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.     

Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells

Graphite oxide/polybenzimidazole synthesized by 3, 3′-diaminobenzidine and 5-tert-butyl isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI) composite membranes were prepared for high temperature polymer proton exchange membrane fuel cells (PEMFCs). All membranes were loaded with different content of phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the chemical stability of BuIPBI membrane and support a higher acid content. The proton conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and 0.027 S/cm, respectively, at 140 °C and without humidity.     

Modeling of high temperature proton exchange membrane fuel cells with novel sulfonated polybenzimidazole membranes

In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.     

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.     

Effects of mesoporous fillers on properties of polybenzimidazole composite membranes for high-temperature polymer fuel cells

In this study, we elucidated the effects of the addition of various mesoporous silicates (0–20 wt%) to the membranes used for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) on cell performance. Two types of polybenzimidazole (PBI)-based hybrid membranes were prepared by homogeneously dispersing a predetermined amount of MCM-41 or SBA-15 within the PBI matrix. Compared to the pure PBI membrane, those with MCM-41 and SBA-15 exhibited significantly enhanced phosphoric acid doping and better mechanical properties, leading to improved HT-PEMFC performance and reduced acid migration. However, the membranes with 20 wt% silicate showed inferior performance compared to those with 10 wt% silicate. In addition, the membranes with SBA-15 exhibited noticeable aggregation, lower phosphoric acid doping, and greater phosphoric acid migration during the leaching test than did the membranes with MCM-41. Finally, during the short-term durability test, the PBI/MCM-41 (10 wt%) membrane showed the best performance (maximum power density of 310  mW cm^?2).     

Alkali doped polybenzimidazole membrane for high performance alkaline direct ethanol fuel cell

An anion exchange membrane for alkaline direct ethanol fuel cell (ADEFC) was prepared by doping KOH in polybenzimidazole (PBI) membrane. The distributions of nitrogen, oxygen and potassium in the membrane were analyzed by means of XRD and SEM-EDX, respectively. It was found that free or combined KOH molecules may exist in the PBI matrix, which was helpful for the ionic conductivity of PBI/KOH. Ethanol permeability through this membrane was much lower than that of Nafion^®. For ADEFC based on this PBI/KOH membrane electrolyte, the power density was 3 to 6 times of the results in literatures. In addition, the micro-structure of alkali doped PBI and the interaction between KOH and PBI matrix were also speculated logically.     

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.     

Simulation and experimental analysis of the clamping pressure distribution in a PEM fuel cell stack

High performance and efficiency are often reported in single-cell polymer electrolyte membrane (PEM) fuel cell (FC) experiments. This however, can reduce substantially when moving from single-cell experiments to multiple cells. Fuel cell performance is degraded for many reasons when adding cells, but; possibly the most important, is contact resistance between the bipolar plate and gas diffusion layer (GDL). Contact resistance is in direct relation to the clamping configuration and clamping pressure applied to a FC stack. Simulation of a single cell and 16-cell FC was performed at various clamping pressures resulting in detailed 3D plots of stress and deformation. The stress on the GDL, for any value of clamping pressure simulated in this study, is around 1.5 MPa for the 16-cell stack and around 4 MPa in single cell simulations. Experimental testing of clamping pressure effects was performed on a 16-cell stack by placing a thin pressure-sensitive film between GDL and bipolar plate. Clamping pressure was applied using various loads, durations, and two types of GDLs. The results from experimental testing show that pressure on the GDL is in the range of 0–2.5 MPa. When using rectangular cells, experimental results show nearly zero pressure in the center of each cell and the center cells of the stack, regardless of clamping method.     

Gravity effect on the performance of PEM fuel cell stack with different gas manifold positions

The importance of gravity effect on the performance of proton exchange membrane fuel cell (PEMFC) has recently been recognized. In this paper, the effect of gravity on the performance of PEMFC has been investigated associating with different gas intake modes. The polarization curves of the stack with different positions of reaction gas inlet and outlet at varied gravitational angles are addressed in detail. The results indicate that the output power of PEMFC stack can be greatly enhanced at the optimized gravitational angle. Gas intake modes that were realized by varying the gas inlet and outlet positions strongly affect the stack performance as well. The optimized performance can be reached at the tilted angle of 90° when both air and hydrogen inlets are placed at the upper side of the stack, whereas the worst performance occurs at the tilted angle of 90° when air and hydrogen flow into the channel from the bottom side of the stack. These results have important implications for PEM fuel cell design and operational strategies. In order to improve the performance, fuel cells should be designed and operated at the optimized gravitational angle and gas inlet/outlet position. Copyright © 2011 John Wiley & Sons, Ltd.     

Feasibility of using thin polybenzimidazole electrolytes in high-temperature proton exchange membrane fuel cells

The use of thin polybenzimidazole membranes in high-temperature polymer electrolyte membrane fuel cells is explored. Membranes in thickness of 10–40 μm are prepared, doped and characterized, including fuel cell test. High molecular weight polymers enable fabrication of membranes as thin as 10 μm with sufficient mechanical strength. The thin membranes, upon acid doping, exhibit comparable conductivity and hence decreased ohmic resistance. Membrane electrode assemblies with thin membranes down to 10 μm show slightly lower open-circuit voltages than that for reference 40 μm but all above 0.97 V. This is in good agreement with the hydrogen permeability measurements, which show a value around 10^?12 mol cm^?1 s^?1 bar^?1, corresponding to a crossover current density of <1 mA cm^?2. The acid transferred from the membrane to the catalyst layer seems constant, as the iR-free polarization plots are nearly the same for membranes of varied thicknesses. The acid remaining in the membrane after the break-in period is estimated, showing an acid inventory issue when thin membranes are used. This is verified by using the membranes of higher acid doping levels.     

Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane

Electrochemical hydrogen pumping using a high-temperature (>100 °C) polybenzimidazole (PBI) membrane was demonstrated under non-humidified and humidified conditions at ambient pressures. Relatively low voltages were required to operate the pump over a wide range of hydrogen flow rates. The advantages of the high-temperature capability were shown by operating the pump on reformate feed gas mixtures containing various amounts of CO and CO₂. Gas purity measurements on the cathode gas product were conducted and significant reductions in gas impurities were detected. The applicability of the PBI membrane for electrochemical hydrogen pumping and its durability under typical operating conditions were established with tests that lasted for nearly 4000 h.     

Effects of temperature and humidity on the cell performance and resistance of a phosphoric acid doped polybenzimidazole fuel cell

Performance and electrochemical impedance spectroscopy (EIS) tests were performed at different temperatures and humidity levels to understand the effects of temperature and humidity on the performance and resistance of a PBI/H₃PO₄ fuel cell.The results of the performance tests indicated that increasing the temperature significantly improved the cell performance. In contrast, no improvement was observed when the gas humidity was increased. On the other hand, the EIS results showed that the membrane resistance was reduced for elevated temperatures. This development can be interpreted by the increase in membrane conductivity, as reflected by the Arrhenius equation. As the formation of H₄P₂O₇ and the self-dehydration of H₃PO₄ start around 130-140 °C, in PBI, they increase the membrane resistance at temperatures that are higher than 130 °C. In addition, the membrane resistance was reduced for elevated gas humidity levels. This is because an increase in humidity leads to an increase of the membrane hydration level.The resistance of the catalyst kinetics mainly contributes to the charge transfer resistance. However, under certain conditions, the interfacial charge transfer resistance is also important. It was concluded that the gas diffusion is the main contributor to the mass transfer resistance under dry conditions while it is the gas concentration under humid conditions.     

Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Recent advances in cathode electrocatalysts for PEM fuel cells

Great progress has been made in the past two decades in the development of the electrocatalysts for proton exchange membrane fuel cells (PEMFCs). This review article is focused on recent advances made in the kinetic-activity improvement on platinum- (Pt-) based cathode electrocatalysts for the oxygen reduction reaction (ORR). The origin of the limited ORR activity of Pt catalysts is discussed, followed by a review on the development of Pt alloy catalysts, Pt monolayer catalysts, and shape- and facet-controlled Pt-alloy nanocrystal catalysts. Mechanistic understanding is reviewed as well on the factors contributing to the enhanced ORR activity of these catalysts. Finally, future directions for PEMFC catalyst research are proposed.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

A review of high-temperature proton exchange membrane fuel cell (HT-PEMFC) system

This paper provides information encompassing the recent discovery of the High Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC) focusing on systems requirement. To have a reliable power production and higher durability level, a proper system must be applied in both normal and especially in transient operations. To date many issues of HT-PEMFC especially in durability and performance still unsolved. This article is written to provide clear information about the research undergo and must be the focus in order to produce an efficient performance. Information about the advantages towards Low Temperature Proton Exchange Membrane Fuel Cell (LT-PEMFC), the main components, and the mode of operation also discussed. In-depth research needs to be conducted into the innovative design and development of HT-PEMFC components and its system since these are the key factors for optimum performance.     

Graphite oxide-incorporated SnP2O7 solid composite electrolyte for high-temperature proton exchange membrane fuel cells

Proton conductors operating in a high temperature range (over 200 °C) have attracted much interest. The promising candidates for inorganic composite membranes for proton exchange membrane fuel cells (PEMFCs) using solid SnP₂O₇ and SnP₂O₇/GO at temperatures between 150 °C and 250 °C are reported. Graphite oxide (GO) incorporated with SnP₂O₇ improves the proton conductivity and reduces the crossover. The structure and phase stability of the membranes are analysed by X-ray diffraction and thermos-gravimetric analysis (TGA), and the microstructure morphology is analysed by scanning electron microscopy (SEM). The SnP₂O₇/GO composite electrolyte exhibits the proton conductivities of 0.0076 cm^?1 at 225 °C and a peak power density of 18 mW cm^?2 at 220 °C.     

Numerical analysis of effects of gas crossover through membrane pinholes in high-temperature proton exchange membrane fuel cells

Durability is a major issue in the widespread commercialization of proton exchange membrane fuel cells (PEMFCs). Various failure modes have been identified over their long runtime. These mainly originate from membrane and catalyst layer failures. One of the most common failure modes in PEMFCs is due to pinhole formation in the membrane and resultant reactant gas crossover through the membrane. Gas crossover induces several critical problems in PEMFCs, including severe reactant depletion in the downstream regions, mixed potential at the electrodes, and formation of local hot spots by hydrogen/oxygen catalytic reaction, which indicates that the cell performance decreases with increasing gas crossover. In this study, we numerically investigate the effects of gas crossover on the performance of a high-temperature PEMFC based on a phosphoric-acid-doped polybenzimidazole (PBI) membrane. In contrast to previous gas-crossover studies 1 and 2 in which uniform gas crossover throughout the entire membrane has been simply assumed, our focus is on examining the impacts of localized gas crossover due to membrane pinholes. Numerical simulations are carried out via arbitrarily assuming pinholes in the membrane. The simulation results clearly show that the presence of pinholes in the membrane significantly disrupts the species, current density, and temperature distributions. Our findings may improve the fundamental and detailed understanding of localized gas-crossover phenomena through the membrane pinholes and the influence of these phenomena on high-temperature PEMFC operation.     

Externally cooled high temperature polymer electrolyte membrane fuel cell stack

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