NOVELTY - A fuel cell system (10) comprises a fuel cell stack (14) including a cathode having an inlet and an outlet; a compressor (12) for compressing the first fluid; an expander (20) coupled to the compressor, and is in fluid communication with the outlet of cathode to recover mechanical energy from the second fluid and generate torque to power the compressor; and a freewheel mechanism (22) coupled to the compressor and the expander for transferring the torque from expander to the compressor, and militates against the expander acting as a restrictor to the compressor.    USE - Fuel cell system.    ADVANTAGE - The system employs an expander for recovering mechanical energy from a cathode exhaust fluid produced by the fuel cell system to generate torque. The expander can assist the compressor over broad flow and pressure ranges without requiring additional components such as actuators (e.g. waste gate valve, variable turbine geometry). The freewheeling expander is a simple, cost effective, self-regulating means to improve an efficiency of the fuel cell system.    DETAILED DESCRIPTION - A fuel cell system (10) comprises:    (A) a fuel cell stack (14) including a cathode having an inlet and an outlet, where a first fluid flows through the inlet of the cathode and a second fluid flows through the outlet of the cathode;    (B) a compressor (12) for compressing the first fluid, an inlet of the compressor in fluid communication with a source of the first fluid and an outlet of the compressor in fluid communication with the inlet of the cathode;    (C) an expander (20) coupled to the compressor, and is in fluid communication with the outlet of cathode to recover mechanical energy from the second fluid and generate torque to power the compressor; and    (D) a freewheel mechanism (22) coupled to the compressor and the expander for transferring the torque from the expander to the compressor, and militates against the expander acting as a restrictor to the compressor.    An INDEPENDENT CLAIM is included for method of operating a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic flow diagram of a fuel cell system.    Fuel cell system (10)    Compressor (12)    Fuel cell stack (14)    Expander (20)    Freewheel mechanism (22)