April 2006

Researchers Develop Solar Water-Splitting Cycle for Hydrogen Production

Nazim Muradov and Olawale Adebiyi  in laboratory

Dr. Nazim Muradov tests a pilot-scale hydrogen unit at FSEC's Hydrogen R&D Laboratory.
(Photo: Nick Waters)

Hydrogen production from solar-driven thermochemical water splitting cycles (TCWSCs) provides an approach that is both energy-efficient and environmentally attractive.  Of particular interest are TCWSCs that utilize both thermal (i.e. high temperature) and light (i.e. quantum) components of the solar resource, boosting the overall solar-to-hydrogen conversion efficiency compared to those with heat-only energy input. 

FSEC researchers Ali T-Raissi, Nazim Muradov, Cunping Huang and Olawale Adebiyi analyzed two solar-driven TCWSCs: a carbon dioxide (CO2)/carbon monoxide cycle and a sulfur dioxide (SO2)/sulfuric acid cycle.  The first cycle is based on the premise that CO2 becomes susceptible to near-ultraviolet and even visible radiation at high temperatures (greater than 1300K).  The second cycle is a modification of the well-known Westinghouse hybrid cycle, in which the electrochemical step is replaced by a photocatalytic step. 

Their research has led to the development of a novel hybrid photo-thermochemical sulfur-ammonia (S-A) cycle.  The main reaction (unique to FSEC's S-A cycle) is the light-induced photocatalytic production of hydrogen and ammonium sulfate from an aqueous ammonium sulfite solution.  The ammonium sulfate product is processed to generate oxygen and recover ammonia and SO2 that are then recycled and reacted with water to regenerate the ammonium sulfite.

The main advantages of the proposed S-A cycle over existing hybrid and solar high-temperature TCWSCs include the following:

  1. The S-A cycle includes a step in which the energy of solar photons is directly converted into the chemical energy of hydrogen (i.e., without the use of intermediate devices such as photovoltaic cells).
  2. There is no need for electrical energy input.
  3. The maximum temperature of the S-A cycle is below 1170K (which allows the use of less costly materials of construction), and
  4. The thermochemical step of the process (i.e. decomposition of sulfuric acid) is a well-developed process since it is common to all sulfur-family cycles.  As a result, S-A cycle has the potential to attain higher solar-to-hydrogen energy conversion efficiencies as compared to the state-of-the-art solar concentrator-turbine-electrolyzer systems.  The search for more efficient photocatalysts as well as thermodynamic and process flowsheet analyses of the proposed cycle are presently underway.

Click here for a copy of their complete paper describing their research and this new hydrogen production cycle.

This material was presented in a poster at the National Hydrogen Association annual conference in Long Beach, CA, in March.  Click here for an abstract of a second poster presented at the conference: “Zero Emission Production of Hydrogen via Catalytic Dissociation of Hydrocarbons” by N.Z. Muradov, F. Smith and A. T-Raissi.

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