I recommend you to use various available relations as function of temperature, It seem that you can consider them ideal gas.

You can find this kind of relations in Appedinx of thermodynamic references like Çengel's or Van Wylen's book.

Cp of mixture is sum of individual Cp of parts of mixture with using molar fractions.

To obtain the detailed knowledge about PEM fuel cell, i suggest you to read the mentioned book.

"Fuel Cell Formulary" by Dr. Alexander Kabza.

Also, this could be achieved in Appedinx of Van Wylen's and yunus A cengel book.