Pressure, fuel cell performance

CO2, and 1.5% N2 by volume along with trace levels of sulfur odorants. The odorants must be reduced to 1 ppmv before entrance into the fuel cell to prevent performance and cell life deterioration. Because the desulfurization requires elevated temperatures, the fuel (streams 3 and 5) is fed through a heat exchanger that recovers heat from the fuel cell exhaust stream (stream 15). The hot desulfurized fuel stream (stream 4) enters the anodes of the high-pressure fuel cell at approximately 399°C (750°F) and 9.3 atmospheres. The fuel entering the low-pressure fuel cell (stream 6) is approximately 399°C (750°F) and 3.1 atmospheres. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Anfolini et al. [161] also compared SAB and Vulcan XC-72 as possible candidates in MPLs, but in this case they used carbon cloth DLs with two MPLs. From their results, it was concluded that the Vulcan XC-72 gave slightly higher electrocatalytic activity for fhe ORR. On the other hand, MPLs near the FF that used SAB had better performance. Thus, it was suggested that for improved fuel cell performance af high pressures (around 3 atm), the ideal cathode MPL compositions would use the Vulcan XC-72 in the MPL next to the CL and SAB in the MPL next to the flow fields. 

G. Velayutham, J. Kaushik, N. Rajalakshmi, and K. S. Dhathathreyan. Effect of PTFE content in gas diffusion media and microlayer on the performance of PEMFC tested under ambient pressure. Fuel Cells 7 (2007) 314—318. 

Bearing in mind that phenomena occurring in nature are too complex to be completely described by mathematical equations, the required details to be described by the model must be goal-driven, i.e. the complexity of the model, and the related results, must be strictly connected to the main goal of the analysis itself. When, for example, the main purpose of the model is to provide the fuel cell performance, in order to analyze the whole system in which it is embedded, the spatial variation in the physical and chemical variables (such as gas concentration, temperature, pressure and current density, for example) are not relevant however the performances, in terms of efficiency, electrical and thermal power and input requirements are important [1-4],... 

The analysis of the conditions within a gas channel can also be assumed to be onedimensional given that the changes in properties in the direction transverse to the streamwise direction are relatively small in comparison to the changes in the stream-wise direction. In this section, we examine the transport in a fixed cross-sectional area gas channel. The principle conserved quantities needed in fuel cell performance modeling are energy and mass. A dynamic equation for the conservation of momentum is not often of interest given the relatively low pressure drops seen in fuel cell operation, and the relatively slow fluid dynamics employed. Hence, momentum, if of interest, is normally given by a quasi-steady model,... 

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

Figure 3.46. Conductivity of Nafion-115 as a function of humidity (partial pressure of water relative to saturation pressure, the latter corresponding to 18% water by weight or 11 water molecules per sulphonate molecule). (From C. Yang, S. Sriniva-san, A. Bocarsly, S. Tulyani, J. Benziger (2004). A comparison of physical properties and fuel cell performance of Nafion and zirconium phosphate/Nafion composite membranes. /. Membrane Sci. 737,145-161. Used with permission from Elsevier.)...

From the above consideration it is clear that the exchange current density is the main factor affecting the activation overpotential, then the optimization of a PEM fuel cell performance requires the maximization of io- This can be obviously accomplished by increasing the catalyst activity, that means to raise the surface area, cell temperature, and reactant pressure (this last effect should also favor gas adsorption on catalyst sites). 

The critical issues in designing a vehicular fuel cell power system are the selection of an optimal operating pressure, oxidant flow rate and the choice of an adequate control strategy. Computer models have been developed to simulate fuel cell performance under various operating conditions. 

Figure 2.12. Capillary pressure as a function of position in the CCL for different fuel cell current densities as indicated on the graphs. For small jo, Tc z) indicating operation in the optimal wetting state. For jo >parts of the CCL are completely flooded, i.e. r z)>ryi, and the fuel cell performance is critically impaired [25],...

Fig. 7.19 Fuel cell performance comparison at 60 °C for a CNT GDL against a Toray GDL. Anode operation conditions 1.0 M methanol at 1 ml min . Cathode operation conditions ambient pressure air at 80 ml min [117] (Reprinted by permission of the publishtr)...

The positive effect of water on the fuel cell performance was further proven by introducing several H2O/H2 mixtures into the anode compartment. I-V curves were recorded for each water vapor partial pressure as shown in Fig. 22. I-V curves were recorded fast, within a time interval less than 30 s, so that the water produced at the cathode would not equilibrate with the membrane. It is evident in Fig. 22 that by increasing the water partial pressure a threefold increase of the cell performance is observ ed, thus proving the vital importance of steam for the efficient operation of the phosphoric acid imbibed high temperature MEA. 

Fig. 15.8 Fuel cell performance at 50 °C, 20 psi back pressure for both H2 and O2, anode 1.25 mg cm Pt/C cathode 2.5 mg cm catalyst loading. With (red filled circles) Pt/C (blue...

Beming et al. [58] performed a parametric study using their previously described singlephase, three-dimensional model [55]. The effect of various operational parameters such as the temperature and pressure on the fuel cell performance was investigated. In addition, geometrical and material parameters such as the gas diffusion electrode thickness and porosity as well as the ratio between the channel width and the land area were investigated. It was found that in order to obtain physically realistic results experimental measurements of various modeling parameters were needed. In addition, the contact resistance inside the cell was found to play an important role for the evaluation of impact of such parameters on the fuel cell performance. The impact of liquid water on transport in the gas-diffusion electrode was, however, not account for. 

To prevent electrode burning during the production process, the Raney nickel is passivated by oxidation at a controlled temperature and pressure. To recover the catalytic activity and fuel-cell performance, the catalyst has to be reactivated. 

No exemplary simulation results are presented here. Anyway, these would only be applicable for a certain MCFC system and under certain conditions, and they would not be representative for the broad range of available models. Nevertheless, MCFC models have been applied for various purposes Toshiba et al. [5] compared different flow configurations, Koh and Kang [10] predicted the impact of pressurized operation on fuel-cell performance, Park et al. [14] and Heidebrecht and Sundmacher [56] applied MCFC models to evaluate the effect of the reforming process on the fuel cell and to optimize it, and Bosio et al. [8] studied the application of nonuniform gas distributions with regard to the temperature distribution in MCFCs. 

The state-of-the-art gas diffusion media are hydrophobized to such an extent that they allow transport of liquid water, an important mechanism at near-saturated conditions, as well as of water vapor and reactant gases. An important role is played by the micro porous layer (MPL). Because of the presence of small hydrophobic pores, a substantial hquid water capillary pressure can be bruit up, enabling a good gradient in the chemical potential of water to drier sections [10]. The optimization of gas diffusion media and the application of the MPL have led to significant improvement of the fuel cell performance at saturated conditions, showing their critical role. 

Air is supplied to the cell by means of a blower or a compressor (depending on operating pressure) whose power consumption is directly proportional to the flow rate. Therefore, at higher air flow rates the fuel cell may perform better, but power consumption of a blower or particularly of a compressor may significantly affect the system efficiency. There are at least two reasons why fuel cell performance improves with excess air flow rate, namely ... 

In this thesis study, a PEM fuel cell was modeled. Using the model, the most important performance parameters of the PEM fuel cell, namely operation temperature and pressure were investigated and their affects were determined on the PEM fuel cell performance. The performance is reflected primarily in the PEM fuel cell output voltage, electrical power output and efficiencies. The basic results of the PEM fuel cell modeling (Table VII. 1) are presented below ... 

Higher operation temperatures and pressures advance the fuel cell performance the effect of the pressure increase is more pronounced. 

The Nemst equation provides a relationship between the ideal standard potential (E°) for the cell reaction and the ideal equilibrium potential (E) at other partial pressures of reactants and products. For the overall cell reaction, the cell potential increases with an increase in the partial pressure (concentration) of reactants and a decrease in the partial pressure of products. For example, for the hydrogen reaction, the ideal cell potential at a given temperature can be increased by operating at higher reactant pressures, and improvements in fuel cell performance have, in fact, been observed at higher pressures. This will be further demonstrated in Chapters 3 through 7 for the various types of fuel cells. 

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