High Oxide Ion Conduction in Molten Na2W2O7

Recently, oxide ion transport in molten electrolytes has attracted increased attention, and molten electrolytes based on Bi2O3, V2O5 and TeO2 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 Na2W2O7 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 Al2O3 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 TeO2‐based electrolyte. The W L3‐edge extended X‐ray absorption fine structure spectra suggests that the disordered coordination‐number‐variable WO4+x mixed polyhedral network plays an important role in the oxide ion conduction in the molten Na2W2O7. 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. Molten Na2W2O7 displays high oxide ion conductivity. When supported by an insulating matrix, such as Al2O3 in this work, it has a great application prospect in the more recently proposed concept of molten oxide fuel cells. The disordered coordination‐number‐variable WO4+x mixed polyhedral network is revealed to show an important role in the oxide ion migration.