Consider a PEFC operating with the following specifications:
= 40% and the pressure is 4 atm at the anode, and = 20% and the pressure is 6 atm at the cathode.
Assume that the gibbs free energy of formation at standard pressure for at 90o C is -233,265 joules.
a) Calculate the Nernst voltage of the PEFC if the operating temperature is 90o C.
b) Find the change in Nernst voltage if the cathode pressure is reduced to 3 atm, all else the same.
c) Find what the pressure at the cathode would have to be to double the Nernst voltage (yes, it is a big number).
Hint: Check the Mench notes posted on blackboard for help. In that book gibbs free energy is calculated from the specific heat. That is not necessary here since it is given, so the problem should be much easier and shorter than what you see in the book.
The stoichiometry, , is defined as the rate of the reactant supplied divided by the theoretical rate required.
For example suppose 1 mole of hydrogen is being supplied per second. Then we would expect the amount of charge per second (i.e. the current) to be 2*F= 192,970 columbs/second (obviously that is a unrealistically large).
Suppose instead that the current is only about 100,000 A. Then theoretically we should expect that only 100,000/(2*F)=0.52 moles of hydrogen is going through the system. However, due to leakage, crossover, and other losses the theoretical amount is usually not equal to the actual amount.
The stoichiometry is then 1.0/0.52=1.92