Ceria and Metal Ferrite Core-Shell Nanoparticle for Solar Thermochemical Fuel Production

Two-step metal oxide-based Solar Thermochemical Hydrogen Production (STCH) is an emerging technology that directly utilizes high-temperature heat to split water and carbon dioxide, producing hydrogen and carbon monoxide, respectively. Using heat can be a more cost-effective source than electricity, particularly for high-capacity factors and continuous fuel production. However, to achieve ambitious targets like the US DOE HydrogenShot ($1/kg-H2), significant efficiency gains and cost reductions are essential. Optimizing the redox material is pivotal in advancing STCH fuel production.<br/><br/>Ceria (CeO₂) is the state-of-the-art redox material in STCH systems due to its high-temperature stability, fast reduction and water-splitting kinetics, and thermodynamic properties. Yet, its limited oxygen-carrying capacity (OCC), compared to perovskites and iron oxides, results in diminished fuel productivity and solar-to-fuel efficiency. Larger quantities of ceria are needed, leading to larger reactors and higher costs.<br/><br/>Metal-substituted ferrites are another STCH material class with 3-5 times higher OCC compared to ceria. However, ferrites can experience sintering and rapid deactivation at high temperatures (~1500°C), causing a marked decrease in fuel productivity after a few cycles. This sintering effect enlarges particle size, and the inherent low ion diffusivity in spinel slows water-splitting kinetics, ultimately leading to material deactivation. Doping ferrites with different cations or combining with inert supports like zirconia has proven inadequate in preventing ferrite agglomeration [1].<br/><br/>In our previous work, we introduced a composite STCH redox material combining magnesium ferrite (MgFe₂O₄) and ceria. We hypothesize that such a composite can combine the high OCC of ferrites with the thermal stability and high oxygen diffusivity of ceria. Our system-level thermodynamic analysis showed that a composite with 50 wt-% ferrite produces twice as much hydrogen per unit mass as ceria and increases STCH efficiency from 33% to 35.3% [2]. <br/><br/>In this presentation, we will discuss our experimental efforts with ceria-ferrite composites. We will present the synthesis and characterization of core-shell composites, with a Mg-ferrite core and a ceria shell. Ceria was chosen as the shell material because of its low sintering, fast redox kinetics, and high oxygen diffusivity. We compare the performance of the core-shell composite with that of simple mechanical mixtures in which ferrite and ceria powders are mixed and pressed into a porous pellet. A combination of Infrared furnace experiments and thermogravimetric analysis is used to measure the OCC of composites at STCH-relevant conditions. The experiments involve thermochemical cycles by switching the temperature between reduction(1400°C-1500°C) and water/CO2 splitting(700°C-900°C), changing the gas environment, and monitoring the real-time gas composition using an in-line mass spectrometer. We measure OCC and fuel production profiles over several consecutive STCH cycles to study the sintering or other forms of degradation composites. The ferrite content in the composite is varied to measure its impact on OCC and degradation. <br/><br/>Ex-situ X-ray diffraction (XRD), SEM and TEM imaging, and surface area measurements will be used to characterize the evolution of composite chemistry and morphology during STCH cycles. This work aims to demonstrate the potential of ferrite-ceria composites and core-shell structures as leading an STCH redox material, setting the stage for scaled-up studies in the future.<br/><br/>[1] Miller et al. <i>Advanced Energy Materials</i>, <i>4</i>(2), 1300469 (2014).<br/>[2] Patankar, PhD thesis, MIT (2023)