Facile synthesis of Co/Pd supported by few-walled carbon nanotubes as an efficient bidirectional catalyst for improving the low temperature hydrogen storage properties of magnesium hydride

Abstract

Catalytic doping is important for enhancing the hydrogen storage performance of metal hydrides, but it is challenging to develop a single catalyst to enhance both hydrogen desorption and absorption to a certain degree. Herein, a bidirectional Co/Pd catalyst, homogeneously loaded on bamboo-shaped carbon nanotubes (Co/Pd@B-CNTs), showed superior catalytic effects, improving both the hydrogen desorption and absorption properties of MgH₂ at relatively low temperatures. The MgH₂–Co/Pd@B-CNTs composite starts to release hydrogen at 198.9 °C, which is 132.4 °C lower than as-milled MgH₂. The hydrogen desorption activation energy for MgH₂ is reduced from 178.00 to 76.66 kJ mol⁻¹ by the catalytic effects of Co/Pd@B-CNTs. The MgH₂–Co/Pd@B-CNTs composite shows dramatically improved absorption kinetics; it rapidly uptakes 6.68 wt% H₂ within 10 s at 250 °C, and quickly absorbs 1.91 wt% H₂ within 100 s, even at a temperature as low as 50 °C. More importantly, a special mechanism for the “bidirectional catalyst” Co/Pd is proposed for the first time and discussed in detail. During the hydrogenation process, elemental Pd plays a dominant role in accelerating the preferential diffusion of hydrogen atoms at the Pd/Mg interface, while during dehydrogenation, phase transformation between Mg₂Co and Mg₂CoH₅ as well as a Mg–Pd alloy becomes the crucial factor, facilitating the release of hydrogen atoms by decreasing the diffusion barrier. Moreover, novel structures of bamboo-shaped carbon nanotubes with a large diameter (>100 nm) and high specific surface area (146.8 m² g⁻¹) allow the homogenous dispersion of Co/Pd NPs and enhance the direct contact with MgH₂ particles.