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 NaAlH4 is not completely understood. Here, we studied the local structure of scandium and the formation of different species in ScCl3-doped NaAlH4 by Na-23, Al-27, and Sc-45 NMR. The Al-27 and Na-23 solid-state NMR spectra of ball-milled ScCl3-doped NaAlH4 mainly show resonances corresponding to NaAlH4, Na3AlH6, Al, and NaCl. The three decomposition products are formed by partial dehydrogenation and reactions with the dopant. The Sc-45 NMR spectrum shows resonances corresponding to a distorted ScCl3 phase, a Sc-Al solid solution, and ScH2. After cycling eight times with hydrogen, the Na3AlH6 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 Sc-45 spectra now mainly show the presence of a distorted ScAl3 phase, and a smaller contribution attributed to a distorted ScCl3 phase. After partial dehydrogenation at room temperature, metallic Al and Na3AlH6 are observed. The static Sc-45 Carr-Purcell-Meiboom-Gill (CPMG) experiment confirms the presence of the distorted ScAl3 phase. A possible role of Sc in the kinetic enhancement of desorption and uptake of hydrogen in NaAlH4 is that it can act as a grain refiner for metallic Al.