rates among the activation surface is critical for improving the performance and durability of automotive proton exchange membrane fuel cells (PEMFCs). Segmented measurement technology is commonly used to characterize the local physical-parameter distribution in the PEMFC. In this study, the local current density (LCD) and temperature distributions of a PEMFC with the activation area of 108 cm2 and various cathode flow fields are experimentally investigated. A homogeneity parameter is introduced to evaluate the homogeneity of LCD distribution. The results show that the performance and LCD distribution uniformity of the cell with dot-parallel flow field are much better than that with parallel and parallel-serpentine flow fields. The temperature distribution is generally positively correlated with LCD distribution. For the LCD distribution, the high LCD region firstly appears in the cathode downstream region, and gradually transfers upstream with increasing the current load. With the increase of inlet humidity, the LCD near the cathode inlet is improved due to the improvement of membrane hydration. Increasing the cathode stoichiometry can effectively improve the uniformity of LCD distribution, and mitigate the local oxygen starvation, especially at high loads.

The Real Current Density Distribution on Mg Surface

To quantitatively analyze the real current density distribution and electrochemical activity on a Mg electrode, the electronic current densities distributed on a multi-Mg wire array electrode were compared with scanning vibration electrode technique (SVET) mapped ionic current density distributions. The SVET measured current densities were found to be nearly ten times lower than the electronic ones. The cathodic current densities detected in SVET mapping could be a result of hydrogen bubbles trapped on the electrode surface. The measurement of the real current density distribution on Mg surface helps deepen the understanding of Mg anode dissolution.

Simulative Investigation on Local Hydrogen Starvation in PEMFCs: Influence of Water Transport and Humidity Conditions

Durability targets of automotive polymer electrolyte membrane fuel cells (PEMFCs) could be crucially threatened by local hydrogen starvation, typically induced by local blockage of gas channels. To gain a deep insight on the evolving of such starvation events and related carbon corrosion losses, we have developed a numerical model with transient nature that includes detailed transport phenomena and electrochemistry. Special focus is on water transport and sensitivity of relative humidity (RH) on both anode and cathode sides, whose influences were commonly neglected in starvation-related modeling studies. Utilizing the model, we show the dominating effect of in-plane hydrogen convection within the anode gas diffusion layer, which is again determined by the accumulation of other gas species including water vapor. We demonstrate how this is again linked with the water management throughout the fuel cell. Furthermore, water transport is shown to affect local current density and membrane oxygen permeability, both being critical influential factors regarding the severity of a local starvation event. The developed model is validated by conducting transient current density distribution measurements. As RH levels are crucial operational conditions within automotive PEMFCs, this work serves as useful input towards development of future operation strategies for better PEMFC durability.

A pluggable current collector for in-operando current measurements in all-vanadium redox flow batteries with flow field

Understanding current distribution in operating Vanadium redox flow batteries (VRFBs) is vital for improving battery design and performance as the non-uniform current density distribution is known as a major cause of the corrosion and side reactions in VRFBs. However, the existing method for current density measurement in the VRFB with flow field is complex and the rib conductive structure of graphite plate is changed which reduces the measurement accuracy and reliability. In this study, we propose a novel pluggable current collector for in-operando current distribution measurements which greatly simplifies the current density distribution measurement process in VRFBs with flow field. The new method relies on eight pluggable graphite units in a flow plate frame to replicate the actual serpentine flow field structure, thereby providing higher measurement reliability compared to the existing methods which are mostly invasive. Using the new method, current density distributions at different electrode compression ratios (CR) and inflow electrolyte rates are investigated. It is found that insufficient supplemental reactant in electrode border region is the main reason for the uneven current density distribution and the current density non-uniformity is intensified in the initial and final stages of cycling process. Increasing CR can improve the current density in the electrode core region. The overall current density uniformity in a VRFB with serpentine flow field can be enhanced by increasing the inflow electrolyte rate. The developed method is believed to facilitate the understanding of the coupled local reactions and transport processes and guide on the future design of VRFBs.

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.

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.

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.

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.

Analytical solutions and dimensional analysis of pseudo 2D current density distribution model in PEM fuel cells

Abstract In this paper, a new dimensionless pseudo 2D steady state current density distribution model along the channel of a polymer electrolyte membrane (PEM) fuel cell is presented. This model includes four fundamental phenomena observed in PEM fuel cell cathode such as the air concentration depletion along the channel, the mass transport through the gas diffusion layer (GDL), the charge transport through the membrane and the electrochemical transfer in the catalyst layer (CL). A dimensional analysis is performed, and three dimensionless parameters are found to govern the current density distribution along the channel: a Peclet number at the channel/GDL interface, a Damkholer number at the GDL/CL interface, and a Wagner number at the CL/PEM interface. Four regimes of operation are defined based on the values of these dimensionless numbers, and for each of them new analytical solutions are developed. A good agreement with the experimental measurements of current density distribution reported in literature is found, concluding that the macroscopic PEM fuel cell physic is well described by our model. The specific operating regime described with relatively simple equations of current density distributions paves the way for a better control of fuel cell operation and performance, and in situ characterisation of fuel cell material properties.

Ionomer equivalent weight structuring in the cathode catalyst layer of automotive fuel cells: Effect on performance, current density distribution and electrochemical impedance spectra

To improve the performance of proton exchange membrane fuel cells, membrane electrode assemblies (MEAs) with segmented cathode electrodes have been manufactured. Electrodes with a higher and lower ionomer equivalent weight (EW) were used and analyzed using current density and temperature distribution, polarization curve, temperature sweep and electrochemical impedance spectroscopy measurements. These were performed using automotive metallic bipolar plates and operating conditions. Measurement data were used to manufacture an optimized segmented cathode electrode. We were able to show that our results are transferable from a small scale hardware to automotive application and that an ionomer EW segmentation of the cathode leads to performance improvement in a broad spectrum of operating conditions. Furthermore, we confirmed our results by using in-situ electrochemical impedance spectroscopy.

In-plane structuring of proton exchange membrane fuel cell cathodes: Effect of ionomer equivalent weight structuring on performance and current density distribution

Abstract Within the active area of a fuel cell inhomogeneous operating conditions occur, however, state of the art electrodes are homogenous over the complete active area. This study uses current density distribution measurements to analyze which ionomer equivalent weight (EW) shows locally the highest current densities. With this information a segmented cathode electrode is manufactured by decal transfer. The segmented electrode shows better performance especially at high current densities compared to homogenous electrodes. Furthermore this segmented catalyst coated membrane (CCM) performs optimal in wet as well as dry conditions, both operating conditions arise in automotive fuel cell applications. Thus, cathode electrodes with an optimized ionomer EW distribution might have a significant impact on future automotive fuel cell development.

Measurements of current density distribution in shaped e-beam writers

In this paper, we present four measurement methods for the two-dimensional mapping of the current density distribution in variable-shaped e-beam writers. All the methods use only the native equipment of the e-beam writer which provides the advantage of relatively easy implementation. The first and second measurement methods are based on the knife-edge technique in which the e-beam is scanned in two directions across a Faraday cup opening and across shaping shutters of the forming system, respectively. In the third method, a phosphor screen is irradiated by a magnified e-beam spot and the current density distribution is represented by the luminescence image on the screen. The fourth method uses electron sensitive resist which is exposed by the e-beam. The developed resist is observed under a microscope and the current density distribution is judged from the resist profile. All the methods are demonstrated in an example, compared, and their spatial resolution and accuracy evaluated.

Current Density Distribution in the Lateral Direction of Conventional and Interdigitated Flow Field

The design of flow field significantly affects the mass transport in an operating PEMFC. One example of such design is interdigitated flow field. In this kind of flow field depending on the location both convective and diffusive mass transfer occur in the land-channel direction. These two modes of mass transfer influence the local current and water generation in the land-channel direction. To understand the difference between these two mass transport mechanism the comparison of local current density and ohmic resistance distribution in the land-channel direction of conventional and interdigitated flow fields is imperative. This work is focused on measurement of current density and ohmic resistance distribution in the land-channel direction of a conventional and an interdigitated flow fields. A customized catalyst coated membrane of active area 9 mm2 was prepared and a segmented anode was employed to measure the parameters mentioned at 350 µm resolution in the land-channel direction of an interdigitated flow field. At 60 oC and 90 % RH current density and ohmic resistance distribution were measured for both kind of flow fields, and discussion is presented.

Anode Current Density Distribution Measurements for Different Vacuum Arc Modes Subjected to Axial Magnetic Field

We report our measurements of anode current density distributions for different modes in vacuum arc discharge using a split-anode and a cup-shaped axial magnetic field cathode configuration system. The modes investigated included the diffuse arc and anode spot. The anode surface was divided into four regions: one central region and three symmetrical-disposed peripheral annular-arc regions. Three different-sized central regions were selected with diameters of 12, 18, and 20 mm. The contact material was CuCr25 (25% Cr). The arc current in the tests ranged from 6 to 14 kA (rms) at 50 Hz. The opening velocities were 1.8 and 2.4 m/s, respectively. The currents of the four areas on the anode contact were measured by four Rogowski coils situated outside the vacuum chamber. The experimental results show the current density distribution in different arc modes. For the diffuse arc mode with arc currents of 6 and 8 kA (rms), the average current density in the central region at current peak ranged from 8.5 to 15.9 A/mm $^{2}$ . The anode spot current density ranged from 31.9 to 37.7 A/mm $^{2}$ , estimated from the current density in the eroded region caused by the anode spots covering the entire central region of diameters 12 and 18 mm, at currents of 12 and 14 kA, respectively, regarded as the lower limit current density of the anode spot. Moreover, the dependence of the current density distribution on opening velocity in diffuse arcs was slight. For the anode spots, the current density distribution with higher opening velocity was more concentrated.

Experimental and Numerical Characterization of Corona Discharge Generated by a Triode Electrode System

The triode electrode system is frequently employed for accurately controlling the corona charging of insulating materials. The primarily aim of this paper is to characterize this electrode system by current–voltage and current density distribution measurements. The electrostatic aspect of the electric field generated by the triode electrode system is also numerically analyzed using a charge simulation program. The ionizing element of the electrode system is a thin tungsten wire (diameter: 200 $\mu\hbox{m} $ ) 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 grounded plate electrode (aluminum, 250 mm $\times$ 130 mm) . The interelectrode spacing has a very significant influence on the current–voltage characteristics of the triode system and on the extension of the corona discharge generated by it. The experimental results obtained for different geometries of the electrode system are interpreted in the light of the numerical modeling of the electric field, in order to make some recommendations regarding its design: reduce/increase the wire-grid distance in order to diminish the operating high voltage/expend the surface affected by the corona discharge.

Analysis of the Correlation Between Local Temperature and Observed Degradations during Durability PEMFC Tests for Automotive Application

In Proton Exchange Membrane Fuel Cells, local temperature is a driving force for many degradation mechanisms such as hydrothermal deformation, creep, platinum dissolution, and bipolar plates corrosion [1, 2]. In order to investigate and quantify those effects, durability testing in automotive-related operating conditions is conducted in this work. In particular, two ageing tests have been conducted: a stationary test over 2000 h under nominal operating conditions and a load/thermal cycling test (1500 cycles) which corresponds to peaks in power demand. During the ageing tests, punctual characterizations like polarization curves, Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS) and Fluoride Release Rate (FRR) are performed in order to investigate respectively the overpotentials (and global performance), the electrochemical surface area, the hydrogen crossover though the membrane, the transfer resistances and the membrane chemical degradation. Continuous in situ measurements of current density distribution and local temperature are made with a printed circuit board (Current Scan Lin S++ device [3]) inserted in the middle of 30 cells stacks. At the end of life, post-mortem analysis of the aged components via optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS) are conducted in order to investigate their microstructure and chemical composition. Infrared Imaging (IR) is used to study the local permeation in the aged membrane. In order to study the correlation between the observed degradation and the local conditions inside the PEMFC (temperature, relative humidity, molar fraction of hydrogen and oxygen, water fluxes…), a pseudo-3D physic based model has been developed at the cell scale (220 cm²) and validated against the in situ experimental data of the printed circuit board. Given the aspect ratio of the different cell components, the idea of the pseudo-3D approach is to consider each component as a plane layer, which is coupled to the other components through appropriate exchange conditions. With this approach, it is thus possible to compute the 2D (in-plane) distribution of the different physical parameters (temperature, species concentration…) in each cell component (active layers, gas diffusion layers, membrane, bipolar plates, cooling circuit…) at a reasonable cost. The model takes into account the thermal effects, including the exchanges with the cooling circuit, species transport and the electrochemical behavior of the electrodes. The model is implemented in the software Comsol Multiphysics. The simulation results highlight different zones where temperature and linked parameters, such as humidity, have extreme values causing notably lower local performance and increasing risks of water flooding. The post-mortem analysis mainly point out heterogeneous bipolar plates and membrane degradations that are different in the anode and cathode sides both at the cell and at the channel/rib scale but quite reproducible along the stack (See Figure 1). The correlation of the simulations with the experimental results highlight the effects of local temperature and gases humidity on in-situ performance losses and ex-situ observed degradations from the cell scale to the channel/rib scale. Furthermore, the impact of dynamic thermal cycles on degradation acceleration is quantified. [1] P. Pei et al. Main factors affecting the lifetime of Proton Exchange Membrane fuel cells in vehicle applications: A review, Applied Energy 125 (2014) 60–75 [2] S.F. Burlatsky et al. A mathematical model for predicting the life of polymer electrolyte fuel cell membranes subjected to hydration cycling, Journal of Power Sources 215 (2012) 135 - 144 [3] S++ Simulation Services, Waldstraße 5, 82418 Murnau-Westried, Germany. www.splusplus.com Figure 1