Please see attached. The switch is in its "discharge" mode. And yes, I did that in Altium
What you are looking for is a standard "switched cap boost" circuit. Usually, they do this to create a supply that is (nearly) double the input voltage. This is done with a few more diodes and FETs, and fewer switches. Also, they flip the "switch" at about 100kHz (or more).
Be sure that the Fuel cell doesn't mind getting double its output voltage shoved in its face.
Tie this circuit in parallel to your battery.
Good luck!