(339z) Solar Water Splitting Using Cobalt Ferrites Deposited On Al2O3 Substrates Via Atomic Layer Deposition
We are reporting the results of an alternative ferrite cycle in which the ferrite (CoFe2O4) is deposited on a porous Al2O3 support via atomic layer deposition (ALD). Rather than a traditional cycle, in which the ferrite is deposited on an inert support or is unsupported, the ferrite reacts with Al2O3 during thermal reduction to form hercynite (FeAl2O4). This is advantages because the reaction between the ferrite and Al2O3 is favored at lower temperatures than the reduction of Fe3+ to Fe2+ on an inert support. Subsequent water oxidation is then achievable at 1000 oC, similar to other ferrite cycles. This redox cycle is shown in the two step reaction below:
CoFe2O4 + Al2O3 + solar thermal energy => CoO + 2FeAl2O4 + 0.5 O2 (1)
CoO + 2FeAl2O4 + H2O => CoFe2O4 + Al2O3 + 2H2 (2)
In the first step, solar thermal energy is used to reduce the ferrite, and oxygen is evolved. In the second step, the reduced material is reacted with water to generate hydrogen and is reoxidized. Therefore, the only inputs are solar energy and water, and the only outputs are H2 and O2.
Thermal reduction was attempted at temperatures ranging from 1200 oC to 1500 oC, and results indicate that reduction occurs at lower temperatures than traditional ferrite cycles. It was observed that hercynite (FeAl2O4) was formed after reduction, due to a reaction between FeO and the Al2O3 substrate. Subsequent water oxidation was achievable at 1000 oC and the reaction was shown to be repeatable over 8 cycles. Experimental results are compared to thermodynamic modeling performed using FactSage and results agree well with thermodynamic predictions, as seen in figure 1.
Figure 1. Left) Thermodynamic modeling results predict more H2 generation for CoFe2O4 on Al2O3 over reduction temperatures from 1200 to 1500 oC. Right) Experimental results predict more H2 generation for CoFe2O4 on Al2O3 over reduction temperatures from 1200 to 1400 oC
H2 conversions varied between 18% and 25 % when cycling was performed at 1200 oC reduction and 1000 oC water oxidation. Crystallinity changes were measured before and after reduction using powder x-ray diffractometry, and conversions were calculated via in situ mass spectrometry. Film morphology and composition were measured via high resolution transmission electron microscopy and induced coupled plasma-atomic emission spectroscopy, respectively.