The thermochemical cycle involving the interconversion between sulfur dioxide and sulfuric acid is a promising method for efficient, large-scale production of hydrogen. A key step in the process is the oxidation of sulfur dioxide to sulfuric acid in an electrolyzer. Gaseous SnormalO2 fed to a proton exchange membrane (PEM) electrolyzer was previously investigated and was shown to be a promising system for the electrolysis step. A critical factor in the performance of this gas-fed electrolyzer is the management of water since it: (i) is needed as a reactant, (ii) determines the product sulfuric acid concentration, (iii) affects SnormalO2 crossover rate, and (iv) serves to hydrate the membrane. Therefore, we present a coupled mathematical and experimental study on the effect of water on the production of sulfuric acid in a gas-phase PEM electrolyzer. The model is shown to successfully predict the concentration of sulfuric acid as a function of temperature, current density, pressure differential across the membrane, and membrane thickness.

The hybrid sulfur thermochemical cycle has been proposed as a means to produce efficiently massive quantities of clean hydrogen using a high-temperature heat source like nuclear or solar.The cycle consists of two steps, one of which is electrolytic. The reversible cell potential for this step and, hence, the resulting operating potential will depend on the concentrations of dissolved SO 2 and sulfuric acid at the electrode. To understand better how these are related as functions of temperature and pressure, an Aspen Plus phase equilibrium model using the OLI Mixed Solvent Electrolyte physical properties method was employed to determine the activities of the species present in the system. These activities were used in conjunction with the Nernst equation to determine the reversible cell potential as a function of sulfuric acid concentration, temperature and pressure. A significant difference between the reversible and actual cell potentials was found, suggesting that there may be considerable room for reducing the operating potential.

The hybrid sulfur cycle has been investigated as a means to produce clean hydrogen efficiently on a large scale by first decomposing H 2 SO 4 to SO 2 , O 2 , and H 2 O and then electrochemically oxidizing SO 2 back to H 2 SO 4 with the cogeneration of H 2 . Thus far, it has been determined that the total cell potential for the hybrid sulfur electrolyzer is controlled mainly by water transport in the cell. Water is required at the anode to participate in the oxidation of SO 2 to H 2 SO 4 and to hydrate the membrane. In addition, water transport to the anode influences the concentration of the sulfuric acid produced. The resulting sulfuric acid concentration at the anode influences the equilibrium potential of and the reaction kinetics for SO 2 oxidation and the average conductivity of the membrane. A final contribution to the potential loss is the diffusion of SO 2 through the sulfuric acid to the catalyst site. Here, we extend our understanding of water transport to predict the individual contributions to the total cell potential. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3397901͔ All rights reserved. The hybrid sulfur ͑HyS͒ cycle has gained attention due to the possibility of using this process to produce clean hydrogen on a large scale at efficiencies higher than those using water electrolysis.1-19 The high temperature decomposition of H 2 SO 4 to SO 2 , O 2 , and H 2 O is suited for use with advanced gas-cooled nuclear reactor heat sources or solar receiver arrays.1-7 The electrolysis step described here is coupled with the high temperature step to complete the cycle. We developed a gas-fed anode electrolyzer in which SO 2 is oxidized to H 2 SO 4 via the following reaction 11,[14][15][16][17][18] We have successfully carried out Reactions 1 and 2 over a range of operating conditions ͑i.e., temperature, flow rate, and membrane pressure differential͒ and design variations ͑i.e., catalyst loading and membrane type and thickness͒. [14][15][16][17] We have also accurately predicted water transport and correlated the operating potential to the sulfuric acid concentration produced at the anode. [15][16][17] We have shown that the concentration of sulfuric acid produced at the anode increases with current density and that the sulfuric acid concentration at the anode influences the cell potential via the reversible cell potential U eq . 18Although we have previously correlated cell potential to acid concentration and hence water transport, a quantitative measure of the various potential losses has never been made. In this discussion, we present a comprehensive investigation of the components that make up the total cell potential ͑i.e., reversible cell potential, membrane resistance, and catalyst activity͒ to better understand and improve electrolyzer performance and operation. ExperimentalThe experimental setup was the same as that described previously.14-17 The cell was a standard 10 cm 2 cell from Fuel Cell Technologies, Inc. Reactants and products were fed to and from the cell through Kynar manifolds inst...