Analysis of transient response of a unit proton-exchange membrane fuel cell with a degraded gas diffusion layer

The transient response characteristics and durability problems of proton-exchange membrane fuel cells are important issues for the application of PEM fuel cells to automotive systems. The gas diffusion layer is the key component of the fuel cell because it directly influences the mass transport mechanism. In this study, the effects of GDL degradation on the transient response of the PEM fuel cell are systematically studied using transient response analysis under different stoichiometric ratios and humidity conditions. With GDLs aged by the accelerated stress test, the effects of hydrophobicity and structural changes due to carbon loss in the GDL on the transient response of PEM fuel cells are determined. The cell voltage is measured according to the sudden current density change. The degraded GDLs that had uneven hydrophobicity distributions cause local water flooding inside the GDL and induce lower and unstable voltage responses after load changes.     

The onset of transient convection in bottom heated porous media

The theory of transient convection in bottom heated porous media under constant heat flux (CHF) condition or fixed surface temperature (FST) condition is advanced and verified by computational fluid dynamics (CFD) simulations. The use of κ∗, instead of κ_m tends to artificially inflate the value of Rayleigh number by about 30%. A new transient Rayleigh number for unsteady-state heat conduction was defined to predict the onset of transient convection in porous media, which were successfully simulated. The critical transient Rayleigh number from the simulation for CHF was about 29.60, which is close to the theoretical value of 27.1 calculated by Ribando and Torrance in 1976. In the case of FST, the critical transient Ra_c was found to be 30.9, which is close to the theoretical value of 32.3. The critical times of onset for simulations were predicted with good accuracy. The prediction of the critical wavelengths of the emerging plumes were fair for the 2D simulations. Any experiment to verify the linear stability analysis for thermal instability must simultaneously concur in the three eigenvalue parameters, namely the Biot number, the critical wavenumber and the corresponding critical Rayleigh number, apart from the physical boundaries. The average maximum transient Nusselt number was found to be 3.41 for CHF and 3.5 for FST respectively.     

Rectified mass diffusion of gas bubbles in liquids under acoustic field with dual frequencies

In this letter, rectified mass diffusion of gas bubbles in liquids under acoustic field with dual frequencies is theoretically investigated. Comparing with gas bubbles under single-frequency acoustic field, if the acoustic pressure amplitude is above a certain value determined in the present work, a wider range of bubbles can grow through rectified mass diffusion with more rapid growth rate under dual-frequency acoustic field.     

Tomographic analysis of porosity variation in gas diffusion layer under freeze-thaw cycles

Synchrotron X-ray micro-computed tomography (X-ray μCT) is employed to measure the volume variation of gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). In the present study, 3D structures are reconstructed by merging orthogonal-plane images. Using the 3D reconstruction, the variation of structural parameters such as the porosity in GDL is investigated under freeze-thaw cycles. The freez-thaw cycles are established using cryo system and light source, respectively. As a result, a structural transformation is observed at the interface between GDL and micro porous layer (MPL). In addition, the porosity is critically changed with irreversible transition under freeze-thaw cycles.     

Imaging of desaturation of the frozen gas diffusion layers by synchrotron X-ray radiography

The visualization of the thawing and desaturation process on an initially saturated, frozen gas diffusion layer (GDL) with a serpentine gas flow channel was performed based on synchrotron X-ray computed tomography images. High speed CT scanning during the experiments allowed the dynamic desaturation process to be quantified under the cold-start with air purging condition. The saturation profiles and the desaturation rates were studied over the entire GDL domain, through-plane, and in selected regions of interest for localized behavior. Sigracet 35AA and 35BA GDLs were selected for the experiments to study the effects of GDL hydrophobicity. Along with the real-time saturation profiles, the average desaturation rates for the entire GDL domain over the whole purging process were 0.000186 μL cm^?2 s^?1, 0.000470 μL cm^?2 s^?1, 0.000516 μL cm^?2 s^?1 and 0.000901 μL cm^?2 s^?1 with the superficial gas velocity of the purging air at 2.88 m/s, 4.26 m/s, 5.98 m/s and 9.02 m/s, respectively. In addition, the dynamic saturation contours and 3-D GDL geometry models were constructed to show the liquid water movement through a GDL. Although the GDL desaturation curves for each experiment share similar trends, the results show that different conditions including air flow rate, GDL geometric location, initial water saturation, and GDL boundary condition could cause heterogeneous desaturation behavior on both overall and localized GDL regions. These data provide valuable information for future modeling studies that involve the thawing process in the GDL, and could be used to optimize the cell design and develop cold-start protocols.     

Estimation of flooding in PEMFC gas diffusion layer by differential pressure measurement

The flooding, especially in gas diffusion layer (GDL), is one of the critical issues to put PEMFC to practical use. However, the experimental data of the flooding in GDL is so insufficient that the optimization design related to the water management for GDL has not established. In this study we developed a method to estimate the water saturation, namely the ratio of liquid water to pore volume in GDL. We fabricated a simple interdigitated cell where the supply gas is enforced to flow under rib. This structure enables to estimate the liquid water ratio in GDL by the measurement of differential pressure through the cell. We operated the cell and measured the differential pressure, and succeeded in estimating the water saturation, which changed largely with changing cell operation condition. In addition to this deferential pressure measurement, we measured the ionic resistance in polymer electrolyte membrane by ac impedance method. We evaluated and discussed the influence of the water saturation on cell voltage.     

Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

Although characteristics of the gas diffusion layer (GDL) affect the performance of a proton exchange membrane fuel cell (PEMFC), mass transfer mechanisms inside the GDL and the performance of the PEMFC have not been directly correlated. To determine the design parameters of the GDL, the effects of substrate design of the GDL on performance of a PEMFC are investigated. By adding an active carbon fiber (ACF), which has a high surface area, the substrate is designed to have a different pore size structure. The results show that steady-state and transient responses are determined by capillary pressure gradient characteristics of the GDL made by pore size distribution of the substrate. The small macro-pore functions as water-retaining passage and the large macro-pore functions as water-removal passage. It is concluded that both small and large macro-pore must be present on the substrate to facilitate its function in a wide range of operating conditions.     

The effects of the gravity on transient responses and cathode flooding in a proton exchange membrane fuel cell

The work presented in this paper includes contributions that provide insight into liquid water transport in the proton exchange membrane fuel cell (PEMFC).     

Effects of porosity gradient in gas diffusion layers on performance of proton exchange membrane fuel cells

A three-dimensional, two-phase, non-isothermal model has been developed to explore the interaction between heat and water transport in proton exchange membrane fuel cells (PEMFCs). Water condensate produced from the electrochemical reaction may accumulate in the open pores of the gas diffusion layer (GDL) and retard the oxygen transport to the catalyst sites. This study predicts the enhancement of the water transport for linear porosity gradient in the cathode GDL of a PEMFC. An optimal porosity distribution was found based on a parametric study. Results show that a optimal linear porosity gradient with ?₁ = 0.7 and ?₂ = 0.3 for the parallel and z-serpentine channel design leads to a maximum increase in the limiting current density from 10,696 Am⁻² to 13,136 Am⁻² and 14,053 Am⁻² to 16,616 Am⁻² at 0.49 V, respectively. On the other hand, the oxygen usage also increases from 36% to 46% for the parallel channel design and from 55% to 67% for the z-serpentine channel design. The formation of a porosity gradient in the GDL enhances the capillary diffusivity, increases the electrical conductivity, and hence, benefits the oxygen transport throughout the GDL. The present study provides a theoretical support for existing reports that a GDL with a gradient porosity improves cell performance.     

Numerical investigations on liquid water removal from the porous gas diffusion layer by reactant flow

Water removal from the gas diffusion layer (GDL) is crucial for the efficient operation of proton exchange membrane (PEM) fuel cell. Static pressure gradient caused by the fast reactant flow in the flow channel is one of the main mechanisms of water removal from GDL. Reactant can leak or cross directly to the neighboring channel via the porous GDL in the cells with serpentine flow channel and many of its modifications. Such cross flow plays an important role for the removal of liquid water accumulated in the GDL especially under land area. To investigate the characteristics of liquid water behavior in the GDL under pressure gradient, the fibrous porous structure of the carbon paper is modeled by three dimensional impermeable cylinders randomly distributed in the in-plane directions and unsteady two-phase simulations are conducted. It is shown that the permeability from the numerical model matches well the experimental measurements of the common GDLs in the literature. The contact angle and pressure gradient are the key parameters that determine the initiation and the process of liquid water transport in the GDL which is initially wet with stagnant liquid water. It has been observed that the larger contact angle results in faster water removal from the GDL. Numerical simulations are performed for a wide range of pressure gradient with different contact angles to determine the minimum pressure gradient that initiates the liquid water transport in the GDL. It is found that the amount of pressure gradient caused by the cross flow is sufficient and effective to get rid of the liquid water accumulated in the GDL. The simulation results are also compared with experimental data in literature showing a good agreement. The characteristics of liquid water discharging from the gas diffusion layer are also described.     

Metal foams as a gas diffusion layer in direct borohydride fuel cells

Renewable energy sources have become an important issue due to global warming and the diminishing of fossil fuel resources that have become a major problem for the world. Fuel cells are one of the most important renewable energy systems. The use of metal foams within the scope of commercialization and cost reduction of fuel cells attracts attention today. Metal foams are a material that provides excellent performance in many engineering applications, especially in fuel cells, thanks to the high permeability provided by the porous structure, narrow flow channels, large specific surface area, capillary, and diffusive forces. Among the metallic foams, the focus is on the use of aluminum foam, due to its low cost, superior strength, and thermal conductivity performance properties as well as ease of production. As an alternative to gas diffusion layers, the use of different metallic foam materials in fuel cell systems and performance improvements were analyzed. The use of aluminum foams as flow distributors and electrodes in the anode diffusion layer, which is one of the DBHYP components, provides significant increases in cell performance, cell weight, and volume. It has been found that open-cell metallic foams used in fuel cell systems cause an increase in performance due to their positive effects on the amount of catalyst coating, flow characteristics, and electrical properties.     

The influence of baffle geometry on mass transfer and performance of proton exchange membrane fuel cells considering the compression of gas diffusion layer

In this paper, the effects of baffle geometric parameters on fuel cell performance and mass transfer are studied considering the geometric deformation of the gas diffusion layer and the changes in porosity and permeability caused by the assembly force of the stack. The parameters include the windward part length, the middle section length, the leeward part length, the height and the number. We find that changing the windward or leeward part length of baffles will not impact the net power density significantly; short leeward part will produce bimodal streamlines and relatively more power; the baffle height close to blockage is undesirable. In addition, we observe that the trapezoidal baffles can not improve the net power density and the distribution uniformity of the physical quantities inside the fuel cell at the same time.     

Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells

The gas diffusion layer (GDL) is important for maintaining the performance of polymer electrolyte membrane (PEM) fuel cells, as its main function is to provide the cells with a path for fuel and water. In this study, the mechanical degradation process of the GDL was investigated using a leaching test to observe the effect of water dissolution. The amount of GDL degradation was measured using various methods, such as static contact angle measurements and scanning electron microscopy. After 2000 h of testing, the GDL showed structural damage and a loss of hydrophobicity. The carbon-paper-type GDL showed weaker characteristics than the carbon-felt-type GDL after dissolution because of the structural differences, and the fuel cell performance of the leached GDL showed a greater voltage drop than that of the fresh GDL. Contrary to what is generally believed, the hydrophobicity loss of GDL was not caused by the decomposition of polytetrafluoroethylene (PTFE).     

Facilitating mass transport in gas diffusion layer of PEMFC by fabricating micro-porous layer with dry layer preparation

For a proton exchange membrane fuel cell (PEMFC), dry layer preparation was optimized and applied to fabricate a micro-porous layer (MPL) for a gas diffusion layer (GDL). The MPLs fabricated by dry layer preparation and the conventional wet layer preparation were compared by physical and electrochemical methods. The PEMFC using dry layer MPLs showed better performance than that using wet layer MPLs, especially when the cells were operated under conditions of high oxygen utilization rate and high humidification temperature of air. The mass transport properties of the GDLs with the dry layer MPLs were also better than with the wet layer MPLs, and were found to be related to the pore size distribution in GDLs. The differences in surface morphology and pore size distribution for the GDLs with the dry layer and wet layer MPLs were investigated and analyzed. The dry layer preparation for MPLs was found to be more beneficial for forming meso-pores (pore size in the range of 0.5–15 μm), which are important and advantageous for facilitating gas transport in the GDLs. Moreover, the GDLs with the dry layer MPLs exhibited better electronic conductivity and more stable hydrophobicity than those with the wet layer MPLs. The reproducibility of the dry layer preparation for MPLs was also satisfying.     

Comparing through-plane diffusibility correlations in PEFC gas diffusion layers using the lattice Boltzmann method

One of the key elements in a polymer electrolyte fuel cell (PEFC) is the gas diffusion layer (GDL). The GDL offers mechanical support to the cell and provides the medium for diffusing the reactant gases from the flow plates to the electrolyte enabling the electrochemical reactions, and therefore the energy conversion. At the same time, it has the task of transporting the electrons from the active sites, near to the electrolyte, towards the flow plates.Describing the fluid flow and mass transport phenomena through the GDLs is not an easy task not only because of their complex geometries, but also because of these phenomena occur at microscale levels. Most of the PEFC models at cell scale make assumptions about certain microscale transport parameters, assumptions that can make a model less close to the reality. The purpose of this study is to analyze five different proposed correlations to estimate the through-plane (TP) diffusibility of digitally created GDLs and using lattice Boltzmann (LB) models. The correlations are ranked depending on their precision, accuracy and symmetry. The results show that the best estimation is given when the porosity and gas-phase tortuosity are taken into account in the correlation.     

Effect of the porous carbon layer in the cathode gas diffusion media on direct methanol fuel cell performances

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm⁻²) at 0.4 V and 70 °C than commercial GDL (SIGRACET^® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm⁻²). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm³ cm⁻² s⁻¹, while the one for SGL 35BC GDLs was 21.7 cm³ cm⁻² s⁻¹. Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.     

A review of gas diffusion layer in PEM fuel cells: Materials and designs

The gas diffusion layer (GDL) plays a key role on reactant gas diffusion and water management in proton exchange membrane (PEM) fuel cells. This paper reviews recent developments of single- and dual-layer GDLs for PEM fuel cells and various materials and approaches used for development of novel GDL. A variety of carbon- and metal-based macroporous substrates are presented. Hydrophobic treatments using different fluorinated polymers are addressed. Engineering parameters which control the performance of microporous layer such as carbon treatment, wettability, thickness, and microstructure are also reviewed. In addition, future prospects for development of new GDL development are discussed.     

Experimental study on carbon corrosion of the gas diffusion layer in polymer electrolyte membrane fuel cells

Generally, the GDL of a PEM fuel cell experiences three external attacks: dissolution of water, erosion of gas flow, and corrosion of electric potential. Of these degradation factors, this study focuses on the carbon corrosion of electric potential and investigates its impact through the accelerated carbon corrosion test. This study confirms that carbon corrosion occurs at the GDL, which decreases the operating fuel cell’s performance. To discover the effects of carbon corrosion, the GDL property changes are measured through various devices, including a scanning electron microscopy, a thermo gravimetric analyzer, and a tensile stress test. Carbon corrosion causes not only loss of weight and thickness but also degradation of mechanical strength in the GDL. In addition, the GDL shows serious damage in its center.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.