MXene-supported VHx nanoparticles enhancing hydrogen storage properties of magnesium hydride

Abstract

Magnesium hydride (MgH2) is a highly attractive candidate for solid-state hydrogen storage because of its high mass density, excellent cyclic stability, and low cost. However, the commercialization of MgH2 has been hindered by its sluggish hydrogen sorption kinetics and elevated operating temperatures. In this study, vanadium hydride nanoparticles (VHx) adhered to Ti3C2 composite catalyst was synthesized by ball milling to improve the hydrogen storage properties of MgH2. The onset dehydrogenation temperature of the MgH2 + 10wt% VHx@Ti3C2 composite decreased from 267.6 to 190.3°C. In addition, the MgH2 + 10wt% VHx@Ti3C2 composite could release 6.72wt% hydrogen within 10 min at 290°C. By comparison, pure MgH2 started to release hydrogen at 267.6°C, whereas only 0.29wt% hydrogen was released under the same conditions. After 20 cycles, more than 95% of the initial hydrogen absorption and desorption capacities were retained, indicating that the MgH2 + 10wt% VHx@Ti3C2 composite exhibited good cycling performance. Investigation of the catalytic mechanism demonstrated that the layered structure of Ti3C2 served as a matrix supplying a large number of active sites, and VHx functioned as a “hydrogen pump” to accelerate the migration of H ions, thereby facilitating the hydrogen absorption and desorption processes of MgH2 and enhancing its hydrogen storage properties. This study provides a viable strategy for designing vanadium-based catalysts to improve solid-state hydrogen storage materials.

1. Introduction

Overreliance on traditional fossil fuels is a major contributor to the global energy crisis and poses significant environmental challenges [1–2]. The escalating severity of these issues has made hydrogen energy a leading research focus owing to its high energy density, environmental friendliness, and versatile applicability [3–5]. Hydrogen storage is key to harnessing hydrogen as a viable clean energy carrier [6–9]. Because of its high volumetric density and inherent safety, solid-state hydrogen storage has considerable potential for future applications [10–13]. In this category, magnesium hydride (MgH2) has garnered significant research attention because of its high gravimetric capacity (7.6wt%), excellent reversibility, and low cost [14–16]. Nevertheless, the practical implementation of this technology poses a major challenge, primarily because of its sluggish kinetics and high operating temperature requirements [17–18]. Several strategies have been pursued to overcome the limitations of MgH2, including alloying [19–21], nanosizing [22–24], and catalyst doping [25–28]. Catalyst doping is a particularly effective method for accelerating the dehydrogenation/hydrogenation kinetics of MgH2 while maintaining a large capacity for hydrogen and ease of operation [29–30].

Transition metal-based catalysts have been extensively utilized for doping MgH2 because of their unique 3d electron orbitals and electronic structures [31–32]. Li et al. [33] reported the development of a novel Ti-based catalyst (Ti4O7), and the corresponding MgH2 + 7wt% Ti4O7 composite released 6.8wt% hydrogen within 18 min at 325°C. Xu et al. [34] reported that the MgH2 + 10wt% PdNi composite reduced the initial dehydrogenation temperature to 149°C while maintaining a hydrogen capacity of 6.3wt%. These studies revealed that while catalysts derived from Ti, Ni, and Pd offered a discernible improvements in the hydrogen storage performance of MgH2, V-based catalysts also appeared to be highly effective and showed promise for superior hydrogen sorption kinetics. Chen et al. [35] reported that the MgH2 + 3mol% VO2F composite achieved a low desorption activation energy of 80.6 kJ·mol−1, combined with a high capacity retention of 97.6% after 50 cycles. Zhang et al. [36] reported that, with the incorporation of nano-VHx, a composite exhibited excellent hydrogen storage properties. Vanadium hydride acts as a protective agent to inhibit the agglomeration of MgH2 and as a hydrogen diffusion channel to accelerate the reaction. The unique interconversion between V and V–H is attributed to the provision of more reactive sites, which acts as “hydrogen pumps” to boost the conversion between MgH2 and Mg. Nevertheless, the performance can be further optimized by incorporating a matrix that provides a high surface area and abundant active sites [37–38], representing a promising strategy for achieving superior catalytic effects.

The unique attributes of MXene, including its exceptional specific surface area, abundance of active sites, and tunable interlayer spacing, enable its dual function as a catalyst and matrix in solid-state hydrogen storage applications [39–40]. Wu et al. [41] reported that the MgH2 + 6wt% Ti3C2 composite initiated hydrogen release at 142°C, achieving a ca...