Air stability enhancement and mechanism of Mg-Ca-hydride-based hydrolysis materials

Abstract

Mg-Ca-hydride-based materials are ideal portable H₂ source owing to excellent hydrolysis properties. However, when exposed to air, there is an explosion risk and serious hydrolysis property degradation due to the reaction with steam in air. In this work, by adding 5 wt% expanded graphite, 8 wt% AlCl₃ and 5 wt% polyethylene glycol 1000, the H₂ yield of a 30 wt% Ca-Mg alloy hydride plate after 4 h air exposure (80 RH%, 25 °C) is increased to 2.3 times that of the original sample without any attenuation. Furthermore, when 15 wt% polyethylene glycol 1000 was added, the attenuation rate after 24 h air exposure is decreased by 43 %. The characterization results show that Ca₄Mg₃H₁₄ is more easily hydrolyzed than MgH₂. First-principles calculations demonstrate that Ca₄Mg₃H₁₄ interacts with H₂O molecules more strongly, owing to larger adsorption energy to H₂O. By adding appropriate amounts of expanded graphite, AlCl₃ and polyethylene glycol 1000, combined with pressing technology, the direct contact between the material and air can be delayed. Simultaneously, the hydrolysis performance of the material itself can be improved contributed to destruction of the Mg(OH)₂ passivation layer by expanded graphite and AlCl₃, thereby jointly enhancing the long-term storage performance. This work may advance the engineering application of Mg-Ca-based hydrolysis materials.