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 in 2006.

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.

SRNL designed, constructed and tested a facility built around a sulfur dioxide depolarized electrolyzer. The facility used in 2005 was upgraded to allow pressures as high as 75 psig and temperatures as high as 80ºC. Increasing the pressure is advantageous because it increases the concentration of the sulfur dioxide reactant, which decreases the cell potential. Increasing the temperature increases reaction rate which also decreases cell potential. The anolyte used in testing was a solution of sulfuric acid in the range of 30 wt% to 70 wt% saturated with sulfur dioxide. The sulfuric acid anolyte was required by balance of plant considerations.

A carbon based electrolyzer cell with an area of 40 cm2 was designed and built. The cell was tested with three different membrane electrode assemblies (MEA). Each MEA consisted of a membrane that can pass hydrogen ions and electrodes with catalyst on either side. One consideration in the selection of membrane was ionic conductivity for protons, which should be high to hold down cell potential. Another consideration was permeability of the membrane to sulfur dioxide, which should be low because sulfur dioxide tends to reduce to elemental sulfur at the cathode.