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JoeMama
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- Homework Statement
- Consider a Fuel cell working via reaction: H_2 + (1/2)O_2 -> H_2 O Derive an expression for the quantity (dE_dP)_T assuming that all the working fluid streams into and out of the fuel cell are at a pressure of 20 bar.
Note this is the rate of change of the EMF with respect to pressure at constant temperature. State clearly your assumptions
- Relevant Equations
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The quantity (dE_dP)_T can be derived using the Gibbs-Helmholtz equation which states that dE = TdS - PdV + μdN where E is the internal energy of the system, S is the entropy, V is the volume, N is the number of particles and μ is the chemical potential. Differentiating this equation with respect to pressure at constant temperature gives (dE_dP)_T = T(d^2S/dPdT) - V.
For a fuel cell working via reaction H2 + 0.5O2 -> H2O, we can use the Nernst equation to calculate the cell potential Ecell = E°cell - (RT/nF)ln(Q) where E°cell is the standard cell potential, R is the gas constant, T is temperature in Kelvin, n is the number of electrons transferred in the reaction and F is Faraday’s constant. Q is the reaction quotient which can be calculated as Q = (PH2O)0.5/(PH2)(PO2)0.5 where PH2O, PH2 and PO2 are partial pressures of water vapor, hydrogen and oxygen respectively.
The change in cell potential with pressure at constant temperature can be calculated using (dEcell/dP)_T = -(RT/nF) [(dlnQ/dP)_T] where Q depends on partial pressures of reactants and products.
Substituting Q into this equation and simplifying gives (dEcell/dP)_T = -(RT/nF) [(1/4)(dln(PH2O)/dP)_T - (1/2)(dln(PH2)/dP)_T - (1/4)(dln(PO2)/dP)_T]
For a fuel cell operating at a pressure of 20 bar, we can substitute PH2O = 20 bar and PH2 = PO2 = 10 bar into this equation to obtain (dEcell/dP)_T = -0.059 V/bar.
For a fuel cell working via reaction H2 + 0.5O2 -> H2O, we can use the Nernst equation to calculate the cell potential Ecell = E°cell - (RT/nF)ln(Q) where E°cell is the standard cell potential, R is the gas constant, T is temperature in Kelvin, n is the number of electrons transferred in the reaction and F is Faraday’s constant. Q is the reaction quotient which can be calculated as Q = (PH2O)0.5/(PH2)(PO2)0.5 where PH2O, PH2 and PO2 are partial pressures of water vapor, hydrogen and oxygen respectively.
The change in cell potential with pressure at constant temperature can be calculated using (dEcell/dP)_T = -(RT/nF) [(dlnQ/dP)_T] where Q depends on partial pressures of reactants and products.
Substituting Q into this equation and simplifying gives (dEcell/dP)_T = -(RT/nF) [(1/4)(dln(PH2O)/dP)_T - (1/2)(dln(PH2)/dP)_T - (1/4)(dln(PO2)/dP)_T]
For a fuel cell operating at a pressure of 20 bar, we can substitute PH2O = 20 bar and PH2 = PO2 = 10 bar into this equation to obtain (dEcell/dP)_T = -0.059 V/bar.