Local Current Distribution Research Articles

Characterization methodology for anode starvation in HT-PEM fuel cells

Degradation caused by fuel starvation may be an important reason for limited fuel cell lifetimes. In this work, we present an analytical characterization of the high temperature polymer exchange membrane fuel cell (HT-PEM FC) behavior under cycled anode starvation and subsequent regeneration conditions to investigate the impact of degradation due to H2 starvation. Two membrane electrode assemblies (MEAs) with an active area of 21 cm2 were operated of up to 550 min, which included up to 14 starvation/regeneration cycles. Overall cell voltage as well as current density distribution (S++ unit) were measured simultaneously each minute during FC operation. The cyclicity of experiments was used to check the long term durability of the HT-PEM FC. After FC operation, micro-computed tomography (μ-CT) was applied to evaluate the influence of starvation on anode and cathode catalyst layer thicknesses.During starvation, cell voltage and current density distribution over the active area of the MEA significantly differed from nominal conditions. A significant drop in cell voltage from 0.6 to 0.1 V occurred after approx. 20 min for the first starvation step, and after 10 min for all subsequent starvation steps. By contrast, the voltage response is immediately stable at 0.6 V during every regeneration step. During each starvation, the local current density reached up to 0.3 A∙point−1 at the area near the gas inlet (9 cm2) while near the outlet it drops to 0.01 A∙point−1. The deviation from a balanced current density distribution occurred after 10 min for the first starvation step, and after ca. 2 min for the subsequent starvation steps. Hence, compared to the voltage drop, the deviation from a balanced current density distribution always starts earlier. This indicates that the local current density distribution is more sensitive to local changes in the MEA than overal cell voltage drop. This finding may help to prevent undesirable influences of the starvation process.The μ-CT images showed that H2 starvation lead to thickness decrease of ca. 20–30% in both anode and cathode catalyst layers compared to a fresh MEA. Despite of the 14 starvation steps and the thinning of the catalyst layers the MEA presents stable cell voltage during regeneration.

Development and application of flexible integrated microsensor as real-time monitoring tool in proton exchange membrane water electrolyzer

The proton exchange membrane (PEM) water electrolyzer has such advantages as simple system, low operating temperature and small-scale hydrogen production according to real time requirement, and the hydrogen production process is clean, meeting the environmental requirements. The PEM water electrolysis hydrogen production is the reverse reaction of fuel cell, but the water electrolysis requires high operating voltage, the resistance is likely to generate a lot of waste heat, and the nonuniform current density results in hot spots, the internal temperature rises, accelerating the decomposition of hydrogen molecules, the water electrolyzer is likely to age and fail. In addition, four important physical parameters (temperature, flow, voltage and current) in the running water electrolyzer can influence its performance and life, but the present bottleneck is external, theoretical, simulated or single measurement, the authentic information in the water electrolyzer cannot be obtained accurately and instantly. This study uses micro-electro-mechanical systems (MEMS) technology to develop a flexible integrated (temperature, flow, voltage and current) microsensor applicable to the high voltage and electrochemical environment in water electrolyzer, which is integrated with a 20 μm thick polyimide (PI) film material. The real-time microscopic diagnosis and measurement in the PEM water electrolyzer can measure the internal local temperature, voltage, current and flow distribution uniformity instantly and accurately, so as to optimize the operating conditions and analysis.

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

Visualized cell characteristics by a two-dimensional model of vanadium redox flow battery with interdigitated channel and thin active electrode

A two-dimensional and steady-state model of the vanadium redox flow battery with an interdigitated channel and a thin active electrode was developed to visualize the flow velocity, concentrations of the reactants, and local current distribution in the through-plane direction of the electrode sheets. The concentration overpotential was increased in the low flow speed region. The low mass transport coefficient by a slow flow speed resulted in a higher concentration overpotential. The mass flux distributions at the state of charge of 20% were strongly dependent on the flow speeds due to low bulk concentration of the reactants. Based on the visualized data, not only the reactivity improvement of the electrode material but also the advanced design of the porous electrode structure for a high flow permeability of the electrolyte solution are essential to improve the cell voltage during discharge.

A Quasi-2D Transient Multiphase Modeling of Cold Start Processes in Proton Exchange Membrane Fuel Cell

<div class="section abstract"><div class="htmlview paragraph">It’s well known that startup process of proton exchange membrane fuel cells (PEMFCs) under subzero temperature is extremely significant because of its influence on fuel cell performance and durability. In the study, a quasi-2D numerical model is developed and dynamic equations of mass conservation, energy conservation, membrane water conservation, ice conservation, species conservation are all considered. Three different hydrogen supply modes are studied in detail: flow-through anode (FTA) mode, dead-ended anode (DEA) mode and off-gas recirculation (OR) mode. It is found that the local current density (LCD) and temperature distribution vary remarkably along flow channel in OR mode as t > 500s due to nitrogen crossover and accumulation. During the cold start operation, the DEA mode and OR mode hold more water in anode catalyst layer (ACL) which reduces the effects of hydraulic permeation, resulting in more ice formation in cathode catalyst layer (CCL) and slower temperature rising. In term of hydrogen utilization ratio, the efficiencies in DEA mode and OR mode are close and much better than that in FTA mode. Compared with DEA mode and FTA mode, characteristics of OR mode can be summarized as the highest operating efficiency, almost the best output performance and the second shortest startup time.</div></div>

Calculation of the local current density in high-temperature superconducting insulated rare earth–barium–copper oxide coils using a volume integral formulation and its contribution to coil protection

High-temperature superconducting materials have remarkable current-carrying capabilities, even when operated under high magnetic fields. Since long rare earth–barium–copper oxide (REBCO)-coated conductors are now available (thanks to improvements in the fabrication process) this material has become an attractive option for high field magnet applications. However, such extreme operating conditions require an efficient quench protection system to prevent the coil from developing damaging hot spots, which greatly depends on the winding used. We focus here on the protection of insulated HTS coils against thermal runaways that can locally destroy the magnet. We developed a transient two-dimensional (2D) axisymmetric model using a volume integral formulation based on generalization of the partial element equivalent circuit method to compute the local current density distribution inside REBCO-insulated coils and account for local performance variations. Indeed, the most interesting property of integral methods is the requirement that only active regions are meshed, which leads to a significant reduction in the size of the problem. The formulation is introduced for general 3D cases and its adaptation to 2D axisymmetric problems is detailed. The formulation has been validated thanks to a bulk magnetization benchmark, the results of which (obtained with the finite element method) were compared with our integral formulation solution. The model has also been compared with experimental data obtained on a double pancake coil. The objective is to study the effects of magnetization on the transient voltage due to dynamic current distribution when ramping up the magnet so as to be able to determine some key parameters associated with coil protection. Such an approach is developed on a small-scale test case and the transient behaviours observed are discussed.

Reduced graphene oxide aerogel as stable host for dendrite-free sodium metal anode

Sodium (Na) metal has attracted great attention as a promising anode for next-generation energy storage systems because of its abundant resources, potentially low cost, and high theoretical capacity. However, severe dendrite growth, large volume expansion during plating/stripping lead to poor cycle performance and limit the practical application of sodium metal anodes. Here, oriented freeze-drying and molten infusion are used to synthesize a Na-infused reduced graphene oxide aerogel (Na@rGa) composite anode using a reduced graphene oxide aerogel (rGa) as a stable host. This unique host shows ultra-light weight which guarantees high capacity (1064 mAh g−1), and the large specific surface area and uniform pores effectively lower the local current density and uniform electrolyte distribution from the inner to the outside of electrode. The Na@rGa anode exhibits low overpotential (50 mV) and stable cycle performance for 1000 cycles at 5 mA cm−2 in carbonate electrolyte system. The electrochemical performance of a full cell using the Na@rGa composite anode is clearly superior to that of a reference cell. This work provides a new horizon for construction of three-dimensional stable hosts and safe sodium metal anodes.

On the distribution of local current density along a PEM fuel cell cathode channel

We report analysis of a recent model for local current density distribution along the air channel in a PEM fuel cell. Analytical solution for the case of small cell ohmic resistivity is obtained. An algorithm for numerical solution of the model equations is developed for practically important case when the cell polarization curve and ohmic resistivity are known. Good agreement between the calculated and experimental shapes of the local current density is demonstrated. Cell ohmic resisitivty greatly homogenizes the distribution of local current. A Python code for local current calculation is available for download.

Local Reactions in Bifunctional Air Electrodes for Aqueous Metal-Air Secondary Batteries

Aqueous metal-air secondary batteries, such as zinc-air secondary batteries, are attractive power sources for large scale energy storage systems since these battery systems potentially satisfy high energy density, high safety standard and low cost. However, large overpotential in bifunctional air electrodes hinders practical applications of the systems.1 In order to reduce the overpotential, plenty of effective reaction sites is necessary as well as highly active bifunctional electrocatalysts. Though it is widely known that relatively many reaction sites are formed in porous gas diffusion electrodes composed of hydrophobic gas diffusion layer and mildly hydrophilic catalyst layer, the overpotentials in the porous gas diffusion electrodes are not low enough for the practical application. Further reduction of the overpotential in the porous gas diffusion electrodes requires not only empirical knowledge but also construction guidelines based on fundamental properties of the reaction sites. However, the microstructures of the porous gas diffusion electrodes are too complicated to be analyzed in detail. In this work, we use a partially immersed electrode system as a model of the porous gas diffusion electrodes to investigate the fundamental properties of the reaction sites. This model system consists of a smooth planer or cylindrical electrode and an electrolyte solution covering the electrode.2-4 Owing to the simple structure of this system, mass transports of the ions and the gases can be easily simulated. In addition, we applied a platinum segmented electrode (Fig. 1(a)) and homemade multichannel current-voltage converter to the partially immersed electrode system to measure current distribution on the electrode directly. First, we measured AC impedance between adjacent segments to characterize electrolyte solution covering the electrode. While blocking electrode behavior was observed at CHs 1-3, transmission line-type frequency dependences were observed at CHs 4-10. This indicates that CHs 1-3 were covered with relatively thick electrolyte solution, so-called meniscus, and CHs 4-10 covered with thin liquid electrolyte film. Curve fittings indicated that ionic resistance of the film was almost same on CHs 4-10, and the thickness of the film was calculated to be around 3 μm from the conductivity of the electrolyte solution. Figure 1(b) shows local current distributions on the partially immersed platinum segmented electrode with constant current (±0.400 mA) applied for oxygen reduction reaction (ORR) and oxygen evolution reaction(OER). In the case of ORR, larger currents were observed at CHs 4-6. On the other hand, relatively lower currents were observed at CHs 4-10 for OER. These results show that electrochemically active regions were different for ORR and OER. Therefore, two different regions, hydrophobic region and hydrophilic region, should be formed in the catalyst layer of the porous gas diffusion electrodes to reduce the overpotential for both ORR and OER. Based on this finding, porous gas diffusion electrodes are prepared and the electrochemical properties will be presented at the meeting.

High Speed Video Investigation of Bubble Dynamics and Current Density Distributions in Membraneless Electrolyzers

Gas evolving electrodes are found in many electrochemical systems, such as water electrolyzers, where the dynamics of multiphase flow at these electrodes can strongly impact cell performance. This is especially true for membraneless water electrolyzers that have emerged in recent years as a promising approach to low cost hydrogen production. However, experimental and modeling efforts to characterize the multiphase dynamics in these systems can be non-trivial due to the complexity of the coupled chemistry and physics that underlie their operation. In this study, we utilize a high speed video (HSV) camera as a non-invasive analytical tool to better understand bubble dynamics in membraneless electrolyzers and to better quantify the void fraction of gas bubbles in the region directly downstream of the mesh flow-through electrodes. By detecting and quantifying the void fraction of bubbles immediately downstream of an operating electrode, the local current density distribution can be determined along the length of the electrode. This HSV-measured void fraction distribution is in good agreement with the modeled primary current distribution. This study also highlights the ability to utilize in situ HSV analysis to monitor gas evolution efficiency yields and bubble size distributions under varying operating conditions.

Electroplating Fundamentals for Coplanarity Improvement

As the definition of 3D packaging softens to include an exhaustive array of inexpensive alternatives to TSV and interposers, adjustments to processes and materials are being made to enable more and more complex 3D integration schemes. For chip stacking in SIP integration, there are at least two planarity concerns. Overall die parallelism issues lead to uneven stacks, and plated bump to bump variability across the die creates standoff issues that could result in open circuit (a short pillar) or shorting across multiple pillars in the case that solder volume or pillar height is abnormally large. Both issues can be strongly influenced by electroplating uniformity. Both potentially also create stresses in the bonded dies that drive warp and reliability failures. This fact is creating a general demand for better coplanarity for stacked dies. Measuring uniformity of plated films has a long history starting with mechanical stylus profilometers. Normal specifications for within wafer (WIW) and within die (WID) uniformity have been refined as metrology systems stepped up to full wafer measurement (all bumps) and to allow statistical calculations using huge data sets. A plated wafer may have up to 20 million pillars. As solder bumps transitioned to copper pillars, and pitches and solder volumes decreased, the possibility of pillar height variation causing failures at assembly also increased. Devices were seldom laid out with plating uniformity in mind and as issues arose, dummy features were sometimes added to aid uniform plating. While WiW and WiD uniformity are global and local versions of the same measurement respectively, the length scale difference creates dramatically different influences on each response. At the global (wafer) scale, relatively coarse (millimeter to tens of millimeter) scale uniformity of electric field drives plating rate. Note here that plating rate variations exactly correlate to uniformity variations. Plating rate is driven by current, generally governed definitively by Faraday's Law: the amount of a substance deposited on each electrode of an electrolytic cell is directly proportional to the amount of electricity passed through the cell. The goal of chamber design in electroplating involves delivery of current in as precise a manner as necessary to create uniform deposition across the wafer. Design elements include various current distribution techniques, mass transfer, RPM, electrolyte flow control, etc. Ideally these devices allow uniform plating such that the average thickness of each die is the same, or that a similarly arranged feature on each die is the same height. If these factors are well controlled, they have little effect on the final WiD uniformity (where length scales vary across tens to hundreds of microns), and all dies across the wafer will have a similar plated thickness variation. This “residual” or intrinsic WID uniformity is governed by a variety of factors including current density, pattern geometry, bath conductivity, bath temperature, and functionality of bath additives. Ultimately, these factors govern where metal ions at the plating interface relinquish their autonomy to become part of a stable plated metal film. The combination of these factors determines the Wagner number, a quantity that describes the local current distribution uniformity. Like the term “throwing power” used in parts plating applications, a higher Wagner number indicates better plated thickness uniformity. Predicting, validating, and improving the intrinsic WID uniformity requires attention to the relationship between the plating chamber, the chemistry, and the pattern being plated. Die specific modelling of the plating system, including calculation of the Wagner number, allows prediction of the WID coplanarity. This information can be used as feedback to refine die layout or to optimize chemistry functionality to plating tool parameters to drive WID uniformity to the lowest possible value.

Poly(vinylidene fluoride)-based nanocomposite employing oriented Bi2S3 nanorods with double-shell structure for high dielectric performance and loss suppression

For polymer-based nanocomposites, designing the nanostructures of fillers, their distribution inside matrix as well as interfaces with the polymer is of crucial importance to achieve high dielectric performances. One-dimensional semiconductor Bi2S3 nanorods coated with homogeneous SiO2 and polydopamine (PDA) double shell layers were imported into the poly(vinylidene fluoride) (PVDF) with parallel arrangement via uniaxial stretching to form Bi2S3@SiO2@PDA/PVDF nanocomposite. The dielectric performances of oriented Bi2S3@SiO2@PDA/PVDF were studied in comparison with its counterparts employing Bi2S3 fillers without shell layers or alignment. A microcapacitor model was used to accurately estimate the dielectric constant along perpendicular direction. Coating conductive Bi2S3 nanorods with insulating SiO2@PDA double layer remarkably reduces the dielectric loss of the nanocomposite, while alignment of one-dimensional Bi2S3@SiO2@PDA nanorods endows electrical properties anisotropy, which was understood from J-V curves measured along two directions and moreover, via three-dimensional finite element analysis, unambiguously revealing the effects of aligned core-shell structured fillers on local electric field and current density distribution. This study provides a promising and facile approach for designing high performance dielectrics.

Spatial point pattern analysis of the local current distribution on the surface of multi-tip field emitters

A further development of the method for obtaining local characteristics of multi-tip field emitter using spatial point pattern analysis was presented. The field emission characteristics of nanocomposite emitter single-walled carbon nanotubes / polystyrene were calculated. Two sets of emission sites with different current load distribution were discovered. The application of the of spatial point pattern analysis allow to determine the spatial distribution of emission sites over the cathode surface. The results of the analysis confirm the cluster character of the emitting sites distribution on the cathode surface.

Numerical study and optimization of the formation and sustainment of a coaxial helicity injection spheromak

Nonlinear, numerical computation with the NIMROD code is used to explore magnetic self-organization during the formation, sustainment, and relaxation of a spheromak with coaxial helicity injection. The formation process is largely insensitive to the local current distribution in the annular current column and details of the injector current trace, instead depending on the peak injected current (specifically λinj/λeig) and the bias flux Ψbias. The injected flux Φinj needed to trigger the column mode instability, the resulting local poloidal flux amplification AΨ, the amount of injected energy retained in the plasma as magnetic energy ηM, and the magnetic helicity per unit of injected energy ηK all scale with λinj/λeig. Both Φinj and ηK scale with the bias flux Ψbias, and ηM and ηK scale inversely with the pre-fill density. Sustained relaxation occurs when the injector current is maintained between two threshold values, 0.8 < λinj/λeig < 1 for the geometry studied. Overall, our results suggest that the performance of the spheromak configuration is very robust and therefore allows for flexibility in the design and operation of a Proof-of-Principle spheromak concept.

Current and potential distribution in electrochemical reactors with activated or resistive electrodes. A multiregion and open source approach

An open source compact and general tool, implemented in OpenFOAM including a novel solver with a new boundary condition and post-processing utilities, is derived to enable calculations of local current and potential distributions in electrochemical systems with activated or resistive electrodes and also leakage currents in the case of a bipolar connection. The algorithm allows the calculations for a given local potential in any electrode, for a fixed cell potential difference and also for a current flowing through the cell under galvanostatic control. In order to validate the algorithm, a detailed comparison between the suggested strategy with experimental results and some simplified theoretical models is made. It was concluded that the proposed mathematical treatment is reliable for the modelling of these electrochemical systems due to the good agreement between theoretical and experimental values with a mean relative percent error of 8.8 ± 2.1% for the current distribution for the whole set of experiments.

EIS and local resolved current density distribution analysis on effects of MPL on PEMFC performance at varied humidification

The performance of two MEA samples with different kinds of gas diffusion layer (with and without micro-porous layer) were investigated at different humidification conditions. Throughout the experiment, segmented cell technique was applied to record the current density distribution. EIS results were recorded combined with the equivalent circuit method to study the internal electrochemical mechanism on performance evolution. The performance of the MEA sample with the micro-porous layer furnished gas diffusion layer improved significantly at both low and high humidification condition. At low load and low humidification condition, the micro-porous layer could retain the product water increasing the hydration level of the membrane and the ionomer in catalyst layers, which greatly reduce the ohmic and activation resistance. Apart from that, the reactant transport resistance is also reduced when product water is retained to hydrate the ionomer, which could be confirmed by the EIS results. At high load and high humidification condition, the micro-porous layer could help curb the flooding in catalyst layer and gas diffusion layer with its excellent water removal capability thus improve the cell performance.

Noise signatures for determining chiral Majorana fermion modes

The conductance measurement of a half quantized plateau in a quantum anomalous Hall insulator-superconductor structure is reported by a recent experiment [Q. L. He \textit{et al.}, Science 357, 294-299 (2017)], which suggests the existence of the chiral Majorana fermion modes. However, such half quantized conductance plateau may also originates from a disorder-induced metallic phase. To identify the exact mechanism, we study the transport properties of such a system in the presence of strong disorders. Our results show that the local current density distributions of these two mechanisms are different. In particular, the current noises measurement can be used to distinguish them without any further fabrication of current experimental setup.

On the Effect of Bipolar Plate Mechanical Properties on the Current Distribution of Proton Exchange Membrane Water Electrolysis

The durability and lifetime of the proton exchange membrane water electrolysis (PEMWE) is directly affected by local current distribution (LCD). Uniform LCD indicates uniform electrochemical reaction and vice versa. LCD might be affected by the mechanical properties of the bipolar plate (BP) such as the elasticity and bristliness. In this work, the LCD is measured at the cathode side using S++® segmented plate with100 segments over the 50 cm2 active area. To study the effect of BP mechanical properties on LCD of the PEMWE, graphite and titanium BPs materials are used in this work. The titanium BP has shown a non-uniform LCD at higher clamping torque (CT), and a uniform LCD at lower one. Meanwhile, the graphite BP has shown a uniform LCD at higher CT and less uniform LCD at lower CT. Also, it is concluded that higher PEMWE cell performance not necessary means more uniform LCD.

Simple and Complex Polymer Electrolyte Fuel Cell Stack Models: A Comparison

The High Temperature Polymer Electrolyte Fuel Cell (HT-PEFC) converts chemical energy to electricity and heat. Operating at around 160°C, the heart of a HT-PEFC is a phosphoric-acid-doped polybenzimidazole membrane, which exhibits good protonic conductivity. HT-PEFCs may readily operate with either hydrogen or reformate as fuel. As is the case with most fuel cells, HT-PEFCs are operated in stacks in order to increase the overall electric potential. However, there are few comprehensive models of HT-PEFC stack performance. The results of this research program are among the first to obtain performance calculations for HT-PEFC at the stack scale with experimental validation. HT-PEFC stacks designed at the Forschungszentrum Jülich are actively cooled with polyalkyline glycol liquid (oil) coolant, flowing in internal passages within the solid bi-polar plates, upon the surfaces of which a complex pattern consisting of straight and serpentine passages have been machined, in order to supply the air and fuel to the cell. Because of the elevated temperature, liquid water is not generally found in the gas channels or porous layers. Considerable experimental data have been previously gathered, in-house, for a 5-cell stack based on this design. Two high temperature polymer electrolyte fuel cell (HT-PEFC) models of a HT-PEFC stack are described in detail: (i) A detailed cell-level model where a set of conformal meshes are body-fitted to all the different parts of the stack. The code typically runs, in parallel, on 1000-2000 cores at the Jülich Supercomputer Centre; and, (ii) A coarse-grid stack model based on a ‘distributed resistance analogy’ whereby rate equations supplant local diffusion terms in selected locations and directions to reduce computational requirements. A multiply-shared space (MUSES) method is employed to obtain simultaneous solutions for concentration, heat and momentum for the different phases occupying the ‘same’ global space. Both (i) detailed and (ii) MUSES methods are implemented by instantiating 5 distinct meshes corresponding to (i) air, (ii) fuel, (iii) oil (fluid), and (iv) membrane electrode assembly, (v) bipolar plate (solid) regions, and obtaining solutions of the governing equations on these meshes. For both methods a solution is obtained simultaneously for the stack manifolds, cell manifolds (entrance regions) as well as the ‘core’ of the fuel cell stack. Thermal equilibrium between the 3 fluid phases is not presumed, a priori. In both methods, the electric field potential is expressed as the ideal (Nernst) potential less activation, ohmic, and transport losses. Individual cell potentiostatic boundary conditions are iteratively corrected until the desired overall galvanostatic condition is obtained for all cells in the stack. Both codes are developed in the modern open-source object-oriented library, OpenFOAM. The results of the two models are compared with each other in terms of local current density and species partial pressure distributions. Both models are then compared with experimental data. The results show that the coarse-grid stack model (ii) agrees well, both quantitatively and qualitatively, with the detailed model (i) and experimental results and calculations are performed in two orders of magnitude less computation time. However, neither the experimental results nor the stack model (ii) are able to resolve local extrema in the current density and species mole fraction values that are observed with the detailed model (i). These are due to complex flow regimes associated with the meandering channels and flow bypassing in the porous transport layers. Importantly, these lead to significant local extrema in the local current density and other parameters of significance. These are among the first fully-comprehensive physicochemicohydrodynamic models of a HT-PEFC stack which allows calculations to be performed simultaneously for all the fluid and solids in the manifolds, entrance regions and stack core, together with experimental validation.

Simple and Complex Polymer Electrolyte Fuel Cell Stack Models: A Comparison

In this paper, two distinct polymer electrolyte fuel cell stack models are constructed: a detailed numerical model (DNM) employing a fine-scale computational mesh and a coarse-mesh approach based on a distributed resistance analogy (DRA) where diffusion terms in the transport equations are replaced by rate terms. Both methods are applied to a 5-cell, high-temperature polymer electrolyte fuel cell stack with an active area of 200 cm2 per cell. The polarization curve and local current density distributions from both the DRA and DNM are compared with experimental data, finding good agreement. Temperature, pressure, Nernst potential, and species distributions are also exhibited. The DNM displays details of fine-scale local extrema not captured by the DRA; however, the latter requires orders of magnitude less computer processor power and memory for execution. Both methods provide much finer-scale results than present experimental techniques.