Hydrogen production from water and solar energy by the Ni-I-S thermochemical cycle

  • Dr Pier Paolo Prosini, Italy
  • Salvatore Sau, ENEA R.C. Casaccia, Rome, Italy
  • Giampaolo Caputo, ENEA R.C. Casaccia, Rome, Italy
  • Dr Cinzia Cento, ENEA R.C. Casaccia, Italy
  • Alberto Giaconia, ENEA R.C. Casaccia, Rome, Italy

    • Thermochemical water-splitting cycles (TWSC) are promising solution to the challenging objective of hydrogen production from water and renewable energy sources. After the recent closed-loop demonstration achieved in Japan, research on the iodine-sulfur (I-S) TWSC is now mainly focused on engineering issues regarding the coupling with the heat source, the equipment design and the efficiency improvements.
    Gaseous HI must be separated from a liquid H2O/HI/I2 solution prior the decomposition to produce hydrogen: this is a critical step in terms of process management, kinetics, energy losses, corrosion and costs.
    Here we present a new approach making use of Ni/NiI2/NiO recycling agents, where hydrogen is not directly generated by the thermodynamically-limited HI decomposition:

    5 I2 + SO2(gas) + 16 H2O(liq.) → (H2SO4+ 4 H2O)H2SO4-phase + (2 HI + 10 H2O + 4 I2)HIx-phase (70-120°C) (1)

    (2 HI + 10 H2O + 4 I2)HIx-phase + NiO(sol) → NiI2(sol) + 4 I2(liq) + 11 H2O(vap) (120-150°C) (2)

    NiI2(sol) → Ni(sol) + I2(gas) (650-750°C) (3)

    (H2SO4+ 4 H2O)H2SO4-phase + Ni(sol) → (NiSO4 + 4 H2O)NiSO4-phase + H2 (gas) (25-80°C) (4)

    (NiSO4 + 4 H2O)NiSO4-phase → NiSO4∙H2O(sol) + 3 H2O(vap) → NiO(sol) + SO2(gas) + O2(gas) + 4 H2O(vap) (800-900°C) (5)

    Bunsen reaction (1) produces two liquid phases: HIx-phase reacts with NiO (2) to obtain NiI2 that is afterwards decomposed (3); H2SO4-phase reacts with metallic Ni to produce hydrogen (4), and the NiSO4 decomposed (5).
    These reactions were experimentally demonstrated. Results obtained will be discussed.
    The use of biomass to drive reaction (5) was studied too.