PEFC Stack Research Articles

Self-Supporting Microporous Layer for Polymer Electrolyte Fuel Cells

The gas diffusion layer (GDL) used in a PEFC is thicker than the electrode catalyst layer and electrolyte membrane. Thinning down the GDL can reduce gas diffusion resistance and volumetric power density of PEFC stacks. In this study, MPL/GDL is prepared by printing microporous layers (MPLs) on carbon meshes of several tens of micrometers thick as substrates for thin-layer GDLs. Through various current-voltage and overvoltage measurements and microstructural analysis of the cells using these thin-layer MPL/GDLs, cell performance has been improved, equivalent to that of the state of the MPL/GDL.

A Compact, Self-Sustaining Fuel Cell Auxiliary Power Unit Operated on Diesel Fuel

A complete fuel cell-based auxiliary power unit in the 7.5 kWe power class utilizing diesel fuel was developed in accordance with the power density and start-up targets defined by the U.S. Department of Energy. The system includes a highly-integrated fuel processor with multifunctional reactors to facilitate autothermal reforming, the water-gas shift reaction, and catalytic combustion. It was designed with the help of process analyses, on the basis of which two commercial, high-temperature PEFC stacks and balance of plant components were selected. The complete system was packaged, which resulted in a volume of 187.5 l. After achieving a stable and reproducible stack performance based on a modified break-in procedure, a maximum power of 3.3 kWe was demonstrated in a single stack. Despite the strong deviation from design points resulting from a malfunctioning stack, all system functions could be validated. By scaling-up the performance of the functioning stack to the level of two stacks, a power density of 35 We l−1 could be estimated, which is close to the 40 We l−1 target. Furthermore, the start-up time could be reduced to less than 22 min, which exceeds the 30 min target. These results may bring diesel-based fuel cell auxiliary power units a step closer to use in real applications, which is supported by the demonstrated indicators.

Numerical Simulation of Full-Scale Cell and Stack at Very High Current Density: Influence of Heat and Liquid Water Saturation

In order to further improve power density of PEFC stack, the operations at high current density and high voltage are required in addition to downsizing. To achieve 6.0kW/L around 2030 formulated by NEDO in Japan, the extreme targets such as 0.7V@3.0A/cm2 and 0.66V@3.8A/cm2 being proposed. However, such operations cannot be achieved by the currently common MEA and GDL, and it is difficult to experimentally investigate the issues of internal states at such very high current density. In the present study, we carried out simulation of fuel cell performance by our originally developed software P-Stack. The results indicated that improving GDL properties is very important in addition to enhance performances of PEM and catalyst layer.The most important feature of this software is the employed macroscopic model comprehensively dealing with all the relevant transport phenomena coupled with electrochemical reactions. In anode and cathode channels and GDLs, mass transport of gas and liquid phase are solved by considering the two-phase fluid dynamics with taking into account the effects of phase changing. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Electrochemical reactions are modeled by Butler-Volmer equation. The parameters in the equation and other models need to be fitted to reproduce the fuel cell performance with target MEA and GDL for different operating conditions.First, we assembled 1cm2 cell with a typical MEA (PEM: Nafion NR211, Ca/An CL: TKK TEC10E50E 0.3mgPt/cm2, Ca/An GDL: SGL28BC), and then measured I-V and I-R curves under various operating conditions and transient behavior when current density is rapidly increased assuming GDL flooding. These experiments were carried out with massive flow rate and constant temperature, and therefore it can be assumed the in-plane distributions are uniform. In order to reproduce the measured results of the 1cm2 cell by simulation, we second fitted the various simulation parameters including electrochemical parameters and mass transport resistance of cathode catalyst layer and the parameters related to GDL flooding. As a validation of the fitted parameters, we compared measured and calculated I-V and I-R curves for 25cm2 dual serpentine cell with the reference MEA. These calculated results are in good agreement with the measured values.300cm2 cell with reference MEA were calculated under the conditions: anode RH is 80 %, cathode RH is 40%, cathode stoichiometry is 2.5, back pressure is 150kPa(g) and coolant flow rate is 1.0L/min/cell. The results indicated that the cell voltage decreases more than 200mV at 3.0A/cm2 due to the influence of GDL flooding. We also calculated 300cm2 cell with hypothetically modified MEA in which proton conductivity, catalyst activity, less mass transport resistance, through-plane electric/thermal conductivity of GDL and gas diffusivity of GDL are highly enhanced to achieve 0.7V@3.0A/cm2. Although the details will be discussed in the presentation, it was indicated that the further improvement of gas diffusivity and through-plane thermal conductivity are very important to reduce the GDL flooding, respectively. In addition, short stacks with 30 cells were also calculated to discuss distributions along the stack direction.

Numerical Simulation of Full-Scale Cell and Stack at Very High Current Density: Influence of Heat and Liquid Water Saturation

In order to further improve power density of PEFC stack, the operations at high current density and high voltage are required in addition to downsizing. To achieve 6.0kW/L around 2030, the extreme targets such as 0.7V@3.0A/cm2 and 0.66V@3.8A/cm2 being proposed. However, such operations cannot be achieved by the currently common MEA and GDL, and it is difficult to experimentally investigate the issues of internal states at such very high current density. In the present study, we carried out simulation of fuel cell performance by our originally developed software P-Stack. The results indicated that improving GDL properties is very important in addition to enhance performances of PEM and catalyst layer. The most important feature of the software is the employed macroscopic model comprehensively dealing with all the relevant transport phenomena coupled with electrochemical reactions. In anode and cathode channels and GDLs, mass transport of gas and liquid phase are solved by considering the two-phase fluid dynamics with taking into account the effects of phase changing. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Electrochemical reactions are modeled by Butler-Volmer equation. The parameters in the equation and other models need to be fitted to reproduce the fuel cell performance with target MEA and GDL for different operating conditions. First, we assembled 1cm2 cell with a typical MEA (PEM: Nafion NR211, Ca/An CL: TKK TEC10E50E 0.3mgPt/cm2, Ca/An GDL: SGL28BC), and then measured I-V and I-R curves under various operating conditions and transient behavior when GDL flooding occurred. These experiments were carried out with massive flow rate and constant temperature, and therefore it can be assumed the in-plane distributions are uniform. In order to reproduce the measured results of the 1cm2 cell by simulation, we second fitted the various simulation parameters including electrochemical parameters and mass transport resistance of cathode catalyst layer and the parameters related to GDL flooding. As a validation of the fitted parameters, we compared measured and calculated I-V and I-R curves for 25cm2 dual serpentine cell with the reference MEA. The figure (a) and (b) show the measured and calculated I-V curves under the different RHs, stoichiometry and back pressure, and it is indicated that the calculated results are in good agreement with the measured values. Figure (c) shows the simulated I-V curves of 300cm2 cell under the conditions: anode RH is 80 %, cathode RH is 40%, cathode stoichiometry is 2.5, back pressure is 150kPa(g) and coolant flow rate is 1.0L/min/cell. In order to clarify the influence of GDL flooding, we carried out calculations with and without the consideration of liquid phase changing. The blue broken line and dots represent the results of the reference MEA. The calculations with GDL flooding (dots) were performed at the few points because they are very time consuming. The results indicated that the cell voltage of the reference MEA decreases more than 200mV at 3.0A/cm2 due to the influence of GDL flooding. In figure (c), red broken line and dots represent the result of hypothetically modified MEA in which proton conductivity, catalyst activity, less mass transport resistance, through-plane electric/thermal conductivity of GDL and gas diffusivity of GDL are highly enhanced to achieve 0.7V@3.0A/cm2. The figure (d) and (e) show the in-plane distribution of temperature of PEM and liquid water saturation in cathode GDL at 3.0A/cm2. Although the details will be discussed in the presentation, it was indicated that the further improvement of gas diffusivity and through-plane thermal conductivity are very important to reduce the GDL flooding and overheat of PEM, respectively.

(Invited) My First 35 Years in R&D of Polymer Electrolyte Fuel Cells

I deeply appreciate the significant efforts of all colleagues and friends involved in organizing this Symposium and the very impressive list of speakers who agreed to come and actively participate. I am expected to offer here the perspective of an old timer looking at the developments in the field of polymer electrolyte R@D over the last 35 years. So, let me share with you some observations from that perspective, pointing to highlights in the history of this technology and adding some personal insights. 1. The Vision and the Relentless Pursuit: The use of PEFCs as green and efficient power source for terrestrial transport applications, was proposed early in the 1980's by leaders of the Electronics Engineering (EE) Division at LANL. The idea faced strong opposition from the main US centers of R@D in electrochemistry and electrochemical engineering of the 1980's , including Berkeley, Case Western Reserve and Texas A&M. Their strong opposition was based on the very wide gap between the status of PEFC technology in the 1980's and a mature PEFC technology for transportation applications , as demonstrated 35 years later in the form of 100kW PEFC stacks powering commercial FCEVs. The visionaries from LANL did recognize the width of this gap and went on to hire into group EE-11 a team of electrochemists and materials scientists who would prime the technical and scientific work in this new area. Some PEFC technology advancements to be described, made by the LANL fuel cell team back in the 1980's and 1990's , helped to establish recognition of the value and viability of PEFC technology for transport application. The leaders as well as the technical staff at LANL remained strongly committed in spite of the tall technical and budgetary challenges expected and in spite of the very significant opposition -- to large degree because of a strong belief in the high value of this technology for green and efficient transportation. This high value justified , in their view, a long-term , costly development. The outcome , as seen 35 years and >10B$ later, has been the successful development of a new type of electrochemical power source for transport applications, culminating with the market entry of FCEVs in 2015. This outcome has been the result of technical work of the highest level carried out in several R@D@E centers in US, Japan and Europe, however, at the outset, in the early 1980's, the key prerequisite for such outcome was recognition of the high value of the technology and the willingness to pursue it in spite of all sort of barriers. Are there any general conclusions that can be drawn from the history of PEFC technology, that would apply to other fuel cell or electrochemical technology developments being considered, or having just been proposed. ? 2. A Perspective of a Dramatic Drop in Cost : With first market entry of FCEVs in 2015, the relative merits of FCEVs and BEVs is an ongoing discussion/debate and the lower projected cost of the FCEV vs. the BEV, is an important factor in this comparison. From my personal perspective , it is interesting to examine how the production cost of FCEVs, as projected by automotive OEMs for year 2025, has fallen under the projected production cost of not just BEVs , but also under the cost of an equivalent ICE-powered vehicle. In this discussion of the cost of PEFC technology, the potential of further cost lowering by using HEMFCs instead of PEMFCs, will be highlighted. 3. On "fundamental" and "Applied" aspects of PEFC technology : The newest fuel cell technology challenge I have been involved with, is the direct ammonia fuel cell (DAFC). This last chapter of work called for detailed consideration of the kinetics of the ammonia oxidation reaction (AOR) , in order to resolve the very practical question of the probability of reaching , in a DAFC operating around 100 degC, the power density level required for transport applications. The kinetic analysis offered for the AOR , revealed a quadratic dependence of the fuel cell current on [NH3] , supporting achievability of the DAFC power densities targeted when operating on anode feeds of high concentrations/vapor pressures of NH3. Insights into distinguishable effects of the M-N bond strength in the catalysis of the AOR, on: (i) the onset potential and, (ii) the activation energy in the RDS, will be described, followed by description of a common pattern identified in mechanisms of electrocatalytic processes [1]. [1] Shimshon Gottesfeld, J Electrochem.Soc.,165 J3405-J3412 ( 2018)

Towards Full-Stack Scale Simulation of PEFC System Transient Control

Overview We will show results of computer simulations on full-stack scale behavior of the PEFC stack system, which also take into account models of auxiliary systems including hydrogen injectors, recirculation pumps, exhaust valves and radiators of coolant. Our simulations focus on transient internal states of the PEFC stack system under driving conditions of FCVs. In particular, we assumed transient loading profiles in similar to automotive uses, and various controlling ways of the stack system (control of injector and/or exhaust valves, recirculation pump and coolant flowrate, for example). Results of simulations show detailed distribution of gas species concentration, temperature, liquid water and electrochemical reaction current. We can discuss possibilities of hydrogen or oxygen local starvation and effect of temperature difference between stacked cells. We also try to investigate the behavior of condensed liquid water. Finally it is discussed how we utilize these results for developments of control systems of PEFC stacks. Numerical Modeling and Results Simulations are performed by our own simulation software for PEFCs, which can deal with full-stack PEFC systems for automotive uses (up to 100s cells) under transient operating conditions. The most important feature of the software is the employed macroscopic model comprehensively dealing with all the relevant transport phenomena coupled with electrochemical reactions. In anode and cathode channels and porous structures in GDL and MPL, mass transport of gas and liquid phase are solved by considering the two-phase fluid dynamics with taking into account the effects of phase changing. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. In the software, compared with ordinary fully-resolved CFD-based models, the computational cost is greatly reduced by use of macroscopic engineering models. For example, two-phase fluid flow is solved by Darcy’s law on the coarse-grained mesh instead of solving Navier-Stokes equation. In order to recover appropriate flow and pressure distribution in channels, the distribution of the equivalent hydraulic parameter in flow channels is estimated by detailed CFD in advance, and mapped to the coarse mesh. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from gas phase to reaction cites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. The detail of computational models of a cell stack system (300 cm2 reaction area in each cell) are followings. The numerical model for anode system includes transient boundary conditions for an injector and an exhaust system with an ejector valve. An interconnection between inflow and outflow manifolds by recirculation pump is also taken into account by adding interconnecting computational elements representing volume of pipes and recirculation pump with forced flow conditions. Recirculation of coolant is also considered in the same way, in addition with modeled effects of the radiator. The transient behavior of the stack system is calculated under transient loading profiles in similar to automotive driving conditions, where the following elements of stack system conditions are taken into account; controlled hydrogen and air feeding, purging of gas and liquid water, recirculation pumps and coolant flow. Simulation results show behaviors of transient internal states of the PEFC stack system under driving conditions. Possibilities of starvation of hydrogen and oxygen at high loading current are ones of interesting results. Temperature distribution between stacked cells and dependency on coolant flow control are also discussed. We also try to work on accumulation and purging of liquid water and effects on cell performance. These discussions can provide useful information for developments of efficient control system for PEFC stack, aiming to avoid possible drawbacks on durability and cost issues.

(Invited Plenary) Fuel Cell Stack Development in Toyota

After commercializing MIRAI in 2014, Toyota is focusing more on further cost reduction, and improvement in performance, durability and reliability, which are still challenging for wide-spreading commercialization enabling PEFC technology for next generation FCVs. To enhance PEFC performance directly impacts towards cost reduction due to reduce an amount of PEFC stack materials. Significant R&D efforts and advancements have been made towards filling the gap to the target on PEFC performance. For example, catalyst research aims not only to improve ORR activity itself but also to enhance mass transport performance in the high current region. The mass transport performance depends greatly on its microscopic structure of electrodes. Electrodes are fablicated from catalyst ink, which is a multi-component system, consisting of catalyst carbon, ionomer, and solvent. We employed several visualization techniques to unveil the structure of catalyst ink and electrodes respectively and discuss the relationship between the structure of catalyst ink and electrodes, and its performance. The technical prospects and R&D approaches for the remaining technical challenges on durability and reliability for the next generation FCVs will be also discussed.

Development of Numerical Models for Simulations of Transient Internal States of PEFC Stack System

Overview A computational model is developed for simulations of transient behavior of PEFC stack under realistic driving conditions. This model is applied to simulations of internal states of cells stack (maximum output about 100kW) with an injector, an exhaust and a recirculation system. Transient internal states under pressure swing and purging process are calculated. The physical model is based on so-called 2D + 1D model, which takes into account overall transport phenomena in PEFC fuel cell systems, such as transport of gas species and liquid water, thermal balance, electrochemical reactions and current distributions. Injector and exhaust systems are modeled by boundary conditions. Transient internal states, including fuel flow distributions between stacked cells and cell performance change at purging process will be demonstrated. Possible applications of those simulations to designing fuel cell stack control system will be discussed. Numerical Modeling and Results Simulation is performed by our own simulation software for PEFCs, which is capable of simulating full-stack fuel cell system for fuel cell vehicles (up to 400 cells) under transient operating conditions. An important feature of this software is that macroscopic models are applied for electrochemical reactions and transport phenomena in the MEA. Heat transfer and fluid dynamics of liquid-gas two phase fluid with phase changing are also taken into account. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel. This software can also simulate cathodic degradation by carbon corrosion reactions. Computational models of cell stack (250 cm2 reaction area in each cells) are constructed. A numerical model of anode system is constructed by adding transient boundary conditions for an injector and an exhaust system with an ejector valve. An interconnect boundary condition between inflow and outflow manifolds are also introduced as a computational model of recirculation system. These transient boundary conditions represent open / close timings of injector and ejector under actual driving conditions. Distributions and consumptions of hydrogen, build-up of cross-leak nitrogen and condensate liquid water can be calculated under transient (maximum output is about 100kW) loading. Simulated results show transient behavior of the stacked cell system, for example, Build-up of cross-leak nitrogen and condensate liquid water under transient loading conditions, and their effects on the distribution of performances between stacked cells.Evolution of fuel flow distribution between stacked cells under pressure swing.Efficiency of purging process and change of performance before and after purging. Those results can be calculated for various configurations of cells and driving conditions, and relations between stacked cell behaviors and those conditions can be discussed. Those discussions can provide useful information for improving durability of stacked cells by avoiding depletion of hydrogen, and / or examinations of control schemes which can efficiently utilize hydrogen fuel.

4-20 固体高分子形燃料電池スタック主要部材の高信頼化に関する研究開発((6)燃料電池2,Session 4 新エネルギー)

4-20 固体高分子形燃料電池スタック主要部材の高信頼化に関する研究開発((6)燃料電池2,Session 4 新エネルギー)

Numerical Simulation of PEFC Stack System for Design of Configuration and Control of Anode Systems

Overview Performances of cell stacks are simulated, focusing on driving state of anode system. This simulation technique takes overall transport phenomena in PEFC fuel cell systems, such as transport of gas species and liquid water in both anode and cathode channels and across MEA, thermal balance, electrochemical reactions and current distributions. Injector and exhaust systems are modeled by boundary conditions, mimicking actual driving conditions. Relation between cell performances and driving state of anode system, for example, consumption of hydrogen, build-up of liquid water and cross-over nitrogen and purging by control of injector and/or ejector can be discussed based on simulated results. Over-all performance of anode system is one of crucial issue for durability and cost-reduction of fuel cell systems. These results show good applicability of this kind of simulations on design of PEFC stack system.Numerical Modeling and Results Simulation is performed by our own simulation software for PEFCs, which is capable of simulating full-stack fuel cell system for fuel cell vehicles (up to 400 cells) under transient operating conditions. An important feature of this software is that macroscopic models are applied for microscopic phenomena in the MEA such as electrochemical reactions and proton/water transport. Heat transfer and fluid of liquid-gas two phase fluid with phase changing are also taken into account. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel. This software can also simulate cathodic degradation by carbon corrosion reactions. Performances of up to 400 cells stacks are simulated. A numerical model of anode system is constructed by adding transient boundary conditions for an injector and an exhaust system with an ejector. These transient boundary conditions represent open / close timings of injector and ejector under actual driving conditions. Distributions and consumptions of hydrogen, build-up of cross-leak nitrogen and condensate liquid water can be shown under constant (up to 30A) loading. Simulated results show difference of cell potential between of stacked cells, and relation between performance distributions and balances of transport phenomena in stack systems. Effects of configuration of stack systems are also shown. Based on these results, design of stack systems and control schemes of anode system can be discussed. Those discussion can provide useful information for improving durability of stacked cells by avoiding depletion of hydrogen, and / or examination of control schemes which can efficiently utilize hydrogen fuel.

J0560204 磁気センサを用いた固体高分子形燃料電池における内部診断方法の検討

The current distribution is an evaluation index of PEFC because non-uniform current distribution leads to low utilization of reactants and catalysts. Additionally, current distribution is affected by the reactant flow arrangement and operation conditions. Therefore, it is important to measure and evaluate the current distribution of PEFCs. Many methods to measure current distribution exist, but most modify components of the PEFC. For instance, a method may segment the flow field and measure current distribution using the printed circuit board and resistors network. In order to measure the current distribution of PEFC systems, which are typically stacked, the methods must be non-destructive. In order to realize a non-destructive current distribution measurement approach for use in PEFC stacks, we use some magnetic sensor probes. Here, we described the measurement system and the method using these proves. And current distribution in PEFC was evaluated in its operating condition. The study showed that the current distribution changes by air flow rates and relates the performance of PEFC.

Influence of failure modes on PEFC stack and single cell performance and durability

The paper reports on the influence of low humidity, short-circuiting, hydrogen starvation and open circuit potential on the performance and service life of 11 kW PEFC stack and 25 cm2 single cell. The failure modes in PEFC stack were caused by series of laboratory and software shortcomings during the tests, while tests in a single PEFC were performed for comparison purpose. Due to the impossibility to predict the influence of the failure modes on PEFC stack performance and durability based solely on the results obtained in a single PEFC demonstrated in the paper and having in mind the cost of a high power PEFC stack there is a lack of systematic knowledge in the field. The failure modes' influence was investigated in detail on the performance of the overall PEFC stack, individually on each membrane electrode assembly (MEA) in the stack and on the single PEFC. The reasons for MEAs failures were explained and recommendations for PEFC stack rehabilitation were given. By analyzing the reasons and shortcomings causing the failure modes the paper also provides information on some specifics in coupling and communication between some conditioning and testing devices.

Endurance of Nafion-composite membranes in PEFCs operating at elevated temperature under low relative-humidity

PEFCs employing Nafion–silica (Nafion–SiO2) and Nafion-mesoporous zirconium phosphate (Nafion–MZP) composite membranes are subjected to accelerated-durability test at 100°C and 15% relative humidity (RH) at open-circuit voltage (OCV) for 50 h and performance compared with the PEFC employing pristine Nafion-1135 membrane. PEFCs with composite membranes sustain the operating voltage better with fluoride-ion-emission rate at least an order of magnitude lower than PEFC with pristine Nafion-1135 membrane. Reduced gas-crossover, fast fuel-cell-reaction kinetics and superior performance of the PEFCs with Nafion-SiO2 and Nafion-MZP composite membranes in relation to the PEFC with pristine Nafion-1135 membrane support the long-term operational usage of the former in PEFCs. An 8-cell PEFC stack employing Nafion–SiO2 composite membrane is also assembled and successfully operated at 60°C without external humidification. PEFCs with Nafion-SiO2 and Nafion-MZP composite membranes exhibit better performance with reduced gas-crossover, faster fuel-cell-reaction kinetics, and lower fluoride-ion-emission rate in relation to the PEFCs with pristine Nafion-1135 membrane.

Design and testing of a fuel cell powertrain with energy constraints

This paper reports a methodology to design a high efficiency fuel cell powertrain. The powertrain equips a prototype car that runs energy-efficient races where the objectives are to go the furthest with the lowest quantity of fuel (Shell Eco Marathon). In the design process, the starting point was the evaluation of the car's energy demand to run on a specified race track by developing a dynamic mechanical model of the car. A dedicated inertial test bench was constructed to reproduce the car's behavior in the laboratory. This was used to determine under which operating conditions an electric motor has to be powered to reach the highest efficiency. The introduction of ultracapacitors was envisaged and, based on efficiency arguments, a drive train arrangement was chosen composed solely of a 500 W PEFC stack, a DC/DC converter operated in a current regulation mode and an electric motor. Final tests of the powertrain revealed the most efficient operating conditions. The present work is the necessary first stage for both the design of the complete powertrain and its detailed energy analysis. It forms the basis for future improvements.

Preferred test conditions for measuring flow rate distribution between cells in a polymer electrolyte fuel cell stack

A new and practical testing technique was developed for measuring the flow rate distribution between cells in a stack that did not contain any internal sensors. The flow rate distribution is obtained by measuring the hydrogen limiting current of each cell in the stack while a mixed gas of hydrogen and dimethyl ether is supplied to the anode and hydrogen to the cathode. In order to measure large flow rate deviations between cells, it is necessary to decrease the flow rate of the anode hydrogen and to sufficiently humidify the cells. The faster the increasing rate of the current, the more the apparent hydrogen limiting current increases than the theoretical electrochemical equivalent current. However, the relative flow rate deviations between cells can be obtained by a practical accuracy using the ratio of the apparent hydrogen limiting current. Humidification of the cell is indispensable for the measurement and a method using dry anode gas and humidified cathode gas is recommended. The preferred test conditions for measuring the flow rate distribution between cells in a PEFC stack are proposed.

Development of a Dynamic CT System for Neutron Radiography and Consecutive Visualization of Three-Dimensional Water Behavior in a PEFC Stack

A dynamic CT system was developed for visualization of consecutive three-dimensional water behavior in a PEFC stack for neutron radiography. The system is composed of a neutron image intensifier and a C-MOS high speed video camera. An operating stack with three cells based on the Japan Automobile Research Institute standard was visualized using the neutron radiography system at a research reactor JRR-3 in Japan Atomic Energy Agency. The dynamic water behavior in channels in the operating PEFC stack was clearly visualized every 15 seconds by using the system. The water amount in each cell was evaluated by the CT reconstructed images. It was shown that a cell voltage decreased gradually when the water increased and increased rapidly when the water was evacuated. It was estimated that the power generation stopped when the channel of a cell was partly filled with the water because the air supply was blocked to a cell in the stack.

An integrated modeling approach for temperature driven water transport in a polymer electrolyte fuel cell stack after shutdown

The concept of using controlled temperature gradients to non-parasitically remove excess water from porous media during PEFC stack shutdown has been numerically investigated. An integrated modeling approach focusing both at stack and single cell level is presented. The stack thermal model is developed to obtain detailed temperature distribution across the PEFC stack. The two-phase unit fuel cell model is developed to investigate the detailed water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, and thermo-osmotic and diffusive flow in the polymer membrane. The single cell model is used to estimate the local water distribution with land or channel boundary condition, and the experimentally validated stack thermal model provided the transient temperature boundary conditions. Two different stack designs are compared to quantify the residual water in the stack. Model results indicate that a favorable temperature gradient can be formed in the stack to enhance the water drainage rate, esp. at anode end cell locations, where freeze/thaw damage has been observed to occur.

Stability and performance improvement of a polymer electrolyte membrane fuel cell stack by laser perforation of gas diffusion layers

The performance and stability of a hydrogen-driven polymer electrolyte membrane fuel cell stack (6-cell PEFC stack) are investigated with regard to pore flooding within the gas diffusion layers (GDLs). Two short stacks with various GDLs (Toray TGP-H-060 untreated and laser-perforated) were characterized at different operating conditions by several characterization techniques such as constant current load, polarization curve, chronoamperometry and chronovoltammetry. The experimental results reveal that the perforation of the cathode GDLs improves the water transport in the porous media and thus the performance as well as the stability of the operating stack in medium and high current density range. A reduced pore flooding is verified when using the customized laser-perforated GDLs. The GDL perforation has a huge potential to balance the inhomogeneous in-plane saturation conditions between the inlet and outlet area of the cell and to compensate to a certain degree the effects of temperature distribution within a stack regarding the water management.

Analytical Investigation of Fuel Cells by Using <i>In Situ </i>and <i>Ex Situ </i> Diagnostic Methods

The study of the behaviour of fuel cells by using various in-situ and ex-situ diagnostic methods is a main topic at the German Aerospace Center (DLR). The degradation of cell components of polymer electrolyte fuel cells (PEFC, DMFC) and of solid oxide fuel cells (SOFC) are of special interest. For this purpose physical and electrochemical methods are used individually as well as in combination. In addition to routinely applied electrochemical methods different methods for locally resolved current density measurements by means of segmented cell technology and integrated temperature sensors have been developed. The latest development with segmented bipolar plates based on printed circuit boards (PCB) is used both in single PEFC cells and stacks. Furthermore, a measuring system for segmented SOFC cells has been developed allowing for the spatially resolved characterisation of cells in terms of current density/voltage characteristics, impedance spectroscopy data, operating temperature and gas composition. The paper summarises the capabilities at DLR with respect to the analysis of fuel cells’ behaviour and gives examples of analytical studies to discuss the potentials and limitations of the diagnostic methodology that is applied.

Modelling compression pressure distribution in fuel cell stacks

A general purpose 3D finite element method model has been developed for the estimation of the compression pressure distribution in fuel cell stacks. The model can be used for the optimisation of any type of fuel cell structure at any temperature. The model was validated with pressure sensitive film measurements using PEFC stack components that had low rigidity and were highly deformable.