Thermochemical Hydrogen Production Using Copper-Chlorine Cycles

Thermochemical hydrogen production via the copper–chlorine (Cu–Cl) cycle is a multi‐step process that harnesses high‐temperature heat to split water into hydrogen and oxygen without direct electrical input. The cycle typically comprises four core reactions: hydrolysis of copper(I) chloride to produce hydrogen and copper oxychloride; thermal decomposition of the oxychloride to yield oxygen and copper(II) chloride; reduction of copper(II) chloride back to copper(I) chloride using hydrogen chloride gas; and regeneration of hydrogen chloride via reaction of metallic copper with chlorine. By operating between 450 °C and 550 °C, the Cu–Cl cycle offers the prospect of integration with industrial waste heat, concentrated solar thermal systems or high‐temperature nuclear reactors. Key advantages include lower maximum temperatures than alternative water‐splitting cycles, the use of relatively abundant materials and continuous internal recycling of chemical reagents. Challenges centre on materials corrosion, efficient heat recovery between steps, catalyst development for the electrolytic sub‐process and reliable management of multi‐phase reaction streams. Recent efforts have focused on optimising reactor designs, advancing electrocatalysts for the aqueous electrolysis stage and securing robust safety systems for continuous operation. Realising a commercially viable Cu–Cl process promises a scalable route to low‐carbon hydrogen with global relevance for decarbonising industrial feedstocks, transport fuels and grid balancing services.

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Thermochemical Hydrogen Production Using Copper-Chlorine Cycles publication trend

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Technical terms

Thermochemical cycle: A sequence of chemical reactions driven by heat that decomposes water into hydrogen and oxygen without net consumption of reagents.

Cuprous chloride (CuCl): A copper(I) chloride intermediate that undergoes hydrolysis to produce hydrogen in the Cu–Cl cycle.

Redox looping: A process in which a metal or metal compound alternately oxidises and reduces to split water, transferring oxygen between cycle steps.

Membrane electrode assembly (MEA): A component of electrolysers where catalysts, ion‐conducting membranes and gas diffusion layers are combined for efficient electrode reactions.

Hydrolysis reactor: The vessel in which cuprous chloride reacts with water to yield hydrogen and copper oxychloride under acidic conditions.

References

1. Thermochemical looping technologies for clean hydrogen production – Current status and recent advances. Journal of Cleaner Production (2023).
2. Investigations on CuCl/HCl Electrolysis Using a Pt/C Electrocatalyst-based Membrane Electrode Assembly. Journal of The Electrochemical Society (2023).
3. Safety Analysis of the Hydrolysis Reactor in the Cu-Cl Thermochemical Hydrogen Production Cycle—Part 1: Methodology and Selected Top Events. Energies (2024).