Anodic stoichiometry

A generation 10-kW class PEM stack with compression molded bipolar plates has been completed and tested, and anode stoichiometry of 1.15 has been demonstrated. 

In order to test the performance of the compression-molded bipolar plates, a 63-cell subscale stack was built initially and tested. The performance of the subscale stack at the nominal power condition is shown in Figure 5. The test conditions are described in detail in Table 1. At a current density of 0.115 A/cm, the stack was able to stably operate at an anode stoichiometry of 1.15. The single cell voltage at this point was measured at 0.74 V. This results in slightly higher performance than previously seen with machined bipolar plates. 

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. 

Fig. 21.22 HTPEM short stack voltage response to a step in the anode stoichiometry...

The reforming process requires heat, which is delivered by a burner that can either be fed with fuel from the tank or with recycled anode waste gas from the fuel cell [19]. Supplying the reformer with heat solely from the combustion of recycled anode waste gas is the basis for an efficient fuel cell reformer process. The anode stoichiometry (Aa,min) required to satisfy the heat demand is determined by the specific enthalpy of the reforming reaction (AH°) for each fuel (see Table 23.2). The efficiency of heat transfer fi-om the burner gas and the reformer is not included in this theoretical value because it depends on the individual hardware design. 

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...

The CO-AB optimization process involves three steps (1) determine an AB range for a reformate of certain CO and concentrations (2) measure VDR and ERR at selected AB levels (3) select the optimum AB set point on the basis of VDR/FRR data and cell lifetime expectations. The optimization should consider two additional factors hydrogen safety and anode stoichiometry. The hydrogen safety requirement (Air Prodncts and Chemicals 2004) limits the use of AB to less than 15% as an effective mitigation method for no higher than 200 ppm CO at a Pt-Ru anode. When high AB is nsed, the anode stoichiometry should be adjusted to compensate for the consnmed by the excess to avoid fuel starvation. 

Figure 4b shows an AB-VDR chart from endurance tests using a lOOppm CO reformate. It shows a distinct minimum near 10% AB. The minimum FRR is between 9 and 10% (see Table 1). At 12% AB, the drastic increase in VDR is attributed to the stoichiometry effect the actual anode stoichiometry is approximately 1.09, significantly lower than the 1.2 set point because of significant consumption by the excess O from the AB. At an anode stoichiometry of 1.09, the cell is at the edge of fuel starvation. Indeed, the cell tripped multiple times because of low cell voltage as a result of partial fuel starvation. 

Consider a 10-plate fuel cell stack at an anode stoichiometry of 1.2 with 20 A current generated in the stack and a stack voltage of 6.0 V. As an engineer, you have a choice to install a recirculation pump to recycle the unused hydrogen from the anode exhaust back into the anode to increase the effective fuel utilization to 100%. However, the pump required 60 W of parasitic power to operate continuously. Is installation of the pump justified Explain. At what value of parasitic power does the addition of the pump become unjustified ... 

Example 3.8 Calculatton of Maximum Water Uptake in a Flow Given a flow inlet to a 5-cm active area fuel cell at 3 atm, 50% RH at 80°C, and an anode stoichiometry of 3.0. Determine the maximum possible molar rate of water uptake from the incoming anode flow if the fuel cell is operating at a current density of 0.8 A/cm. You can assume the flow rate, pressure, and temperature are constant in the fuel cell. 

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.

Example 7.2 Loss of PAFC Electrolyte by Vaporization Over Time Given a 0.5-m -active-area PAFC operating at 150 mA/cm and an anode stoichiometry of 1.4, determine the amount of electrolyte lost to the anode flow over 40,000 h operation. Assume the equilibrium concentration of H3PO4 in vapor form is 3 ppm at the operating temperature of 200°C. The anode flow is pure hydrogen. 

Estimate the maximum rate of hydrogen availability that would be required to supply a 100-kWe maximum output PEFC stack for an automotive application in grams in per second. Assume an anode stoichiometry of 1.2. 

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