Solid oxide fuel cell system and a method of operating a solid oxide fuel cell system

A solid oxide fuel cell system (10) comprises a solid oxide fuel cell stack (12) and an electrochemical device (14). The solid oxide fuel cell stack (12) comprises at least one solid oxide fuel cell (16) and each solid oxide fuel cell (16) comprises an electrolyte (18), an anode (20) and a cathode (22). An oxidant supply (24) is arranged to supply oxidant to the cathode (22) of the at least one solid oxide fuel cell (16) and a fuel supply (26) is arranged to supply fuel to the anode (20) of the at least one solid oxide fuel cell (16). The electrochemical device (14) comprises an electrolyte (34), an anode (36) and a cathode (38). Means (28, 50, 52) to supply a portion of the unused fuel from the anode (20) of the at least one solid oxide fuel cell (16) to the anode (36) of the electrochemical device (14), means (32, 50, 58) to supply a portion of the unused fuel from the anode (20) of the at least one solid oxide fuel cell (16) to the cathode (38) of the electrochemical device (14). In use the electrochemical device (14) is arranged to remove oxygen from the unused fuel at the cathode (38) of the electrochemical device (14) and the electrochemical device (14) is arranged to transfer oxygen through the electrolyte (34) from the unused fuel at the cathode (38) of the electrochemical device (14) to the unused fuel at the anode (36) of the electrochemical device (14). The electrochemical device (14) is arranged to oxidise the unused fuel at the anode (34) of the electrochemical device (14) and means (32, 60, 62, 68) to supply the portion of oxygen depleted unused fuel from the cathode (38) of the electrochemical device (14) to the anode (20) of the at least one solid oxide fuel cell (16).