Invited Talk: Entropy Source Analysis for Ferrites in Two-Step Thermochemical Splitting of Water and Carbon Dioxide

Greenhouse gases are being emitted at a rate of ~40 Gt(gigaton)-CO₂-equivalent/year. To keep the global average temperature rise to less than 2°C above that of the pre-industrial age, zero or even negative emissions need to be achieved within a couple of decades. Gt-scale carbon-free hydrogen (H₂) with cost below $2/kg has the potential to decarbonize industrial heat and feedstocks, electricity generation, and transportation, and it is also a potential energy carrier for intermittent renewables. Water splitting can produce carbon-free H₂ through electrolysis, photo(electro)chemistry and thermochemistry. The current chemical and energy industry relies almost exclusively on thermochemical processes because they are the only ones reaching Gt-scale with economies of scale.

Two-step thermochemical splitting of water and CO₂ has shown promise for large-scale operation due to its relative simplicity. The main challenges for reducing cost in the two-step thermochemical cycle include very high thermal reduction temperatures, low H₂O-to-H₂ (and CO₂-to-CO) conversion and limited oxygen-exchange reaction capacity. The thermodynamics of metal oxides for these reactions holds the key to achieving the needed performance. This research will present a deeper thermodynamic understanding of solid-solid metal oxide phase transitions, which forms the foundation for controlling these reactions.

For a metal oxide to spontaneously conduct both thermal reduction and water splitting reactions, increasing its h_O and s_O (partial molar enthalpy and entropy of oxygen, respectively) is necessary for lowering temperatures and improving H₂O-to-H₂ (and CO₂-to-CO) conversion. While state-of-the-art metal oxides for these reactions have h_O values spanning a large range, s_O values usually fall in a narrow range and their manipulation has remained poorly understood and difficult. Therefore, analyzing sources of s_O is critical for directing materials design.

Recently, iron-poor (Fe-poor) ferrites (Fe_yM_1-yO_x where y < 2/3 and M is another one or more metals) were found to split CO₂ with unusually high yield compared to traditional Fe-rich ferrites, where Fe is the redox active element. The exceptional performance is achieved through tuning solid-solid phase transitions by changing the Fe:M ratio; specifically, Fe-poor ferrites show advantageous thermodynamic properties compared to the traditional Fe-rich ones. However, the fundamental sources of solid-state entropy change have not yet been analyzed. Here we have used Fe_0.45Co_0.55O_x1 and Fe_2/3Co_1/3O_x2 as model materials to quantify the entropic driving forces of the oxygen exchange reactions. Cation configurational entropy is determined by resonant x-ray diffraction measurements and CALPHAD-FactSage thermodynamic simulations. Lattice vibrational entropy is quantified by first-principles calculations using Hubbard-corrected density functional theory. Both sources are found to be important as components of entropic driving forces and the analysis explains the advantageous performance of Fe-poor ferrites compared to traditional Fe-rich ones.