U.S. Department of Energy - Energy Efficiency and Renewable Energy
Fuel Cell Technologies Office – Hydrogen Production
High-Temperature Water Splitting
High-temperature water splitting (a "thermochemical" process) is a long-term technology in the early stages of development.
How Does It Work?
High-temperature heat (500°C–2000°C) drives a series of chemical reactions that produce hydrogen.
Chemicals used in the process are reused within each cycle, creating a closed loop that consumes only water and produces hydrogen and oxygen.
The high-temperature heat needed can be supplied by next-generation nuclear reactors under development (up to about 1000°C) or by using sunlight with solar concentrators (up to about 2000°C).
Researchers have identified cycles appropriate to specific temperature ranges and are examining these systems in the laboratory. The more than 200 possible cycles identified have been screened and down-selected to about twelve for initial research.
High-temperature water splitting is most suitable for large-scale, centralized production of hydrogen even though semi-central production from solar driven cycles might be possible.
High-Temperature Water Splitting Using Solar Concentrators
A solar concentrator uses mirrors and a reflective or refractive lens to capture and focus sunlight to produce temperatures up to 2,000°C. This high-temperature heat can be used to drive chemical reactions that produce hydrogen.
Chemical cycle example: zinc/zinc oxide cycle
Zinc oxide powder passes through a reactor heated by a solar concentrator operating at about 1,900°C. At this temperature, the zinc oxide dissociates to zinc and oxygen gases. The zinc cools, separates, and reacts with water to form hydrogen gas and solid zinc oxide. The net result is hydrogen and oxygen, produced from water. The hydrogen can be separated and purified. The zinc oxide can be recycled and reused to create more hydrogen through this process.
2ZnO + heat → 2Zn + O2
2Zn + 2H2O → 2ZnO + 2H2
High-Temperature Water Splitting Using Nuclear Energy
Similar to a solar concentrator, a nuclear reactor produces energy as high-temperature heat that can be used to drive high-temperature thermochemical water splitting cycles. The next-generation nuclear reactors under development could generate temperatures of 800°C to 1,000°C—these temperatures are much lower than those produced by a solar concentrator. A thermochemical process based on nuclear heat would use a different set of chemical reactions to produce hydrogen. This area of research is being pursued by DOE's Office of Nuclear Energy, Science, and Technology.
Chemical cycle example: sulfur-iodine cycle
Sulfuric acid, when heated to about 850°C, decomposes to water, oxygen, and sulfur dioxide. The oxygen is removed, the sulfur dioxide and water are cooled, and the sulfur dioxide reacts with water and iodine to form sulfuric acid and hydrogen iodide. The sulfuric acid is separated and removed, and the hydrogen iodide is heated to 300°C, where it breaks down into hydrogen and iodine. The net result is hydrogen and oxygen, produced from water. The hydrogen can be separated and purified. The sulfuric acid and iodine are recycled and used to repeat the process.
2H2SO4 +heat at 850°C → 2H2O + 2SO2 + O2
4H2O + 2SO2 + 2I2 → 2H2SO4 + 4HI
4HI + heat at 300°C → 2I2 + 2H2
Why Is This Technology Being Considered?
Near-zero greenhouse gas emissions. Solar- and nuclear-driven high-temperature thermochemical water splitting cycles produce hydrogen with near-zero greenhouse gas emissions using water and either sunlight or nuclear energy.
Research Focuses On Overcoming Challenges
This technology is in the early stages of development, and the high temperatures required present many challenges. Specifically, research is needed to:
Identify appropriate materials for construction for these high-temperature operations.
Demonstrate feasibility—several of these technologies have been demonstrated with some level of feasibility in the laboratory but require significantly more research and development.
Reduce the cost of solar concentrators, identify suitable solar receivers, and develop a heat-transfer medium.
Develop next-generation nuclear reactor technology to supply the high temperatures needed for this process.