Fuel cell illustration

The example of a hydrogen/oxygen fuel cell illustrates this relationship. 

Figure 2. Typical potential current curve for the hydrogen-oxygen fuel cell illustrating the different power losses as the current drain increases...

Figure 33 Typical plot of cell potential vs. current for fuel cells illustrating regions of control by various types of overpotentials [317].

Figure 735 Tubular, monolithic and planar geometries of solid oxide fuel cells. Illustration courtesy of M. Muller (Muller, 2001).

Fig. 15.15 (a) Enhanced activity (x3) under H2IO2 for pre-leached PtCo/C electrocatalysts compared to Pt/C as tested in subscale fuel cells [42] (b) electrochemical area loss forPt/C versus PtCo/C catalysts over 10,000 cycles in subscale fuel cells illustrates lower degradation for the PtCo/C [39]... 

Raising the cell temperature. This fully explains the different shape of the voltage/current density graphs of low- and high-temperature fuel cells illustrated in... 

These various miniature fuel cells illustrate some of the work that is ongoing, but which is still in the very early development stages. Although initial prototypes have been built, none has yet been definitely demonstrated as a viable, compact device that would be competitive in performance, cost and convenience with a battery. The objective remains a challenging, but potentially fruitful goal. 

Although flooding is a localized phenomenon, for a fuel cell to operate at steady state, an overall state of water balance must be achieved. Consider a control volume mass balance of the water in a single fuel cell, illustrated in Figure 6.18. 

A complete fuel cell system, even when operating on pure hydrogen, is quite complex because, like most engines, a fuel cell stack cannot produce power without functioning air, fuel, thermal, and electrical systems. Figure 3 illustrates the major elements of a complete system. It is important to understand that the sub-systems are not only critical from an operational standpoint, but also have a major effect on system economics since they account for the majority of the fuel cell system cost. 

Several of the gas turbine cycle options discussed m this section (intercooling, recuperation, and reheat) are illustrated in Figure 4. These cycle options can be applied singly or in various combinations with other cycles to improve thermal efficiency. Other possible cycle concepts that are discussed include thermochemical recuperation, partial oxidation, use of a humid air turbine, and use of fuel cells. 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

The single cell thus fabricated was placed in a single chamber station as illustrated in Fig. 2. A humidified mixture of methane and oxygen was supplied to the station so that both electrode compartments were exposed to the same composition of methane and oxygen. For the measurement of the cell temperature, a thermocouple (TC) was placed approximately 4 mm away from the cathode site. For the evaluation of the fuel-cell performance, Ft wires and Inconel gauzes were used as the output terminals and electrical collectors, respectively. 

The Brouwer diagram approach can be illustrated with reference to the perovskite structure oxide system BaYbvPr VC>3, which has been explored as a potential cathode material for use in solid oxide fuel cells. The parent phase... 

The use of this approach can be illustrated by the perovskite structure proton conductor BaYo.2Zro.gO3 g- This material has been investigated for possible use in solid oxide fuel cells, hydrogen sensors and pumps, and as catalysts. It is similar to the BaPr03 oxide described above. The parent phase is Ba2+Zr4+03, and doping with... 

In the EPD process, a DC electric field is used to deposit charged particles from a colloidal suspension onto an oppositely charged substrate, as illustrated in Figure 6.8. The graphite rod used for the deposition substrate is later burned out prior to cell operation, leaving a hollow tube. The other fuel cell layers can be deposited by a similar process onto the anode support tube. 

One way to illustrate the effect of the EDL is to compare in situ electrochemical reactions with their equivalent UHV counterparts. Due to their roles in fuel cells, the methanol oxidation reaction and the oxygen reduction reaction are two such reactions for which numerous in situ and UHV experiments have been performed. 

The SEA method can also be applied for the synthesis of bimetallic catalysts. For illustration, the potentially high impact area of bimetallic catalysts for fuel cells will be discussed. 

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential, thereby reducing the cell voltage. Figure 2-3 illustrates the contribution to polarization of the two half cells for a PAFC. The reference point (zero polarization) is hydrogen. These shapes of the polarization curves are typical of other types of fuel cells. 

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

The MCFC provides a good example to illustrate the influence of the extent of reactant utilization on the electrode potential. An analysis of the gas composition at the fuel cell outlet as a function of utilization at the anode and cathode is presented in Example 10-5. The Nemst equation can be expressed in terms of the mole fraction of the gases (Xi) at the fuel cell outlet ... 

Component enthalpies are readily available on a per mass basis from data such as JANAF (4). Product enthalpy usually includes the heat of formation in published tables. A typical energy balance calculation is the determination of the cell exit temperature knowing the reactant composition, the temperatures, H2 and O2 utilization, the expected power produced, and a percent heat loss. The exit constituents are calculated from the fuel cell reactions as illustrated in Example 10-3, Section 10. 

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