Milling time effect of Reactive Hydride Composites of NaFNaHMgB2 investigated by in situ powder diffraction

• NaFNaHMgB2 composites milled for 87 h and 5 h show 6.0 and 6.3 wt% H uptake when hydrogenated at 325 °C and 50 bar H2.
• NaBH4 is forming upon hydrogenation at 206 °C compared to 380 °C for the fluoride free NaHMgB2-system.
• The presence of fluoride greatly improves the kinetics with about twice as fast hydrogen absorption.
• Formation of NaBH4 is observed at 368 °C and 1 bar hydrogen upon rehydrogenation.

Light metal complex borohydrides have high hydrogen storage capacities but suffer from drawbacks of slow hydrogen sorption kinetics, poor reversibility and high thermodynamic stability. The NaF + 9NaH + 5MgB2 composite has a theoretical hydrogen capacity of 7.7 wt% H assuming the formation of 10NaBH3.9F0.1 + 5MgH2. Hydrogenation and dehydrogenation properties as well as the effect of different ball milling times have been investigated. The in situ hydrogenation is faster in the composite ball milled for 87 h than the 5 h milled composite. A boron-rich phase with space group Pa-3, a = 7.4124(5) Å was formed during hydrogenation at 325 °C and 50 bar hydrogen for both short and long milling times. In the long milled composite the boron-rich phase disappeared after 3 h of hydrogenation, whereas it became a major phase in the short milled composite after 1.5 h of hydrogenation. NaBH4 was formed at 206 °C. NaMgH1-xFx was formed at 290 °C instead of the assumed MgH2. The same phases formed at 268 °C and 325 °C, respectively, and only in minor amounts in the short milled composite. Ex situ hydrogenation in a Sieverts' type apparatus at the same temperature and hydrogen pressure conditions followed a different reaction pathway with formation of MgH2 in addition to NaBH4 and NaMgH3-xFx (0 ≤ x ≤ 1). The measured hydrogen uptake was 6.0 and 6.3 wt% for the long and short milled composites, respectively.