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

Abstract

The water transport and SO 2 crossover in the hybrid sulfur cycle electrolyzer were quantified for a poly͑phenylene͒-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 SO 2 crossover, which can modify the electrode composition, consume current that should be used for hydrogen production, introduce SO 2 to the hydrogen stream, and result in a loss of sulfur from the system. Recent research has focused on poly͑phenylene͒-based membranes that have exhibited high current density with good stability ͑both chemical and temperature͒ while limiting SO 2 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 SO 2 crossover. We show the ability of poly͑phenylene͒ membranes to operate at elevated temperatures with improved performance over lower temperatures; the high temperature performance exceeds that of Nafion membranes.

Characterizing Voltage Losses in an SO2Depolarized Electrolyzer Using Sulfonated Polybenzimidazole Membranes

“…[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.…”

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

“…11,13,15,16 We showed that water transport affected the electrolyzer performance ͑i.e., cell voltage͒ by controlling the sulfuric acid concentration at the anode. We in turn developed a model to predict water flux through the membrane as a function of membrane thickness, temperature, current density, and pressure differential.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In this thermochemical cycle, sulfuric acid is decomposed at high temperature ͑ϳ850°C͒ to SO 2 and water, and the SO 2 is converted back to sulfuric acid in a proton exchange membrane ͑PEM͒ electrolyzer. Hence, the sulfur compounds are internally recycled, such that the overall process decomposes water into hydrogen and oxygen.…”

“…Hence, the sulfur compounds are internally recycled, such that the overall process decomposes water into hydrogen and oxygen. We developed a gas-fed anode electrolyzer in which SO 2 is oxidized to H 2 SO 4 via the following reaction 11,13,15,16 …”