27Al, 23Na, and 45Sc Solid-State NMR Studies of ScCl3-Doped NaAlH4

The exact role of catalysts, such as titanium, scandium, or cerium, in improving the hydrogen kinetics in NaAlH 4 is not completely understood. Here, we studied the local structure of scandium and the formation of different species in ScCl 3 -doped NaAlH 4 by 23 Na, 27 Al, and 45 Sc NMR. The 27 Al and 23 Na solid-state NMR spectra of ball-milled ScCl 3 -doped NaAlH 4 mainly show resonances corresponding to NaAlH 4, Na 3 AlH 6, Al, and NaCl. The three decomposition products are formed by partial dehydrogenation and reactions with the dopant. The 45 Sc NMR spectrum shows resonances corresponding to a distorted ScCl 3 phase, a Sc–Al solid solution, and ScH 2 . After cycling eight times with hydrogen, the Na 3 AlH 6 and Al intensities increase. This can be explained by an incomplete rehydrogenation that is due to a partial reversibility of the first desorption reaction. After cycling under hydrogen, the NaCl intensity strongly increases. This indicates that the doping reaction is not completed after ball-milling. The 45 Sc spectra now mainly show the presence of a distorted ScAl 3 phase, and a smaller contribution attributed to a distorted ScCl 3 phase. After partial dehydrogenation at room temperature, metallic Al and Na 3 AlH 6 are observed. The static 45 Sc Carr–Purcell–Meiboom–Gill (CPMG) experiment confirms the presence of the distorted ScAl 3 phase. A possible role of Sc in the kinetic enhancement of desorption and uptake of hydrogen in NaAlH 4 is that it can act as a grain refiner for metallic Al.