The doping of coral-ball-shaped heterojunction NiO/NiFe2O4 as a catalyst for MgH2: improvement of hydrogen storage performance

Magnesium hydride (MgH 2 ), recognized as a promising alternative energy material to replace fossil fuels, possesses distinct advantages, including higher stability and hydrogen storage capacity. Nevertheless, its widespread application remains limits due to intrinsic drawbacks such as high thermodynamic stability and sluggish hydrogen adsorption-desorption kinetics. Coral-like NiO/NiFe 2 O 4 heterojunction catalysts were efficaciously synthesized through a conventional hydrothermal-calcination method in our work. These catalysts were subsequently ball-milled with MgH 2 to obtain the MgH 2 –NiO/NiFe 2 O 4 composite. A comprehensive series of hydrogen storage performance (HSP) evaluations demonstrated that the composite exhibits markedly enhanced hydrogen adsorption-desorption characteristics. Specifically, MgH 2 –NiO/NiFe 2 O 4 initiated hydrogen release at 188 °C, which represents a 133 °C decrease compared with ball-milled MgH 2 . At 275 °C, its composite released 6.2 wt% H 2 within 20 min, while at a hydrogen absorption temperature of 100 °C, it absorbed 4.4 wt% H 2 within 30 min. Besides, the composite dehydrogenation activation energy was declined to 86.97 kJ/mol, a decrease of 62.17 kJ/mol relative to ball-milled MgH 2 . Mechanistic investigations revealed that NiO/NiFe 2 O 4 underwent in situ transformation into Mg 2 Ni and FeO during hydrogen adsorption-desorption process. Simultaneously, the reversible “Mg 2 NiH 4 /Mg 2 Ni” phase functioned as hydrogen pump, thereby accelerating the kinetics. First-principles calculations further demonstrated that Fe atoms in the in situ-generated FeO acquired electrons from the MgH 2 interface, which weakened the Mg–H bonds and facilitated hydrogen desorption. This electronic modulation of MgH 2 effectively enhanced its hydrogen adsorption-desorption dynamics and substantially lowered hydrogen desorption activation energy. To sum up, our study reports successful preparation of a binary transition-metal heterojunction catalyst, NiO/NiFe 2 O 4 , derived from Ni and Fe. The outcomes offer new thoughts into design and classification of transition-metal-based catalysts for hydrogen storage applications.