Thermochemical Hydrogen Production using the Iodine-Sulfur Cycle

The iodine-sulfur cycle is a closed-loop thermochemical process that uses heat to split water into its constituent gases without direct electrical input. It comprises three core reactions: the high-temperature decomposition of sulphuric acid into sulphur dioxide, water vapour and oxygen; the hydrolysis of sulphur dioxide in the presence of iodine to regenerate sulphuric acid and produce hydrogen iodide; and the catalytic splitting of hydrogen iodide into hydrogen gas and iodine. Operating temperatures range from around 800 °C for the sulphuric acid step to 450–550 °C for the hydriodic acid decomposition. Heat sources such as high-temperature nuclear reactors or concentrated solar systems can drive these reactions, enabling large-scale, low-carbon hydrogen production. Key challenges include managing corrosive acid mixtures, optimising separations of mixed acid streams, and developing materials that withstand repeated thermal cycling. Advances in reactor design, catalyst development and heat-transfer technologies are steadily improving cycle efficiency. The global transition to a hydrogen economy and stringent carbon-reduction targets highlight the importance of this cycle as a sustainable route to clean hydrogen for industrial processes, fuel-cell vehicles and energy storage.

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Technical terms

Thermochemical cycle: A series of high-temperature chemical reactions that split water into hydrogen and oxygen using heat rather than electricity.

Iodine-sulfur cycle: A three-step thermochemical water-splitting cycle involving the conversion of sulphuric acid and hydrogen iodide to produce hydrogen, oxygen and recyclable iodine and sulphur dioxide.

Sulfuric acid decomposition: The endothermic reaction in which H2SO4 is thermally broken down into SO2, H2O and O2, typically occurring above 800 °C.

Hydriodic acid decomposition: The catalytic reaction in which HI is split into I2 and H2, usually conducted at temperatures around 500 °C.

Bayonet heat exchanger: A concentric-tube heat exchanger design that optimises heat transfer for corrosive high-temperature fluids by routing hot and cold streams through inner and outer tubes.

Catalytic decomposer: A reactor unit containing a solid catalyst that lowers the activation energy of chemical decomposition reactions, enhancing reaction rates at reduced temperatures.

References

1. Review of Sulfuric Acid Decomposition Processes for Sulfur-Based Thermochemical Hydrogen Production Cycles. Processes (2020).
2. Structural Design Simulation of Bayonet Heat Exchanger for Sulfuric Acid Decomposition. Energies (2021).
3. Thermodynamics and Kinetic Modeling of the ZnSO4·H2O Thermal Decomposition in the Presence of a Pd/Al2O3 Catalyst. Energies (2022).