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.     

A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique

A novel ultrasonic-spray method for preparing gas diffusion electrodes (GDEs) for proton exchange membrane fuel cell (PEMFC) is described. Platinum (Pt) loaded on Nafion^®-bonded GDEs were prepared by the ultrasonic-spray method on various commercial woven and non-woven gas diffusion layers (GDLs) at several Pt loadings in the range of 0.40-0.05 mg cm⁻². The ultrasonic-sprayed GDEs were tested and compared to commercial and hand-painted GDEs. It was found that the GDEs prepared by the ultrasonic-spray method exhibited better performances compared to those prepared by the hand-painting technique, especially at low Pt loadings. GDEs fabricated by the ultrasonic-spray method with a platinum loading of 0.05 mg cm⁻² exhibited a peak power rating of 10.9 W mg⁻¹ compared to 9.8 W mg⁻¹ for hand-painted GDEs. For all experiments using various GDLs, Sigracet SGL 10BC exhibited the best performance with a peak power of 0.695 W cm⁻².     

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.     

Preparation and operation of gas diffusion electrodes for high-temperature proton exchange membrane fuel cells

Gas diffusion electrodes for high-temperature PEMFC based on acid-doped polybenzimidazole membranes were prepared by a tape-casting method. The overall porosity of the electrodes was tailored in a range from 38% to 59% by introducing porogens into the supporting and/or catalyst layers. The investigated porogens include volatile ammonium oxalate, carbonate and acetate and acid-soluble zinc oxide, among which are ammonium oxalate and ZnO more effective in improving the overall electrode porosity. Effects of the electrode porosity on the fuel cell performance were investigated in terms of the cathodic limiting current density and minimum air stoichiometry, anodic limiting current and hydrogen utilization, as well as operations under different pressures and temperatures.     

Novel epoxy-based cross-linked polybenzimidazole for high temperature proton exchange membrane fuel cells

An approach has been proposed to prepare the reinforced phosphoric acid (PA) doped cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells (HT-PEMFCs), using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker. FT-IR measurement and solubility test showed the successful completion of the crosslinking reaction. The resulting cross-linked membranes exhibited improved mechanical strength, making it possible to obtain higher phosphoric acid doping levels and therefore relatively high proton conductivity. Moreover, the oxidative stability of the cross-linked membranes was significantly enhanced. For instance, in Fenton’s reagent (3% H₂O₂ solution, 4 ppm Fe²⁺, 70 °C), the cross-linked PBI-NGDE-20% membrane did not break into pieces and kept its shape for more than 480 h and its remaining weight percent was approximately 65%. In addition, the thermal stability was sufficient enough within the operation temperature of PBI-based fuel cells. The cross-linked PBI-NGDE-X% (X is the weight percent of epoxy resin in the cross-linked membranes) membranes displayed relatively high proton conductivity under anhydrous conditions. For instance, PBI-NGDE-5% membrane with acid uptake of 193% exhibited a proton conductivity of 0.017 S cm⁻¹ at 200 °C. All the results indicated that it may be a suitable candidate for applications in HT-PEMFCs.     

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.     

Carbon film coating on gas diffusion layer for proton exchange membrane fuel cells

This study discusses a novel process to increase the performance of proton exchange membrane fuel cells (PEMFC). In order to improve the electrical conductivity and reduce the surface indentation of the carbon fibers, we modified the carbon fibers with pitch-based carbon materials (mesophase pitch and coal tar pitch). Compared with the gas diffusion backing (GDB), GDB-A240 and GDB-MP have 32% and 33% higher current densities at 0.5 V, respectively. Self-made carbon paper with the addition of a micro-porous layer (MPL) (GDL-A240 and GDL-MP) show improved performance compared with GDB-A240 and GDB-MP. The current densities of GDL-A240 and GDL-MP at 0.5 V increased by 37% and 31% compared with GDL, respectively. This study combines these two effects (carbon film and MPL coating) to promote high current density in a PEMFC.     

Lattice Boltzmann method modeling and experimental study on liquid water characteristics in the gas diffusion layer of proton exchange membrane fuel cells

Water transport through the gas diffusion layer (GDL) is vital to proton exchange membrane fuel cells (PEMFCs), especially under flooding conditions. In this paper, a two-dimensional (2D) lattice Boltzmann method (LBM) is applied to reveal the water dynamic characteristics in GDL, and the computational domain is reconstructed based on the experiment. In-situ experiments, including I–V performance and electrochemical impedance spectroscopy (EIS) tests under flooding conditions, are carried out and analyzed. It is found that the porosity distribution inside the GDL is a crucial factor in water dynamic behavior research. The horizontal liquid water saturation (HS_w) under the channel of real GDL (with porosity distribution) at 0.4 relative thickness are 3.2 times, 2.1 times and 3.4 times higher than the ideal GDL (without porosity distribution) in the case of 0.8 mm, 1.2 mm and 2.0 mm, respectively. The numerical simulation and experimental study show that water dynamic characteristics under the rib influence cell performance directly. In our LBM model, the GDL water distribution inconsistency (Var_w) under 2.0 mm width rib is 43.1% and 28.0% higher than that under the 0.8 mm and 1.2 mm rib, respectively. With the rib wider from 0.8 mm to 2.0 mm, some parts of cell impedance such as R_mt, R_ct, and L_mt increase 64.22%, 98.89%, and 47.46%, respectively. However, GDL under the channel shows no influence on water transport process.     

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).     

A review of water flooding issues in the proton exchange membrane fuel cell

We have reviewed more than 100 references that are related to water management in proton exchange membrane (PEM) fuel cells, with a particular focus on the issue of water flooding, its diagnosis and mitigation. It was found that extensive work has been carried out on the issues of flooding during the last two decades, including prediction through numerical modeling, detection by experimental measurements, and mitigation through the design of cell components and manipulating the operating conditions. Two classes of strategies to mitigate flooding have been developed. The first is based on system design and engineering, which is often accompanied by significant parasitic power loss. The second class is based on membrane electrode assembly (MEA) design and engineering, and involves modifying the material and structural properties of the gas diffusion layer (GDL), cathode catalyst layer (CCL) and membrane to function in the presence of liquid water. In this review, several insightful directions are also suggested for future investigation.     

Fabrication of a carbon nanofiber sheet as a micro-porous layer for proton exchange membrane fuel cells

A carbon nanofiber sheet (CNFS) has been prepared by electrospinning, stabilisation and subsequent carbonisation processes. Imaging with scanning electron microscope (SEM) indicates that the CNFS is formed by nonwoven nanofibers with diameters between 400 and 700 nm. The CNFS, with its three-dimensional pores, shows excellent electrical conductivity and hydrophobicity. In addition, it is found that the CNFS can be successfully applied as a micro-porous layer (MPL) in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC). The GDL with the CNFS as a MPL has higher gas permeability than a conventional GDL. Moreover, the resultant cathode GDL exhibits excellent fuel cell performance with a higher peak power density than that of a cathode GDL fabricated with a conventional MPL under the same test condition.     

Preparation of microporous layer for proton exchange membrane fuel cell by using polyvinylpyrrolidone aqueous solution

A new method of preparing microporous layer (MPL) for proton exchange membrane fuel cell (PEMFC) was presented in this paper. Considering the bad dispersion of PTFE aqueous suspension in the carbon slurry based on ethanol, polyvinylpyrrolidone (PVP) aqueous solution was used to prepare carbon slurry for microporous layer. The prepared gas diffusion layers (GDLs) were characterized by scanning electron microscopy, contact angle system and pore size distribution analyzer. It was found that the GDL prepared with PVP aqueous solution had higher gas permeability, as well as more homogeneous hydrophobicity. Moreover, the prepared GDLs were used in the cathode of fuel cell and evaluated with fuel cell performance and EIS analysis, and the GDL prepared with PVP aqueous solution indicated better fuel cell performance and lower ohmic resistance and mass transfer resistance.     

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.     

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.     

Gas diffusion layer for proton exchange membrane fuel cells—A review

Gas diffusion layer (GDL) is one of the critical components acting both as the functional as well as the support structure for membrane-electrode assembly in the proton exchange membrane fuel cell (PEMFC). The role of the GDL is very significant in the H₂/air PEM fuel cell to make it commercially viable. A bibliometric analysis of the publications on the GDLs since 1992 shows a total of 400+ publications (>140 papers in the Journal of Power Sources alone) and reveals an exponential growth due to reasons that PEMFC promises a lot of potential as the future energy source for varied applications and hence its vital component GDL requires due innovative analysis and research. This paper is an attempt to pool together the published work on the GDLs and also to review the essential properties of the GDLs, the method of achieving each one of them, their characterization and the current status and future directions. The optimization of the functional properties of the GDLs is possible only by understanding the role of its key parameters such as structure, porosity, hydrophobicity, hydrophilicity, gas permeability, transport properties, water management and the surface morphology. This paper discusses them in detail to provide an insight into the structural parts that make the GDLs and also the processes that occur in the GDLs under service conditions and the characteristic properties. The required balance in the properties of the GDLs to facilitate the counter current flow of the gas and water is highlighted through its characteristics.     

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.     

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.     

The effect of materials on proton exchange membrane fuel cell electrode performance

This paper describes the optimisation in the fabrication materials and techniques used in proton exchange membrane fuel cell (PEMFC) electrodes. The effect on the performance of membrane electrode assemblies (MEAs) from the solvents used in producing catalyst inks is reported. Comparison in MEA performances between various gas diffusion layers (GDLs) and the importance of microporous layers (MPLs) in gas diffusion electrodes (GDEs) are also shown. It was found that the best performances were achieved for GDEs using tetrahydrofuran (THF) as the solvent in the catalyst ink formulation and Sigracet 10BC as the GDL. The results also showed that our in-house painted GDEs were comparable to commercial ones (using Johnson Matthey HiSpec™ and E-TEK catalysts).     

Liquid transport in gas diffusion layer of proton exchange membrane fuel cells: Effects of micro-porous layer cracks

Micro porous layer (MPL) is a carbon layer (～15 μm) that coated on the gas diffusion layer (GDL) to enhance the electrical conduction and membrane hydration of proton exchange membrane fuel cell (PEMFC). However, the liquid transport behavior from MPL to GDL and its impact on water management remain unclear. Thus, a three-dimensional volume of fluid (VOF) model is developed to investigate the effects of MPL crack properties on liquid water saturation, liquid pathway formation, and the two-phase mass transport mechanism in GDL. Firstly, a stochastic orientation method is used to reconstruct the fibrous structure of the GDL. After that, the liquid water saturation calculated from the numerical results agrees well with the experimental data. With considering the full morphology of the overlap between MPL and GDL, it's found that this overlap determines the preferred liquid emerging port of both MPL and GDL. Three crack design shapes in MPL are proposed on the base of the similarity crack formation processes of soil mud. In addition, the effects of crack shape, distance between cracks, and crack number on liquid water transport from MPL to GDL are investigated. It is found that the liquid water saturation of GDL increases with crack number and the distance between cracks, while presents little correlation to the crack shape. Hopefully, these results can help the development of PEMFC models without reconstructing full MPL morphology.