•Reviewed methods for the humidification of polymer electrolyte membrane fuel cells.•Categorized into internal and external humidification methods.•Presented advantage and drawback of each ...humidification method.•Summarized suitable applications for each humidification method.
Polymer electrolyte membrane fuel cells are promising power sources because of their advantage such as high efficiency, zero emission and low operating temperature. Water management is one of the critical issues for polymer electrolyte membrane fuel cells and has received significant attention. The membrane within the fuel cell needs to stay in hydrated state to have high ion conductivity and durability, which requires proper humidification. Both internal and external methods have been utilized to humidify the polymer electrolyte membrane. Numerous studies on fuel cell humidification have been conducted in the past decades, especially in recent years. This review aims to summarize the main humidification methods and the related studies. The internal humidification methods are classified as physical methods and chemical methods. The external humidification methods include gas bubbling humidification, direct water injection, enthalpy wheel humidification, membrane humidifiers, and exhaust gas recirculation. The working principle and performance of each method are introduced and the advantage and drawback are summarized. Further, the humidification methods for alkaline anion exchange membrane fuel cells are also briefly reviewed, because of more recent studies showing their potential of using non-precious metal catalysts. This review can help to choose proper humidification strategy for specific polymer electrolyte membrane fuel cell application and may inspire further investigations.
Proton exchange membranes with short-pathway through-plane orientated proton conductivity are highly desirable for use in proton exchange membrane fuel cells. Magnetic field is utilized to create ...oriented structure in proton exchange membranes. Previously, this has only been carried out by proton nonconductive metal oxide-based fillers. Here, under a strong magnetic field, a proton-conducting paramagnetic complex based on ferrocyanide-coordinated polymer and phosphotungstic acid is used to prepare composite membranes with highly conductive through-plane-aligned proton channels. Gratifyingly, this strategy simultaneously overcomes the high water-solubility of phosphotungstic acid in composite membranes, thereby preventing its leaching and the subsequent loss of membrane conductivity. The ferrocyanide groups in the coordinated polymer, via redox cycle, can continuously consume free radicals, thus helping to improve the long-term in situ membrane durability. The composite membranes exhibit outstanding proton conductivity, fuel cell performance and durability, compared with other types of hydrocarbon membranes and industry standard Nafion
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•Influence of humidity changes on catalyst layers (CL) studied by accelerated stress tests (AST).•ASTs at 99% relative humidity induce apparent changes in CL structure.•No obvious ...changes were observed for AST at 20% RH.•Structural changes of CL are related to ionomer migration during humidity changes.
The degradation of the membrane electrode assembly, originating from microstructural changes in the catalyst layer, inhibits the commercialization of polymer electrolyte membrane fuel cell. In particular, changes in relative humidity during starting/working conditions cause crack growth and propagation within the catalyst layer, but the reason is still not clear. Here, accelerated stress tests are designed from starting conditions (25 °C, 45% RH) to working conditions with different RHs (20%, 45% and 99%) at 85 °C for different cycles. For low working RH of 20%, no obvious change can be observed, while the accelerated stress test to high working RH induces apparent changes in the catalyst layer structure, with ionomer aggregation and migration leading to crack generation. The obtained results indicated that ionomer migration plays an important role in the structure changes of catalyst layer, suggesting that the design of ionomer with high stability and reliable water retention is necessary to improve the structural stability of the catalyst layer.
•Water removal and transport in a novel PEMFC flow channel is analyzed numerically.•The novel flow channel consists of a hydrophilic plate in the flow channel.•Water removal is facilitated ...significantly by the hydrophilic plate.•The plate should have a contact angle larger than bottom channel but less than MEA.•Optimal surface contact angle, length and height of the plate are investigated.
Effective removal and transport of water in the flow channel of a proton exchange membrane (PEM) fuel cell (PEMFC) is significantly important to the critical water management in PEMFCs. In this study, the process of water removal and transport is investigated numerically by using the volume-of-fluid method for a flow channel having a hydrophilic plate in the middle of the channel. The results show that the liquid water droplet on the membrane-electrode assembly (MEA) surface can be removed effectively, and the removal process is facilitated significantly by the hydrophilic plate which should have a surface contact angle larger than the bottom channel surface but less than the MEA surface. Once the liquid water contacts the plate, it is detached from the MEA surface, and transported to the channel surface along the plate surface; whereas without the plate the water droplet is transported along the MEA surface under the same flow condition. The pressure drop associated with the flow in the channel can be reduced substantially by the presence of the plate due to a characteristic change in the water removal and transport process, when compared to the pressure drop in a conventional flow channel or a channel with a needle shown in literature. The wettability, the length and the height of the plate all can have an impact on the water transport and dynamics as well as the associated pressure drop in the flow channel. A parametric study is carried out to determine the optimal values for the surface contact angle, the length and height of the plate.
Based on a coupled finite element method (FEM) and computational fluid dynamics (CFD) model, the structural deformation and performance of a proton exchange membrane fuel cell (PEMFC) under different ...membrane water contents are studied. The water absorption behavior of the membrane is investigated experimentally to obtain its expansion coefficient with water content, and the Young’s modulus of the membrane and catalyst (CL) are obtained through a tensile experiment. The simulation results show that the deformation of the membrane increases with water content, and membrane swelling under the channel is larger than that under the rib, forming a surface bump under the channel. The structural changes caused by the membrane water content have little effect on the performance of PEMFC in the low-current density range; while its influence is significant in the medium- and high-current density range. A medium membrane water content value of 12 achieves the best fuel cell performance due to the balance of membrane resistance and mass transport.
A fuel cell is an energy conversion device that utilizes hydrogen energy through an electrochemical reaction. Despite their many advantages, such as high efficiency, zero emissions, and fast startup, ...fuel cells have not yet been fully commercialized due to deficiencies in service life, cost, and performance. Efficient evaluation methods for performance and service life are critical for the design and optimization of fuel cells. The purpose of this paper was to review the application of common machine learning algorithms in fuel cells. The significance and status of machine learning applications in fuel cells are briefly described. Common machine learning algorithms, such as artificial neural networks, support vector machines, and random forests are introduced, and their applications in fuel cell performance prediction and optimization are comprehensively elaborated. The review revealed that machine learning algorithms can be successfully used for performance prediction, service life prediction, and fault diagnosis in fuel cells, with good accuracy in solving nonlinear problems. Combined with optimization algorithms, machine learning models can further carry out the optimization of design and operating parameters to achieve multiple optimization goals with good accuracy and efficiency. It is expected that this review paper could help the reader comprehend the state of the art of machine learning applications in fuel fuels and shed light on further development directions in fuel cell research.
Water transport and removal in the proton exchange membrane fuel cell (PEMFC) is critically important to fuel cell performance, stability, and durability. Water emerging locations on the ...membrane-electrode assembly (MEA) surface and the channel surface wettability significantly influence the water transport and removal in PEMFC. In most simulations of water transport and removal in the PEMFC flow channel, liquid water is usually introduced at the center of the MEA surface, which is fortuitous, since water droplet can emerge randomly on the MEA surface in PEMFC. In addition, the commonly used no-slip wall boundary condition greatly confines the water sliding features on hydrophobic MEA/channel surfaces, degrading the simulation accuracy. In this study, water droplet is introduced with various locations along the channel width direction on the MEA surface, and water transport and removal is investigated numerically using an improved model incorporating the sliding flow property by using the shear wall boundary condition. It is found that the water droplet can be driven to the channel sidewall by aerodynamics when the initial water location deviates from the MEA center to a certain amount, forming the water corner flow in the flow channel. The channel surface wettability on the water transport is also studied and is shown to have a significant impact on the water corner flow in the flow channel.
Liquid water transport and removal is one of the critical issues in the proton exchange membrane fuel cell (PEMFC) for achieving good performance and durability. In this study, two novel channels ...with different blocks are designed to study their effects on water removal using the volume of fluid (VOF) model considering the dynamic contact angle effect. It is found that compared with the conventional straight channel, both the one-block and two-block channels can promote liquid water removal. The one-block channel leads to faster water movement and removal on the gas diffusion layer (GDL) surface, but results in a much higher pressure drop. The separated two-block channel directly drags water away from the GDL surface by the capillary wicking effect of the block surface, achieving both faster water removal and smaller pressure drop. Effects of the droplet size, air velocity and static contact angle of GDL surface on water removal are investigated comprehensively in both the novel channels, as well as the conventional straight channel, with particular attention on the variations of water removal time, water coverage ratio and pressure drop.
•Two novel fuel cell channels with baffle blocks are designed for fast water removal.•One-block channel promotes water transport and removal by the narrowing effect.•Two-block channel promotes water removal by direct block drag effect.•Two-block channel achieves both faster water removal and smaller pressure drop.•Two-block channel is effective to remove large droplet on more hydrophobic surface.
Water dynamics in the flow channel of a proton exchange membrane fuel cell is significantly important to water management and removal. In this study, volume-of-fluid method is used to investigate ...numerically the three-dimensional water dynamics in a flow channel with a hydrophilic needle. It is found that water transport and dynamics in this novel flow channel are quite different from the conventional channel. Liquid water droplet, introduced on the electrode surface, is removed through capillary effect once touching the hydrophilic needle. This is desirable since the electrode surface becomes free of liquid water, avoiding the flooding and blockage of reactant gas transport into the electrode. Increasing the contact area between the water droplet and needle, through an increase in the diameter or length of the needle, can facilitate water removal from the electrode surface because of greater capillary effect, but it also increases the pressure drop in the channel due to greater blockage by the needle. Overall, the pressure drop in the modified channel is still small compared to the pressure drop in a serpentine flow channel, making the present approach viable for use in the conventional parallel flow channels for proton exchange membrane fuel cells.
► Water transport and dynamics in a novel PEMFC flow channel is simulated numerically. ► The novel flow channel consists of a hydrophilic needle in the flow channel. ► Water emerging from MEA surface is removed by the hydrophilic needle upon contact. ► Optimal needle diameter and length are determined.
The performance of a polymer electrolyte membrane fuel cell (PEMFC) closely depends on internal reactant diffusion and liquid water removal. As one of the key components of PEMFCs, bipolar plates ...(BPs) provide paths for reactant diffusion and product transport. Therefore, to achieve high fuel cell performance, one key issue is designing BPs with a reasonable flow field. This paper provides a comprehensive review of various modifications of the conventional parallel flow field, interdigitated flow field, and serpentine flow field to improve fuel cells’ overall performance. The main focuses for modifications of conventional flow fields are flow field shape, length, aspect ratio, baffle, trap, auxiliary inlet, and channels, as well as channel numbers. These modifications can partly enhance reactant diffusion and product transport while maintaining an acceptable flow pressure drop. This review also covers the detailed structural description of the newly developed flow fields, including the 3D flow field, metal flow field, and bionic flow field. Moreover, the effects of these flow field designs on the internal physical quantity transport and distribution, as well as the fuel cells’ overall performance, are investigated. This review describes state-of-the-art flow field design, identifies the key research gaps, and provides references and guidance for the design of high-performance flow fields for PEMFCs in the future.
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