The Magnesium-Iodine (Mg-I) thermochemical cycle represents a promising pathway for hydrogen production through a series of chemical reactions that split water into hydrogen and oxygen using intermediate compounds. This cycle leverages the reactivity of magnesium and iodine to achieve water decomposition at relatively lower temperatures compared to other thermochemical processes, making it a candidate for integration with solar thermal energy systems. The cycle operates through distinct reaction steps, each with specific energy requirements and challenges, particularly in handling iodine and managing efficiency losses.

The Mg-I cycle consists of three primary reactions. The first step involves the reaction between magnesium oxide (MgO) and iodine (I₂) to produce magnesium iodide (MgI₂) and oxygen (O₂). This reaction occurs at temperatures around 850°C and is endothermic, requiring significant heat input. The second step hydrolyzes magnesium iodide to produce hydrogen iodide (HI) and magnesium hydroxide (Mg(OH)₂). This step proceeds at lower temperatures, typically below 400°C. The final step decomposes hydrogen iodide into hydrogen (H₂) and iodine (I₂), which is recycled back to the first reaction. The decomposition of HI is endothermic and occurs at approximately 450°C. The net reaction of the cycle is the decomposition of water into hydrogen and oxygen, with magnesium and iodine acting as recyclable intermediates.

Energy requirements for the Mg-I cycle are a critical consideration. The first reaction, involving MgO and I₂, demands high temperatures, but these are still lower than those required by sulfur- or metal oxide-based cycles, which often exceed 1000°C. This temperature range aligns well with concentrated solar power (CSP) systems, which can deliver heat at 800–1000°C. Solar thermal integration could thus provide the necessary heat without relying on fossil fuels, reducing the carbon footprint of hydrogen production. The second and third steps operate at moderate temperatures, allowing for efficient heat recovery and reuse within the system. However, the overall efficiency of the cycle is limited by heat losses, incomplete reactions, and the energy penalty associated with HI decomposition.

Iodine handling poses significant challenges in the Mg-I cycle. Iodine is corrosive and volatile, requiring specialized materials for reactors and piping to prevent degradation. Its high reactivity can also lead to side reactions that reduce process efficiency. For instance, iodine may react with construction materials or form unwanted byproducts, complicating separation and recycling. Effective containment and purification systems are essential to mitigate these issues. Additionally, the toxicity of iodine necessitates stringent safety protocols to protect workers and prevent environmental release.

Efficiency barriers further constrain the Mg-I cycle. The thermal efficiency of the cycle, defined as the energy content of produced hydrogen relative to the total energy input, is influenced by reaction yields and heat recovery effectiveness. Incomplete conversion of MgO to MgI₂ or HI to H₂ can lower overall efficiency, while heat losses during transfers between steps reduce the usable energy. Research indicates that optimizing reaction conditions, such as pressure and catalyst use, could improve yields, but achieving high efficiency remains a challenge. The cycle’s theoretical efficiency is estimated at around 40–50%, but practical systems may achieve lower values due to operational losses.

Comparing the Mg-I cycle with other iodine-based cycles highlights its relative advantages and drawbacks. The Sulfur-Iodine (S-I) cycle, for example, operates at higher temperatures (up to 900°C) but achieves higher efficiency due to more favorable thermodynamics. The S-I cycle also benefits from extensive research and pilot-scale demonstrations, whereas the Mg-I cycle remains less mature. However, the Mg-I cycle avoids the use of sulfuric acid, which simplifies corrosion management compared to the S-I process. Another iodine-based cycle, the Copper-Chlorine (Cu-Cl) cycle, incorporates electrochemical steps and operates at lower temperatures, but it introduces complexity with additional chemicals and separation requirements. The Mg-I cycle’s simplicity and lower temperature demands make it an attractive alternative, provided iodine-related challenges can be addressed.

Solar thermal integration offers a pathway to enhance the sustainability of the Mg-I cycle. CSP systems can supply the high-temperature heat needed for the MgO-I₂ reaction, while mid- and low-temperature heat for subsequent steps can be sourced from waste heat or secondary solar collectors. Coupling the cycle with thermal energy storage would enable continuous operation despite solar intermittency. Pilot studies have demonstrated the feasibility of solar-driven thermochemical cycles, though scaling the Mg-I process requires further development in reactor design and heat management.

Material compatibility is another critical factor for the Mg-I cycle. Reactor components must withstand corrosive iodine and high temperatures without degrading. Nickel-based alloys and ceramics have shown promise in laboratory settings, but long-term durability under operational conditions remains unproven. Advances in materials science, such as protective coatings or composite materials, could extend equipment lifetimes and reduce maintenance costs.

The Mg-I thermochemical cycle presents a viable route for hydrogen production with potential advantages in temperature requirements and chemical simplicity. Its compatibility with solar thermal energy makes it a candidate for sustainable hydrogen generation, but challenges in iodine handling, efficiency, and materials must be overcome. Compared to other iodine-based cycles, it offers a balance of moderate temperatures and reduced chemical complexity, though further research is needed to validate its practicality at scale. Addressing these hurdles could position the Mg-I cycle as a complementary technology in the future hydrogen economy.