Study of the impact of water management on the performance of PEMFC commercial stacks by impedance spectroscopy

Water management is critical for PEMFCs, especially at system level. To study its impact on the performance and obtain useful indicators for fault detection, two commercial stacks were characterized by impedance spectroscopy under various humidities and current densities. One was fresh while the other was operated on-field during 10 000 h. Four capacitive and one inductive loops are identified. The capacitive loops are attributed to charge transfer kinetics at both anode and cathode and to cathodic and anodic mass transfer limitations. The inductive loop is attributed to oscillations in the ionomer water content induced by oscillations in the amount of produced water during the measurement. The size of the cathodic charge transfer loop and its departure angle at high frequencies increase as humidity decreases because the ionomer conductivity decreases. These parameters are potential indicators of the cathode drying. The angles are below 45° at low humidities but close to 90° in sur-saturated conditions.     

Effect of channel-rib width ratio and relative humidity on performance of a single serpentine PEMFC based on electrochemical impedance spectroscopy

The geometry configuration of proton exchange membrane fuel cell (PEMFC) which is considered as a promising energy conversion device has great influence on PEMFC performance. In this paper, effect of channel-to-rib width ratio and relative humidity of reactant gas on the performance are compared based on two single PEMFCs. The EIS testing results below 50 A are given and analyzed. The results obtained from polarization curves, power density curves and EIS fitting results prove that: 1. Compared with cell 3:4, the anode high humidification has a greater addition to the performance of cell 1:1; 2. PEMFCs with different geometry configurations of flow field have their own suitable working condition ranges; 3. The charge transfer resistance is the dominating factor when current loading is below 2.0 A cm^?2.     

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.     

Mechanism analysis of starvation in PEMFC based on external characteristics

As an energy conversion device that converts hydrogen energy into electrical energy, the fuel cell is one of the most promising. Starvation is the main reasons that shortens the lifetime of Proton Exchange Membrane Fuel Cell (PEMFC) for vehicle usage. Therefore, combining the experimental and simulation results, as well as previous studies, based on the external characteristics of PEMFCs, the mechanism of judging the starvation in PEMFCs is explored. It provides a theoretical basis for judging the starvation using external characteristics. In addition, the starvation index is proposed to solve the problem that the starvation in the process of fuel cell loading, which could not be evaluated in the past. This factor can provide guidance for matching and controlling the fuel cell supply system, as well as optimizing the internal structural parameters of the fuel cell.     

The impact of air contaminants on PEMFC performance and durability

The impact of air contaminants such as sulfur compounds (SO₂, H₂S) and nitrogen compounds (NO_x and NH₃) was investigated using subscale fuel cells. The severity of the effect of these impurities varies depending on the contaminants. Among air contaminants, sulfur compounds cause the most severe performance loss due to decrease of available Pt sites for oxygen reduction reaction (ORR). We found that sulfur compounds adsorbed on Pt surface tend to be oxidized to sulfate at 0.9 V and higher potentials. The cell performance can be recovered partially by excursions to high potentials due to increase of available Pt sites. Furthermore, flushing the cathode with high humidity gases results in almost complete recovery of the cell performance. We conclude that these recovery effects are due to oxidation/removal of the contaminants from the Pt surface.     

The impact of flow field pattern on concentration and performance in PEMFC

In this study, we present a rigorous mathematical model, to treat prediction and analysis of proton exchange membrane fuel cells gas concentration and current density distribution in mass transfer area and chemical reaction area performed in 3‐D geometry. The model is based on the solution of the conservation equations of mass, momentum, species, and electric current in a fully integrated finite‐volume solver using the CFDRC commercial code. The influences of fuel cell performance with two kinds of flow channel pattern design are studied. The gas concentration of the straight flow pattern appears excessively non‐uniform, resulting in a local concentration polarization. On the other hand, the gas concentration is well distributed for the serpentine flow pattern, creating a better mass transfer phenomena. The performance curves (polarization curves) are also well correlated with experimental data. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of gas permeation on the sealing performance of PEMFC stacks

The sealing performance of gaskets is critical to the safety of PEMFC stacks. Leak through the interface between gasket and BPP or MEA frame has been widely investigated, while gas permeation through gaskets is ignored in most studies. In this work, helium mass spectrometry leak measurement and finite element calculation were used to investigate the leaking and sealing mechanisms. It shows that interfacial leak and gas permeation of gaskets could be distinguished by their difference in terms of steady-state flow rate and stabilization process. Gas permeation is the main leak path of gaskets. The permeation flow rate is non-negligible for the safety of fuel cell stacks and needs to be considered when designing gaskets.     

Examination of compression effects on PEMFC performance by numerical and experimental analyses

In the present study, the effects of compression method on Proton Exchange Membrane Fuel Cell (PEMFC) performance were investigated both numerically and experimentally. Total deformation of the components within the PEMFC was simulated by ANSYS three-dimensional finite element analysis (3D FEA). Moreover, geometrical and material properties of all components of PEMFC such as bipolar plates (BPP), membrane electrode assembly (MEA), gasket, current collector plate (CCP), screw and nut were implemented for accurate simulation of compression. In the experimental part, PEMFC tests were performed with 25 cm² active area single cell having 3 channel parallel in series (3 PS) flow channel via PEMFC test station with H₂ and air at 60 °C. The maximum power density was achieved as 0.458 W/cm² and 0.480 W/cm² for bolt compression and clamping plates compression, respectively. The equivalent stress values were found as 120 MPa that under 4389 N the clamping plates and 1600 MPa under bolt compression with 1.3 Nm torque. When numerical and experimental studies are examined together, it is seen that bolt compression has higher deformation and less equivalent stress than clamping plates compression.     

Numerical investigation on the performance of PEMFC with rib-like flow channels

An innovative flow channel inspired by the physical structure of the human rib was developed in this paper. The performance of a proton exchange membrane fuel cell (PEMFC) with the proposed rib-like flow channels under different flow patterns and relative humidity of anode (RH_a) was investigated. Compared with the conventional interdigitated flow channel (CIFC) and cross-flow channel (CRFC), the maximum current density of the counter-flow channel (COFC) was 1.06 A/cm² at 0.4 V, with enhancements of 4.95% and 2.91%, respectively. In addition, the quantity referred to as non-uniformity N was introduced to quantify the concentration distribution of oxygen, the minimum non-uniformity N of 0.17 was obtained for CRFC, and the COFC exhibited a more uniform concentration distribution of temperature as compared with the CIFC and CRFC, indicating that the COFC would prevent the occurrence of local hot spots. The maximum net power density of COFC was 6.0% and 3.0% higher than that of the CIFC and CRFC. Finally, the maximum current density of RH_a = 30% was 1.06 A/cm², which was 3.9% and 7.1% higher than that of RH_a = 60% and RH_a = 100%. The temperature with RH_a = 100% was more uniform in comparison with RH_a = 30% and RH_a = 60%, and the mass fraction of H₂ decreased with the increase of values of RH_a. The proposed rib-like flow channel can further enrich PEMFC flow channel design and afford novel insights into the application of bionics in fuel cells.     

Numerical study on the compression effect of gas diffusion layer on PEMFC performance

A numerical method is developed to study the effect of the compression deformation of the gas diffusion layer (GDL) on the performance of the proton exchange membrane fuel cell (PEMFC). The GDL compression deformation, caused by the clamping force, plays an important role in controlling the performance of PEMFC since the compression deformation affects the contact resistance, the GDL porosity distribution, and the cross-section area of the gas channel. In the present paper, finite element method (FEM) is used to first analyze the ohmic contact resistance between the bipolar plate and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and products. We investigate the effects of the GDL compression deformation, the ohmic contact resistivity, the air relative humidity, and the thickness of the catalyst layer (CL) on the performance of the PEMFC. The numerical results show that the fuel cell performance decreases with increasing compression deformation if the contact resistance is negligible, but an optimal compression deformation exists if the contact resistance is considerable.     

Analyze the effects of flow mode and humidity on PEMFC performance by equivalent membrane conductivity

A three‐dimensional and two‐phase numerical model is developed for a 25‐cm² proton exchange membrane fuel cell (PEMFC) to investigate the effects of flow mode (coflow and counterflow) and relative humidity (anode 0%/100%; cathode 60%/100%) on the cell performance. Experimental studies are performed to validate this developed model. An equivalent membrane conductivity is proposed to describe the match level between current flux and membrane conductivity. It is found that the cell performance is enhanced under low relative humidity conditions because of the optimized equivalent membrane conductivity. More specifically, the voltage is improved from 0.611 to 0.637 V, and the equivalent membrane conductivity is enhanced from 10.35 to 11.11 S m^?1 by replacing the coflow mode with counterflow mode at 1000 mA cm^?2 when anode gas is dry and cathode gas is 100% hydrated. Both the anode and cathode relative humidities show an obvious influence on the PEMFC performance, and a suitable inlet humidity could ensure adequate hydration of membrane and avoid water flooding in gas diffusion layers simultaneously.     

Study of PEM fuel cell performance by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a suitable and powerful diagnostic testing method for fuel cells because it is non-destructive and provides useful information about fuel cell performance and its components. This paper presents the diagnostic testing results of a 120 W single cell and a 480 W PEM fuel cell short stack by electrochemical impedance spectroscopy. The effects of clamping torque, non-uniform assembly pressure and operating temperature on the single cell impedance spectrum were studied. Optimal clamping torque of the single cell was determined by inspection of variations of high frequency and mass transport resistances with the clamping torque. The results of the electrochemical impedance analysis show that the non-uniform assembly pressure can deteriorate the fuel cell performance by increasing the ohmic resistance and the mass transport limitation. Break-in procedure of the short stack was monitored and it is indicated that the ohmic resistance as well as the charge transfer resistance decrease to specified values as the break-in process proceeds. The effect of output current on the impedance plots of the short stack was also investigated.     

Visualization of flooding in a single cell and stacks by using a newly-designed transparent PEMFC

The flow phenomenon in the fuel-cell channels is difficult to understand and predict because of the two-phase flow. Proton exchange membrane fuel cells (PEMFCs) with transparent windows are widely used for visualizing the two-phase flow in the channels. In this paper, the visualization of the two-phase flow in the channels was accomplished under various current-density conditions using a transparent cell. The visualization of the single serpentine flow field clearly reveals that anode flooding is more severe than cathode flooding. The main cause for anode flooding is a low gas-flow rate in the channel because of the absence of the carrier gas. In addition, flooding is more significant under a low current-density condition than under a high current-density condition; under the latter condition, there is significantly more reaction heat that prevents flooding. The flow phenomena in the PEMFC stack were also visualized by electrically connecting three transparent cells in series and supplying fuel to each cell from a manifold. Sudden voltage drops and overshoots were detected, and the voltage fluctuations were found to be strongly related to flooding.     

Experimental study on rapid cold start-up performance of PEMFC system

The cold start-up of a proton exchange membrane fuel cell is considered one of the main factors affecting the commercialization of fuel cell vehicles. In this study, an automotive fuel cell system was designed and tested for cold start-up at low temperatures. In the absence of PTC (Positive Temperature Coefficient) heating device, the stack was directly loaded to generate heat, which provided the cold start-up characteristics of system at low temperatures. Cold start-up process and purging control strategies were analyzed at −20 °C and −30 °C. It was found that the fuel cell system could produce 50% power in 25 s at −20 °C, the coolant temperature's heating rate was 0.78 °C/s, the coolant outlet temperature could reach 20 °C within 40 s and no apparent low voltage of single cell occurred. While, the cell close to the end plate had low cell voltage and reverse polar phenomena throughout the −30 °C cold start-up process. The heating rate of the coolant temperature was 0.44 °C/s, and the temperature of coolant outlet reached 20 °C within 90 s. The purging time ranged from 180 to 260 s according to the voltage drop value of stack and the ohmic resistance of stack was 360–470 mΩ after the high-volume air purging at different tests. After 30 cold start-up tests, the rated point performance of the stack declined by about 1%, and the consistency of cell voltages did not change significantly. Future work will focus on optimizing cold start-up strategy and speeding up purging time to minimize the performance impact of the cold start-up.     

Experiment and simulation investigations for effects of flow channel patterns on the PEMFC performance

Experiments and simulations are presented in this paper to investigate the effects of flow channel patterns on the performance of proton exchange membrane fuel cell (PEMFC). The experiments are conducted in the Fuel Cell Center of Yuan Ze University and the simulations are performed by way of a three‐dimensional full‐cell computational fluid dynamics model. The flow channel patterns adopted in this study include the parallel and serpentine flow channels with the single path of uniform depth and four paths of step‐wise depth, respectively. Experimental measurements show that the performance (i.e. cell voltage) of PEMFC with the serpentine flow channel is superior to that with the parallel flow channel, which is precisely captured by the present simulation model. For the parallel flow channel, different depth patterns of flow channel have a strong influence on the PEMFC performance. However, this effect is insignificant for the serpentine flow channel. In addition, the calculated results obtained by the present model show satisfactory agreement with the experimental data for the PEMFC performance under different flow channel patterns. These validations reveal that this simulation model can supplement the useful and localized information for the PEMFC with confidence, which cannot be obtained from the experimental data. Copyright © 2007 John Wiley & Sons, Ltd.     

Study of the effect of mechanical uncertainties parameters on performance of PEMFC by coupling a 3D numerical multiphysics model and design of experiment

As the PEMFC is a complex multi-physics device whose reliability and durability depend on the thermal-mechanical-electrical and chemical parameters. In this paper, theoretical and numerical studies is proposed to optimize the fuel cell performance using multiphysics model and design of experiments. 3D finite element analysis including a fully coupling of thermal-electrical-mechanical model is proposed to predict the electrical resistance of fuel cell. As the mechanical parameters (bending radius of the bipolar plate, thickness of the GDL and clamping pressure) remain uncertain, the design of experiments procedure is used to optimize the fuel cell behavior under several conditions.     

Implications of electronic short circuiting in plasma sprayed solid oxide fuel cells on electrode performance evaluation by electrochemical impedance spectroscopy

Electronic short circuiting of the electrolyte in a solid oxide fuel cell (SOFC) arising from flaws in the plasma spray fabrication process has been found to have a significant effect on the perceived performance of the electrodes, as evaluated by electrochemical impedance spectroscopy (EIS). The presence of a short circuit has been found to lead to the underestimation of the electrode polarization resistance (R_p) and hence an overestimation of electrode performance. The effect is particularly noticeable when electrolyte resistance is relatively high, for example during low to intermediate temperature operation, leading to an obvious deviation from the expected Arrhenius-type temperature dependence of R_p. A method is developed for determining the real electrode performance from measurements of various cell properties, and strategies for eliminating the occurrence of short circuiting in plasma sprayed cells are identified.     

Development of new test instruments and protocols for the diagnostic of fuel cell stacks

In the area of fuel cell research, most of the experimental techniques and equipments are still devoted to the analysis of single cells or very short stacks. However, the diagnosis of fuel cell stacks providing significant power levels is a critical aspect to be considered for the integration of fuel cell systems into real applications such as vehicles or stationary gensets. In this article, a new instrument developed in-lab is proposed in order to satisfy the requirements of electrochemical impedance studies to be led on large FC generators made of numerous individual cells. Moreover, new voltammetry protocols dedicated to PEMFC stack analysis are described. They enable for instance the study of membrane permeability and loss of platinum activity inside complete PEMFC assemblies.     

A numerical study on the performance of PEMFC with wedge-shaped fins in the cathode channel

The gas flow field design has a significant influence on the overall performance of a proton exchange membrane fuel cell (PEMFC). A single-channel PEMFC with wedge-shaped fins in the cathode channel was proposed, and the effects of fin parameters such as volume (0.5 mm³, 1.0 mm³, and 1.5 mm³), number (3, 5, and 9), and porosity of the gas diffusion layer (GDL) (0.2, 0.4, 0.6, and 0.8) on the performance of PEMFC were numerically examined based on the growth rate of power density (GRPD) and polarization curve. It was shown that wedge-shaped fins could effectively improve the PEMFC performance. With an increase in fin volume, the distributions of oxygen mass fraction in the outlet area of the cathode channel were lower, the drainage effect of the PEMFC improved, and GRPD also increased accordingly. Similar results were obtained as the number of fins increased. The GDL porosity had a greater effect than the wedge-shaped fins on the improvement in PEMFC performance, but the influence of GDL porosity weakened and the GRPD of porosity decreased as the porosity increased. This study provides an effective guideline for the optimization of the cathode channel in a PEMFC.     

Effect of operational parameters on the performance of PEMFC assembled with Au-coated Ni-foam

An innovative proton exchange membrane fuel cell was assembled using Au-coated nickel foam instead of the conventional flow field (carbon plate). The effect of operational parameters on the performance of this cell was investigated by DC polarization and electrochemical impedance spectroscopy techniques. Parameters such as cell operating temperature, cathode humidification temperature, and cathode-gas stoichiometry were of concern.