I am modeling a Nafion 117 proton exchange membrane and I want to determine the concentration of hydrogen ions inside this membrane when it is fully hydrated.
You can calculate this from the lambda values (water molecules per sulfonic acid) you're using, i.e. around 8 or 9 for fully hydrated Nafion, depending on what reference you use. Considering the pKa is -3 (give or take) in PFSAs, you can assume full dissociation.