Microwave-assisted reduction of Ti species in MgH2-TiO2 composite and its effect on hydrogen storage

The goal of this work is to shed light on the significant dehydrogenation behavior of MgH2-TiO2 composite exposed to microwave irradiation, and the changes in chemical state of titanium species that occur as a result. It is discovered that the reduced titanium oxide (TiO2-x) formed during ball milling can significantly absorb microwave radiation and act as "hot spots", such that the MgH2 with 25 wt% TiO2 sample is nearly completed dehydrogenated at a constant temperature of 220 °C in a microwave field. Upon full dehydrogenation, the microwave irradiated sample significantly absorbs 1.25 wt% H2 within 600 sec and 4.25 wt% H2 within 10 h at 25 °C under 1 bar of hydrogen. Characterization results of these samples show that microwave irradiation assists the uniform dispersal of defective TiO2-x species around Mg/MgH2 particles and further promotes the reduction of Ti4+ to lower valence states, for better electron transfer between Mg2+ and H-. HRTEM images as well reveal that part of Mg grain size falls around 10 nm, which is believed to significantly enhance the hydrogen sorption as observed. Our findings show that the synergistic effects of microwave irradiation and heat may advance MgH2 further to practical applications.