In this study, liquid-gas two-phase flow pressure drops were measured in an ex-situ PEM fuel cell test section. Pressure drop signatures were studied for three nominal air flow rates and different water flow rates within a flow channel. The pressure drop signatures showed an increasing trend at the beginning of the experiments which were followed by a drop to lower values before reaching uniform patterns. It was observed that as the water flow rate increased, the time interval at which pressure signatures reached uniform patterns decreased. In addition, a qualitative comparison with Mishima-Hibiki model [13] revealed that this two-phase flow pressure drop model showed the best prediction capability for the medium air flow rate used in this study, ∼300mℓ/min inflow channel, corresponding to ∼220 Reynolds number.

Water management is a critical component of extracting optimum performance and efficiency from polymer electrolyte membrane (PEM) fuel cells. During fuel cell operation, a balance needs to be maintained between excess water blocking the reactant pathways through the gas diffusion layer, and the requirement for membrane hydration. The ionic conductivity through the membrane depends strongly on the hydration of the membrane. The reactant gases in a PEM fuel cell are supplied through a humidification system to maintain appropriate levels of hydration in the membrane. The removal of the anode humidifier would significantly reduce the balance of plant costs and reduce the volume required for the fuel cell in an automotive setting. However, removing the anode humidification system could have adverse effects on membrane hydration and on fuel cell performance. In this study, the anode humidification was varied and the cell performance and the membrane resistance were monitored. Synchrotron X-ray radiography was conducted simultaneously to visualize the water distribution in the membrane, the gas diffusion layer, and the associated interfaces. It was observed that the anode humidification had a strong impact on the performance of the fuel cell, with the dry condition leading to voltage instability at a current density below 1.0 A/cm 2 . The membrane water content was observed to decrease with increases in operating current density.

The free energy based lattice Boltzmann method (LBM) for two-phase flow with large density ratio is used to simulate droplet dynamics in the polymer electrolyte fuel cell (PEFC). The shape deformation of a static water droplet in the gas channel occurred in the simulations was eliminated. In this LBM model, two types of staggered grids which respectively make use of the velocity components from the orthogonal and diagonal directions are blended to calculate the hydrodynamic pressure from the Poisson equation, with the successive over-relaxation method (SOR). It is found that the simulated water droplet shape is determined by both the blending factor of the two types of staggered grids and the radius length. The appropriate blending factor for each radius length is summarized to optimize the simulation. The dependence of shape deformation on the blending factor and the radius length is further validated while considering the wettability effect of the solid wall of the gas channel. It is proved that the summarized appropriate blending factors are still practical when the concept of equivalent radius length is adopted.

The water balance in proton exchange membrane (PEM) fuel cells still remains a topic of much investigation in order to maintain satisfactory cell performance. One specific water management issue relates to the gas-liquid flows that occur when water enters the reactant flow field channels, which are typically microchannels or minichannels. Due to its unique water introduction, the Lockhart-Martinelli (LM) approach has been revised for its applicability in predicting the two-phase pressure drop in these channels where water emerges from a gas diffusion layer perpendicular to the direction of gas flow. In the revised LM approach, the Chisholm parameter C is found not to vary strongly as a function of key fuel cell operating variables (relative humidity, temperature, materials, gas stoichiometry), whereas it does vary as a function of flow regime and current density. A new flow regime map was proposed based on all pressure drop data collected from active fuel cells, where an accumulating flow regime is presented in addition to single-phase, film/droplet, and slug flow. The proposed accumulating regime is linked to water droplet dynamics, namely, water droplet emergence, growth, and detachment. A force balance approach shows when detachment will occur, which clarifies the bounds of the accumulating regime in terms of superficial gas velocity (gas stoichiometry ratio) and liquid velocity (current density). The balance considers different wetting scenarios in the channels and a range of superficial velocities of importance to PEM fuel cells.

Membrane Bio Reactor (MBR) technology is a promising alternative to municipal and industrial wastewater treatment owing to low sludge production and wide range of acceptable influents. Biofouling in MBRs hampers long term functionality of the system through reduction in permeate flux over time. Membrane biofouling could necessitate periodic membrane backwashing or even require membrane replacement, thus increasing operational cost for the systems. Microbe-secreted extracellular polymeric substance (EPS) forms a complex matrix on the surface; is persistent against physical removal and tends to resist high concentrations of antimicrobial agents, thus playing a major role in membrane biofouling. There is a need for developing methods towards efficient removal of biofoulants from surfaces. In tandem with low DC current, the synergistic effect of antimicrobial agents has been reported successful towards reducing biofilm formation leading to biofouling. This paper discusses the application of in-plane bioelectric effect as a solution to biofouling in MBRs; especially Microbial Fuel Cells and Microbial Desalination Cells towards harnessing in-situ current for tackling biofouling, thus facilitating longer system functionality.

Proton exchange membrane fuel cells (PEMFCs) are considered one of the most promising alternatives for the automotive industry owing to their high energy efficiency, zero emission at the vehicle use stage, and low temperature operation. Water as a byproduct plays a complex role in fuel cell operation. In particular, the inevitable occurrence of liquid water leads to gas-liquid two-phase flows in various components of PEMFCs including flow channels of which diameters range from micrometers to millimeters. In conventional minichannels and microchannels, the Lockhart-Martinelli (LM) approach has been employed to predict the two-phase pressure drop of gas-liquid systems. This approach has previously been updated by our group to more accurately reflect the introduction of liquid water into the flow channels of a PEMFC i.e. from a porous media perpendicular to the gas flow. Importantly, the LM method normalizes the data independent of the flow field design and operating conditions like temperature, pressure, and relative humidity. This paper analyzes the increasing amount of experimental data on two-phase flow pressure drops/two-phase flow multipliers in the literature with these approaches. The focus is the cathode side (therefore an air/water system), and data is collected from multiple research groups using active fuel cells (electrochemically produced water). The traditional LM approach greatly under-predicts the two-phase pressure drop at low current densities. However, the analysis is applied over a range of current densities, and it better predicts results at higher current densities (>600 mA cm −2 ). Literature correlations for the Chisholm parameter C , a flow regime dependent parameter in the LM equation, have been proposed for non-active (external water injection) fuel cells but do not match the results from operating fuel cells. C is shown here to vary with current density, flow stoichiometry (gas velocity), gas diffusion layer, and slightly with relative humidity.

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.

Proton Exchange Membrane fuel cells are being considered as the powertrain of choice for automotive applications. Automotive fuel cells experience transients during start-up, shut-down and changing load conditions, which constitute a significant part of the drive cycle. Transient behavior of PEMFCs can be classified into three categories: electrochemical, thermal and two-phase flow. Two-phase transients require a longer time to return to steady state than the electrochemical transient (which typically requires less than 1 second). Experiments have shown two-phase transients to be more prominent at the lower temperatures due to the increased presence of liquid water. Overshoot / undershoot behavior of current and voltage has been observed during investigations of electrochemical transients. This study investigates similar overshoot / undershoot behavior in the two-phase pressure drop in the reactant channels. An increase in the current drawn from the PEMFC is accompanied by larger air flow rates and greater water generation. An in situ setup is utilized to measure the pressure drop in the reactant channels across the length of the channel, when the electrical load drawn from the PEMFC is changed. This pressure drop measurement along the length of the reactant channels is used to characterize the overshoot / undershoot behavior. A parametric study is conducted to identify the factors which influence the overshoot / undershoot in two-phase flow pressure drop. The transient behavior is explored at the temperatures of 40, 60 and 80°C. Transient behavior is more pronounced at the lower temperature. Five different ramp rates have been used to show that faster ramp rates results in larger overshoot. The effect of magnitude of current change is investigated using four levels of load change. It was observed that increased magnitude of change results in increased overshoot behavior. However, no direct relationship has been observed between the magnitude of overshoot and the time required to return to steady state.

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.

Polymer electrolyte fuel cells (PEFCs) are highly expected as a next-generation power supply system due to the purity of its exhaust gas, its high power density and high efficiency. The polymer electrolyte membrane is a critical component for the performance of the PEFCs and it is important to understand the nanostructure in the membrane to enhance proton transport. We have performed an atomistic analysis of the vehicular transport of hydronium ions and water molecules in the nanostructure of hydrated Nafion membrane by systematically changing the hydration level which provides insights into a connection between the nanoscopic and mesoscopic structure of ion clusters and the dynamics of hydronium ions and water molecules in the hydrated Nafion membrane. In this study, classical molecular dynamics simulations are implemented using a model of Nafion membrane which is based on DREIDING force field and newly modified and validated by comparing the density, water diffusivity, and Nafion morphology with experimental data. The simulated final density after the annealing procedure agrees with experiment within 1.3 % for various water contents and the trends that density decreases with increasing hydration level are reproduced. In addition to determination of diffusion coefficients of solvent molecules as a function of hydration level (from λ = 1 up to λ = 18), we have also calculated radial distribution functions and static structure factors not only to clarify the structure of water molecules and hydronium ions around the first solvation shell of sulfonate groups but also to validate the mesoscopic periodic structure among water clusters. The diffusion coefficient of water molecules increases with increasing hydration level and is found to be in good agreement with experimental data. The diffusion coefficient of hydronium ions has showed that general trends in the experimental data are reproduced by the simulations although the classical models have the limitation of probing hydronium dynamics. The static structure factors of liquid molecules at low wave length provide insights into the periodic structure of the inter-water clusters. These results are consistent with the Gebel’s model based on small-angle X-ray scattering that considers the dry membrane to be made of isolated spherical ionic clusters of radius ∼7.5 Å that swell with increasing hydration.

The studies on microfluidics have advanced with the use of microchannels in micro bioreactors, applications in biomedical engineering, bioengineering and pharmaceuticals, fuel cells and compact heat exchangers for heating and cooling of micro electro mechanical systems. For a wide application of microchannels, it is very important to elucidate the effect of the tube diameters on the flow phenomena of gas liquid two-phase flow in microchannels. The flow phenomena and the frictional pressure drop of gas-liquid two-phase flow were investigated experimentally by using three kinds of circular microchannels made of fused silica tubes, with inner diameters of 0.1 mm, 0.15 mm and 0.25mm. In these channels, bubbly flow, slug-churn flow, and annular flow were observed. In annular flow, two characteristic flows of ring film flow and disturbed ring film flow were observed. For the 0.25 mm diameter microchannel, several conductance probe sensors were flush mounted to the channel wall in order to measure the very thin mean film thickness or liquid holdup in a microchannel. The flow patterns maps were also made by using the high speed video images and the holdup wave signals. The effects of tube diameter on the flow patterns were investigated.

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.

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 the present work, a chip embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed. Embedded current collectors are expected to increase the number of contact points with the porous electrodes and thereby reduce the overall contact resistance and ohmic losses of the cell. The micromachining based thin film process is compatible with the overall cell fabrication scheme, which is based on soft lithography, and does not require a substantial modification of the original cell design. The flow-through microfluidic fuel cell architecture with porous carbon paper electrodes is used as a proof-of-concept in this study. Our preliminary study shows that the peak power density of the cell with the current collector is increased by 79% at 300 μL min −1 compared to an otherwise identical cell without current collector, indicating that the contact resistance is significantly reduced. Electrochemical impedance spectroscopy analysis is carried out to estimate the overall cell ohmic resistance and confirms a 32% reduction using the current collectors.

Water management issues continue to be a major concern for the performance of polymer electrolyte membrane (PEM) fuel cells. Maintaining the optimal amount of hydration can ensure that the cell is operating properly and with high efficiency. There are several components that can affect water management, however one area that has received increased attention is the interface between the gas diffusion layer (GDL) and the gas reactant channels where excess water has a tendency to build up and block reactant gasses. One key parameter that can affect this build up is the geometry of the microchannels. The work presented here proposes an optimal trapezoidal geometry which will aid in the removal of excess water in the gas channels. The Concus-Finn condition is applied to the channel surfaces and GDL to ensure the water will be drawn away from GDL surface and wicked to the top corner of the channel. An ex situ setup is designed to establish the validity of the Concus-Finn application. Once validated, this condition is then used to design optimal channel geometries for water removal in a PEM fuel cell gas channel.

The appropriate blend proportional factor value which combines two kinds of staggered grids used in Lattice Boltzmann Method (LBM) for simulating the multiphase flow phenomena with large density ratio in the Polymer electrolyte fuel cell (PEFC) is fixed on. The shape deformation of the water droplet is found when using the two kinds of staggered grids to prevent the pressure oscillation when solving the Poisson equation of this LBM model and the shape of the water droplet varies with the changes of the blend proportional factor values. Two methods are adopted to find out the two staggered grids’ appropriate blend proportional factor value that can diminish or minimize the deformation of the droplet. The first one is to compare the simulation results of different blend proportional factors with the theoretical value and find the one mostly approaches the theoretical value; the second one is to compare the current velocity divergences of the two staggered grids using the results calculated by different blend proportional factor values. A water droplet resting in a tunnel is simulated with different blend proportional factor values and the appropriate value is decided.

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.

Gas-liquid two-phase flow in rectangular minichannels of polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on the fuel cell performance and durability. Different from traditional two-phase flow in other applications, water in the PEMFCs is introduced into the minichannel from the gas diffusion layers (GDLs) through random pores of different sizes. Meanwhile, the four channel surfaces may have different wettabilities due to the different materials used. Thus, the microstructure of GDLs and the surface wettability should be considered in investigating the two-phase flow in PEMFC channels. One challenge in simulating PEMFCs is that, full consideration of detailed microstructure of GDL needs extremely large computational time. In this work, we simplified the microstructure of GDL to a number of representative pores on the 2D GDL surface. A 3-D minichannel with 1.0 mm × 1.0 mm square cross section and 100 mm long was used in the simulation. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. Volume of fluid (VOF) method was employed to explicitly track the droplet surfaces emerging from the non-uniform GDLs. Simulation results show that, as the flow develops along the channel, the flow pattern evolves from corner flow on the bottom and side wall to corner flow on the top wall, annular flow and slug flow. The effects of liquid injection rates were studied, and it is found that the high liquid flow rate would accelerate the flow pattern development. The effect of wall surface material wettability was also studied by changing the hydrophobicity of GDL surface and side walls, separately. Simulation results show that the material wettability has a strong impact on the two-phase flow pattern, with a more hydrophilic side walls and/or a more hydrophobic GDL surface being more beneficial for expelling water out of the channel.