(157b) Solar Hydrogen Production Using Ferrite-Based Water Splitting Cycles On Al2O3 Substrates

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.