Investigation on the nature of active species in the CeCl3-doped sodium alanate system

Research highlights: → The additive of CeCl3 is reduced in doping process, causing the formation of NaCl and highly dispersed Ce species. → After dehydrogenation, Al-Ce alloy with a structure of CeAl4 comes into being and remains almost unchanged in the following cycles. → Directly doping CeAl4 into the system results in much similar kinetics. - Abstract: CeCl3-doped NaAlH4 was directly synthesized in hydrogenation process using NaH/Al with 2 mol% CeCl3 under ball-milling. X-ray diffraction was utilized to unveil the nature of cerium during NaAlH4 synthesis process and succedent cycling. It is found that, CeCl3 is reduced in the ball-milling process and following cycles, causing the formation of NaCl and Al-Ce alloy with a structure of CeAl4. The catalytic enhancement arising upon doping the ball-milled CeAl4 alloy is quite similar to that achieved in the CeCl3-doped sodium alanate. Because the CeAl4 dopant does not consume the effective hydrogen storage component, the CeAl4-doped NaAlH4 exhibits more hydrogen storage capacity. Moreover, CeCl3-doped NaAlH4 and CeAl4-doped NaAlH4 exhibit similar apparent activation energies estimated from Kissinger's method, suggesting the reactions are all determined by the same rate-limiting step. These results clearly demonstrate that the in situ formed CeAl4 acts as active species to catalyze the reversible dehydriding/rehydriding of NaAlH4.