Anchoring Mo single atoms on N-CNTs synchronizes hydrogenation/dehydrogenation property of Mg/MgH₂

Highlights

• MoSA and Mg nanoparticles both distribute uniformly on the N-CNTs surface.

• Mg/MgH₂ @MoSA-N-CNTs shows superior performance on de/hydrogenation kinetics.

• The mechanism on H₂ ab-/de-sorption properties of Mg/MgH₂ is proposed.

Abstract

MgH₂ has attracted substantial consideration mainly due to its sufficient content and high gravimetric density etc. However, its high thermodynamic stability and poor dynamics hinder its practical application. N-doping carbon nanotubes (N-CNTs) supported Mo single atoms (MoSA-N-CNTs) was constructed to facilitate the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system. According to the synergistic effect of N-CNTs nanoconfinement and MoSA catalysts, Mg@MoSA-N-CNTs can absorb 7.37 wt.% H₂ during the milling within 15 h and the corresponding activation energy for hydrogenation can be reduced to 21.2 kJ·mol⁻¹. MgH₂ @MoSA-N-CNTs releases 7.23 wt.% H2 at 325 ℃ for 5 min, nearly 3 times pristine MgH₂. Meanwhile, its initial dehydriding temperature decreases from 265.7°C to 204.6°C, and its apparent activation energy for dehydrogenation also greatly decreases from 108.6 kJ/mol to 68.4 kJ/mol. The above values are much lower than the pure MgH₂. The synergistic effect mechanism of N-CNTs and MoSA on hydrogenation/dehydrogenation properties of Mg/MgH₂ is proposed systemically. It is revealed that the MoSA and the N-CNTs not only accelerate H₂ dissociation and diffusion but also effectively prevent the Mg/MgH₂ nanoparticles agglomeration. The above results highlight the great promise of Mg/MgH₂ @MoSA-N-CNTs composite for high-performance Mg-based hydrogen storage materials.

Introduction

In recent years, a number of environmental issues, such as the greenhouse effect, have become increasingly problematic as a result of excessive exploitation and consumption of fossil energy [1]. Researchers have been searching for practical ways to use green energy to minimize carbon emissions, relieve the burden on natural resources, and achieve carbon neutrality through a low-carbon economy [2], [3]. As a potential energy carrier, hydrogen can effectively combat the aforementioned pollution issues due to its lightweight, renewability, and absence of carbon emissions [4]. Therefore, efficient utilization of hydrogen is bound to become the ultimate goal of new energy development in the future [5]. Solid-state hydrogen storage is superior to gas-phase storage in terms of efficiency, safety, and usage [6], [7], [8], [9], [10], [11]. In recent years, there has been a lot of focus on solid-state hydrogen storage technologies [12], [13], [14]. Hydrogen storage technology has been hampered by issues including poor storage density and difficult use at room temperature, although solid-state hydrogen storage materials are a crucial part of hydrogen utilization [6].

Magnesium hydride (MgH₂), a traditional family of solid-state hydrogen storage materials, has plenty of advantages, including an excellent gravimetric capacity (7.6 wt.%) and 111 kg m⁻³ of high volumetric capacity [15], [16], [17], [18], [19]. Unfortunately, its thermodynamic and kinetic problems, eg. high operating temperatures and thermal stability (∆H=76 kJ/mol) limit the MgH₂ applications for onboard H₂ storage [20], [21]. Thus, several efforts have been performed to solve the above-mentioned problems, including nanosizing [22], [23], [24], catalyzing [25], [26], [27], [28], and nanoconfinement [29], [30], [31]. Among those methods studied so far, nano-structuring and catalyst doping have resulted in a substantial promotion of the dehydrogenation kinetics of MgH₂ [32], [33], [34], [35], [36], [37], [38], [39], [40]. However, dehydrided MgH₂ nanocrystals agglomerate during the H₂ absorption/desorption process, resulting in a poor reversibility.

Several 3D carbon materials, including ordered mesoporous carbon [29], CNTs [30], graphene [31], etc. were employed to efficiently prevent MgH₂ from aggregating. It was shown by our team that CNTs, as a highly effective framework, can further enhance the cycle characteristics of MgH₂. This core-shell nanostructured composite has better hydrogen storage capabilities than pure MgH₂ because the particle size of the MgH₂ is restricted to the nanometer range [41]. Furthermore, Liu et al. observed that CNTs effectively inhibit MgH₂ aggregation and that MgH₂ @bamboo-shaped CNTs exhibit remarkable dehydriding and rehydriding capabilities [42]. In-situ loading of a nanocatalyst onto a carbon nanotube-confined material has been shown to enhance the H₂ adsorption/desorption performance of MgH₂.

Catalyst loading is considered as an effective strategy to improve the thermodynamic and kinetic properties of hydride, especially transition metal catalysts because of their high catalytic activity. The activity of transition metal catalysts is highly influenced by their size. It has been established that metal atoms with unsaturated coordination are likely to the active catalytic centers [43]. Therefore, when optimization catalysts, it is highly desired to simultaneously decrease the size of a catalyst and increase the fraction of metal atoms that has unsaturated coordination. On the basis of this aspect, single non-precious-metal atoms anchored on supports would be a promising candidate for catalysts, because of their maximum atom efficiency, unique catalytic performances, and unsaturated metal coordination environment [44]. It is worth noting that rational introducing a small amount of metal single atoms (SAs) into the carbon skeleton, not only changes the polarity of carbon and thus enhances physical adsorption, but also ensures the effective exposure surface sites for catalytic conversion without reducing its specific surface area and pore volume [45]. However, due to the high surface free energy of SAs metal center, it is chemically unstable and even tends to aggregate into nanoparticles, which significantly reduces the catalytic activity and leads to sluggish conversion reaction [46]. At present, the coordination of adjacent dopants (C, N, etc.) on the carrier with metal (M) atoms (M=Mo, Fe, Co, Ni, etc.) can significantly solve the problem of monatomic agglomeration, thereby realizing flexible controllable reaction activity [47]. Especially, the N-doped carbon material anchored metal SAs can be used as excellent catalysts for the conversion because of their special electronic structure and maximum atom utilization rate [48]. Meanwhile, the M-Nx SAs catalyst possesses abundant active sites to effectively accelerate the kinetic conversion in the reaction process [49]. Compared with traditional transition metals (Ti, Ni, Co, etc.), Molybdenum (Mo), exhibits superior catalytic efficacy in energy-related conversions as a low-cost catalyst [50], [51]. Actually, Mo-based catalysts have been widely used to replace Pt or Ni-based catalysts mainly due to a similar d electron structure [52]. Among these transitional metal (M) atoms, the orbitals of Mo atoms on the structure present a half-full state and are easily activated, making Mo-based SACs the most promising catalysts. Its catalytic activity is closely related to the unoccupied d orbital which can accept electrons from the hydride. This action not only reinforces the H dissociation. A weaker metal-H bond is another effect of the π-back electron donation. However, its catalytic mechanism on the hydrogenation of metal is still unclear. Moreover, a carbonization process is inevitable for preparing Mo particles, which always leads to the aggregation of Mo nano-particles. Consequently, the catalytic performance of Mo for de/rehydrogenation is inferior to that of the comprehensively evaluated TM catalysts. Based on the above analysis, confining nano-sized metal on the N-CNTs surface, can not only prohibit metal from aggregation but also increase its specific surface areas resulting in stability and enhancement of the nanostructure and catalytic activity. In addition, doping CNTs with metal-free heteroatoms, such as N, can significantly improve their catalytic activity mainly because of the modified electronic state of adjacent carbon [53]. Therefore, in-situ loading of single atoms on the surface of modified CNTs is an effective way to develop efficient hydrogenation/dehydrogenation catalysts, in which synergistic effects of different components can further improve the catalytic activity.

In the present work, in view of the electrostatic attraction between the basic groups in the solvent of ammonia molybdate and the acidic group on the surface of N-CNTs triggered by hydroxylamine hydrochloride, an effectively assembling MoSA on the nitrogen-doped carbon nanotubes (MoSA-N-CNTs) was constructed. After ball milling, the MgH₂ loaded on the N-CNTs (MgH₂ @MoSA-N-CNTs) was obtained. This nanocomposite presents a high catalytic activity for hydrogenation and dehydrogenation. The outstanding activity is mainly ascribed to the catalytic activity of MoSA and the nanoconfinement of N-doped CNTs. The localized catalyst prepared in this study provides a new strategy for improving the performance of hydrogen storage materials and promotes the promotion and application of high-performance hydrogen storage technology.