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

• Published in: Journal of power sources 2007-02, Vol.164 (2), p.782-791
• Main Authors: Calabretta, Daniel L., Davis, Boyd R.
• Format: Article
• Language: English
• Subjects: Applied sciences; Energy; Energy. Thermal use of fuels; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Fuel cells; Hydrogen storage; Molten salts; Reciprocal system; Regeneration; Sodium borohydride; Sodium hydride
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2006.11.023

Although sodium borohydride (NaBH 4) 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 NaBH 4 from sodium metaborate (NaBO 2) is introduced following a literature review of NaBH 4 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), NaBO 2 and NaBH 4. We found that molten NaBH 4 is not stable with NaBO 2 above 600 °C due to the formation of sodium orthoborate (Na 4B 2O 5), hydrogen and boron. However, the quasi-reciprocal ternary system, (4/5)NaH–NaBO 2–(1/5)NaBH 4–(2/5)Na 4B 2O 5, that was discovered, proves that molten Na 4B 2O 5 is miscible and stable with molten NaBH 4 to at least 650 °C under the hydrogen pressures used in this study. As well, the compound Na 6B 2O 5H 2 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.