Isothermal activation, thermodynamic and hysteresis of MgH₂ hydrides catalytically modified by high-energy ball milling with MWCNTs and TiF₃

MgH₂-M (M = MWCNTs or TiF₃ or both of them) composites prepared by high-energy ball milling (HEBM) are used in this work to illustrate the effect of catalysts on isothermal activation, thermodynamic and hysteresis of MgH₂ hydrides. The phase compositions, microstructures, particle morphologies and distributions of MgH₂ with catalysts have been evaluated. The isothermal synergetic catalytic-activation and dehydrogenation effect of MWCNTs and TiF₃ evaluated by P-C-T give the evidences that the addition of catalysts is an effective strategy to destabilize MgH₂ and reduce the dehydrogenation temperatures. The isothermal activation process can be remarkly accelerated by adding MWCNTs or TiF₃. It's worthnoting that fast initial absorption rate and high hydrogenation capacity are obtained for modified MgH₂ when adding MWCNTs coupling with TiF₃ and no special activation treatment is needed. The activation effect is mainly attributed to the large contraction/expansion stresses caused by accelerated catalytic desorption/absorption cycles. The catalytic effect on thermodynamic is mainly attributed to electronic exchange reactions with hydrogen molecules during the dissociation-absorption or recombination-desorption process. The thermodynamic hysteresis of catalyzed MgH₂ is also investigated to evaluate the energy consumption and estimate the efficiency of the isothermal hydrogen absorption/desorption process. A probable synergetic catalytic-activation mechanism is probed.