Effect of Water on the Electrochemical Oxidation of Gas-Phase SO2 in a PEM Electrolyzer for H2 Production

Water plays a critical role in producing hydrogen from the electrochemical oxidation of SO 2 in a proton exchange membrane (PEM) electrolyzer. Not only is water needed to keep the membrane hydrated, but it is also a reactant. One way to supply water is to dissolve SO 2 in sulfuric acid and feed that liquid to the anode, but this process results in significant diffusion resistance for the SO 2 . Alternatively, we have developed a process where SO 2 is fed as a gas to the anode compartment and reacts with water crossing the membrane to produce sulfuric acid. There was concern that the diffusion resistance of water through the membrane is as significant as SO 2 diffusion through water, thus limiting the benefit of a gas-phase anode feed. We show here that water diffusion through the membrane is not as limiting as liquid-phase SO 2 diffusion. Therefore, we can control the cell voltage, the limiting current, and the sulfuric acid concentration by varying the diffusion resistance of the membrane via thickness or temperature. Catalyst loading, however, has a negligible effect on cell performance.