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

Paper Number: 130646

130646 - Materials Thermodynamic Limits in Thermochemical Fuel Production 

Abstract: 

Solar-thermochemical (STC) splitting of water and carbon dioxide shows promise to be a crucial technology in the global shift away from fossil fuels, pivotal in the pursuit of sustainable and renewable energy carriers and feedstocks. As the world increasingly seeks to reduce its dependence on fossil fuels, STC presents a pathway to produce solar fuels, which is essential in this transition. The process, which utilizes solar energy to generate hydrogen (H₂) and/or carbon monoxide (CO), aligns with broader environmental objectives of reducing carbon emissions and fostering a sustainable energy economy.

The most extensively studied STC implementation, the two-step redox-active metal oxide cycle, is at the forefront of this technological shift. This approach involves metal oxides undergoing redox reactions, which are key to efficiently splitting water and carbon dioxide. The success of this technology depends heavily on advanced reactor designs and the development of effective redox-active metal oxides, that perform better than the state-of-the-art, ceria (CeO₂). These advancements are geared towards enhancing the energy efficiency of the STC process, addressing issues such as high energy consumption and the durability of materials under operational conditions.

Historically, research in this field has often focused on optimizing specific material attributes, such as lowering the reduction temperature, without fully considering the impact on other reactions within the cycle. This approach neglected the intricate interdependencies between various reactions, reduction, re­oxidation, and separations, where altering one aspect could inadvertently adversely affect others beyond what is tolerable. Recognizing this, our paper introduces a comprehensive thermodynamic model that is independent of the materials used. This model provides clarity on the trade-offs between different performance metrics, such as the enthalpy of reduction, reduction temperature, reduction extent, and overall conversion (splitting) yield.

Applicable to a broad range of non-stoichiometric metal oxides, particularly those without significant phase changes during redox cycles or where such changes are energetically and structurally negligible, the model offers insights critical for enhancing STC efficiency. It can also be extended to materials experiencing modest phase changes, thus widening its scope of application. The findings from this model underscore the importance of operating parameters, such as temperature and oxygen partial pressure during the reduction phase. These parameters set firm limits on the cycle's yield and the thermodynamic requirements of the redox-active materials.

In summary, the advancements in STC technology discussed in this presentation represent significant progress toward achieving the global objective of transitioning away from fossil fuels. By enhancing the efficiency and understanding of the STC process, this research contributes to the development of sustainable solar fuel production, a key element in the future of sustainable energy systems and chemical feedstocks.

Presenting Author: James Miller Arizona State University

Presenting Author Biography: James E. Miller (Jim) is a chemical engineer who has directed his efforts towards energy, materials, and chemical processing research. Prior to joining ASU LightWorks® in 2018, he worked at Sandia National Laboratories for over 25 years where his work touched on diverse topics ranging from catalysis for a variety of reactions to desalination. He served as the principle investigator of the large multi-institution and –discipline solar thermochemical efforts for solar fuels (Sunshine to Petrol) and thermochemical energy storage (PROMOTES). He led and contributed directly to efforts in materials development and characterization, development of new high temperature thermochemical reactors and systems, and thermodynamic, engineering and technoeconomic analysis. Miller has co-authored over 120 technical documents, holds 10 patents, and is the recipient of two R&D 100 Awards.

Miller received his BS in Chemical Engineering from Texas A&M University in 1986, and his PhD in Chemical Engineering from the University of Texas at Austin in 1992.

Authors:

Ivan Ermanoski Arizona State University
James Miller Arizona State University
Ellen Stechel Arizona State University