Pore-Scale Modeling of Liquid Water Transport in Compressed Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells Considering Fiber Anisotropy.

Water management of the gas diffusion layer (GDL) is crucial to the performance of proton exchange membrane fuel cells (PEMFCs). Appropriate water management ensures efficient transport of reactive gases and maintains wetting of the proton exchange membrane to enhance proton conduction. In this paper, a two-dimensional pseudo-potential multiphase lattice Boltzmann model is developed to study liquid water transport within the GDL. Liquid water transport from the GDL to the gas channel is the focus, and the effect of fiber anisotropy and compression on water management is evaluated. The results show that the fiber distribution approximately perpendicular to the rib reduces liquid water saturation within the GDL. Compression significantly changes the microstructure of the GDL under the ribs, which facilitates the formation of liquid water transport pathways under the gas channel, and the increase in the compression ratio leads to a decrease in liquid water saturation. The performed microstructure analysis and the pore-scale two-phase behavior simulation study comprise a promising technique for optimizing liquid water transport within the GDL.

Effects of Compression and Porosity Gradients on Two-Phase Behavior in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells.

Water management within the gas diffusion layer (GDL) plays an important role in the performance of the proton exchange membrane fuel cell (PEMFC) and its reliability. The compression of the gas diffusion layer during fabrication and assembly has a significant impact on the mass transport, and the porosity gradient design of the gas diffusion layer is an essential way to improve water management. In this paper, the two-dimensional lattice Boltzmann method (LBM) is applied to investigate the two-phase behavior in gas diffusion layers with different porosity gradients under compression. Compression results in an increase in flow resistance below the ribs, prompting the appearance of the flow path of liquid water below the channel, and liquid water breaks through to the channel more quickly. GDLs with linear, multilayer, and inverted V-shaped porosity distributions with an overall porosity of 0.78 are generated to evaluate the effect of porosity gradients on the liquid water transport. The liquid water saturation values within the linear and multilayer GDLs are significantly reduced compared to that of the GDL with uniform porosity, but the liquid water within the inverted V-shaped GDL accumulates in the middle region and is more likely to cause flooding.

Numerical Study on Thermal Stress of High Temperature Proton Exchange Membrane Fuel Cells during Start-Up Process.

High-temperature proton-exchange membrane fuel cells (HT-PEMFCs) with phosphoric-doped polybenzimidazole (PBI) membranes have a higher operating temperature compared to the PEMFCs operating below 373.15 K. The fuel cell is first heated from room temperature to the minimum operating temperature to avoid the generation of liquid water. The existence of liquid water can result in the loss of phosphoric acid and then affect the cell performance. In this study, the start-up process of HT-PEMFCs is numerically studied by establishing a three-dimensional non-isothermal mathematical model. Preheated gas is supplied into gas flow channels to heat the fuel cell, and then voltage load is applied to accelerate the start-up process. Effects of voltage (0.9 V, 0.7 V and 0.5 V) and flow arrangement (co-flow and counter flow) on temperature, current density, proton conductivity and stress distributions of fuel cells are examined. It is found that the maximum stress is increased when a lower voltage is adopted, and the counter-flow arrangement provides a more uniform stress distribution than that of co-flow arrangement.

Impact of Membrane Phosphoric Acid Doping Level on Transport Phenomena and Performance in High Temperature PEM Fuel Cells.

In this work, a three-dimensional mathematical model including the fluid flow, heat transfer, mass transfer, and charge transfer incorporating electrochemical reactions was developed and applied to investigate the transport phenomena and performance in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) with a membrane phosphoric acid doping level of 5, 7, 9, 11. The cell performance is evaluated and compared in terms of the polarization curve. The distributions of temperature, oxygen mass fraction, water mass fraction, proton conductivity, and local current density of four cases are given and compared in detail. Results show that the overall performance and local transport characteristics are significantly affected by the membrane phosphoric acid doping level.

Effects of Hydration and Temperature on the Microstructure and Transport Properties of Nafion Polyelectrolyte Membrane: A Molecular Dynamics Simulation.

To investigate the effects of temperature and hydration on the microstructure of polymer electrolyte membrane and the transport of water molecules and hydronium ions, molecular dynamics simulations are performed on Nafion 117 for a series of water contents at different temperatures. The interactions among the sulfonate groups, hydronium ions, and water molecules are studied according to the analysis of radial distribution functions and coordination numbers. The sizes and connectivity of water clusters are also discussed, and it is found that the hydration level plays a key role in the phase separation of the membrane. However, the effect of the temperature is slight. When the water content increases from 3.5 to 16, the size of water clusters in the membrane increases, and the clusters connect to each other to form continuous channels for diffusion of water molecules and hydronium ions. The diffusion coefficients are estimated by studying the mean square displacements. The results show that the diffusion of water molecules and hydronium ions are both enhanced by the increase of the temperature and hydration level. Furthermore, the diffusion coefficient of water molecules is always much larger than that of hydronium ions. However, the ratio of the diffusion coefficient of water molecules to that of hydronium ions decreases with the increase of water content.

Numerical study on the effect of discrete catalytic layer arrangement on methane steam reforming performance.

The arrangement of catalytic layers inside the reactor is an important factor that affects the efficiency of methane steam reforming to produce hydrogen, and the traditional continuous catalytic layer structure is limited by the heat and mass transfer, resulting in unbalanced heat distribution inside the reactor and poor reaction performance. In order to improve the performance of methane reforming and balance the internal temperature of the reactor, different catalytic layers were designed based on 2D numerical simulation, and different numbers of discrete catalytic layers were modeled to compare the heat and mass transfer, methane conversion rate and hydrogen yield between the walls and inside the reactor. The results show that the increase in the number of catalyst gaps improves the temperature gradient inside the reactor, reduces the average cold point temperature difference inside the reactor by up to 7.2%, maintains a better thermal balance inside the reactor, improves the reaction rate inside the reactor, and the methane conversion rate and hydrogen yield after the reaction have been improved by 28.46% and 12.7% respectively.