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tential starts to increase, as does the Nafion conductivity. The conductivity of the Nafion peaks at about 2.5 M and then starts to fall off again with decreasing molarity, whereas Eeq continues to rise. The competition between these two effects after the peak Nafion conductivity results in a maximum at about 2.3 M. It is also important to note that the maximum power surfaces and the surfaces repre- senting the power density at 90% efficiency are roughly the same shape for both the Base Case cell and the More Optimal case cell. This intuitively makes sense: operating condi- tions that permit high-efficiency operation will also provide the highest possible maximum power densities. In Figure 8, we report the variation of the maximum peak power (the height of the highest point in Figure 7) as each of the engineering parameters (other than the hydrogen exchange current density, which is fixed at iH = 250 mA ) is varied away from the Base 0 cm2 Case while keeping the other four EPs fixed. For each set of EPs a surface similar to that in Figure 7 is generated, and the “Best OPs” and “Worst OPs” for that set of EPs are recorded in Figure 8. The latter are included as an indication of the performance sensitivity to OPs. In Figure 8a we varied iCl from the certainly attainable value of 1 mA , to the likely 0 cm2 unattainable value of 1000 mA . For this set of EPs there are significantly diminishing cm2 returns for efforts to increase iCl beyond the Base Case, but decreasing iCl below the Base 00 Case causes a degradation in performance. In Figure 8b we show how performance rapidly increases with decreasing membrane thickness, due to decreasing membrane resistance. Nafion is currently commercially avail- able in thicknesses from 25 to 250 μm, but engineering a fuel cell with the thinnest mem- brane can be difficult, as it serves as the only separator between two highly reactive gases. As we refer back to Figure 6, we see that, in the absence of significant mass transport loss, the resistance overpotential dominates the other overpotentials at high current densities, including current densities at which the maximum power densities are located. In Figure 8c we show that the power performance declines considerably for large val- ues of the acid layer thickness ε; this occurs because of small limiting current densities. As ε is decreased, the gain in power is roughly linear until a rapid upturn at about 2 μm. This results from a trade-off between mass transport and resistance loss. For ε > 2 μm, mass transport loss dominates the temperature sensitivity; hence the optimal operating temperature is driven down for increased Cl2 solubility. As ε drops below 2 μm, the mass transport loss has become small enough that higher cell temperatures are selected to lower the membrane resistance. When ε = 0.5 μm, the optimal temperature has increased to 67.5 ◦C, where the membrane conductivity is significantly higher over the entire molarity range. We thus see that, as mass transport is effectively made less important by decreasing the size of ε, the optimal temperature for cell operation increases. In Figure 8d we show how the cell performance depends on the pressure of H2 and Cl2 gases (we always maintain pCl2 = pH2 ). Higher pressure gives a modest boost to the open circuit potential. However, this effect is dominated by a covariant conductivity increase in the PEM. Increasing the pressure raises the solubility of Cl2(g) proportionally, which decreases the need to go to low temperature for solubility enhancement, thereby allowing 20PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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