Recent progress of mechanically activated Mg-based materials to promote hydrogen generation via hydrolysis

Urbanization and industrialization have led to increases in energy utilization, and large-scale non-renewable fossil fuel use has caused environmental pollution, global warming, and climate change; thus, research on green and renewable energy sources is essential. Researchers have suggested that a shift from a fossil fuel economy to a “green” energy system by adopting renewable energy approaches. The implementation of renewable energy sources, including hydrogen [1], [2], [3], biomass [4], [5], hydropower [6], geothermal [7], wind [8], solar thermal [9], and solar PV [10], [11], can decrease or eliminate the reliance on fossil fuels.

Hydrogen possesses several advantages over traditional fuels. Its byproducts are environmentally friendly, and it has abundant reserves and 142 MJkg⁻¹ energy density [12], [13]. John and Haldane proposed the concept of a hydrogen economy [14], [15] and extensive hydrogen use was made possible in the mid-2000 s by the introduction of proton exchange membrane fuel cells (PEMCs) for portable devices and automobiles [16], [17]. However, the lack of reliable techniques for hydrogen production, storage, and delivery is a major drawback for the application of hydrogen fuel [18], [19], [20].

At the same time, despite being the most abundant and inexhaustible element in the universe, hydrogen is generally not found in a free state. Rather, it occurs in a combined form, such as in water, hydrocarbon fuels, and other organic compounds. To date, fossil fuels including coal, oil, gas, and naphtha have been used to produce hydrogen, as well as by green generation methods, such as photocatalysis [21], [22], photo-electrochemical [23], photochemical [24], [25], biomass [26], [27] and water electrolysis methods [28], [29]. Natural gas reforming and petroleum cracking are mature large-scale production techniques. A strong network is required for the transportation of natural gas because the source is located far away from the consumption point [30]. Unfortunately, these processes will be restricted in the future because of their consumption of nonrenewable fossil fuels and the release of harmful pollutants into the atmosphere [31]. In term of greenhouse gases, CO₂ is responsible for approximately 76 % [32]. Water electrolysis is another well-known industrial-scale technique for producing hydrogen [33]. However, this method has limitations in terms of storage and transportation, which represent the current challenges associated with meeting the hydrogen demand. Biomass, photocatalysis, and photochemical processes all produce hydrogen on a laboratory scale, and further research is required [34], [35]. The availability of safe, effective, and on-demand hydrogen remains an issue that must be resolved immediately.

Various factors like cost effectiveness, amiable operating conditions, and improved theoretical hydrogen yield has increased the interest of scientists to produce hydrogen through metal hydrolysis [36], [37], [38], [39]. Magnesium (Mg)-based materials have been the subject of widespread attention and research as sources for hydrogen hydrolysis owing to their comprehensive advantages, however, the dense passivation film that forms during the hydrolysis process will prevent direct contact between unreacted Mg/MgH₂ and water which leads to poor kinetics or termination of the hydrolysis reaction. Therefore, eliminating the adverse effects of the dense layer is essential for promoting the Mg and MgH₂ hydrolysis rates (Fig. 1(A)).

This review focuses on the recent research progress on nanosized Mg-based materials for hydrogen production by hydrolysis, additionally, it outlines the particular obstacles that must be tackled in order to facilitate their practical implementation. First, we briefly describe the development of a mechanochemical method to activate Mg-based materials for hydrolysis to produce hydrogen. In the ball-milling technique, catalysts may be added, alloys may be formed, and material modifications may be produced to control the slow reaction rate and poor conversion yields seen in Mg-based materials. The role of ball milling parameters, size of particles, material morphology, and different hydrolysis mechanisms are also discussed in detail. Furthermore, a detailed overview of the applications, regeneration, and cost-efficiency of Mg used for hydrogen generation is provided.