Semester

Spring

Date of Graduation

2022

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Wei Li

Committee Member

Wei Li

Committee Member

Wenyuan Li

Abstract

In this project, we designed, synthesized, and tested the transformative (La0.8Sr0.2)(Mn[1-x]/3Fe[1-x]/3CoxAl[1-x]/3)O3 (0 ≤ x ≤ 1.0) high-entropy perovskite oxides (HEPOs) as redox active oxygen carriers for thermochemical hydrogen production with improved stability, kinetics, and H2 yield. The developed HEPOs form an R3c (hexagonal) phase and were successfully synthesized using both the Pechini and solid-state reactions with the latter synthesis being the primary source for testing. These innovative perovskites demonstrated an improved kinetics with oxygen surface exchange coefficient, k, greater than 7.5 x 10-4 cm/s, with a maximum average value of 3.0 x 10-2 cm/s, which relatively decreased with increasing Co content. This behavior is attributed to Co being the primary redox active element beginning its activity around 800 °C. Furthermore, structural stability was seen for samples with x < 0.61 upon thermochemical looping cycles, while samples with x ≥ 0.61 formed the La2CoO4 secondary phase. The oxygen nonstoichiometry, δ, is almost linearly influenced by Co content, growing with increasing x under a relatively low reduction temperature (Tred < 1400 °C). As the x value increased from 0 to 0.4, the H2 production yield rose. The optimal composition, (La0.8Sr0.2)(Mn0.2Fe0.2Co0.4Al0.2)O3 demonstrated the highest H2 yield of more than 400 μmol/g and remarkable cycling stability for 14 effective cycles alternated with 30-minute reduction at 1350 °C and 60-minute water splitting at 1100 °C. With further increase of x beyond 0.4, H2 yields decreased. Despite the high content of redox-active Co elements, the (La0.8Sr0.2)Co0.4O3 showed the lowest H2 yield and poor stability. The tradeoff between increasing the oxygen vacancy amount and maintaining proper thermodynamic barrier for water splitting was met to reach a peak H2 yield when x = 0.4, demonstrating an effect of high entropy on the thermodynamics of thermochemical water splitting. This work provides guidance and new understanding for the development of new oxygen carriers for thermochemical water splitting and other chemical looping reactions.

Embargo Reason

Publication Pending

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