The final step in the proposed reaction cycle is the use of SO 2 to electrochemically produce H 2 SO 4 and the H 2 product in an aqueous solution. While direct electrolysis of water to form H 2 and O 2 is very slow due to the complex kinetics of the 4-electron oxidation to form O 2 , the introduction of SO 2 into water depolarizes the system and allows for rapid oxidation of the SO 2 , producing H 2 SO 4 and H 2 . This reaction requires application of at least 0.29 V, much smaller than 1.23 V required for water and the more typical values of ~ 1.5-1.8 V needed to electrolyze water to O 2 . SO 2 -depolarized water electrolysis: