Direct water electrolysis is commercially viable but too inefficient for large-scale hydrogen production. Hence the interest in using thermochemical cycles for producing hydrogen from renewable energy [1]. Thermochemical cycles produce hydrogen through a series of chemical reactions that result in the splitting of water at much lower temperatures (~500-1000ºC) than direct thermal dissociation (>2500^oC). All other chemical species in these reactions are recycled resulting in the consumption of only heat and water to produce hydrogen and oxygen. Since water rather than hydrocarbons are used as the source of hydrogen in these thermochemical cycles, no carbon dioxide emissions are produced and the hydrogen is highly pure.

Although there are hundreds of possible thermochemical cycles, the hybrid-sulfur (HyS) process is the only practical, all-fluid, two-step thermochemical cycle [2-5]. The high temperature step (850-950°C) involves the decomposition of H₂SO₄ to produce oxygen and sulfur dioxide via the following reaction:

H₂SO₄ => SO₂ + ½ O₂ + H₂O [1]

The SO₂ is separated, cooled, and sent to the SO₂-depolarized electrolyzer (SDE). The resulting reactions at the anode and cathode, respectively, are:

SO₂ + 2H₂O => H₂SO₄ + 2H⁺ + 2e^- U⁰_SO2 = 0.158 V vs. SHE [2]

2H⁺ + 2e^- => H₂ U⁰_H2 = 0 V vs. SHE [3]

Thus, the overall reaction in the electrolyzer is represented as:

SO₂ + 2H₂O => H₂SO₄ + H₂ [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 [6] to a perfluorinated sulfonic acid membrane (e.g., DuPont’s Nafion®) [7-10]. For example, Westinghouse was only able to get the cell voltage down to 1.0 V at 400 mA/cm², where we achieved 500 mA/cm² at 0.71 V and 1.2 A/cm² 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. The key issue when using membranes like Nafion that rely on water for their proton conductivity is that high acid concentrations dehydrate the membrane and dramatically increase membrane resistance.

Therefore, we have recently developed an electrolyzer that uses sulfuric acid-doped polybenzimidazole (s-PBI) membranes as an alternative to membranes like Nafion because they do not rely on water for their proton conductivity [3, 11-14]. Here, we analyze the voltage losses and acid concentration from an SDE operated using s-PBI membranes under a range of operating conditions. From the voltage data and thermodynamic modeling, kinetic parameters and membrane conductivity were obtained to better understand and quantify the individual potential contributions to the cell voltage. The measured acid concentrations were also compared to predictions from water balances coupled to a non-ideal vapor-liquid equation of state. The physical parameters obtained here enable the prediction of cell voltage and acid concentrations over a wide range of operating temperatures, pressures, currents and reactant flow rates.

References

1. Nuclear Hydrogen Research and Development Plan, Department of Energy, Office of Nuclear Energy, Science and Technology (2004).
2. C. Corgnale and W. A. Summers, Int J Hydrogen Energ, 36, 11604 (2011).
3. J. W. Weidner, J Appl Electrochem, 46, 829 (2016).
4. C. Corgnale, S. Shimpalee, M. B. Gorensek, P. Satjaritanun, J. W. Weidner and W. A. Summers, Int J Hydrogen Energ (2017).
5. M. B. Gorensek and W. A. Summers, Int J Hydrogen Energ, 34, 4097 (2009).
6. P. W. T. Lu, E. R. Garcia and R. L. Ammon, J Appl Electrochem, 11, 347 (1981).
7. Staser, R. P. Ramasamy, P. Sivasubramanian and J. W. Weidner, Electrochem Solid St, 10, E17 (2007).
8. J. A. Staser, M. B. Gorensek and J. W. Weidner, J Electrochem Soc, 157, B952 (2010).
9. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B16 (2009).
10. J. A. Staser and J. W. Weidner, J Electrochem Soc, 156, B836 (2009).
11. J. V. Jayakumar, A. Gulledge, J. A. Staser, C. H. Kim, B. C. Benicewicz and J. W. Weidner, Ecs Electrochem Lett, 1, F44 (2012).
12. T. R. Garrick, A. Gulledge, J. A. Staser, B. Benicewicz and J. W. Weidner, ECS Transactions, 61, 11 (2014).
13. T. R. Garrick, A. Gulledge, J. A. Staser, B. Benicewicz and J. W. Weidner, ECS Transactions, 66, 31 (2015).
14. T. R. Garrick, C. H. Wilkins, A. T. Pingitore, J. Mehlhoff, A. Gulledge, B. C. Benciwicz, and J. W. Weidner, J. Electrochem. Soc., 164(14), F1591-F1595 (2017).