Hybrid sulfur cycle
Encyclopedia
The hybrid sulfur cycle (HyS) is a two-step water-splitting
process intended to be used for hydrogen production
. Based on sulfur
oxidation and reduction, it is classified as a hybrid thermochemical cycle
because it uses an electrochemical (instead of a thermochemical) reaction for one of the two steps. The remaining thermochemical step is shared with the sulfur-iodine cycle
.
The HyS cycle was initially proposed and developed by Westinghouse Electric Corp. in the 1970s, so it is also known as the "Westinghouse" cycle. Current development efforts in the United States are being led by the Savannah River National Laboratory
.
Sulfur dioxide acts to depolarize the anode
of the electrolyzer. This results in a significant decrease in the reversible cell potential (and, therefore, the electric power requirement) for reaction (2). The standard cell potential for reaction (2) is -0.158 V at 298.15 K, compared to -1.229 V for the electrolysis of water (with oxygen evolution as the anodic reaction).
Water splitting
Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy. Various techniques for water splitting have been issued in water splitting patents in...
process intended to be used for hydrogen production
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
. Based on sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
oxidation and reduction, it is classified as a hybrid thermochemical cycle
Thermochemical cycle
Thermochemical cycles combine solely heat sources with chemical reactions to split water into its hydrogen and oxygen components . The term cycle is used because aside of water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled.If work is partially used...
because it uses an electrochemical (instead of a thermochemical) reaction for one of the two steps. The remaining thermochemical step is shared with the sulfur-iodine cycle
Sulfur-iodine cycle
The sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled...
.
The HyS cycle was initially proposed and developed by Westinghouse Electric Corp. in the 1970s, so it is also known as the "Westinghouse" cycle. Current development efforts in the United States are being led by the Savannah River National Laboratory
Savannah River National Laboratory
The Savannah River National Laboratory is the applied research and development laboratory at the U.S. Department of Energy’s Savannah River Site near Aiken, South Carolina. SRNL was founded in 1951 as the Savannah River Laboratory. It was certified as a national laboratory on May 7, 2004...
.
Process Description
The two reactions in the HyS cycle are as follows:- H2SO4(aq) → H2O(g) + SO2(g) + ½ O2(g) (thermochemical, T > 800°C)
- SO2(aq) + 2 H2O(l) → H2SO4(aq) + H2(g) (electrochemical, T = 80-120°C)
- Net reaction: H2O(l) → H2(g) + ½ O2(g)
Sulfur dioxide acts to depolarize the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
of the electrolyzer. This results in a significant decrease in the reversible cell potential (and, therefore, the electric power requirement) for reaction (2). The standard cell potential for reaction (2) is -0.158 V at 298.15 K, compared to -1.229 V for the electrolysis of water (with oxygen evolution as the anodic reaction).
See also
- Cerium(IV) oxide-cerium(III) oxide cycleCerium(IV) oxide-cerium(III) oxide cycleThe cerium oxide–cerium oxide cycle or CeO2/Ce2O3 cycle is a two step thermochemical process based on cerium oxide and cerium oxide for hydrogen production...
- Copper-chlorine cycleCopper-chlorine cycleThe copper–chlorine cycle is a four-step thermochemical cycle. It has a maximum temperature requirement of about 530 degrees Celsius. The Cu–Cl cycle is one of the prominent thermochemical cycles under development within the Generation IV International Forum...
- Iron oxide cycleIron oxide cycleThe iron oxide cycle is a two-step thermochemical cycle proposed for use for hydrogen production.-Process description:The thermochemical two-step water splitting process uses redox systems...
- Sulfur-iodine cycleSulfur-iodine cycleThe sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled...
- Zinc zinc-oxide cycle