An electrochemical energy conversion device 10 comprising a stack of solid oxide electrochemical cells 12 alternating with gas separators 14, 16, wherein scavenger material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the negative electrode-side of the cell 12, the adjacent gas separator 16 and any other structure of the device 10 forming a gas chamber 66 between the cell and the gas separator. The invention also extends to the treated cell 12.