Membrane in fuel cell performance modeling

Finally, there are some miscellaneous polymer-electrolyte fuel cell models that should be mentioned. The models of Okada and co-workers - have examined how impurities in the water affect fuel-cell performance. They have focused mainly on ionic species such as chlorine and sodium and show that even a small concentration, especially next to the membrane at the cathode, impacts the overall fuelcell performance significantly. There are also some models that examine having free convection for gas transfer into the fuel cell. These models are also for very miniaturized fuel cells, so that free convection can provide enough oxygen. The models are basically the same as the ones above, but because the cell area is much smaller, the results and effects can be different. For example, free convection is used for both heat transfer and mass transfer, and the small... 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

There are different approaches that incorporate the water balance in the membrane into models of fuel cell performance. They rest on different concepts of membrane microstructure. As a common feature they use local values of transport parameters which are functions of the local water content, w (volume fraction of water relative to the total membrane volume). 

The influence of CO poisoning at the anode of an HT-PEFC was investigated by Bergmann et ul. [28]. The dynamic, nonisothermal model takes the catalyst layer as a two-dimensional plane between the membrane and gas diffusion layer into account. The effects of CO and hydrogen adsorption with respect to temperature and time are discussed in detail. The CO poisoning is analyzed with polarization curves for different CO concentrations and dynamic CO pulses. The analysis of fuel-cell performance under the influence of CO shows a nonlinear behavior. The presence of water at the anode is explicitly considered to take part in the electrooxidation of CO. The investigation of the current response to a CO pulse of 1.31% at the anode inlet showed a reversible recovery time of 20 min. 

Higher dimension fuel cell models can be considered to study PEM fuel cell performance. As shown previously, two-dimensional models can be used to characterize fuel cell performance along the channel length (x-z plane) or across channels (y-z plane). In the y-z plane, the model can focus on the membrane electrolyte assemble, or include the effect of gas transport in the porous diffusion layer and gas channel. In the y-z plane, several channels can be considered in order to investigate the effects such as gas mixing between the chaimels occurring in the diffusion layer. A three-dimensional model can be used to consider all the aforementioned phenomena, but computational hmitations often limit the model s fidelity. The equations are the same as those in 1-D and 2-D models, but all the differential equations are applied in all three directions. Consequently, the boundary conditions must be applied for all three dimensions. 

As an important part of the PEMFC, PEM is used to separate the cathode and anode reaction gas, while the proton is conducting from the anode to the cathode. Good performance of the membrane is a significant property of the film that largely depends on the water content. Proper control of the water distribution can significantly improve the fuel cell performance. Many researchers regard water transportation phenomena in PEMFC as a research focus. A water diffusion model [12,13] of... 

In conclusion, PEM is a core part of PEMFCs. The membrane resistance and water content of the membrane is crucial to the fuel cell performance. This model describes the dynamic process of the membrane variables (e.g., the flux density of HjO, the flux density of H+ and water content). 

The fault of membrane dehydration can be indnced by inappropriate temperature. Therefore, a temperature ramp signal is put into the model. From 0 to 60 s, the fuel cell stack works under the normal condition of 60°C. From 60 to 80 s, the temperature rises linearly to 107°C (180 K). The voltage degradation is distinct, shown in Figure 12.8. In addition, the water content in membrane has a critical effect on the fuel cell performance. The fault occurs when the water content decreases to 0.1% of the initial value at 60 s, shown in Figure 12.9. 

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