Method to Prevent Sulfur Accumulation Inside Membrane Electrode Assembly

摘要

HyS is conceptually the simplest of the thermochemical cycles and involves only sulfur chemistry. In the HyS Cycle hydrogen gas (H(sub 2)) is produced at the cathode of the electrochemical cell (or electrolyzer). Sulfur dioxide (SO(sub 2)) is oxidized at the anode to form sulfuric acid (H(sub 2)SO(sub 4)) and protons (H(sup +)). A separate high temperature reaction decomposes the sulfuric acid to water and sulfur dioxide which are recycled to the electrolyzers, and oxygen which is separated out as a secondary product. The electrolyzer includes a membrane that will allow hydrogen ions to pass through but block the flow of hydrogen gas. The membrane is also intended to prevent other chemical species from migrating between electrodes and undergoing undesired reactions that could poison the cathode or reduce overall process efficiency. In conventional water electrolysis, water is oxidized at the anode to produce protons and oxygen. The standard cell potential for conventional water electrolysis is 1.23 volts at 25 C. However, commercial electrolyzers typically require higher voltages ranging from 1.8 V to 2.6 V (Kirk-Othmer, 1991). The oxidation of sulfur dioxide instead of water in the HyS electrolyzer occurs at a much lower potential. For example, the standard cell potential for sulfur dioxide oxidation at 25 C in 50 wt % sulfuric acid is 0.29 V (Westinghouse, 1980). Since power consumption by the electrolyzers is equal to voltage times current, and current is proportional to hydrogen production, a large reduction in voltage results in a large reduction in electrical power cost per unit of hydrogen generated.