Water transport in polymer electrolyte membrane fuel cells

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

Free vibration analysis of a polymer electrolyte membrane fuel cell

A free vibration analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed by modelling the PEMFC as a 20 cm × 20 cm composite plate structure. The membrane, gas diffusion electrodes, and bi-polar plates are modelled as composite material plies. Energy equations are derived based on Mindlin's plate theory, and natural frequencies and mode shapes of the PEMFC are calculated using finite element modelling. A parametric study is conducted to investigate how the natural frequency varies as a function of thickness, Young's modulus, and density for each component layer. It is observed that increasing the thickness of the bi-polar plates has the most significant effect on the lowest natural frequency, with a 25% increase in thickness resulting in a 17% increase in the natural frequency. The mode shapes of the PEMFC provide insight into the maximum displacement exhibited as well as the stresses experienced by the single cell under vibration conditions that should be considered for transportation and stationary applications. This work provides insight into how the natural frequencies of the PEMFC should be tuned to avoid high amplitude oscillations by modifying the material and geometric properties of individual components.     

A review of polymer electrolyte membrane fuel cell stack testing

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.     

A nonlinear viscoelastic-viscoplastic constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells

This paper describes a phenomenological constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells (PEMFCs). Unlike the existing approaches of elasto-plastic, viscoelastic, and viscoplastic model, the proposed model was inspired by micromechanisms of polymer deformation. The constitutive model is a combination of the nonlinear visco-elastic Bergström-Boyce model and hydration-temperature-dependent empirical equations for elastic modulus of ionomer membranes. Experiment results obtained from an uniaxial tension test for Nafion NR-111 membrane under well controlled environments were compared with simulated results by the finite element method (FEM) and the proposed model showed fairly good predictive capabilities for the large deformation behavior of the Nafion membrane subjected to the uniaxial loading condition in a wide range of relative humidity and temperature levels including liquid water.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

Neoteric advancements in polybenzimidazole based polymer electrolytes for high-temperature proton exchange membrane fuel cells - A versatile review

The world's dependence on hydrocarbon fuel to generate power has proven to be the primary source of energy production. The emission of dangerous toxic and effluent gases during the process of hydrocarbon extraction and utilisation poses a massive threat to the environment and human life. This has driven the research into green energy technology; polymer electrolyte membrane fuel cells (PEMFCs) is one of the future century's bright green and clean energy producers. The most critical factor in the fuel cell is the polymer electrolyte membrane (PEM), which is the heart of the fuel cell assembly. Recently, polybenzimidazole (PBI)-based high-temperature polymer electrolytes have attracted researchers because they have high chemical and thermal stability combined with fillers that can control proton mobility. There are several limitations to the usage of PBI in high-temperature PEMFCs and this review summarizes the various structural modification: phase inversion, semi-interpenetrating IPNs, branched blocks and physical modification methods like crosslinking, blending, doping, progress done by various researchers to tackle the drawbacks in PBI-based polymer electrolyte membranes.     

Cold start analysis of polymer electrolyte membrane fuel cells

Cold start is critical to the commercialization of polymer electrolyte membrane fuel cell (PEMFC) for practical applications such as backup power and automotive applications. In this study, various numerically simulated PEMFC cold start processes are analyzed. The success of the cold start process depends on the competition between how fast the cell is heated up to the freezing point temperature and how fast ice is formed and built up in the pores of the cathode catalyst layer (CL) blocking oxygen transport to the reaction sites; the success of the cold start process thus depends on the product water (i) that is absorbed into the ionomer in the CL and membrane, (ii) that is taken away in vapour form by the gas flows (can be neglected), and (iii) that is frozen into ice in the CL pores. It is found that the membrane thickness and the ionomer volume fraction in the CL play pivotal roles in reducing the amount of ice formation. A thicker membrane leads to a larger water capacity but a slower water absorption process, and increasing the ionomer volume fraction in the CL enlarges the ionomer water capacity and enhances the membrane water absorption. Starting the cell under the potentiostatic condition is confirmed to be superior to the galvanostatic condition. Heating up the external surfaces and the inlet air enhances the temperature increment of the cell. However, the external heating methods have negligible improvement in reducing the amount of ice formation. Even though heating the inlet air is more effective in increasing the cell temperature than heating the outer surfaces, the heat capacity of the inlet air is low.     

New materials for polymer electrolyte membrane fuel cell current collectors

Polymer Electrolyte Membrane Fuel cells for automotive applications need to have high power density, and be inexpensive and robust to compete effectively with the internal combustion engine. Development of membranes and new electrodes and catalysts have increased power significantly, but further improvements may be achieved by the use of new materials and construction techniques in the manufacture of the bipolar plates. To show this, a variety of materials have been fabricated into flow field plates, both metallic and graphitic, and single fuel cell tests were conducted to determine the performance of each material. Maximum power was obtained with materials which had lowest contact resistance and good electrical conductivity. The performance of the best material was characterised as a function of cell compression and flow field geometry.     

Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification

The performance of polymer electrolyte membrane fuel cells is highly influenced by the water content in the membrane. To prevent the membrane from drying, several researchers have proposed extra humidification on the input reactants. But in some applications, the extra size and weight of the humidifier should be avoided. In this research a control technique, which maintains the relative humidity on saturated conditions, is implemented by adjusting the air stoichiometry; the effects of drying of membrane and flooding of electrodes are considered, as well. For initial analysis, a mathematical model reveals the relationship among variables that can be difficult to monitor in a real machine. Also prediction can be tested optimizing time and resources. For instance, the effects of temperature and humidity can be analyzed separately. For experimental validation, tests in a fault tolerant fuel cell are conducted.     

Hybrid electrode effects on polymer electrolyte membrane fuel cells

Research on membrane electrode assemblies (MEA) is focused on reducing cost and increasing durability in polymer electrolyte membrane fuel cells (PEMFC). Development of the electrode structure and reduction of platinum (Pt) contents are studied to improve the efficiency of Pt catalysts. We studied the combined effects of improved electrode structure and reduced Pt loading. To enhance the performance of an MEA, a commercial Pt/C catalyst with micro graphite (MG) was used. The 40 wt% Pt/C catalyst content was reduced about 5, 15, 30 and 60 wt% at the cathode. MG was added as a reduced weight percent of Pt/C. Cell performance was significantly dependent on the content of MG. The MEA with 15 wt% of MG was seen to best performance compare with other MEA. These results showed that the catalyst with mixed MG improved both performance and cost savings with reduced Pt content of PEMFC.     

质子交换膜燃料电池的稳态分析

通过建立质子交换膜燃料电池稳态模型,考察了电堆温度和反应压力对电堆性能的影响。仿真结果表明,升高电堆温度使得氢气分压和氧气分压下降,但氢气分压下降的更快;在电堆工作温度范围内,电堆温度升高,热动力电势、欧姆极化电势和活化极化电势均下降,但电堆总输出电压上升;提高阴极侧压力有利于提高热动力电势,同时使得活化极化电势降低,有利于电堆整体性能的改善;提高阳极侧压力对电堆性能改善影响不大。     

Development of shut-down process for a proton exchange membrane fuel cell

Several different shut-down procedures were carried out to reduce the degradation of membrane electrode assembly (MEA) in a proton exchange membrane fuel cell (PEMFC). The effects of close/open state of outlets of a single cell and application of a dummy load during the shut-down on the degradation of the MEA were investigated. Also, we elucidated the relationship between the thickness of the electrolyte membrane and the degradation of the MEA for different shut-down procedures. When a thin electrolyte membrane was used, the closer of outlets mitigated the degradation during on/off operation. For the thicker electrolyte membrane, the dummy load which eliminates residual hydrogen and oxygen in the electrodes should be applied to lower the degradation.     

Investigating effect of different gas diffusion layers on water droplet characteristics for proton exchange membrane (PEM) fuel cells

In this work, advanced x-ray radiographic techniques available at the Canadian Light Source (CLS) were utilized to study water droplet dynamics in a serpentine flow channel mimicking a proton exchange membrane fuel cell (PEMFC). High spatial and temporal resolution coupled with high energy photons of an x-ray beam provided high-resolution images of water droplets. This technique solved the problem caused by the opaqueness of fuel cell materials including the gas diffusion layer by providing a unique way to study water droplet dynamics at different operating conditions. From the captured images, droplet emergence and formation on porous gas diffusion layers (GDLs) were analyzed. Three commercially available GDLs (Sigracet AA, Sigracet BA, and Sigracet BC) were used and droplet detachment height was found to decrease in the following order AA < BA < BC under the same flow condition. Increasing the superficial gas velocity was found to decrease the droplet detachment height for all GDLs tested. Average droplet cycle for various operating conditions was obtained. It was found that humidified air did not show a difference in droplet dimensions at detachment compared to dry air used at the inlet gas. However, it did show an impact on droplet cycle time, which might be due to condensation.     

Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells

The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve.     

Nonlinear frequency response analysis of dehydration phenomena in polymer electrolyte membrane fuel cells

Dehydration phenomena in a PEM fuel cell were investigated by nonlinear frequency response analysis (NFRA) in a differential H₂/H₂ cell. The linear H_1,0 spectra, which are equal to classic EIS spectra, showed not only an increase of the membrane resistance but also an increase of the anode reaction resistance, caused by dehydration leading to the decrease of the protonic conductivity of the polymer network in the catalyst layer. With this, active sites with long protonic pathes to the membrane become inactive. In order to further clarify this effect, modelling work was used. Therefore, proton transport was incorporated into an existing model of a differential H₂/H₂ cell. Finally, the key features of NFRA spectra under dehydration and CO poisoning are compared in order to discuss the suitability of NFRA for unambiguous diagnosis of PEMFC. It can be seen that while the linear spectrum is not sufficient to distinguish between both cases, the second order frequency response functions can be used for discrimination.     

Specific approaches to dramatic reduction in stack activation time and perfect long-term storage for high-performance air-breathing polymer electrolyte membrane fuel cell

Air-breathing polymer electrolyte membrane fuel cell (PEMFC) systems without the humidifier and air blower have been developed to overcome the cost and complexity of balance of plants (BOPs). Until now, there has been no specific way to improve the stack's initial performance through the specific activation protocol and maintain the initial performance for a very long time. Herein, we studied a technique for finishing the total activation within 1 h by using a pre-activation process (i.e., soaking the stack in a DI-water reservoir) and applying current at 0.65 V. The pre-activation procedure significantly increased the swelling of the polymer membrane and the Nafion binder in the catalyst layer, reducing the total activation time. Also, we showed that long-term storage using humidified N₂ gas in a closed box did not hinder the electrocatalytic activity of the Pt and the drying of the polymer membrane for 60 days.     

Analysis of the spatially distributed performance degradation of a polymer electrolyte membrane fuel cell stack

Herein we report the spatially uneven degradation of a polymer electrolyte membrane fuel cell (PEMFC) stack operated under load variation. Fifteen sub-membrane electrode assemblies (sub-MEAs) at various cell positions and various points within each cell were obtained from the original MEAs employed in the fuel cell stack. Polarization curves and the voltammetric charge of these MEAs were measured in order to correlate localized performances with the redistributed electrochemically active surface on Pt using the polarization technique and cyclic voltammetry. Several ex situ characterizations including electron probe microanalysis, environmental scanning electron microscopy, and X-ray diffraction were also performed to find evidence, supporting the inhomogeneous degradation of the fuel cell stack. Possible routes and processes for the non-uniform stack degradation during the PEMFC stack operation will also be discussed.     

Novel technique for measuring oxygen crossover through the membrane in polymer electrolyte membrane fuel cells

In this study, the exact amount of oxygen crossover that reacts with hydrogen has been investigated using a mass spectrometer system. By measuring the amount of oxygen crossover that reacts with hydrogen, the exact amount of oxygen crossover that affects membrane degradation and/or water generation can be calculated under the fuel cell operating conditions. The amount of oxygen crossover that reacts with hydrogen is expressed as an effective oxygen crossover ratio, which is in a range between 0.927 and 0.933 under the fuel cell operating temperature conditions. This means that approximately 93% of the entire oxygen crossover through the membrane can affect membrane degradation and/or water generation at the anode catalyst layer. Thus, the effective oxygen crossover ratio should be considered as a novel index of oxygen crossover because it represents the exact amount of oxygen crossover that reacts with hydrogen.     

Optimisation of carbon nanofiber based electrodes for polymer electrolyte membrane fuel cells prepared by a sedimentation method

Tubular carbon nanofibers with an average diameter of 150 nm are investigated as a possible material for the electrodes preparation for polymer electrolyte membrane fuel cells. Well-dispersed platinum particles with an average crystallite size of 4.6 nm are deposited on surface-oxidised fibers to be used as a catalyst support with an electroless plating method. The carbon nanofiber-based electrodes are prepared by a sedimentation method without the use of organic solvents. This method allows an exact setting of the fiber and binder content and the catalyst loading. The electrodes are optimised by varying the thickness of the gas diffusion layer and its binder content as well as the thickness of the active layer. These optimised electrodes show a considerably better performance when compared to carbon black-based electrodes with the same catalyst loading prepared by a spraying process using the same type and amount of electrolyte in the membrane electrode assembly. By reducing the platinum content from 0.7 to 0.2 mg cm⁻², catalyst utilisation is significantly increased.