Effect of Ce0.6Zr0.4O2 nanocrystals on boosting hydrogen storage performance of MgH2

Abstract

Magnesium hydride (MgH2) has been widely recognized as a highly promising solid-state media for hydrogen storage. However, the high operating temperature and the intrinsic sluggish kinetics hinder the practical application as a portable solid storage carrier. To address this issue, we have successfully fabricated a novel rare earth-containing bimetallic oxide additive, namely Ce0.6Zr0.4O2 nanocrystals, which exhibits a significant catalytic effect in enhancing the hydrogen storage properties of MgH2. By incorporating 7 wt% Ce0.6Zr0.4O2 into MgH2, the modified composite releases hydrogen as low as 201 °C. Furthermore, in an isothermal dehydrogenation test conducted at 270 °C for 8 min, approximately 6.15 wt% of H2 was desorbed. The composites can rapidly recharge hydrogen at a low temperature of 50 °C and 6.02 wt% H2 being absorbed within 3 min at 150 °C under 50.0 bar. Comparatively, the dehydrogenation activation energy of the Ce0.6Zr0.4O2-modified MgH2 significantly decreased compared to MgH2 by ball milling. In addition to its improved hydrogen storage properties, the composite also exhibits excellent cycling stability. Through cyclic de-/rehydrogenation experiments conducted at 270 °C, a capacity retention of 98.9 % was achieved over 20 cycles. This exceptional performance can be attributed to the in-situ formation of the CeH2.73/CeO2-x and ZrO2, which originated from the Ce0.6Zr0.4O2 additive following the first de-/rehydrogenation cycle. The uniform dispersion of the multiphase component nano-sized active species within the MgH2 matrix facilitates charge transfer and accelerates hydrogen diffusion. Density functional theory calculations revealed the dissociation energy barrier and dissociation energy of H2 were alleviated by integrating Zr into CeO2. This work provides valuable insights for further rational designs of novel catalysts by transition metals and rare earths in promoting the commercialization of MgH2.