Electrolyte Membrane Fuel Cell Stack Research Articles

Fault detection for polymer electrolyte membrane fuel cell stack by external magnetic field

An original non-invasive approach of fuel cell diagnosis is proposed in order to locate different kinds of faults in PEMFC stacks from magnetic field measurements. The method is based on the solving of an inverse linear problem linking the magnetic field signature outside of the fuel cell to the current density distribution inside. The searched solution is a linear combination of conservative current distribution obtained by a set of electrokinetic problems solved by a finite face element method. As the problem is ill-posed, the solution is stabilized using a truncated singular value decomposition. In this work, 30 sensors are used to perform the magnetic tomography of a PEMFC stack consisting of 100 cells with a large active area of 220 cm2. External magnetic measurement makes possible to identify 2D or 3D changes of current density distribution induced either by a cell flooding or membrane drying as well as by material degradation in a PEMFC stack.

Consistency analysis of polymer electrolyte membrane fuel cell stack during cold start

This paper investigates the cold start characteristic of the polymer electrolyte membrane fuel cell stack in the constant voltage startup mode. The stack is used in actual vehicle working conditions, so it is practical significance to study the cold start characteristics of the stack. The evolution of the temperature and current density distribution at different regions in the constant voltage mode of the stack are studied. Consistency analyzes from two dimensions, in-plane and through-plane are studied. Then, the performance degradation of the stack after the cold start is explored through the polarization curve and the voltage consistency.The cold start characteristics and performance degradation mechanism of the stack are found. The stack has better cold start performance than that of the single cell, which can realize a rapid cold start at −15 °C in 95 s. Also, the performance of the single cell in the middle position is obviously better than that of the single cell on both sides. The internal behavior of each single cell during cold start process is different and the performance consistency is poor after the failed cold start. The performance degradation gradually decreases from the single cell near the inlet to the single cell far away from the inlet. This paper can provide direction for the optimization cold start strategy.

Numerical and experimental investigation on an ejector designed for an 80 kW polymer electrolyte membrane fuel cell stack

An ejector aiming at an 80 kW polymer electrolyte membrane fuel cell system designed based on the Sokolov 1-D ejector mode is evaluated numerically and experimentally. A two-dimensional axisymmetric model for the ejector is established based on the Shear Stress Transport model and the Re-Normalization Group model and validated by comparing simulated results and experimental data. Effects of the ejector's mixing chamber diameter and diffusion chamber angle and humidity of the secondary flow on its performance are investigated by a Computational Fluid Dynamics method. The results show that the primary mass flow rate presents a linear relationship with the primary pressure and that the Re-Normalization Group model is more accurate than the Shear Stress Transport model in predicting the ejector's performance. When the diffusion chamber angle is 11° and 13°, the ejector exhibits the best performances. The mixing chamber diameter and the secondary flow humidity significantly influence entertainment ratio and hydrogen recirculation of the ejector.

A DC – DC converter for PEMFC stack power conditioning applications

Fuel cell technology is one of the most sought – after renewable technology. However, the output voltage of fuel cell (FC) stacks is inherently unstable, as such; it is of little or no use for most power supply applications. In addition to the unstable output voltage, FC stacks are susceptible to high current ripples, which can reduce the system’s life expectancy. It is therefore a requirement that the output voltage is stabilised and the high current ripple reduced in order to make FC technology useful. In order to achieve this, a power-conditioning unit (PCU) is interfaced directly to the FC stack module. The PCU is realised by means of a DC – DC converter or DC – AC inverter or a combination of both. This paper presents a PWM DC – DC Interleaved Boost Voltage Multiplier (IBVM) converter as the PCU for a Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack. The mathematical modelling of the converter is validated by simulation in MATLAB/SIMULINK. Important simulation results are presented herein: which show that the output voltage of a PEMFC stack can be stabilised.

Operation Strategies Based on Carbon Corrosion and Lifetime Investigations for High Temperature Polymer Electrolyte Membrane Fuel Cell Stacks

AbstractThis paper is aimed to develop operation strategies or high temperature polymer electrolyte fuel cells (HT‐PEMFCs) stacks in order to enhance the endurance by mitigating carbon oxidation reaction. The testing protocols are carefully designed to suit the operating cycle for the realistic application. A 5 cell co‐flow stack is assembled with BASF Celtec® P membrane electrode assembly (MEA) with an active area of 163.5 cm2. The oxidation rate of carbon is systematically investigated employing potentiostatic experiments under variation of both cell voltage and temperature using on‐line mass spectrometry.The experimental results show that more CO2 is measured for the open circuit voltage (OCV) operation, indicating that the lifetime of the stack is strongly affected by a factor of approximately 12–26 between OCV and 700 mV depending on temperature. Protective start‐stop algorithms are developed to avoid the formation of aggressive cell potentials. The startup procedures let degrade the catalyst support to a higher extent than the stop procedures, which is presumably due to both OCV exposure and hydrogen front passing through the anode. A model for lifetime prediction is developed from carbon corrosion experiments and validated with a durability test for 1,562 cycle events.

Reliability prediction of large fuel cell stack based on structure stress analysis

The aim of this paper is to improve the reliability of Proton Electrolyte Membrane Fuel Cell (PEMFC) stack by designing the clamping force and the thickness difference between the membrane electrode assembly (MEA) and the gasket. The stack reliability is directly determined by the component reliability, which is affected by the material property and contact stress. The component contact stress is a random variable because it is usually affected by many uncertain factors in the production and clamping process. We have investigated the influences of parameter variation coefficient on the probability distribution of contact stress using the equivalent stiffness model and the first-order second moment method. The optimal contact stress to make the component stay in the highest level reliability is obtained by the stress-strength interference model. To obtain the optimal contact stress between the contact components, the optimal thickness of the component and the stack clamping force are optimally designed. Finally, a detailed description is given how to design the MEA and gasket dimensions to obtain the highest stack reliability. This work can provide a valuable guidance in the design of stack structure for a high reliability of fuel cell stack.

Experimental validation of the polarization curve and the temperature distribution in a PEMFC stack using a one dimensional analytical model

This paper presents the experimental validation of a one dimensional analytical model developed for a Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack. The analytical model is developed to simulate a stack formed by N cells, where the experimental validation has been carried out for a stack formed by 3, 5 and 7 cells. The main physical phenomena in a PEMFC such as electrochemistry, mass transfer (hydrogen, oxygen and water – both liquid and gas) have been modeled. The experimental validation has been done with two different variables in order to provide good quality results. The polarization curve and the stack temperature distribution have been selected as validation variables. In order to compare the experimental and the numerical results, a set of experimental tests modifying the main operating conditions such as the cathode stoichiometric, relative humidity in anode and cathode and operating pressure have been performed. The model results show a good agreement also for the different configurations of the stack (3, 5 and 7 cells). The average error is always less than 2% for the polarization curve and for the stack temperature distribution, so the accuracy of the one dimensional analytical model is verified.

Variability and comparability of testing procedures for PEMFC stacks and modules regarding performance aspects

Reliable and reproducible testing protocols are needed for fuel cell stacks, modules and subsystems in order to reach comparable results for example in performance measurements. A testing protocol was developed which aims at the performance measurement of fuel cell stacks and modules. Measurements for the reproducibility and comparability were performed on a low temperature polymer electrolyte membrane fuel cell stack and on a fuel cell subsystem. The resulting voltages at the different load steps show a difference in adaption to load changes form the stack and the subsystem. In most cases the stack adapted faster with a more stable voltage. The repeatability of the testing protocols was tested which resulted in a higher degradation of the fuel cell subsystem compared to the stack. The measurements in comparison between two laboratories showed a clear decrease in voltages at the second laboratory. The measurement of the test protocol influences the fuel cell stack with an increase in voltage whereas the voltages decrease for the fuel cell subsystem.

Simulation and in situ measurement of stress distribution in a polymer electrolyte membrane fuel cell stack

A critical parameter for PEM fuel cell's electric contact is the nominal clamping pressure. Predicting the mechanical behavior of all components in a fuel cell stack is a very complex task due to the diversity of materials properties. Prior to the integration of a 3 kW PEMFC power plant, a numerical simulation was performed in order to obtain the mechanical stress distribution for two of the most pressure sensitive components of the stack: the membrane, and the graphite plates. The stress distribution of the above mentioned components was numerically simulated by finite element analysis and the stress magnitude for the membrane was confirmed using pressure films. Stress values were found within the elastic zone which guarantees mechanical integrity of fuel cell components. These low stress levels particularly for the membrane will allow prolonging the life and integrity of the fuel cell stack according to its design specifications.

In-situ electrochemically active surface area evaluation of an open-cathode polymer electrolyte membrane fuel cell stack

Abstract The ability to evaluate the electrochemically active surface area (ECSA) of fuel cell electrodes is crucial toward characterising designs and component suites in-situ, particularly when evaluating component durability in endurance testing, since it is a measure of the electrode area available to take part in the fuel cell reactions. Conventional methods to obtain the ECSA using cyclic voltammetry, however, rely on potentiostats that cannot be easily scaled to simultaneously evaluate all cells in a fuel cell stack of practical size, which is desirable in fuel cell development. In-situ diagnostics of an open-cathode fuel cell stack are furthermore challenging because the cells do not each possess an enclosed cathode compartment; instead, the cathodes are rather open to the environment. Here we report on a diagnostic setup that allows the electrochemically active surface area of each cell anode or cathode in an open-cathode fuel cell stack to be evaluated in-situ and simultaneously, with high resolution and reproducibility, using an easily scalable chronopotentiometry methodology and a gas-tight stack enclosure.

Study of an Isolated DC-DC Converter for Fuel Cell Applications

This paper presents the analysis and design of a dual active bridge DC-DC converter for fuel cell applications. The zero voltage switching operating condition of such converter is analyzed to select an appropriate turn ratio of the high frequency transformer for a high efficiency operation. The ratio between the output voltage to the fuel cell voltage should be close to the transformer turn ratio to guarantee the zero voltage switching regimes at a light load. The prototype converter was designed to be suitable for the input voltage of 40 to 65 V and output voltage of 360 to 400 V with the transformer turn ratio of 7.33. The converter was tested with a 48 V DC power supply and with a polymer electrolyte membrane fuel cell stack. The maximum power of 700 W was delivered and the efficiency was better than 94% for the whole load range.

A novel cooling flow field design for polymer electrolyte membrane fuel cell stack

Efficient operation of a proton exchange membrane fuel cell (PEMFC) is hugely dependent on an effective cooling system. Non-uniformity of temperature causes a varying rate of electrochemical reaction at different places leading hot spot formation which decrease the lifetime of PEM fuel cell. The most part of heat generated in the PEM fuel cell is removed by cooling fluid (forced convection) while the remainder is dissipated by natural convection, radiation and temperature difference between the inlet and outlet of reaction gases. The reaction rate and current density are related to temperature distribution on flow field, therefore it has to be kept uniform at the surface of active area to achieve a uniform current density. Designing a cooling system requires significant challenges in regard to the narrow range of operating temperatures. In this study, new cooling flow fields are numerically investigated in order to clarify the sufficient design for the heat removal. Numerical simulation is employed to investigate the coolant flow distribution, pressure drop and thermal behavior of different designs. The temperature contours represent the temperature uniformity for the assessed case.

Improved electrochemical in-situ characterization of polymer electrolyte membrane fuel cell stacks

In-situ diagnostics for single polymer electrolyte membrane fuel cells are well known and established. Comparable stack level techniques are urgently needed to enhance the understanding of degradation during real system operation, but have not yet reached a similar level of sophistication. We have therefore developed a new method for the quantification of the hydrogen crossover current in stacks, which in combination with a previously published technique now allows a clear quantitative characterization of the individual cells' membranes and electrodes. The limits of the reported methods are theoretically assessed and application is then demonstrated on automotive short stacks. The results prove to be highly reproducible and are validated for individual cells of the respective stacks by direct comparison with cyclic voltammetry results, showing good quantitative agreement for the hydrogen crossover current, the double layer capacitance and the electrochemically active surface area.

Development of a polymer electrolyte membrane fuel cell stack for an underwater vehicle

This paper presents a polymer electrolyte membrane (PEM) fuel cell stack that is specifically designed for the propulsion of an underwater vehicle (UV). The stack for a UV must be continuously operated in a closed space using hydrogen and pure oxygen; it should meet various performance requirements such as high hydrogen and oxygen utilizations, low hydrogen and oxygen consumptions, a high ramp-up rate, and a long lifetime. To this end, a cascade-type stack design is employed and the cell components, including the membrane electrode assembly and bipolar plate, are evaluated using long-term performance tests. The feasibility of a fabricated 4-kW-class stack was confirmed through various performance evaluations. The proposed cascade-type stack exhibited a high efficiency of 65% and high hydrogen and oxygen utilizations of 99.89% and 99.68%, respectively, resulting in significantly lesser purge-gas emissions to the outside of the stack. The load-following test was successfully performed at a high ramp-up rate. The lifetime of the stack was confirmed by a 3500-h performance test, from which the degradation rate of the cell voltage was obtained. The advantages of the cascade-type stack were also confirmed by comparing its performance with that of a single-stage stack operating in dead-end mode.

Evaluation of a 2.5 kWel automotive low temperature PEM fuel cell stack with extended operating temperature range up to 120 °C

Nowadays, the operating temperature of polymer electrolyte membrane fuel cell stacks is typically limited to 80 °C due to water management issues of membrane materials. In the present work, short-term operation at elevated temperatures up to 120 °C and long-term steady-state operation under automotive relevant conditions at 80 °C are examined using a 30-cell stack developed at DLR. The high temperature behavior is investigated by using temperature cycles between 90 and 120 °C without adjustment of the gases dew points, to simulate a short-period temperature increase, possibly caused by an extended power demand and/or limited heat removal. This galvanostatic test demonstrates a fully reversible performance decrease of 21 ± 1% during each thermal cycle. The irreversible degradation rate is about a factor of 6 higher compared to the one determined by the long-term test. The 1200-h test at 80 °C demonstrates linear stack voltage decay with acceptable degradation rate, apart from a malfunction of the air compressor, which results in increased catalyst degradation effects on individual cells. This interpretation is based on an end-of-life characterization, aimed to investigate catalyst, electrode and membrane degradation, by determining hydrogen crossover rates, high frequency resistances, electrochemically active surface areas and catalyst particle sizes.

Novel approach to mathematical modeling of the complex electrochemical systems with multiple phase interfaces

This study presents a novel, macrohomogeneous concept of multi-dimensional mathematical modeling of industrial-scale electrochemical systems based on a plate‑and‑frame design. The main idea of the model concept presented is that it considers the studied system as an anisotropic continuum in all spatial directions. The anisotropy of transport in the individual system layers is obtained by effective anisotropic transport parameters. Interlayer/interfacial fluxes are described by source terms in the conservation equations. This model concept is theoretically straightforward, avoiding both excessive simplification of the physical definition of the problem and complex numerical algorithms. It enables the calculation of the local fields of the physical properties inside the apparatus even with industrial-scale dimensions by means of conventional computational hardware. The capabilities and limits of this approach are demonstrated by two examples: (a) a stationary isothermal two-dimensional mathematical model of a high temperature polymer electrolyte membrane fuel cell stack (100cells) with approximately 4 kW power output and (b) a stationary isothermal three-dimensional mathematical model of a pilot-scale electrodialysis unit (200 membrane pairs with a total membrane area of 82m2).

Thermal management of high temperature polymer electrolyte membrane fuel cell stacks in the power range of 1–10 kWe

Maintaining optimal temperature of the stack is critical for efficient operation of high temperature polymer electrolyte membrane fuel cells. While a number of possibilities of thermal management exist for small stacks, the problem becomes more complicated for larger stacks. In the present study, the thermal management of stacks in the power range of 1–10 kWe is considered through computational fluid dynamics simulations. It is shown that large stacks need to have dedicated cooling plates through which a coolant is circulated. Further, stacks of the size of 10 kWe can have reasonably low cell temperature variations (∼20 K) only by passing pre-heated liquid coolant through the coolant plates. Estimates show that the concomitant increase in the coolant flow rate induces large pressure drops, of the order of 30 bar, if a four-parallel serpentine is used on an active cell area of 30 cm × 30 cm. It is therefore necessary to use parallel channel flow fields with carefully designed feeder manifolds to maintain optimal cell temperatures and reasonably low coolant pressure drops in large stacks.

PEMFC stack voltage singularity measurement and fault classification

The study summarized in this paper deals with non-intrusive fault diagnosis of Polymer Electrolyte Membrane Fuel Cell (PEMFC) stack. In the proposed approach, the diagnosis operation is based on the stack voltage singularity measurement and classification. To this aim, wavelet transform-based multifractal formalism, named WTMM (Wavelet Transform Modulus Maxima), and pattern recognition methods are combined to realize the identification of the PEMFC faults. The proposed method takes advantage of the non-linearities associated with discontinuities introduced in the dynamic response data resulting from various failure modes. Indeed, the singularities signature of poor operating conditions (faults) of the PEMFC is revealed through the computing of multifractal spectra. The obtained good classification rates demonstrate that the multifractal spectrum based on WTMM is effective to extract the incipient fault features during the PEMFC operation. The proposed method leads to a promising non-intrusive and low cost diagnostic tool to achieve on-line characterizations of dynamical FC behaviors.

Leak diagnosis of polymer electrolyte membrane fuel cell stacks

Pressure decay test is usually used to highlight leakages inside the stack between anode, cathode and cooling compartments. Although this test can satisfactorily detect leaks occurring within the stack, it does not help in localization of the leak source in the stack. This paper presents some descriptions of two PEMFC stacks which have leakage between anode/cathode or cathode/cooling compartments. A variety of diagnosis techniques including IR thermography, Open Circuit Voltage decay, Crossover Current, Open Circuit Voltage growth are employed with the goal to locate the leak sources in the stacks. Furthermore, some techniques are presented to find the leak types and sources in the defective cells in the stack. Since hydrogen and air are not used simultaneously in the stack, the crossover current test is safer than the other methods. However, results of this test are strongly dependent on the nitrogen flow rate. The results of IR thermography show that this technique has a good potential to be used along with the other methods in the leak diagnosis of the PEMFC stacks. In fact, this technique can be used to build confidence in the results of the other leak diagnosis techniques.

Effectiveness of heat-integrated methanol steam reformer and polymer electrolyte membrane fuel cell stack systems for portable applications

Efficiently combining proton exchange membrane fuel cell (PEMFC) stack with methanol steam reformer (MSR) into a small portable system is still quite a topical issue. Using methanol as a fuel in PEMFC stack includes a series of chemical processes where each proceeds at a unique temperature. In a combined MSR–PEMFC-stack system with integrated auxiliary fuel processors (vaporizer, catalytic combustor, etc.) the processes are both endothermic and exothermic hence their proper thermal integration can help raising the system efficiency. A concept of such fully integrated and compact system is proposed in this study. Three separate systems are designed based on different PEMFC stacks and MSR. Low-temperature (LT) and conventional high-temperature (cHT) PEMFC stack characteristics are based on available data from suppliers. Also, a novel high-temperature (nHT) PEMFC stack is proposed because its operating temperature coincides with that of MSR. A comparative study of modelled systems is performed using a mass and energy balances zero-dimensional model, which is interdependently coupled to a physical model based on finite element method (FEM). The results indicate that a system with nHT PEMFC stack is feasible and has the potential to reach higher system efficiencies than systems with LT or cHT PEMFC stacks.