Characterizations of composite NaNH2–NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH2–NaBH4 (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na3(NH2)2BH4, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na3(NH2)2BH4. TG-DTA demonstrates that the phase change temperature of the composite NaNH2–NaBH4 (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na3BN2; while above 400 °C, the previous by-product Na3BN2 continues to decompose so as to give metal Na gradually.

► A new phase of Na3(NH2)2BH4 is detected, and the accumulated mechanical energy may cause the phase change of Na3(NH2)2BH4.
► The decomposition mechanisms of NaNH2–NaBH4 (2/1) are analyzed and illustrated.
► Based on the mass conservation, the theoretic H2 yield of NaNH2-NaBH4 (2/1) exceeds 98%.
► It is an efficient way to improve the H2 yield by suppressing the N2 generation in NaNH2-NaBH4 (2/1).