The present paper investigates the ionic species coexisting in the HI 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 , 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. 1. Introduction Thermochemical cycles (TC), which decompose feedwater to generate hydrogen and oxygen, are a promising route to the large-scale hydrogen production. The primary product of these closed-cycle chemical reactions, hydrogen, is both a carbon-free energy source and a valuable reagent gas (N.B. the current industry annual demand for hydrogen is of about 0.1？Gton, of which only 2% is produced from renewable sources) [1–3]. The TC that has attracted the most attention worldwide is the sulfur-iodine cycle (SI-TC), the first stage of which is the exothermic Bunsen reaction: This is followed by two endothermic reactions, one for each of the two reaction products, H2SO4 and HI. The decomposition reaction for H2SO4 requires high temperatures whilst the decomposition reaction for HI requires only modest temperatures. Overall, this three-step scheme is seen to regenerate the reagents, SO2 and I2, whilst splitting water into H2 and O2 gases. It was discovered by General Atomics in the late 1970s that when the Bunsen reaction was operated with excess H2O and I2, a spontaneous separation into two aqueous acid phases occurred. The lighter phase contained predominantly H2SO4, and the heavier phase containing most of the HI [4, 5]. This gravity-driven phase separation makes an important contribution to the overall efficiency of the SI-TC. Under these reaction conditions it is more realistic to represent the Bunsen separation, Reaction (1), as follows: where (i) represents the lighter H2SO4-rich phase, and (ii) represents the heavier HI-rich phase, referred to here as the HI phase, or simply HI [4, 6]. The two phases (i) and (ii) are the respective feeds to the separate H2SO4 and HI decomposition stages of the SI-TC. The
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