Metal-Organic Frameworks Promoted Hydrogen Storage Properties of Magnesium Hydride for In-Situ Resource Utilization (ISRU) on Mars

Hydrogen is regarded as a promising solution to fulfill the energy demand of Mars human base in the future. Through in-situ resource utilization (ISRU) on Mars, the composite of metal-organic frameworks (MOFs) and magnesium hydride which demonstrates synergistic effect of physi- and chemisorption has been proposed to be an attractive approach for long-term hydrogen storage. MOFs may act either as scaffolds to confine Mg nanoparticles or as catalysts/precursors to lower the energy barrier for hydrogen dissociation on Mg surfaces. The corresponding mechanisms for faster hydrogenation/dehydrogenation kinetics and lower operation temperature were further discussed and analyzed.


INTRODUCTION
In 2020, United Arab Emirates (UAE), China, the United States launched Mars probes successively. Mars has become a major concern in space exploration in the 21st century. Several countries have already seriously put crewed missions to Mars on the agenda. Considering the high cost of launching resources from Earth, in-situ resource utilization (ISRU) has become increasingly important to make these missions economically feasible and sustainable.
Evidence showed that there was water resource on Mars, in the form of water ice in polar regions, subglacial liquid water, or mineral hydrate in lithosphere. In 2015, National Aeronautics and Space Administration (NASA) confirmed evidence that liquid salt water flowed on Mars (NASA, 2015). In 2018, European Space Agency (ESA) announced the detection of a 20 km-wide lake of liquid water underneath solid ice in the Planum Australe region, according to the radar profiles from the Mars Express spacecraft (Orosei et al., 2018). If the extraterrestrial water is efficiently acquired and utilized, sufficient hydrogen will be provided for permanent human base, which is regarded as an ideal fuel with high energy density (142 MJ/kg) and no harmful emission (water as the only combustion product).
However, hydrogen is too small and too active to be steadily stored, so that various hydrogen storage approaches have been developed. Conventional phase conversion processes, such as liquefied storage and cryo-compressed storage, have low volumetric capacity and demand high infrastructure cost (Thornton et al., 2017). The temperature on Mars varies greatly from −133 to 27°C, which is demanding for the thermal control of cryogenic storage system. The ultralow atmospheric pressure on Mars (less than 1% on Earth) may greatly increase the cost of the pressure vessel as well. Therefore, it is essential to find an efficient hydrogen storage solution to meet the requirements of longterm in-space application. Adsorption in solid materials is regarded as the most safe hydrogen storage method, and there are two strategies: associated adsorption and dissociated adsorption of hydrogen (Seayad and Antonelli, 2004).
Light metals can form chemical bonds with dissociated hydrogen atoms, generating stable metal hydrides, e.g. LiH, MgH 2 , AlH 3 (Duan et al., 2015;Wan et al., 2015;Wang et al., 2016;Sadhasivam et al., 2017;Duan et al., 2018). This is a typical dissociated adsorption strategy. Among metal hydrides, MgH 2 has been regarded as one of the most promising hydrogen storage carriers owing to its non-toxicity, low cost and excellent reversibility (Grochala and Edwards, 2004). The theoretical hydrogen uptake capacity of Mg/MgH 2 matrix is as high as 7.6 wt%. Moreover, abundant Mg content within Martian soil makes it an attractive option for ISRU on Mars (Robinson and Morrison, 2018). According to the chemical compositions of Martian soil detected with the Alpha Particle X-ray Spectrometer on the Opportunity rover (Rieder et al., 2004), MgO has an abundance of 7.2%, mainly in the form of olivine.
However, the slow sorption kinetics and high dehydrogenation thermal stability (ΔH 75 kJ/mol H 2 ) of MgH 2 attributed to the strong Mg-H chemical bond impedes its large-scale application. In order to further enhancing the hydrogen uptake capacity and accelerating hydrogen de/ adsorption kinetics of MgH 2 , various strategies have been applied. For example, multiple catalytic metals, metal oxides, and metal halides have been added into Mg/MgH 2 matrix to improve the sorption kinetic performances.
Carbon nanoarchitectures, e.g. activated carbons, graphene, carbon nanotubes (CNTs), and metal-organic frameworks (MOFs), have been regarded as a promising class of associated adsorption hydrogen storage materials due to their good chemical stability, high specific surface area, and low density (Xia et al., 2013). It is favorable that more than 95% of Martian atmosphere is composed of carbon dioxide, which guarantees the easily and sustainably accessible carbon source. In this case, promoting hydrogen storage properties of MgH 2 with carbon nanoarchitectures seems to be an attractive solution for the in-space application of hydrogen energy.

The Promotion Effect of Carbon Nanoarchitectures for Hydrogen Storage
In earlier researches (before 2010), porous carbon materials were regarded as a kind of good additives to improve hydrogen storage properties by enhancing the hydrogen diffusion in MgH 2 -C systems. Graphite was firstly used as anti-sticking agent to enhance the ball milling efficiency (Imamura et al., 2003;Imamura et al., 2005). Interestingly, comparing with noncarbon additives, all the carbon additives showed positive effects on the hydrogen storage properties of Mg, especially on the increment of hydrogenation capacity (Wu et al., 2006a). Whereafter, carbon materials with threedimensional structures have attracted more attention. For instance, a C 60 fullerene molecule is capable of storing up to 58 H atoms remaining a metastable structure, which is equivalent to a storage capacity of 7.5 wt% (Pupysheva et al., 2008). Owing to the strong adsorption ability for hydrogen and the excellent heat transfer performance, CNTs may enhance the hydrogen storage capacity and lower the hydrogen desorption platform temperature of Mg (Lyu et al., 2019).
The corresponding hydrogenation mechanism was reported to compose of three consecutive steps: 1) the dissociation of hydrogen molecules on Mg or alloy surfaces; 2) the diffusion of atomic hydrogen along grain boundaries; 3) the interaction between metal and atomic hydrogen. CNTs played a dominated role in the second step by forming thousands of 10-50 nm grains in the Mg particle (500-1,000 nm) (Yao et al., 2007), which built hydrogen diffusion channels to drive hydrogenation/ dehydrogenation reactions towards complete (Wu et al., 2006b). Besides, absorbed carbon component on freshly exposed Mg surfaces inhibits the reform of oxide layers. In the carbon fragments produced from ball milling during preparation, unhybridized π electrons are delocalized and then interact with hydrogen appropriately (Wu and Cheng, 2010).
However, fullerenes or CNTs have an inherent spatial structure, so that their properties can only be adjusted using physical approaches like ball milling. In order to lower the energy barrier for hydrogen dissociation, the addition of metallic catalyst is supposed to be achieved by alloying with Mg, which demands stringent preparation conditions for melting (Xu et al., 2019). Therefore, a class of more flexible and reproducible carbon-based materials with exposed metal sites is in demand. MOFs, which are assembled by various inorganic nodes and organic linkers through coordination bonds, have become a more competitive option for hydrogen storage in the past decade.
The interaction between hydrogen and adsorbent surfaces are mainly van der Waals force, the energy of which is as low as 4-8 kJ/mol. Adsorbent pore sizes close to the kinetic diameter of hydrogen molecule (2.89 Å) can promote the interaction with hydrogen, when the potential fields from opposite walls overlap (Perles et al., 2005). Although MOFs are favorable owing to their tunable porosities, high hydrogen storage capacities (up to 4.5-7.5 wt%) are normally achieved at cryogenic temperature (77 K) and high pressure, which is not desirable for practical use (Thomas, 2007;Shet et al., 2021). In order to improve the hydrogen storage capacities of MOFs, several approaches to introduce dissociated adsorption of hydrogen by forming hybrid composites have been come up with.
The hydrogen spillover mechanism has aroused wide interest. Hydrogen molecules are firstly dissociated on the catalyst, and then newly generated hydrogen atoms will more easily migrate to the substrate. The introduction of chemisorption hence enables to store hydrogen at room temperature. The addition of metal dopants, e.g. Pd, Pt, Ni, working as hydrogen spillover receptor can effectively enhance hydrogen storage capacity (Zhou et al., 2015;Guo et al., 2020). This mechanism gives a new perspective on the interaction between MOFs and magnesium for hydrogen storage. Rowsell and Yaghi (2005) proposed that combining light metals with MOFs was one of the most effective strategies to promote hydrogen adsorption in MOFs. Even substituting metal nodes within MOFs with Mg can enhance hydrogen uptake. According to the theoretical calculation result given by Han et al. (2009), Mg 4 O(CO 2 ) 6 H 6 cluster showed a stronger H 2 binding energy than original Zn 4 O (CO 2 ) 6 H 6 and Be-substituted one, so that Mg-MOF had the highest hydrogen uptake at low pressure (1 bar).

Metal-Organic Frameworks as Scaffolds to Enhance Hydrogen Storage Capacity of Magnesium
The diffusion path of hydrogen can be prominently shortened when Mg nanoparticles are confined inside a scaffold with fine porosity, which promotes the thermodynamics, kinetics, and reversibility of hydrogen storage (Nielsen et al., 2009;Zhang et al., 2017). The imposed nanostructural geometry of Mg/MgH 2 matrix creates a great number of defects, forming specific interactions between Mg-H bonds (Wu, 2008). For hydrogen storage, chemisorption positively correlates to the temperature while physisorption inversely correlates. A balance of high storage capacity and low operation temperature is supposed to be achieved by appropriate combination of associated adsorption and dissociated adsorption. In this way, magnesium showed considerable synergistic effect in the presence of MOFs for hydrogen de/adsorption properties.
Compared with bare Mg, the Mg/ZIF-67 composite lowered dehydrogenation activation energy by 43.2 kJ/mol. When hydrogen adsorbed by Mg/ZIF-67 was completely released at 338°C, only 44% hydrogen within Mg was released. Microstructural characterization revealed the formation of core-shell structure within the Mg/ZIF-67 nanocomposite (Figure 1). The MOF scaffold impeded the growth and agglomeration of Mg/MgH 2 nanocrystals as an "aggregation blocker" in the hydrogen de/adsorption processes and ensured outstanding cyclic stability. Therefore, the Mg/ZIF-67 composite showed no degradation in hydrogenation/dehydrogenation capacity even after 100 cycles (Wang et al., 2019a).
The research of Lim et al. (2012) showed that Mg embedded MOF (Mg@SNU-90′) had lower desorption temperature and higher hydrogen adsorption isosteric heat, owing to significantly reduced chemisorption temperature of Mg nanocrystals within SNU-90′ comparing with pure Mg powder. The estimated hydrogen uptake capacity of Mg alone within the composite reached 7.5 wt% at 473 K and 30 bar, indicating 99% of the Mg nanocrystals were fully utilized, which was very close to the theoretical limit of 7.6 wt%. Desorption experiments showed that the organic ligands of SNU-90′ were not hydrogenated, indicating that Mg@SNU-90' demonstrated synergistic behavior of physisorption by MOF and chemisorption by Mg. Jia et al. (2013) found that a fraction of hydrogen was desorbed below 80°C from MgH 2 clusters which were confined to mesoporous carbons, owing to partially transferring electrons from the Mg/MgH 2 matrix. Electron transfer capability between Mg and appropriate hosts hence lowers energy barriers for hydrogenation/dehydrogenation processes.

Metal-Organic Frameworks as Catalysts or Precursors to Modify Hydrogen Storage Kinetics of Magnesium
It is reported that the size effect of catalytic active sites plays an important role in preparing Mg-based nanocomposites (De Jongh and Adelhelm, 2010). MOFs not only adsorb hydrogen through their porosities, but also act as excellent catalyst carriers to enhance hydrogenation thermodynamic properties. Owing to the self-assembled inerratic spatial structures, the metal active sites within MOFs may achieve quite high dispersion. In this way, a fraction of transition metals is introduced into the nanocomposites to modify the high energy barrier for hydrogen dissociation on Mg surfaces with no obvious density increasement. Ma et al. (2019) used trimasic acid (TMA) as organic linker to prepare TMA-TM MOF (TM Co., Fe). After ball milled with MgH 2 , the activation energy (Ea) for hydrogenation of the dehydrogenated MgH 2 -Fe MOF dropped to 66.8 kJ/mol H 2 , which was much lower than that of pure Mg (100.7 kJ/mol H 2 ). The dehydrogenation activation energy (Ed) of MgH 2 -Fe MOF composites (142.3 kJ/mol H 2 ) was also much lower than that of pure MgH 2 (181.4 kJ/mol H 2 ). MOFs showed considerable catalytic effects in hydrogen storage. Jia et al. (2015) fabricated ultrafine (2-3 nm) metallic Ni nanoparticles embedded in the crushed MgH 2 matrix by insitu bottom-up reduction of the Ni-MOF-74 precursor. The composite adsorbed 6.2 wt% hydrogen within only 30 s at a temperature as low as 150 C and fully desorbed within just 20 min at 300 C, which exhibited faster kinetics than conventional MgH 2 . Theoretical calculation showed that dissociated hydrogen was capable of shifting quickly from Ni active sites to pure Mg with an energy increment of only 0.07 eV. Hydrogen diffusion on Ni cluster hence became the rate limiting step, so that limited Ni nanoparticle size significantly promoted hydrogenation/dehydrogenation kinetics.
In the Mg embedded Ni-MOF composite, in-situ formation of Mg 2 NiH 4 around the MgH 2 nanocrystals (∼3 nm) was observed (Ma et al., 2020). The "hydrogen pump" effect owing to mutual phase transformation between Mg 2 Ni and Mg 2 NiH 4 made it easier to release and uptake hydrogen for MgH 2 matrix. Huang et al. (2021) further studied the catalytic mechanism of MOF-derived Ni nanoparticles on monolayer Ti 3 C 2 MXene carrier. DFT calculations showed that Mg 2 Ni (Ti) H 4 had lower formation energy than the neat one, so that Mg 2 Ni/ Mg 2 NiH 4 transformation was facilitated by in-situ formed Ti 0 from MXene during ball milling, which kept a steady state with no phase or valence change. Comparing with the pristine MgH 2 , over 120 C decrease in desorption peak temperature and over 60% reduction in dehydrogenation activation energy were achieved owing to the "facilitated hydrogen pump" effect. MOF-promoted hydrogen storage properties of MgH 2 have been summarized in Table 1.

CONCLUSIONS AND PERSPECTIVES
The storage and utilization of hydrogen have great potential to meet the energy demand not only on Earth but also on Mars. Promoting hydrogen storage properties of MgH 2 with MOFs is regarded as an attractive solution for long-term in-space application of hydrogen energy. The rationality and feasibility to imply this concept on Mars have been demonstrated in this article. Mechanisms and effects of enhanced hydrogen storage when combining associated adsorption with dissociated adsorption have been discussed and analyzed. In particular, the introduction of MOFs as catalysts or precursors during preparation seems to be more promising. On the one hand, it has great potential to simplify the synthesis procedure into one step, which reduces resource consuming resulting from byproducts. On the other hand, the in-situ formed matrix of Mg and metal nodes within MOFs is easily designed and modified to achieve lower activation energy. Moreover, the hydrogenation/ dehydrogenation kinetics under the condition of weak gravity on Mars are supposed to be further studied to bring this concept into practice.

AUTHOR CONTRIBUTIONS
YW and QL contributed the conception of the study. ZZ produced and wrote the article. HW and XX edited the article. All authors contributed to the article and approved the submitted version.

FUNDING
This study was sponsored by Shanghai Sailing Program (21YF1430200).