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the electrical energy per charge spent in doing the electrolysis, E(i): 1 : i = 0; E(i) :i>0; cell efficiency = Eeq p=i·E(i). [2] Because of the high cost of fuel cells per unit cell area, cells having high efficiencies at large power densities are desirable. The cell potential deviates from its equilibrium value due to several loss mechanisms, all of which lead to the generation of heat in the cell. We identify four overpotentials in the cell: the hydrogen and chlorine electrode overpotentials (ηH and ηCl; the latter actually includes two losses: one due to electrode activation and one due to mass transport limitations at the chlorine electrode), and the membrane resistance overpotential (ηR). Each overpotential is a function of current density and depends on the OPs and a subset of the EPs. The overall cell potential can thus be expressed as the equilibrium cell potential minus the individual losses: E(i) = Eeq −ηR(i)−ηH(i)−ηCl(i), [3] where all of the above quantities are in volts. Eeq is a function of temperature and of the activities of the reactants and products, which themselves depend on temperature, pressure, and concentration. ηR is determined by the conductance of the membrane, which depends on temperature, acid concentration, and membrane thickness. The overpotentials at the two electrodes arise from two different effects: an activation loss due to the kinetics involved with the electron transfer at the surface, and a mass transport loss due to the depletion of the reactants and enrichment of the products near the electrode surface at non-zero current densities. We ignore the mass transport effect at the hydrogen electrode due to presumed fast transport of gaseous hydrogen, whereas we include the chlorine mass transport because of the relatively slow transport of Cl2 (aq) in aqueous solution. The equilibrium potential, Eeq The equilibrium potential is that of the combined half-cell reactions, where all poten- tials are relative to that of a standard hydrogen electrode (SHE): H2(g) 2H+(aq) + 2e− Eeq = 0.000 V; [4a] Cl2(aq) + 2e− 2Cl−(aq) Eeq = 1.358 V. [4b] [1] :i<0. The power density, in mW , is the amount of power produced per cell area. It is equal to the cm2 cell potential multiplied by the current density: Eeq E (i) 7PDF Image | Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale
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