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sets the equilibrium partial pressure of HCl(g) in the bubble, which in turn maintains the thin film interface concentration at the bulk molarity. We assume the addition of new HCl(g) does not appreciably affect the partial pressure of HCl(g) in the bubble, due to the bubble’s large volume with respect to the HCl(g) generated and the bubble’s ability to expand longitudinally to take up any small pressure increase. The surface concentration is then modeled by: CRs(i)=M+ε i [28] 1000 FDHCl 1000 Note that the sign has changed (relative to Eq. 20) because positive fluxes of Cl- require negative current densities. Also, a factor of two is absent from the second term because there is one charge per chloride ion, as opposed to two charges per chlorine molecule in the previous case. We fit the data of James and Gordon (27) for the diffusion coefficient of Cl- in 0.1 M hydrochloric acid to an Arrhenius function, resulting in −0.170 kB(T +273.15) DCl− = 0.0217exp where the prefactor is in cm2 and the activation energy is in eV. We assume the diffusion s coefficient is independent of HCl(aq) concentration (27). Model Results and Discussion The model calculates the cell potential according to Equation 3, accounting for the various effects of concentration, temperature, and pressure on each of the overpotentials. We first consider the variation of the current density-voltage relationship with operating parameters for “Base Case” engineering parameters: iCl = 10 mA , ε = 3 μm, l = 0.178 cm 0 cm2 (7 mil), and pCl2 = pH2 = 1 atm. Figure 5a shows calculated cell potentials for the Base Case EPs for three sets of OPs. The curve labeled “Standard OPs” is the cell potential at standard conditions of 25 ◦Cand 1 M HCl. The two most relevant performance characteristics are the cell efficiency-vs.-current- density function and the maximum cell power density. On one hand, high cell efficiency is paramount for energy storage devices because lost energy is lost revenue. It is imperative that a storage device be able to operate at high efficiencies at reasonable current densities. On the other hand, operating at higher power densities reduces the capital cost for a given power-delivery capability, because one may buy less cell area for an equivalent power. The maximum power density also permits a determination of the minimum membrane area (and associated cost) necessary to achieve a required system power. In any real storage system (used to levelize wind power, for example), the cell will operate over a distribution of current densities, constantly ramping up and down, depending of course on how much power is being generated by the turbines. Thus, both the maximum power density and the cell efficiency are important, so we explore both of these characteristics in this model. The maximum power also serves as a convenient scalar proxy for the cell efficiency- vs.-current-density function, as the two performance characteristics are correlated, as is 16 , [29]PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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