FeNbO4-based oxide cathode for steam electrolysis

Abstract

FeNbO4 is a layered oxide containing Fe3+ and Nb5+ cations on either side of a hexagonal close-packed O2– plane. This layered oxide can be reduced at 800 °C under hydrogen to form a composite of Fe0 and a FeNb2O6 tri-rutile of similar structure. The isothermal conductivity of FeNbO4 against oxygen partial pressure, PO2, indicates that the critical phase-switching oxygen pressure is around 10−16 bar at 800 °C and the conductivity of the resultant composite shows a p-type conductivity under a PO2 between 10−20 and 10−17 bar. The composite of FeNb2O6 and Fe0 from FeNbO4 reduced in 5% H2/Ar gas can maintain a stable performance as the cathode for the electrolysis of steam. In addition, we demonstrate that FeNbO4 can be transformed to FeNb2O6 and Fe/Fe(O) under a cathodic current without prior H2 reduction and a high current density of 0.32 A cm−2 can be obtained at 1.5 V for the electrolysis of 5% H2O/Ar. Nb5+ in FeNb2O6 can be reduced to Nb4+ under a bias of 1.5 V at 800 °C.