The hybrid sulfur cycle has been investigated as a means to produce CO 2 -free hydrogen efficiently on a large scale through the decomposition of 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 . The net effect is the production of hydrogen and oxygen from water. Recently, sulfonated polybenzimidazoles (s-PBI) have been investigated as a replacement for Nafion due to the ability to offer increased process efficiency through the generation of higher acid concentrations at lower potentials. Here, we measure the acid concentrations and individual potential contributions toward the overall operating voltage seen in the SO 2 -depolarized-electrolyzer. We then determine model parameters necessary to predict voltage losses in a cell over a wide range of operating temperatures, pressures, currents and reactant flow rates. The hydrogen production program at the U. S. Department of Energy is examining an array of distributed and centralized hydrogen facilities that could contribute to the hydrogen generation infrastructure.1 Thermochemical cycles are being considered for large scale, centralized facilities due to their potential for high efficiencies at low costs. These cycles involve a series of chemical reactions that result in the splitting of water at much lower temperatures (∼500-1000• C) than direct thermal dissociation (>2500 • C) and at much higher efficiencies than direct water electrolysis.2 Chemical species in these reactions are recycled resulting in the consumption of only energy and water to produce hydrogen and oxygen. Although there are hundreds of possible thermochemical cycles, the hybrid-sulfur (HyS) process is the only all-fluid, two step thermochemical cycle. 3-6The high temperature step (850-950• C) involves the decomposition of H 2 SO 4 to produce oxygen and sulfur dioxide via the following reaction:The SO 2 is separated, cooled, and sent to the SO 2 -depolarized electrolyzer (SDE). The resulting reactions at the anode and cathode, respectively, are:Thus, the overall reaction in the electrolyzer is represented as: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 * Electrochemical Society Member.* * Electrochemical Society Student Member. * * * Electrochemical Society Fellow.z E-mail: taylor.garrick@gm.com; weidner@cec.sc.edu necessary. The key issue when using membranes like Nafion that rely on water for their proton conductivity is that high acid concentrations dehydrate ...

The hybrid sulfur thermochemical cycle has seen much attentional recently due to its potential to enable the production of clean hydrogen on a large scale with a higher efficiency than water electrolysis.[1-9] The two step hybrid sulfur (HyS) process involves the high temperature decomposition of sulfuric acid to produce sulfur dioxide, oxygen, and water, as well as a low temperature electrochemical oxidation of sulfur dioxide in the presence of water to produce sulfuric acid and gaseous hydrogen. Due to the internal recycling of sulfur compounds in the HyS process, the overall balance is the decomposition of water to form gaseous hydrogen and oxygen. This process is interesting because the high temperature decomposition step could be coupled to next generation solar power plants or high temperature nuclear reactors in order to produce hydrogen for other applications. Using a proton exchange membrane such as Nafion in the HyS electrolyzer has been thoroughly examined via the prediction of mass transport through the membrane as a function of operating potential and other design variables. However, Nafion presents several drawbacks, including the inability to operate at elevated temperatures and the decreased performance seen when exposed to high acid concentrations.[7, 8] Previously we showed that acid doped polybenzimidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for proton conductivity, and therefore offer the possibility of operating at higher acid concentrations in order to minimize energy requirements necessary for water separation, as well as operation at higher temperatures in order to minimize voltage losses.[8] Previous operation has focused on low temperatures in for comparison to Nafion, however, high temperature operation offers a better utilization of the advantages of PBI. Here, we present the results of high temperature operation of the HyS electrolyzer using sulfonated PBI. The acid concentration and hydrogen production rates are also quantified. Current interrupt was used to determine the membrane resistance under different operating parameters. A discussion of economic operating targets will also be provided. 1. H. R. Colon-Mercado and D. T. Hobbs, Electrochem Commun, 9, 2649 (2007). 2. J. Staser, R. P. Ramasamy, P. Sivasubramanian and J. W. Weidner, Electrochem Solid St, 10, E17 (2007). 3. S. K. Lee, C. H. Kim, W. C. Cho, K. S. Kang, C. S. Park and K. K. Bae, Int J Hydrogen Energ, 34, 4701 (2009). 4. J. A. Staser, K. Norman, C. H. Fujimoto, M. A. Hickner and J. W. Weidner, J Electrochem Soc, 156, B842 (2009). 5. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B16 (2009). 6. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B836 (2009). 7. J. A. Staser, M. B. Gorensek and J. W. Weidner, J Electrochem Soc, 157, B952 (2010). 8. J. V. Jayakumar, A. Gulledge, J. A. Staser, C. H. Kim, B. C. Benicewicz and J. W. Weidner, Ecs Electrochem Lett, 1, F44 (2012). 9. J. L. Steimke, T. J. Steeper, H. R. Colon-Mercado and M. B. Gorensek, Int J Hydrogen Energ, 40, 13281 (2015).

The hybrid sulfur thermochemical cycle has seen much attentional recently due to its potential to enable the production of clean hydrogen on a large scale with a higher efficiency than water electrolysis.[1-9] The two step hybrid sulfur (HyS) process involves the high temperature decomposition of sulfuric acid to produce sulfur dioxide, oxygen, and water, as well as a low temperature electrochemical oxidation of sulfur dioxide in the presence of water to produce sulfuric acid and gaseous hydrogen. Due to the internal recycling of sulfur compounds in the HyS process, the overall balance is the decomposition of water to form gaseous hydrogen and oxygen. This process is interesting because the high temperature decomposition step could be coupled to next generation solar power plants or high temperature nuclear reactors in order to produce hydrogen for other applications. Using a proton exchange membrane such as Nafion in the HyS electrolyzer has been thoroughly examined via the prediction of mass transport through the membrane as a function of operating potential and other design variables. However, Nafion presents several drawbacks, including the inability to operate at elevated temperatures and the decreased performance seen when exposed to high acid concentrations.[7, 8] Previously we showed that acid doped polybenzimidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for proton conductivity, and therefore offer the possibility of operating at higher acid concentrations in order to minimize energy requirements necessary for water separation, as well as operation at higher temperatures in order to minimize voltage losses.[8] Through the successful operation of the HyS electrolyzer using sulfuric acid doped PBI membranes, we have determined that despite the relative thickness of s-PBI, the area-specific resistance of s-PBI compares favorably with Nafion and is not adversely affected by the sulfuric acid concentration at the anode. Through further characterization of the membrane and electrolyzer, we have been able to refine a model for high temperature and high pressure operation of the electrolyzer allowing for further analysis of the system in order to determine operating conditions that allow for economically viable operation of the HyS electrolyzer. 1. H. R. Colon-Mercado and D. T. Hobbs, Electrochem Commun, 9, 2649 (2007). 2. J. Staser, R. P. Ramasamy, P. Sivasubramanian and J. W. Weidner, Electrochem Solid St, 10, E17 (2007). 3. S. K. Lee, C. H. Kim, W. C. Cho, K. S. Kang, C. S. Park and K. K. Bae, Int J Hydrogen Energ, 34, 4701 (2009). 4. J. A. Staser, K. Norman, C. H. Fujimoto, M. A. Hickner and J. W. Weidner, J Electrochem Soc, 156, B842 (2009). 5. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B16 (2009). 6. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B836 (2009). 7. J. A. Staser, M. B. Gorensek and J. W. Weidner, J Electrochem Soc, 157, B952 (2010). 8. J. V. Jayakumar, A. Gulledge, J. A. Staser, C. H. Kim, B. C. Benicewicz and J. W. Weidner, Ecs Electrochem Lett, 1, F44 (2012). 9. J. L. Steimke, T. J. Steeper, H. R. Colon-Mercado and M. B. Gorensek, Int J Hydrogen Energ, 40, 13281 (2015).

The hybrid sulfur thermochemical cycle has seen much attention recently due to its potential to enable the production of clean hydrogen on a large scale with a higher energy efficiency than water electrolysis. The two step hybrid sulfur (HyS) process involves the high temperature decomposition of sulfuric acid to produce sulfur dioxide, oxygen, and water, as well as a low temperature electrochemical oxidation of sulfur dioxide in the presence of water to produce sulfuric acid and gaseous hydrogen. Due to the internal recycling of sulfur compounds in the HyS process, the overall balance is the decomposition of water to form gaseous hydrogen and oxygen. This process is interesting because the high temperature decomposition step could be coupled to next generation solar power plants or high temperature nuclear reactors in order to produce hydrogen for other applications. Using a proton exchange membrane such as Nafion in the HyS electrolyzer has been thoroughly examined via the prediction of mass transport through the membrane as a function of operating potential and other design variables. However, Nafion presents several drawbacks, such as the inability to operate at elevated temperatures and the decreased performance seen when exposed to high acid concentrations. Previously, we showed that acid doped polybenzimidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for proton conductivity, and therefore offer the possibility of operating at higher acid concentrations in order to minimize energy requirements necessary for water separation, as well as operation at higher temperatures in order to minimize voltage losses. Through the successful operation of the HyS electrolyzer using sulfuric acid doped PBI membranes, we have determined that despite the relative thickness of s-PBI, the area-specific resistance of s-PBI compares favorably with Nafion and is not adversely affected by the sulfuric acid concentration in the electrolyzer. Through further characterization of the membrane and the electrolyzer, we have been able to refine a model for high temperature and high pressure operation of the electrolyzer allowing for further analysis of the system in order to determine operating conditions that allow for economically viable operation of the HyS electrolyzer. The dependence of the individual potential contributions on temperature and pressure will be discussed, as well as trends seen in the membrane conductivity as a function of operating conditions and produced acid concentrations. A discussion of model results illustrating future membrane design considerations will also be presented.