(581d) Generation of Hydrogen Using Electrolyzer with Sulfur Dioxide Depolarized Anode

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. Thermochemical water-splitting cycles are leading contenders within the NHI program because they have the potential for high efficiencies with favorable scale-up characteristics. One process under development, the Hybrid Sulfur (HyS) Process, generates hydrogen using electrolyzers with sulfur dioxide depolarized anodes. SRNL received funding to test and develop this type of electrolyzer.

Traditional water electrolyzers have an anode and cathode separated by an electrically insulating membrane that does not pass gases. Water is fed to the anode where it is oxidized to oxygen and hydronium ions. The ions cross the membrane where they are reduced to hydrogen gas. The required voltage is between 1.5 volts and 2 volts. The anode of a sulfur dioxide depolarized electrolyzer is fed water and sulfur dioxide, which is oxidized to sulfuric acid and hydronium ions. Similarly to the traditional electrolyzer, the ions cross the membrane and are reduced to hydrogen. The cell voltage can be significantly less than one volt.

SRNL designed, constructed and tested a facility built around a sulfur dioxide depolarized electrolyzer. The electrolyzer was purchased from Proton Energy Systems and was modified by them from one of their standard water electrolyzers. The other primary components were a sulfur dioxide absorber, a hydrogen collector to allow accurate measurement of hydrogen generation rate, pumps, flowmeters, pressure transducers, thermocouples and a computer based data acquisition system.

The first testing was standard electrolysis of water as a baseline. The cell voltage ranged from 1.57 volts at a current density of 60 mA/cm2 to 1.82 volts at 600 mA/cm2. Sulfur dioxide was absorbed in the water for the second set of tests. Using water saturated with sulfur dioxide decreased the cell voltage by about a volt. Increasing the absorber pressure from one bar to two bar increased the concentration of sulfur dioxide and decreased the cell voltage by an additional 0.1 volt. The water was replaced by sulfuric acid solutions for the third set of tests, because balance of plant considerations require the use of sulfuric acid solutions. The voltages measured were slightly higher than for water.

It was observed that sulfur dioxide crossed the Nafion membrane from the anode to the cathode and was reduced to elemental sulfur, which partially clogged the cathode flow passages and was also a parasitic reaction. Future work will research a membrane having greatly reduced permeability for sulfur dioxide.