Quantifying Individual Potential Contributions of the Hybrid Sulfur Electrolyzer

Abstract

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...

A thermodynamic analysis of the SO2/H2SO4 system in SO2-depolarized electrolysis

“…The differences between the observed potential and that calculated by the Nernst equation Results of current interrupt and hydrogen pump experiments at USC suggest that the two main contributions to the overpotential are kinetic losses at the anode and the ohmic resistance of the membrane in the presence of concentrated sulfuric acid [23]. Therefore, it should be possible to lower the operating cell potential (i.e.…”

Characterizing Voltage Losses in an SO2Depolarized Electrolyzer Using Sulfonated Polybenzimidazole Membranes

“…8,9,11,12 However, an increase in acid concentration at the anode in the SDE operated with s-PBI shows no adverse effect on membrane resistance because the conductivity of s-PBI is not dependent on water to facilitate proton conduction. In addition, the measured resistance was not a function of temperature between 70…”

“…[8][9][10][11][12] For example, Westinghouse was only able to get the cell voltage down to 1.0 V at 400 mA/cm 2 , where we achieved 500 mA/cm 2 at 0.71 V and 1.2 A/cm 2 at 1.0 V using Nafion 212 (N212). However, to achieve overall process efficiency, concentrated sulfuric acid as well as low cell voltage at high current densities are * Electrochemical Society Member.…”

“…The resulting reactions at the anode and cathode, respectively, are: 2H + + 2e − → H 2 U 0 H2 = 0 V vs. SHE [3] Thus, the overall reaction in the electrolyzer is represented as: SO 2 + 2H 2 O → H 2 SO 4 + H 2 [4] Considerable progress was made in the last decade in lowering the operating voltage and increasing the current density of the SDE by moving from a microporous rubber diaphragm separator used by Westinghouse 7 to a perfluorinated sulfonic acid membrane (e.g., DuPont's Nafion). [8][9][10][11][12] For example, Westinghouse was only able to get the cell voltage down to 1.0 V at 400 mA/cm 2 , where we achieved 500 mA/cm 2 at 0.71 V and 1.2 A/cm 2 at 1.0 V using Nafion 212 (N212). However, to achieve overall process efficiency, concentrated sulfuric acid as well as low cell voltage at high current densities are necessary.…”

Solar Thermochemical Hydrogen (STCH) Processes

“…[15][16][17] However, water management was quickly identified as a key driver of electrolyzer performance. 15 Water sent to the electrolyzer cathode diffuses toward the anode due to a concentration difference, while at the same time, water is transported back across the membrane from the cathode to the anode due to electro-osmotic drag with H + .…”