(685b) Two-Step Solar Thermochemical Cycle for Splitting CO2 Via Ceria Redox Reactions - Experimental Investigation with a 3.8 Kw Solar Reactor

Philipp Furler ¹, Jonathan Scheffe ¹, Aldo Steinfeld ^1,2

¹ Department of Mechanical and Process Engineering, ETH Zurich
Zurich 8092, Switzerland

² Solar Technology Laboratory, Paul Scherrer Institute
Villigen PSI 5232, Switzerland

Abstract

Solar thermochemical approaches for CO₂ splitting inherently operate at high temperatures and utilize the entire solar spectrum. As such, they provide an attractive path to solar fuels production with high energy conversion efficiencies without the use of precious metal catalysts. In contrast to direct thermolysis of CO₂, two-step thermochemical cycles further bypass the CO/O₂ separation problem. Cerium oxide (ceria) has emerged as a highly attractive redox intermediate because of its favorable thermodynamics and kinetics at moderate temperatures. Reduction proceeds via the formation of oxygen vacancies and the release of gaseous O₂, resulting in the subsequent change in stoichiometry (δ). Oxidation is capable of proceeding with CO₂, thereby releasing CO and re-incorporating oxygen into the lattice. The two-step CO₂splitting solar thermochemical cycle based on oxygen-deficient ceria is represented by:

High-T reduction:                   CeO₂ → CeO₂-δ + δ/2 O₂                          (1)
Low-T oxidation with CO₂:    CeO₂-δ + δ CO₂ → CeO₂+ δ CO                (2)

We report on recent experimental studies for splitting CO₂ using concentrated solar energy. A novel, macro-porous CeO₂ structure was developed and tested in a 3.8 kW solar reactor prototype which was previously described in detail.^{1, 2} Experiments were conducted at ETH’s High-Flux Solar Simulator under conditions that closely approximates the heat transfer characteristics of highly concentrating solar systems such as solar towers and parabolic dishes. The reactor engineering design, experimental setup, and the novel CeO₂structure are described in detail and measured product compositions and solar-to-fuel energy conversion efficiencies are presented.

1.    W. C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S. M. Haile and A. Steinfeld, Science, 2010, 330, 1797-1801.
2.    P. Furler, J. R. Scheffe and A. Steinfeld, Energy & Environmental Science, 2012, 5, 6098-6103.