First Observation of Defined Structural Motifs in the Sulfur-Iodine Thermochemical Cycle and Their Role in Hydrogen Production

The present paper investigates the ionic species coexisting in the HIx feed of the sulfur-iodine thermochemical cycle. For this purpose, Raman and inelastic neutron scattering as well as molecular modelling were applied to the study of the binary HI-H2O system and the ternary HI-I2-H2O and KI-I2-H2O systems. Raman spectra, obtained at 298 K, strongly suggest the coexistence of I3−, I−(I2), and I−(I2)2 species. Whereas on the other hand, inelastic neutron scattering spectra (20 K) revealed, for the first time, evidence for the presence of discrete water structural motifs under specific conditions. Molecular modelling of two idealized structures has allowed us to establish a reasonable interpretation of the important structural motifs in these systems, in terms of the azeotrope of the HI-H2O system and the pseudoazeotrope of the HI-I2-H2O system.