The present disclosure relates to solid oxide fuel cells (SOFCs) exploiting gas separation devices in which a first gas mixture including components A (for example hydrogen) and B (for example carbon dioxide) is to be separated so that a first product of the separation is enriched in component A, while component B is mixed with a third gas component C (for example air, oxygen-enriched air or oxygen-depleted air) contained in a displacement purge stream to form a second gas mixture including components B and C, and with provision to prevent cross contamination of component C into the first product containing component A, or of component A into the second gas mixture containing component C. The invention may be applied to hydrogen (component A) enrichment from fuel cell anode exhaust, where dilute carbon dioxide (component B) is to be rejected such as to the atmosphere by purging with cathode exhaust oxygen-depleted air (as component C).