Fuel crossover

Fuel crossover, 72 214 Fuel economy, 72 388-389, 414 diesel engine, 72 420 Fuel efficiency, in furnaces, 72 332-333 Fuel-fired furnaces, 72 318-336 analysis of, 72 332-333 classification of, 72 320-321 development of, 72 319-320 industrial furnaces, 72 327-330 power-plant furnaces, 72 323-327 Fuel gas... 

The emergence of commercial fuel cell cars will depend on developments in membrane technology, which are about one third of the fuel cell cost. Improvements are desired in fuel crossover from one side of a membrane to the other, the chemical and mechanical stability of the membrane, undesirable side reactions, contamination from fuel impurities and overall costs. 

Even if the polymeric membrane in a PEM fuel cell is designed to permit only the passage of hydrated hydrogen ion, some electric conductivity and gas permeation cannot be avoided. Then at open-circuit, when no current can be observed through the external circuit, two phenomena can occur at the anode side of a PEM fuel cells, called fuel crossover and internal current. They can be described as follows ... 

Ways to increase membrane durability have been examined by various researchers across the country. Maurtiz et al. examined the use of metal-oxide metal particles to increase the properties of the membrane. A titanium isopropoxide (Figure 11.4) addition to Nation membranes generates quasi-network particles this improves membrane modulus and dimensional stability [17], In addition, the titanium matrix reduces fuel crossover and minimizes chemical degradation. Table 11.2 shows the increase in modulus along with stress and strain and stress changes after the addition of the titanium matrix [17], With a 20% load of the titanium matrix, performance criteria remain comparable. Acid functionality remains intact however, water uptake is reduced as volume inside clusters is occupied. Conductivity is reduced due to chain mobility [17],... 

Produce anhydrous solid membranes having minimal fuel crossover capability and address many of the problems associated with current hydrogen fuel cell technologies. 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

Micro fuel cell designs without polymeric membranes can overcome some PEM-related issues such as fuel crossover, anode dry-out or cathode flooding. In these membraneless laminar flow-based fuel cells (LF-EC) two or more liquid streams merge into a single microfluidic channel. The stream flows over the anode and the cathode electrodes placed on opposing side walls within the channel. The reaction of fuel and oxidant takes place at the electrodes while the two liquid streams and their liquid-liquid interface provide the necessary ionic transport [122,123]. 

As evident from the i-E response, the 1.0 M solution of methanol delivered better performance at low current densities compared with all concentrations of TMM studied at 90 C. However, at very high current densities (>750 mA/cm ) the 0.5 and 1.0 M solutions of TMM shows improved performance with respect to methanol. This type of behavior was observed at a number of different ceil operating temperatures. When the effect of TMM concentration upon cell performance was investigated, it was observed that at low current densities the solutions of low fuel concentration showed less polarization, whereas at higher current densities solutions of higher concentrations showed better performance. This trend in performance is due to fuel crossover effects which dominate at low... 

In order to evaluate the impact of the relative amount of fuel crossover upon the cathode performance, and thus the overall cell voltage, it is necessary to compare the crossover current densities, rather than on a molar basis. In Fig. 1.50, the crossover current densities are compared for ail three fuels, dimethoxymethane, trimethoxymethane. and methanol, of varying concentrations. When the behavior of TMM is contrasted with that of methanol, it is apparent that a 1.0 M methanol solution displays similar crossover characteristics compared with a - 0.3 M solution of trimethoxymethane. This trend is understandable in that one trimethoxymethane molecule is converted to four carbon dioxide molecule in an overall 20 electron electro-oxidative... 

When the fuel crossover rates of a 4" x 6" cell operating on solutions of trimethoxymethane are compared with that of methanol, the same trends are observed as seen in 2 x 2 -size cell designs. As shown in Fig 1.62, solutions... 

To summarize, all of the PSSA-PVDF membrane sample incorporated into both 2 x 2 -size and 1 x 1 -size MEAs and tested in operating direct methanol fuel cells displayed fuel crossover rates which were lower than that displayed by Nation under similar conditions. As illustrated in Table 1.11, the... 

As illustrated in Figs. 1.107 and 1.108, high fuel efficiencies in operating 2 X 2 -size fuel cells equipped with PSSA-PVDF membranes are obtainable, especially at lower temperatures, due to the decreased methanol crossover obsen/ed. The fuel efficiency was determined for all samples at a number of different operating temperatures and ail displayed decreasing fuel efficiency with an increase in temperature, as shown in Fig. 1.107. This effect can be attributed solely to the amount of fuel crossover occurring as a function of... 

The methanol crossover rates present in operating fuel cells was measured by analyzing the CO2 content present in the cathode exit stream. This was accomplished by utilizing an on-line analyzer, which measures the CO2 volume percent in the cathode stream. Before each measurement, the instrument was calibrated with gases of known COg content. The way in which the fuel crossover current density was calculated from the carbon dioxide content detected in the cathode stream is shown below. 

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