Effect of inlet hydrogen temperature

Great question!
First off, what "high temperature" are you running the cell at? I assume somewhere in the range of 300C... that's roughly normal operating temps for PEMFC's if I remember correctly, but I could be wrong.
I work on SOFC's, and operating temperature is normally about 800C for those types of cells. If you have high enough flow rates at higher temperatures, then preheating inlet fuel gas is necessary for optimal performance of a SOFC.
I don't know the exact adverse effect colder H2 would have on a PEMFC, basically because I would need more info (flow rates, working temp of the cell, hydrogen concentration [if you are diluting H2 in N2 or some other inert gas]), but I would assume that if temperatures and flow rates are low enough, it wouldn't have too much of an adverse affect on the cell.

- Dglock18