Oxidation Kinetics of Hercynite Alloys for Solar Thermochemical Fuel Production

Hydrogen (H₂) is a clean energy source that can replace fossil fuels in the transportation and power sectors, two of the largest contributors of anthropogenic greenhouse gas emissions. Globally, more than 95% of H₂ is produced from fossil fuels, resulting in carbon dioxide CO₂ emissions and a significant carbon footprint. To address this, researchers have sought to produce fuels, such as H₂ and carbon monoxide (CO), via solar thermochemical fuel (STCF) production using solar energy to drive the endothermic gas splitting reaction via a reduction/oxidation cycle. The development of an economically viable solar thermochemical fuel production process relies largely on identifying redox active materials with optimized thermodynamic and kinetic properties. Hercynite (FeAl₂O₄) has been demonstrated as a viable redox-active material for this process with higher H₂ productivities than its cobalt-iron aluminate counterpart (Co_xFe_1-xAl₂O₄). However, a qualitative tradeoff between the favorable thermodynamic properties of hercynite and its slower kinetic properties was observed when cobalt is introduced.

In this work, we evaluate four spinel aluminate materials with varying cobalt contents from 0 to 40% (FeAl₂O₄, Co_0.05Fe_0.95Al₂O₄, Co_0.25Fe_0.75Al₂O₄, and Co_0.40Fe_0.60Al₂O₄) in order to further understand the role of cobalt in these materials and to quantify its effect on the thermodynamic and kinetic properties for CO₂ reduction. A solid state kinetic analysis was performed on each sample to model its oxidation kinetics in CO₂ splitting experiments at temperatures ranging from 1200°C to 1350°C using a thermogravimetric analyzer (TGA). An F1 model representing first-order reaction kinetics was found to most accurately represent the experimental data for all materials evaluated. The computed rate constants, activation energies, and pre-exponential factors all increase with increasing cobalt content. However, lower productivities are seen with increasing cobalt content. High temperature in-situ XPS study was utilized, for the first time on these oxides, to characterize their surfaces and indicated the presence of metallic states of the reduced cobalt-iron alloys, which are not present in hercynite. These species provide a new site for the CO₂ reduction reaction and enhance its rate through an increased pre-exponential factor.