Hydrogen Production via a Sulfur-Sulfur Thermochemical Water-Splitting Cycle

Thermochemical water splitting cycles have been conceptualized and researched for over half a century, yet to this day none are commercially viable. The heavily studied Sulfur-Iodine cycle has been stalled in the early development stage due to a difficult HI-H2O separation step and material compatibility issues. In an effort to avoid the azeotropic HI-H2O mixture, an imidazolium-based ionic liquid was used as a reaction medium instead of water. Ionic liquids were selected based on their high solubility for SO2, I2, and tunable miscibility with water. The initial low temperature step of the Sulfur-Iodine cycle was successfully carried out in ionic liquid reaction medium. Kinetics of the reaction were investigated by I2 colorimetry. The reaction also evolved H2S gas, which led to the conceptual idea of a new Sulfur-Sulfur thermochemical cycle, shown below: 4I2l+4S O2l+8H2 Ol↔4H 2SO4l+8HI l 8HIl+H 2SO4l↔ H2Sg +4H2Ol +4I2l 3H2SO4g ↔3H2Og +3SO2g+1 12O2g H2Sg +2H2Og ↔SO2g +3H2g .;The critical step in the Sulfur-Sulfur cycle is the steam reformation of H2S. This highly endothermic step is shown to successfully occur at temperatures in excess of 800°C in the presence of a molybdenum catalyst. A parametric study varying the H2O:H2S ratio, temperature, and residence time in a simple tubular quartz reactor was carried out and Arrhenius parameters were estimated.;All reactive steps of the Sulfur-Sulfur cycle have been either demonstrated previously or demonstrated in this work. A theoretical heat-to-hydrogen thermal efficiency is estimated to be 55% at a hot temperature of 1100 K and 59% at 2000 K. As a highly efficient, all-fluid based thermochemical cycle, the Sulfur-Sulfur cycle has great potential for feasible process implementation for the transformation of high quality heat to chemical energy.