• Energy Research
• 6. Clean water close
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An in-depth insight in the role of gas diffusion layers (GDLs) and its impact on the water management is a key issue for the optimization of fuel cells. A new ex situ test method is developed to investigate the water transport in gas diffusion media for polymer electrolyte membrane fuel cells (PEMFCs). This research is focused on properties of GDLs, which influence the water removal and water retention in the cell. Gas diffusion media are evaluated ex situ in terms of liquid water and water vapor transport employing a conventional PEMFC setup. The amount of water transported through a GDL and out of the cell is determined by the properties of the gas diffusion medium. GDL properties such as the GDL thickness have a significant impact on water transport behavior. Furthermore, high impregnation weight or an additional micro-porous layer (MPL) reduces water removal due to enhanced mass transport resistances. The composition and distribution of the impregnation material in the GDL substrate also play a crucial role. Water transport rates depend not only on the GDL properties but increase exponentially with cell temperature. Finally, a two-phase water transport model is proposed taking into account both diffusive gas phase and liquid water transport in diffusion media. Based on this model, ex situ data set in correlation with in situ performance in PEMFCs on dry operating conditions and guidelines towards new design concepts for gas diffusion media are deduced.

Abstract Recently, flow fields of proton exchange membrane fuel cell with three-dimensional (3-D) structure are attracting attentions due to their merits on mass transfer and water management. Among them the most representative one is 3-D fine mesh flow field (3-D flow field). In this study, the morphology of 3-D flow field is reconstructed based on optical microscope image to discuss the single- and two-phase flow characteristics. It is found that the air guidance effect of 3-D baffles contributes to the reactant transport. Meanwhile, a separated liquid-gas transport phenomenon is observed, which reduces the liquid coverage area on gas diffusion layer surface and provides a larger passage area for mass transfer. Furthermore, the effects of air inlet velocity, droplet size and baffle contact angle are discussed. It is found that unless the air velocity is too low, the droplet tends to overcome surface tension and move to above-baffle area. In addition, the water retention capacity of 3-D flow field is found limited and influenced by air velocity and baffle contact angle. Besides, a super hydrophobic or hydrophilic surface of 3-D baffles is likely to cause problems on water management.

Water management is one of the most important issues in Proton Exchange Membrane Fuel Cells (PEMFC). Flooding and drying out are the two main degradation mechanisms that occur when water management is not adequate. This paper overviews the underlying phenomena linked to water management and some characterization methods or strategies to prevent their occurrence. Finally, a fault tree is built, in order to discriminate the different defaults and better understand the relationship between the causes and symptoms.

Abstract Through-plane liquid water distributions recently visualized by Manahan et al. [1] and Turhan et al. [2] using the neutron radiography (NR) technique show that the peaks of the water distributions are located near the center of a gas diffusion layer (GDL). We suggest that the distinctive water profiles are caused by incomplete polytetrafluoroethylene (PTFE) treatment of the GDL and the resultant spatial variation of GDL wettability in the through-plane direction. Based on this hypothesis, we improve the macroscopic two-phase fuel cell model to describe two-phase transport through GDLs with variation of spatial wettability [3] . The proposed model successfully reproduces the shape of through-plane water profiles obtained from the NR experiments [1] , [2] . Therefore, the centrally located liquid saturation peak in the GDL can be attributed to incomplete PTFE treatment of the GDL. This occurs because liquid water is more easily accumulated in the relatively hydrophilic GDL pores encountered in the inner GDL region (rather than the outer GDL region) due to its incomplete PTFE treatment. Our results indicate that the overall characteristics of liquid water distribution in a GDL under an inhomogeneous wetting condition can be macroscopically predicted using the two-phase model presented here.

Abstract A novel water management system for micro passive DMFC is fabricated and characterized in this paper. This system consists of both a cathode current collector made of a 316L sintered stainless fiber felt (SSFF) and an aluminum-based end plate fabricated with a perforated flow field. Besides, some water-collecting channels were fabricated on the surface of the cathode end plate and then covered by the plasma electrolytic oxidation (PEO) coating. The results show that the PEO coating plays crucial roles in the water management system. Because of the highly hydrophilic property of the coating, the channels work well in collecting the liquid water from the current collector, and water accumulation along the air-breathing holes can be well prevented, which improves the stability of the micro DMFC.

Abstract Handling of water accumulation is still a key issue in fuel cell research. The presented study evaluates the condensate removal capability of three different flow field designs. The designs are compared regarding cell voltage at different current densities using the same operating conditions. The investigated type of meander-shaped channels with a high degree of parallelization shows the best performance with stationary water thickness inside channels throughout the analyzed current densities. To develop and evaluate a condensate removal criterion for fuel cell construction, the pressure drop of each flow field is correlated to the water appearance visualized with neutron radiography. For a further insight, computational fluid dynamics simulation is used to calculate pressure drops present inside the characteristic regions of each flow field. Thus, a characteristic design limit of 20 mbar m −1 for meander-shaped channels is proven to ensure the absence of channel blockage. The meander-shaped channels show specific pressure drops around this limit depending on water production and gas supply. The two other analyzed flow fields suffer from higher channel filling rates: the investigated straight channels with less parallelization fill up with time, while the pattern-structured flow field demonstrates gravity as an additional influence on condensate removal.

Abstract This review concentrated on proton exchange membrane fuel cell (PEMFC) water management, which introduces the latest research contents and progresses on membrane electrode assembly (MEA) components, and summarizes the influencing factors about the properties of them. The PEMFC's core component is MEA which consists of proton exchange membrane, catalyst layer, microporous layer and carbon fiber paper. The properties of each component have significant impacts on the fuel cell (FC) performance, and there will be strong interactions between some properties. Therefore, the FC performance is determined by the properties of each component and their interactions. It is the basis for the design of the high-performance FCs to clarify the influence mechanism of various properties and their interactions on the FC performance, but there is currently no summary on this aspect. Therefore, the conclusions about the same component or property drawn from all literatures are sorted and combined, so that we can intuitively know which literatures have studied and reached the conclusions, and comprehensively realize the influence mechanism of a certain property. Based on the development and achievement of each component in some aspects, this article makes some predictions and discussions on the future research direction and development prospects of each component.

The effect of wettability on water transport dynamics in gas diffusion layer (GDL) is investigated by simulating water invasion in an initially gas-filled GDL using the multiphase free-energy lattice Boltzmann method (LBM). The results show that wettability plays a significant role on water saturation distribution in two-phase flow in the uniform wetting GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially. The resulting water saturation distributions show that the saturation in the GDL has little change after water breaks through the GDL, and further confirm that the selective introduction of hydrophilic passages in the GDL would facilitate the removal of liquid water more effectively, thus alleviating the flooding in catalyst layer (CL) and GDL. The LBM approach presented in this study provides an effective tool to investigate water transport phenomenon in the GDL at pore-scale level with wettability distribution taken into consideration.

Abstract Water management has a major impact in the solid polymer fuel cell on overall system power, cost and efficiency. Single cell and stack performance may be adversely affected by the formation of liquid water, the dilution of reactant gases by water vapour, or by the dehydration of the solid polymer membrane. Peak fuel cell power is achieved typically at current densities at which performance is limited by mass transport. Improved water management at higher current densities not only increases peak power and efficiency but changes the profile of the power curve resulting in improved stability near the peak power operating point. Fuel cell water management can be accomplished by a number of approaches which include system design, stack operating conditions, stack hardware and membrane electrode assembly design. A number of these techniques have been successfully applied to both single cells and stacks. However, the options available for water management have to be assessed from an overall engineered system point of view.