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We present a hydrogen storage mechanism of the surface and bulk NaeLieAl hydrides substituted by the transition metal (TM) dopants such as Ni, Cu, Ag, and Zn. The host hydrides of interest, namely, NaAlH4, LiAlH4, Na3AlH4, Li3AlH4, and Na2LiAlH4 are found to be stable compositions at ambient pressure. Hydrogen vacancy mechanisms of the host hydrides with the TM dopants are investigated using ab initio calculations. Remarkably, the results show the enhancement of the internal mechanism for hydrogen storage in the Na-Li-Al complex hydrides. Doping of Ni or Zn mainly reduces the energy barrier of diffusion kinetics in the host Na-Li-Al hydrides, leading to the improvement of the hydrogen storage efficiency of the host Na-Li-Al hydrides. Therefore, hydrogen vacancy diffusion kinetics in the Na-Li-Al hydrides can be induced by adding the Ni and Zn dopants. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.