(277k) H2 Production from So2 Oxidation in a Pem Electrolyzer

Efficient production of clean hydrogen is important for the hydrogen economy. In this study the electrochemical production of hydrogen by the oxidation of sulfur dioxide to sulfuric acid is reported. This approach is novel because the anode reaction for the oxidation of sulfur dioxide was carried out in the gas phase, which leads to a reduction in the resistance to transport of the reactants to the electrode surface.

Sulfur dioxide gas at room temperature and atmospheric pressure was fed to the anode of a proton exchange membrane (PEM) fuel cell operating essentially in reverse. Water at elevated temperatures and atmospheric pressure was fed to the cathode. The sulfur dioxide was fed to the anode dry. In order to oxidize to sulfuric acid, water must also be present at the anode with the sulfur dioxide; this was achieved by the diffusion of water across the PEM due to a concentration gradient. With the application of current the sulfur dioxide was oxidized in the presence of water to form sulfuric acid, and hydrogen was evolved at the cathode.

Several factors affecting the performance of the electrolyzer have been investigated, including temperature effects and stoichiometric flow rate of sulfur dioxide to the anode. It has been found that increasing the temperature increases the performance of the electrolyzer and allows for operation at higher current density; this is due to increased diffusion of water through the membrane to the anode to facilitate in sulfur dioxide reduction. It was determined that decreasing the stoichiometric flow of sulfur dioxide down to 1.2 times excess does not significantly affect the electrolyzer performance.

In addition, the effect of catalyst loading on electrolyzer performance was investigated, and it was determined that increasing the catalyst loading from 0.5mg/cm2 Pt to 1.5mg/cm2 does not significantly improve electrolyzer performance.

Also, membrane thickness was investigated as a cause for increased electrolzyer performance. Because a thinner membrane allows for increased water diffusion, an electrolyzer utilizing a thinner membrane will be able to operate at higher current densities. This is because as the current density increases, so too does the requirement for water at the anode. Once the maximum diffusion of water across the membrane has been reached, there is not sufficient water at the anode to achieve the required rate of sulfur dioxide oxidation, and the electrolyzer performance decreases rapidly.

However, just because a thinner membrane allows for operation at higher currents does not mean that this membrane is the best choice. It is desired that the sulfuric acid leaving the anode be concentrated, because this sulfuric acid will be sent to a thermal cycle for decomposition to sulfur dioxide. While a thinner membrane does allow for operation at higher currents, the sulfuric acid leaving the anode is less concentrated than from an electrolyzer utilizing a thicker membrane, and so the excess water must be separated from the sulfuric acid downstream.

In this study hydrogen production will be reported for several different membrane types and operating temperatures, as well as sulfuric acid production rate and concentration. Also, the electrolyzer performance will be reported in standard polarization curves for comparison of performances based on membrane thickness, catalyst loading and operating temperature.