Effect of bimetallic nitride NiCoN on the hydrogen absorption and desorption properties of MgH2 and the catalytic effect of in situ formed Mg2Ni and Mg2Co phases

Magnesium hydroxide has attracted extensive attention in the past few years due to its excellent hydrogen storage capacity, good reversibility, and low cost, but its high thermodynamic stability and low rate of kinetic activity hinder its application. Therefore, in order to improve the hydrogen storage capacity of MgH2, we prepared ternary transition metal nitride NiCoN to catalyze MgH2 by hydrothermal and calcination techniques. Owing to the addition of 6 wt% NiCoN, the dehydrogenation process of MgH2 started at 164.4 °C (ball-milled MgH2 started dehydrogenation at 287.6 °C). At 285 °C, 4.95 wt% of hydrogen gas was desorption within 30 min, while 4.6 wt% of hydrogen gas was absorbed at 125 °C. And calculated by the Kissinger method, the activation energy required for the dehydrogenation reaction of MgH2-6NiCoN is reduced to 56.5 kJ/mol (ball-milled MgH2 is 127.3 kJ/mol). It is found that MgH2-6NiCoN forms Mg2Co/Mg2Ni in situ during the first dehydrogenation process, and then transforms into Mg2CoH5/Mg2NiH4 after rehydrogenation, implying a reversible phase transition between the Mg2Co/Mg2CoH5 and Mg2Ni/Mg2NiH4 phases. Due to the formation of intermediate products, it still maintains a good hydrogen desorption capacity after 40 cycles. The dehydrogenation retention rate is 94 %. The excellent behavior of MgH2 is attributed to the formation of Mg2Ni/Mg2NiH4 and Mg2Co/Mg2CoH5, which are responsible for the improved hydrogen absorption/desorption behavior of MgH2. These findings provide new insights into the hydrogen storage behavior of the catalytic MgH2 system.