Fluid displacement apparatus with a set of opposing pistons in traveling chambers and a set of opposing pistons in stationary bores; a second embodiment with all pistons in traveling chambers; a third embodiment with all pistons in stationary bores. In each embodiment the displacement of the pistons can be different and the displacement can be variable. The apparatus is connected to a fuel cell stack to supply compressed gaseous fluid from a portion of the chambers and receive exhaust from the fuel cell stack in other chambers to perform an expander function. The apparatus can also be connected to a fuel cell stack and a reformer to supply compressed gaseous fluid to both and to receive exhaust from both as an expander. The chambers are sized differently to provide a pressure ratio across the fuel cell stack.