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One complicating factor is that the real surface area of the electrode is typically dif- ferent, and sometimes vastly different, than the projected surface area of the electrode. In a PEM fuel cell, electrodes are usually made up of finely dispersed catalyst particles, which have a collective surface area much larger than the geometric area of the electrode. Technically, i0 may also depend on temperature, but we ignore this dependence because the uncertainty in the catalytic activity and because the area multiplier is much more sig- nificant. The exchange current density of real hydrogen electrodes has been studied in detail in the context of hydrogen-oxygen PEM fuel cells. Neyerlin et al. report that for a structured fuel cell electrode, the increase in the effective hydrogen exchange current density over that of a single crystal surface can be as large as a factor of 500 (15). They measured iH0 mA. cm2 The chlorine electrode overpotential, ηCl In galvanic mode, the consumption of Cl2 and production of Cl- result in a depletion of Cl2 near the electrode and an enrichment of Cl-. The opposite occurs in electrolytic mode. For a given current density, the transport behavior of the system stabilizes at a steady-state concentration of reactant and product, so long as there exists a boundary somewhere in the system with a stable concentration and enough time is allowed to reach this steady state. In this case, we can express the concentrations of Cl2 and Cl- near the electrode as a function of current density. The full, concentration-dependent Butler-Volmer equation describes the total chlorine electrode overpotential: i =COs(i)exp(−αfηCl)−CRs(i)exp((1−α)fηCl), [17] iCl Cbulk Cbulk values in the range 250-600 mA . For the base case in this study, we set iH equal to 250 cm2 0 0OR where Cbulk and Cbulk are the bulk concentrations of the oxidized and reduced forms, re- OR spectively, and COs (i) and CRs (i) are their respective concentrations near the electrode sur- face,allinmol.TheoxidizedformisCl andthereducedformisCl-. cm3 2 Measurements in our laboratory indicate that iCl on a flat platinum surface is about 0 half that of hydrogen on the same surface. We are aware of no extensive study examining chlorine exchange current densities in real, structured chlorine fuel cell electrodes. Mea- surements from Thomassen et al. indicate a value for iCl on smooth RuO of 0.01 mA 0 2 cm2 (16). For a structured, high surface-area electrode, we expect a surface area enhancement factor (defined as the real electrode surface area over the projected electrode surface area) of about 1000 is reasonably attainable. Consequently, we assume a value of 10 mA for the cm2 chlorine exchange current density in the “Base Case”, and later we vary this value from 1 to 1000 mA . We expect this range to include values that will be observed in engineered cm2 (i.e. high surface area) fuel cell electrodes. We are interested in separating the losses that arise from mass transport and those that arise from activation of the surface reaction. In order to do this, we define the mass 12PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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