Analysis and design of flow fields for proton exchange membrane fuel cell (PEMFC) require coupled solution of the flow fields, gas transport and electrochemical reaction kinetics in the anode and the cathode. Computational cost prohibits the widespread use of three-dimensional models of the anode and cathode flow fields, gas diffusion layers (GDL), catalyst layers (CL) and the membrane for fluid flow and mass transport. On the other-hand, detailed cross-sectional two-dimensional models cannot resolve the effects of the anode and cathode flow field designs. Here, a two-dimensional in-plane model is developed for the resolution of the effects of anode and cathode flow channels and GDLs, catalyst layers are treated as thin-layers of reaction interfaces and the membrane is considered as a thin-layer that resist the transfer of species and the ionic current. Brinkman equations are used to model the in-plane flow distribution in the channels and the GDLs to account for the momentum transport in the channels and the porous GDLs. Fick’s law equations are used to model transport of gas species in the channels and GDLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by Butler-Volmer equations. Complete features of the flow in the channels and inlet and outlet manifolds are included in the model using resistance relationships in the through-plane direction. The model is applied to a small cell having an active area of 1.3 cm 2 and consisting of 8 parallel channels in the anode and a double serpentine in the cathode. Effects of the anode and cathode stoichiometric ratios on the cell performance and hydrogen utilization are investigated. Results demonstrate that for a sufficiently high cathode stoichiometric ratio enough, anode stoichiometric ratio can be lowered to unity to obtain very high hydrogen utilization and output power.

One of the most important objectives of the GDL in a PEM fuel cell is the transport of reactant gases from the gas flow channels to the reaction sites at the catalyst layer. Most state-of-the-art GDLs are composed of a carbon fiber paper coated with a microporous layer composed of carbon nano-particles. This bi-layered GDL structure has been proven to provide significant improvement to the performance of PEM fuel cells. In order to improve our understanding of reactant transport through these GDL materials, it is important for us to characterize the structure of these materials. In this study, we use X-ray Computed Tomography (X-CT) to study the structure of the bi-layered GDL at the microscale. This work presents a unique segmentation routine developed in-house to identify the distinct components of the bi-layer GDL, isolating the carbon fiber, the microporous layer and the void regions as individual phases. Two commercially available GDL samples, SGL 35BA and SGL 35BC are segmented with this novel algorithm to obtain unique porosity profiles. The MPL is identified separately in SGL 35BC along with the fibrous substrate region. It is observed that in the case of this sample, there is no region where only the MPL is present. The entire thickness of the MPL region is within the substrate region with fibers present throughout the MPL region. The substrate region is 300 μm thick while the MPL is present up to 200 μm from one side.

High utilization of hydrogen is desired in operation of PEM fuel cells. Typically, additional devices for hydrogen recovery at the anode exit are necessary. Alternatively, dead-ended anode (DEA) operation may be considered, however this mode causes severe voltage transients and loss of catalyst support in hydrogen-depleted regions of the active area. Here. ultra-low stoichiometric (ULS) flow conditions that deliver very high hydrogen utilization is considered, however uniform flow distribution is necessary in this case. In order to obtain the flow distribution in the channels and in the inlet and exhaust headers in the anode, a three-dimensional CFD model is developed based on the finite-element method to solve Stokes equations subject to no-slip boundary conditions on the walls, specified pressure at the inlet and specified flow rate at the exit, which is set to a very low value comparable to the typical rate of nitrogen accumulation in the anode side in order to simulate the ULS flow. Uniformity of the flow distribution between the channels is quantified by means of two performance metrics: (i) the root-mean-square (rms) of the channel average velocities; (ii) the ratio of maximum and minimum values of the channel averages. Effects of geometric parameters, such as the widths of the channels and ribs and the position and lengths of the baffles in the inlet and exhaust headers are studied. The final design has less than 5% rms, and less than 1.2 for maximum to minimum average channel velocity ratio.

In order to prevent membrane dehydration and flooding in proton exchange membrane fuel cells (PEMFC), appropriate water management must be done. Therefore, accurate knowledge of the effects of various parameters on cell water content is needed. This knowledge helps to achieve optimum cell performance. The effects of both anode and cathode stoichiometry and relative humidity on single serpentine PEMFC performance and flow channel water content are surveyed experimentally. All tests conducted on transparent fuel cell with serpentine flow field channels and 25cm 2 MEA active areas. Digital image processing is performed on the captured images of cathode channel to detect accumulated water at that side. The results indicate that there is a correlation between water content of cathode flow channels and cell performance. Increasing cathode water content causes membrane hydration which leads to better cell performance, but high water content causes flooding. It is concluded that both stoichiometry and relative humidity have significant effect on cell performance. Increasing reactant relative humidity improves water transport mechanisms within the cell, but fully humidified reactants may causes flooding. Although high stoichiometry rate prevents flooding and facilitate reactants transport within catalyst layer, high anode and cathode stoichiometry leads to membrane dehydration. Experimental observation shows that both stoichiometry and relative humidity have optimum value.

In this article we presented the recent activities in the field of gas flow in the micro channel and porous media of a polymer electrolyte fuel cell (PEFC). The gas flow behavior in the micro-channel, especially in the case of serpentine channel is very complex due to the appearance of cross flow through the gas diffusion layer (GDL). The gas flow behavior in the separator channel and GDL of a PEFC has been studied by using a transient, isothermal and three dimensional numerical models. To predict gas flow phenomena accurately the precise calculation of mass conversation is necessary which is strictly maintained in our present simulation. The effects of physical characteristics and geometrical properties have been investigated to quantify the amount of cross flow and pressure loss. The cross flow has been investigated in terms of volume mass flux through the GDL under the rib. The ratio cross flow rate to the total flow rate increases when gas channel pitch length decreased. Moreover, with increasing of permeability this ratio also increases. The effect of cross flow and bend region characteristics on the pressure loss has been identified. In addition, to isolate the contribution of cross flow on the performance of fuel cell, the simulation was carried out with electrochemical reaction using parallel straight channel. We designed a parallel flow field to induce artificial cross flow through the GDL. The numerical results show that the flow cross-over through the GDL under the rib significantly facilitate the oxygen transport towards the catalyst layer. Therefore, it is possible to overcome the oxygen transport limitation. Consequently, the cross flow can increase the current density by reducing the oxygen transport limitation, although this also increases the non-uniformity in current density.

This paper examines the role of micro porous layers (MPLs) in Polymer Electrode Fuel Cells (PEFCs) by observing the cross-sectional distribution of condensed water inside a cathode side MPL In addition, the forms of water condensation in the vicinity of a MPL are also compared between two places, under flow channels and under lands, by observing both inside the MPL and an interface between the MPL and a catalyst layer (CL). The freezing method and a cryo-scanning electronic microscope (cryo-SEM) are used for the observation. The result under the non-flooded condition shows that condensed water does not accumulate inside the MPL. This result indicates that the water produced by PEFC power generation passes through the MPL as vapor state under non-flooded conditions.

In a polymer electrolyte fuel cell (PEFC), the catalyst degradation on cathodic side is one of the fatal problems caused by mal-distributed hydrogen supply into each channel on active area in a fuel cell, especially in a fuel cell stack for automotive fuel cell systems which consist of several hundreds of fuel cells stacked. For example, before getting the fuel cell system started-up, the gas in all the anodic flow passage including channels in each fuel cell is occupied by air instead of hydrogen due to cross leak from cathodic side to anodic side through the membrane employed as an electrolyte. In this situation, if hydrogen is supplied partially or unevenly between cells to start up the system, a concentration interface of air and hydrogen will be made within a fuel cell. This causes a state of local cell within a single fuel cell and the catalyst degradation (carbon corrosion or Pt dissolution) occurs. In this paper, to avoid this catalyst degradation, the gas distribution is investigated with pressurized hydrogen supply into channels located on the hundreds stacked fuel cells statically filled with air initially. A transient computational fluid analysis was applied to the flow fields of anodic side which consist of channels on fuel cells, both distributing and collecting manifold connected to the fuel cells under parameters: 1) number of stacked fuel cells (i.e. manifold length), 2) the rate of pressure rising (Pa/sec.) which makes the gas flow velocity. A gas analysis experiment was also carried out for a validation with mass spectrometer taking gas sample from several points along the gas channels on alternative fuel cells which are made of transparent acrylic resin. The results show that the uniform distribution in concentration between cells and its profile within the channels along the flow direction are strongly affected by flow field formed within the distributing manifold located upstream of stacked plates with channels.

The analysis and control of transport phenomena in fluidic nanopores and nanochannels is important in applications such as biochemical analysis, power generation and environmental protection. A unique aspect of nanofluidics is that the relevant length scale is comparable to the range of various surface and interfacial forces in liquids (such as electrostatic, van der Waals and steric interactions). Thus, to obtain an adequate description of transport phenomena in nanospace, it is necessary to understand the discreteness of molecules, especially when the size decreases to 2 nm. Micelle-templated mesoporous silicas (MPSs) possess highly ordered structures such as 2D hexagonal and 3D cubic structures and pores within the 2–50 nm range. In particular, 2D hexagonal films that generally have pore channels parallel to the surface plane have been widely synthesized by using various types of template molecules. If the pore channels of such materials are aligned in a certain direction, these materials can be employed for various purposes such as the fabrication of oriented nanowires, optoelectronic devices, recording media, selective separations, and nanofluidic systems. 3D cubic structures give large surface areas and become good candidates for highly efficient catalysts and sensors. Advances in the synthesis, measurement and analysis of nanotubes and nanochannels have allowed ion and liquid transport to be routinely examined and controlled in spaces with dimensions that range from 10 to 100 nm. The ability to now explore transport and adsorption phenomena in confined spaces of around 2 nm offers a range of possibilities. We have investigated several unique transport and adsorption phenomena in mesopores measuring a few nanometers in diameter, including nonlinear I – V curves of ionic current passing through MPS thin films filled with aqueous solutions, humidity-dependent adsorption rate of water into MPS, and the reduction of melting and freezing temperature of water in MPS.

Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte fuel cells (PEFCs), but details of the mechanism that works to suppress water flooding has not been fully understood. In this study, the authors investigated water distribution at the interface between the MPL and the catalyst layer (CL) at the cathode side to clarify the effect of the MPL on the discharge of produced water. A freezing method was applied to observe the distribution of the condensed water, and the ice distribution on the CL surface was quantified by image processing. The effects of operating conditions on the water distribution were examined at normal temperature conditions. The distribution of ice formed on the surface of the CL with and without MPL after −10°C cold start operation was also established. Water transport rate in the vapor phase was analyzed based on temperature and vapor pressure gradient considerations. The experiments and the analysis showed that the MPL functions to prevent accumulation of water on the surface of the CL, resulting in less water flooding.

In Polymer electrolyte membrane fuel cells (PEMFCs), the generated water transfers from the catalyst layer to the gas channel through micro channels of different scales in a two phase flow. It is important to know details of the water transport phenomena to realize better cell performance, as the water causes flooding in the high current density conditions and start up problem under freezing temperatures. The keynote speech presents specifics of the ice formation characteristics in the catalyst layer and in the gas diffusion layer (GDL) with photos taken with an optical microscope and a CRYO-SEM. The observation results show that cold starts at −10°C results in ice formation at the interface between the catalyst layer and the micro porous layer (MPL) of GDL, at −20°C most of the ice is formed in the catalyst layer. Water transport phenomena through the micro porous layer and GDL are also a matter of interest, because the role of the MPL is not well understood from the water management angle. The speech will discuss the difference in the water distribution at the interface between the catalyst layer and the GDL arising from the presence of such a micro porous layer.

Lab-on-a-chip microchemical systems were fabricated from PDMS, polyimide (PI) film and functional polymers by different lithographic techniques to look for microreactor applications in the areas of organic syntheses. In detail, efficient gas-liquid reactions in dual-channel microreactor, continuous recovery and recirculation of catalyst-immobilized magnetic particles in microfluidic system, and a flexible film microreactor will be discussed.

The reduction of effective transfer length on microscale eliminates the external diffusion limitation on reaction rate and makes it possible to realize the non-equilibrium chemical reactions. The peculiarities of methane and carbon monoxide steam reforming in a minichannel reactor with activation of reactions on thin film catalyst prepared by nanotechnology are considered in this paper. Consistent accomplishment of these reactions can increase the hydrogen yield and reduce the concentration of carbon monoxide in the product. Steam reforming of methane was studied on Rh/Al 2 O 3 nanocatalyst deposited on the inner wall of the annular minichannel. Steam reforming of carbon monoxide was studied at Pt/CeO 2 /Al 2 O 3 nanocatalyst deposited on the walls of the minichannel plate. The procedure of catalyst preparation which makes the nanoparticles of two nanometers in size is developed. The catalyst has uniform fraction of nanoparticles and optimal oxygen mobility in the lattice of carrier. During tests the data on the composition of the reacting gas mixture in temperature range from 200 C to 940 C were obtained including data on conversion in controlled temperature field when hydrogen content in the product reaches 68% and carbon monoxide content reduces to 1%. Methane steam reforming and water gas shift reaction in the minichannel were modeled numerically. The detailed information on the temperature and species concentration fields has been obtained, and kinetics of multistage reactions was defined when the external heat is supplied to proceed the steam reforming. The temperature regimes of high conversion of methane and carbon monoxide were defined and discussed in connection with the experimental data.

The polymer electrolyte membrane (PEM) fuel cell is a zero emission power generation system that has long been considered as a replacement for conventional fossil fuel combustion systems. However, before constituting a viable market for commercial use, the fuel cell’s efficiency and reliability need to be improved significantly. It has been shown that water management has a significant effect on the power and reliability of the cell as the electrolyte membrane must be well hydrated to allow for ion transfer while excess water blocks the activation sites on the cathode side. The latter effect is known as flooding which occurs at large current densities and compromises the normal operation of the fuel cell. To enhance water management, a prodigious amount of studies have been conducted to optimize the properties and structures of different layers. One of the key results of these studies has been the design of a flow field pattern on the relatively hydrophobic surface of a graphite plate which is believed to provide a better mechanism for removing water droplets from the cathode flow channel. However, the wettability gradient between the catalyst layer (i.e., hydrophilic) and the flow channel (which is currently more hydrophobic) introduces problems as the water droplets formed at the catalyst layer will not likely detach, and hence create a film of liquid that will block the activation sites. If the flow channel is made out of a material that is more hydrophilic than the catalyst layer, water removal and transport will be enhanced as water naturally moves from low surface energy to high surface energy sites. However, recent numerical studies conducted on simulation of water transport in the channels show that removing the water film formed on the hydrophilic channels is limited due to the pressure of the gas flow in the channels. To resolve this problem, the use of compact aluminum foams in the flow channels is studied in this paper. It is shown that the hydrophilicity of the foam-filled flow channel helps the transport of the water droplets at the catalyst layer to the channel in which a liquid film is formed. This film is then removed due to the increased pressure developed in the porous media of the foam (as opposed to the regular open flow channel). The paper includes the experimental results obtained for the fuel cell performance using the new geometry with and without the gas diffusion layers (GDLs). These results will be compared to a similar flow channel that does not include the compressed aluminum porous structure. This work will result in finding the optimum geometry for achieving maximum performance in the flooding regime.

Porous transport layers are an integral part of polymer electrolyte fuel cells (PEMFC). In order to optimize the catalyst layer performance and reduce catalyst consumption, a thorough understanding of mass transport through porous media is necessary. Currently, there is a lack of experimental measurements of effective mass transport properties of porous transport layers. Further, mass transport theories in the literature, such as the binary friction model by Kerkhof [1], have not been extensively validated for porous media. In the present study, mass transport measurements have been performed on the porous media of a PEMFC, namely a GDL and an MPL. The experimental setup described by Pant et al. [2] has been used. The setup uses the diffusion bridge/counter-diffusion technique for the mass transport measurements. The experimental setup has the advantage that it can be used to perform studies for pure diffusion and convection-diffusion mass transport. The setup also facilitates measurement of permeability of porous media, which can then be used in convection-diffusion studies. Preliminary permeability measurements of GDL and MPL from the setup show good agreement with values available in literature. In preliminary experimentation, the conventional diffusivity correlations like Bruggeman equation have been found to overpredict the diffusivities.

Numerical modeling of methane-steam reforming is performed in a mini/microchannel with heat input through Nickel-deposited channel walls. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The surface reactions in the presence of Nickel catalyst are modeled as Neumann boundary conditions to the governing equations. Effects of the total heat input, heat flux profile, and inlet methane-steam molar concentration on production of hydrogen are investigated in detail.

Hydrogen purification through water-gas shift (WGS) is a favored option in fuel processing for hydrogen fuel cells. A three-dimensional single channel model is developed to simulate the behavior of a water-gas shift micro reactor. The flow regime is assumed to be steady and laminar; furthermore, it is presumed that the walls are isothermal. A water-gas shift reaction rate model is utilized to simulate the surface reaction on Pt/TiO 2 catalyst. The gas feed composition is taken as the efflux of a typical auto-thermal reforming (ATR) reactor. A parametric study is conducted to investigate the effect of gas feed temperature, gas space velocity and channel length on water-gas shift micro reactor performance. The study resulted in an optimum water-gas shift micro reactor design. It should be noted that a water-gas shift micro reactor is an essential part of a reactor train to remove carbon monoxide from a hydrogen rich mixture. Such a mixture can be used as fuel for a PEM fuel cell in portable devices. The results of these simulations revealed that the optimum reactor consists of a square cross section channel with 100 μm hydraulic diameter, 20 mm length, space velocity of 1000 h −1 and gas feed temperature of 270°C. The carbon monoxide mole fraction in the efflux is in a range suitable for a typical preferential oxidation (PROX) reactor.

Hydrogen production through autothermal reforming (ATR) of hydrocarbons, such as methane, is one option of interest for mobile applications of hydrogen fuel cells. In the present study, a numerical investigation of catalytic autothermal reforming of methane in a surface microreactor is presented. A three-dimensional ATR reactor model is developed to simulate the flow and surface reactions in a microchannel of rectangular cross section with 340-μm sides, and total length of 8.5 mm. A four-reaction mechanism is implemented to simulate the surface reactions on a Ni/Al 2 O 3 catalyst. The governing equations in the model include conservations of mass, momentum, energy and chemical species. A CFD code based on the finite-volume method has been developed in-house to solve the governing equations. Validation of the results against available data confirms the accuracy of the numerical approach. The simulation results reveal the dependency of hydrogen yield on space velocity (SV), air/fuel molar ratio (A/F), water/fuel molar ratio (W/F), and the gas feed temperature.

A pore-network model is developed to study numerically the transient flow of fluid through the gas diffusion layer (GDL) of the PEM fuel cell. It is shown that the agglomeration of water droplet on the interface of the GDL and catalyst layer occurs faster for the samples with smaller pore diameters and lower contents of the hydrophobic agent. The study suggests that analysis of the temporal response of the GDL is a useful tool to evaluate its performance against transporting liquids.

We introduce the successful fabrication of inorganic polymer derived microchannels with organic solvent resistance and optical transparency, via economic micro-molding process by using two types of source materials: commercial polyvinylsilazane (HTT1800 Kion Corp.), or allylhydropolycarbosilane (SMP-10, Starfire Co.). And we demonstrated the reliable microchemical performance in various organic solvents such as THF, DMF and acetonitrile at elevated temperatures. Knovenagel and Diels-Alder reactions were successfully run by pressured-driven flow in 2 cm and 16 cm long channel, respectively. It is proven that the developed inorganic polymer-based microchannels were obviously performed as a niche material-based microfluidic device between plastic and glass based device. In addition, we present the fabrication and characterization of ceramic microreactors composed of inverted beaded silicon carbide (SiC) monoliths with interconnected pores as catalyst supports, integrated within high-density alumina housings obtained via an optimized gel-casting procedure. These tailored macroporous SiC monoliths deposited Ru as a catalyst was run for the decomposition of ammonia with at temperatures between 450 and 1000 °C, which demonstrated a high temperature fuel cell reformer.

The PEMFC (Polymer Electrolyte Membrane Fuel Cell) requires well hydration for acceptable protonic conductivity, but liquid water in the catalyst layers and gas diffusion layers can cause performance loss due to blockage of reactants to the catalysts. Many activities have been done on the water management in PEMFC stack to guaranty better performance and its longevity. Some approaches for PEMFC stack in Hyundai-motor will be shown in this presentation based on analytic modeling, CFD, and experiment, then some challenges for better understanding of water behavior in PEMFC will be shown at the end of this paper.