Chloride substitution in sodium borohydride

The dissolution of sodium chloride and sodium borohydride into each other resulting in formation of solid solutions of composition Na(BH4)1−xClx is studied. The dissolution reaction is facilitated by two methods: ball milling or combination of ball milling and annealing at 300 °C for three days of NaBH4–NaCl samples in molar ratios of 0.5:0.5 and 0.75:0.25. The degree of dissolution is studied by Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data. The results show that dissolution of 10 mol% NaCl into NaBH4, forming Na(BH4)0.9Cl0.1, takes place during ball milling. A higher degree of dissolution of NaCl in NaBH4 is obtained by annealing resulting in solid solutions containing up to 57 mol% NaCl, i.e. Na(BH4)0.43Cl0.57. In addition, annealing results in dissolution of 10–20 mol% NaBH4 into NaCl. The mechanism of the dissolution during annealing and the decomposition pathway of the solid solutions are studied by in situ SR-PXD. Furthermore, the stability upon hydrogen release and uptake were studied by Sieverts measurements.