Synergetic effect of Co and carbon nanotubes on MgH2 sorption properties

This paper analyses the synergetic effect of the coupled addition of Co particles (Co, 5 wt%) and pure multi-walled carbon nanotubes (MWCNTs, 5 wt%) on hydrogen sorption kinetics of MgH2 (M). Experimental results show that MCo co-milled 50 h plus admixed MWCNTs for 5 h exhibits enhanced absorption/desorption performance, which remains stable after 15 hydrogen sorption cycles. About 6.5 wt% of hydrogen was absorbed after 100 s and desorbed in 85 min at 250 °C, being one of the best conditions reported for Mg-based composites. Furthermore, desorption temperature after cycling was reduced by 55 °C due to the introduction of Co, and 136 °C by the combined Co and MWCNT addition as compared to free MgH2. Activation energy analysis demonstrates that although the MWCNT addition has a slight influence on the energy barrier, the Co particles show a good catalytic effect. Moreover, the coupled effect of Co and MWCNTs reduces the activation energy and it is maintained after cycling. As a consequence of cycling, some nanograins of MgH2 appear covered with a thin external layer of amorphous carbon while some part of the MWCNTs remains intact in the material. An interpretation of the kinetics effect of Co particles and MWCNT coupled addition is proposed.