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.     

Numerical study of thermal stresses in high-temperature proton exchange membrane fuel cell (HT-PEMFC)

The purpose of this work is to numerically examine the thermal stress distributions in a high-temperature proton exchange membrane fuel cell (HT-PEMFC) based on a phosphoric acid doped polybenzimidazole (PBI) membrane. A fluid structure interaction (FSI) method is adopted to simulate the expansion/compression that arises in various components of a membrane electrode assembly (MEA) during the HT-PEMFC assembly processes, as well as during cell operations. First, three-dimensional (3-D) finite element method (FEM) simulations are conducted to predict the cell deformation during cell clamping. Then, a nonisothermal computational fluid dynamic (CFD)-based HT-PEMFC model developed in a previous study [1] is applied to the deformed cell geometry to estimate the key species and temperature distributions inside the cell. Finally, the temperature distributions obtained from these CFD simulations are employed as the input load for 3-D FEM simulations. The present numerical study provides a fundamental understanding of the stress–temperature interaction during HT-PEMFC operations and demonstrates that the coupled FEM/CFD HT-PEMFC model presented in this paper can be used as a useful tool for optimizing HT-PEMFC clamping and operating conditions.     

Investigation of parameter effects on the performance of high-temperature PEM fuel cell

The performance of high-temperature PEM fuel cell (HT-PEMFC) is substantially influenced by physical parameters. In this work, the effects of three parameters on the performance of HT-PEMFC are studied by using a 3D model in COMSOL. The parameters are the operating temperature, membrane's thickness and catalyst layer's thickness. The polarization curves are adopted to analyze the effects on the performance. The results show that the increase of temperature can enhance the performance of fuel cell. For the effect of catalyst layer thickness, the cell performance is promoted as the catalyst layer's thickness decreases. For the effect of the thickness of membrane, it is found that the thinner membrane of fuel cell can achieve better performance. These findings can be further extended to guide the operation and design of HT-PEMFC in practical applications.     

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.     

Modeling and analysis of a 5 kWe HT-PEMFC system for residential heat and power generation

We present a high-temperature proton exchange membrane fuel cell (HT-PEMFC) system model that accounts for fuel reforming, HT-PEMFC stack, and heat-recovery modules along with heat exchangers and balance of plant (BOP) components. In the model developed for analysis, the reaction kinetics for the fuel reforming processes are considered to accurately capture exhaust gas compositions and reactor temperatures under various operating conditions. The HT-PEMFC stack model is simplified from the three-dimensional HT-PEMFC CFD models developed in our previous studies. In addition, the parasitic power consumption and waste heat release from the various BOP components are calculated based on their heat-capacity curves. An experimental fuel reforming reactor for a 5.0 kW_e HT-PEMFC system was tested to experimentally validate the fuel reforming sub model. The model predictions were found to be in good agreement with the experimental data in terms of exhaust gas compositions and bed temperatures. Additionally, the simulation revealed the impacts of the burner air-fuel ratio (AFR) and the steam reforming reactor steam-carbon ratio on the system performance and efficiency. In particular, the combined heat and power efficiency of the system increased up to 78% when the burner AFR was properly adjusted. This study clearly illustrates that an HT-PEMFC system requires a high degree of thermal integration and optimization of the system configuration and operating conditions.     

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.     

Experimental and numerical investigation on effects of cathode flow field configurations in an air-breathing high-temperature PEMFC

Air-breathing high-temperature proton exchange membrane fuel cell (HT-PEMFC) gets rid of the cumbersome air supplying systems and avoids the water flooding problem by directly exposing the cathode to air and operating the fuel cell at elevated temperature. Performance of the air-breathing HT-PEMFC is dependent on many factors particularly the cathode flow field configurations. However, studies about air-breathing HT-PEMFCs are quite limited in the literature. In the present study, an experimental testing system was setup for the performance measurement of the air-breathing HT-PEMFC. A 3D numerical model was established and validated by the experimental data. Effects of the cathode flow field configurations including the opening shape, end plate thickness, open ratio and opening direction on performance of the air-breathing HT-PEMFC were experimentally and numerically investigated. It was found that the cathode end plate thickness and upward or sideways orientation have the least effect on the performance. The maximum power density of 160 mW/cm² at the current density of 394 mA/cm² can be achieved for the cathode flow field with slot holes and an open ratio of 75%.     

Development of a one-dimensional and semi-empirical model for a high temperature proton exchange membrane fuel cell

High temperature proton exchange membrane fuel cells (HT-PEMFC), which operate between 160 °C and 200 °C, can be generally used in portable and stationary power generation applications. In this study, a one-dimensional, semi-empirical, and steady-state model of a HT-PEMFC fed with a gas mixture consisting of hydrogen and carbon monoxide is developed. Some modeling parameters are adjusted using empirical data, which are obtained conducting experiments on a HT-PEMFC for different values of Pt loading and cell temperature. For adjusting these parameters, the total summation of the square of the difference between the cell voltages found using the experimental and theoretical methods is minimized using genetic algorithm. After finding the values of the adjusted parameters, the effects of different cell temperature, Pt loading, phosphoric acid (PA) percentage, and different binders (PBI and PVDF) on the performance of the fuel cell are examined. It was found that, the performance of the fuel cell using PVDF binder exhibited better performance as compared to that using PBI binder.     

Numerical simulation of proton exchange membrane fuel cells at high operating temperature

A three-dimensional, single-phase, non-isothermal numerical model for proton exchange membrane (PEM) fuel cell at high operating temperature (T ≥ 393 K) was developed and implemented into a computational fluid dynamic (CFD) code. The model accounts for convective and diffusive transport and allows predicting the concentration of species. The heat generated from electrochemical reactions, entropic heat and ohmic heat arising from the electrolyte ionic resistance were considered. The heat transport model was coupled with the electrochemical and mass transport models. The product water was assumed to be vaporous and treated as ideal gas. Water transportation across the membrane was ignored because of its low water electro-osmosis drag force in the polymer polybenzimidazole (PBI) membrane. The results show that the thermal effects strongly affect the fuel cell performance. The current density increases with the increasing of operating temperature. In addition, numerical prediction reveals that the width and distribution of gas channel and current collector land area are key optimization parameters for the cell performance improvement.     

Performance prediction of PEM fuel cell with wavy serpentine flow channel by using artificial neural network

Effects of serpentine flow channel having sinusoidal wave at the rib surface on performance of PEMFC having 25 cm² active area are investigated at different flow rates, three different amplitudes changing from 0.25 mm to 0.75 mm and three different cell operation temperatures. A proton exchange membrane fuel cell (PEMFC) is modeled for the prediction of the output current by using artificial neural network (ANN) that is utilized the aforementioned experimental parameters. Effect of hydrogen and air flow rate, the fuel cell temperature, amplitude of channel is tested. The results indicated that model C1 having lowest amplitude is enhanced maximum power output up to 20.15% as compared to indicated conventional serpentine channel (model C4) for 0.7 SLPM H₂ and 1.5 SLPM air and also model C1 has better performance than C2, C3 and C4 models. The maximum power output is augmented with increasing the cell temperature due to raising the fuel and oxidant diffusion ratio. Cell temperature, amplitude, H₂ and air flow rate and input voltage is used as input variables in train and test of the developing ANN model. MAPE of training and testing is determined as 2.89 and 2.059, respectively. Prediction results of developed ANN model including two hidden layer shows similar trend with experimental results. Developed ANN model can be used to both decrease the number of required experiments and find the optimum operation condition within the range of input parameters.     

Composite membrane by incorporating sulfonated graphene oxide in polybenzimidazole for high temperature proton exchange membrane fuel cells

The objective of this work is to examine the polybenzimidazole (PBI)/sulfonated graphene oxide (sGO) membranes as alternative materials for high-temperature proton exchange membrane fuel cell (HT-PEMFC). PBI/sGO composite membranes were characterized by TGA, FTIR, SEM analysis, acid doping&acid leaching tests, mechanical analysis, and proton conductivity measurements. The proton conductivity of composite membranes was considerably enhanced by the existence of sGO filler. The enhancement of these properties is related to the increased content of –SO₃H groups in the PBI/sGO composite membrane, increasing the channel availability required for the proton transport. The PBI/sGO membranes were tested in a single HT-PEMFC to evaluate high-temperature fuel cell performance. Amongst the PBI/sGO composite membranes, the membrane containing 5 wt. % GO (PBI/sGO-2) showed the highest HT-PEMFC performance. The maximum power density of 364 mW/cm² was yielded by PBI/sGO-2 membrane when operating the cell at 160 °C under non humidified conditions. In comparison, a maximum power density of 235 mW/cm² was determined by the PBI membrane under the same operating conditions. To investigate the HT-PEMFC stability, long-term stability tests were performed in comparison with the PBI membrane. After a long-term performance test for 200 h, the HT-PEMFC performance loss was obtained as 9% and 13% for PBI/sGO-2 and PBI membranes, respectively. The improved HT-PEMFC performance of PBI/sGO composite membranes suggests that PBI/sGO composites are feasible candidates for HT-PEMFC applications.     

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.     

Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells

Electrochemical carbon corrosion occurring in a high temperature proton exchange membrane fuel cell (HT-PEMFC) operating under non-humidification conditions was investigated by measuring CO₂ generation using on-line mass spectrometry and comparing the results with a low-temperature proton exchange membrane fuel cell (LT-PEMFC) operated under fully humidified conditions. The experimental results showed that more CO₂ was measured for the HT-PEMFC, indicating that more electrochemical carbon corrosion occurs in HT-PEMFCs. This observation is attributed to the enhanced kinetics of electrochemical carbon corrosion due to the elevated operating temperature in HT-PEMFCs. Additionally, electrochemical carbon corrosion in HT-PEMFCs showed a strong dependence on water content. Therefore, it is critical to remove the water content in the supply gases to reduce electrochemical carbon corrosion.     

Energy and exergy performance assessments of a high temperature-proton exchange membrane fuel cell based integrated cogeneration system

High-temperature proton exchange membrane fuel cell (HT-PEMFC), which operates between 160 °C and 200 °C, is considered to be a promising technology, especially for cogeneration applications. In this study, a mathematical model of a natural gas fed integrated energy system based on HT-PEMFC is first developed using the principles of electrochemistry and thermodynamics (including energy and exergy analyses). The effects of some key operating parameters (e.g., steam-to-carbon ratio, HT-PEMFC operating temperature, and anode stoichiometric ratio) on the system performance (electrical, cogeneration, and exergetic efficiencies) are examined. The exergy destruction rates of each component in the integrated system are found for different values of these parameters. The results show that the most influential parameter which affects the performance of the integrated system is the anode stoichiometric ratio. For the baseline conditions, when the anode stoichiometric ratio increases from 1.2 to 2, the electrical, cogeneration, and exergetic efficiencies decrease by 42.04%, 33.15%, and 37.39%, respectively. The highest electrical power output of the system is obtained when the SCR, operating temperature, and anode stoichiometric ratio are taken as 2, 160 °C, and 1.2, respectively. For this case, the electrical, cogeneration, and exergetic efficiencies are found as 26.20%, 70.34%, and 26.74%, respectively.     

Development and performance evaluation of a high temperature proton exchange membrane fuel cell with stamped 304 stainless steel bipolar plates

In this work, a high temperature proton exchange membrane fuel cell (HT-PEMFC) with stamped SS304 bipolar plates is successfully developed. Its performance was evaluated under two types of gaskets at different assembly torques and air stoichiometric ratios. The rates of pressure loss at a torque of 7 N-m with 50 Shore A hardness gaskets was 2.0 × 10^?3 MPa min^?1, which is acceptable. The best performance of the developed HT-PEMFC with stamped SS304 bipolar plates was 228.33 mW cm^?2, which approaches the performance of HT-PEMFCs with graphite bipolar plates. The optimal air stoichiometric ratio for the HT-PEMFC with stamped SS304 bipolar plates was 4.0, which is higher than that for proton exchange membrane fuel cells with CNC milled graphite bipolar plates. This is probably because of the deformation of the flow channels under the assembly compression force, which causes an elevated gas-diffusion drag in the flow channels. After the test, it was observed that some products of corrosion reaction formed on the surface of the SS304 bipolar plate. This phenomenon may lead to a decrease in the operating life of the HT-PEMFC.     

Artificial intelligence based structural optimization of solid oxide fuel cell with three-dimensional reticulated trapezoidal flow field

Three-dimensional Reticulated trapezoidal flow field (RTFF) is promising in improving the performance and durability of the solid oxide fuel cell (SOFC). However, the structural complexity makes it challenging for the geometry configuration of the splitter and mixer. To this end, an intelligent optimization framework is proposed by coupling artificial neural network (ANN) and non-dominated sorting genetic algorithm-II (NSGA-II), in order to maximize the net power density and oxygen uniformity simultaneously. The ANN prediction model is trained to obtain the computationally efficient surrogate model of the computational fluid dynamics (CFD) numerical simulation. NSGA-II is used for the multi-objective optimization of the RTFF structural parameters. The results illustrate that the prediction model is of high prediction precision and generalization capability. In comparison to SOFC with conventional parallel flow fields (CPFF), the degree of the performance improvement of SOFC with optimized RTFF depends on the working condition, i.e., fuel and air flow rates and operating temperatures. The SOFC with the optimal RTFF achieves a higher molar concentration of oxygen and a more uniform distribution of oxygen and current density than the CPFF SOFC. The proposed optimization framework provides an efficient design method for the development of the next-generation SOFC flow field.     

A hybrid neural network model for PEM fuel cells

The goal of this paper is to discuss a neural network modeling approach for developing a quantitatively good model for proton exchange membrane (PEM) fuel cells. Various ANN approaches have been tested; the back-propagation feed-forward networks and radial basis function networks show satisfactory performance with regard to cell voltage prediction. The effects of Pt loading on the performance of the PEM fuel cell have been specifically studied. The results show that the ANN model is capable of simulating these effects for which there are currently no valid fundamental models available from the open literature.

Two novel hybrid neural network models (multiplicative and additive), each consisting of an ANN component and a physical component, have been developed and compared with the full-blown ANN model. The results from the hybrid models demonstrate comparable performance (in terms of cell voltage predictions) compared to the ANN model. Additionally, the hybrid models show performance gains over the physical model alone. The additive hybrid model shows better accuracy than that of the multiplicative hybrid model in our tests.     

Utilizing coke oven gases as a fuel for a cogeneration system based on high temperature proton exchange membrane fuel cell; energy,exergy and economic assessment

Based on a high temperature proton exchange membrane fuel cell (HT-PEMFC), a cogeneration system is proposed to produce heat and power. The system includes a coke oven gas steam reformer, a water gas shift reactor, and an afterburner. The system is analyzed in detail considering the energy, exergy and economic viewpoints. The analyses reveal the importance of HT-PEMFC in the system and according to the results, 9.03 kW power is generated with energy and exergy efficiencies of 88.2% and 26.2%, respectively and the total product unit cost is calculated as 91.8 $/GJ. Through a parametric study the effects on system performance are studied of such variables as the current density, fuel cell and reformer operating temperatures, and cathode stoichiometric ratio. It is found that an increase in the fuel cell temperature and/or a decrease in the reformer temperature enhance the exergy efficiency. The exergy efficiency is also maximized at the cathode stoichiometric ratio of 2.4. By performing a two-objective optimization using genetic algorithm, the best operating point is determined at which the exergy efficiency is (32.86%) and the total product unit cost is (78.68 $/GJ).     

1D and 3D numerical simulations in PEM fuel cells

The potential of fuel cells for clean and efficient energy conversion is generally recognized.The proton-exchange membrane (PEM) fuel cells are one of the most promising types of fuel cells. Models play an important role in fuel cell development since they enable the understanding of the influence of different parameters on the cell performance allowing a systematic simulation, design and optimization of fuel cells systems. In the present work, one-dimensional and three-dimensional numerical simulations were performed and compared with experimental data obtained in a PEM fuel cell. The 1D model, coupling heat and mass transfer effects, was previously developed and validated by the same authors [1] and [2]. The 3D numerical simulations were obtained using the commercial code FLUENT - PEMFC module.The results show that 1D and 3D model simulations considering just one phase for the water flow are similar, with a slightly better accordance for the 1D model exhibiting a substantially lower CPU time. However both numerical results over predict the fuel cell performance while the 3D simulations reproduce very well the experimental data. The effect of the relative humidity of gases and operation temperature on fuel cell performance was also studied both through the comparison of the polarization curves for the 1D and 3D simulations and experimental data and through the analysis of relevant physical parameters such as the water membrane content and the proton conductivity. A polarization curve with the 1D model is obtained with a CPU time around 5 min, while the 3D computing time is around 24 h. The results show that the 1D model can be used to predict optimal operating conditions in PEMFCs and the general trends of the impact on fuel cell performance of several important physical parameters (such as those related to the water management). The use of the 3D numerical simulations is indicated if more detailed predictions are needed namely the spatial distribution and visualization of various relevant parameters.An important conclusion of this work is the demonstration that a simpler model using low CPU has potential to be used in real-time PEMFC simulations.     

3D non-isothermal dynamic simulation of high temperature proton exchange membrane fuel cell in the start-up process

High temperature proton exchange membrane fuel cell (HT-PEMFC) with phosphoric acid doped polybenzimidazole (PBI) electrolyte shows multiple advantages over conventional PEMFC working at below 373 K, such as faster electrochemical kinetics, simpler water management, higher carbon monoxide tolerance. However, starting HT-PEMFC from room temperature to the optimal operating temperature range (433.15 K–453.15 K) is still a serious challenge. In present work, the start-up strategy is proposed and evaluated and a three-dimensional non-isothermal dynamic model is developed to investigate start-up time and temperature distribution during the start-up process. The HT-PEMFC is preheated by gas to 393.15 K, followed by discharging a current from the cell for electrochemical heat generation. Firstly, different current loads are applied when the average temperature of membrane reaches 393.15 K. Then, the start-up time and temperature distribution of co-flow and counter-flow are compared at different current loads. Finally, the effect of inlet velocity and temperature on the start-up process are explored in the case of counter-flow. Numerical results clearly show that applied current load is necessary to reduce start-up time and just 0.1 A/cm² current load can reduce startup time by 45%. It is also found that co-flow takes 18.8% less time than counter-flow to heat membrane temperature to 393.15 K, but the maximum temperature difference of membrane is 39% higher than the counter-flow. Increasing the inlet gas flow velocity and temperature can shorten the start-up time but increases the temperature difference of the membrane.