Dissertation Title: LiquidWaterTransport in the ReactantChannels of ProtonExchangeMembraneFuelCellsWater management has been identified as a critical issue in the development of PEM fuelcells for automotive applications. Water is present inside the PEM fuel cell in three phases, i.e. liquid phase, vapor phase and mist phase. Liquidwater in the reactantchannels causes flooding of the cell and blocks the transport of reactants to the reaction sites at the catalyst layer. Understanding the behavior of liquidwater in the reactantchannels would allow us to devise improved strategies for removing liquidwater from the reactantchannels. In situ fuel cell tests have been performed to identify and diagnose operating conditions which result in the flooding of the fuel cell. A relationship has been identified between the liquidwater present in the reactantchannels and the cell performance. A novel diagnostic technique has been established which utilizes the pressure drop multiplier in the reactantchannels to predict the flooding of the cell or the drying-out of the membrane. An ex-situ study has been undertaken to quantify the liquidwater present in the reactantchannels. A new parameter, the Area Coverage Ratio (ACR), has been defined to identify the interfacial area of the reactant channel which is blocked for reactanttransport by the presence of liquidwater. A parametric study has been conducted to study the effect of changing temperature and the inlet relative humidity on the ACR. The ACR decreases with increase in current density as the gas flow rates increase, removing water more efficiently. With increase in temperature, the ACR decreases rapidly, such that by 60°C, there is no significant ACR to be reported.
The table contains the most relevant information for each design reported. The fuel cell type (hydrogen PEMFC or DMFC) and the Bipolar Plate design are first specified. Then it is indicated whether the study is using CFD techniques for the design and investigation and/or experimental prototypes. In case CFD simulations are reported, it is indicated whether the model was including liquidwatertransport and its effects, and whether there is a validation of the model. The table also includes the basic information about the cell components being used (membrane, GDL, and catalysts) if available in the original publication. The range of the operating conditions is also provided. Finally, in order to assess the benefits of the novel biomimetic designs, the gain in performance with respect to the conventional designs is indicated in the table. This information has been extracted from the polarization curves reported in the different works. The performance gain is indicated as the current density improvement for the biomimetic bipolar plate with respect to the conventional design, at 0.6 V (considered to be a nominal point) and at 0.4 V (more representative of operating conditions at high current densities where the influence of the Bipolar Plate design is much more pronounced). Thus, the value of
However, the performance of the contemporary PEM fuel cell still needs to be significantly improved toward engineering optimization and cost reduction [1-4].
Water management in a PEM fuel cell has been one of the critical challenging issues. In practices, the water vapor in the fuelcells is produced from the electrochemical reaction taking place in the catalyst layer on the cathode side, it penetrate through the gas diffusion layer and condenses on the walls of the gas channel on the bipolar plate if the partial pressure of the water vapor exceeds the saturation pressure or when the operating temperature of a PEM fuel cell is lower than the dew point temperature of the gas mixture. Besides, additional water may be supplied by an external humidifier which is used to add moisture into the hydrogen gas on the anode side so as to optimize the hydration of the protonexchangemembrane. When the water vapor or the liquid droplets are not able to be readily removed from the channels by the gas flow, the liquidwater condensate may be accumulated to cause water flooding and retard the mass transport by occupying the pores of the gas diffusion and catalyst layers. Therefore, maintaining the proper balance in fuel cell between water production and removal is important in improving the PEM fuel cell performance. Thus, water management has become a critical issue for fuel cell design [5-7].
steady-state condition, the best fuel cell performance is when the air flow rate is 0.20 SLPM and is the worst when the air flow rate is 0.10 SLPM.
The wetted bend ratio and wetted area ratio for both sides have been calculated from the captured images and plotted in Fig. 5. It can be seen that they both decrease as the air flow rate increases. As indicated earlier, this is most likely to be due to the increase in the ability to remove liquidwater from the flow channel with increasing air flow rate. Since water is produced at the cathode, the wetted ratio numbers at the anode are smaller than those of the cathode. However, their trends are similar to each other even though the hydrogen flow rate has been kept constant. This may be attributed to the net watertransport across the membrane through the electro-osmotic drag and the back diffusion. This indicates that the wetted ratio numbers at the anode side can also be employed to interpret the relationship between the fuel cell performance and the air flow rate.
safety are some issues which needed to be accounted for designing a fuel cell assembly. There is a need for development in catalyst layer. Further studies are needed in characterization of pore size distribution as well as hydrophilicity and hydrophobicity distributions and using this information to develop pore level models. The realistic and accurate simulation of liquidwater and gas transport through gas diffusion layers with highly non-uniform pore sizes and wettability can be done by such studies. For success in direction of wide areas of applications, a number of important rather complex problems must first be solved including cost, environmental stability and longer lifetime of the cell.
Third, the PEMFCs brought no air-pollution with zero-emission. Because of the only product are water and heat, the DHFCs are considered to produce drinking water.
DHFC is more matured with longer history and more research concern because of its higher power-to-weight ratio. Nevertheless, for the application of small and portable devices, DMFCs showed their merit. Compare with hydrogen, methanol is much more easy to produced and transport and could be utilized in the fuelcells without passing through an expensive reformer. Although the power-to-weight ratio of methanol is only 1/5 of hydrogen, methanol could offer 4 times power density per volume under 250 atm because it is liquid.
50%). Meanwhile, Ge and Wang  justified the 35% reduction in Pt surface area of their tested cell at – 30˚C through the formation of an ice sheet between Pt particles and ionomers in the cathode catalyst layer.
Not all the residual water in the membrane freezes when the temperature reaches 0˚C. Three types of water molecules known as non-freezing water, bond freezing water, and free water were distinguished by researchers during subfreezing temperatures. Each of these types has different freezing points depending on the nature of their molecular chemical bonds. For instant, Lu et al  detected three distinctive water states in subfreezing temperatures using a dielectric relaxation spectroscopy technique. They found in the kHz frequency region that water molecules are hydrogen-bonded to the sulfonates whereas in the GHz domain water molecules are either in liquid bulk-like or loose bond forms.
(Source from: http://www.energi.kemi.dtu.dk/Projekter/fuelcells.aspx)
1.3. Water Management Problem in ProtonExchangeMembraneFuelCells
As the main product in the cathode, water plays an important role in PEMFC’s performance.
Because the operating temperature of a PEMFC is under 100 ℃ (usually 50-100 ℃ ), water generated from the electrochemical reaction easily condenses into liquidwater and accumulates in the cathode. Too much liquidwater may flood the GDL and channels, hinder mass transport of gas from reaction, resulting in an increase of voltage loss. On the other hand, if there is too little water so that the membrane is unable to get hydrated enough to ensure adequate protons go through it; the generated voltage is decreased as well. Therefore, the amount of water and distribution is a key point affecting the performance of PEMFC, either too much or too little will drop the power output. That is why water management, especially liquidwater management, is a critical issue in the improvement of PEMFC’s performance and the commercialization that attracts interests of academic researchers and engineers. Thus, it is important to get an understanding of gas-liquid dynamics inside PEMFCs with different flow field designs.
3.6. Direct methanol fuelcellsFuel cell technology, particularly for transport applications, would take a leap forward if a viable system were to be developed that could use a liquidfuel without the need for reformation. The prospects for anode catalysts being developed having the activity to operate on petroleum derived hydrocarbon fuels are poor. However, Shell and others in the 1960s established that Methanol, with anode catalysts such as Pt/Ru, had some potential. The early work utilized sulphuric acid as the electrolyte. With the introduction of proton conducting membranes, interest in DMFC systems in the 1990s has been renewed with projects in America, Japan and Europe. Of particular significance has been the work of Los Alamos National Laboratory. If the power density required for vehicle applications are to be achieved, further improvements to anode catalyst performance are necessary. In addition, existing membrane materials are subject to what is known as ‘methanol crossover’, which in turn contributes to poor cell performance. In this context, it is interesting to speculate on how high temperature membranes such as that developed by Celanese would perform in a DMFC fuel cell (23) .
In this study, the effect of electrical contact resistance on cell performance and local transport characteristics of protonexchangemembranefuelcells (PEMFCs) are numerically investigated by using a two-dimensional, non-isothermal and two-phase flow fuel cell model. The conservation equations of species, temperature, charge, liquidwater and dissolved water were solved to investigate the transport processes of heat and mass transfer, electron and proton transports, liquidwater formation and transport, and watertransport through the membrane. The mathematical model was validated against the experimental data reported in the open literature. Results showed that the performance is significantly affected by the electrical contact resistance, especially at low cell voltages. In addition, the temperature, liquidwater saturation and solid phase potential distribution profiles are greatly influenced by the existence of electrical contact resistance.
The effect of mechanical vibration on the watertransport
In order to explain the reason that mechanical vibration causes the voltage fluctuation, the experiments watertransport in an anode flow channel were conducted for the transparent PEMFC under mechanical vibration and no mechanical vibration, respectively. The watertransport progress in the anode flow channels under no vibration During this process, the gas was not humidified at the anode and the cathode, the flow rate was 70 mL/min at the anode and 1 L/min at the cathode, the temperatures was 40 _C, the transparent PEMFC was loaded at a constant current 5 A. At first, a water droplet suddenly migrates into and through the diffusion layer at the 5 min, From the enlarged picture shows, the water droplet is appearing close to the channel. After 8 min the water droplet grow larger, which lead to a complete blockage of the channel.
accumulates at the interface between the CL and GDL before draining from the MEA through the GDL by capillary action .
GDLs commonly consist of a substrate and microporous layer (MPL). To achieve water removal and gas diffusion, the substrate is typically waterproofed by introducing polytetra‐ fluoroethylene (PTFE) within micropores. This prevents the GDL surface and pores from being clogged with liquidwater and facilitates gas transport to the CL. The treatment process normally results in inhomogeneous PTFE distribution across the GDL substrate, with higher concentrations at the surface. Neutron radiography reportedly indicated that water concen‐ trates in the center of the carbon paper when the fuel cell is operating [10–12], reflecting the fact that GDLs are easily flooded.
Intuitively, this shear stress seems to be much weaker than that in the channels. As a result, the growth and spread of water droplets in the porous media has a longer time evolution compared to the channels as shown in Figures from t = 0.54 s to t = 0.66 s. In other words, the growth and spread of water droplets are quite slow in the GDL as well as the catalyst layer. In Figure 4.6, it also can be clearly seen that there is a large amount of liquidwater concentrated in the region under the channel-end. Initially, liquidwater is pushed from the channel to the porous media by the secondary flow in the Y- direction, the liquidwater region is formed with a circular shape due to uniform velocity distribution. Furthermore, this region is influenced by the primary flow field distributed in the porous media (X-Z planes) instead of the secondary flow. The radial distribution of the flow velocity in the GDL region under the channel-end would make the liquid region deformed and elongated in the Z- direction as time progressed. Meanwhile, a small portion of liquidwater in the GDL is pushed to the cathode catalyst layer by secondary flow in the cathode GDL, and also is deformed under the primary flow in the catalyst layer. The presence of too much liquidwater in the GDL and especially in the catalyst layer would severely affect the fuel cell performance. A large amount of liquidwater should be somehow removed from the porous media to prevent flooding but a small amount should be kept to maintain a high conductivity of the membrane. Interestingly, the simulation results show that it is feasible to remove the liquidwater by a higher velocity field in the porous media of an interdigitated channel PEMFC as compared to the serpentine or parallel channel PEMFCs. The deformed liquidwater would continuously elongate until it is partially removed to the outlet channel. This is illustrated by a decrease of the volume fraction of liquidwater in the liquid region shown in Figure 4.6b at t = 0.66 s. It could be more clearly seen in the catalyst layer region that a large amount of liquidwater first is pushed to the catalyst layer from the GDL at t = 0.54 s, then it is deformed and elongated under the impact of gas flow from t = 0.56 s to t = 0.62 s, and eventually is taken away from the catalyst layer at t = 0.64 s.
In this manuscript Dynamic behavior of a fuel cell which is a complex phenomenon which is modeled in mathematical equation including all loses which arises in PEM Fuel cell and simulated using mat lab / Simulink package. A PEM fuel cell has been modelled with this model exhaustively using more parameters. Dynamical structure of the model can be obtained to change the input parameters as required in this model the factors in the dynamic behavior of a PEM fuel cell are the reactant gases humidity change, various load changes and liquidwater formation in cathode channel.
2. MODEL DESCRIPTION
Lumped model has been developed in this study. Lumped model is model with zero dimensions. The model was developed with some assumptions such as, the transport process is steady- state which resolves coupled transport in membrane, all gases obey the ideal gas low, the gas flow channels is laminar ,the catalysts is very thin ,the change phase of water was neglected ,the heat transport cross solid medium as gas medium, the output temperature is cell temperature. The developed model has five major sections. Theses included mass transport, heat transport, electrical characterization, water management and losses product.
All research reported in – presents a summary of critical parameters for the cold startup of fuelcells, and some conclusions are summarized in Table 1. In this context, many studies showed that purging residual liquidwater at shutdown is the key to a successful cold start. Other studies focused on finding heating solutions at startup to raise the stack temperature above the freezing point of water and prevent ice blockage from occurring. On the one hand, the differ- ent studies aim to obtain a successful cold start and avoid performance degradation. On the other hand, they attempt to satisfy the specific targets for cold startup time and energy requirement.
Fig. 5 shows, in general, a similar behavior for the leading materials in a number of citations, suffering an apparent absence of trend that does not de ﬁne a turning point for the time interval under study (2008–2018). The articles with the highest number of citations on the subject under investigation are mostly publications that are outside the range under review, which explains the initial citation behavior (2008) where except the authorship article by Wang Y (TC2008 ¼ 0), the others have citation values above zero. Without a doubt the most critical piece on PEMFC for most of the years under study was that of Springer TE (TC2018 ¼ 802), published more than two decades ago, the research entitled "Polymer Electrolyte Fuel Cell Model" as shown in Table 4 , developed and pro- posed a simple, one-dimensional, isothermal model of a complete poly- mer electrolyte fuel cell that has provided useful information on the cell's watertransport mechanisms and their effect on cell performance. In this study, membranewater/electrode water steam balance conditions were applied to the membrane/electrode interfaces and the electroosmotic and diffusion driving forces for water in the membrane and diffusion of water steam and reactive gases in the electrodes were considered to obtain material balances throughout the cell  . Another important document that has laid the foundation for PEMFC research is the article by Gasteiger et al. (TC2018 ¼ 705) entitled " Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs ".
Keywords: protonexchangemembranefuel cell; numerical simulation; computation fluid dynamics
A protonexchangemembranefuel cell has many prominent characteristics, especially short startup time under low temperature, which makes it become a promise power source for future transport tools (such as electrical vehicles). General speaking, PEMFCs as power sources of transport tools will often operate dynamically, for example during startup and stop, acceleration and deceleration of electrical vehicles. It is well known that good gas flow fields design and water balance is useful for the enhancement of fuel cell.
The conductivities of the membranes are calculated from the impedance data from 180°C to 280°C shown in fig. 7. The proton conductivity of the mesoporous SnP 2 O 7 was around one order of magnitude higher than that of the nonporous SnP 2 O 7 ranging from 180°C to 280°C. This is explained by assuming that the proton conductivity is mainly enhanced by the presence of proton ions dissolved in water molecules filled in the mesopores in favour of protontransport [11, 23-25]. The mesoporous SnP 2 O 7 exhibits an increasing conductivity from 0.0091 S cm -1 to 0.015 S cm -1 in the temperatures range of 180-220 °C in Fig. 7. However, there is an obvious reduction in the conductivity when the temperature increases over 220°C, which could be attributed to the water lost from the capillary pores.
Figure 2: electrical behavioral models – left (Fouquet et al. 2006), right (Asghari et al., 2010)
3.3 Losses in PEMFC
Another efficient way to study PEMFC is to empirically characterize them, namely with polarization curves for the static behavior and Nyquist plots for the dynamical one. These curves give useful information regarding losses and internal resistances in the system. If we take a closer look at the polarization curve on figure 3 for example, we can distinguish four zones corresponding to different types of losses. The Nyquist plot, as for it, can be used to study the behavior evolution during ageing process (Hissel et al., 2007). Five parts are identified due to several phenomena: (1) polarization resistance, (2) mass transport, (3) charges transport, (4) resistance and (5) pseudo inductance due to metallic components.