Investigation of the anhydrous molten Na–B–O–H system and the concept: Electrolytic hydriding of sodium boron oxide species

Although sodium borohydride (NaBH4) can act as an excellent hydrogen storage material, its cost renders it impractical for automotive applications. In this paper the concept of electrolytic production of NaBH4 from sodium metaborate (NaBO2) is introduced following a literature review of NaBH4 synthesis. By deduction, we assert that only by employing dense solid oxide ion electrolytes and a molten salt solution containing the two constituents would such a process be possible. We investigated the molten anhydrous Na–B–O–H system by pressure differential thermal analysis (PDTA), X-ray diffraction (XRD) and gas evolution analysis (GEA) using the starting reagents sodium hydride (NaH), NaBO2 and NaBH4. We found that molten NaBH4 is not stable with NaBO2 above 600 °C due to the formation of sodium orthoborate (Na4B2O5), hydrogen and boron. However, the quasi-reciprocal ternary system, (4/5)NaH–NaBO2–(1/5)NaBH4–(2/5)Na4B2O5, that was discovered, proves that molten Na4B2O5 is miscible and stable with molten NaBH4 to at least 650 °C under the hydrogen pressures used in this study. As well, the compound Na6B2O5H2 was discovered and a substantial portion of the anhydrous Na–B–O–H phase diagram has been experimentally deduced. There is a large ionic liquid composition domain within the system that would allow for the electrolytic hydriding of sodium boron oxide species to be tested.