Fuel cell effectiveness factor

Practically every battery system uses carbon in one form or another. The purity, morphology and physical form are very important factors in its effective use in all these applications. Its use in lithium-ion batteries (Li-Ion), fuel cells and other battery systems has been reviewed previously [1 -8]. Two recent applications in alkaline cells and Li-Ion cells will be discussed in more detail. Table 1 contains a partial listing of the use of carbon materials in batteries that stretch across a wide spectrum of battery technologies and materials. Materials stretch from bituminous materials used to seal carbon-zinc and lead acid batteries to synthetic graphites used as active materials in lithium ion cells. 

In order to make catalyst layers with high platinum utilization and better performance, we need to determine how various factors affect Pt utilization. Although this objective has been receiving more attention, we have not achieved a fundamental understanding of the relationships of composition, structure, effective properties, and fuel cell performance—a fact that may limit the optimal design and fabrication of CLs. 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

Within the last five years, many fuel-cell models have come out of the Research Center in Julich, Germany. These models have different degrees of complexity and seek to identify the limiting factors in fuel-cell operation. The model of Kulikovsky et al. examined a 2-D structure of rib and channel on the cathode side of the fuel cell, and is similar to that of Springer et al. Other models by Kulikovsky included examination of depletion along long feed channels and effects in the catalyst layers.The most recent model by Kulikovsky relaxed the assumption of constant water content in the membrane and examined quasi 3-D profiles of it. Also at the research center, Eikerling et developed many... 

The inclusion of multidimensional effects is important to realistically mimic transport in the fuel cell. This is not to say that certain cases and factors cannot be collapsed to lower dimensionality, but one must be aware of higher dimensional effects, lest they become important. 

Hence these three key points will determine the energy efficiency and the specific power of the elementary fuel cell an improvement in each component of the cell will increase the power density from 0.175 to 1.05 Wcm, that is, an increase by a factor of 6. As a consequence, for the fuel cell systems the weight and volume will be decreased by a similar factor, for a given power of the system, and presumably the overall cost will be diminished. The improvement in the components of the elementary fuel cell thus has a direct effect on the system technology and therefore on the overall cost. 

Using the 2-D saturation maps from the two-phase LB simulation, shown in Fig. 14, the effective ECA can be evaluated and correlated according to Eq. (26). Based on several liquid water saturation levels, the catalytic surface coverage factor for the CL microstructure is estimated and the following correlation can be constructed, which can be used as valuable input to macroscopic two-phase fuel cell models.27,62... 

In a second prototype, the reaction temperature was reduced to 250 °C, which reduced the carbon monoxide concentration from 1.2 to < 1%. Later, the first fuel processor prototype was linked to a meso-scale high-temperature fuel cell developed at Case Western University by Holladay et al. [117], which was tolerant to carbon monoxide concentrations up to 10%. Hence no CO clean-up was necessary to run the fuel processor. A 23 mW power output was demonstrated according to Holladay et al. [118], This value was lower than expected, which was attributed to several factors. First, the hydrogen supply was lower in the reformate. Second, the presence of carbon monoxide (2%) lowered the cell voltage. Third, the presence of carbon dioxide (25%) generated a magnified dilution effect at the gas diffusion layer material of the fuel cell, which was considerably less porous than conventional materials. 

Fuel-cell manufacturing costs and the directiy related retail prices are expected to be strongly affected by the established production volumes and the corresponding learning effects. These volumes depend on the rate of fuel-cell commercialisation, which is expected to be determined by their price competitiveness. In this context, the fuel-cell-related industry, the most cmcial leveraging factor for fuel- ced market development, is of major importance, and also represents an indicator of the situation so far and its near-term future potential (Pehnt et al., 2004). 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

It is favorable for fuel cell operation when reduced methanol transport across the membrane is accompanied by proper water management. In particular, a low water crossover from the anode to the cathode is necessary to avoid flooding of the cathode. The dependence of water permeation on the membrane thickness is weak. Only a small decrease in water permeation is observed for the commercial Nafion membranes, whereas the thickness of the recast membranes has no significant influence on the water transport rate. In contrast, the effect of temperature on water permeation is strong. At 65°C, the rates are higher by a factor of 5 compared to those at 25°C. 

Designing alloy electrocatalysts by the so-called ad-atom method, and by alloy sputtering for oxidation of CH3OH and CO, and for CO tolerance in H2 oxidation, respectively, as well as for O2 reduction are discussed. Many years of experience are summarized and collaborations with other groups are highlighted. The particle size effect in electrocatalysis by small particle electrodes, and the effect of corrosion of carbon-black supported nanoparticles on the electrocatalytic activity are also discussed. All these factors, as well as catalyst lifetimes, are very important in fuel cell performance and in the final cost estimates for the practical fuel cell applications. 

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