Surface roughness dominated wettability of carbon fiber in gas diffusion layer materials revealed by molecular dynamics simulations

High water contact angle in carbon fiber can facilitate water removal ability of gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). Water contact angle is intensively dependent on the surface hydrophobicity of carbon fiber in GDL. In this study, the hydrophobicity of commercial GDL is enhanced through the immersion and hydrothermal methods. The porosity decreases slightly while the surface roughness and surface topology diversity increase significantly in hydrothermal GDL compared with commercial reference and immersion GDL samples. The molecular dynamics simulations show that the water contact angle increases significantly with the increasing surface roughness but varies slightly with different surface topology, indicating that the water contact angle is dominated by the surface roughness. This study's findings are expected to offer an approach that can effectively enhance the water removal capacity by tailoring the surface roughness of carbon fibers in GDL materials.     

Numerical simulation of two-phase flow in gas diffusion layer and gas channel of proton exchange membrane fuel cells

Liquid water within the cathode Gas Diffusion Layer (GDL) and Gas Channel (GC) of Proton Exchange Membrane Fuel Cells (PEMFCs) is strongly coupled to gas transport properties, thereby affecting the electrochemical conversion rates. In this study, the GDL and GC regions are utilized as the simulation domain, which differs from previous studies that only focused on any one of them. A Volume of Fluid (VOF) method is adopted to numerically investigate the two-phase flow (gas and liquid) behavior, e.g., water transport pattern evolution, water coverage ratio as well as local and total water saturation. To obtain GDL geometries, an in-house geometry-based method is developed for GDL reconstruction. Furthermore, to study the effect of GDL carbon fiber diameter, the same procedure is used to reconstruct three GDL structures by varying the carbon fiber diameter but keeping the porosity and geometric dimensions constant. The wall wettability is introduced with static contact angles at carbon fiber surfaces and channel walls. The results show that the GDL fiber microstructure has a significant impact on the two-phase flow patterns in the cathode field. Different stages of two-phase flow pattern evolution in both cathode domains are observed. The liquid water in the GDL experiences water invasion, spreading, and rising, followed by the droplet breakthrough in the GDL/GC interface. In the GC, the water droplets randomly experience accumulation, combination, attachment, and detachment. Due to the difference in surface wettability, the water coverage of the GDL/GC interface is smaller than that of the channel side and top walls. It is also found that the water saturation inside the GDL stabilizes after the water breakthrough, while local water saturation at the interface keeps irregular oscillations. Last but not the least, a water saturation balance requirement between the GDL and GC is observed. In terms of varying fiber diameter, a larger fiber diameter would result in less water saturation in the GDL but more water in the GC, in addition to faster water movement throughout the total domain.     

Study of the mechanical behavior of paper-type GDL in PEMFC based on microstructure morphology

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.     

Melting heat transfer characteristics of a horizontal ice cylinder immersed in quiescent saline water

Experiments were performed to determine the effect of salinity level on the melting heat transfer characteristics of a horizontal ice cylinder immersed in quiescent saline water. Emphasis was placed on interpreting the heat transfer mechanism which dominates the solid-liquid interface situation. Measurements were carried out for saline water of 0.5–3.5 wt% in salinity, while the ambient temperature ranged from 1.8 to 24.0°C. Flow visualization was employed to investigate the transient flow patterns and corresponding solid-liquid interface locations. It was found that the flow patterns around the ice cylinder were a strong function of the saline water concentration, which then considerably affected the local heat transfer coefficient along the melting ice cylinder.     

Ultra thin gas diffusion layer development for PEMFC

This research studies an ultra-thin carbon fiber paper fabrication process for proton exchange membrane fuel cells (PEMFCs). Polyacrylonitrile (PAN) based carbon fibers 6 mm long were dispersed and formed at aerial weights of 15 and 20 g/m² using a slurry molding machine. Polyscrylamide (PAM) and polyvinyl alcohol (PVA) dispersal agent solutions for fiber binding were added to evenly distribute the carbon fibers and increase the paper mechanical strength. The carbon fiber papers were dried after resin impregnation using a convective oven at 120 °C temperature for 10 min. The hot press machine was heated to 160 °C temperature and the workpieces were pressed for 5 min. Graphitization completed the gas diffusion substrate (GDS) process. GDL involves immersing the paper in a 5% polytetrafluoroethylene (PTFE) solution, coating the paper with a micro porous layer (MPL). This study shows the proposed ultra-thin GDL fabrication method is suitable for PEMFC applications and exhibits feasible functionality for fuel cells.     

Dynamic behavior of droplet transport on realistic gas diffusion layer with inertial effect via a unified lattice Boltzmann method

The dynamic behavior of liquid droplets on a reconstructed real gas diffusion layer (GDL) surface with the inertial effect produced by the three dimensional (3D) flow channel is investigated using an improved pseudopotential multiphase model within the unified lattice Boltzmann model (ULBM) framework, which can realize thermodynamic consistency and tunable surface tension. The microstructure of the GDL (Toray-090) including carbon fibers and polytetrafluoroethylene (PTFE) is reconstructed by a stochastic and mixed-wettability model. The critical force formulation for the Cassie-Wenzel transition of a droplet on GDL surface is derived. The effects of inertia and contact angles on the liquid droplet transport process on a reconstructed real GDL surface with a 3D flow channel are investigated. The results show the normalized center-of-mass coordinate X may enter the channel wall area or fluctuate around the initial position. With increased inertia applied on the droplet, the normalized center-of-mass coordinate Y grows faster and the normalized center-of-mass coordinate Z decreases. It is found by the ULBM for the first time that the liquid droplet is pushed back into the GDL by inertial effect. With the increase of inertia and the decrease of contact angle of GDL, both the droplet penetration depth in GDL and the droplet invasion fraction increase. The droplet invasion fraction in GDL is up to 30%.     

Effect of electrochemical aging on the interaction between gas diffusion layers and the flow field in a proton exchange membrane fuel cell

Gas diffusion layer (GDL) is a porous medium placed between the flow field and the catalyst layer in a proton exchange membrane fuel cell (PEMFC), and experiences electrochemical aging and mechanical stresses during usage. In the present work carbon cloth and carbon paper, two commonly used GDLs in PEMFC, are electrochemically aged in a simulated PEMFC environment. The results indicate that carbon paper is less prone to oxidation when compared to carbon cloth, which can be attributed to higher degree of graphitization of carbon fibers in the paper. However, carbon paper suffers greater loss of structural stability due to the adverse effects of aging on the fiber matrix interface. Increased weakening of paper when compared to cloth, after electrochemical aging, results in higher residual strain when subjected to cyclic compression and an increased intrusion of paper into the flow field channel when compared to cloth GDL.     

Super-cooled water behavior inside polymer electrolyte fuel cell cross-section below freezing temperature

This study investigated the phenomenon of water freezing below freezing point in polymer electrolyte fuel cells (PEFCs). To understand the details of water freezing phenomena inside a PEFC, a system capable of cross-sectional imaging inside the fuel cell with visible and infrared images was developed. Super-cooled water freezing phenomena were observed under different gas purge conditions. The present test confirmed that super-cooled water was generated on the gas diffusion layer (GDL) surface and that water freezing occurs at the interface between the GDL and MEA (membrane electrode assembly) at the moment cell performance deteriorates under conditions when remaining water was dry enough inside the fuel cell before cold starting. Moreover, using infrared radiation imaging, it was clarified that heat of solidification spreads at the GDL/MEA interface at the moment cell performance drops. Compared with a no-initial purge condition, liquid water generation was not confirmed to cause ice growth at the GDL/MEA interface after cell performance deterioration. Each condition indicated that ice formation at the GDL/MEA interface causes cell performance deterioration. Therefore, it is believed that ice formation between the GDL/MEA interface causes air gas stoppage and that this blockage leads to a drop in cell performance.     

Effect of deformation on electrical properties of carbon fibers used in gas diffusion layer of proton exchange membrane fuel cells

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials.     

Melting of a vertical ice cylinder inside a rotating cylindrical cavity filled with binary aqueous solution

The melting of a vertical ice cylinder into a homogeneous calcium chloride aqueous solution inside a rotating cylindrical cavity with several rotating speeds is considered experimentally. The melting mass and temperature are measured on four initial conditions of the solution and four rotating speeds of the cavity. The temperature of the liquid layer becomes uniform by the mixing effect resulting from cavity rotation and it enhances the melting rate of the ice cylinder. As the cavity‐rotating speed increases, the melting rate increases. The dimensionless melting mass is related to the Fourier number and the rotating Reynolds number in each initial condition, therefore an experimental equation that is able to quantitatively calculate the dimensionless melting mass is presented. It is seen that the melting Nusselt numbers increase again in the middle of the melting process. The ice cylinder continues to melt in spite of the small temperature difference between the ice cylinder and the solution. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(6): 359–373, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20211     

Effect of CaO mineral change on coal ash melting characteristics

The change of mineral composition in ash and the effect of CaO on the melting characteristics of coal ash were studied by adding different contents of CaO to coal ash. At the same time, the Factsage thermodynamics software was used to simulate the mineral changes in the synthetic ash to support and verify the experimental results. The results show that with the increase of CaO content, the melting characteristic temperature of coal ash first decreases and then rises. After adding a certain amount of CaO, the quartz with higher melting point completely disappears, and the melting point of the coal ash reaches a minimum value. And with the increase of CaO content, the appearance of wollastonite and single crystal calcium oxide mineral makes the melting point of coal ash gradually increase. It can be seen from the phase diagram calculated by Factsage that as the CaO content increases, the corresponding position of the coal ash gradually moves from the hematite region to the calcareous region. And the phenomenon of low temperature eutectic occurs when the CaO content is 35%, which is consistent with the trend of temperature change of the melting characteristics. These phenomena all indicate that the change in the melting point of coal ash is nonlinear as the content of CaO minerals increases.     

Visualization of the solid-liquid interface morphology formed by natural convection during melting of a solid from below

Experiments have been performed to obtain quantitative solid-liquid interface morphology data for melting of n-octadecane from below. Flow visualization experiments have revealed that natural convection flow was established as soon as the critical Rayleigh number was exceeded for the melt layer. For a given bottom temperature, the Benard-like, three-dimensional convection cells increased in size, merged with the neighboring ones and formed nearly two-dimensional rolls as the melting progressed. The mean diameter increased and the number of cells per unit projected area decreased with the melting time. The effect of initial subcooling of the solid was only to delay the development of the solid-liquid interface morphology.     

In-plane gas permeability of proton exchange membrane fuel cell gas diffusion layers

A new analytical approach is proposed for evaluating the in-plane permeability of gas diffusion layers (GDLs) of proton exchange membrane fuel cells. In this approach, the microstructure of carbon papers is modeled as a combination of equally-sized, equally-spaced fibers parallel and perpendicular to the flow direction. The permeability of the carbon paper is then estimated by a blend of the permeability of the two groups. Several blending techniques are investigated to find an optimum blend through comparisons with experimental data for GDLs. The proposed model captures the trends of experimental data over the entire range of GDL porosity. In addition, a compact relationship is reported that predicts the in-plane permeability of GDL as a function of porosity and the fiber diameter. A blending technique is also successfully adopted to report a closed-form relationship for in-plane permeability of three-directional fibrous materials.     

Effect of wettability heterogeneity and compression on liquid water transport in gas diffusion layer coated with microporous layer of PEMFC

This study applied the pseudo-potential Lattice Boltzmann method (LBM) for investigating liquid water transport in the microporous layer (MPL) and gas diffusion layer (GDL) of polymer electrolyte fuel cell. The MPL and GDL reconstruction is performed by using a stochastic method. Unlike previous studies that examined the GDL as two distinct layers of hydrophilic and hydrophobic, this study considered the wettability heterogeneity. In the present study, some of the carbon fibers in the GDL are randomly considered hydrophilic. Moreover, liquid water transport in the compressed and uncompressed GDL with different hydrophilic fibers percentage are compared. The effect of hydrophilic fibers percentage and the compression ratio of the GDL on the liquid water saturation level, the steady-state time, and the formation and growth of droplets in the gas channel (GC) are investigated. The results indicated that more than 10% of hydrophilicity of the fibers lead to the formation of discontinuous water clusters. This phenomenon increased the steady-state time and water saturation level significantly. The simulation showed that compression increased the number of discontinuous water clusters in the GDL. The obtained results demonstrated that the hydrophilic fibers may have positive or negative effects on water transport in the GDL due to their location. In addition, this study indicated that for 10% of hydrophilic fibers with 10% compression, water saturation level and time required to reach steady-state decreased by 5.2% and 22% respectively compared to purely hydrophobic GDL.     

Effects of gas diffusion layer structure on the open circuit voltage and hydrogen crossover of polymer electrolyte membrane fuel cells

The clamping pressure of polymer electrolyte membrane fuel cells for vehicle applications should be typically high enough to minimize contact resistance. However, an excessive compression pressure may cause a durability problem. In this study, the effects of gas diffusion layer (GDL) structure on the open circuit voltage (OCV) and hydrogen crossover have been closely examined. Results show that the performances of fuel cells with GDL-1 (a carbon fiber felt substrate with MPL having rough surface) and GDL-3 (a carbon fiber paper substrate with MPL having smooth surface) are higher than that with GDL-2 (a carbon fiber felt substrate with MPL having smooth surface) under low clamping torque conditions, whereas when clamping torque is high, the GDL-1 sample shows the largest decrease in cell performance. Hydrogen crossover for all GDL samples increases with the increase of clamping torque, especially the degree of increase of GDL-1 is much greater than that of the other two GDL samples. The OCV reduction of GDL-1 is much greater than that of GDL-2 and GDL-3. It is concluded that the GDL-3 is better than the other two GDLs in terms of fuel cell durability, because the GDL-3 shows the minimum OCV reduction.     

Analytic determination of the effective thermal conductivity of PEM fuel cell gas diffusion layers

Accurate information on the temperature field and associated heat transfer rates are particularly important in devising appropriate heat and water management strategies in proton exchange membrane (PEM) fuel cells. An important parameter in fuel cell performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL). Estimation of the effective thermal conductivity is complicated because of the random nature of the GDL micro structure. In the present study, a compact analytical model for evaluating the effective thermal conductivity of fibrous GDLs is developed. The model accounts for conduction in both the solid fibrous matrix and in the gas phase; the spreading resistance associated with the contact area between overlapping fibers; gas rarefaction effects in microgaps; and salient geometric and mechanical features including fiber orientation and compressive forces due to cell/stack clamping. The model predictions are in good agreement with existing experimental data over a wide range of porosities. Parametric studies are performed using the proposed model to investigate the effect of bipolar plate pressure, aspect ratio, fiber diameter, fiber angle, and operating temperature.     

BOUNDARY ELEMENT SOLUTIONS FOR FREE BOUNDARY CONVECTION-DIFFUSION PROBLEMS

The boundary element technique is used to solve the steady state convection-diffusion problem with constant velocity in a two-dimensional domain with a free interface. These problems arise in a number of important heat transfer applications involving melting or solidification, such as bulk crystal growth in Bridgman furnaces. The boundary element approach reduces the dimension of the problem, thereby improving the computational efficiency, and is particularly well suited to free-surface problems in which the position and shape of the solid-liquid interface are of primary importance. Results are presented for a case study problem representing solidification in a two-dimensional, rectangular configuration.     

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.     

阳极焙烧温度场的数值模拟

以FLUENT 6.3为计算平台,建立了阳极焙烧温度场的计算模型,采用数值模拟的方法研究了不同焙烧曲线对阳极升温过程、终焙温度和内外温差的影响.结果表明:数值模拟结果与实测值极为接近,所建模型具有较高的可信度.在其它参数一定的条件下,随火焰周期的延长,阳极表面和中心的升温速率变小,终焙温度升高且出现的位置后移,阳极内外温差缩小,阳极内部温度均匀性得到改善；随火焰周期的变短,阳极在各焙烧阶段的平均升温速率增大,保温时间减少,操作参数的控制难度增大.     

Performance and liquid water distribution in PEFCs with different anisotropic fiber directions of the GDL

To maintain the efficiency of proton exchange membrane fuel cells (PEFCs) without flooding, it is necessary to control the liquid water transport in the gas diffusion layer (GDL). This experimental study investigates the effects of the GDL fiber direction on the cell performance using an anisotropic GDL. The results of the experiments show that the efficiency of the cell is better when the fiber direction is perpendicular to the channel direction, and that the cells with perpendicular fibers are more tolerant to flooding than cells with fibers parallel to the channel direction. To determine the mechanism of the fiber direction effects, the liquid water behavior in the channels was observed through a glass window on the cathode side. The observations substantiate that the liquid water produced under the ribs is removed more smoothly with the perpendicular fiber direction. Additionally, the water inside the GDL was frozen to observe its distribution using a specially made cell broken into two pieces. The photographic results show that the amount of water under the ribs is larger than that under the channels using the parallel fiber direction GDL while the water distributions in these two places are almost equal level with the perpendicular fiber direction GDL. This freezing method confirmed the better liquid water removal ability and better reactant gas transportation in the GDL with the fiber direction perpendicular to the channel direction.