Evaluation of the Productivity and Kinetics of Hercynite for Solar Thermochemical Water Splitting

As the world’s dependence on fossil fuels is being reevaluated due to their harmful side effects, the search for promising renewable sources of energy has become increasingly important. Although there are several potential “green” sources of energy, the use of hydrogen as a fuel is particularly intriguing due to its high energy density. Solar thermochemical water splitting via redox reaction offers a two-step process in which specific metal oxide intermediates are subjected to a redox scheme, producing hydrogen from steam during oxidation. These redox active materials feature specific thermodynamic and kinetic properties, and iron aluminate (FeAl₂O­₄), or hercynite, has been identified as a viable material for this process under both temperature-swing and isothermal conditions. In this study, three iron aluminate materials with different iron loadings (FeAl2O4, Fe1.2Al1.8O4, and Fe1.4Al1.6O4) are evaluated to compare the effects of iron content on the abilities of the materials to reduce CO₂, an analogous reaction to water-splitting. The various samples in this work are subjected to reduction at 1400 ^oC and oxidation at temperatures ranging from 1000-1400 ^oC. Kinetic analyses of each sample are performed to identify and model the kinetics of CO₂ reduction and determine the effects of iron loading. Comparisons of the CO productivities of the materials at various oxidation temperatures also quantify the role of iron loading in determining the ability of the materials to effectively reduce CO₂.