Due to their low melting point and high conductivity molten hydroxides are interesting electrolytes, or additive to other molten electrolytes for high-temperature electrochemical devices. There is nowadays a revival of such reactive media, first of all for their significant role in the electrode mechanisms in molten carbonate fuel cells (MCFCs) and the reverse co-electrolysis of water and carbon dioxide process, but also in different applications, among which direct carbon fuel cells (DCFCs), hybrid carbonate/oxide fuel cells. This overview shows the properties and interest of molten hydroxides and their use in relevant energy devices, pointing out their direct use as electrolytic media or as key species in complex kinetic processes. A thorough understanding of their behavior should allow improving and optimizing significantly fuel cells, electrolyzers, and probably also CO2 capture and valorization.

Sodium ion batteries (NIBs) are gaining importance as an energy storage technology complementary to Li-ion batteries. In this regard, sodium-based layered oxides are recognized as one of the most suitable positive electrode candidate for NIBs, but the vast variety of their chemical compositions and crystal structures complicates the selection of the best material. To shed light on different figures of merit that control the selection of layered oxide material for NIB applications, we compare here a P2, P2-O3 intergrowth and two O3 sodium layered oxides composed of the same transition metals (Ni, Mn, Ti and Zn) but in differing stoichiometries. The results show that the new P2-type Na0.67Ni0.3Zn0.03Mn0.52Ti0.15O2 phase reported here combines most of the advantages, such as stability towards moisture, reduced surface reactivity, better thermal stability, and high rate capability, which are essential for commercialization, except for a limited capacity (~125 mAh/g) compared to the O3 NaNi0.45Zn0.05Mn0.4Ti0.1O2 phase (~180 mAh/g), reported by our group previously. Altogether, this work enlarges the family of attractive Na-based positive electrodes and teaches us that we should not be solely obsessed with the capacity of a material to decide of its applicability.