Near-Isothermal Doped-Hercynite Redox Cycle for Solar-Thermal Water Splitting

Abstract

Hydrogen productivity exceeding 350 micromoles H2/g total redox material has been demonstrated for near-isothermal processing using the doped-hercynite cycle for oxidation with steam carried out at 1350oC following 1500oC reduction. This temperature difference driving the redox is quite narrow compared to standard 500oC temperature swing (T-swing) redox processing. Such near-isothermal processing substantially reduces the difficult solid/solid heat recuperation, the energy required to heat back up to reduction temperatures following oxidation and the thermal stresses associated with heating/cooling active materials associated with standard T-swing redox cycling. Evidence is provided indicating that the cycle occurs via an oxygen-vacancy formation process. Efficiency analysis identifies isothermal processing with perfect steam/steam heat exchange as providing for the highest theoretically possible efficiency. Although it may be impossible to operate isothermally with perfect gas/gas heat exchange in practice, it does provide for an optimal “stretch” target to aim for. Near-isothermal operation seems achievable in practice and may provide for the most realistic efficient mode of operation.