Cathodic stoichiometry

Figure 22. Local current density profiles along the channel direction for different humidification levels at Ueii = 0.65 V. Anode and cathode stoichiometries are 1.4 at 1.0...

Figure 34. Comparison of calculation (lines) and experimental (symbols) results for cathode stoichiometry of 2.0 at 0.75 A/cm and fully humidified anode and cathode (a) average polarization curve and (b) current distribution. Different colors in panel b represent various cell voltages as defined in the figure legend. ...

Figure 43. Calculated current—voltage characteristics and power density curve of electrolyte-supported co-flow SOFC at an operating temperature of 1000 °C and anode and cathode stoichiometry of 1.5 and 2.0 at 0.4 A/cm, respectively.

Figure 45. Current distribution (A/m ) at a cell potential of 0.4 V in the five-channel cross-flow electrolyte-supported SOFC (A/m ) under anode and cathode stoichiometries of 1.5 and 2.0, respectively, and a cell temperature of 1000 °C.

Figure 31.10 Calculated polarization and power curves of two-channel serpentine PEFC at a cell temperature of 80°C, a pressure of 1.5 atm, fully humidified inlets, and an-ode cathode stoichiometry 2 at lAcm [33].

Figure 31.20 Comparison of model prediction (lines) and experimental (symbols) results for cathode stoichiometry of 2.0 at 0.75 A cm and full humidification [48].

Other factors affecting the water balance are the hydrogen utilisation in the fuel cell anode and the oxygen stoichiometry on the cathode side. Increasing hydrogen utilisation requires a surplus of cathode air and consequently cathode stoichiometry needs to be increased. This dilutes the burner off-gas, which has a detrimental effect on the water balance of the fuel cell/fuel processor system [435]. 

Recommended fuel cell test conditions include the use of pure hydrogen and air/oxygen, humidified at 80 °C to assure 100 % relative humidity at an anode stoichiometry of 2 and cathode stoichiometry of 9.5. Both electrodes should be maintained at a backpressure that results in a 1.0 bar partial pressure of the gases. Testing is typically carried out a cell temperature of 80 °C. 

As Table 20.3 shows, the necessary cathode stoichiometry is between 11 and 18 for a temperature difference of 135 K between incoming and exiting air. Using a meander type flow field with... 

FIGURE 14.95 Discharge of lithium/silver vanadium oxide. Voltage is shown versus equivalents of lithium for a cathode stoichiometry of AgV205 5. Ref. 46.)... 

Fig. 4 Long-term operation of identical metallic bipolar plates uncoated 316L, low-cost coating and gold plated. DANA standard conditions were as follows constant current operation at 250m cm , 80°C, fuUy humidified, anode stoichiometry 2, cathode stoichiometry 5...

These membranes also showed excellent high-temperature performance in fuel cells when tested with dry hydrogen/air and hydrogen/oxygen gases at 160 °C. The polarization curves are shown in Fig. 18 using anode and cathode stoichiometries of 1.2 and 2.5, respectively. Exceptional long-term stability was demonstrated in cell performance tests. 

To avoid confusion the reader should be aware that other symbols for stoichiometty, besides A, are commonly used in the literature, including f and The theoretical rate of reactant required is calculated by Faraday s law, and the actual rate of reactant dehvered is a funchon of the fuel or oxidizer delivery system. One important point is worth mentioning Fuel cells must always have an anode and cathode stoichiometry greater than 1. For a value less than unity, the current specified could not be produced. For reasons explained in Chapter 4, a stoichiometry of exactly 1 is not possible either, so that a Faradic efficiency of 100% is not possible on the anode or cathode for a single pass of reactant. ... 

Example 2.4 Stoichiometry and Utilization Consider a portable 20 cm active area fuel cell operating steadily at 0.75 V, 0.6 A/cm. The fuel utilization efficiency is 50%, and the cathode stoichiometry is 2.3. The fuel ceU is expected to run for three days before being recharged. The cathode operates on ambient air, and the anode runs off of compressed hydrogen gas. 

Faradic efficiency is often called the fuel utilization efficiency (p./) when applied to the fuel in a galvanic redox reaction. The anode and cathode stoichiometries are defined as follows ... 

Given a flow inlet to a fuel ceU cathode at 2 atm, 75% RH, 90°C, and at a cathode stoichiometry of 2.0, determine the maximum possible molar rate of water uptake into the cathode flow if a 100-cm active area fuel ceU is operating at 1.2 A/cm. You can assume the flow rate, pressure, and temperature are constant in the fuel ceU. 

Given a fuel ceU stack with each cell generating 1 A/cm and a geometric area of 150 cm per plate, if the air flow in the cathode inlet RH is 25% at 80° C and 1.5 atm, what must the cathode stoichiometry be to remove exactly the amount of water generated by reaction as water vapor at 80°C You can assume the flow rate, pressure, and temperature are constant in the fuel ceU and the exit RH is 100%. 

Figure 4.18 Typical polarization curve for low-temperature PEFC. Despite the use of an expensive platinum catalyst, there is still significant activation polarization. The fuel cell is operating at 65°C, with zero back pressure, 100% RH on anode and cathode, anode stoichiometry of 1.5, and cathode stoichiometry of 2.0.

Given a 0.5-m -active-area PAFC 250-cell stack operating at 150 mA/cm, a cathode stoichiometry of 2.0, and an anode stoichiometry of 1.3, (a) plot the amount of electrolyte lost to the anode and cathode flow over 40,000 h operation as a function of H3PO4 ppm vapor pressure and (b) use another resource to find the vapor pressure of H3 PO4 as a function of temperature from 160 to 200°C and discuss the difference in onboard stored H3PO4 that can be achieved from reducing operating temperature from 200 to 175°C. 

In another study by Wu et al. (2005), the optimal operating conditions based on validated multiresolution fuel cell simulation tool has been developed with four control parameters including cell temperature, cathode stoichiometry, pressure, and humidity. The study shows that different optimal solutions exist for different system assumptions, as well as different current loading levels, classified into small, medium, and large current densities. This design can be readily applied to a larger number of control parameters and further to the fuel cell design optimizations. 

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