Pressure, fuel cell performance optimization

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],...

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. 

Research and development effort has been concentrated on the bipolar plate designs to reduce the cost and increase the performance of the fuel cell. Improvements can occur in the performance of a fuel cell through optimization of the channel dimensions and shape in the flow field of bipolar plates. The contact surface area of the reactant gas on the bipolar plates has an effective contribution on the overall reaction of the gases. The reactant gas pressure has an important role in the overall functioning of the fuel cell. Consideration of fluid flow, heat, and mass transfer phenomenon is impor-fanf while designing the bipolar plate channels. 

Fuel cell pressurization is typical of many optimization issues, in that there are many interrelated factors that can complicate the question of whether to pressurize the fuel cell. Pressurization improves process performance at the cost of providing the pressurization. Fundamentally, the question of pressurization is a trade-off between the improved performance (and/or reduced cell area) and the reduced piping volume, insulation, and heat loss compared to the increased parasitic load and capital cost of the compressor and pressure-rated equipment. However, other factors can further complicate the issue. To address this issue in more detail, pressurization for an MCFC system will be examined. 

It is important to operate the fuel cell at different compression pressures in order to determine the correct compression pressure for a DL material. If the applied compression pressures are too high, the DLs may deform, both the porosity and permeability of the DL decrease, and the probability of failure modes increases significantly. On the other hand, if the pressures are too low, then gas leaks and serious contact resistance between the components of the cell may be present. Various studies have been presented in which the compression pressure of the fuel cell is varied in order to observe how the cell s performance is affected [25,183,252]. In general, there is an optimal compression pressure range in which the cell s performance is the highest however, this depends on the DL material and on the MPL thickness (see Figure 4.21). 

Separation of the individual contributors can provide useful information about performance optimization for fuel cells, helping to optimize MEA components, including catalyst layers (e.g., catalyst loading, Nafion content, and PTFE content), gas diffusion layers, and membranes. It assists in the down-selection of catalysts, composite structure, and MEA fabrication methods. It also helps in selecting the most appropriate operating conditions, including humidification, temperature, back-pressure, and reactant flow rates. 

In fuel cell catalysis, finely divided platinum particles are used on suitable support materials such as special grades of carbon blacks. Techniques for investigating the interactions between the platinum particles and protons under hydrogen pressures typical of catalyst operation are needed for optimization of catalyst morphology and performance as well as development of economic catalysts. 

The optimization of the air pressure influences significantly the fuel cell stack performance. The optimization should be done individually for each fuel cell system, since the pressure depends on the fuel cell technology and the performance of the compressor depends on the compressor technology and its compositirai (with or without energy recuperation). 

In conclusion, all of these observations indicate that there is still much room to improve ADAFC performance by developing novel materials and, on the other hand, by optimizing the operational conditions of the fuel cell. Future work should look into a wider range of potential low-cost materials and composites with novel structures and properties, presenting catalytic activity comparable to that of noble metals. The development of new catalyst systems is more likely in alkaline media because of the wide range of options for the materials support and catalyst, as compared to acidic media which offer more limited materials choice. Moreover, efforts have to be addressed to meet the durability targets required for commercial application. More work is needed to optimize the operational fuel cell conditions, by achieving suitable chemical (OH concentration, hydroxyl/alcohol ratio in the fuel stream) and physical (temperature, pressure, flow rate) parameters. 

The purpose of this section is to describe the chemical and thermodynamic relations governing fuel cells and how operating conditions affect their performance. Understanding the impacts of variables such as temperature, pressure, and gas constituents on performance allows fuel cell developers to optimize their design of the modular units and it allows process engineers to maximize the performance of systems applications. 

The principle of how a PEM fuel cell generates electricity is straightforward. However, the cell power output depends on material properties, cell design and structure, and operation conditions, such as the gas flow, pressure regulation, heat, and water management. High performance of a PEM fuel cell requires maintaining optimal temperature, membrane hydration, and partial pressure of the reactants... 

Xing, X. Q., Lum, K. W, Poh, H. J. et al. 2010. Optimization of assembly clamping pressure on performance of proton-exchange membrane fuel cells. Journal of Power Sources 195 62-68. 

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