Achieving superior cycling stability by in situ forming NdH2–Mg–Mg2Ni nanocomposites

Abstract

Magnesium hydrides have great potential as hydrogen storage materials for fuel cell technologies, but the poor cycling stability and slow kinetics have severely restrained their commercial applications. In this work, we report the Nd_4.3Mg_87.0Ni_8.7 alloy formed by hydrogen induction with remarkably fast hydrogen storage kinetics and high hydrogen storage capacity (78.6% of the maximum value) even after 819 hydriding/dehydriding (H/D) cycles. In situ synchrotron powder X-ray diffraction and three-dimensional atom probe tomography analysis reveal that the catalytic NdH₂ nanocrystallites are in situ formed during the first hydrogenation of Nd₄Mg₈₀Ni₈ and densely distribute in the matrix of α-Mg and Mg₂Ni, which plays the key role in achieving excellent hydrogen storage properties. Analysis using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model suggests that the diffusion rate of H atoms during hydrogenation is greatly enhanced by the high-density grain boundaries in the NdH₂–Mg–Mg₂Ni nanocomposites. The pumping effect of NdH_3−x, which captures H atoms and transfers them from the 4b sites of NdH_3−x to the octahedral interstitial sites of the NdH_3−x/α-Mg interface along the [100]_Mg direction, is demonstrated via first-principles calculations. The pioneering work presented in this paper with the proposed microstructure evolution mechanism is of great significance to the design and fabrication of new hydrogen storage materials with superior hydrogen storage performances.