Behavior Of Liquid Water Research Articles

Effect of GDL Structure and Operating Conditions on PEMFC Performance and Liquid Water Removal

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Effect of GDL Structure and Operating Conditions on PEMFC Performance and Liquid Water Removal

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Micro-CT structures facilitated lattice Boltzmann simulations on the water dynamics in PEMFCs GDL-Gas channel

This study employed Micro X-ray computed tomography (Micro-CT) to reconstruct the gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) and used lattice Boltzmann method (LBM) simulations to investigate the dynamic behavior of liquid water in the multiscale space of the GDL and gas channel (GC). The results showed that the transport of liquid water from GDL to GC through the interface of GDL-land and GDL-GC were divided into four stages: large pore filling, through-plane breakthrough, in-plane diffusion, and rapid transport into GC. The wettability of GC wall and GDL affects the transport and distribution of liquid water. When the contact angle on the GC wall was less than 90°, it promoted liquid water transport to the top GC wall, resulting in faster internal flow formation. Conversely, contact angles exceeding 90° caused liquid water to break through the GDL-GC interface from pores away from the GC wall. When the contact angle of the GDL was in the range of 110°–150°, both excessively large and excessively small contact angles hindered water transport and management. Maintaining the contact angle of the GDL around 130° enhanced the breakthrough path and transport speed of liquid water into the GC, and it also reduces localized liquid water accumulation.

Numerical analysis of thermal-hydraulic influence of geometric flow baffles on multistage Tesla valves in printed circuit heat exchangers

The innovative structure and flexible functionality of Tesla valves have attracted significant attention across various industries. Unlike conventional check valves, which open in response to fluid movement and pressure and close to prevent backflow, Tesla valves utilize interconnected pathways to create a highly efficient and reliable method for regulating fluid flow. However, the thermal–hydraulic influence associated with the utilization of different geometric configurations of flow baffles in Tesla valves remain incompletely understood. This study employs numerical simulations to assess the effectiveness of a printed circuit heat exchanger (PCHE) incorporating three layers of multistage Tesla valves with geometric flow baffles. The simulations, based on solving the Navier-Stokes and energy equations, focus on analyzing the behavior of liquid water within a temperature range of 30 to 90 °C, pertinent to applications such as geothermal heating systems. The employed Tesla valve configuration demonstrates superior performance, exhibiting a notable enhancement in thermal effectiveness ranging from 13 % to 55 % compared to configurations utilizing zigzag, separate, and overlapping NACA 0020 airfoils under otherwise identical conditions. Three crucial geometric parameters are introduced to characterize the shape of flow baffles in multistage Tesla valves: the length from the baffle apex to the baffle half-front sphere, the radius of the baffle half-front sphere, and the distance from the baffle apex to the centerline of the entrance flow channel. Performance evaluation criteria (PEC) indicate that while the overlapping airfoil configuration performs better than both zigzag and separate configurations, the current Tesla valve design achieves approximately a 10–14 % higher PEC value than the overlapping airfoil configuration. The manipulation of geometric parameters for flow baffles is generally believed to be a critical step in the advancement of multistage Tesla valves, contributing to the enhancement of heat transfer and potentially broadening the applicability of these valves to various practical heat transfer systems.

Effects of Gas-Diffusion Layers and Water Management on the Carbon Corrosion of a Catalyst Layer in Proton-Exchange Membrane Fuel Cells

Carbon corrosion in a catalyst layer (CL) deteriorates the performance and durability of proton-exchange membrane fuel cells (PEMFCs), which are closely related to water management within these cells. This study investigates the characteristics of water behavior of two gas diffusion layers (GDLs) and compares their influence on degrees of degradation in the CL. First, the properties of the GDLs, including their thickness, pore size distribution, gas permeability, electrical resistance, contact angle, and polytetrafluoroethylene (PTFE) content, are evaluated. The dynamic behavior of liquid water is observed using a visualization cell and synchrotron X-ray imaging. Second, a modified accelerated stress test (AST), which includes a water generation reaction within the catalyst support protocol of the US Department of Energy (DOE), is performed. For assessing the degradation, we utilize polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, and field emission transmission electron microscopy. The results reveal that, even though GDL B contains a higher hydrophobic content than GDL A, it exhibits lower water discharge, indicating a reduced performance at high relative humidity (RH) levels. This is attributed to a low capillary pressure gradient, which is influenced not only by PTFE but also by the overall pore structure (i.e., porosity and pore size). Consequently, a high capillary pressure gradient can enhance water discharge and thereby mitigate carbon corrosion and Pt agglomeration in the CL. In addition, the application of the modified AST induces carbon corrosion with fewer cycles than that achieved using the DOE carbon support protocol.

Study on the Relationship between GDL Wettability and Liquid Water Behavior in Polymer Electrolyte Fuel Cells

Liquid water that accumulates inside gas diffusion layers (GDL) prevents oxygen transport and degrades the performance of polymer electrolyte fuel cells (PEFCs). Therefore, improving the drainage of GDL is neccesary. One factor that directly affects drainage is the wettability of GDLs. In this study, the relationship between wettability of GDL and drainage was investigated through a numerical simulation using the volume of fluid (VOF) method. The simulation incorporated condensation model to mimic the actual operation of a PEFC. As a result of the simulation, it was found that condensation occurs near the catalyst layer (CL), and that liquid water accumulation develops from the vicinity of the CL side to the channel side. Simulations for different contact angles of θ contact = 60°, 90°, 110°, and 150° were performed, and showed that the liquid water volume inside the GDL decreased in the case of higher contact angle. In addition, we found that the hydrophobicity of the GDL promotes the movement of liquid water and hastened liquid-water discharge from the GDL surface. In the case of higher contact angle, the proportion of the gas-liquid interface in the GDL and its curvature were found to increase. Furthermore, the liquid water was discharged from greater number of pores on the GDL surface.

Pore-scale study of two-phase flow in the gas diffusion layer of proton exchange membrane fuel cells: The impact of polytetrafluoroethylene content and gradient distribution

The polytetrafluoroethylene (PTFE) is commonly used to improve the hydrophobic of the gas diffusion layer (GDL). In this study, impacts of PTFE content and piecewise/linear gradient distributions on the transport behavior of liquid water were numerically studied by using a multiphase lattice Boltzmann method. A two-dimensional microstructure of the GDL was reconstructed by stochastic algorithm. The two-phase flow in the GDLs with PTFE contents of 0, 10, 20, 30 and 40 wt% were evaluated and compared. The liquid water transport process can be greatly affected by the PTFE content and the best water drainage performance was provided by the GDL with PTFE content of 30 wt%. Subsequently, the GDLs with PTFE piecewise/linear gradient distributions were designed and investigated. It was found that the water drainage performance can be further improved when the PTFE content is well distributed. This study can provide guidelines for design of GDL for high performance proton exchange membrane fuel cells.

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry.

We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces.

Numerical Simulation of Liquid Water Behavior in PEFCs with Different GDL Wettability

The performance of polymer electrolyte fuel cells (PEFCs) is greatly influenced by internal transport phenomena. Particularly, accumulation of liquid water due to electrochemical reaction is one of the main factors that prevents the transport of oxygen and increases the concentration overpotential, and thus, quick rejection of liquid water accumulation is important issue for improving performance. This study focuses on liquid water accumulation inside gas diffusion layers (GDLs), and investigates how wettability of the substrate surface alters the liquid water behavior using numerical simulation. In this study, a numerical simulation incorporating condensation phenomena was performed using three-dimensional structure of GDL substrate obtained from CT measurements. The cell temperature and the current density were set to 40° and 0.6A/cm2, and the contact angles of the substrate was set to 60°, 110°, 150°. The simulation results showed that in the hydrophobic cases of 110°, 150°, the droplets began to move between pores to minimize their surface energy as their size grew into pore scale of the substrate. While in the hydrophilic 60° case, the liquid water condensed near the catalyst layer only moved in such a way that the thickness of the liquid water increased, and no droplet-like structure were observed. In the 150° case, the contact area between the substrate and the droplets was smaller than that in the case of 110°, and the surface energy of the droplets was assumed to be larger. It was found that the liquid water accumulation is highest at the 60° case followed by 110°, 150°. Figure 1

Influence and Optimization of Gas Diffusion Layer Porosity Distribution along the Flow Direction on the Performance of Proton Exchange Membrane Fuel Cells

Influence and Optimization of Gas Diffusion Layer Porosity Distribution along the Flow Direction on the Performance of Proton Exchange Membrane Fuel Cells

Numerical Simulation of Liquid Water Behavior in PEFCs with Different GDL Wettability

In this study, an unsteady numerical simulation of PEFCs using the VOF method was performed to investigate the relationship between wettability of gas diffusion layer (GDL) substrates and liquid water drainage. The simulations were conducted with contact angle of 60°, 90°, 110°, 150°. The simulation results showed that the more hydrophobic is, the more liquid water movement was facilitated. Also, in the case of higher contact angles, the contact area between liquid water and substrates was decreased, and the area of gas-liquid interface was increased, and it was observed that the curvature of gas-liquid interface increased.

Effect of multi-channel shape design on dynamic behavior of liquid water in PEMFC

A well-designed flow channel is crucial for effective water management in proton exchange membrane fuel cell. This study presents novel simulation investigation of the dynamic behavior of liquid water in three-dimensional multi-channels with various shape designs, including straight channel (SC), blocked channel (BC), wave channel (WC), and tapered channel (TC). The first, second, and third single channels are arranged in order from gas inlet in multi-channel. The results indicate that drainage rate of first channel in multi-channel is slower than that of second and third channel. Among the four designs, BC and TC reduced water ejection time by 7 ms and 6.5 ms respectively compared to SC, and reduced the time by 39 ms and 38.5 ms respectively compared to WC. In base case, drag reduction rate of TC compared to BC and WC increased by 4.12% and 20.98% respectively. However, WC has the smallest water coverage and average drag reduction ratio, while SC has a lower average gas flow resistance coefficient. This research offers innovative perspectives and theoretical foundations for current flow field design.

Effects of polarizability and charge transfer on water dynamics and the underlying activation energies.

A large number of force fields have been proposed for describing the behavior of liquid water within classical atomistic simulations, particularly molecular dynamics. In the past two decades, models that incorporate molecular polarizability and even charge transfer have become more prevalent, in attempts to develop more accurate descriptions. These are frequently parameterized to reproduce the measured thermodynamics, phase behavior, and structure of water. On the other hand, the dynamics of water is rarely considered in the construction of these models, despite its importance in their ultimate applications. In this paper, we explore the structure and dynamics of polarizable and charge-transfer water models, with a focus on timescales that directly or indirectly relate to hydrogen bond (H-bond) making and breaking. Moreover, we use the recently developed fluctuation theory for dynamics to determine the temperature dependence of these properties to shed light on the driving forces. This approach provides key insight into the timescale activation energies through a rigorous decomposition into contributions from the different interactions, including polarization and charge transfer. The results show that charge transfer effects have a negligible effect on the activation energies. Furthermore, the same tension between electrostatic and van der Waals interactions that is found in fixed-charge water models also governs the behavior of polarizable models. The models are found to involve significant energy-entropy compensation, pointing to the importance of developing water models that accurately describe the temperature dependence of water structure and dynamics.

Effect of Gas Diffusion Layer Notch Arrangement and Gradient Depth on the Performance of Proton Exchange Membrane Fuel Cells in the Serpentine Flow Field.

The structure of the gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) affects the transfer of the reaction gas and the water flooding phenomenon. First, the three-dimensional numerical model of the GDL was reconstructed by the stochastic reconstruction method, and the kinetic behaviors of liquid water in the conventional GDL, circular groove GDL, and elliptical groove GDL were compared and analyzed, based on which the liquid water penetration time, the effective diffusion rate of oxygen, and relative permeability of water in the three GDLs were analyzed, and the results of the study found that the circular groove GDL had the best drainage effect. Second, based on the PEMFC with serpentine channels, an electrochemical model of the GDL with circular grooves was arranged in the flow field, and the effects of the groove depth distribution and center spacing of circular grooves on its performance were numerically investigated. It was found that the presence of circular grooves enhanced the transfer rate of reactants from the GDL to the catalytic layer and increased the current density. Oxygen concentration uniformity and under-rib convection were enhanced when the groove depth was designed to have a power function distribution with an exponent close to 1, effectively reducing flooding. The best drainage effect was achieved when the groove spacing was 2 mm, and also the cell performance was more stable.

2D Numerical investigation of surface wettability induced liquid water flow on the surface of the NACA0012 airfoil

The accumulation of ice on wind turbine blades is a serious threat to the safety of wind power generation. Anti-icing techniques with low energy consumption, especially when combined with superhydrophobic surfaces, have recently attracted increasing levels of interest. In this work, the anti-icing mechanism of superhydrophobic surfaces was investigated by conducting the two-dimensional numerical analysis of the flow patterns of liquid water on surfaces with different wettability, which was based on the phase field method (PFM) and a dynamic contact angle (DCA) model. It is found that the wettability of surfaces significantly influences the flow, break, and shedding behaviors of liquid water on the surfaces. An increase in contact angle (CA) leads to more frequent break and shedding behaviors, and the flow pattern of liquid water gradually transformed from continuous water film to fragmented droplets because of the vortical flow field. In addition, it is also investigated that when the CA of surfaces exceeded 145°, the broken liquid water completely shed away in 0–0.1 chord length. Moreover, the average chordwise location (x‾/c) and maximum chordwise location (xmax/c) of the liquid water that had traveled the farthest on the surface in 30 ms gradually decreased with increasing contact angle, which is beneficial to superhydrophobic-dry anti-icing. What's important, the wettability characteristics should be responsible for the unique water flow patterns coupled with shear stress, which has been validated again, and previously proposed by our work. The results of this study provide theoretical support for the incorporation of superhydrophobic surfaces in anti-icing technology applied to wind turbine blades, which is promising for solving the icing issue of wind turbine blades with low energy consumption.

Effects of pin shapes on gas-liquid transport behaviors in PEMFC cathode

In this work, three pin-type flow field (FF) designs for a proton exchange membrane fuel cell (PEMFC) cathode with the circle, diamond, and square pins are proposed. The gas-liquid transport phenomena inside the PEMFC cathode are investigated to evaluate the water management performance of the designs. The volume of fluid model is used for the observance of the liquid water behaviors. A validated dynamic contact angle model is applied to improve the accuracy of the numerical simulation. From the results, it can be concluded that the pin shape affects greatly the emergence of liquid water from the catalyst layer, gas diffusion layer (GDL) to the FF. The study also offers insights into the airflow and liquid behaviors inside the pin-type FFs. Among the three FFs, the diamond pins remove water fastest with the lowest pressure drop. The square-pin FF also has low water mass in the computational domains but suffers from the highest pressure drop and uneven velocity, water, and pressure distributions. The circle pins have the most water content but can provide low pressure drop with the most uniform distributions. Thus, considerations for water removal ability should be taken for the pin-type FF designs regarding PEMFC performance and reliability.

Investigation of Liquid Water Behavior and Performance of PEFC Catalyst Layers Using a Microdevice and in-Operando Infrared Microscopy

Studies on water behavior in the vicinity of a catalyst layer (CL) and its effect on the cell performance are important challenges because water management is a key factor to achieve higher performance of polymer electrolyte fuel cells (PEFCs). In this study, a microdevice that contains a catalyst coated membrane (CCM) was developed (1) and in-operando infrared (IR) microscopy was conducted to visualize water behavior on the CL. Two types of CLs contain either carbon (2) or tin oxide (3) as supporting material to investigate the effects of materials on the water behavior and cell performance.The microdevice, which was made of a silicon wafer, contains two microchannels (100 μm-width, 200 μm-pitch) and a current-collecting layer (30 μm-width) on a rib of the cathode plate. The CCM was sandwiched by two plates. The anode channel faced the rib of the cathode. The surface of a cathode catalyst layer was coated with thin Au by sputtering to enhance IR reflectance, which did not change the porous structure of the CL significantly. The surface of the cathode catalyst layer was observed by IR microscopy through the cathode plate.Figure 1 shows liquid water generated during the constant current operation of the device. The catalyst layer contains carbon as the supporting material in this case. The effects of the materials and flow conditions on the water behavior and cell performance were investigated.AcknowledgmentsThis work was supported by NEDO ECCEED’30.References(1) T. Suzuki et al., J. Therm. Sci. Technol., 11 (2016), 16-00370.(2) T. Suzuki et al., Int. J. Hydrogen Energy, 41 (2016), 20326.(3) Y. Chino et al., J. Electrochem. Soc., 162 (2015), F736. Figure 1

3D Visualization of Liquid Water in Catalyst Layer Using X-Ray CT

To clarify the behavior of liquid water in polymer electrolyte fuel cell (PEFC), X-ray computed tomography (XTM) is used. In the past, 3D visualization experiments of liquid water in substrate layer and microporous layer (MPL) were conducted. However, 3D visualization of liquid water in catalyst layer (CL) has not been conducted yet. It is important to clarify liquid water in CL because water is generated in CL. We improved two things. First, we used the CL with low platinum percentage because its X-ray absorption coefficient is low. Second, 20.5-hour CT scan was conducted to improve spatial resolution. Therefore, we succeeded the detection of liquid water in CL.

Liquid-liquid criticality in the WAIL water model.

The hypothesis that the anomalous behavior of liquid water is related to the existence of a second critical point in deeply supercooled states has long been the subject of intense debate. Recent, sophisticated experiments designed to observe the transformation between the two subcritical liquids on nano- and microsecond time scales, along with demanding numerical simulations based on classical (rigid) models parameterized to reproduce thermodynamic properties of water, have provided support to this hypothesis. A stronger numerical proof requires demonstrating that the critical point, which occurs at temperatures and pressures far from those at which the models were optimized, is robust with respect to model parameterization, specifically with respect to incorporating additional physical effects. Here, we show that a liquid-liquid critical point can be rigorously located also in the WAIL model of water [Pinnick et al., J. Chem. Phys. 137, 014510 (2012)], a model parameterized using ab initio calculations only. The model incorporates two features not present in many previously studied water models: It is both flexible and polarizable, properties which can significantly influence the phase behavior of water. The observation of the critical point in a model in which the water-water interaction is estimated using only quantum ab initio calculations provides strong support to the viewpoint according to which the existence of two distinct liquids is a robust feature in the free energy landscape of supercooled water.

New notions on water and possibilities of application

In earlier times, some researchers who studied magnetic field effects related them to a psychic power influencing different systems.However, over the last decades it was realized that the alterations observed on these systems are due to the presence of water. Among the different processes that can alter the behavior of liquid water there are the action of magnetic fields and the successive dilution process of aqueous solutions. There are many evidences that the properties of magnetically treated water are different from the ones of untreated water. This fact is usually attributed to the weakness or breaking of intermolecular interactions (hydrogen-bond). It is observed that diluted solutions, prepared by dinamization process, absorbe UV light in a concentration range where the solute is no longer detected by spectrometry, which indicates a probable absorption by rearranged solvent after dinamization process.