Synthesis of Nanoscale Cobalt-Iron Spinel Oxides Via Atomic Layer Deposition and Their Applications for Solar Thermochemical Water Splitting

Nano-thick cobalt-iron spinel oxides (cobalt ferrites) have been synthesized via atomic layer deposition (ALD) on yttria-stabilized zirconia (YSZ) and Al₂O₃ nanoparticles. Multilayers of cobalt(II) oxide and iron(II,III) oxide were deposited and subsequently heat treated at 900 ^oC. X-ray diffractometry (XRD) was utilized to determine film crystallinity and oxidation state, and induced coupled plasma - atomic emission spectroscopy (ICP-AES) was verified the relative amounts of cobalt and iron.

The kinetics of the thermochemical 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.