Experimental modeling of hydrogen producing steps in a novel sulfur–sulfur thermochemical water splitting cycle

• Experimental verification of novel sulfur–sulfur cycle feasibility.
• Monitoring experimental progression of two step chemical reaction to produce hydrogen sulfide via UV absorption of iodine.
• Development of a predictive model to track iodine in the batch reaction system.

One of the more well-known examples of thermochemical water splitting cycles is the sulfur–iodine cycle, which utilizes iodine and sulfur dioxide to transform water into hydrogen and oxygen. A novel sulfur–sulfur cycle has been developed based on the original sulfur–iodine cycle, seeking an overall more favorable chemical process. The initial stages of the original and novel cycles are the same, in using sulfur dioxide and iodine with an excess of water to form two strong acids, hydrogen iodide and sulfuric acid. The secondary reactions differ greatly and are based on a previously unwanted side reaction where the two strong acids regenerate iodine and water along with hydrogen sulfide, as shown in the equations below. The hydrogen sulfide can then be steam reformed to hydrogen and sulfur dioxide, completing the cycle. Excess sulfuric acid can also be treated to regenerate water and sulfur dioxide.I2(l)+SO2(g)+2H2O(l)→2HI(l)+H2SO4(l)I2(l)+SO2(g)+2H2O(l)→2HI(l)+H2SO4(l)8HI(l)+H2SO4(l)→4I2(l)+4H2O(l)+H2S(g)8HI(l)+H2SO4(l)→4I2(l)+4H2O(l)+H2S(g)This study seeks to explore the reaction kinetics of this two-step reaction by altering the various reaction temperatures and the initial concentration of water and to develop a robust and simple predictive model based on the resulting experimental data. The results suggest that an increase in the initial concentration of water will increase the rate of both reactions substantially and that it is beneficial to carry them out in the same vessel. A predictive model was successfully developed that allows for monitoring the progression of iodine through the reaction system in a batch setting. The results of the experimental and modeling work warrants further development of this novel sulfur–sulfur thermochemical water splitting cycle.

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