Jonathan R. Scheffe

This work reports kinetics and improved long-term stability of A-site deficient Ga-doped La–Sr–Mn (LSM) perovskite for two-step thermochemical H ₂ O splitting over 100 redox cycles (163 h).

Hydrogen production via multi‐step thermochemical cycles may be an affordable, low‐carbon alternative to steam methane reforming or water electrolysis. Conventional thermochemical water splitting generates hydrogen while coproducing an oxidized product, oxygen, that has little value. Instead, we use elemental sulfur as an input and coproduce sulfur dioxide. This process, which we call Steam Sulfur Reforming (SSR), could function as a value‐added substitute for the sulfur burning step of sulfuric acid manufacturing. SSR is demonstrated with experiments using ceria and iron oxide, thermodynamic modeling, and technoeconomic analysis. Sulfur oxidation is favorable at temperatures near 1200°C, comparable to conventional sulfur burning and lower than many thermochemical water splitting cycles. Experiments with ceria demonstrate stability over dozens of cycles, with sulfur conversions of up to 90% at 1200°C and steam conversions of up to 50% between 700°C and 900°C. Experiments with iron oxide show the intended cycle but are challenged by sulfidation side reactions and low steam conversion, highlighting opportunities for metal oxide modification. SSR may achieve costs below 2$/kg H ₂ with CeO ₂ or Fe ₃ O ₄ , making the process cost‐competitive with fossil hydrogen and a potential drop‐in hydrogen replacement for the fertilizer industry.

Solar thermochemical hydrogen (STCH) production via thermochemical redox cycling of metal oxides is a promising pathway for production of green hydrogen. Understanding the intrinsic thermodynamic properties of metal oxide candidates employed in these cycles is vital for better understanding process technoeconomics and reactor design. Herein, for the candidate Ga- doped La-Sr-Mn perovskite water splitting oxide (La_0.6Sr_0.4)_0.95Mn_0.8Ga_0.2O_3-δ (LSMG6482), we present a detailed characterization of the partial molar thermodynamic properties, defect equilibria, and water splitting behavior. Measurements of equilibrium oxygen non-stoichiometry (δ) were obtained from thermogravimetric relaxation experiments to describe the equilibrium behavior of LSMG6482 (and CeO_2-δ as a reference) in a pO₂ range of 10^-6.84 atm ≤ pO₂ ≤ 10^-2.94 atm and a temperature range of 1200 °C ≤ T ≤ 1400 °C. From this data, the partial molar changes of enthalpy and entropy (∆h_o and ∆s_o), were extracted by fitting the experimental data to an oxygen defect model, and through Van’t Hoff analysis were compared to prior published Al- doped La-Sr-Mn perovskites. It was found that the magnitude of ∆h_o and ∆s_o was greater than those of Al- doped LSM perovskites at δ ≥ 0.1, with ∆h_o (δ=0.1) = 286 kJ mol^-1 and ∆s_o (δ=0.1) = 132 J mol^-1 K^-1. An Ellingham diagram and water splitting model was developed using ∆h_o and ∆s_o to observe trends in H₂ yield and steam conversion to identify suitable operating conditions for LSMG6482 and contrasted with the state-of-the-art CeO_2-δ. Overall, it was found that LSMG6482 is most suitable for cycling under smaller temperature swings than ceria and is suitable for lower reduction temperature operation (~1300 °C vs. ~1500 °C), but at the expense of low steam conversion to H₂.

Solar-driven chemical looping technologies are promising pathways to convert CO₂ and other carbonaceous feedstocks such as biogas or methane to synthesis gas, a building block for renewable ‘drop-in’ fuels. Herein, a novel non-steady operating strategy, driven by computational insight, is presented to improve syngas selectivity during chemical looping reforming of methane while maintaining high conversion. We show that periodic longer reaction times enable non-steady cycling, where an optimal metal oxide oxygen deficiency at the outlet of a fixed bed reactor and lower oxygen deficiency elsewhere is key for improved performance. Using the candidate nonstoichiometric metal oxide Ni-CeO_2−δ, experiments demonstrate syngas selectivity as high as 98% and above the 95% achieved via non-steady cycling strategies, all while maintaining approximately 100% conversion of methane and 96% conversion of CO₂ at 800 °C. Advantages of this cycling strategy are projected to further increase with scale. Additionally, insights from this work can be generalized to numerous other industrially relevant chemical looping processes.

Synthesis of solar thermochemical H₂ (STCH) production redox materials with engineered structures, for example replica foams, can enable efficient heat and mass transport and are critical for scaled-up systems. Prior work has motivated the use of LSM-type perovskites as foamed STCH materials, but the effect of their morphology on bulk and kinetic behavior has not been reported. In this work, replica and direct foamed samples of L_a0.65Sr_0.35MnO_3-δ (LSM35) were fabricated and compared to synthesized powders and dense monoliths, and similarly synthesized CeO_2-δ (ceria) foams, regarding their specific reaction rates and bulk oxygen capacity/H₂ yields. Changes in oxygen capacity (Δδ) and reaction rates were measured between 1200 °C −1400 °C by using fixed ratios of steam and hydrogen during both reduction and oxidation steps, allowing for analysis under practical high conversion conditions. Results suggest bulk behavior and reaction rates of the foamed LSM materials are comparable to their powder analogues. Differences in reaction rates were observed only when replica foamed samples were subjected to rapid laser heating (emulating conditions expected in solar furnaces), which is expected but has not been demonstrated at such a small scale. Foamed samples were further subjected to 50 redox cycles at 1400 °C to evaluate their stability. Results show no statistically significant decrease in hydrogen production for any of the foamed samples, but the direct foamed samples became brittle with time. Together, these results demonstrate the viability of replica foamed LSM perovskites for integration in scaled up STCH systems.

Chemical looping reforming of methane utilizing the oxygen exchange material CeO ₂ is catalytically enhanced with metallic Ni to improve reaction rates at low operating temperatures. A kinetic study and comparison to noncatalyzed CeO ₂ is presented.