Advances in fuel-cell technology and an increasing demand for hydrogen are driving the need for the development of more efficient methods to produce hydrogen. Thermochemical cycles, which produce pure hydrogen by splitting water through a series of chemical reactions, are being investigated as avenues for producing hydrogen efficiently on a large scale. Although there are hundreds of possible thermochemical cycles, the hybrid-sulfur process is the only practical, all-fluid, two-step thermochemical cycle. In a solar-driven process, solar radiation is used in a solar receiver/reactor to provide the energy needed to vaporize and decompose sulfuric acid. The resulting sulfur dioxide (SO2) is used in the second step consisting of an SO2-depolarized electrolyzer (SDE) that electrochemically oxidizes SO2 with water to form sulfuric acid at the anode and hydrogen at the cathode. All the sulfur species are internally recycled and the overall reaction is the splitting of water to form hydrogen and oxygen. We report here on our patented gas-fed SDE that was tested over a range of operating conditions (e.g., current, temperature) and design variations (e.g., membrane type and thickness). A key insight from our work is that perfluorinated sulfonic acid membranes like Nafion(A (R)) dehydrate at high acid concentrations, leading to high membrane resistance. Using acid-doped polybenzimidazole membranes represent an alternative material because they do not rely on water for their proton conductivity, and they can operate at temperatures above 100 A degrees C.