Electrolyte Membrane Fuel Cell Stack Research Articles

Turbulent flow in the distribution header of a PEM fuel cell stack

A numerical investigation of the flowfield in a model distribution header manifold of a polymer electrolyte membrane fuel cell stack is conducted. The computational model simulates two segments of an experimental setup of a pair of model headers which replicate the headers of a fuel cell stack. The model headers consist of an inlet and outlet sections connected with a plate containing an array of holes that replicate the unit cells. The flow structures in the outlet header are rather complex and are the result of the superposition of a series of impinging jets in a confined space in the presence of crossflow. The flow from each hole, which represents an individual cell outlet, enters the outlet header as a jet stream and is subjected to a crossflow. Large Eddy Simulations (LES) are performed for a portion of the outlet header to investigate the complex turbulent flow and related structures under different crossflow conditions, and are complemented by Particle Image Velocimetry (PIV) measurements. The LES results show that two large vortical structures are formed in the header cross-section, with a high-speed round jet from the cell outlet holes forcing a diversion of the crossflow, dividing it into two separate branches. Investigation of the flow restructuring after a blockage of one of the jets is performed. Simulation results using a slot opening for the jet show flow instabilities. The results of this study highlight the unsteady and highly turbulent nature of the flow in the header and provide a characterization of the complex three-dimensional structure of the flow. The flowfield and flow structures may impact the overall pressure drop along the header and the effective cross-sectional area for the flow leaving the header. The observations and insights obtained from the LES simulation and PIV measurements point to the need to further investigate the impact on flow sharing in a stack of the flowfield development in the outlet header.

Analysis and control of a fuel delivery system considering a two-phase anode model of the polymer electrolyte membrane fuel cell stack

The fuel delivery system using both an ejector and a blower for a PEM fuel cell stack is introduced as a fuel efficiency configuration because of the possibility of hydrogen recirculation dependent upon load states.A high pressure difference between the cathode and anode could potentially damage the thin polymer electrolyte membrane. Therefore, the hydrogen pressure imposed to the stack should follow any change of the cathode pressure. In addition, stoichiometric ratio of the hydrogen should be maintained at a constant to prevent a fuel starvation at abrupt load changes.Furthermore, liquid water in the anode gas flow channels should be purged out in time to prevent flooding in the channels and other layers. The purging control also reduces the impurities concentration in cells to improve the cell performance.We developed a set of control oriented dynamic models that include a anode model considering the two-phase phenomenon and system components The model is used to design and optimize a state feedback controller along with an observer that controls the fuel pressure and stoichiometric ratio, whereby purging processes are also considered. Finally, included is static and dynamic analysis with respect to tracking and rejection performance of the proposed control.

Longevity test results for reformate polymer electrolyte membrane fuel cell stacks

Polymer electrolyte membrane fuel cell (PEMFC) stacks offer a great potential for combined heat and power (CHP) applications because of their good performance and technical maturity of the key components. Nonetheless, some developmental issues have remained open. Among those are the long-term stability with respect to performance degradation and sudden death phenomena like membrane rupture. In a development program for domestic CHP systems, PEMFC stacks intended for long-term operation on reformate were developed. Development targets were high performance, high media utilization, good longevity and low degradation rates. In this paper, results on long-term performance tests of these stacks are reported. Operating times of more than 15,000 h with degradation rates of approx. 10 μV h −1 have been achieved.

Distribution Study of Species and Current Density During Oxygen Starvation

In a fuel cell system the stack is strongly coupled with the main system components, among which the compressor is one of the most important. Malfunction of this auxiliary device (delay during peak power, low stoichiometry operation, emergency stop, etc.) is directly responsible for bad oxygen distribution in the cathode (substoichiometry reactants feeding). This phenomenon is usually called oxygen starvation. In this study we want to identify the consequences of oxygen starvation on the performance and durability of polymer electrolyte membrane fuel cell stacks, and more particularly, on the current distribution along the cell. The oxygen concentration decreases along the channel and induces a change in the local electrochemical response; it means that the local current density on the cell is redistributed on the surface. This bad distribution of reactive gas (in a transient time or long time) decreases the performance but may also have an effect on cathode degradation such as carbon corrosion and platinum dissolution/oxidation. The current distribution along the cell is studied by two approaches (modeling and experiments). The 3D model using serpentine bipolar plate meshing is adapted to dynamically compute for the catalyst layer local conditions (local current, temperature, gases partial pressure, and water activity). It is able to reproduce the conditions of low or high oxygen concentration in the cathode side. The experiments are performed with a bi-cell stack developed by CEA with specific design for the magnetic sensors (the local current is computed by measuring the local induced magnetic field and using the Maxwell equations).

Optimization of PEM Fuel Cell System Using Dynamic Model

The output power efficiency of the fuel cell system depends on the anode pressure, cathode pressure, temperature, demanded current, air and hydrogen humidity. Thus, it is necessary to determine the optimal operation condition for maximum power efficiency. In this paper, we developed a dynamic model of fuel cell system which contains mass flow model, membrane hydration and electro-chemistry model. Experiments have been performed to evaluate the dynamical Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack model. In order to determine the maximum output power and minimum use of hydrogen in a certain condition, response surface methodology optimization based on the proposed PEMFC stack model is presented. The results provide an effective method to optimize the operation condition under varied situations.

Nitride films as protective layers for metallic bipolar plates of polymer electrolyte membrane fuel cell stacks

The idea of using nitride films coated by sputtering Nb and Cr, using N 2 as a reaction gas, as protective layers for metallic bipolar plates (BP) of polymer electrolyte membrane fuel cell (PEMFC) stacks, was explored by experiments. Specimens were fabricated from austenite 304 stainless steels, which are frequently used as bipolar plate materials for fuel cells due to their good corrosion resistance and high strength at elevated temperature. The results of XRD analysis and Gaussian function analysis of the coated films suggest that NbN or NbN/NbCrN films were induced depending on the process parameters. The NbN/NbCrN multiphase films were induced at high Cr target powers and low gas ratios among the process parameters selected in this investigation. The result of ICR measurement of the films suggests that the effect of the Cr target power, i.e. the effect of the Cr amount in the film, on the ICR of the films is not significant, while the effect of the gas ratio on the ICR of the films is noticeable. For the films deposited at different gas ratios, the ICR of the film generally decreases as the gas ratio increases. In general, all the 304 SS specimens coated by the NbN single phase or NbN/NbCrN multiphase films at the given process parameters showed significantly improved corrosion resistance in comparison with the bare 304 SS. Among NbN single phase and NbN/NbCrN multiphase films, the performance of NbN/NbCrN multiphase films was more stable. Depending on the process parameters, the polarization curves of the specimens coated with NbN films showed rapid increase of the current density due to the pitting. Therefore, as corrosion resistance coating for metallic BP of PEMFC stacks, NbN/NbCrN multiphase film may be preferred to NbN single phase films.

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.

Durability Test Results of a Polymer Electrolyte Membrane Fuel Cell Operated at Overnominal Temperature With Low Humidified Reactants

A three-cell polymer electrolyte membrane fuel cell stack equipped with perfluorinated ionomer membranes is tested for 1000 h in severe operating conditions. The fuel cell (FC) is fed by dry hydrogen/low humidified air and it is simultaneously operated at overnominal temperature. During the long term experiment, some polarization curves and electrochemical impedance spectra are recorded at regular time-spaced intervals for various stoichiometry rates. The objective is to characterize the degradation of FC performances with the aim to better understand the FC aging mechanisms. Some response surfaces are plotted to display and analyze the stack voltage variations as a function of aging time, static load current, and gas utilization rates. Different points of the FC impedance spectra are also considered. The evolutions of their coordinates in the Nyquist plot can be monitored versus aging time and for different gas utilization rates.

Correlating Nitrogen Accumulation With Temporal Fuel Cell Performance

The permeability or crossover characteristics of a typical perfluorosulfonic acid base type membrane are used for the temporal and spatial estimations of nitrogen concentration along the anode channels of a polymer electrolyte membrane fuel cell stack. The predicted nitrogen accumulation is then used to estimate the impact of local fuel starvation on stack voltage through the notion of apparent current density. Despite the simplifying assumptions on the water accumulation and membrane hydration levels, the calibrated model predicts reasonably well the response of a 20-cell stack with a dead-ended anode. Specifically, the predicted voltage decay and the estimated gas composition at the anode outlet are experimentally validated using the stack-averaged voltage and a mass spectrometer. This work shows that the crossover of nitrogen and its accumulation in the anode can cause a considerable decay in stack voltage and should be taken into account under high hydrogen utilization conditions.

Design and development of a 7kW polymer electrolyte membrane fuel cell stack for UPS application

This work presents the PEMFC stack design methodology developed at CNR-ITAE, in the frame of a collaboration with an industrial partner, Electro Power Systems (EPS), operating in the Uninterruptable Power Supply (UPS) market. A detailed description of the design procedure of a 7 kW PEMFC stack is reported, starting from technical requirements of the UPS system to experimental tests. Bipolar plate layout, active area surface and shape, maximum (OCV) and minimum voltage, maximum cooling circuit pressure drop, maximum cathodic flow-field pressure drop, were the main constraint that influenced the constructive solutions. The electrochemical performances of Gore Primea 5621 MEA with SGL Sigracet GDL were chosen as reference to select the appropriate operating point in terms of current density and single cell voltage. A current density of 800 mA/cm 2 was imposed as operating point of the stack, subsequently main stack parameters were calculated. Three different cathodic flow fields, that were designed to fulfill UPS system requirements, were tested in a single cell arrangement, to find the best gas flow path in terms of compromise between cell performance and pressure drop. Also a specific study was dedicated to the selection of gasket material to find the best compromise between cell performance and limited mechanical stress. The assembled 70 cells unit was tested in a test bench simulating the power system. Preliminary tests of the full unit yielded to a power of 6.2kW at 36 V.

Annealing induced interfacial layers in niobium-clad stainless steel developed as a bipolar plate material for polymer electrolyte membrane fuel cell stacks

Roll-bonded niobium (Nb)-clad 304 L stainless steel (SS) is currently being developed as a metallic bipolar plate material for polymer electrolyte membrane fuel cell stacks. Prior work has shown that post-roll annealing significantly softens the constituent core and cladding materials, enhancing the ductility and formability of each. However under the vacuum annealing condition employed in the previous study (900 s, 982 °C (1800 F)), an interfacial layer was observed to form between the two bonded materials. Subsequent bending and flattening tests indicated brittle failure of this interfacial region under high strain conditions. The present study employs transmission electron microscopy to examine the composition, structure, and thickness of phases generated at the Nb/SS interface as functions of annealing temperature and time. Corresponding selected electron diffraction patterns indicate that above a threshold annealing temperature of ∼950 °C, the formation of (Fe 1− x Cr x ) 2Nb appears to control the failure behavior of the Nb/304 L SS material. Annealing treatments conducted below this temperature generally avoid the formation of this intermetallic layer.

Impact of defective single cell on the operation of polymer electrolyte membrane fuel cell stack

Potential inversion of a single cell in a PEMFC stack reduces its performance and can lead to severe damage in the MEAs. Numerous causes of potential inversion can be considered, from MEA degradation to insufficient feed of reacting gases. The first part of the paper deals with experiments carried out to investigate whether the potential inversion could be suppressed by increasing the inlet gas flow. Then, the defective cell resistances were measured and compared to those exhibited by the other cells in the 23-cell stack. We also studied the influence of this cell on the whole stack resistances. Finally, the defective cell was dismantled and analysed, in order to explain where the defect comes from and whether it has affected each part of the cell.

Spatially resolved degradation effects in membrane-electrode-assemblies of vehicle aged polymer electrolyte membrane fuel cell stacks

State of the art MEAs were aged in a fuel cell vehicle and degradation effects analyzed using electron microscopy and electrochemical methods. All cells of the stack showed a performance decay along with a loss in the electrochemical surface area. This could be correlated to particle growth and carbon corrosion observed by electron microscopy. Spatially resolved investigations showed a significant deterioration of the cathode, which is particularly pronounced at the hydrogen inlet. Differences in the cell performance of the aged cells could not be attributed to a variation in the catalyst degradation, but are linked to an altered ohmic resistance in the cells. The ohmic resistance of the cells is likely to be affected by the formation of precipitates in the membrane and seems to be correlated with their size.

Analysis of the Ripple Current in a 5 kW Polymer Electrolyte Membrane Fuel Cell Stack

AbstractPolymer electrolyte fuel cell systems are increasingly being used in applications requiring an inverter to convert the direct current (DC) output of the stack to an alternating current (AC). These inverters, and other time‐varying inputs to the stack such as the anode feed pressure, cause deviations from the average stack current, or ripple currents, which are undesirable for reasons of performance and durability. A dynamic fuel cell model has been developed and validated against experimental data for a 5 kW fuel cell stack, examining in detail the ripple current behaviour. It was shown that the ripple currents exceed the 2% maximum recommended value, and may lead to long‐term degradation of the fuel cell stack.

Optimal operating points of PEM fuel cell model with RSM

The output power efficiency of the fuel cell system mainly depends on the required current, stack temperature, air excess ratio, hydrogen excess ratio, and inlet air humidity. Therefore, the operating conditions should be optimized to get maximum output power efficiency. In this paper, a dynamic model for the fuel cell stack was developed, which is comprised of a mass flow model, a gas diffusion layer model, a membrane hydration, and a stack voltage model. Experiments have been performed to calibrate the dynamic Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack model. To achieve the maximum output power and the minimum use of hydrogen in a certain power condition, optimization was carried out using Response Surface Methodology (RSM) based on the proposed PEMFC stack model. Using the developed method, optimal operating conditions can be effectively selected in order to obtain minimum hydrogen consumption.

Sliding Mode Strategy for PEM Fuel Cells Stacks Breathing Control Using a Super-Twisting Algorithm

A second-order sliding mode strategy to control the breathing subsystem of a polymer electrolyte membrane fuel cell stack for transportation applications is presented. The controller is developed from a design model of the plant derived from open literature, and well suited for the design of second-order sliding mode strategies. Stability issues are solved using a super twisting algorithm. The resulting approach exhibits good dynamic characteristics, being robust to uncertainties and disturbances. Simulations results are provided, showing the feasibility of the approach.

Modeling and Experimental Analyses of a Two-Cell Polymer Electrolyte Membrane Fuel Cell Stack Emphasizing Individual Cell Characteristics

Performance of individual cells in an operating polymer electrolyte membrane (PEM) fuel cell stack is different from each other because of inherent manufacturing tolerances of the cell components and unequal operating conditions for the individual cells. In this paper, first, effects of different operating conditions on performance of the individual cells in a two-cell PEM fuel cell stack have been experimentally investigated. The results of the experiments showed the presence of a voltage difference between the two cells that cannot be manipulated by operating conditions. The temperature of the supplying air among others predominantly influences the individual cell voltages. In addition, those effects are explored by using a dynamic model of a stack that has been developed. The model uses electrochemical voltage equations, dynamic water balance in the membrane, energy balance, and diffusion in the gas diffusion layer, reflecting a two-phase phenomenon of water. Major design parameters and an operating condition by conveying simulations have been changed to analyze sensitivity of the parameters on the performance, which is then compared with experimental results. It turns out that proton conductivity of the membrane in cells among others is the most influential parameter on the performance, which is fairly in line with the reading from the experimental results.

Externally cooled high temperature polymer electrolyte membrane fuel cell stack

One key issue in high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stack development is heat removal at the operating temperature of 140–180 °C. Conventionally, this process is done using coolants such as thermooil, steam or pressurized water. In this contribution, external liquid cooling designs are described, which are avoiding two constraints. First, in the cell active area, no liquid coolant is present avoiding any sealing problems with respect to the electrode. Secondly, the external positioning allows high temperature gradients between the heat removal zone and the active area resulting in a good adjustability of appropriate reformate conversion temperatures (e.g. 160 °C) and a more compact cell design. Different design concepts were investigated using modeling techniques and a selection of them has also been investigated experimentally. The experiments proved the feasibility of the external cooling design and showed that the temperature gradients within the active area are below 15 K under typical operating conditions.

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.

Model-Based Detection of Hydrogen Leaks in a Fuel Cell Stack

Hydrogen leaks are potentially dangerous faults in fuel cell systems that are fed with hydrogen-rich gas mixtures. This brief presents an approach to hydrogen leak detection and, thus, complements direct detection using hydrogen sensors. It relies on simple mass balance equations of an anode filling volume after taking into account the natural leak of the stack. A hydrogen mass flow, anode pressure, and relative humidity sensor are employed. Hydrogen leak detection without the use of relative humidity sensors is considered by employing adaptive alarm thresholds to eliminate false alarms. The validity of the method is also discussed in terms of common hydrogen supply system configurations. The detection method is validated using a 1.25-kW polymer electrolyte membrane fuel cell stack in a laboratory facility where leaks could be introduced in a controlled manner.