High Oxide Ion Conduction in Molten Na2W2O7

Abstract Recently, oxide ion transport in molten electrolytes has attracted increased attention, and molten electrolytes based on Bi 2 O 3 , V 2 O 5 and TeO 2 oxides have shown excellent oxide ion conductivity and thus possess great potential as electrochemical energy materials. A new concept of molten oxide fuel cells (MOFCs) is proposed as a promising energy technology that can work in the intermediate temperature range of 500–800 °C, complementary to solid oxide fuel cells (SOFCs). Here, oxide ion transport in molten Na 2 W 2 O 7 is presented, with a recorded oxide ion conductivity of ≈0.015 S cm −1 at 750 °C and delivered power density of 121 mW cm −2 as a MOFC electrolyte operating at 750 °C supported by an insulating Al 2 O 3 matrix. This represents a significant advance to the MOFC output compared with the only previous performance reported (11.5 mW cm −2 ) achieved using a molten TeO 2 ‐based electrolyte. The W L 3 ‐edge extended X‐ray absorption fine structure spectra suggests that the disordered coordination‐number‐variable WO 4+ x mixed polyhedral network plays an important role in the oxide ion conduction in the molten Na 2 W 2 O 7 . These results emphasize a great opportunity to exploit oxide ion conductors within the molten oxide family, which can bring a brighter prospect to the MOFCs.