Abstract: The invention relates to a fuel cell system having a pressure swing adsorption (PSA) unit for enriching a reactant stream supplied to the fuel cell, and particularly for enriching an oxidant stream supplied to the fuel cell. The reactant stream flow is compressed in two stages, with one compressor arranged upstream and another compressor arranged downstream of the PSA unit. Further, for improved efficiency, a portion of the compressed reactant from the first stage may bypass the PSA and be combined with enriched reactant upstream of the second compression stage. A third compressor may be used for desorbing the PSA unit under vacuum. An energy recovery device may be employed in the oxidant exhaust line to recover energy in the oxidant exhaust, and the compressors and turbine may be arranged on a common shaft.