(281e) Metal Ferrite and Ceria Composites for Solar Thermochemical Fuel Production

Rapid decarbonization of tough-to-electrify sectors like long-distance transportation and industrial heating can be facilitated by synthetic renewable fuels like hydrogen, syngas and hydrocarbons. Thermochemical (TC) redox cycles is an emerging technology that uses high-temperature heat to split water and carbon dioxide to produce hydrogen and carbon monoxide respectively¹. Two-step, metal-oxide based TC cycles consist of a high temperature reduction step followed by a water-splitting step. Intermittent renewables like solar energy can be stored in the form of thermal energy at low cost, enabling round-the-clock fuel production. This contrasts with electrolysis which uses expensive-to-store electricity. Despite the promise of high capacity factor operation, significant efficiency gains and cost reductions are required to produce green hydrogen by the TC route at the $1/kg price target set by US DOE². Along with more efficient reactors³, improved redox materials are needed to bring TC fuel production technology closer to viability.

State-of-the-art solar TC fuel production systems use ceria as the redox metal oxide because of its high temperature stability, fast reduction and water-splitting kinetics, and fair thermodynamic properties. However, ceria suffers from low oxygen carrying capacity (OCC) compared to other candidate materials like perovskites and iron oxides, with one kilogram of ceria producing less than 0.5 grams of hydrogen per cycle under typical operating conditions. Low fuel productivity limits the solar-to-fuel efficiency of ceria-based cycles. Moreover, it necessitates large quantities of ceria in the TC fuel production facility. The cost penalty associated with low ceria OCC is exacerbated by the high price of ceria and high-temperature TC reactors. Doped ceria (eg. Ceria-zirconia solid solutions) and perovskite materials have higher OCC than ceria⁴. However, their equilibrium conversion during water-splitting or CO₂-splitting is rather low, resulting in large amounts of unconverted oxidizer (overall steam to hydrogen ratio > 500). When the work of separating fuel (H₂/CO) and oxidizer (H₂O/CO₂) is accounted for, doped ceria and perovskites generally have a lower efficiency than pure ceria⁵.

Metal-substituted ferrites are another class of TC materials that have higher OCC than ceria, and can split water with reasonable steam-to-hydrogen conversion. We have previously reported a thermodynamic analysis that showed nickel ferrite (NiFe₂O₄) being 5 percentage points more efficient than ceria, with a heat-to-hydrogen conversion efficiency of 34%⁶. However, metal ferrites undergo severe sintering and deactivation at high reduction temperatures (1500C), leading to reduced fuel productivity after only a few cycles⁷. As particle size increases with sintering, the low mass diffusivity of spinel leads to slower water-splitting kinetics and eventual deactivation of some material⁸. The issue of slow water-splitting kinetics is exacerbated by the fact that equilibrium steam-to-hydrogen conversion is low at temperatures above 1000C. Dispersing the ferrite on a refractory zirconia support reduces the rate of sintering, but cannot entirely prevent ferrite agglomeration. More recently, iron aluminate has been suggested for TC fuel production, and it was shown to be resistant to sintering and deactivation at 1400C⁹. However, behavior at higher reduction temperatures and lower temperature water-splitting kinetics are yet unknown.

We have previously proposed Nickel ferrite and ceria composites for TC fuel production⁶. Such a composite can benefit from the high OCC of ferrites, and the thermal stability and high oxygen diffusivity of ceria. We showed that the efficiency of a composite with 50 wt% ferrite is 3.5 percentage points higher than pure ceria (33% heat-to-hydrogen efficiency for the composite). Moreover, such a composite produces twice as much hydrogen per unit weight as compared to ceria and costs considerably less in terms of raw material. These benefits grow as the percentage of ferrite in the composite increases. However, some minimum amount of ceria will be required to prevent sintering and maintain fast kinetics. This previous study was a theoretical analysis using thermodynamic properties of ceria and Nickel ferrite. In other work ceria-perovskite composites have been reported to have improved TC performance compared to their constituents^10,11. More recently, iron-nickel alloy embedded in a perovskite substrate has been proposed for TC fuel production¹².

In this presentation we will report progress on our work with metal ferrite and ceria composites for TC fuel production. We focus on Nickel and Magnesium ferrites (NiFe₂O₄ and MgFe₂O₄ respectively) because of their lower cost and favorable thermodynamic properties. Mechanical mixture composites of a ferrite and ceria are synthesized and tested under TC conditions in an Infrared Furnace (IRF). The micro-reactor consists of an alumina tube in the IRF holding a porous pellet of the redox material. Thermochemical cycles are executed by varying the IRF temperature and changing the composition of gas flowing through the alumina tube. Reduction temperatures between 1400-1500C are considered, along with water-splitting/CO₂-splitting temperatures between 700-1100C. An in-line mass spectrometer is used to measure the composition of gas leaving the furnace.

Purity of ceria and ferrite phases can be affected by diffusion of cations between the two phases. This will be investigated experimentally, supplemented by thermodynamic calculations using Factsage¹³. Significant intermixing (e.g. dissolution of iron in ceria phase) will alter thermodynamic properties of both phases, and will necessitate a revision of the cycle efficiency analysis we reported earlier. Along with phase composition, the evolution of composite morphology will also be investigated with high-temperature ageing and long-term TC cycling. Such changes will be correlated with variation in fuel production rates observed in the IRF experiments. The effect of varying ferrite-ceria ratio in the composite on long-term stability, kinetics and fuel productivity will be measured. Alternative composite morphologies will be considered for improved resistance to ferrite agglomeration and deactivation. This work will demonstrate the feasibility of ferrite-ceria composites as TC redox materials. Composite thermodynamics and kinetics data obtained in this study will be used to formulate a higher-fidelity system efficiency and optimization model in the future.

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