Thermodynamics and kinetics of nano-engineered Mg-MgH₂ system for reversible hydrogen storage application

Highlights

• Cyclic performance of nano Fe doped MgH₂ was tested.

• Nano Fe doped MgH₂ hydrogenate even below room temperature.

• Activation energies of Nano doped MgH₂/Mg reduced remarkably.

• Nano Fe does not change the thermodynamics of MgH₂.

Abstract

Thermodynamics and kinetics of hydrogenation-dehydrogenation of nanometric iron (nFe) doped Mg-MgH₂ system have been studied. The nFe-doped Mg could be hydrogenated even at 0 °C up to 45% of the theoretical hydrogen storage capacity within an hour. The dehydrogenation of nFe doped MgH₂ starts below 150 °C. The remarkably improved hydrogenation-dehydrogenation kinetics could be attributed to the nano-engineered surface of MgH₂ by nFe. The enthalpies of hydrogenation-dehydrogenation were found to be 76 kJ/mol, and 77 kJ/mol respectively. The activation energy of hydrogenation was evaluated as 41 ± 2 kJ/mol which is same as the diffusion barrier of hydrogen in Mg matrix. The apparent activation energy of dehydrogenation of nFe-doped MgH₂ was found to be 74 ± 1 kJ/mol which is same as the enthalpy of dehydrogenation. The nFe-doped Mg-MgH₂ system has shown cyclic stability up to 50 cycles without significant changes in the kinetics and hydrogen storage capacity. Three-dimensional diffusion seems to be controlling the dehydrogenation process.

Introduction

Hydrogen is considered as one of the best portable energy carriers to utilize the renewable energy for the portable application analogous to gasoline [1]. An efficient and effective hydrogen storage method has to be developed to realize the hydrogen-based energy system [2], [3]. The hydrogen storage in the metal hydrides and chemical hydrides appears promising as far as the safety and cost-effectiveness are concerned. A series of intermetallic compounds such as LaNi₅, FeTi, and vanadium based bcc alloys have been explored in connection to develop the metal hydride based hydrogen storage materials [4], [5], [6], [7]. However, low cyclic hydrogen storage capacity (<2.5 wt.%) and high materials cost limit their practical applications. The development of cost-effective and high hydrogen storage capacity material is essential to fulfilling the requirement of mobile and stationary applications [8], [9]. Low atomic number (Z) elements such as Li, Mg, Na, Ca, and their compounds are the promising candidates [10], [11], [12]. The major issues with these metals and their compounds are the high thermal stability of their hydrides and slow hydrogenation-dehydrogenation kinetics. The slow hydrogenation-dehydrogenation kinetics at ambient conditions is a big concern for onboard application as it required long duration to refuel the fuel cell cartridge [13]. Moreover, hydrogenation of these metals and their compounds are possible under the stringent temperature and pressure conditions. However, the high hydrogen storage capacity has attracted the researchers to fine-tune the performance by tailoring the thermodynamics and kinetics to achieve the sets target of US-Department of Energy (DOE). Among others, magnesium hydride (MgH₂) has been considered as one of the best candidates as far as gravimetric hydrogen storage capacity (>7.6 wt.%), natural abundance, cost-effectiveness, and safety are concerned [14]. Therefore, the hydrogen sorption reaction of metallic Mg and Mg-based alloys & compounds have been extensively studied [15], [16], [17], [18], [19]. The slow hydrogenation-dehydrogenation kinetics of the Mg-MgH₂ system have been resolved, to some extent by oxide and metal based catalysts [20], [21], [22], [23], [24], [25], [26], [27]. The issues with metal oxides based catalyst are their chemical reactivity with magnesium which eventually affects the cyclic performance and hydrogen storage capacity. Although nanometric-metal have shown excellent catalytic activity, cost-effective and eco-friendly nonmetric-iron (nFe) as a catalyst has not been explored sufficiently. In the present study, the effectiveness nFe has been explored as a catalyst for the hydrogenation-dehydrogenation of the Mg-MgH₂ system. It has been found that the doping of mere 5 wt.% nFe has considerably reduced the hydrogenation-dehydrogenation temperature of the Mg-MgH₂ system without affecting much to the hydrogen storage capacity. The hydrogenation-dehydrogenation of the pristine Mg-MgH₂ system is kinetically impossible at such a low temperature and under similar experimental conditions.