Raman scattering studies of the condensed phase of the HI_x feed of the sulfur-iodine thermochemical cycle

The species present in the condensed phase of the HI_x, hydrogen producing, feed in the water-splitting, sulfur-iodine thermochemical cycle have been investigated using spontaneous Raman scattering. Measurements of I₂-containing species in the low Raman-shift region from 50 to 400cm^-1 in samples of the two aqueous binary systems, I₂/H₂O and HI/H₂O, and the ternary system HI/I₂/H₂O with and without the addition of H₂SO₄ have provided a consistent picture of the aqueous iodine and polyiodide chemistry. Samples were contained in sealed silica ampoules and were heated to temperatures in the range 20-300°C. The results, which cover a wide range of I₂ and HI mole fractions, and xI2/x_HI mole ratios, in the HI/I₂/H₂O system, reveal the co-occurrence of H⁺I-3, H⁺-I^-(I₂), and H⁺-I^-(I₂)₂ solvated species in the condensed phase of HI_x. Thus, while the first is mostly evident by its strong fundamental band with Raman shift in the range from 110 to 115cm^-1, the other two species appear convoluted in a broad prominent band whose Raman shift ranges between 153 and 172cm^-1 depending on the xI2/x_HI mole ratio. These well characterized Raman features are proposed as an in situ diagnostic for process control of the cycle.