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Microfluidics Research 

Two-dimensional stilbene MOF
Two-dimensional stilbene MOF

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Research in Microfluidic Transport at Sandia National Laboratories

Hydrogen Production

Thermal redox systems for solar hydrogen generation
We are developing a rational design approach to thermal redox systems for the production of hydrogen. Currently, we are using thermodynamic analysis of mixed-metal ferrite systems (e.g. AFe2O4, A = Mg, Co, Ni, or Zn) to determine optimal conditions for material synthesis, thermal reduction, and hydrogen production. Redox cycles using iron-containing ferrites are being proposed for two-step water-splitting cycles for the production of hydrogen from solar energy. These cycles consist of a thermal reduction step (TR; reaction (1)) in which solar energy is used to reduce the Fe(III) to the Fe(II) state with release of O2, followed by a water oxidation step (WO; reaction (2)) in which the reduced oxide reacts with steam to form hydrogen and regenerate the ferrite:

AxFe3-xO4 right arrow with lambda xAO + (3-x)FeO + 0.5 O2   (1)
xAO + (3-x)FeO + H2Oright arrow AxFe3-xO4 + H2(2)

Our calculations are proving to be a valuable tool for selecting materials. For example, the results suggest that zirconia ceramic supports play both a chemical as well as physical role in the redox process and that NiFe2O4 displays the best overall combination of properties for solar redox cycles (see Fig. 1).

fig. 9
Figure 1. Comparison of oxygen partial pressures in equilibrium with various metal-substituted ferrites (MFe2O4). The dashed blue line indicates p(O2) for air, showing the temperature at which a given ferrite can be reduced to suboxides (FeO + MO) in air.

Publications

R.B. Diver, J.E. Miller, M.D. Allendorf, N.P. Siegel, R.E. Hogan “Solar Thermochemical Water-Splitting Ferrite-Cycle Heat Engines,” J. Solar En. Eng. in press, August 2008.

M. D. Allendorf, R. B. Diver, N. P. Siegel, J. E. Miller “Two-Step Water Splitting Using Mixed-Metal Ferrites: Thermodynamic Analysis and Characterization of Synthesized Materials,” in press, Energy and Fuels Sept. 2008.

J. E. Miller, M. D. Allendorf, R. B. Diver, L. R. Evans, N. P. Siegel and J. N. Stuecker “Metal oxide composites and structures for ultra-high temperature solar thermochemical cycles,” J. Mater. Sci., Volume: 43   Issue: 14  Pages: 4714-4728 2008.

M. D. Allendorf, J. E. Miller, N. P. Siegle, R. B. Diver Jr. "Thermodynamic Analysis Of Mixed-Metal Ferrites For Hydrogen Production By Two-Step Water Splitting," Proc. ISEC 2006, ISEC2006-99114.

J. E. Miller, L. R. Evans, J. N. Stuecker, M. D. Allendorf, N. P. Siegel, and R. B. Diver "Materials Development for the CR5 Solar Thermochemical Heat Engine," Proc. ISEC 2006, ISEC2006-99152.

R. B. Diver Jr., J. A. Miller, M. D. Allendorf, N. P. Siegel, R. E. Hogan "Solar Thermochemical Water-Splitting Ferrite-Cycle Heat Engines," Proc. ISEC 2006, ISEC2006-99147.