John A. Staser

Effect of Water Transport on the Production of Hydrogen and Sulfuric Acid in a PEM Electrolyzer

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 SO2 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 SO2 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.

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.

Quantifying Individual Potential Contributions of the Hybrid Sulfur Electrolyzer

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 S0 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.

Sulfur Dioxide Crossover during the Production of Hydrogen and Sulfuric Acid in a PEM Electrolyzer

A proton exchange membrane PEM electrolyzer has been investigated as a viable system for the electrolysis step in the thermochemical conversion of sulfur dioxide to sulfuric acid for the large-scale production of hydrogen. Unfortunately, during operation, sulfur dioxide can diffuse from the anode to the cathode. This has several negative effects, including reduction to sulfur that could potentially damage the electrode, consumption of current that would otherwise be used for the production of hydrogen,

Transport Properties and Performance of Polymer Electrolyte Membranes for the Hybrid Sulfur Electrolyzer

The water transport and SO2 crossover in the hybrid sulfur cycle electrolyzer were quantified for a polyphenylene-based proton exchange membrane and compared to the performance of industry-standard Nafion membranes. While Nafion exhibits good performance, there exists the possibility of a significant SO2 crossover, which can modify the electrode composition, consume current that should be used for hydrogen production, introduce SO2 to the hydrogen stream, and result in a loss of sulfur from the system. Recent research has focused on polyphenylene-based membranes that have exhibited high current density with good stability both chemical and temperature while limiting SO2 crossover. In this paper, we extend our previous water-transportmodeling work on Nafion membranes to this polymer electrolyte and directly compare the two in terms of electrolyzer performance and SO2 crossover. We show the ability of polyphenylene membranes to operate at elevated temperatures with improved performance over lower temperatures; the high temperature performance exceeds that of Nafion membranes.

Gas-Phase Hybrid Sulfur Electrolyzer Stack

The hybrid sulfur process has attracted considerable attention in recent years due its potential to produce large quantities of clean hydrogen more efficiently than hightemperature water electrolysis. The hybrid sulfur process consists of a high-temperature step for the decomposition of H2SO4 to SO2, and a low-temperature electrochemical step, in which the SO2 is oxidized in the presence of water to produce H2SO4 at the anode with the cogeneration of H2 at the cathode.