Fuel cells have to run hot, which is hard on materials.
To be specific, solid-oxide fuel cells have to run hot enough to mobilize oxygen ions in the ceramic electrolyte. This is the principle that automotive oxygen sensors have long used. Other hydrogen-ion conductors like Nafion can operate fairly close to room temperature.
How a dual oxygen/hydrogen ion conductor avoids the two ions neutralizing each other on their way through the material I don't know.