The increase in hydrogen back pressure unexpectedly enhances the overall dehydrogenation reaction rate of the 4LiBH(4) + YH(3) composite significantly. Also, argon back pressure has a similar influence on the composite. Gas back pressure seems to enhance the dehydrogenation reaction by kinetically suppressing the formation of the diborane by-product.

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Hydrogen back pressure remarkably promotes the formation of metal boride during the dehydrogenation of 4LiBH4 + YH3, 6LiBH4 + CeH2 and 6LiBH4 + CaH2 composites, which seems to be a general phenomenon in LiBH4-based reactive hydride composites that enables mutual destabilization between LiBH4 and metal hydride. The formation of metal boride plays a crucial role in the reversible hydrogen storage properties of these composites. The dependence of the dehydrogenation behavior on hydrogen back pressure might be associated with the microstructural evolution of the dehydrogenation products formed by a solid−liquid reaction.

This paper investigates dehydrogenation reaction behavior of the LiBH 4 −MgH 2 composite at 450 °C under various hydrogen and argon backpressure conditions. While the individual decompositions of LiBH 4 and MgH 2 simultaneously occur under 0.1 MPa H 2 , the dehydrogenation of MgH 2 into Mg first takes place and subsequent reaction between LiBH 4 and Mg into LiH and MgB 2 after an incubation period under 0.5 MPa H 2 . Under 1 MPa H 2 , enhanced dehydrogenation kinetics for the same reaction pathway as that under 0.5 MPa H 2 is obtained without the incubation period. However, the dehydrogenation reaction is significantly suppressed under 2 MPa H 2 . The formation of Li 2 B 12 H 12 as an intermediate product during dehydrogenation seems to be responsible for the incubation period. The degradation in hydrogen capacity during hydrogen sorption cycles is not prevented with the dehydrogenation under 1 MPa H 2 , which effectively suppresses the formation of Li 2 B 12 H 12 . The overall dehydrogenation behavior under argon pressure conditions is similar to that at hydrogen pressure conditions, except that under 2 MPa Ar.

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