Effect of catholyte in a single step electrochemical hydroiodic acid decomposition for hydrogen production using the iodine–sulfur thermochemical cycle

Abstract

This study identifies the most suitable catholyte for the electrochemical HI decomposition process, an emerging single-step alternative to the conventional multistep HI section of the I–S thermochemical cycle for hydrogen production. Four catholytes, H₂O, H₃PO₄, H₂SO₄, and HI, were shortlisted based on compatibility with the I–S cycle and ability to support the hydrogen evolution. Polarization studies in a two-compartment electrochemical cell revealed a similar order of onset potentials for the electrochemical HI decomposition across all four catholyte electrolytes. However, the performance at higher overpotentials following the onset of the electrochemical HI decomposition followed the order H₂SO₄ > HI > H₃PO₄ > H₂O. The polarization behavior was not found to vary significantly with the catholyte concentration. Model fitting revealed that the invariance in polarization behavior with the catholyte concentration arose from the compensation between the changes in activation overpotentials with the open-circuit voltage. The onset potential was predominantly influenced by the I₂/HI ratio in the anolyte, with higher ratios resulting in an increase in the onset potentials for electrochemical HI decomposition. Polarization studies identified H₂SO₄ and HI as the most promising catholytes for further investigation. Consequently, continuous electrochemical HI decomposition and hydrogen production was demonstrated with H₂SO₄ and HI as the catholyte for three I₂/HI ratios of 0.25, 0.5, and 1 at two different current densities, with an average current efficiency of ∼97%. Among the two acids, H₂SO₄ showed higher current efficiency and lower energy consumption per mole of hydrogen compared to HI under the similar anolyte configuration and current density.