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bubble wall thickness. The model predicted the cell potential vs. current density in both galvanic and electrolytic directions. For each set of EPs, we varied the OPs over a range where the model approximations hold, and identified the Best OPs for each set of EPs. We started with “Base Case EPs”, which we believe are technically feasible today: iCl 0 = 10 mA ; ε = 3 μm; l = 0.178 cm (7 mil); and atmospheric pressure. We showed the cm2 contributions to the total loss vs. current density for each of the four loss mechanisms. Chlorine activation overpotential is the largest loss mechanism for low current densities (i < 400 mA , η > 0.7), whereas for higher current densities the ohmic loss in the Nafion cm2 dominates the losses. We varied each EP (except for iH0 ) systematically while holding all the other EPs at their “Base Case” values, found the best and worst OPs for each set of EPs, and report, for the best OPs for each set of EPs, the maximum power density and the power density at 90% galvanic efficiency. Particularly important to improving high-efficiency operation is increasing the chlorine exchange current density, either by improving chlorine catalysis or by increasing the specific surface area of the chlorine electrode. Particularly important to achieving high maximum power densities is to safely use as thin of a membrane as possible. Assuming mass transport is kept insignificant by proper management of the diffusion layer thickness, or by operating the cell at high pressures, then gains in mass transport will not significantly affect operation at peak power or high efficiency. We identified a set of “More Optimal EPs” representing ambitious targets that we believe to be attainable with further R&D: iCl = 400 mA ; ε = 1 μm; l = 25 μm (1 mil); 0 cm2 and 5 atm pressure. This case provides a reasonable estimate of the upper bound on the performance of a hydrogen-chlorine fuel cell of this type: a cell efficiency exceeding 90% one-way at a galvanic power density of about 1150 mW and at an electrolytic power density of about 1650 mW . The maximum power density of such a cell would be about 5400 mW . cm2 cm2 Although these are ambitious figures, we believe that if the performance of a real system were to approach even half of these values, regenerative hydrogen chlorine fuel cells could become economically viable electricity storage devices. Acknowledgments This research was supported by National Science Foundation grant NSF-IIP-0848366 through Sustainable Innovations, LLC. We thank Dr. Trent M. Molter for helpful discus- sions. One of us (B.H.) was supported by an NSF Graduate Research Fellowship. 25 cm2PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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