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Ferrite Based Water Splitting Cycles

Alleviating our dependence on fossil fuels will prove to be a major technological hurdle for the United States in the upcoming decades. Excessive CO2 emissions are having a profound effect upon global warming, and fossil fuels are rapidly diminishing. In order to alleviate our dependence on fossil fuels, and decrease CO2 emissions, it is imperative that alternative energy sources, such as hydrogen and bio-fuels, be utilized. Hydrogen is a particularly promising candidate because it can be produced from water, an essentially inexhaustible resource. In addition, hydrogen is a clean, sustainable energy carrier. Its only byproduct after combustion is water.

Concentrated solar energy, an alternative to fossil fuels, has proven to be capable of providing sufficient energy for producing hydrogen via the splitting of water. However, there are problems associated with this technique, as direct water splitting requires extremely high temperatures (2500 K) and a separation of an explosive mixture of hydrogen and oxygen. Solar thermochemical cycles, avert these obstacles by producing hydrogen and oxygen in separate steps at significantly lower temperatures than 2500 K.

One such thermochemical cycle that we are studying utilizes spinel ferrites of the form MxFe3-xO4, where M generally represents Ni, Zn, Co, Mn, or other transition metals. These have been shown to be capable of splitting water according to the two step cycle shown below:

MxFe3-xO4 + solar thermal energy --> xMO + (3-x)FeO + 0.5 O2  (1)

xMO + (3-x)FeO + H2O -->  MxFe3-xO4 + 2H2                         

This is an inherently clean and sustainable process, as the only net inputs are solar energy and water, and the net outputs are hydrogen and oxygen.

In order to study the efficacy of this reaction, we are utilizing atomic layer deposition (ALD) to synthesize ferrites. This is a self-limiting technique that is capable of depositing ultra-thin, highly conformal films one atomic layer at a time. Operating conditions are controlled in such a way that the precursors of one half-reaction react only with the precursors of the other. This ensures atomic level control, since at most one sub-monolayer is deposited per half-reaction. Therefore, various film thicknesses can be synthesized in a controlled manner. Additionally, ALD offers the advantage of precisely controlling the M/Fe ratio in MxFe3-xO4. An example of the ultra-thin nature of the ALD films is shown in the figure to the right.

ALD has proven to be capable of increasing both reaction rates and conversions of these cycles due to the ultra-thin nature of the deposited films. Generally, ferrites are synthesized via solid state or wet chemistry methods in which the bulk of the particles remains unreacted. This is due to the fact that the surface is oxidized prior to the bulk during the water splitting step, creating a passivating layer preventing further oxidation. ALD circumvents these problems due to the fact that the majority of the reactive material is near the surface, rather than the bulk. This behavior can be seen in the figure above, in which the H2 reaction rate and conversion are both greater for an ALD synthesized ferrite.

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© 2009. Team Weimer

University of Colorado at Boulder University of Colorado at Boulder