When hydrogen and steam are considered as reactants, the overall reaction takes this form:
where the forward reaction occurs during SOFC mode, and the backward reaction during SOEC mode. On the fuel electrode, hydrogen oxidation (forward reaction) takes in SOFC mode and water reduction (backward reaction) takes plain SOEC mode: 
On the oxygen electrode, oxygen reduction (forward reaction) occurs in SOFC mode and oxide ions oxidation (backward reaction) occurs in SOEC mode:
The thermoneutral voltage for steam electrolysis is equal to 1.29 V.