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

  • Type:
    Conference Presentation
  • Conference Type:
    AIChE Annual Meeting
  • Presentation Date:
    November 16, 2020
  • Duration:
    13 minutes
  • Skill Level:
  • PDHs:

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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 hO and sO (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 hO values spanning a large range, sO values usually fall in a narrow range and their manipulation has remained poorly understood and difficult. Therefore, analyzing sources of sO is critical for directing materials design.

Recently, iron-poor (Fe-poor) ferrites (FeyM1-yOx 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 Fe0.45Co0.55Ox1 and Fe2/3Co1/3Ox2 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.

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