Rates fuel-cell operation

The operating temperature also affects the fuel cell operating potential, A high operating temperature accelerates reaction rates but... 

Type of fuel cell Operating fuel and temperature Power rating (kW) Fuel efficiency s (%) Power density (mW/cm ) Lifetime s (hr) Capital cost s ( /kW) Applications... 

Cyclic voltammogram recorded for the cathode of a membrane electrode assembly with a cathode Pt loading of 0.4 mg/cm in a fuel cell operated at 80°C and 100% RH. Cathode Nj anode Hj scan rate 50 mV/s. (Unpublished data from the authors.)... 

For each thickness, at least 10 different flow rate measurements were obtained in order to cover the range of flow rates that a DL experiences during normal fuel cell operation. To obfain fhe corresponding permeabilify, fhe pressure drop resulfs were ploffed as a function of the mass flow rate. After this, the Forchheimer equation was fitted to the plotted data to determine the viscous and inertial permeabilities. As expected, the in-plane permeabilities of each sample DL maferial decreased when the compression pressure was increased. It is also important to mention that these tests were performed in two perpendicular directions for each sample in order to determine whether any anisotropy existed due to fiber orienfation. 

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

Under concentration control, the reversible hydrogen electrode exhibits Nemstian reversibility. This provides for a potential shift of 29.75 mV at room temperature, which translates to a shift of 46.8 mV at 200 °C for each decade of change in hydrogen concentration. Under fuel-cell operating conditions with highly dispersed electrocatalysts, it is possible to approach the kinetic rate determined by the dual-site dissociation of the hydrogen molecule, viz. ... 

When connected through an external circuit, the net result of these two half-cell reactions is the production of H2O and electricity from H2 and O2. Heat is also generated in the process. In the absence of a proper catalyst, however, neither of these two half reactions takes place at meaningful rates under PEM fuel cell operating conditions (50 to 80°C, 1 to 5 atm). Despite decades of effort in search of cheaper alternatives, platinum is still the catalyst of choice for both the HOR and ORR. 

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. 

This oxyhydrogen torch uses hydrogen as its fuel and oxidizes hydrogen to water in a vigorous combustion reaction. Like the torch, fuel cells also oxidize hydrogen to water, but fuel cells operate at a much more controlled rate. 

As discussed above, the electrochemical oxidation of a fuel can theoretically be accomplished at very high efficiencies (e.g. 96% for gas-phase product water or 83% for liquid product water for the H2/O2 reaction at 25 °C, see Fig. 8.3) as compared to heat engines utilizing the combustion of a fuel. However, in practice, fuel cells experience irreversible losses due to resistive and reaction kinetic losses (see Fig. 8.4), and efficiencies of fuel cell stacks rarely exceed 60% at rated load. The irreversible losses appear as heat and, for example, a 1 kW fuel cell operating... 

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