- Hybrid sulfur cycle
The hybrid sulfur cycle (HyS) is a sequence of processes used for
hydrogen production. It is a variant of the sulfur-iodine cycle.
The HyS cycle involves a thermochemical and an electrochemical reaction, where the net reaction is the decomposition of water. All other chemicals are recycled. The HyS process requires an efficient source of heat.
The two reactions in the HyS cycle are as follows: [ Summers, W.A., "Hybrid Sulfur Thermochemical Process Development", Savannah River National Laboratory, May 16, 2006. [http://www.hydrogen.energy.gov/pdfs/review06/pdp_25_summers.pdf (PDF)] .]
#2 H2SO4 → 2 H2O + 2 SO2 + O2 (thermochemical 800 - 900 °C)
# SO2 + 2 H2O → H2SO4 + H2 (electrochemical 80 - 120 °C)
:: "Net reaction: 2 H2O → 2 H2 + O2"
Sulfur dioxide acts as a depolarizer on the anode. This results in a significant decrease of voltage (and thus electrical energy) required for reaction (2). The reversible voltage of reaction (2) is about 0.17 V, compared to 1.23 V required for electrolysis of water (with oxygen evolution as the anodic reaction).
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