The Department of Energy Nuclear Hydrogen Initiative (NHI) is developing production technologies to couple with next-generation nuclear reactors for emission-free production of hydrogen. Current commercial hydrogen production processes primarily use fossil fuels and release carbon dioxide, a greenhouse gas linked to global climate change. The goal of NHI is to demonstrate the economic, large-scale production of hydrogen using nuclear energy to fuel a future hydrogen economy.

Sandia's role in NHI includes both research and technical integration under the DOE Office of Nuclear Energy, Science, and Technology. Sandia's research focuses on thermochemical cycles, specifically the sulfur-iodine (S-I) thermochemical water-splitting cycle (see figure). The nuclear/S-I cycle is one of several options under consideration that use high-temperature heat or efficient electricity from an advanced reactor to produce hydrogen from water. Sandia is working with General Atomics and Commissariat á l'Energie Atomique on this collaborative effort, which involves process chemistry, high-temperature materials, and system integration issues.

Sulfur-iodine thermochemical water-splitting cycle

The NHI will focus on these high-temperature, potentially high-efficiency processes, which have many materials and systems technology issues in common with hydrogen production research underway at other DOE offices. The objective is to develop thermochemical and high-temperature electrolytic hydrogen production processes for use with the Next Generation Nuclear Plant (NGNP) to achieve economically competitive hydrogen production by 2017.

Thermochemical cycles are promising options for hydrogen production because of their potential for high efficiencies and economic scaling to large capacities. However, the technology is relatively immature, requiring further research.

Thermochemical cycles produce hydrogen through a series of chemical reactions. The net result is the production of hydrogen and oxygen from water at much lower temperatures than direct thermal decomposition. Energy in the form of heat is supplied in the temperature range necessary to drive the endothermic reactions, generally 750-1000•À¸C or higher. All process chemicals in the system are fully recycled.

Hybrid thermochemical cycles include both chemical reaction steps and electrolysis of some chemical compound (not water) that produces hydrogen. Both thermal and electrical energy is required to complete the hybrid cycle, but the electrical energy requirement for the hybrid electrolysis step is less than that for conventional electrolysis of water.

High-temperature electrolysis (HTE), or steam electrolysis, has the potential for even higher efficiency than conventional electrolysis. Thermal energy is used to produce high-temperature steam, which reduces the electrical energy required for electrolysis. This method requires low-cost, efficient electricity and an energy source that provides the highest possible temperatures consistent with materials capabilities. A temperature range of up to 950•À¸C is under consideration.

HTE can be accomplished using materials and technology similar to that used in solid-oxide fuel cells (SOFC). Individual electrolyzer cells would be relatively small so that large-scale applications could be composed of many electrolyzer modules. A comparison of the cost effectiveness of scaling the modular electrolysis process with the scaling of thermochemical cycles will be an important result of the R&D program.

Contact:
Paul Pickard
pspicka@sandia.gov
(505) 845-3046