Local Current Distribution Research Articles

Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm−2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.

Three-terminal spin/charge current router

Topological insulator materials have wide applications in electronic and spintronic devices by utilizing the protected edge states. In this paper, based on these topological edge states and energy gaps, we propose some types of spin/charge current router in a three-terminal system consisting of silicene-like nanoribbons (SiNRs). The current is well controlled by the helical edge states of zigzag SiNRs (ZSiNRs) and external fields. Using the tight-binding model and non-equilibrium Green’s function theory, we investigate three types of such router. The first type is a spin current shunter which separates the spin-up and spin-down current into different leads. The second type is a spin filter which separates the spin-polarized electrons into one of those leads. The last type is a charge current switcher which switches the charge current from one lead to the other lead. The local current distribution is calculated for the specific electron path. We find that the small Rashba does not destroy the filtering properties of the system. Besides, as an example, we employ the Landauer–Büttiker formula to obtain the current–voltage curves of the first type router and investigate the transmittance reciprocity relations in such a three-terminal system. We believe these proposed spin/charge current routers, which can separate the specific current into the expected lead, have potential applications in the future spintronics designs.

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.

Topological spin–valley filtering effects based on hybrid silicene-like nanoribbons

Topological edge states have crucial applications in nano spintronics and valleytronics devices, while topological inner-edge states have seldom been extensively researched in this field. Based on the inner-edge states of the hybridized zigzag silicene-like nanoribbons, we investigate their transport properties. We propose two types of spin–valley filters. The first type can generate two different spin–valley polarized currents in output leads, respectively. The second type outputs the specific spin–valley polarized current in only one of the output leads. All these inner-edge states have the spin–valley-momentum locking property. These types of filters can switch the output spin–valley polarizations by modulating the external fields. Besides, we also find that the device size plays a crucial role in designing these spin–valley filters. Moreover, the local current distributions are calculated to visualize the detailed transport process and understand the mechanism. The mechanism lies that the spin–valley polarized current can nearly freely pass through the system with the same momentum, spin and valley degrees of freedom. The small reflection of the current results from the inter-valley scattering. In particular, we also consider the realistic (disorder) effects on the performance of these filters to ensure the robustness of our systems. We believe these spin–valley current filtering effects have potential applications in the future spintronics and valleytronics device designs.

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].

Parametric study on the local current distribution of polymer electrolyte membrane fuel cell with counter flow channel under pressurized condition

Local measurement of current was conducted using a segmented cell to understand local electrochemical characteristics of polymer electrolyte membrane fuel cell with counter flow channel under pressurized condition. We changed stoichiometry ratio (SR), relative humidity (RH), and operating temperature to examine the effect of operating parameters on the performance of fuel cell under pressurized condition. A segmented fuel cell with a divided active area of 25 cm2 was used to measure local current distribution to analyze complex relationships between SR, RH, temperature and pressure. The results show that RH and SR are the main factors for determining current distribution. High SR with low-RH reactants make the membrane of the inlet region more dried out, which leads to less electrochemical reaction. When SR is low, however, less water is evaporated and the effect of RH condition is minimized. On the other hand, current distribution at various RH and SR conditions at 3 bar is relatively similar among the results. Thus, the effects of RH and SR can be neglected for high operating pressure cases. When the temperature is high in pressurized condition, the Ohmic loss decreased and the overall cell performance improved.

An Overview of Major Experimental Methods and Apparatus for Measuring and Investigating Erosion-Corrosion of Ferrous-Based Steels

This paper firstly provides an overview of major efforts made over the past several decades on developing the test apparatus for the study of erosion-corrosion of ferrous-based steels. The merits and the application range of the set-ups, i.e., rotating disc/cylinder electrode system, slurry jet impingement rig, and test flow loop, are introduced. Secondly, the advantages and limitations of both non-electrochemical and electrochemical methods used for the measurement of erosion-corrosion are presented. Thereafter, the interaction between erosion and corrosion using a cyclic erosion-corrosion method was introduced. The synergy between erosion and corrosion for the steels under the passive state were analyzed on the basis of the breakdown and restoration of the passive film. In addition, the difference between the obtained erosion-enhanced corrosion and the corrosion-enhanced erosion using different experimental methods for steels under active corrosion was discussed. Finally, the interaction between the localized corrosion and erosion was studied through the dynamic change of the local current distribution revealed by the wire beam electrode.

Spatially Resolved Analysis of an Organic Alkaline RFB to Investigate the Influence of the Operating Conditions

Designing competitive and reliable Redox Flow Batteries (RFBs) requires the development of specific analytical tools at lab-scale. Specifically, the influence of the cycling conditions on the system performance must be well identified. An instrumented test bench dedicated to the tuning of the operating parameters and the monitoring of the battery internal behavior was developed. The use of a segmented cell with a 1D-like design allowed to focus on the local current distribution along the electrolyte flow. The study started by examining the performances of a RFB coupling the ferri/ferrocyanide couple (Fe(CN)63−/ Fe(CN)64−) to the anthraquinone Alizarin Red S, over 300 cycles. Once cycling in standard conditions was reviewed, a parameter study was performed to scan a varied range of operating points, by changing the current density, the flow rate and the temperature successively. Thanks to the spatially-resolved analysis of the cell, the global performance could be related to local processes. This investigation gave unprecedented insight of the internal operation of a RFB, with peculiar features appearing at the start of the charge and the discharge. It was shown that a same global response of the cell could actually hide very different internal operations.

Modeling of Fuel Cell Cold Start and Dimension Reduction Simplification Method

Sub-zero startup ability remains a key barrier for commercial application of polymer electrolyte fuel cells (PEMFC), especially for automotive applications. In order to improve the startup ability and durability of fuel cells, understanding of the characteristics and mechanisms of cold start is essential, and here modeling of fuel cell cold start plays an important role. In this study, a one-dimensional model is developed to simulate the fuel cell cold start. The model includes mass transport and phase change, heat transfer and electrochemical reaction. Key features such as membrane water and local current distributions are analyzed. Based on the one-dimensional model and simulation results, a spatial reduced simplified model is developed that distinguished only n states across the cell. The simplified model inherits the key features of the one-dimensional model, while the computational cost is significantly reduced to 10% (from 216 s to 20.88 s). The one-dimensional model and simplified model are both validated by the cold start experiment and the voltage error and temperature error are within 15% and 1.2 K respectively. Thus, the proposed simplified model could be used in dynamic simulation and in further multi-scale modeling study to build a stack model.

Fast Analysis of Local Current Distribution for Electromagnetic Scattering Problems of Electrically Large Objects

To improve computational efficiency of obtaining local induced current distribution under multiple incidents angles when electromagnetic scattering problems of electrically large objects are considered, a novel method is proposed to form small-scale impedance matrix and reduce number of repeated calculation for different incident angles, which is implemented by following three steps. Firstly, an undetermined matrix equation including multiple incident angles is constructed by interrelating part field nodes and all source nodes on object surface. Secondly, the undetermined matrix equation is transformed into a form that conforms to the compressive sensing (CS) framework, which is solved by reconstructed algorithm to obtain approximate induced currents of all source nodes under a wide incident angle range. Finally, exact wide-angle currents of a given node are obtained by calculating total magnetic field, and neighborhood wide-angle currents of the given node can be expanded by choosing appropriate technology in different boundary conditions. Numerical results of electrically large objects shown the efficiency of the proposed method.

Study on the performance of proton exchange membrane fuel cell (PEMFC) with dead-ended anode in gravity environment

The accumulation of water on the anode side will have an important impact on the performance of proton exchange membrane fuel cell (PEMFC) with dead-ended anode (DEA). In order to find out the accumulation and distribution characteristics of water in the flow channel and its influence on the performance of PEMFC with DEA, the transparent fuel cell technology and printed circuit board (PCB) segmented measurement technology are used to study the change of water distribution characteristics caused by the placement orientation of the flow channel in gravity environment, and the influence of this change on the performance of PEMFC with DEA is also studied. The experimental results show that the placement orientation of the flow channel has a significant effect on the transient performance of the PEMFC with DEA. When the orientation of the flow channel is changed, not only the accumulated content and distribution state of water in the anode flow channel are significantly affected, but also the working time of the PEMFC with DEA is affected. The change of internal resistance is also one of the important reasons that affect the transient performance of fuel cell, but the accumulation of water in anode flow channel is not the main reason for the performance degradation. In addition, changing the placement orientation of the flow channel will affect the uniformity of the local current density distribution, and when the flow channel is placed in the case of (d), the local current density distribution is the most uniform.

In situ mapping of activity distribution and oxygen evolution reaction in vanadium flow batteries

Understanding spatial distribution difference and reaction kinetics of the electrode is vital for enhancing the electrochemical reaction efficiency. Here, we report a total internal reflection imaging sensor without background current interference to map local current distribution of the electrode in a vanadium redox flow battery during cyclic voltammetry (CV), enabling mapping of the activity and reversibility distribution with the spatial resolution of a single fiber. Three graphite felts with different activity are compared to verify its feasibility. In long-term cyclic voltammetry, the oxygen evolution reaction is proved to enhance activity distribution, and homogeneity of the electrode and its bubble kinetics with periodic fluctuation is consistent with the cyclic voltammetry curve, enabling the onset oxygen evolution/reduction potential determination. Higher activity and irreversibility distribution of the electrode is found in favor of the oxygen evolution reaction. This sensor has potential to detect in situ, among other processes, electrochemical reactions in flow batteries, water splitting, electrocatalysis and electrochemical corrosion.

Conductive AFM study of the local current in thin ferroelectric sol-gel PZT films

The local current distribution across the grains and their boundaries in ferroelectric PZT films are studied using conductive AFM. The films were formed with various grain structures and different lead excess content by chemical solution deposition. C-AFM measurements have shown the influence of the lead excess and seed sublayer crystallization conditions on the grain-boundary conductivity. PZT films with fixed 0-15 wt% Pb excess demonstrate non-conductive grain boundaries, whereas in films with fixed 30 wt% Pb excess the grain-boundary conductivity is found to be much higher than that of the grains themselves. Conductive grain-boundaries was also found in PZT films without lead excess in crystallized sublayer. A study of the transient current at varied preliminary polarization revealed the current peaks in the current-voltage curves. The appearance conditions and magnitude of these peaks depend on the grain-boundary conductivity. The correlation between the grain-boundary conduction and the current polarization dependences is confirmed.

Nondestructive real-space imaging of energy dissipation distributions in randomly networked conductive nanomaterials

For realization the new functional materials and devices by conductive nanomaterials, how to control and realize the optimum network structures are import point for fundamental, applied and industrial science. In this manuscript, the nondestructive real-space imaging technique has been studied with the lock-in thermal scope via Joule heating caused by ac bias conditions. By this dynamical method, a few micrometer scale energy dissipations originating from local current density and resistance distributions are visualized in a few tens of minutes due to the frequency-space separation and the strong temperature damping of conductive heat components. Moreover, in the tensile test, the sample broken points were completely corresponding to the intensity images of lock-in thermography. These results indicated that the lock-in thermography is a powerful tool for inspecting the intrinsic network structures, which are difficult to observe by conventional imaging methods.

Topological Anderson insulator phase in a quasicrystal lattice

Motivated by the recent experimental realization of the topological Anderson insulator and research interest on the topological quasicrystal lattices, we investigate the effects of disorder on topological properties of a two-dimensional Penrose-type quasicrystal lattice that supports the quantum spin Hall insulator (QSHI) and normal insulator (NI) phases in the clean limit. It is shown that the helical edge state of the QSHI phase is robust against weak disorder. Most saliently, it is found that disorder can induce a phase transition from NI to QSHI phase in the quasicrystal system. The numerical results based on a two-terminal device show that a quantized conductance plateau can arise inside the energy gap of NI phase for moderate Anderson disorder strength. Further, it is confirmed that the local current distributions of the disorder-induced quantized conductance plateau are located on the two edges of sample. Finally, we identify this disorder-induced phase as topological Anderson insulator phase by computing the disorder-averaged spin Bott index.

Power Losses and Current Distribution Studies by Infrared Thermal Imaging in Soft- and Hard-Switched IGBTs Under Resonant Load

A test bench is proposed to study, at die-level, the power losses and current distribution in power devices. It is based on an infrared camera and a flexible half-bridge resonant inverter with a tunable resonance frequency f res . With this setup, the die surface temperature is acquired in steady state, while the device is under real operation. The power losses are derived from the temperature mean value averaged, first, over a few switching cycles and, then, across the die surface. By contrast, the current distribution is inferred from the spectral component of the surface thermal map at the switching frequency (f Sw ). As a proof of concept, two case studies are reported considering 650 V-40 A insulated gate bipolar transistors (IGBTs) soft- and hard-switched within and outside the zero voltage switching condition. First, the power losses are analyzed under switching conditions representative of domestic induction heating applications (f res = 29.6 kHz) at f Sw = 40 kHz and f Sw = 20 kHz. Second, the power losses and local current distribution are investigated when f res = 9.25 kHz at f Sw ranged from 8.91 to 9.51 kHz. Such results are assessed with power losses electrical measurements and simulations, obtaining a satisfactory agreement. Moreover, hot spots are identified as current crowding points at f Sw , whose location is fixed by the bonding wires attachment to the die and the device edge termination. As main benefits of this technique, a higher spatial resolution is achieved and problems related with noisy electrical measurements resulting from the insertion of the used probes or transducers, power circuit stray elements, or device packaging parasitics are avoided.

Ultra-wide temperature electronic synapses based on self-rectifying ferroelectric memristors

Memristors have been intensively studied in recent years as promising building blocks for next-generation nonvolatile memory, artificial neural networks and brain-inspired computing systems. However, most memristors cannot simultaneously function in extremely low and high temperatures, limiting their use for many harsh environment applications. Here, we demonstrate that the memristors based on high-Curie temperature ferroelectrics can resolve these issues. Excellent synaptic learning and memory functions can be achieved in BiFeO3 (BFO)-based ferroelectric memristors in an ultra-wide temperature range. Correlation between electronic transport and ferroelectric properties is established by the coincidence of resistance and ferroelectricity switch and the direct visualization of local current and domain distributions. The interfacial barrier modification by the reversal of ferroelectric polarization leads to a robust resistance switching behavior. Various synaptic functions including long-term potentiation/depression, consecutive potentiation/depression and spike-timing dependent plasticity have been realized in the BFO ferroelectric memristors over an extremely wide temperature range of −170 °C ∼ 300 °C, which even can be extended to 500 °C due to the robust ferroelectricity of BFO at high temperatures. Our findings illustrate that the BFO ferroelectric memristors are promising candidates for ultra-wide temperature electronic synapse in extreme or harsh environments.

Modeling of Industrial Electroplating Processes with Comsol Multiphysics in Order to Optimize Treatment of Complex Parts

Electroplating processes are widely used on several aeronautical applications to confer functional properties (electrical conductivity, tribological, anti-corrosion). In this industrial context, it is possible to keep the desired properties by respecting specifications such as thicknesses and coating composition for alloys. However, it is well-known that phenomena intrinsic to electrochemical processes create heterogeneities, especially on complex geometries like landing gears. Industrially, the treatment of these disparities is part of the know-how of the workshops, and they are balanced by complex auxiliary tools in order to obtain a more uniform local current densities distribution, and therefore coating thickness. The development of these specific anode shapes is time-consuming and depends strongly of an empirical know-how. The numerical simulation of electrochemical processes is developed at Safran Tech in collaboration with Institute Utinam to optimize the development of plating tools at industrial scale with ComsolÒ Multiphysic software. The global objective is to improve the reliability of the existing systems, and moreover to enhance the innovation by facilitating new coatings integration. This is crucial in the context of tightening of environmental REACH regulations, such as the substitution of cadmium by zinc-nickel. Modeling industrial electroplating processes by a secondary current distribution model is evaluated in terms of thickness prediction by a comparison with experimental results at a laboratory scale pilot. It has been emphasized that the main limitation are due to the failure to take full account of mass transport induced by convection. The study also participate to the development of good practice for auxiliary tools design, and the simulation demonstrate invaluable assistance in industrial operations. Figure 1

Understanding the Interplay between Electrolyte Velocity Distribution and Current Distribution in Vanadium Flow Battery Electrode

Vanadium redox flow batteries (VRFBs) are considered a promising candidate among grid-scale energy storage technologies due to their flexibility and scalability, high coulombic efficiency, and long cycle life. However, system costs and relatively low energy and power density compared to other electrochemical energy storage devices are major challenges to commercialization. At the cell level, mass transport losses are one of the major contributors to performance losses; this mechanism is associated with inadequate delivery of active species to electrode surfaces. Improving flow field design and electrode structure or simply increasing flow rate can mitigate mass transport losses. However, due to parasitic pumping losses, optimization is not straightforward. As yet unpublished research in our lab has shown that increased convection improves mass transport of the active species through the porous electrode and it is possible to achieve better electrochemical performance with lower pressure drop in convection-dominated VRFB cells. In this study, the impact of convection on electrochemical performance is investigated using computational fluid dynamics (CFD) model and in-situ localized current distribution diagnostics. Polarization curve analysis and pressure drop measurements are also used to support the conclusions. Experiments and simulations are conducted on strip cell architecture with varying channel depths (0.25, 0.50, 1.00, 2.50 mm) and flow rates (10-50 ml min-1). The simulation domain includes both channel and porous electrodes seen in Figure 1. Conservation of mass and momentum are solved to obtain velocity and pressure distributions. It is found that there is a strong relationship between electrolyte velocity in the electrode and electrochemical performance. Similar electrolyte velocity distribution (in the electrode) predicted for two configurations: 1 mm channel depth at 40 mL min-1 flow rate and 0.50 mm channel depth at 10 mL min-1 have similar polarization curves and current distributions down the channel. However, the model predicts very different velocity distributions for these configurations in channel. Higher velocity distribution predicted for 1 mm configuration has also higher pressure drop than 0.5 mm configuration. The results of the computational and experimental study as related to the trade-off between pumping power requirements and electrochemical performance in VRFBs will be discussed in this talk. Figure 1

Three-dimensional modeling of a PEMFC with serpentine flow field incorporating the impacts of electrode inhomogeneous compression deformation

The effects of compression deformation of gas diffusion layer (GDL) on the performance of a proton exchange membrane fuel cell (PEMFC) with serpentine flow field were numerically investigated by coupling two-dimensional GDL mechanical deformation model based on Finite Element Analysis and three-dimensional two-phase PEMFC model with incorporating the deformation impacts. Emphasis is located on exploring the influences of assembly pressure on the non-uniform geometric deformation and distributions of transport properties in the GDL, flow behaviors and local distributions of oxygen and current density, cell polarization curves and net power densities of the PEMFC. It was indicated that the non-uniform deformation of GDL results in inhomogeneous distributions of porosity and permeability in the GDL due to the presence of rib-channel pattern, and the transport properties in the under-rib region are greatly reduced with increasing the assembly pressure, consequently weakening the gas flow and oxygen transport in the under-rib region and increasing the non-uniformity of local current density distribution. As for the overall cell performance, however, attributed to the tradeoff between the adverse impacts of GDL compression on mass transport loss and positive effects on reducing ohmic loss, the overall cell performance is firstly increased and then decreased with increasing assembly pressure from 0 MPa to 5.0 MPa, and the maximum cell performance can be achieved at the assembly pressure of about 1.0 MPa for all cases studied. As compared with the case for zero assembly pressure, the maximum net power density of the cell can be improved by about 7.7%, 9.9%, 10.5% and 10.7% for the cathode stoichiometry ratios of 2.0, 3.0, 4.0 and 5.0@iref = 1 A·cm−2, respectively. Practically, it is suggested that the assembly pressure is controlled in an appropriate range of 0.5 MPa–1.5 MPa such that the cell net power can be boosted and pressure head requirement for the pump can be maintained in a appropriate level.