Coupled mechanical stress and multi-dimensional CFD analysis for high temperature proton exchange membrane fuel cells (HT-PEMFCs)

We use a combined finite element method (FEM)/computational fluid dynamics (CFD) methodology to numerically investigate the effects of gas diffusion layer (GDL) compression/intrusion on the performance of a phosphoric acid-doped polybenzimidazole (PBI) membrane-based high temperature proton exchange membrane fuel cell (HT-PEMFC). Three-dimensional (3-D) FEM simulations are conducted under various displacement clamping conditions to analyze cell deformation characteristics. Then, a multi-dimensional HT-PEMFC CFD model is applied to the deformed cell geometries to study transport and electrochemical processes during HT-PEMFC operations. Our numerical simulation results reveal that the maximum stresses in the deformed GDLs always occur near the edge of the ribs. The combined effects of GDL compression/intrusion considerably increase spatial non-uniformity in the species and current density distributions, and reduce cell performance.     

Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.     

Effects of inlet relative humidity (RH) on the performance of a high temperature-proton exchange membrane fuel cell (HT-PEMFC)

A high temperature-proton exchange membrane fuel cells (HT-PEMFC) based on phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane is able to operate at elevated temperature ranging from 100 to 200 °C. Therefore, it is evident that the relative humidity (RH) of gases within a HT-PEMFC must be minimal owing to its high operating temperature range. However, it has been continuously reported in the literature that a HT-PEMFC performs better under higher inlet RH conditions. In this study, inlet RH dependence on the performance of a HT-PEMFC is precisely studied by numerical HT-PEMFC simulations. Assuming phase equilibrium between membrane and gas phases, we newly develop a membrane water transport model for HT-PEMFCs and incorporate it into a three-dimensional (3-D) HT-PEMFC model developed in our previous study. The water diffusion coefficient in the membrane is considered as an adjustable parameter to fit the experimental water transport data. In addition, the expression of proton conductivity for PA-doped PBI membranes given in the literature is modified to be suitable for commercial PBI membranes with high PA doping levels such as those used in Celtec^® MEAs. Although the comparison between simulations and experiments shows a lack of agreement quantitatively, the model successfully captures the experimental trends, showing quantitative influence of inlet RH on membrane water flux, ohmic resistance, and cell performance during various HT-PEMFC operations.     

Effect of cooling surface temperature difference on the performance of high-temperature PEMFCs

Internal temperature distribution of the high-temperature proton exchange membrane fuel cell (HT-PEMFC) is affected by the cooling temperature, heat generation and reactant gas flow. Reasonable temperature control is helpful to improve the fuel cell performance and durability. In this work, a three-dimensional model that couples the reactant flows, species transport, heat transfer, charge transfer, and electrochemical reaction, was developed to simulate the HT-PEMFC operation. A solid mechanics model was established to analyze the stress distribution of the fuel cell. The polarization curves, distributions of temperature, membrane proton conductivity, current density and stress are investigated for different cooling surface temperature. Furthermore, the effect of assembly temperature on the stress of phosphoric acid-doped polybenzimidazole (PBI) membrane is discussed. Results reveals that the peak power density and uniformity of current density decrease with the increase of cooling surface temperature difference. The peak power density decreases by 9.14% when the temperature difference increases from 0 K to 40 K. The cooling surface temperature difference of less than 10 K and the voltage range in 0.5–0.7 V can achieve better current density uniformity and smaller current density change rate. In addition, the membrane in fuel cell has the highest stress, and increasing the assembly temperature is helpful to reduce the membrane stress. When the assembly temperature increases from 293.15 K to 343.15 K, the max and min compressive stresses in membrane in-plane decrease from 39.436 MPa to 31.416 MPa–24.934 MPa and 17.369 MPa at the temperature difference of 30 K, which decreases by 36.77% and 44.7%, respectively.     

Investigating the transport characteristics and cell performance for a micro PEMFC through the micro sensors and CFD simulations

The local transport characteristics and the global polarization curve for a self-made micro proton exchange membrane fuel cell (PEMFC) have been experimentally and numerically investigated in this paper. The micro-sensors are developed to measure the local fluid temperature, cell voltage, and current density and the fuel cell test system is used to measure the polarization curve. A three-dimensional (3-D) non-isothermal compressible computational fluid dynamics (CFD) full-cell model is also adopted to simulate the test micro PEMFC. This CFD model has been validated with these global and local data. The ionic conductivity is increased as the water content in the membrane increases, enhancing the cell performance. This positive effect of inlet fuel humidity on the cell performance is also captured by the CFD simulation model.     

The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells

The effect of relative humidity of the cathode (RHC) on proton exchange membrane (PEM) fuel cells has been studied focusing on automotive operation. Computational fluid dynamics (CFD) simulations were performed on a 300-cm² serpentine flow-field configuration at various RHC levels. The dependency of current density, membrane water contents, net water flux on the performance and the uniformity was investigated. The uniformity of current density and temperature was evaluated by employing standard deviation. The water balance inside a fuel cell was examined by describing electro-osmotic drag and back diffusion. It was concluded that the RHC has strong effect on the cell performance and uniformity. The dry RHC showed low cell voltage and non-uniform distributions of current density and temperature, whereas high RHC presented increased cell performance and uniform distributions of current density and temperature with well-hydrated membrane electrode assembly (MEA). Also the local current density distribution was strongly dependent on the local membrane water contents distribution that has complex phenomena of water transport. The elimination of external humidifier is desirable for the automotive operation, but it could degrade cell performance and durability due to dehydration of the MEA. Therefore a proper humidification of the reactant is necessary.     

Combined effect of channel to rib width ratio and gas diffusion layer deformation on high temperature – Polymer electrolyte membrane fuel cell performance

The present study investigates the combined influence of Channel to Rib Width (CRW) ratio and clamping pressure on the structure and performance of High Temperature-Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) using a three-dimensional numerical model developed previously. It also considers the impact of interfacial contact resistance between the Gas Diffusion Layer (GDL) and Bipolar Plate (BPP). The structural analysis of the single straight channel HT-PEMFC geometry shows that the von-Mises stress greatly increases in the GDL under the ribs as the CRW ratio increases resulting in considerably high deformation. The cell performance analysis depicts the significance of ohmic resistance and concentration polarization for different CRW ratios, particularly at higher operating current densities. However, in low to medium current density regions, the CRW ratio has little influence on cell performance. A substantial impact on the species, overpotential, and current distributions is observed. The findings also reveal that the CRW ratio significantly affects the temperature distribution in the cell.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

Stress and plastic deformation of MEA in fuel cells: Stresses generated during cell assembly

A linear elastic–plastic 2D model of fuel cell with hardening is developed for analysis of mechanical stresses in MEA arising in cell assembly procedure. The model includes the main components of real fuel cell (membrane, gas diffusion layers, graphite plates, and seal joints) and clamping elements (steel plates, bolts, nuts). The stress and plastic deformation in MEA are simulated with ABAQUS code taking into account the realistic clamping conditions. The stress distributions are obtained on the local and the global scales. The first one corresponds to the single tooth/channel structure. The global scale deals with features of the entire cell (the seal joint and the bolts). Experimental measurements of the residual membrane deformations have been provided at different bolts torques. The experimental data are in a good agreement with numerical predictions concerning the beginning of the plastic deformation.     

An integrated model of the water transport in nonuniform compressed gas diffusion layers for PEMFC

Water transport in gas diffusion layer (GDL) is a very important issue for high power density Proton Exchange Membrane Fuel Cell (PEMFC). During the GDL and bipolar plate (BPP) assembly process, the water transport behavior is greatly influenced by the nonuniform compression on the GDL, which leads to uneven distribution of the internal mass transport pores. In this study, an integrated model is developed to predict the water transport in nonuniform compressed GDL. Firstly, a GDL compression deformation model is built to obtain the relationship between the GDL deformation and assembly clamping force based on energy method. Then, a water transport model is established by considering the probability density function (PDF) of the pore size for the compressed GDL. The accuracy of the integrated model has been verified by comparing with the finite element method (FEM) and the computational fluid dynamics (CFD) simulation results. The influence of assembly clamping force, GDL thickness and channel geometry are analyzed based on the integrated model. Drainage pressure increases monotonically with the assembly clamping force and is divided into three stages. For the baseline case, 0.2 mm of GDL thickness and small rib-channel ratio is conducive to improving drainage capacity. It provides the guidance for matching of GDL/BPP assembly condition and performance prediction of PEMFC.     

Influence of the phosphoric acid-doping level in a polybenzimidazole membrane on the cell performance of high-temperature proton exchange membrane fuel cells

The acid migration in phosphoric acid-doped polybenzimidazole (PBI) membrane high-temperature proton exchange membrane fuel cells (HT-PEMFC) during operation is experimentally evaluated to clarify the influence of the acid balance between the membrane and electrodes on cell performance. A method for controlling the amount of phosphoric acid doped in PBI membranes is investigated, and PBI membranes with various amounts of phosphoric acid are prepared. Cell operation tests and AC impedance spectroscopy of cells fabricated with these membranes are conducted. It was found that the amount of phosphoric acid doped in the membranes can be controlled by changing the solution temperature and the immersion time in phosphoric acid solution. It was also found that the HT-PEMFC performance can be improved by optimizing the amount of phosphoric acid doped in the membrane and by diffusion of phosphoric acid into the catalyst layer during the initial stage of cell operation.     

Mechanical behavior of fuel cell membranes under humidity cycles and effect of swelling anisotropy on the fatigue stresses

The mechanical response of proton exchange membranes in a fuel cell assembly is investigated under humidity cycles at a constant temperature (85 °C). The behavior of the membrane under hydration–dehydration cycles is simulated by imposing a humidity gradient from the cathode to the anode. Linear elastic, plastic constitutive behavior with isotropic hardening and temperature and humidity dependent material properties are utilized in the simulations for the membrane. The evolution of the stresses and plastic deformation during the humidity cycles are determined using finite element analysis for two clamping methods and various levels of swelling anisotropy. The membrane response strongly depends on the swelling anisotropy where the stress amplitude decreases with increasing anisotropy. These results suggest that it may be possible to optimize a membrane with respect to swelling anisotropy to achieve better fatigue resistance, potentially enhancing the durability of fuel cell membranes.     

Thermal and electrochemical performance analysis of a proton exchange membrane fuel cell under assembly pressure on gas diffusion layer

In this study, the effect of clamping pressure on the performance of a proton exchange membrane fuel cell (PEMFC) is investigated for three different widths of channel. The deformation of gas diffusion layer (GDL) due to clamping pressure is modeled using a finite element method, and the results are applied as inputs to a CFD model. The CFD analysis is based on finite volume method in non-isothermal condition. Also, a comparison is made between three cases to identify the geometry that has the best performance. The distribution of temperature, current density and mole fraction of oxygen are investigated for the geometry with best performance. The results reveal that by decreasing the width of channel, the performance of PEMFC improves due to increase of flow velocity. Also, it is found that intrusion of GDL into the gas flow channel due to assembly pressure deteriorates the PEMFC performance, while decrease of GDL thickness and GDL porosity have smaller effects. It is shown that assembly pressure has a minor effect on temperature profile in the membrane-catalyst interface at cathode side. Also, assembly pressure has a significant effect on ohmic and concentration losses of PEMFC at high current densities.     

Parameter study of high-temperature proton exchange membrane fuel cell using data-driven models

In this paper, a numerical model of high-temperature proton exchange membrane fuel cell (HT-PEMFC) was developed, in which the thermal and electrical properties were treated as temperature dependent. Based on the numerical simulation, the needed training data was acquired and used for the development of data-driven model via the artificial neural network (ANN) algorithm. The developed data-driven model was then used to predict the performance of HT-PEMFC. The simulation results indicated that the deviation of ANN prediction was less than 2.48% compared with numerical simulation. The effects of various influential factors, such as the geometry size of the gas flow channel, the thickness of the membrane and the operating temperature, could be predicted easily by using the ANN model. The ANN model prediction results showed that the more compact fuel cell and the higher operating temperature improved the performance of HT-PEMFC. The proposed ANN model and the parameters study will contribute to the further design and operation of HT-PEMFC.     

Phosphosilicate nano-network (PPSN)-Polybenzimidazole (PBI) composite electrolyte membrane for enhanced proton conductivity,durability and power generation of HT-PEMFC

Here we report a composite electrolyte membrane of Polybenzimidazole (PBI) with Phosphosilicate nano-network (PPSN) for enhanced proton conductivity, durability and power generation of high temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Solid state proton conductor three dimensional Phosphosilicate nano-network (average particle size <10 nm) is synthesized using easy and low-cost sol gel method followed by ball milling and composited with PBI at different loading employing methane sulfonic acid (MSA) as solvent. The electrolyte membrane is characterized using FESEM, XRD, FTIR, TGA; proton conductivity, ion exchange capacity, water uptake and acid doping level, chemical stability and mechanical yield strength are measured and the membrane is tested for HT-PEMFC application. Property and performance mapping reveals that with 10% PPSN loading, composite (PPSN-PBI-10) membrane offers the maximum enhancement of all properties and power generation of HT-PEMFC, while beyond a critical loading (～22%) properties and performance deteriorate below that of pristine PBI. Using optimum loading of PPSN, compared to pristine PBI, a remarkable rise in water uptake and acid doping level is achieved that facilitates proton conduction; also in spite of the presence of Phosphoric acid in the PPSN filler, the maximum 47.5% enhancement of ultimate strength is attained. The performance of HT-PEMFC using composite PPSN-PBI unveil that almost 2 times (100%) enhancement of peak power generation (～0.73 W cm^?2) is achieved using PPSN-PBI-10 at 170 °C operating temperature compared to pristine PBI. This may be attributed to the facilitated proton conduction through the extended tunnelling network offered by PPSN. Incorporation of PPSN improves the durability; over 48 h only 16% decay in voltage is noticed using PPSN-PBI-10 membrane which is remarkably lower than the 31% decay of pristine PBI membrane.     

Comparison of temperature distributions inside a PEM fuel cell with parallel and interdigitated gas distributors

A comparison of the temperature distributions in a proton exchange membrane (PEM) fuel cell between the parallel-flow gas distributors and the interdigitated gas distributor has been discussed in detail. An electrochemical–thermal coupled numerical model in a five-layer membrane-electrode assembly (MEA) is developed. The temperatures for the reactant fuels as well as the carbon fibers in the porous electrode are predicted by using a CFD technique. The overpotential across the MEA is varied to examine its effect on the temperature distributions of the PEM fuel cell. It is found that both the fuel temperature and the carbon fiber temperature are increased with increasing the total overpotential. In addition, the fuel and carbon-fiber temperature distributions are significantly affected by the flow pattern that cast on the gas distributor. Replacing the parallel-flow gas distributor by the interdigitated gas distributor will increase the local maximum temperature inside the PEM fuel cell.     

Study of different designs of methanol steam reformers: Experiment and modeling

Three reformers with different designs (multi-channel, radial and tubular) were developed for thermal integration with a high temperature polymeric electrolyte membrane fuel cell (HT-PEMFC). They were characterized experimentally at temperatures between 443 K and 473 K, using the commercial catalyst G66 MR from Süd-Chemie (CuO/ZnO/Al₂O₃). The reactors were modeled and simulated using a computational fluid dynamics (CFD) analysis. The models were validated using experimental data.The results showed that the multi-channel design is the best solution for thermal integration with a HT-PEMFC, presenting high methanol conversion and low pressure drop. Regarding the heat transfer ability, the multi-channel showed also the best performance, presenting the lowest temperature sink among the studied reformers. The low flow velocities and the absence of metallic surfaces in the radial reformer had detrimental effect on the heat transfer. Concerning the flow distribution a coefficient of variation of 0.6% was observed in the multichannel reformer. A quasi plug flow behavior was found in the tubular and a multichannel (channels region only) reformer, while in the radial a not fully developed laminar flow was found.At temperatures lower than 473 K was found that the reformate stream did not require further purification to be fed to a HT-PEMFC due to the low CO concentration (<1600 ppm).The advantages and limitations of each design is discussed based on experimental data and CFD modeling.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Modeling and simulation of a PEM fuel cell stack considering temperature effects

Management of the water and heat ejected as byproducts in an operating PEM fuel cell stack are crucial factors in their optimal design and safe operations. Models currently available for a PEM fuel cell are based on either empirical or 3-D computational fluid dynamics (CFD). Both models do not fully meet the need to represent physical behavior of a stack because of either their simplicity or complexity. We propose a highly dynamic PEM fuel cell stack model, taking into account the most influential property of temperature affecting performance and dynamics. Simulations have been conducted to analyze start-up behaviors and the performance of the stack in conjunction with the cells. Our analyses demonstrate static and dynamic behaviors of a stack. Major results presented are as follows: (1) operating dependent temperature gradient across through-plane direction of the fuel cell stack, (2) endplate effects on the temperature profile during start-up process, (3) temperature profile influences on the output voltage of individual cells and the stack, (4) temperature influence on the water content in membranes of different cells, and (5) cathode inlet relative humidity influence on the temperature profile of the stack.     

In-situ diagnosis on performance degradation of high temperature polymer electrolyte membrane fuel cell by examining its electrochemical properties under operation

Ex-situ electrochemical characterization techniques could significantly alter or misrepresent the materials of high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) to the point where they are not reflective of their conditions during operation, resulting in difficulties in obtaining realistic fuel cell durability. To minimize this disturbance, we proposed an in-situ low-invasive technique of electrochemical impedance spectroscopy (EIS), combining with polarization curve and Tafel slope analysis, to investigate the performance degradation of HT-PEMFC. The membrane electrode assemblies (MEAs) used in the HT-PEMFC were lab-made but with commercial catalyst and poly(2,5-benzimidazole) (ABPBI) membrane. Two common test modes, i.e. steady-state operation and dynamic-state operation, were employed to mimic practical HT-PEMFC operation. By examining the changes of electrochemical properties of the HT-PEMFC under steady- or dynamic-state operation, the main mechanism for the performance degradation can be determined. The results from the study suggests that a high cell performance decay rate cannot be directly attributed to materials degradation, especially in a short-term steady-state operation. In contrast, the change of Tafel slope can be seen as a clear indicator to determine the extent of catalyst degradation of HT-PEMFC, no matter which test protocol was applied. Post-analysis of TEM on the catalysts before and after tests further confirmed the main mechanism for the performance losses of the HT-PEMFCs underwent two test protocols, while acid loss and membrane degradation were considered to be negligible during the short-term tests.