Active species of CeAl4 in the CeCl3-doped sodium aluminium hydride and its enhancement on reversible hydrogen storage performance

Abstract

By directly doping CeAl(4) into sodium aluminium hydride, which probably serves as the active species in the hydriding and dehydriding processes of CeCl(3)-doped NaAlH(4), a high reversible hydrogen capacity of 4.77-4.92 wt% (close to expected capacity of 5.13 wt%) can be achieved in less than 20 min under moderate conditions.

Nanoscopic Al1−xCex phases in the NaH + Al + 0.02CeCl3 system

“…Recently it has been reported that Al 4 Ce is the catalytically active phase for CeCl 3 enhanced NaAlH 4 [20,21], which is further supported by the direct addition of Al 4 Ce to NaAlH 4 and CaAlH 4 [32]. The diffraction pattern of arc melted and milled Al 4 Ce in [21] is consistent with the reported structure type for Al 4 Ce [33].…”

“…The hydrogen cycled NaH þ Al þ 0.02CeCl 3 sample in [20,21] clearly shows the broad Al 4 Ce doublet of (103)/(112) at the correct d-spacing (d ¼ 2.63 Å , or ca. 34 2q for Cu radiation), accurate to within 0.005 Å by interpolating the d-spacing axis directly from the diffraction patterns in [20,21] according to the well known d-spacing standard of the Al unit cell, a ¼ 4.049 Å .…”

“…8 Our high resolution data clearly shows that Al 4 Ce is not present in our sample. As both the (110) AlCe and (300) cubic Al 1Àx Ce x reflections are already significantly broadened, it is impossible to incorrectly index either of these phases, even for medium resolution data, such as that collected in [20,21]. The hydrogen cycled NaH þ Al þ 0.02CeCl 3 sample in [20,21] clearly shows the broad Al 4 Ce doublet of (103)/(112) at the correct d-spacing (d ¼ 2.63 Å , or ca.…”

“…As both the (110) AlCe and (300) cubic Al 1Àx Ce x reflections are already significantly broadened, it is impossible to incorrectly index either of these phases, even for medium resolution data, such as that collected in [20,21]. The hydrogen cycled NaH þ Al þ 0.02CeCl 3 sample in [20,21] clearly shows the broad Al 4 Ce doublet of (103)/(112) at the correct d-spacing (d ¼ 2.63 Å , or ca. 34 2q for Cu radiation), accurate to within 0.005 Å by interpolating the d-spacing axis directly from the diffraction patterns in [20,21] according to the well known d-spacing standard of the Al unit cell, a ¼ 4.049 Å .…”

“…X-ray absorption fine structure (XAFS) analysis suggests that for CeCl 3 enhanced NaAlH 4 , the Ce oxidation state remains constant as Ce(III) after milling and H cycling, and that Al 1Àx Ce x formation is inhibited [19]. Laboratory source X-ray diffraction measurements of NaH þ Al þ 0.02CeCl 3 milled under a hydrogen atmosphere reveal the formation of tetragonal Al 4 Ce [20,21], further supported by the direct addition of Al 4 Ce (arc melted, then milled 12 h under Ar) as NaH þ Al þ 0.02Al 4 Ce yielding an identical rate of absorption. The XAFS study in [19] is also in conflict with [15], the only other study to use NaAlH 4 rather than NaH þ Al.…”