(391e) Kinetics of the Thermochemical Ferrite Water Splitting Cycle Using Nano-Thick Cobalt-Iron Spinel Oxide Films Synthesized Via Atomic Layer Deposition

Nano-thick cobalt-iron spinel oxides (cobalt ferrites) have been synthesized via atomic layer deposition (ALD) on nano-scale yttria-stabilized zirconia (YSZ) and Al₂O₃ supports. ALD has the advantage of being able to precisely control the thickness and Co/Fe ratio on the atomic level, which is significant because it has been suggested that this two-step cycle is a surface dominated reaction. The kinetics of the water splitting step has been investigated and results indicate that there is a correlation between film thickness and hydrolysis reaction rates. Reaction rates were more rapid as film thickness was decreased, indicating that this reaction is limited by diffusion through the bulk of the film. Additionally, results were directly compared to samples synthesized via traditional methods (coprecipitation, etc), and the amount of hydrogen generated per mole of ferrite was shown to be greater and more rapid for samples prepared via ALD. Conversion and kinetics were enhanced with a cobalt stoichiometry of x = 1, in CoxFe3-xO4, which is in good agreement with thermodynamic data. The effect of water concentration, pressure, and residence time was explored for chemically reduced ferrites and it was determined that water concentration and pressure have a significant effect on the H2 reaction rate. Hydrogen evolution was measured in situ using a residual gas analyzer, and changes in crystallinity were measured using a powder x-ray diffractometer.