Does Ionization of an atom expell heat?

[Jonas Knudsen]

So I was looking into Hydrogen fuel cells, and it says that there is a loss of energy.

But I don't see where the loss is unless thermal energy is expelled in the ionization of the Hydrogen.

[exchemist]

1 hour ago, Jonas Knudsen said:

So I was looking into Hydrogen fuel cells, and it says that there is a loss of energy.

But I don't see where the loss is unless thermal energy is expelled in the ionization of the Hydrogen.

OK this sounds a bit confused. Ionisation of an atom requires an input of energy, because an electron has to be pulled out of the atom, against the electrostatic attraction of the positively charge nucleus. But what you seem to be concerned with the energy losses in the operation of fuels cells. That is another matter.

It is to do with inefficiencies in the conversion of chemical potential energy into electricity. No conversion process is 100% efficient in practice. In fuel cells the theoretical output voltage cannot be obtained in practice due to some irreversible features of the process that occur when current is drawn. Irreversible processes involve entropy increase and that reduces thermodynamic efficiency.  I don't know the ins and outs of fuel cell operation, though maybe someone else here does. For now the best I can so is give you a link that may give you an idea of the kinds of thing involved: https://www.fuelcellstore.com/blog-section/explanation-of-the-thermodynamics-behind-fuel-cell-and-electrolyzer-design