During thermochemical water splitting, water is separated into oxygen and hydrogen using chemical reactions. These reactions are initiated by heat or by combination of heat and electric energy using hybrid cycles. This process only needs source of water and heat as the rest of the chemical substances are reused. The final products are hydrogen and oxygen.

One of the cycles is the sulphur-iodine thermochemical cycle. It is an inexpensive and efficient method of hydrogen production using nuclear power. The first step of the process is the reaction of water with iodine and sulphur dioxide [9]. The outcomes of this reaction are sulfuric acid and hydrogen iodine. The next step is the endothermic decomposition of sulphuric acid and hydrogen iodine [10 and 11], which requires high temperatures (800 - 1200°C and 450°C respectively).

 

[9] I2 + SO2 + H2O ⟶ 2HI + H2SO4

[10] H2SO4 ⟶ SO2 + H2SO4 + 1/2O2

[11] 2HI ⟶ I2 + H2

 

The efficiency of such a complex process is difficult to establish. Generally, it varies between 40 – 52% (50% at 950°C). Higher temperature means higher efficiency of the cycle. Compared to electrolysis, the energy loss during electric energy production is eliminated. Disadvantages lay in high temperatures and aggressive chemicals such as sulphuric acid and hydroiodic acid. Therefore, containers must be made of – materials with high resistance to these chemicals. Same as electrolysis, theoretically no waste is produced during the thermochemical splitting. However, certain amount of chemical compounds is lost during this process and needs to be regularly refilled. This long-term technology pathway has potentially low or no greenhouse gas emissions despite the process control in mass production remains challenging to translate this technology into industry.