Greenhouse gases are being emitted at a rate of \~40 Gt(gigaton)-CO~2~-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 (H~2~) with cost below \$2/kg has the potential to decarbonize industrial heat and feedstock (steel, concrete, ammonia, petrochemical), electricity generation, and transportation (fuel cells for long-haul trucks), and it is also a potential energy carrier for intermittent renewables. Water splitting can produce carbon-free H~2~ 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 the key advantage of achieving economies of scale. Two-step thermochemical splitting of water and CO~2~ 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 H~2~O-to-H~2~ (and CO~2~-to-CO) conversion and limited oxygen-exchange reaction capacity.  Metal oxides have been shown to achieve these reactions and their thermodynamics holds the key to achieving the needed temperature and conversion 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  and  (partial molar enthalpy and entropy of oxygen, respectively) is necessary for lowering temperatures and improving H~2~O-to-H~2~ (and CO~2~-to-CO) conversion. While state-of-the-art metal oxides for these reactions have  values spanning a large range,  values usually fall in a narrow range and their manipulation has remained poorly understood and difficult. Therefore, analyzing sources of  is critical for directing materials design.

Recently, iron-poor (Fe-poor) ferrites (Fe~y~M~1-y~O~x~ where y \< 2/3 and M is another one or more metals) were found to split CO~2~ 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. Van’t Hoff thermodynamic analysis was conducted on various ferrites for their  and , and Fe-poor ferrites show advantageous entropic 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~0.45~Co~0.55~O~x1~ and Fe~0.67~Co~0.33~O~x2~ (i.e., CoFe~2~O~x3~) as model materials to quantify the entropic driving forces of the oxygen exchange reactions. Cation configurational entropy is determined by resonant x-ray diffraction (resonant XRD) measurement and CALPHAD-FactSage thermodynamic simulation. Lattice vibrational entropy is quantified by first-principles calculations using Hubbard-corrected density functional theory (DFT). Both sources are found to be important as components of entropic driving forces.


Entropic Driving Forces of Ferrites for Two-Step Thermochemical Water and Carbon Dioxide Splitting

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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