Water splitting, thermochemical

Various thermochemical water-splitting processes have been proposed to address hydrogen production on a large scale. Most of these cycles utilize a high-temperature heat source for rapid chemical kinetics and high conversion efficiencies. In the thermochemical processes, a set of coupled, thermally-driven chemical reactions decompose water into H2 and O with various intermediate regents. All reagents returned within the cycle and 

These thermochemical water-splitting processes have been explored extensively in the 1970s and some 25 key cycles have been identified (Besenbruch, 1982 Brown et al., 2003). The sulfur-iodine (SI) cycle is currently the leading candidate. The SI cycle consists of three chemical processes expressed as sections  

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Multistep Thermochemical Water Splitting. Multistep thermochemical hydrogen production methods are designed to avoid the problems of one-step water spHtting, ie, the high temperatures needed to achieve appreciable AG reduction, and the low efficiencies of water electrolysis. Although water electrolysis itself is quite efficient, the production of electricity is inefficient (30—40%). This results in an overall efficiency of 24—35% for water electrolysis. 

A detailed discussion of thermochemical water splitting is available (155,165—167). Whereas many problems remain to be solved before commercia1i2ation is considered, this method has the potential of beiag a more efficient, and hence more cost-effective way to produce hydrogen than is water electrolysis. 

Non-electrolytic sources of hydrogen have also been studied. The chemical problem is how to transfer the correct amount of free energy to a water molecule in order to decompose it. In the last few years about I0(X)0 such thermochemical water-splitting cycles have been identified, most of them with the help of computers, though it is significant that the most promising ones were discovered first by the intuition of chemists. 

Thermochemical Water Splitting by Iodine-Sulfur Cycle.137... 

Norman, J.H. et al., Thermochemical water-splitting for hydrogen production, Gas Research Institute report GRI-80/0105,1980. 

Kubo, S. et al., A demonstration study on a closed-cycle hydrogen production by thermochemical water-splitting Iodine-Sulfur process, Nucl. Eng. Des., 233, 347, 2004. 

Kubo, S. et al., Corrosion test on structural materials for iodine-sulfur thermochemical water splitting cycle, in Proc. 2nd Topical Conf. on Fuel Cell Tech., AIChE 2003 Spring National Meeting, New Orleans, March 30-April 3, 2003. 

Sulfur-Iodine Thermochemical Water-Splitting Process... 

The sulfur-iodine thermochemical water-splitting cycle (S-1 cycle) developed for hydrogen production from water is fundamentally based on the following three chemical reactions (Wang, 2007) ... 

Fig. 2.6 General representation of the thermochemical water splitting process...

Alvani C, Ennas G, La Barbara A, Marongiu G, Padella F, Varsano F (2005) Synthesis and characterization of nanociystalline MnFe204 advances in thermochemical water splitting. Int J Hydrogen Energy 30 1407-1411... 

Kodoma T, Kondoh Y, Kiyama A, Shimizu K (2003) Hydrogen production by solar thermochemical water-splitting/methane-reforming process. International Solar Energy Conference pp 121-128... 

Norman JH, Mysels KJ, Sharp R, Williamson D (1982) Studies of the sulfur-iodine thermochemical water-splitting cycle. Int J Hydrogen Energy 7 545-556... 

Kubo S, Kasahara S, Okuda H, Terada A, Tanaka N, Inaba Y, Ohashi H, Inagaki Y, Onuki K, Hino R (2004) A pilot test plan of the thermochemical water-splitting iodine-sulfur process. Nucl Eng Des 233 355-362... 

Kubo, S., et al. (2004), A Pilot Test Plan of the Thermochemical Water-splitting Iodine-Sulfur Process , Nuclear Engineering and Design, 233 (1-3), 355-362. 

O Keefe, D.R., et al. (1982), Preliminary Results from Bench-scale Testing of Sulphur-iodine Thermochemical Water-splitting Cycle , Int. J. Hydrogen Energy, 7 (5), 381-92. 

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