Fuel cell voltage

Electrical management, or power conditioning, of fuel cell output is often essential because the fuel cell voltage is always dc and may not be at a suitable level. For stationai y applications, an inverter is needed for conversion to ac, while in cases where dc voltage is acceptable, a dc-dc converter maybe needed to adjust to the load voltage. In electric vehicles, for example, a combination of dc-dc conversion followed by inversion may be necessary to interface the fuel cell stack to a, 100 V ac motor. 

Fuel cell vehicles, 73 800-801 Fuel cell voltage equation, 72 208 Fuel cell voltages, 72 206-209 versus current densities, 72 208 Fuel cooling, in pressurized water reactors, 77 544... 

Ohmic losses, in fuel cell voltages, 12 207 Ohmic polarization, batteries, 3 425—426 Ohnesorge number, 23 183, 190 Oil absorption, by silica, 22 371 Oil additives... 

Figure 2-2 Ideal and Actual Fuel Cell Voltage/Current Characteristic...

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

Pt is, however, an expensive and limited resource. For a 60 kW fuel cell vehicle, the cost of Pt would be over 2,400 at current cost and loading of Pt. Even worse, replacing combustion engines in all existing vehicles by fuel cell drive systems at no penalty in power would exceed the known reserves of Pt. Catalyst layer design, therefore, strives to reduce the Pt loading markedly at no penalty in the fuel cell voltage. 

Plot of fuel cell voltage versus current density showing the transition between two principal states of operation corresponding to ideally wetted conditions with primary pores filled and secondary pores available for gas diffusion and fully flooded conditions. In the depicted case, the transition involves a bistability. 

Figure 15.10 Hydrogen/oxygen fuel cell voltage and efficiency as a function of the current drawn.

Here, the cathode potential is dependent on the water phase. If the water is liquid, its activity is usually considered to be 1. In this case, the fuel cell voltage... 

The OCV seems to be of less practical use. It is much more useful for a fuel cell to work under a load, with power being delivered to the users. With the electric energy output, the fuel cell voltage will decrease as the electric current load increases. A popular way to evaluate a fuel cell is to measure its polarization curve (abbreviated as I-V curve ), which is a plot showing the cell voltage change with current or current density. Figure 1.20 shows a typical polarization curve obtained with a PEMFC. 

In a fuel cell, Ia should equal Ic, and then the fuel cell voltage (VceU) can be expressed as... 

FIGURE 12.2 Schematic view of various overpotential losses ideal and apparent fuel cell voltage-current characteristics. 

The theory was also used to explore novel design ideas. It was predicted that functionally graded layers with enhanced ionomer content near the membrane interface and correspondingly reduced ionomer content near the GDL side would result in better performances compared to standard CCLs with uniform composition [122]. This prediction was recently verified in experiment [123]. Correspondingly, fabricated catalyst layers with a three-sublayer structure result in enhancements of fuel cell voltage by 5-10%. 

Figure 21. Long-term test of the performance stability of the PtRu2o electrocatalyst in an operating fuel cell. The fuel cell voltage at constant current of 0.4 A cm is given as a function of time for the electrode of 50 cm with an anode containing to 0.18 mg Ru cm and 0.018 mg Pt cm / (approximately 1/10 of the standard Pt loading) and a standard air cathode with a Pt/C electrocatalyst. The fuel was clean H2 or H2 with 50 ppm of CO and 3% air temperature 80 C.

Figure 12.6 Fuel cell voltage and power density versus current density.

The performance of a fuel cell is characterized by its output voltage and current density, which is defined as the current per unit area of the cell. The fuel cell voltage drops at higher currents due to increasing catalytic activation losses, ionic and electronic resistances in the cell, and mass transport limitations. The cell efficiency is therefore proportional to the ratio of measured voltage to the ideal cell voltage (1.23 V and 1.21 V for hydrogen and methanol at 25 °C, respectively). 

The H30 -P7P-Al203 of the 6N3-type was used in a reversible steam electrolysis/fuel cell unit and the results are shown in Fig. 33.9. The efficiency at the high temperatures (200-300 °C) is substantially below theoretical due to the dehydration of the electrolyte. As this is reversible, a high-pressure steam cell unit was constructed and initial trials showed a substantial improvement in steam-electrolysis/fuel-cell voltage/current-density characteristics. 

The fuel cell voltage as a function of current density can be seen in Figure 2. The value of 1,2 V represents a theoretically loss-free voltage. We can see that the actual cell voltage (incl. the open circuit voltage) is always lower than this value. The curve shown in Figure 2 is important with respect to efficiency as the rule states the directly proportional relationship between the efficiency and the fuel cell voltage. 

The shape of the fuel cell voltage-current density characteristic shown in Figure 2 is the result of four basic losses which are in detail described below. 

Fig. 8.16 Bottom H2-O2 fuel cell polarization plots recorded with 4 mg cm of a PANI-FeCo-C catalyst in the cathode. Performance of an H2-air fuel cell with a Pt cathode (0.2 mgp, cm ) is shown for comparison. Top Long-term perframance stability test of the PANI-FeCo-C catalyst in a H2-air fuel cell at a constant fuel cell voltage of 0.40 V. Anode and cathode gas pressure 2.8 bar, anode loading 0.25 mgp, cm cell temperature 80 °C (reprinted from ref [57] with permission from AAAS)...

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