Solar thermochemical conversion of CO2 and water into fuel using non-stoichiometric oxygen-exchange redox materials – SUNFUEL

The project is focused on two-step cycles involving non-stoichiometric solid-gas reactions. The proposed approach makes use of a cyclical redox process involving metal oxide systems for the separate production of H2/CO and O2 in two distinct steps: (1) reduction of the oxide releasing O2, (2) oxidation of the reduced oxide with H2O/CO2 to form H2 and CO, where the mixture can serve as the gaseous feedstock for the synthesis of liquid hydrocarbon fuels. The energy required in the reactor is supplied by concentrating solar radiation. The high-temperature reactions are carried out at the focal point of solar concentrators at PROMES laboratory, enabling concentration of direct sunlight radiation. A solar reactor in which the active material is integrated is used to perform both reaction steps of the cycle.
The R&D studies performed in this project are related to the identification, chemical synthesis and shaping of redox active materials, the characterization of thermochemical reactions resulting in CO2 and H2O conversion, and the design/development of suitable solar reactors.
The main locks to be lifted for the development of this new pathway towards solar fuels production are the optimization of the materials reactivity and stability during cycling, and the development of novel solar reactor technologies based on different concepts (porous media, membranes) for the continuous production of CO/H2.

The obtained results are related to the optimization of materials reactivity by modification of their composition and morphology, and their utilization in solar processes, which requires their suitable shaping for integration in a solar reactor. The most stable and best-performing non-stoichiometric oxides were shaped in the form of porous structures for volumetric radiation absorption (ceramic foams) or dense membranes for their integration in reactor. Two innovative solar reactor prototypes were designed and constructed, then tested to optimize their operating conditions, fuel yields, and thermal efficiencies. The H2O and CO2 conversion process was experimentally demonstrated in the reactor prototypes under real solar irradiation to investigate reaction yields, fuel productivities, and energy conversion efficiencies. The solar facilities of PROMES laboratory (solar furnaces equipped with parabolic concentrators) were extensively used for the experimental study.
The obtained scientific and technological results of the project are the followings:
(1) identification and synthesis of innovative redox materials, and comparison of their performance (H2/CO productivity, chemical kinetics, cycling stability under high-flux concentrated solar radiation),
(2) shaping of materials as porous structures able to efficiently absorb the concentrated solar radiation, and as dense oxygen-conducting membranes,
(3) design and construction of 2 prototype reactors to demonstrate new solar reactor concepts operating continuously (porous media and dense membranes), with optimized thermochemical efficiencies and potentially scalable.

Ceria reticulated foams showed good stability during thermochemical cycling under concentrated solar flux. The main prospects in this field will be related to the optimization of the foams composition to maximize H2/CO productivity, as well as their geometry to maximize solar radiation absorption (minimization of thermal gradients).
The production of MIEC membranes was challenging and required specific development for their integration in a high temperature solar reactor. Dense and stable tubular membranes have been for the first time tested under real solar conditions and the solar reactor isothermal continuous operation has been demonstrated for a total testing duration above about 20h at temperatures between 1450 and 1550°C. These results are promising because the preparation of a dense membrane remaining stable above 1400°C under a partial pressure of oxygen was challenging. The optimization of the thermo-mechanical resistance and the structure of membranes could be considered in future studies.

- 12 international publications:
3 ‘review’ articles were published on redox materials, perovskites structures and doped ceria applied to thermochemical cycles (Catalysts, 2018, 8(12), 611), (AIMS Materials Science, 2019, 6(5), 657-684), (ChemEngineering, 2019, 3(3), 63).
2 publications were published on the synthesis/characterization and performance of different perovskite oxide formulations for CO2 dissociation (Ceramics International 45 (2019) 15636–15648), (Sustainable Energy & Fuels (RSC) (2018), 2, 843-854).
5 articles were published on the performance of ceria-based foams in solar reactor as well as porous biomimetic materials (Chemical Engineering Research and Design, 2020, 156, 311-323), (Energy, 2020, 201, 117649), (Energy & Fuels, 2020, 34, 7, 9037–9049), (Sustainable Energy & Fuels, 2020, 4, 3077-3089), (Journal of CO2 Utilization, 2020, 41, 101257).
2 articles were published on the membrane solar process (Chemical Engineering Journal, 422, 2021, 130026), (Journal of Membrane Science, 634, 2021, 119387).

- Conferences: one poster on the development of dense membranes for solar fuels production was presented at ICIM 2018 (Dresde, Allemagne), at the Matériaux 2018 congress (Strasbourg, France) and at the Journées Annuelles du GFC 2019 (Montpellier) and GFC 2020/2021 (Caen, virtuel, France). An oral presentation was given at IUPAC 2019 congress (Paris), a keynote at the SFGP 2019 congress (Nantes) and an invited talk is planned at the AMC 2021 (Stockholm, Sweden).

The SUNFUEL project addresses the solar thermochemical conversion of H2O and captured CO2 for the production of high-value solar fuels and CO2 valorization. The production of alternative synthetic fuels to replace fossil fuels is a promising path to reduce CO2 emissions associated with the problems of global warming and depletion of carbon-based energy resources. The project targets clean, efficient and low cost solar fuel production from H2O and captured CO2 by using renewable solar energy in high-temperature thermochemical cycles. Solar-driven H2O and CO2-splitting using redox materials constitute an attractive option for massive synthetic fuel production, avoiding greenhouse gas emission and allowing complete recycling of chemical intermediates. Solar thermochemical approaches to split CO2 and H2O inherently operate at high temperatures and utilize the entire solar spectrum and, as such, provide an attractive path to solar fuels production with high energy conversion efficiencies and without any precious metal catalysts. The two-step process is based on the thermal reduction of a metal oxide to generate an oxygen-deficient active material that is subsequently oxidized with H2O and/or CO2 to generate syngas, the building block for a wide variety of synthetic fuels. This method for fuel production uses concentrated solar radiation as the source of high temperature process heat, and also results in the recycling and upgrading of CO2 into synthetic fuels. Moreover, the conversion of concentrated solar heat to chemical fuels has the advantage of producing long-term storable energy carriers.
This project addresses the investigation of innovative redox materials based on non-stoichiometric metal oxides as chemical intermediates. Two kinds of oxygen ion-conducting oxides are proposed, involving perovskite (ABO3) and ceria-based fluorite compounds (MxCe1-xO2), applied to solar fuel production. The novel class of materials based on perovskite structure is today emerging as a very promising option for solar thermochemical application. Sustained research efforts are required to evaluate their potential compared to conventional ceria-based materials (originally published by the project coordinator) and to integrate them in solar thermochemical fuel production processes. The research will first focus on the synthesis and characterization of redox active materials based on perovskite and fluorite structures suitable for H2O and CO2 splitting, and the determination of their thermochemical performance for fuel production. The O2 releasing thermal reduction and the H2O/CO2-splitting reactions will be studied with special emphasis on the oxygen storage capacities and exchange capabilities, reaction kinetics and chemical activity, as well as stability of thermochemical performance upon cycling. The best performing and stable materials will then be selected for the development of macroporous structures and dense membranes with enhanced transport properties for their integration in suitable solar reactors. Accordingly, two innovative solar reactor prototypes will be designed, constructed, tested and modelled to optimise operating conditions, chemical conversions, and thermal efficiencies. The developed solar reactors will be based on two very different concepts and operation modes: (1) monolithic porous structures with enhanced surface area for gas-solid reactions and efficient solar radiation absorption (temperature-swing redox cycling), and (2) mixed ionic-electronic conducting membranes for isothermal and continuous operation. The reactors will be simulated with models that couple transport processes, radiative heat transfer, and chemical reaction. One key objective of this project is the on-sun demonstration of the H2O/CO2-splitting process in the prototype reactors that will be experimentally studied to compare their performance concerning reaction yields, fuel production rates and solar-to-fuel energy conversion efficiencies.