Step concentrated (typically from 30 -65 % wt/wt) and thermally decomposed at temperature above 800 o C to remove oxygen from the system and regenerate the SO 2 . This step is the most energy-intensive of the cycle, and the highly corrosive nature of H 2 SO 4 and the SO 3 reaction intermediate limits potential reactor materials to specialized ceramics such as SiC. To circumvent this energy-intensive and technically challenging step, we propose chemical reduction of sulfuric acid, similar to a method previously proposed for use in the similar sulfur-iodine cycle. the most optimal conditions would be to hold the aqueous H 2 SO 4 under reflux at its boiling point (337 °C, for concentrated H 2 SO 4 ). Notably, this temperature is approximately the high-temperature limit (~330 °C) for the catalytic Claus process utilized in Step 2. Holding the solution under reflex would be expected to increase reaction rates and remove generated H 2 O that otherwise would dilute the H 2 SO 4 solution. Figure shows the free energy and enthalpy of likely reactions for the chemical reduction of H 2 SO 4 . These data show that at a reflux temperature of 337 °C, the reduction of H 2 SO 4 is exergonic with both H 2 S and S feedstocks, but only exothermic when H 2 S is used. The reaction between H 2 S and SO 2 is also favorable and exothermic at 337 °C, and the S product formed in solution will be consumed by the H 2 SO 4 . These above factors suggest that this Step 3 in the proposed reaction cycle is expected to be rapid and run to completion, but may need additional thermal energy input to keep the system under reflux. Step 4: Generation of H2 using a SO 2 -depolarized electrochemical cell: