Thermodynamic and microstructural basis for the fast hydrogenation kinetics in Mg–Mg2Ni-carbon hybrids

Abstract

Mg-based hybrids have shown promise via enhanced hydrogen storage properties. The Mg–Mg2Ni-carbon hybrid can be synthesized by accumulative roll bonding (ARB), which is amenable to ‘scaled-up’ synthesis. In spite of the ‘bulk’ nature of the samples synthesized, they display fast kinetics of absorption and desorption of hydrogen. In the current work, we try to comprehend the basis for the same in terms of the activation energy of the underlying processes involved; via desorption curves (wt.% H - time curves) in a Sievert's apparatus and differential scanning calorimetry (heat evolved - T plots). Analysis invoking the Johnson-Mehl-Avrami model and Kissinger plots show that the significantly reduced activation energy for the dehydrogenation process in the hybrid is responsible for the rapid kinetics. It is evinced that admixing the additives with Mg, coupled with fine scale microstructure rich in interfaces is responsible for the fast kinetics. It is established that the rate limiting step for hydrogen desorption is interface migration and not the diffusion of hydrogen, which is governed by the JMA-3D model.