Solar Thermal Production of Hydrogen Using Alumina Supported Ferrites: Thermodynamic Evaluation and On-Sun Validation

Alumina supports were coated with CoFe₂O₄ using an alternating atomic layer deposition process. Nano-powder and hollow skeletal shells were used as supports. Large, high surface area polymer particles (~600µm, 43.5 m²/g) have been coated via ALD with several layers of alumina. The alumina layers were applied using TMA/water ALD chemistry. The particles were then heated to 800°C in air to burn out the polymer substrate. Thermodynamic modeling of the particles shows the creation of a hercynite/spinel phase that is capable of being cycled with water to produce hydrogen. The process requires reduction temperatures several hundred degrees below those of conventional ferrites supported on inert materials.  Cycling has been completed on-sun at the National Renewable Energy Laboratory in Golden, CO. Powders were housed in a prototype concentrated solar reactor and subjected to cycling. Hydrogen production was quantified using a mass spectrometer. Results were used to estimate solar to hydrogen efficiency of the prototype reactor.