Hierarchical Structure Carbon-Coated CoNi Nanocatalysts Derived from Flower-Like Bimetal MOFs: Enhancing the Hydrogen Storage Performance of MgH2 under Mild Conditions

Abstract

The construction of transition metal nanocatalysts has become an attractive way to improve the hydrogen storage performance of MgH2. However, it is still a great challenge to obtain multielement transition metal nanocatalysts, improve the hydrogen storage capacity of MgH2 under mild conditions (<573 K), and reduce the apparent activation energy of hydrogen absorption and desorption. In this work, a flower-like CoNi-MOF was successfully synthesized by sacrificing templates. After further pyrolysis, we obtained hierarchical structure flower-like MOF derivatives assembled by CoNi@C nanoparticles with an average particle size of 15.1 nm. Then, MgH2-x wt % CoNi@C nanocomposites are fabricated through the ball milling process. Due to the unique hierarchical flower-like structure of MOF derivatives, the inhibition of an amorphous carbon shell on CoNi alloy growth and agglomeration, and the synergistic catalysis of Mg2Co and Mg2Ni, the MgH2-10 wt % CoNi@C nanocomposite exhibits excellent hydrogen storage capacity under mild conditions and low apparent activation energy: At 473 K, the nanocomposite can quickly absorb 5.0 wt % H2 in 5 min, and even at 373 K, it can still absorb 4.2 wt % H2 in 60 min. At 573 and 548 K, it can release 4.8 and 3.1 wt % H2, respectively. The apparent activation energies for hydrogen absorption and desorption decrease remarkably to 24.9 and 67.3 kJ/mol, respectively, which are significantly better than those for MgH2-10 wt % Co@C (44.7 and 79.8 kJ/mol) and MgH2-10 wt % Ni@C (39.0 and 78.9 kJ/mol) nanocomposites. The first-principles calculation indicated that the hydrogen adsorption energy of the Mg–CoNi model is as low as −5.87 eV. This work provides a new strategy for the design of highly efficient multielement transition metal hydrogen storage material catalysts with a hierarchical structure.