[eloiamb]

First time post here, I hope this is a suitable forum.

To find a maximum theoretical fuel cell efficiency, it is common to take the Gibbs free energy of formation and divide by the heat of formation.

For a hydrogen oxygen fuel cell this equation results in -237.192 [kJ/mol] / -285.840 [kJ/mol] = 0.8298 or 82.98% theoretical maximum efficiency.

For a hydrogen chlorine fuel cell, Gibbs free energy of formation is -95.30 [kJ/mol] and the heat of formation is -92.31 [kJ/kmol]. This results in a maximum theoretical efficiency of 103.2%. This of course is not possible.

Where is the discrepancy? I know a lot of reasons why a fuel cell could never reach its maximum efficiency however I can't figure out why the heat of formation is greater than the Gibbs free energy of formation for HCl.

If it helps the half reactions are: H2(g) -> 2H+(g) + 2e- 2Cl2(g) + 2H+ (g) + 2e- -> 2HCl(g)

Any thoughts would be greatly appreciated.

[dipesh747]

I don't know about this at all to be honest. But what came across my mind is 2 things,

1. Is the equation you are using correct for this instance?
2. Where did you get the numbers from? If they are experimental have you calculated standard error and seen if it goes below 100% when you include the low errors?