The quest for sustainable and environmentally friendly energy sources has led to significant research and development efforts in the field of renewable energy. In this context, the production of renewable drop-in fuels for the transportation sector has garnered considerable attention. A recent study published in the peer-reviewed journal Advanced Materials Interfaces by a team of researchers from ETH Zürich in Switzerland, led by Dr. Sebastian Sas Brunser, Dr. Fabio L. Bargardi, Dr. Rafael Libanori, Dr. Noëmi Kaufmann, Dr. Hugo Braun, Professor Aldo Steinfeld, and Professor André R. Studart, presents a novel approach to produce these fuels using concentrated solar energy and ceria (CeO2) as the key redox material.
Ceria-based thermochemical redox cycles have emerged as a promising avenue for producing renewable fuels due to their rapid kinetics and crystallographic stability. It results in the production of a tailored syngas mixture of H2 and CO, which can be further processed into liquid hydrocarbon fuels such as kerosene, diesel, and gasoline. What sets this approach apart is its direct utilization of concentrated solar energy to drive the redox cycle, bypassing several intermediate steps involved in conventional methods, such as solar electricity generation, water electrolysis, and reverse-water gas shift reactions. The cornerstone of this solar thermochemical fuel production process is the solar reactor, which plays a critical role in achieving high solar-to-fuel energy efficiency. However, prior to the research conducted by the ETH Zürich team, the solar-to-fuel energy efficiency of existing solar reactors remained relatively low, with maximum measured values in the single digits. These values indicated that there was significant room for improvement in the design and performance of solar reactors for this purpose.
One of the key challenges in designing efficient solar reactors is to ensure uniform heating of the redox material throughout the structure. Previous reactor designs, including those utilizing reticulated porous ceramic (RPC) structures made of ceria, faced issues related to non-uniform temperature distribution and inefficient utilization of the redox material. The homogeneous porosity of RPC-type structures resulted in exponential attenuation of incident solar radiation, leading to overheating of regions near the irradiated front face and insufficient heating of regions deeper within the structure. This inefficiency not only reduced the overall fuel yield but also adversely affected the solar-to-fuel energy efficiency.
Additionally, there was a trade-off between increasing the effective density of the structure (ρeff) and achieving volumetric radiative absorption. Higher ρeff, which represents the mass of ceria per total volume, was desirable to maximize the redox active material loading inside the solar reactor. However, increasing ρeff often led to greater optical thickness and hindered volumetric radiative absorption, resulting in a more pronounced temperature gradient and inefficient use of the redox material.
To address these challenges, the researchers at ETH Zürich developed a novel approach based on hierarchically ordered channeled structures made of pure ceria. These structures were manufactured using the direct ink writing (DIW) technique with a specially formulated ink that exhibited optimal rheological behavior. The rheological properties of the ink were critical in achieving proper extrusion and bonding between printed layers while preventing shape distortion. The key advantage of hierarchically ordered channeled structures is their ability to stepwise attenuate incident radiation, resulting in a more uniform temperature distribution throughout the structure. Unlike the RPC-type structures, which exhibited monotonically decreasing temperature profiles, the hierarchically ordered structures achieved higher and more uniform temperature values, even with higher values of ρeff. This uniform heating of the redox material led to significantly improved specific fuel yield, indicating that a larger portion of the ceria structure contributed to fuel generation.
The authors findings demonstrated that the hierarchically ordered channeled structures, referred to as “Gradient-1” and “Gradient-2,” outperformed the state-of-the-art RPC structures in terms of specific fuel yield. These structures produced approximately three times more CO with only a modest increase in mass, highlighting their superior efficiency. Moreover, the stepwise radiative attenuation in these structures allowed for deeper penetration of incident radiation, reducing emissive radiative losses and further enhancing overall temperature levels. Long-term stability testing of these structures under consecutive redox cycles validated their mechanical and chemical stability. The vacuum coating technique was employed to address imperfections, such as cracks, ensuring the structural integrity of the hierarchically ordered channeled structures.
The research conducted by the ETH Zürich researchers represents a significant advancement in the field of solar thermochemical fuel production. The development of hierarchically ordered channeled structures with optimized rheological properties opens the door to more efficient and scalable solar reactor designs. By addressing the challenges of non-uniform heating and inefficient utilization of redox material, these structures have the potential to significantly increase the solar-to-fuel energy efficiency of the entire process. The implications extend beyond the laboratory setting. Implementing hierarchically ordered channeled structures in commercial-scale solar reactors could lead to a substantial increase in the production of renewable drop-in fuels from CO2 and H2O using concentrated solar energy. Furthermore, the enhanced energy efficiency could make this approach economically competitive with other methods of producing solar drop-in fuels, providing a viable and sustainable alternative to conventional fossil fuels.
Sebastian Sas Brunser, Fabio L. Bargardi, Rafael Libanori, Noëmi Kaufmann, Hugo Braun, Aldo Steinfeld, André R. Studart. Solar-Driven Redox Splitting of CO2 Using 3D-Printed Hierarchically Channeled Ceria Structures. Advanced Materials Interfaces 10, 30, 2023, 2300452