The DOE Office of Nuclear Energy, Science & Technology has established the Nuclear Hydrogen Initiative (NHI) to develop technologies that can be coupled with next generation nuclear reactors for hydrogen production. One process under development, the Hybrid Sulfur (HyS) Process, generates hydrogen using electrolyzers with sulfur dioxide depolarized anodes. The Savannah River National Laboratory (SRNL) first received funding to test and develop this type of electrolyzer in 2005 and received funding for continued testing through FY 08.

Traditional water electrolyzers have an anode and cathode separated by an electrically insulating membrane that does not pass product gases. Water is fed to the anode where it is oxidized to oxygen and hydrogen ions. The ions cross the membrane where they are reduced to hydrogen gas. The required voltage is between 1.5 volts and 2 volts. For a sulfur dioxide depolarized electrolyzer, the reactants water and sulfur dioxide are fed to the anode where they react to form sulfuric acid and hydrogen ions (protons). Similarly to the traditional electrolyzer, the hydrogen ions cross the membrane and are reduced to hydrogen. The cell potential can be significantly less than one volt, which is important because electrolyzer power cost is proportional to cell potential. A separate high temperature reaction decomposes sulfuric acid to regenerate sulfur dioxide.

The SRNL electrolyzer system operates by feeding SO2 dissolved in sulfuric acid to the anode. The anode of the electrolyzer is bathed in a sulfuric acid solution saturated in sulfur dioxide. Sulfur dioxide is oxidized to sulfuric acid at the anode. Hydrogen ions cross a membrane to the cathode where they are reduced to hydrogen gas. The cathode is flushed with water for temperature control and to sweep away any material that crosses the membrane.

SRNL designed, constructed and tested a facility built around a single cell sulfur dioxide depolarized electrolyzer. As part of the continued development of the HyS program, the test facility was modified to accept a three cell stack electrolyzer, expanding the active cell area from 60 cm2 480 cm2. The facility allows pressures as high as 75 psig and temperatures as high as 80ºC. Modifications to the facility included increasing the power input, adding additional flow lengths to provide current isolation for each cell, and the ability to preheat the cathode water flush.

The Three Cell Stack was designed jointly by Giner Compay and SRNL and was constructed by Giner Co. The stack has three cells and a total area of 480 cm2. The three cells are connected electrically in series and hydraulically in parallel. A challenge for the stack was to limit leakage currents through the conductive fluid in the anolyte flow loop. Testing includes two different membrane electrode assembly (MEA) configurations and under different operating conditions. The maximum hydrogen production rate was 80 standard liters per hour. Individual cell voltages were measured and ranged between 0.61 and 1.2 volts with current densities up to 400 mA/cm2. After completing tests with the first set of MEAs, the stack was removed from the test stand, disassembled and inspected. The MEAs were removed and analyzed by Scanning Electron Microscopy (SEM) for the potential deposition of elemental sulfur. The electrolyzer was then re-assembled using a second set of MEAs with different catalyst loadings and testing continued.