Amorphous scaly high-entropy borides with electron traps for efficient catalysis in solid-state hydrogen storage

Abstract

Owing to the orbital hybridization between the transition metal and the B element and the electron-trapping effect of the B element, transition metal borides are considered very promising materials for energy catalysis. In this work, an amorphous scaly high-entropy boride (HEB) with electron traps was designed and fabricated via a facile reduction method to improve the hydrogen storage properties of magnesium hydride (MgH2). For dehydrogenation, the onset temperature of MgH2 + 10wt% HEB was dropped to 187.4°C; besides, the composite exhibited superior isothermal kinetics and the activation energy of the composite was reduced from (212.78 ± 3.93) to (65.04 ± 2.81) kJ/mol. In addition, MgH2 + 10wt% HEB could absorb hydrogen at 21.5°C, and 5.02wt% H2 was charged in 50 min at 75°C. For reversible hydrogen storage capacity tests, the composite maintained a retention rate of 97% with 6.47wt% hydrogen capacity after 30 cycles. Combining microstructure evidence with hydrogen storage performance, the catalytic mechanism was proposed. During ball milling, scaly high-entropy borides riveted a large number of heterogeneous active sites on the surface of MgH2. Driven by the cocktail effect as well as the orbital hybridization of metal borides, numerous active sites steadily enhanced the hydrogen storage reactions in MgH2.

1. Introduction

As a highly promising energy source, hydrogen can power environmentally friendly technologies such as hydrogen engines and hydrogen fuel cells [1–4]. However, the current development of hydrogen storage technology is lagging behind, which limits the practical application of hydrogen energy. Magnesium hydride (MgH2) has the advantages of good reversibility, convenient storage and transportation due to its high hydrogen storage capacity of 7.6wt% [5–6]. However, MgH2 has high thermodynamic stability and slow kinetic properties [7–9]; in addition, the cyclic property of MgH2 is relatively disadvantageous [10–11]. Past studies have shown that nanosizing [12], alloying [13–14], adding catalysts [15–17], and severe plastic deformation (SPD) methods [18] can improve the hydrogen storage properties of MgH2. Among them, catalytic doping is feasible and effective in improving the hydrogen storage properties of MgH2.

Transition metal compounds such as metal oxides, metal sulfides, and metal hydroxides have been used to improve the hydrogen storage properties of metal hydrides [19–20]. It is worth mentioning that metal boride catalysts are promising catalysts with a small radius of B atoms, which allows the B atoms to occupy interstitial positions within the metal lattice and maintain the integrity of the metal bonding [21–22]. Meanwhile, electronegativity and a unique outer electron configuration determine the diversity of bonding modes and bonding strengths in the transition metal borides [23–24]. Metal borides not only take into account the multielemental synergistic effects, but also have the electron-deficient property of B-atoms, which can accelerate the rate of electron migration in the reaction, and thus they have great application prospects in the field of catalytic hydrogen storage [25]. In the previous explorations, Liu et al. uncovered that TiB2 [26] and NiB [27] can effectively improve the ability of MgH2 to store hydrogen. Gao et al. demonstrated that onset temperatures of hydrogen desorption for MgH2–CoB [28] and MgH2–FeB [29] were reduced to 214°C and 196°C; besides, with the addition of FeB, the desorption rate of the composite at the temperature of 400°C was rose to 23.1wt%/h. Zhang et al. discovered that MgH2’ onset temperature was reduced to 190°C by adding amorphous NiCoB, and MgH2–NiCoB could release 6.5wt% H2 in 200 s [30].

High entropy materials are defined as materials consisting of at least five elements with an atomic ratio (at%) between 5% and 35% of each element, or the materials with a configurational entropy (ΔSconf) > 1.5R (R is the gas constant) [31]. Recently, high entropy materials are highly potent due to their highly editable and unlimited creativity [32], and catalysts of high entropy materials are emerging for energy applications [33–35]. The high entropy materials have been studied as hydrogen storage media. Dangwal et al. predicted the formation enthalpy of TixZr2−xCrMnFeNi by machine learning and synthesized Ti0.5Zr1.5CrMnFeNi with room temperature hydrogen storage performance [36]. Edalati et al. prepared TiZrCrMnFeNi hydrogen storage alloy, and the alloy can absorb and desorb hydrogen at room temperature without activation [37]. Additionally, high entropy materials have shown promise in various catalytic applications, but their use in hydrogen storage, particularly as borides, remains underexplored. This work introduces amorphous scaly high-entropy borides with electron traps, which exhibit exceptional catalytic performance for MgH2, offering a new avenue for efficient solid-state hydrogen storage.