Non-Stoichiometric Redox Active Perovskite Materials for Solar Thermochemical Fuel Production: A Review

Abstract

Due to the requirement to develop carbon-free energy, solar energy conversion into chemical energy carriers is a promising solution. Thermochemical fuel production cycles are particularly interesting because they can convert carbon dioxide or water into CO or H2 with concentrated solar energy as a high-temperature process heat source. This process further valorizes and upgrades carbon dioxide into valuable and storable fuels. Development of redox active catalysts is the key challenge for the success of thermochemical cycles for solar-driven H2O and CO2 splitting. Ultimately, the achievement of economically viable solar fuel production relies on increasing the attainable solar-to-fuel energy conversion efficiency. This necessitates the discovery of novel redox-active and thermally-stable materials able to split H2O and CO2 with both high-fuel productivities and chemical conversion rates. Perovskites have recently emerged as promising reactive materials for this application as they feature high non-stoichiometric oxygen exchange capacities and diffusion rates while maintaining their crystallographic structure during cycling over a wide range of operating conditions and reduction extents. This paper provides an overview of the best performing perovskite formulations considered in recent studies, with special focus on their non-stoichiometry extent, their ability to produce solar fuel with high yield and performance stability, and the different methods developed to study the reaction kinetics.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature

“…Additionally, many SOFC cathode materials are reversibly reduced and oxidized, as revealed by their electrochemical properties . Reversibility of reduction/oxidation and catalytic activity are highly desirable in a material to be used for water splitting . Thus, a large number of materials studied as potential SOFC cathodes could be useful for hydrogen production.…”

“…For example, MgO and Fe 2 O 3 are reduced at approximately 3400 °C, TiO 2 at approximately 3700 °C, and CeO 2 at approximately 2000 °C . Such high temperatures are owing to the stability of the metal oxidation states (Mg 2+ , Fe 3+ , Ti 4+ , and Ce 4+ ) and/or the difficulty of the corresponding structures (rock‐salt, corundum, rutile, and fluorite, respectively) to tolerate oxygen sub‐stoichiometry, in contrast to the perovskite structure, which can accommodate large degrees of oxygen sub‐stoichiometry . For example, only approximately 2 % of the oxygen sites in ceria can be actively used for splitting of water .…”

“…These oxides display advantageous properties such as a large degree of reduction at moderate temperatures, rapid oxidation and reduction kinetics, favorable oxidation thermodynamics, and stability at high temperatures . In this context, perovskite oxides may stand as promising candidates for hydrogen production by thermochemical water splitting . The non‐stoichiometric perovskite acts as a solid‐state oxygen carrier, which facilitates water reduction into hydrogen …”

“…In particular, those perovskite oxides displaying both mixed ionic–electronic conductivity (MIEC) and high catalytic activity for oxygen reduction are of considerable interest as air electrode (cathode) materials in solid‐oxide fuel cells (SOFC) . MIEC is related to the presence of mobile oxygen vacancies and the ability to gain/lose oxygen under different oxygen partial pressures ( p O 2 ); because metal ions present mixed oxidation states, this is also associated with catalytic properties . Additionally, many SOFC cathode materials are reversibly reduced and oxidized, as revealed by their electrochemical properties .…”

Metal Oxides Applied to Thermochemical Water-Splitting for Hydrogen Production Using Concentrated Solar Energy

“…Based on the possible number of doping schemes, series of even more attractive materials certainly remain to be discovered. More recently, perovskites of the form ABO 3−δ have emerged as a new attractive class of nonstoichiometric oxides that are widely unexplored for thermochemical cycles [161][162][163][164][165][166][167][168][169][170][171][172][173]. These oxygen-deficient materials exhibit interesting characteristics concerning oxygen storage capacities and transport properties through oxygen vacancies formation.…”