## 1 Introduction  
Hydrogen is considered as a sustainable energy carrier owing to the near-zero carbon emission that is possible through its use. However, methods to enable the safe and efficient storage of hydrogen with high density remain a major obstacle for its widespread use [1; 2; 3]. The U.S. Department of Energy has outlined a technical target for the on-board hydrogen storage system of 5.5 mass % H\({}_{2}\) by 2025 [4]. A range of materials including metal-organic frameworks, carbon nanostructures, liquid organic hydrogen carriers, metal hydrides, and complex metal hydrides have been proposed as promising hydrogen storage materials [5; 6; 7; 8; 9]. In particular, NaAlH\({}_{4}\) and LiAlH\({}_{4}\) have garnered immense attention due to their high theoretical hydrogen storage capacity (7.4 and 10.5 mass% H\({}_{2}\), respectively) and relatively low decomposition temperature (<300 \({}^{\circ}\)C) [10; 11]. Unfortunately, the poor hydrogen sorption kinetics of these materials and the lack of hydrogen reversibility in particular for LiAlH\({}_{4}\) remain severe drawbacks when considering any applications [12; 13]. Typically, the regeneration of LiAlH\({}_{4}\) from the decomposition products would require an extreme pressure, more than 1000 MPa at 170 \({}^{\circ}\)C, due to the thermodynamic constraints [12; 14; 15]. In contrast, the regeneration of NaAlH\({}_{4}\) from NaH and Al is easier, but still remains sluggish without appropriate modifications [16]. Extensive efforts have shown that the kinetics, thermodynamics, and reversibility of NaAlH\({}_{4}\) can be significantly improved by adding catalysts such as transition metal halides [17; 18], lanthanide oxides [19], titanium compounds [20; 21], or carbon materials [22; 23]. Unfortunately, the positive catalytic effect on reversibility observed with NaAlH\({}_{4}\) have been difficult to translate to LiAlH\({}_{4}\) due to the exothermic nature of hydrogen release from pristine LiAlH\({}_{4}\)[24]. An alternative approach to improve the hydrogen storage properties is through nanosizing. Several studies have shown significant improvements in hydrogen sorption properties upon nanosizing of hydrides [25; 26]. For example, [PERSON] et al. synthesized NaAlH\({}_{4}\) nanoparticles (<20 nm) supported on carbon nanofibers and achieved a low desorption temperature of less than 160 \({}^{\circ}\)C [27]. In this case, reducing the particle size of NaAlH\({}_{4}\) to the nanoscale leads to a modification of NaAlH\({}_{4}\) thermodynamics due to their increased surface area and number of hydrogen adsorption sites [28; 29]. Additionally, smaller hydride particles induce shorter diffusion paths, which results in faster hydrogen kinetics and improved reversibility [29; 30]. Currently, the simplest approach to synthesize nanoparticles of alanates is through confinement within the porosity of high surface area materials and carbons in particular, via melt infiltration or solvent impregnation [29; 30; 31; 32]. For example, the nanoconfinement of LiAlH\({}_{4}\) in high surface area graphite led to drastic improvements with an onset hydrogen desorption occurring at 135 \({}^{\circ}\)C instead of 200 \({}^{\circ}\)C and rehydrogenation observed at 300 \({}^{\circ}\)C under 7 MPa of hydrogen pressure [30]. Similarly, [PERSON] et al. reported on an improved cycling stability of NaAlH\({}_{4}\) upon confinement in mesoporous carbons [33]. Unfortunately, these nanoconfinement approach suffers from significant drawbacks including the dead weight and volume of the scaffold and the difficulties in filling the pores. This inevitably reduces the practical hydrogen storage capacity to <1 mass%, which is far below the practical requirements [25; 29; 30]. An alternative strategy is through the synthesis and stabilization of the isolated nanoparticles in a core-shell manner as previously reported [34; 35].