The effect of different Co phase structure (FCC/HCP) on the catalytic action towards the hydrogen storage performance of MgH2

Abstract

The close contact of fine Co particles with MgH 2 created numerous active sites and provided sufficient channels for hydrogen diffusion, contributing to the accelerated de/rehydrogenation kinetics. High hydrogen desorption temperature and sluggish reaction kinetics are the major limitations for the practical application of MgH 2 . In this study, Co particles with a face centered cubic (FCC) structure and a hexagonal close packed (HCP) structure were prepared facilely and proved to be good catalysts for magnesium hydride. Co particles with FCC structure presented better catalytic effect on MgH 2 than that with HCP structure. Both 7% (mass) Co FCC and HCP particle modified MgH 2 decreased the initial dehydrogenation temperature from 301.3 °C to approximately 195.0 °C, but 7% (mass) Co with FCC structure modified MgH 2 has a faster desorption rate, and around 6.5% (mass) H 2 was desorbed in 10 min at 325 °C. Hydrogen uptake was detected at 70 °C under 3.25 MPa hydrogen pressure and 6.0% (mass) H 2 was recharged in 40 min at 150 °C. The hydrogen desorption and absorption activation energy for 7% (mass) FCC Co modified MgH 2 was significantly decreased to (76.6±8.3) kJ·mol −1 and (68.3±6.0) kJ·mol −1 , respectively. Thermodynamic property was also studied, the plateau pressures of MgH 2 + 7% (mass) FCC Co were determined to be 0.14, 0.28, 0.53 and 0.98 MPa for 300 °C, 325 °C, 350 °C and 375°C. The decomposition enthalpy of hydrogen (Δ H ) for MgH 2 + 7% (mass) FCC Co was (80.6±0.1) kJ·mol −1 , 5.8 kJ·mol −1 lower than that of as-prepared MgH 2 . Moreover, cycling performance for the first 20 cycles revealed that the reaction kinetics and capacity of MgH 2 -FCC Co composite remained almost unchanged. The result of density functional theory calculation demonstrated that cobalt could extract the Mg H bond and reduced the decompose energy of magnesium hydride. Our paper can be presented as a reference for searching highly effective catalysts for hydrogen storage and other energy-related research fields.