Session: 17-02: Symposium Steinfeld - Solar fuels via two-step cycles + the addition

Paper Number: 142198

142198 - Compositional Exploration of La-Sr-Mn-Ga Perovskites and Thermodynamic Activity Modeling for Solar Thermochemical Water Splitting 

Abstract: 

Solar thermochemical redox cycling of Sr doped LaMn (LSM) perovskite type metal oxides has been shown to be a promising pathway for thermochemically splitting water to produce solar H₂. Recent experimental work has revealed that doping the B-site of the LSM perovskite with Ga results in desirable high H₂ yields, equal to or exceeding that of more well-known Al- doped LSM perovskites, under relevant solar thermochemical cycling conditions. In this work, we examine the influence of varying A-site and B-site doping concentrations of Sr and Ga, respectively, in a La-Sr-Mn-Ga (LSMG) perovskite on water splitting behavior and H₂ yields. Powder samples of (La_1-xSr_x)_0.95Mn_1-yGa_yO_3-δ (x = 0.2, 0.4; y = 0.1, 0.2, 0.3) were synthesized and subjected to temperature swing cycling, at a reduction temperature of T_red = 1400°C and oxidation temperature of T_ox = 1200°C, and high conversion cycling at temperatures between 1300°C and 1400°C with nH₂O/nH₂ ranging from 150 to 500 in the Sandia National Labs Stagnation Flow Reactor (SFR). Peak oxidation rates and total H₂ yields were observed and compared over a total of 12 cycles throughout the experimental campaign. Overall, the compositions demonstrated steady water splitting behavior and it was revealed that when increasing Sr concentration from 20% to 40% with all else held constant, there was a 2.85x increase in H₂ yield from 57.9 to 165.1 µmol g^-1. Similarly, when increasing Ga concentration from 10% to 20% with all else held constant, there was a 1.44x increase in H₂ yield from 114.6 to 165.1 µmol g^-1. This was not observed when increasing Ga concentration from 20% to 30%, however the Sr concentration was decreased simultaneously to the Ga increase due to solubility limits during synthesis. During high conversion experiments, the peak oxidation rates of the 40% Sr doped samples were consistently greater than the 20% samples, likely due to operation at a deeper reduction extent promoted by the Sr. Under all the conditions investigated, (La_0.6Sr_0.4)_0.95Mn_0.8Ga_0.2O_3-δ, or LSMG6482, produced the most H₂. Motivated by this insight, LSMG6482 and its Al- doped analogue, LSMA6482, partial molar thermodynamic properties, extracted in a parallel study, were used to probe thermodynamic activity and favorability toward redox reactions under various conditions to gauge the role of Ga- doping. Specifically, we analyze Gibbs free energy changes, nH₂ yield, and steam conversion compared to the well-known Al- doped LSM. The results from this study help guide the doping strategies used in synthesis of LSMG type perovskites and provide insight toward the ideal operating conditions for their use in efficient thermochemical H₂ production.

Presenting Author: Caroline Hill University of Florida

Presenting Author Biography: Caroline Hill graduated from the University of Tennessee, Knoxville in 2017 with a B.S. in mechanical engineering. Following graduation, she spent a year working with the Siemens Healthcare division in Knoxville, TN. There she assisted with the research and development of cutting-edge PET/CT scanners used primarily for the diagnosis and treatment of cancer. In August 2018, she enrolled in the PhD program at the University of Florida and joined the Renewable Energy Conversion Laboratory. Working as a research assistant under the advisement of Dr. Jonathan Scheffe, she studied candidate redox materials and novel operating methods to improve solar thermochemical looping reforming of methane. She earned her M.S. in mechanical engineering from the University of Florida in May 2022 and her Ph.D. in May 2023. Currently, she is a postdoctoral associate at the University of Florida continuing research in solar fuels.

Authors:

Dylan McCord University of Florida
Caroline Hill University of Florida
Elizabeth Gager University of Florida
Juan Nino University of Florida
Anthony Mcdaniel Sandia National Laboratories
Jonathan Scheffe University of Florida