Question 2 (25%). Alkaline hydrogen fuel cells have been used by NASA for their

Question

Question 2 (25%). Alkaline hydrogen fuel cells have been used by NASA for their Gemini, Apollo and the space shuttle programs (https://www.nasa.gov/content/space-applications-of-hydrogen-and-fuel cells). The anode and cathode reactions in an alkaline hydrogen fuel cell are given by: Cathode: a,0,tb, H20+c,e-d, OH. The enthalpy of formation, standard molar entropy and Gibbs free energy of formation for reactants and products at 298K and latm is given in Table 2 Table 2: Enthalpy of formation, standard molar entropy and Gibbs free energy of formation at SATP Compound Absolute entropy, [J/(mol K)] 130.57 70.0 205.0 Enthalpy of formation, [kJ/mol] Gibbs free energy of formation, [kJ/mol] 2(g) 285.83 237.18 Answer the following a) Determine the stoichiometry of the reactants and products in the anode and cathode reaction. b) Determine the open cell voltage of the fuel cell assuming that it is operating at 25°C and 1 atm. c) Determine the theoretical efficiency of the fuel cell when it is operating at 80°C, 1 atm H2 in the anode, and 1 atm air in the cathode.

Explanation / Answer

i) the stoichiometry of the reactants and products in the anode and cathode reaction are

The balanced equaiton will be:

H2 + 2OH- ---> 2H2O + 2e

aa=1

ba=2

ca= 2

da=2

O2 + 2H2O + 4e ---> 4OH-

ac= 1

bc=2

cc=4

dc = 4

Overall equation will be:

2H2 + O2 ---> 2H2O

DeltaG = [DeltaG of products - Delta G of reactants] = [2 X -237.18 ]= -474.36 kJ / mole

DeltaG = -nFEcell

Ecell = 474.36 X 1000 / 4 X 96485 = 1.94 V

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