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.     

Recent approaches to improve Nafion performance for fuel cell applications: A review

Among various polymeric materials applied as polymer electrolyte membrane (PEM) for fuel cells, Nafion is known as a standard polymer so that any new material needs to be compared with it. However, low proton conductivity under anhydrous conditions at elevated temperatures due to water evaporation and consequent need for water management, and its low tolerance for fuel impurities still remain as Nafion's major shortcomings. Development of new PEM with higher conductivity at high temperatures and stability for application at anhydrous conditions is highly desired. The aim of this review is thus to specify the latest achievements made to improve the Nafion performance by means of pretreatments, incorporation of reinforcing agents, nanocomposites, blending, ionic liquids, and so on. In recent years, a significant enhancement in Nafion performance, at humidified conditions, has been reported and many efforts have been conducted to improve its performance under dried conditions. Although impressive enhancements in Nafion performance have been reported, a need for revolutionary approaches to overcome its drawbacks is undeniably needed for future works.     

Investigation and analysis of proton exchange membrane fuel cell dynamic response characteristics on hydrogen consumption of fuel cell vehicle

The number of working points and response speed are two essential characteristics of proton exchange membrane fuel cell (PEMFC). The improper setting of the number of working points and response speed may reduce the life of PEMFC and increase the hydrogen consumption of the vehicle. This paper explores the impact of the response speed as well as the working points of the PEMFC on the hydrogen consumption in the real-system level. In this paper a dynamic model of the PEMFC system is established and verified by experiments. The model is able to reflect the dynamic response process of PEMFC under a series different number of working points and different response speed. Based on the proposed model, the influence of working points and the response speed of PEMFC on the hydrogen consumption in the vehicle under different driving cycles is analyzed and summarized, for the first time, in the open literature. The results highlight that the hydrogen consumption will decreases in both cases that with the increase of working point number and increase of response speed. However, the reduction range of hydrogen consumption trends to smaller and may reach to an optimal level considering the trade-off between the hydrogen saving and the other costs, for example the control cost. Also, with a more complex driving cycle, the working points and response speed have a greater the impact on the hydrogen consumption in the vehicle applications.     

Homopolymer-like sulfonated phenyl- and diphenyl-poly(arylene ether ketone)s for fuel cell applications

Two series of homopolymer-like sulfonated aromatic poly(ether ketone)s (SPEKs) were readily prepared and post-sulfonated using mild conditions. The homopolymer-like SPEKs exhibited advantages in synthesis and physical properties over typical post-sulfonated random copolymers, such as rapid and mild sulfonation conditions, high molecular weights, site specificity and control over IEC, as well as an excellent combination of dimensional swelling stability, low methanol permeability and high proton conductivity. These beneficial membrane properties are reflected in the attractive direct methanol fuel cell (DMFC) and polymer electrolyte membrane fuel cell (PEMFC) performance of these homopolymer-like SPEKs as compared with typical random copolymer SPEKs.     

An overview: Current progress on hydrogen fuel cell vehicles

The harmful consequences of pollutants emitted by conventional fuel cars have prompted vehicle manufacturers to shift towards alternative energy sources. Currently, fuel cells (FCs) are commonly regarded as highly efficient and non-polluting power sources capable of delivering far greater energy densities and energy efficiency than conventional technologies. Proton exchange membrane fuel cells (PEMFC) are viewed as promising in transportation sectors because of their ability to start at cold temperatures and minimal emissions. PEMFC is an electrochemical device that converts hydrogen and oxidants into electricity, water, and heat at various temperatures. The pros and cons of the technology are discussed in this article. Various fuel cell types and their applications in the portable, automobile, and stationary sectors are discussed. Additionally, recent issues associated with existing fuel cell technology in the automobile sector are reviewed.     

Dynamic modeling and simulation of air-breathing proton exchange membrane fuel cell

Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

Modeling and simulation of a proton exchange membrane fuel cell using computational fluid dynamics

An isothermal, three dimensional, single phase model was presented to evaluate a proton exchange membrane fuel cell (PEMFC) with serpentine flow. The mass, momentum and electrochemical equations were solved simultaneously for the steady state condition using computational fluid dynamics (CFD) software based on the finite element method. The model considered reactions as mass source/sink terms, and electron transport in the catalyst layers and GDLs. To validate the model, the numerical results were compared to the experimental data collected from the fabricated membrane electrode assemblies. The exchange current density parameter of the catalysts was fitted by the model to calibrate the results. The model showed good agreement with experimental data and predicted a higher current density for the catalyst with a higher surface area and Pt content. The oxygen, hydrogen and water mass fraction distribution, velocity magnitude and pressure distribution were estimated by the model. Moreover, the effect of pressure and temperature, as two important operating conditions, on the current density was predicted by the validated model.     

Double cross-linked polyetheretherketone proton exchange membrane for fuel cell

The proton exchange membrane based on polyetheretherketone was prepared via two steps of cross-linking. The properties of the double cross-linked membrane (water uptake, proton conductivity, methanol permeability and thermal stability) have been investigated for fuel cell applications. The prepared membrane exhibited relatively high proton conductivity, 3.2 × 10⁻² S cm⁻¹ at room temperature and 5.8 × 10⁻² S cm⁻¹ at 80 °C. The second cross-linking significantly decreased the water uptake of the membrane. The performance of direct methanol fuel cell was slightly improved as compared to Nafion^® 117 due to its low methanol permeability. The results indicated that the double cross-linked membrane is a promising candidate for the polymer electrolyte membrane fuel cell, especially for the direct methanol fuel cell due to its low methanol permeability and high stability in a methanol solution.     

Current status of cross-linking and blending approaches for durability improvement of hydrocarbon-based fuel cell membranes

Due to the demand for developing reliable and economical fuel cells, many researchers have focused on durability improvement of hydrocarbon-based proton exchange membranes (PEMs), without compromising performance. Among various techniques, cross-linking and blending show promising potentials by introducing physical and/or chemical bonds between polymer chains and creating a 3D network within their structure. Cross-linking is accomplished through thermal, solvothermal, radiation-assisted, and cross-linker assisted methods, whereas blending is categorized based upon the existing interactions between polymers, namely acid-acid, acid-base, and charge transfer network. This review article discusses the recent achievements of cross-linked and blend hydrocarbon-based PEMs in long-term stability tests and durability studies. Additionally, their salient dimensional, thermo-chemical, and transport properties are highlighted, including the in-situ fuel cell performance and electrochemical diagnostics. Accordingly, cross-linked and blend membranes have shown over 4000 h durability in hydrogen fuel cells and more than 100 times lower methanol cross-over compared to conventional fluorinated membranes, implying significant potential for commercialization.     

The computer-aided analysis for the driving stability of a plug-in fuel cell vehicle using a proton exchange membrane fuel cell

In order to analyze the driving stability of a plug-in fuel cell vehicle (PFCV), a computer-aided simulator for PFCVs has been developed. PFCVs have been introduced around the world to achieve early commercialization of an eco-friendly and highly efficient fuel cell vehicle. The plug-in option, which allows the battery to be recharged from the electricity grid, enables a reduction in size of the fuel cell system (FCS) and an improvement of its durability. As such, the existing limitations of the fuel cell - such as its high cost, poor durability, and the insufficient hydrogen infrastructure – can be overcome. During the design phase of PFCV development, simulation-based driving stability test is necessary to determine the sizes of the electric engine of the FCS and the battery. The developed simulator is very useful for analyzing the driving stability of the PFCV with respect to the capacities of the FCS and battery. The simulation results are in fact very close to those obtained from a real system, since the estimation accuracy of PFCV component models used in this simulator, such as the fuel cell stack, battery, electric vehicle, and the other balance of plants (BOPs), are verified by the experiments, and the simulator uses the newly-proposed power distribution control logic and the pre-confirmed real driving schedule. Using these results, we can study which one will be the best in terms of driving stability.     

Internal currents in response to a load change during fuel cell start-up

The transient response of proton exchange membrane fuel cells during start-up is an important issue for backup power systems. These require a very short start-up time which limits the use of batteries during a blackout. In this study the fuel cell was initially inerted with nitrogen at the cathode and thus the start-up procedures occurred in two stages: gas supply in open-circuit and load connection. The influence of the current time-profile, the cell voltage at the connection and the gas flow rates on the voltage variation were investigated using a segmented fuel cell permitting the measurements of the internal local currents. We found that the voltage during the filling of the cathode is not sufficient to determine which fraction of the cathode was filled with oxygen. In the case of a step change in current, the start-up time decreases as the voltage at the moment the cell is connected increases. In response to a ramp, the asymptotic power value is reached quickly.     

Sulfonated poly(fluorenyl ether ketone) ionomers containing aliphatic functional segments for fuel cell applications

A series of Sulfonated Poly (fluorenyl ether ketone) ionomers containing aliphatic functional segments were synthesized and characterized. The monomer 4,4′-Dihydroxy-α, ω-diphenoxydecane with aliphatic group was conveniently prepared from hydroquinone and 1,10-dibromodecane. A series of sulfonated aliphatic functional groups containing poly(fluorenyl ether ketone)s with different aliphatic group content were successfully synthesized and characterized in detail, in particular with respect to properties relevant for their application as membrane materials in proton exchange membrane (PEM) fuel cells. Tough and transparent membranes were formed by casting from their solutions. The effects of alkyl groups were investigated by comparison of the PEM properties of the copolymers with different content of aliphatic component on the copolymer chain. The introduction of aliphatic segments can provide an enhanced water uptake, increased proton conductivities, but worse oxidative stability. Transmission electron microscope (TEM) images of j and f revealed an improved phase separation structure under the effect of aliphatic groups. The as-made membranes can also exhibit comparable or much better single cell performance than Nafion^@ 117 at 75 °C–95 °C under full or partial relative humidification.     

Validation of a fuel cell compression spring equivalent model using polarisation data

Fuel cell stack compression is a vital part of the manufacturing process, however limited research exists in predicting the optimal compression force to maximise fuel cell performance. This paper validates a spring equivalent model proposed in a previous publication which, when coupled with literature derived gas diffusion layer (GDL) optimal compression data, can predict the compression force required based on gas diffusion layer and gasket properties. The error between the model and the optimal performance of the stack is a maximum of 6.4%. This is a positive indication as to the model's validity. In addition, the compression homogeneity applied by the compression system to the flow field plate is measured to confirm the GDL is experiencing the predicted compression force. The impact of this research is a reduction in development time and cost as less empirical testing will be required to identify optimal fuel cell stack compression.     

Silica-related membranes in fuel cell applications: An overview

Silica is the most common inorganic filler used in fuel cells, especially for proton exchange membrane fuel cell and direct alcohol fuel cell applications. Silica has played an important role in improving the performance of fuel cells by enhancing their membrane properties. Recently, silica has been widely implemented in different types of membranes, such as fluorinated membranes (Nafion), sulfonated membranes (SPEEK, SPS, SPAES, SPI) and other organic polymer matrixes. The incorporation of silica into membrane matrices has improved the thermal stability, mechanical strength, water retention capacity and proton conductivity of the membrane. This review describes the interactions between silica and different types of polymer matrices in fuel cells and how they boost fuel cell performance. In addition, this review also discusses the current challenges of silica-related membrane-based fuel cells and predicts the future prospects of silica in membrane-based fuel cell applications.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

Experimental study of the start-up of a fuel cell stack for backup power application

The transient response of proton exchange membrane fuel cells during start-up is an important issue for backup power systems which require a very short start-up time in order to limit the use of batteries during a blackout. The start-up procedure of a ten cells stack was studied: in the first stage the cathode channel initially filled with nitrogen was supplied with oxygen in open circuit then in the second stage it was connected to the load. The influences of the current time-profile (step or ramp), the cell voltage at the connection and the gas flow rates on the voltage variation were investigated. It was found that the voltage value during the filling of the cathode is not sufficient to determine which fraction of the cathode was filled with oxygen. In most cases, high oxygen flow rates allow reducing the start-up time of the stack. Furthermore, for fixed current density and stoichiometric coefficients it was found that a minimum start-up time exits. The analysis of transient response to current steps showed that around 70% of the maximum electrical power was available less than 2 s after the beginning of the start-up procedure.     

Investigation of membrane electrode assembly (MEA) hot-pressing parameters for proton exchange membrane fuel cell

The hot-pressing conditions for fabricating the membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC) was investigated by using a 2ⁿ full factorial design. Time, temperature and pressure were key parameters that were varied from 500 to 1500 psi, 1 to 5 min and 100 to 160 °C, respectively. The results from the full factorial analysis indicated that the order of significance of the main MEA fabricating effects was temperature, pressure, time–temperature interaction and pressure–time–temperature interaction. By examining the cell performance curves, the lower fabrication conditions of temperature and pressure were suitable for MEA preparation. The conductive layer between the membrane and the catalyst layer became thin at high pressure and high temperature, as seen from scanning electron microscopy (SEM) images. In the ranges of condition studied, the most suitable hot-pressing condition for MEA fabrication was at 100 °C, 1000 psi and 2 min. This condition provided the highest maximum power density from the MEA and the best contact at the interfaces between the gas diffusion layer, the active layer and the electrolyte membrane. The experimental results were verified by testing with a commercial MEA in the same operating condition and with the same equipment. The performance of the fabricated MEA was better than that of the commercial one.     

Recent advances in designing and tailoring nanofiber composite electrolyte membranes for high-performance proton exchange membrane fuel cells

As the critical component of proton exchange membrane fuel cell (PEMFC), proton exchange membrane (PEM) determine its overall performance. Current PEMs hardly meet the operating requirements of fuel cells, limiting their commercial applications. With the development of nanocomposite technology, nanofibers introduced into PEMs to prepare nanofiber composite proton exchange membranes (NCPEMs) have been widely studied. In an NCPEM, nanofibers can form long-range channels for proton transport, and reinforced skeleton to reach the target performance of PEMFCs. Focusing on NCPEM, this paper reviews on recent progresses in nanofiber preparation and NCPEM preparation techniques. Furthermore, different types of nanofibers incorporated into NCPEMs are reviewed in terms of fiber composition. The challenges and future perspectives regarding NCPEMs are also discussed.